Each chapter is divided into the following categories: Clinical Example. A short case report that serves to dramatize the relevance of neuroanatomy introduces. LWBKG-FM[i-xviii] 10/17/08 AM Page i Aptara (PPG-Quark) CLINICAL NEUROANATOMY S E V E N T H E D I T I O N Richard S. Snell. CLINICAL. NEUROANATOMY. S E V E N T H E D I T I O N. Richard ruthenpress.info sional responsibility of the practitioner; the clinical treatments described and.
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to succeed in college The book you are holding in your hands is now in its seventh edition,. How to Study Java The Complete Reference - 7th Edition. S. Mark Williams, Ph.D. Department of Neurobiology. Duke University Medical Center [email protected] Clinical Neuroanatomy for. Undergraduates. a LANGE medical book. Clinical. Neuroanatomy. Twenty-Seventh Edition. New York Chicago San Francisco Lisbon London Madrid Mexico City. Milan New.
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For Wendy and Rosalie, new lights in my life. Key Features of this Edition! Discussion of the latest advances in molecular and cellular biology in the context of neuroanatomy. Coverage of the basic structure and function of the brain, spinal cord, and peripheral nerves as w ell a clini al pre entation of di ea e proce e invol ing pe ific tructure. Clinical Correlations and case studies to help you interpret and remember e entia!
FIIJ "ln.. All iD diM. E -. L q Development ,. CCIN talftlii'Cffl TbtfUnn n Tables encapsulate. UlloM do.. Introduction to Clinical Thinking ecrion e plain how to u e neuroanatomy a a ba i for analyzing the di ordered nervou sytem.
AI Cll 6fJ! JII"d rn. Development and Cellular Constituents What is the lesion? Vascular Supply of the Brain 7. Imaging of the Brain References Electrodiagnostic Tests Discussion of Cases The Location of Lesions Questions and Answers Very few organ systems, if any, present as fascinating an array explicative, and memorable. This book is not meant to sup of structures and mechanisms as the human brain and spinal plant longer, comprehensive handbooks on neuroscience and cord.
Furthermore, it is hard to think of a clinical field that neuroanatomy. On the contrary, it has been designed to pro does not encompass at least some aspect of the neurosciences, vide a manageable and concise overview for busy medical stu from molecular and cellular neurobiology through motor, dents and residents, as well as trainees in health-related fields sensory, and cognitive neuroscience, to human behavior and such as physical therapy; graduate students and postdoctoral even social interactions.
It is the brain, in fact, that makes us fellows with an interest in neuroanatomy and its functional uniquely human. No surprise, then, that neuroscience has underpinnings; and clinicians in practice, for whom minutes emerged as one of the most exciting fields of research and now are precious.
This book is unique in containing a section entitled One of the unique things about the nervous system is its "Introduction to Clinical Thinking;' which introduces the exquisite architecture. The nervous system contains more cell reader, early in the text, to the logical processes involved in types than any other organ or organ system, and its con using neuroanatomy as a basis for thinking about patients. Many disease processes affect, in a direct or indirect way, acteristics of patients selected from an extensive clinical expe the nervous system.
Thus, every clinician, and every basic sci rience. Also included are illustrative clinical images including entist with an interest in clinical disease, needs an understand computer tomography CT and magnetic resonance imaging ing of neuroanatomy. Stroke remains the most frequent cause MRI , both of normal brain and spinal cord, and of common of death in most industrialized societies; mood disorders such clinical entities that trainees will likely encounter.
Joachim Baehring, MD, hospital stay. An understanding of neuroanatomy is crucial and Joseph Schindler, MD, of Yale, as well as Catharina Faber, not only for neurologists, neurosurgeons, and psychiatrists MD, at the University of Maastricht contributed invaluable but also for clinicians in all subspecialties, since patients of clinical illustrations. Over the years, these colleagues and every stripe will present situations that require an understand friends have helped to create an environment where learning is ing of the nervous system, its structure, and its function.
I hope that read This book, now in its 27th edition, is designed as an ers will join me in finding that neuroanatomy, which provides accessible, easy-to-remember synopsis of neuroanatomy and much of the foundation for both neuroscience and clinical its functional and clinical implications. Since many of us learn medicine, can be enjoyable, memorable, and easily learned.
Waxman, MD, PhD brain scans and pathological specimens but also with hun New Haven, Connecticut dreds of diagrams and tables that are designed to be clear, April It re Main Divisions ceives and interprets an immense array of sensory information, A. Anatomy controls a variety of simple and complex motor behaviors, and engages in deductive and inductive logic. The brain can make Anatomically, the human nervous system is a complex of two complex decisions, think creatively, and feel emotions.
It can gen subdivisions. The human nerv enclosed in bone and wrapped in protective coverings ous system, for example, can immediately identify a familiar face meninges and fluid-filled spaces.
It can carry out many of these demanding tasks in a nearly simultaneous manner. Indeed, neuroscience has begun to provide an under B. Physiology standing, in elegant detail, of the organization and physiology of the nervous system and the alterations in nervous system Functionally, the nervous system is divided into two systems.
This understanding is 7. Somatic nervous system-This innervates the structures firmly based on an appreciation of the structure of the nervous of the body wall muscles, skin, and mucous membranes. The complexity of the nervous system's actions is r eflected 2. Autonomic visceral nervous system ANS -The ANS by a rich and complex structure-in a sense, the nervous system contains portions of the central and peripheral systems.
It can be viewed as a complex and dynamic network of interlinked controls the activities of the smooth muscles and glands of the computers. Nevertheless, the anatomy of the nervous system internal organs viscera and the blood vessels and returns can be readily understood. Since different parts of the brain and sensory information to the brain. An understanding of neuroanatomy is The central portion o f the nervous system consists o f the immediately relevant to both basic neuroscience and clinical brain and the elongated spinal cord Fig 1 -2 and Table 1 - 1.
Clinical neuroanatomy i. Telencephalon cerebral hemisphere Skull"'lfl Brain Two cranial Cranial nerve X FIGURE 1 - 1 The structu re of the central nervous system and the periphera l nervous system, showi ng the r elationsh i p between FIGURE 1 -2 The two major divisions of the centra l nervous sys the centra l nervous system and its bony coverings.
Neurons, or nerve cells, are and the diencephalon; the telencephalon includes the cerebral specialized cells that receive and send signals to other cells cortex the most highly evolved part of the brain, sometimes through their extensions nerve fibers, or axons. The infor called "gray matter" , subcortical white matter, and t he basal mation is processed and encoded in a sequence of electrical or ganglia, which are gray masses deep within the cerebral hemi chemical steps that occur, in most cases, very rapidly in mil spheres.
The white matter carries that name because, in a freshly liseconds. Many neurons have relatively large cell bodies and sectioned brain, it has a glistening appearance as a result of its long axons that transmit impulses quickly over a considerable high lipid-rich myelin content; the white matter consists of distance.
Interneurons, on the other hand, have small cell myelinated fibers and does not contain neuronal cell bodies or bodies and short axons and transmit impulses locally. Nerve synapses Fig 1 The major subdivisions of the diencephalon cells serving a common function, often with a common target, are the thalamus and hypothalamus. The brain stem consists of are frequently grouped together into nuclei.
Nerve cells with the midbrain mesencephalon , pons, and medulla oblongata. The together outside the CNS are called ganglia. These spaces are filled with cere Glial cells within the brain and spinal cord outnumber brospinal fluid CSF Figs and 1 -5; see also Chapter 1 1. Cerebellar cortex Bra i n Cerebel l u m encepha lon Cerebellar nuclei. Midbrain mesencephalon. Bra i n stem Pons. Med u l la oblongata. Dorsal col umns. White matter Lateral col umns Spinal cord.
Anterior col u m n s. Glial cell N erve N erve Neuron nucleus fibers fibers. Gray matter Wh ite matter. FIGURE 1 -3 Cross section through the spinal cord, showing gray matter which contains neuronal and g l i a l cel l bodies, axons, dend rites, and synapses and wh ite matter which contains myelinated axons and associated g l i a l cel l s.
Basic Histology: McGraw-Hill, Cerebral hemisphere Cerebral hemisphere. Brain stem. Com pare with Fig u re 1 This reflects the fact that the nervous system extracts different aspects of its sensory surround eg, the shape, FIGURE 1 -4 Photogra p h of a midsagitta l section through the weight, and temperature of an object touching the body head and u pper neck, showing the major d ivisions of the centra l and encodes them separately and that it controls specific as nervous system.
Reproduced, with permission, from deGroot J: Correlative Neu pects of motor behavior posture, muscle tone, delicate roanatomy of Computed Tomography and Magnetic Resonance Imagery. After partial destruction of the nervous sys tem, only some functions will be lost; other functions may Com putation in the Nervous System be retained, increasing the probability that the organism Nerve cells convey signals to one another at synapses see will survive.
Chapters 2 and 3. Chemical transmitters are associated with the function of the synapse: A neu ron may receive thousands of synapses, which bring it infor Sym m etry of the Nervous System mation from many sources. By integrating the excitatory and A general theme in neuroanatomy is that, to a first approxima inhibitory inputs from these diverse sources and producing tion, the nervous system is constructed with bilateral symme its own message, each neuron acts as an information-pro try.
This is most apparent in the cerebrum and cerebellum, cessing device. On ini Some very primitive behaviors eg, the reflex and uncon tial consideration, these hemispheres appear symmetric. More complex behaviors, however, require larger polysynap Even in more caudal structures, such as the brain stem and tic neural circuits in which many neurons, interconnected by spinal cord, which are not organized into hemispheres, there synapses, are involved.
Tracts and Com missures Crossed Representation The connections, or pathways, between groups of neurons in Another general theme i n the construction o f the nervous the CNS are in the form of fiber bundles, or tracts fasciculi. Neu Aggregates of tracts, as seen in the spinal cord, are referred to roanatomists use the term "decussation" to describe the cross as columns funiculi.
Tracts may descend eg, from the cere ing of a fiber tract from one side of the nervous system right brum to the brain stem or spinal cord or ascend eg, from the or left to the other. The right side of the brain receives infor spinal cord to the cerebrum.
These pathways are vertical con mation about, and controls motor function pertaining to, the nections that in their course may cross decussate from one left side of the world and vice versa. Visual information about side of the CNS to the other. Horizontal lateral connections the right side of the world is processed in the visual cortex on are called commissures. Similarly, sensation of touch, sensation of heat or cold, Multiple tracts connect many parts of the nervous and joint position sense from the body's right side are system.
For example, multiple ascending and descending processed in the somatosensory cortex in the left cerebral tracts connect the PNS and lower spinal centers with the hemisphere. This includes, a particular type of axon, and t hey are laid down in gradients of course, control of the muscles of the right arm and leg, such of varying concentration. In many parts of the developing as the biceps, triceps, hand muscles, and gastrocnemius.
There nervous system, there is initially an overabundance of young are occasional exceptions to this pattern of "crossed innerva axons, and those that do not reach the correct targets are sub tion'': For example, the left sternocleidomastoid muscle is con sequently lost by a process of pruning.
However, even this exception Although the structural organization of the brain is well makes functional sense: As a result of its unusual biomechan established before neural function begins, the maturing brain ics, contraction of the left sternocleidomastoid rotates the neck is susceptible to modification if an appropriate stimulus is ap to the right.
Even for the anomalous muscle, then, control of plied or withheld during a critical period, which can last only movements relevant to the right side of the world originates in a few days or even less. As a result of the organization of cerebellar inputs and outputs, each cerebellar hemisphere controls coordination The peripheral nervous system PNS consists of spinal and muscle tone on the ipsilateral side of the body see nerves, cranial nerves, and their associated ganglia groups Chapter 7.
The nerves contain nerve fibers that conduct information to afferent or from effer ent the CNS. In general, efferent fibers are involved in mo Maps of the World Within the B ra i n tor functions, such as the contraction of muscles or secre At each of many levels, the brain maps contain a representa tion of glands; afferent fibers usually convey sensory tion of various aspects of the outside world.
For example, stimuli from the skin, mucous membranes, and deeper consider the dorsal columns which carry sensory informa structures. Axons within the dorsal columns are arranged in an or of the body innervated by that particular nerve. Some systemic derly manner, with fibers from the arm, trunk, and leg form illnesses such as diabetes, or exposure to toxins or drugs that ing a map that preserves the spatial relationship of these body are neurotoxic can injure nerves throughout the body, produc parts.
Within the cerebral cortex, there is also a sensory map ing a peripheral polyneuropathy; in these cases the longest which has the form of a small man and is, therefore, called a nerves those innervating the feet are affected first. There are multiple maps of the visual world within the occipital lobes and within the temporal and parietal lobes as well. These maps are called PLANES AND TERMS retinotopic because they preserve the geometrical relation ships between objects imaged on the retina and thus provide Neuroanatomists tend to think of the brain and spinal cord spatial representations of the visual environment within the in terms of how they appear in slices, or sections.
The planes brain. Each map contains neurons that are devoted to extract of section and terms used in neuroanatomy are shown in ing and analyzing information about one particular aspect Figure and Table 1 Development Coronal Su perior.
The earliest tracts of nerve fibers appear at about the second plane month of fetal life; major descending motor tracts appear at about the fifth month. Myelination sheathing with myelin of the spinal cord's nerve fibers begins about the middle of fe Rostra l.
Dorsa l The oldest tracts those common to all animals myelin ate first; the corticospinal tracts myelinate largely during the first and second years after birth. Ventral Growing axons are guided to the correct targets during Caudal development of the nervous system by extracellular guidance molecules including the netrins and semaphorins. Some of FIGURE 1 -6 Planes coronal, horizontal, transverse and d i rec these act as attractants for growing axons, guiding them to tions rostra l, caudal, etc.
Others act as repellants. There are bra i n and spinal cord. The plane of the d rawing is the m i d sagitta l. Geschwind N, Galaburda AM: Cerebral Lateralization. Harvard Univ Press, 1 Principles of Neural Science. Atlas of the Human Brain. Elsevier, Brain Mapping: The Disorders. Nervous System Atlas and Annotations. Vol l: From Neuron to Brain, 3rd ed.
Parent A, Carpenter MC: Carpenter's Human Neuroanatomy, 8th ed. Contralateral On the opposite side Romanes GJ: Cunningham's Textbook ofAnatomy, 1 8th ed. Oxford Univ Press, Bilatera l On both sides Shepherd GM: Neurobiology, 2nd ed. Oxford Univ Press, 1 Toga A, Mazziotta J: The Systems. The Central Nervous System: Structure and Function. Oxford Univ Press, 1 98 1. Damasio H: Human Brain Anatomy i n Computerized Images.
Motor neurons are usually larger than sensory neurons. The cellular elements of the tube appear undifferentiated lower spinal cord, a distance of less than 2 ft in infants or 4 ft at first, but they later develop into various types of neurons or more in adults; others have very short processes, reaching, and supporting glial cells.
These small neurons, with short axons that terminate locally, are called interneurons. Layers of the Neura l Tu be Extending from the nerve cell body are usually a num The embryonic neural tube has three layers Fig 2- 1: Most neu tricular zone, later called the ependyma, around the lumen rons give rise to a single axon which branches along its central canal of the tube; the intermediate zone , which is course and to many dendrites which also divide and subdi formed by the dividing cells of the ventricular zone including vide, like the branches of a tree.
The receptive part of the the earliest radial glial cell type and stretches between the neuron is the dendrite, or dendritic zone see Dendrites ventricular surface and the outer pial layer; and the external section. The conducting propagating or transmitting part marginal zone, which is formed later by processes of the is the axon, which may have one or more collateral branches. The downstream end of the axon is called the synaptic ter The intermediate zone, or mantle layer, increases in cellu minal, or arborization.
The neuron's cell body is called the larity and becomes gray matter. The nerve cell processes in the soma, or perikaryon. Cel l Bodies The cell body is the metabolic and genetic center of a neuron Differentiation and M i g ration see Fig Although its size varies greatly in different neu The largest neurons, which are mostly motor neurons, differen ron types, the cell body makes up only a small part of t he neu tiate first.
Sensory and small neurons, and most of the glial cells, ron's total volume. Newly formed neurons may The cell body and dendrites constitute the receptive pole of migrate extensively through regions of previously formed neu the neuron. Synapses from other cells or glial processes tend to rons. When glial cells appear, they can act as a framework that cover the surface of a cell body Fig Because the axonal process of a neuron may begin growing toward its target Dend rites during migration, nerve processes in the adult brain are often curved rather than straight.
The newer cells of the future cere Dendrites are branches of neurons that extend from the cell bral cortex migrate from the deepest to the more superficial lay body; they receive incoming synaptic information and thus, ers. The small neurons of the incipient cerebellum migrate first together with the cell body, provide the receptive pole of the to the surface and later to deeper layers, and this process con neuron. Most neurons have many dendrites see Figs , , tinues for several months after birth.
The receptive surface area of the dendrites is usually far larger than that of the cell body. Because most dendrites are long and thin, they act as resistors, isolating electrical NEURONS events, such as postsynaptic potentials, from one another see Chapter 3. The branching pattern of the dendrites can be Neurons vary in size and complexity. For example, the nuclei very complex and determines how the neuron integrates of one type of small cerebellar cortical cell granule cell are synaptic inputs from various sources.
Some dendrites give rise only slightly larger than the nucleoli of an adjacent large to dendritic spines, which are small mushroom-shaped. Mantle layer Dend rites cellular: Peri karyon Axon hillock I n itial segment of axon. Early stag e with large centra l canal. Later stage with smal ler central canal. Col lateral branch. Dendritic spines are currently of great interest to researchers. The shape of a spine regulates the strength of the synaptic signal that it receives.
A synapse onto the tip of a spine with a thin "neck'' will have a smaller influ ence than a synapse onto a spine with a thick neck. Dendritic spines are dynamic, and their shape can change. Changes in dendritic spine shape can strengthen synaptic connections so as to contribute to learning and memory. The mye l i n sheath is prod uced by oligodendrocytes in the centra l nervous system and by Schwa n n cel ls in the peri p heral nerv Receptive zone Transm ission Terminal zone axon ous system.
N ote the three motor end-p lates, which transmit the nerve i m pu l ses t o striated skeleta l m u scle fi bers. Arrows show the d i rection of the nerve i m p u l se.
A Axon s A single axon or nerve fiber arises from most neurons. The axon is a cylindrical tube of cytoplasm covered by a membrane, t he axolemma. A cytoskeleton consisting of neurofilaments and microtubules runs through the axon. The rnicrotubules provide a framework for fast axonal transport see Axonal Transport sec tion. Specialized molecular motors kinesin molecules bind to Preganglionic vesicles containing molecules eg, neurotransmitters destined B cel l for transport and "walk'' via a series of adenosine triphosphate FIGURE Schematic illustration of nerve cel l types.
ATP -consuming steps along the microtubules. Central nervous system cells: Autonomic cel l s to smooth m u scle. Notice how the po axon, near the cell body to the synaptic terminals. The initial sition of the cell body with r espect to the axon varies. The neuronal su rface is completely cov e red by either syna ptic endings of other neurons 5 or p rocesses of glial cells. Many other p rocesses a round this cel l a re myeli nated axons M.
Courtesy of Dr. DM McDonald. The axolemma of the initial by Schwann cells in the peripheral nervous system PNS and segment contains a high density of sodium channels, which by oligodendrocytes a type of glial cell in the central nervous permit the initial segment to act as a trigger zone. In this system CNS Figs to 2- 1 1. The myelin sheath is divided zone, action potentials are generated so that they can travel into segments about 1 mm long by small gaps 1 11m long along the axon, finally invading the terminal axonal branches where myelin is absent; these are the nodes of Ranvier.
The and triggering synaptic activity, which impinges on other neu smallest axons are unmyelinated. As noted in Chapter 3 , rons. The initial segment does not contain Nissl substance myelin functions as a n insulator.
I n general, myelination see Fig In large neurons, the initial segment arises con serves to increase the speed of impulse conduction along the spicuously from the axon hillock, a cone-shaped portion of axon. Axons range in length from a f ew microns in interneurons to well over a meter ie, in a lumbar motor neu B.
Axonal Tra nsport ron that projects from the spinal cord to the muscles of the In addition to conducting action potentials, axons transport foot and in diameter from 0. Myelin body retrograde transport.
Because ribosomes are not Many axons are covered b y myelin. The myelin consists of present in the axon, new protein must be synthesized and multiple concentric layers of lipid-rich membrane produced moved to the axon. Retrograde transport is similar to rapid anterograde transport. Fast transport involves microtubules extending through the cytoplasm of the neuron.
An axon can be injured by being cut or severed, crushed, or compressed. After injury to the axon, the neuronal cell body responds by entering a phase called the axon reaction, or chromatolysis. In general, axons within peripheral nerves can regenerate quickly after they are severed, whereas those within the CNS do not tend to regenerate. The axon reaction and axonal regeneration are further discussed in Chapter Syna pses Transmission of information between neurons occurs at synapses.
Communication between neurons usually occurs from the axon terminal of the transmitting neuron presyn aptic side to the receptive region of the receiving neuron postsynaptic side Figs and This specialized in terneuronal complex is a synapse, or synaptic j unction.
As outlined in Table 2- 1 , some synapses are located between an axon and a dendrite axodendritic synapses, which tend to be excitatory , or a thorn, or mushroom-shaped dendritic spine. Note the spi nes on the main dend rite and on its smaller branches.
Micrograph courtesy of Dr. Andrew Tan, Yale University. Dend rites 1 rad iate from the neuronal cell B. Note the i ncreased n u m be r of dendritic spi nes and their alte red body, which conta ins the nucleus 3. The axon arises from the ce l l shape fol l owing nerve i nj u ry. Modified from Tan AM et al: Axodend ritic 4 a n d axosomatic Rac1 -reg u l ated dendritic spine remodeling contributes to neuropathic pain after 5 synapses a re p resent. M ye l i n sheaths 6 a re present a round some peripheral nerve i nju ry, Exper Neurol 1 ; Schwann cell nucleus have several distinctive characteristics: Synaptic vesicles contain neurotransmitters, and each vesicle contains a small packet, or quanta, of transmitter.
When the synaptic terminal is depo larized by an action potential in i ts parent axon , there is an influx of calcium.
This calcium influx leads to phosphoryla tion of a class of proteins called synapsins. After phosphory lation of synapsins, synaptic vesicles dock at t he presynaptic membrane facing the synaptic cleft, fuse with i t, and release their transmitter see Chapter 3. Synapses are very diverse in their shapes and other prop erties.
Some are inhibitory and some excitatory; in some, the Schwann cell nucleus transmitter is acetylcholine; in others, it is a catecholamine, A amino acid, or other substance see Chapter 3. Some synaptic vesicles are large, some small; some have a dense core, whereas others do not.
Flat synaptic vesicles appear to contain an in hibitory mediator; dense-core vesicles contain catecholarnines. In addition to calcium-dependent, vesicular neurotrans mitter release, there is also a second, nonvesicular mode of neurotransmitter release that is not calcium-dependent.
This mode of release depends on transporter molecules , which usually serve to take up transmitter from the synaptic cleft. B Inner mesaxon Outer mesaxon Nerve cell bodies are grouped characteristically in many parts of the nervous system. In the peri phera l nervou s system PN S , cortices, cell bodies aggregate to form layers called laminas.
These axons a re not, however, insu lated by a mye l i n brum form compact groups, or nuclei. Each nucleus contains sheath. Mye l i nated PNS fi bers a re su rrounded b y a mye l i n sheath that is formed by a spiral wra pping of the axon by a Schwann cell.
Reproduced, with permission, from J u nq ueira LC, short relays within the nucleus. Basic Histology, 1 1 th ed. McGraw Hill, Groups of nerve cells are connected by pathways formed which protrudes from the dendrite Fig 2- 1 3. Other synapses by bundles of axons. In some pathways, the axon bundles are are located between an axon and a nerve cell body axoso sufficiently defined to be identified as tracts, or fasciculi; in matic synapses, which tend to be inhibitory.
Still other others, there are no discrete bundles of axons. Aggregates of synapses are located between an axon terminal and another tracts in the spinal cord are referred to as columns, or funi axon; these axoaxonic synapses modulate transmitter release culi see Chapter 5. Within the brain, certain tracts are re by the postsynaptic axon. Synaptic transmission permits in ferred to as lemnisci. In some regions of the brain, axons are formation from many presynaptic neurons to converge on a intermingled with dendrites and do not r un in bundles so that single postsynaptic neuron.
Some large cell bodies receive sev pathways are difficult to identify. These networks are called eral thousand synapses see Fig Impulse transmission at most synaptic sites involves the release of a chemical transmitter substance see Chapter 3 ; at other sites, current passes directly from cell to cell through NEUROGLIA specialized junctions called electrical synapses, or gap junctions. Electrical synapses are most common in inverte Neuroglial cells, commonly called glial cells, outnumber brate nervous systems, although they are found in a small neurons in the brain and spinal cord 1 0: They do not form number of sites in the mammalian CNS.
Chemical synapses synapses. Schwann cel l s 5 may s u rround one myeli nated or severa l u n myel i nated axons. X 1 6, There are two cytes, both of which are derived from ectoderm. In contrast broad classes of glial cells, macroglia and microglia to neurons, these cells may have the capability, under some Table Axodend ritic Axon terminal Dend rite Usually excitatory.
Axosomatic Axon terminal Cell body Usually inhibitory. Axoaxonic Axon terminal Axon term inal Presynaptic i n h ibition modu l ates transmitter release in postsynaptic axon.
Astrocytes provide structural support to nervous tissue and act during development as guidewires that direct neu ronal migration. Astrocytes may also play a role in synaptic transmission. Many synapses are closely invested by astrocytic processes, which appear to participate in the reup take of neurotransmitters. Astrocytes also surround endothe lial cells within the CNS, which are joined by tight junctions that impede the transport of molecules across the capillary ep ithelium, and contribute to the formation of the blood-brain barrier see Chapter 1 1.
Although astrocytic processes around capillaries do not form a functional barrier, they can selectively take up materials to provide an environment opti mal for neuronal function. Astrocytes form a covering on the entire CNS surface and proliferate to aid in repairing damaged neural tissue Fig 5. These reactive astrocytes are larger, are more eas ily stained, and can be definitively identified in histological sections because they contain a characteristic, astrocyte-spe cific protein: Chronic astrocytic proliferation leads to gliosis, sometimes called glial scarring.
Whether glial scarring is beneficial, or inhibits regeneration of injured neurons, is currently being FIGURE 2 - 1 0 Oligodendrocytes form myelin i n t h e centra l studied. There is l ittl e oligodendrocyte cytop l a s m Cyt i n the o l igodend rocyte p rocesses that s p i ra l around the axon Ol igodendrocytes to fo rm mye l i n , and the mye l i n sheaths a re connected to t h e i r p a r Oligodendrocytes predominate in white matter; they extend ent oligodendrocyte cel l body by o n l y t h i n tongues of cyto p l a s m.
The mye l i n is period ica l l y of myelin which acts as an insulator around axons in the CNS. Redrawn and reproduced with permission from the neurons they envelop.
A single oligodendrocyte may wrap Bunge M, Bunge R. Pappas G: Ultrastructural study of r emyelination in an experimen tal lesion in adult cat spinal cord, J Biophys Biochem Cytol May; 1 0: In peripheral nerves, by contrast, myelin is formed by Schwann cells. Each Schwann cell myelinates a single axon, and remyelination can occur at a brisk pace after Astrocytes injury to the myelin in the peripheral nerves. There are two broad classes of astrocytes: Protoplasmic astrocytes are more delicate, and their many processes are branched.
They occur in gray matter. Fi Microg l ia brous astrocytes are more fibrous, and their processes con Microglial cells are the macrophages, or scavengers, of the taining glial fibrils are seldom branched. Astrocytic processes CNS. They constantly survey the brain and spinal cord, acting radiate in all directions from a small cell body.
They surround as sentries designed so as to detect, and destroy, invaders. Glial cel l s f Macroglia Oligodendrocytes. Astrocytes Myelin formation i n CNS. Reg u late ionic environment; reuptake of neu rotransm itters; guidance of. X The inset shows axon A1 and its mye l i n sheath at higher magn ification. The myel i n is a spira l of ol igodendrocyte mem brane that surro u nds the axon.
M ost of the ol igodendrocyte cytoplasm is extruded from the myelin. Beca use the mye l i n is com pact, it has a high electrical resistance and low capacitance so that it can fu nction as a n insulator around the axon. When an area of the brain or spinal cord is Extrace l l u l a r Space damaged or infected, microglia activate and migrate to the site There is some fluid-fill e d space between the various cellular of injury to remove cellular debris.
Some microglia are always components of the CNS. Microglia play an of the total volume of the brain and spinal cord. Their role after endogenous insults, portant in electrical signaling in the nervous system see including stroke or neurodegenerative diseases such as Chapter 3 , regulation of the levels of these ions in the extra Alzheimer disease, is less well understood, and it is not clear cellular compartment ionic homeostasis is an important at this time whether activation of microglia in these disorders function, which is, at least in part, performed by astrocytes.
Bielschowsky si lver membrane sta i n. Postsynaptic membrane The capillaries within the CNS are completely invested by glial Synaptic cleft or neural processes. Moreover, capillary endothelial cells in the brain in contrast to capillary endothelial cells in other or FIGURE 2 Schematic drawing of a synaptic terminal. This barrier iso molecules i nto the synaptic cleft so that they can bind t o receptors in lates the brain extracellular space from the intravascular the postsyna ptic mem brane.
Postsynaptic cell C l i n ica l Correlation In cerebral edema , there is an increase in the bulk of the. Cerebral edema can be either vasogenic primarily ex tracellular or cytotoxic primarily intracellular. The cell body maintains the functional and anatomic integrity of the axon Fig If the axon is cut, the part distal to the cut degenerates wallerian degeneration , because materials for maintaining the axon mostly proteins are formed in the Axosomatic cell body and can no longer be transported down the axon axoplasmic transport.
FIGURE 3 Axodend ritic synapses termi nate on dend ri ties or mushroom-shaped "dendritic spines;' and tend to be exci tatory. Axosomatic synapses termi nate on neuronal cel l bodies and FIGURE 5 M icrog raphs showi ng astrocytes with i n t h e nor tend to be i n h i bitory. Axoaxonal synapses termi nate o n a n axon, mal human bra i n A , and within glial sca rs in patients with m u ltiple often close to synaptic term i n a ls, and modu late the release of neuro sclerosis B and fol lowi ng stroke C.
Note the hypertrop hied transmitters. Reprod uced, with permission, from Ganong WF: Review of Medical astrocytes with i n g l i a l scars i n B and C. Courtesy of Physiology, 22nd ed. McGraw-H ill, Several months. Normal nerve fi ber, with its perika ryon and the effector cel l stri ated skeletal m u scle.
N otice the position of the n e u ro n n ucleus and the a m o u nt and d i stribution of N i ssl bodies. When the fi ber is i nj u red, the n e u ro n a l n ucleus m oves to the cel l periphery, Nissl bodies beco m e g reatly red uced i n n u m be r chromatolysis , and the nerve fi ber d i sta l to the i nj u ry degenerates a l o n g with its mye l i n sheath. Debris is p h a g ocytized by m a c rophages.
The m uscle fi ber shows p ro n o u nced disuse atrop hy. Schwa n n cel l s prolife rate, forming a com pact cord that is penetrated by the g rowi n g axon. The axon g rows at a rate of 0. In this exa m ple, the n erve fi ber regeneration was successfu l, and the m uscle fi ber was a l s o regenerated after rece iving nerve sti m u l i.
When the axon does not penetrate the cord of Schwa n n ce l l s, its g rowt h i s not organ ized and s u ccessfu l regenera tion does n ot occ u r. The Principles of Pathology and Bacteriology, 3rd ed. Butterworth, 1 Distal to the level of axonal transection when a peripheral swelling of nearby astrocytes, and activation of microglia. To Successful axonal regeneration does not commonly occur af gether with macrophages, they phagocytize the remnants of ter injury to the CNS.
Many neurons appear to be dependent the myelin sheaths, which lose their integrity as the axon on connection with appropriate target cells; if t he axon fails degenerates.
The changes include swelling of the cell body and nucleus, which is usu ally displaced from the center of the cell to an eccentric loca Reg eneration tion. The regular arrays of ribosome-studded endoplasmic A. Periphera l Nerves reticulum, which characterize most neurons, are dispersed Regeneration denotes a nerve's ability to regrow to an appro and replaced by polyribosomes.
The ribosome-studded en priate target, including the reestablishment of functionally doplasmic reticulum, which had been termed the Nissl sub useful connections see Figs 2 - 1 6 and 2 - 1 7. Shortly 1 -3 stance by classical neuroanatomists, normally stains densely days after an axon is cut, the tips of the proximal stumps form with basic dyes.
The loss of staining of the Nissl substance, as enlargements, or growth cones. The growth cones send out a result of dispersion of the endoplasmic reticulum during exploratory pseudopodia that are similar to the axonal growth the axon reaction, led these early scientists to use the term cones formed in normal development.
Each axonal growth "chromatolysis: Axon branch Receptor now appreciated that molecules such as NoGo act as "stop. Neutralization of NoGo has been shown to pro mote the regeneration of axons within t he spinal cord in ex perimental animals. When confronted with a permissive envi Retrograde Receptor ronment eg, when the transected axons of CNS neurons are degeneration hypersensitive permitted to regrow into a peripheral nerve, or transplanted Site of injury into the CNS as a "bridge" , CNS axons can regenerate for dis.
Retrograde Regenerative Orthograde reaction: Remyeli nation In a number of disorders of the peripheral nervous system FIGURE 7 S u m m a ry of changes occurring in a neuron and such as the Guillain-Barre syndrome , there is demyelina the structure it innervates when its axon is crushed or cut at the tion, which interferes with conduction see Chapter 3.
This point ma rked X. Modified from Ries D. Reproduced, with permission, from condition is often followed by remyelination by Schwann cells, Ganong WF: Review of Medical Physiology, 22nd ed. In contrast, remyelination occurs can cross the scar tissue and enter the distal nerve stump, suc much more slowly if at all in the CNS. Little remyelination cessful regeneration with restoration of function may occur. A different form of plasticity Schwarm cell tubes surrounded by basal lamina Biingner's ie, the molecular reorganization of the axon membrane that bands in the distal stump explains the different degrees of re acquires sodium channels in demyelinated zones appears to generation that are seen after nerve crush compared with nerve underlie clinical remissions in which there is neurological transection.
After a crush injury to a peripheral nerve, the ax improvement in patients with multiple sclerosis. Col latera l Sprouti ng sion, facilitating regeneration of axons through the injured This phenomenon has been demonstrated in the CNS as well nerve.
In contrast, if the nerve is cut, the continuity of these as in the peripheral nervous system see Fig It occurs pathways is disrupted. Even with meticulous surgery, it can be when an innervated structure has been partially denervated. This kind of regen Peripheral system axons will reinnervate both muscle and eration demonstrates that there is considerable plasticity in sensory targets; however, motor axons will not connect to sen the nervous system and that one axon can take over the synap sory structures, or sensory axons to muscle.
Although a motor tic sites formerly occupied by another. Such movements include "j aw It has classically been believed t hat neurogenesis-the capa winking;' in which motor axons destined for the jaw muscles bility for production of neurons from undifferentiated, prolif reinnervate muscles around the eye after injury. According to this B. Centra l Nervous System traditional view, after pathological insults t hat result in neu Axonal regeneration is typically abortive in the CNS.
The rea ronal death, the number of neurons is permanently reduced. Classi However, recent evidence has indicated that a small number cal neuropathologists suggested that the glial scar, which is of neuronal precursor cells, capable of dividing and t hen dif largely formed by astrocytic processes, may be partly respon ferentiating into neurons, may exist in t he forebrain of adult sible.
The properties of the oligodendroglial cells in contrast mammals, including humans. These rare precursor cells re to those of the Schwarm cells of peripheral nerves may also side in the subventricular zone. For example, there is some ev account for the difference in regenerative capacity: Recent idence for postnatal neurogenesis in the dentate gy.
An inhibitory fac eration of new neurons in this critical region can be acceler tor produced by oligodendrocytes, CNS myelin, or both may ated in an enriched environment. While the number of new interfere with regeneration of axons through the CNS. Neuroglia, 2nd ed. Oxford Univ Press, suggest strategies for restoring function after inj ury to the This is an area of intense research.
Kordower J, Tuszynski M: CNS Regeneration. Laming PR: Glial Cells. Cambridge Univ Press, 1 The Neuron: Cell and Molecular Biology, 3rd ed. Oxford Univ Press, 1. The Fine Structure o f the peutics: Trends Neurosci Nervous System, 3rd ed. Rakic P: A century of progress in corticoneurogenesis: Cereb Cortex ; 1 6 vol 2. Librairie Maloine, 1 9 1 1. Hall ZW editor: An Introduction t o Molecular Neurobiology. Development of the Nervous System. Sinauer, NY, Strittmatter SM: Remyelination of developmental guidance during recovery from spinal cord the injured spinal cord.
Prog Brain Res ; 1 6 1: Nat Rev Neurosci ;7: Comparative views of neuro Neurochemisry. The Neurologist ;6: Basic Histology, 9th ed. Exp Neurol 2; The Axon: Structure, Injury. Humana, 1. Function, and Pathophysiology. More hippocampal neurons Yuste R: Dendritic spines MIT. Press, Nature 1 ; Si gnaling in the Nervous System Along with muscle cells, neurons are unique in that they are see Table 3 - 1. The Nernst equation, which would be used to excitable; that is, they respond to stimuli by generating elec determine membrane potential across a membrane permeable trical impulses.
Electrical r esponses of neurons modifications only to r ions, is as follows: Propa gated electrical impulses are termed action potentials. Cell membranes are highly permeable to most At physiologic temperatures inorganic ions, but they are almost impermeable to proteins and many other organic ions. For these membranes, potential is the weighted average membrane into the cell. In carrying out this essential activity, of the equilibrium potentials for each permeable ion, with the the pump consumes adenosine triphosphate ATP.
When the chemical and electrical forces are equally strong, an equilibrium potential exists. They are called radicular arteries if they supply only the nerve roots, and radiculospinal arteries if they supply blood to both the roots and the cord. Each radiculospinal artery supplies blood to approximately six spinal cord segments, with the exception of the great radicular artery of Adamkiewicz, which usually enters with the left second lumbar ventral root range T10 to L4 and supplies most of the caudal third of the cord.
What type of information is carried in the dorsal columns? The dorsal columns convey tactile discrimination, vibration, and joint position sense. What types of receptors are stimulated to sense this information? Muscle spindles and Golgi tendon organs perceive position sense, Pacinian corpuscles perceive vibration, and Meissner corpuscles perceive superficial touch sensation needed for tactile discrimination.
Pacinian and Meissner corpuscles are examples of mechanoreceptors. What type of peripheral nerve fiber is involved with transmission of dorsal column information? Large, myelinated, fast-conducting nerve fibers carry dorsal column-type information. What is the pathway by which this information reaches the cerebral cortex? Sensation on skin! Where do dorsal column fibers decussate? At what locations do they synapse? The dorsal columns decussate in the lower medulla, after synapsing in the nucleus gracilis and cuneatus.
They also synapse in the VPL of the thalamus before going to the cortex. What type of information is carried in the spinothalamic tract? The spinothalamic tract conveys pain, temperature, and crude touch. What type of peripheral nerve fiber is involved with transmission of spinothalamic information? Small, myelinated, and unmyelinated fibers carry spinothalamic-type information. Where do the spinothalamic fibers decussate?
Pain and temperature are perceived by naked terminals of A delta and C fibers and by many specialized chemoreceptors that are excited by tissue substances released in response to noxious and inflammatory stimuli. Substance P is thought to be the neurotransmitter released by A delta and C fibers at their connections with the interneurons in the spinal cord.
Where in the internal capsule do the afferents travel from the VPL thalamic nucleus? The sensory tracts from the VPL travel in the posterior aspect of the posterior limb of the internal capsule. To which anatomic locations do the afferents from the VPL project?
Which pathway carries proprioception from the lower limbs to the cerebellum? Proprioception travels from the legs to the cerebellum in the dorsal columns. Where does cerebellar proprioception for the lower limb synapse? These fibers synapse in the midthoracic level of the spinal cord in the nucleus dorsalis of Clarke.
Where is the spinocerebellar tract located? The spinocerebellar tract lies lateral to the corticospinal tract in the cord.
Where do the motor fibers originate? Where do the motor fibers travel in the internal capsule? The corticospinal fibers travel in the anterior portion of the posterior limb of the internal capsule.
The motor fibers to the face corticobulbar fibers travel in the genu of the internal capsule. Which cranial nerve exits the midbrain in close proximity to the corticospinal fibers? Cranial nerve III exits the midbrain in close proximity to the corticospinal fibers, which explains the symptoms of a common vascular syndrome.
Where do the motor fibers decussate? The corticospinal tract decussates in the lower ventral medulla, and most fibers continue in the cord as the lateral corticospinal tract, with a small percentage descending in the ventral corticospinal tract. On what type of neurons in the spinal cord do the corticospinal fibers synapse?
What is the reticulospinal tract? The reticulospinal tract also originates in the precentral gyrus, but instead of descending uninterrupted to the spinal cord, these fibers synapse in the reticular formation of the brain stem as they descend to the spinal cord.
They mainly have an inhibitory effect on the alpha and gamma motor neurons. What is the vestibulospinal tract? The vestibulospinal tract is the efferent from the lateral vestibular nucleus. This tract descends the spinal cord, residing lateral to the spinothalamic tract, and coordinates motor and vestibular performance. What is the MLF? The MLF is primarily an efferent of the lateral vestibular nucleus.
This tract ascends to the sixth, fourth, and third cranial nuclei. What are the three parts of the brain stem? The brain stem consists of the midbrain, pons, and medulla. What is the reticular formation? The reticular formation is a loosely organized longitudinal collection of interneurons that fill the central core of the brain stem, which is concerned with modulating awareness and behavioral performance. Name the 12 cranial nerves.
Olfactory IV. Trochlear V. Trigeminal II. Optic III. Oculomotor VI. Abducens VII. Facial VIII. Auditory IX. Glossopharyngeal X. Vagus XI. Spinal accessory XII. Hypoglossal What are general somatic afferent nerves? Which cranial nerves carry them? General somatic afferent fibers carry exteroceptive pain, temperature, touch and proprioceptive impulses. Cranial nerves for proprioception: What are general visceral afferent nerves?
General visceral afferent fibers carry impulses from the visceral structures, and cranial nerves IX and X contain these fibers. What are special somatic afferent nerves? Special somatic afferent fibers carry sensory impulses from the special senses vision, hearing, equilibrium , and cranial nerves II and VIII contain these fibers. What are special visceral afferent nerves?
Special visceral afferent fibers carry impulses from the olfactory and gustatory senses, and cranial nerves I olfactory and VII, IX, and X gustatory contain these fibers. What are general somatic efferent nerves? General somatic efferent fibers carry motor impulses to somatic skeletal muscles. In the head, the tongue and extraocular muscles are of this type. What are general visceral efferent nerves?
General visceral efferent fibers carry parasympathetic autonomic axons. The following cranial nerves carry general visceral efferent fibers: Cranial nerve VII superior salivatory nucleus: The postganglionic fibers innervate the lacrimal gland from the pterygopalatine ganglion and the submandibular and sublingual gland from the submandibular ganglion.
Cranial nerve IX inferior salivatory nucleus: Cranial nerve X dorsal motor nucleus: What are special visceral efferent nerves? Special visceral efferent fibers innervate skeletal muscle derived from the branchial arches. What are the three anatomic subdivisions of the midbrain? The midbrain can be divided into the tectum, tegmentum, and cerebral crus Figure What is the quadrigeminal plate?
The quadrigeminal plate is formed by the tectum and the superior and inferior colliculi. What is the substantia nigra? The substantia nigra, a motor nucleus in the basal ganglia system, lies anterior to the tegmentum but posterior to the crus pyramidal tract in the midbrain.
Diagram of the midbrain. Which disease affects the substantia nigra? What is the pathology? The primary efferent neurotransmitter from the substantia nigra is dopamine. Pathologically, the neurons lose their melanin and the nucleus becomes depigmented.
Many neurons also contain inclusion bodies called Lewy bodies. What is the red nucleus? The red nucleus is a globular mass located in the ventral portion of the tegmentum of the midbrain. It is a relay center for many of the efferent cerebellar tracts. The crossed fibers of the superior cerebellar peduncle SCP pass through and around its edges.
What is the Edinger-Westphal nucleus? The Edinger-Westphal nucleus, in the posterior midbrain, supplies parasympathetic fibers that terminate in the ciliary ganglion via cranial nerve III. It is mainly involved in pupillary constriction and the light accommodation reflex. What is the function of cranial nerve III? Cranial nerve III innervates all the extraocular muscles except for the lateral rectus and superior oblique. In innervates the medial rectus, superior rectus, inferior rectus, and inferior oblique muscles.
Where does cranial nerve III originate and exit the brain stem? Cranial nerve III, the oculomotor nerve, exits the brain stem medially from the midbrain between the posterior cerebral artery and the superior cerebellar artery. This is important because the nerve can be affected by aneurysms of these arteries. What is the function of cranial nerve IV?
Cranial nerve IV, the trochlear nerve, innervates the superior oblique muscle. What is the route of cranial nerve IV? Cranial nerve IV travels posteriorly and medially, crosses the midline, wraps around the midbrain, and exits the brain stem laterally between the posterior cerebral artery and superior cerebellar artery.
It has the longest intracranial route approximately 7. It then travels through the cavernous sinus and enters the orbit through the superior orbital fissure. Because it crosses the midline, the right trochlear nerve innervates the left superior oblique muscle. In a superior oblique palsy, which way would the patient tilt his or her head?
If the left superior oblique muscle is weak, then tilting the head to the right would reduce the diplopia, and tilting the head to the left would worsen the diplopia. So a patient tilts his or her head away from the affected eye Figure Anatomy of the medulla. PONS Which cranial nerves exit at the pontomedullary junction?
Anatomy of the pons. Where does cranial nerve V exit the brain stem? Cranial nerve V, the trigeminal nerve, exits the brain stem laterally at the mid-pons level.
It divides into three main branches: V1 ophthalmic , V2 maxillary , and V3 mandibular. What are the four subdivisions of the trigeminal nucleus? Mesencephalic nucleus which is a nucleus of unipolar cell bodies similar to the dorsal root ganglion, with no synapse 2.
Chief sensory nucleus 3. Descending spinal nucleus 4. Motor nucleus What type of information does cranial nerve V carry? What is the pathway by which sensation from the face reaches the cortex? After cranial nerve V enters the brain stem, the afferent nerves split into two parts: The former goes to the ipsilateral chief sensory nucleus of V mid-pons!
The pain-carrying fibers become the spinal tract of V! VPM nucleus of the thalamus! What is the function of cranial nerve VI?
Cranial nerve VI, the abducens nerve, abducts the eye. What is the function of cranial nerve VII? Cranial nerve VII, the facial nerve, innervates the muscles of facial expression special visceral efferent ; innervates the lacrimal, submandibular, sublingual, and parotid glands general visceral efferent ; supplies taste sensation to the anterior two-thirds of the tongue special visceral afferent ; and supplies sensation to the external ear general somatic afferent.
How does the nucleus for cranial nerve VII receive higher cortical input? The innervation to the muscles of facial expression can be separated into the muscles of the upper part of the face and the muscles of the lower part of the face. The supranuclear input responsible for the movement of the upper facial musculature is a bilateral input from the cortex to the nucleus.
The supranuclear input responsible for the movement of the lower facial musculature is only a contralateral input from the cortex to the facial nucleus Figure What is the difference between an upper motor neuron central and lower motor neuron peripheral facial weakness? If the patient with a facial droop can move the upper facial muscles i.
The lesion is somewhere in the contralateral corticobulbar tracts above the facial nerve nucleus e. If the patient cannot voluntarily move any muscle involved in facial expression either upper or lower facial musculature , the lesion is localized to the facial nucleus or the peripheral facial nerve on the ipsilateral side. Innervation to muscles of facial expression. Patients also may have associated absence of the abducens nuclei. What is the function of cranial nerve VIII?
Cranial nerve VIII, the vestibulocochlear nerve, has two functionally distinct sensory divisions: The vestibular nerve responds to position and movement of the head, serving functions often identified as equilibrium.
The cochlear nerve mediates auditory functions. What is the nucleus ambiguus? It is a cigar-shaped nucleus that lies in the depths of the medulla. It innervates the volitional muscles of the pharynx by way of both cranial nerves IX and X and the larynx for phonation via cranial nerve X. The larynx and pharynx have bilateral cortical input. What is the nucleus solitarius? It is the nucleus in the medulla that receives afferent information from the larynx via cranial nerve X and posterior pharynx and mediates the gag and cough reflexes cranial nerves IX and X.
Pain sensation from these areas enters the brain stem through cranial nerves IX and X but terminates in the descending spinal tract of the trigeminal nerve.
What is the salivatory nucleus? The superior salivatory nucleus sends efferent autonomic fibers general visceral efferent through cranial nerve VII to innervate the lacrimal, submandibular, and sublingual glands as well as the mucous membranes of the nose and hard and soft palate.
The inferior salivatory nucleus sends efferent autonomic fibers via cranial nerve IX to innervate the parotid gland. What is the gustatory nucleus? The gustatory nucleus is the nucleus in the medulla that receives afferent sensory information for the sensation of taste. Taste from the anterior two-thirds of the tongue is innervated by the chorda tympani cranial nerve VII , the posterior one-third of the tongue is innervated by cranial nerve IX, and the epiglottis is innervated by cranial nerve X.
Describe the function of cranial nerves IX and X glossopharyngeal—vagal complex Cranial nerve IX the glossopharyngeal nerve and cranial nerve X the vagus nerve are usually considered together because of their overlapping functions.
Both cranial nerves travel together intracranially, and both exit the cranial vault through the jugular foramen. The nucleus ambiguus innervates the volitional muscles of the pharynx through both cranial nerves IX and X, and the larynx via cranial nerve X. Sensation from the larynx enters the medulla via cranial nerve X to terminate in the nucleus solitarius.
Taste fibers from the posterior one-third of the tongue travel via cranial nerve IX, and taste from the epiglottis via cranial nerve X. They terminate in the gustatory nucleus. Cranial nerve IX also supplies parasympathetic innervation to the parotid, originating in the inferior salivatory nucleus.
Branches of cranial nerve X, the vagus nerve, continue beyond the larynx to innervate the heart, lungs, and abdominal viscera, providing primarily parasympathetic input. What is the function of cranial nerve XI?
Cranial nerve XI, the spinal accessory nerve, is a small nerve of about motor fibers that arises from the upper cervical and lower medullary anterior horn cells and supplies the SCM and trapezius muscles. It exits the cranial vault via the jugular foramen. What is jugular foramen syndrome? Because cranial nerves IX, X, and XI exit the cranial vault through the jugular foramen, jugular foramen syndrome is a constellation of symptoms arising from a lesion typically a tumor at the level of the jugular foramen that compromises the function of these cranial nerves.
Symptoms include loss of taste to the posterior two-thirds of the tongue; paralysis of the vocal cords, palate, and pharynx; and paralysis of the trapezius and SCM muscles. If the left spinal accessory nerve is cut, which functions are lost? Because the left SCM is involved in turning the head to the right, a lesion of the left cranial nerve XI results in an inability to turn the head to the right.
The left trapezius also loses function, and the patient would not be able to shrug the left shoulder. If the left hypoglossal nucleus is injured, which way does the tongue deviate? Lesioning the nucleus is similar to lesioning the peripheral nerve. The left hypoglossal nerve innervates the left tongue muscles, which, if acting alone, pushes the tongue to the right.
The right hypoglossal nerve innervates the right tongue muscles, which, if acting alone, pushes the tongue to the left. Usually, these muscles work together to push the tongue forward without deviation. If the left hypoglossal nucleus is lesioned, the right hypoglossal muscles act unopposed. The tongue thus deviates to the left, or, in other words, the tongue deviates toward the affected side. Critical in establishing level of damage in coma 2.
Critical to finding a cause focal—structural lesion; nonfocal—metabolic 3. Pupil reaction: II in, III out 4. IX in, X out 6. Able to breathe: What is Cheyne-Stokes breathing? Where is the lesion that causes it? Cheyne-Stokes breathing is a crescendo-decrescendo pattern of periodic breathing in which phases of hyperpnea regularly alternate with apnea. Cheyne-Stokes respirations are seen most often with lesions affecting both cerebral hemispheres. What is central neurogenic hyperventilation?
What causes it? Central neurogenic hyperventilation is a sustained, rapid, deep hyperpnea. It is produced by lesions in the low midbrain to upper one-third of the pons. What is apneustic breathing? Apneusis is a prolonged respiratory cramp, a pause at full inspiration. Apneustic breathing may occur after damage to the mid or caudal pons. What is cluster breathing?
When does it occur? Cluster breathing, a disorderly sequence of breaths with irregular pauses between the breaths, may result from damage to the lower pons or upper medulla. What is ataxic breathing? It is a completely irregular pattern of breathing in which both deep and shallow breaths occur randomly. The respiratory rate tends to be slow.
The lesion that causes it is in the central medulla. What is decorticate posturing? Decorticate posturing is a stereotyped response to noxious stimuli. In the upper extremity, it consists of flexion of the arm, wrist, and fingers; in the lower extremity, it consists of extension, internal rotation, and plantar flexion.
Decorticate posturing most often occurs in comatose patients with lesions below the thalamus but above the red nucleus.
What is decerebrate posturing? In whom does it occur? Decerebrate posturing is a stereotyped response to noxious stimuli. It consists of extension, adduction, and hyperpronation in the upper extremity and extension with plantar flexion in the lower extremity. Comatose patients with lesions below the red nucleus but above the vestibular nucleus may have decerebrate posturing.
What are the five receptors of the vestibular apparatus, and what do they sense? Three semicircular canals that are oriented 90 degrees to each other sense rotational acceleration in all three planes. One horizontally oriented utricle and one vertically oriented saccule sense linear acceleration.
Where does the vestibular information synapse? The vestibular nerve, carrying sensory data from the receptors, divides and synapses in four vestibular nuclei grouped together in the medulla: What is the output from these nuclei? The vestibulospinal tracts and the MLF are the two efferent tracts from the vestibular nuclei. Where do the vestibular nuclei project? Vestibular nuclei project to 1 the oculomotor nuclei cranial nerves III, IV, and VI , 2 cranial nerve XI, 3 cervical nuclei for head and neck position, 4 fastigial nuclei of the cerebellum, and 5 reticular formations of the brain stem.
What is the response of a normal person to cold water injected in the left ear? Injecting cold water in to the left ear causes slow eye movements toward the left, followed by a fast phase of nystagmus back to the right. What is the expected response of a comatose patient with an intact brain stem to cold water in the left ear?
The patient will have slow eye deviation toward the left ear. The fast-phase nystagmus is absent. Which structures constitute the external ear, middle ear, and inner ear? The external ear is composed of the pinna, the external auditory canal, and the tympanic membrane.
The middle ear is composed of the tympanic membrane, ossicles malleus, incus, stapes , and oval window. The ossicles function as an impedance matching device between air and fluid during the travel of the sound wave. The inner ear is composed of part of the oval window, the cochlea, and the round window. Which compartments of the cochlea are filled with perilymph?
It is separated from the scala media by the basilar membrane. Anatomy of the hearing apparatus. From Kandel E.
New York, Elsevier, , p. What is the pathway traveled by the cochlear fluid pressure wave initiated by a sound wave? The basilar membranes move next and transmit the pressure to the scala tympani and from there to the oval window. What is the arrangement of the neuroepithelial cells of the organ of Corti? The outer hair cells arranged in three rows rest on the basilar membrane, with their stereocilia inserted into the tectorial membrane; these cells are able to contract and initiate the flow of endolymph toward the inner hair cells.
The inner hair cells one row sit on the bone; they do not contract. These cells respond to the movement of endolymph and provide most of the afferent input to the spiral ganglion. How does the organ of Corti serve as an audiofrequency analyzer?
The anatomic arrangement allows frequency analysis of sounds: The basilar membrane responds to high frequencies at its base and to low frequencies at its apex. The hair cells in the base of the cochlear duct have short and fat stereocilia, which are stimulated by high frequencies. The hair cells in the apex of the cochlea have long and thin stereocilia, which respond best to low frequencies. What is the anatomy of the auditory pathway?
Spiral ganglion! At what level is there crossing of information between the left and right ascending tracts? The crossing of axons occurs on every level from the trapezoid body to the medial geniculate body. To produce unilateral deafness, where could the lesion be? The lesion must be at the cochlear nucleus or more peripheral because of multiple crossovers above the cochlear nucleus.
A vibrating tuning fork is placed in the middle of the forehead. In patients with sensorineural deafness, the signal is localized to the healthy ear. A vibrating tuning fork is placed on the mastoid bone; when the patient can no longer hear it, it is removed and placed next to the ear.
Thus, bone conduction is compared with air conduction. What is the innervation of the external ear canal? Damage to which structures results in hyperacusis? Facial nerve VII —innervates the stapedius muscle, which retracts the stapes from the round window.
Trigeminal nerve V —supplies the tensor tympani, which inserts into the malleus and tenses the tympanic membrane, thus preventing it from vibrating.
What is the pathway for the feedback loop? When auditory input reaches the superior olive, it sends signals to the olivocochlear bundle through the VIII nerve; the signals then terminate on the outer hair cells or afferent fibers in the spiral ganglia.
What is the paramedian pontine reticular formation PPRF? The PPRF is a collection of cells lying in the pons adjacent to the nucleus of cranial nerve VI, and is an important center for horizontal gaze. What is the difference between saccades and smooth pursuit movements? Saccades are fast conjugate eye movements that are under voluntary control. Smooth pursuits are slow involuntary movements of eyes fixed on a moving target. What is the pathway for saccades? What is the pathway for smooth pursuit?
What is the brain stem area for vertical gaze? Near the superior colliculus, there are subtectal and pretectal centers that control vertical eye movements and project to cranial nuclei III, IV, and VI. What are the pathways for voluntary vertical eye movements? Vertical movements are driven symmetrically from both frontal lobes. Describe the anatomic divisions of the cerebellum.
The cerebellum is anatomically divided into the two hemispheres, the midline vermis and the flocculonodulus. The hemispheres are involved in appendicular control, the vermis is involved in axial control, and the flocculonodular lobe is involved in vestibular balance. What are the three layers of the cerebellar cortex? Outermost molecular cell layer 2. Middle Purkinje cell layer 3. Innermost granular cell layer What types of cells are located in each of these layers?
The molecular layer contains 1 stellate cells, 2 basket cells, 3 dendrites of Purkinje cells, 4 dendrites of Golgi type II cells, and 5 axons of granule cells. The Purkinje layer contains the cell bodies of Purkinje cells. The granular layer contains 6 granule cells, 7 Golgi type II cells, and 8 glomeruli synaptic complexes that contain mossy fibers, axons and dendrites of Golgi type II cells, and dendrites of granule cells.
What is the afferent fiber from the inferior olives? Through which peduncle does it reach the cerebellum? The afferent fiber from the inferior olives is the climbing fiber.
It enters the cerebellum through the inferior cerebellar peduncle. A lesion in this pathway can cause palatal myoclonus. What are the deep nuclei of the cerebellum medial to lateral? Medial to lateral, the cerebellar deep nuclei are fastigial, globus, emboliform, and dentate. What are the primary inputs and outputs of the cerebellum? Cerebellar function can be conceptualized as a feedback loop, with input arriving from an origin, synapsing in a cerebellar nucleus, and then projecting back, often to the same origin Table TABLE What type of fiber originating in the cerebellar cortex is inhibitory on the deep cerebellar nuclei?
Purkinje fibers originate in the cerebellar cortex and synapse on the deep nuclei as an inhibitory neuron. Where does the dentatorubrothalamic tract synapse? These fibers synapse in the ventrolateral VL nucleus of the thalamus before ascending to the cortex.
What are the basal ganglia? The basal ganglia are a collection of nuclei, largely concerned with motor control, composed primarily of the corpus striatum, and the lenticular complex. See Figure What are the parts of the corpus striatum? The corpus striatum is composed of the putamen and caudate. What is the lenticular complex? The lenticular complex, or lentiform nucleus, is composed of the globus pallidus and putamen. Which structure is the lateral border of the caudate?
The anterior limb of the internal capsule is the lateral border of the caudate. What is the major outflow of the basal ganglia? Another bundle from the medial globus pallidus loops around the internal capsule as the ansa lenticularis. These thalamic nuclei then relay information up to the motor cortex. Is there any other output from the medial globus pallidus?
Apart from the lenticular fasciculus and the ansa lenticularis, a third fiber tract leaves the medial globus pallidus as the pallidotegmental tract and descends onto the pedunculopontine nucleus in the midbrain, where neurons help to regulate posture. This is the only descending tract from the basal ganglia. Is there any output from the basal ganglia that does not originate in the medial globus pallidus? The only other output is a small tract pallidosubthalamic fibers that leaves the lateral globus pallidus to synapse in the subthalamic nucleus.
What is the major input to the basal ganglia? The major input is from the motor cortex and the thalamic nuclei. The basal ganglia function, simplistically, as a feedback loop: Which structure lies lateral to the thalamus and medial to the thalamus? The posterior limb of the internal capsule is the lateral border of the thalamus. The third ventricle lies medial to the thalamus.
What is the anatomy of the thalamus? The intermedullary lamina divides the thalamus into anterior, medial, and lateral groups. The lateral group is further divided into ventral and dorsal tiers. Each group contains specific nuclei: Anterior nucleus n Medial group: Dorsomedial DM nucleus n Lateral group: What are the inputs to and from the main thalamic nuclei?
What is the limbic lobe? The limbic lobe is not a true lobe of the brain but rather a functional collection of structures that regulate higher activities such as memory and emotion.
It is commonly said to include 1 cingulate gyrus, 2 parahippocampal gyrus, 3 hippocampal gyrus, and 4 uncus.
This is a route by which the limbic system communicates between the hippocampus, thalamus, hypothalamus, and cortex. It forms a circuit from the hippocampal formation! What are the olfactory receptor cells?
The receptor cells are bipolar neurons that pass from the olfactory mucosa through the cribriform plate to the olfactory bulb. Collectively, the central processes of the olfactory receptor cells constitute cranial nerve I. What is the anatomy of the olfactory pathway? In the olfactory bulb, the axons of receptor cells synapse on dendrites of mitral and tufted cells forming a glomerulus.
The axons of mitral and tufted cells compose the olfactory tract, which soon divides into medial and lateral stria. Medial stria fibers cross to the contralateral side via the anterior commissure, while the lateral stria fibers terminate in the anterior perforated substance, amygdaloid complex, and lateral olfactory gyrus which is the primary olfactory cortex. From the lateral olfactory gyrus prepiriform area , fibers project to the entorhinal cortex, the medial dorsal nucleus of the thalamus, and the hypothalamus.
What is unique about the projection of olfactory information to the cerebral cortex? Unlike other sensory modalities, olfaction reaches the cortex without relay through the thalamus. What are the most common causes of anosmia? Smoking 3. Head injury 4. Craniotomy 5. Subarachnoid hemorrhage 6. Meningiomas of the olfactory groove 7. Zinc and vitamin A deficiency 8. Hypothyroidism 9. What is the arrangement of cones and rods in the retina?
The 6 million cones are concentrated toward the center, and the million rods are in the periphery of the retina. In the fovea, located centrally within the macula, each cone is served by a single ganglion cell, resulting in very high resolution. In the periphery, many rods project to a single ganglion cell, giving high sensitivity but lower resolution. What are the primary functions of rods?
Rods are concerned with night vision and are most sensitive between the blue and green wavelengths. What are the primary functions of cones? Cones are concerned with color vision and daytime vision.
The three types of cones are tuned, via visual pigments, to different frequencies in the blue, green, and red wavelength ranges. What is the afferent pathway for the pupillary light reflex?