Snell's Clinical Neuroanatomy

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NINTH EDITION SNELL’S CLINICAL NEUROANATOMY

RYAN SPLITTGERBER, PhD Associate Professor, Department of Surgery Vanderbilt University Medical Center Office of Health Sciences Education Vanderbilt University School of Medicine Nashville, Tennessee

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Ninth Edition

Copyright © 2019, 2010, 2006, 2001, 1997, 1992, 1987, 1980 Wolters Kluwer Health/Lippincott Williams & Wilkins. All rights reserved. This book is protected by copyright. No part of this book may be reproduced or transmitted in any form or by any means, including as photocopies or scanned-in or other electronic copies, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotations embodied in critical articles and reviews. Materials appearing in this book prepared by individuals as part of their official duties as U.S. government employees are not covered by the above-men tioned copyright. To request permission, please contact Wolters Kluwer at Two Commerce Square, 2001 Market Street, Philadelphia, PA 19103, via email at permissions @lww.com, or via our website at shop.lww.com (products and services).

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Library of Congress Cataloging-in-Publication Data Names: Splitt gerber, Ryan, author. Title: Snell’s clinical neuroanatomy / Ryan Splittgerber. Other titles: Clinical neuroanatomy Description: Ninth edition. | Philadelphia, PA: Wolters Kluwer Health, [2024] | Includes bibliographical references and index. Identifiers: LCCN 2023051269 | ISBN 9781975195946 (hardback) Subjects: MESH: Nervous System--anatomy & histology | Nervous System Diseases--diagnosis | BISAC: MEDICAL / Anatomy | MEDICAL / Neuroscience Classification: LCC QM451 | NLM WL 101 | DDC 616.8--dc23/eng/20231211 LC record availabl e at https://lccn.loc.gov/2023051269

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To learn is not to know; there are the learners and the learned. Memory makes the one, philosophy the other. —Alexandre Dumas

To my wife, Brienne For providing more love and support than I deserve. To my boys, Carter and Caden For providing inspiration and humor … a lot of humor.

Preface

• Clinical Notes. This section provides the practical application of neuroanatomical facts that are essen tial in clinical practice. It emphasizes the structures that the clinician will encounter when making a diagnosis and treating a patient. It also provides the information necessary to understand many proce dures and techniques and notes the anatomical “pitfalls” commonly encountered. • Key Concepts. These quick, bulleted reviews of key topics and information are provided at the end of each chapter. • Clinical Problem Solving. This section provides the student with many examples of clinical situations in which knowledge of neuroanatomy is necessary to solve clinical problems and to institute treatment; solutions to the problems are provided at the end of the chapter. • Review Questions. The purpose of the questions is threefold: to focus attention on areas of importance, to enable students to assess their areas of weakness, and to provide a form of self-evaluation when questions are answered under examination conditions. Some of the questions are centered around a clinical problem that requires a neuroanatomical answer. Solutions to the problem are provided at the end of each chapter. An interactive Review Test , including over 450 questions, is provided online. The book is extensively illustrated. The majority of the figures have been kept simple and are in color. As in the previous edition, a concise Atlas of the dissected brain is included prior to the text. This small but important group of colored plates enables the reader to quickly relate a particular part of the brain to the whole organ.

This book contains the basic neuroanatomical facts necessary for the practice of medicine. It is suitable for medical students, dental students, nurses, and allied health students. Residents find this book useful during their rotations. The functional organization of the nervous system has been emphasized and indicates how injury and disease can result in neurologic deficits. The amount of factual information has been strictly limited to that which is clinically important. Authorship transitioned from the late Dr. Richard Snell who, with brilliance and dedication, fathered the previous seven editions and provided the framework for the eighth. The content of each chapter has been reviewed and edited to be more straightforward and concise. The traditional artwork has been recolored and updated to enhance the clarity and to provide additional information to each image. High-quality magnetic reso nance images (MRIs) and histologic photomicrographs have been updated to provide greater visual details. Each chapter introduces the relevance of neuroanat omy through a short case report. • Clinical Example. A short case report that serves to dramatize the relevance of neuroanatomy introduces each chapter. • Chapter Objectives. This section details the material that is most important to learn and understand in each chapter. • Basic Neuroanatomy. This section provides basic information on neuroanatomical structures that are of clinical importance. Numerous examples of nor mal radiographs, CT scans, MRIs, and PET scans are also provided. Many cross-sectional diagrams have been included to stimulate students to think in terms of three-dimensional anatomy, which is so important in the interpretation of CT scans and MRIs.

R.S. R.S.S.

Acknowledgments

Starting with the first edition of Clinical Neuroanatomy published in 1980, many people have provided their exper tise and should be recognized for their contributions. First and foremost, thanks to Richard S. Snell whose shoulders we stand upon to advance our own intellectual progress. Throughout this text and in previous editions, the fol lowing individuals provided valuable contributions and are gratefully acknowledged: Arafat AL-Boasi, Carolina Assuncao, N. Cauna, L. Clerk, D. O. Davis, H. Dey, M. Feldman, T. M. J. Fitzgerald, I. Grunther, J. M. Kerns, T. McCarthy, Josh Min, Ajitanshu Singh Parihar, A. Peters, João Rui Nunes Branco Polónia dos Santos, G. Sze, and L. Wener. EIGHTH EDITION I am greatly indebted to the staff of Wolters Kluwer, including Crystal Taylor, who brought me in and provided

me with this wonderful opportunity, as well as Debbie Bordeaux, development editor, and Erin Hernandez, edi torial coordinator. Thanks also to freelance development editor Kelly Horvath, who provided invaluable direction and extreme patience with me throughout the entire process. My special thanks to Stephanie Vas, Program Director of the Magnetic Resonance Imaging Program at the University of Nebraska Medical Center, who produced exceptional MR images. I would like to extend my gratitude to my colleagues and mentors for their encouragement and wisdom— especially, Art Dalley, Scott Pearson, Cathy Pettepher, and Eli Zimmerman.

R.S.

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Contents

Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . vi Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . vii ColorAtlasofBrain....................... ix

CHAPTER 1 Organization of the Nervous System 1 CHAPTER 2 Neurons and Neuroglia 33 CHAPTER 3 Nerve Fibers and Peripheral Innervation 71 CHAPTER 4 Spinal Cord and Ascending, Descending, and Intersegmental Tracts 131 CHAPTER 5 Brainstem 185 CHAPTER 6 Cranial Nerve Nuclei 226 CHAPTER 7 Cerebellum and Its Connections 267 CHAPTER 8 Reticular Formation and Limbic System 288 CHAPTER 9 Basal Nuclei (Basal Ganglia) 299 CHAPTER 10 Thalamus 312 CHAPTER 11 Hypothalamus 322 CHAPTER 12 Autonomic Nervous System 336 CHAPTER 13 Cerebrum 366 CHAPTER 14 Structure and Functional Localization of the Cerebral Cortex 397 CHAPTER 15 Meninges 417 CHAPTER 16 Ventricular System and Cerebrospinal Fluid 435 CHAPTER 17 Blood Supply of the Brain and Spinal Cord 463 CHAPTER 18 Nervous System Development 487

Index

505

viii

Color Atlas of Brain

Frontal pole

Frontal lobe

Longitudinal fissure

Precentral sulcus Precentral gyrus Central sulcus Postcentral gyrus Postcentral sulcus

Parietal lobe

Occipital lobe

Occipital pole

Frontal lobe

Longitudinal fissure

Olfactory bulb

Uncus

Mammillary body Posterior perforating substance

Temporal lobe

Pons

Pyramid

Olive

Medulla

Cerebellum

Figure CA-1 Top: Superior view of the brain. Bottom: Inferior view of the brain.

Color Atlas of Brain

Superior frontal gyus

Superior frontal sulcus

Middle frontal gyus

Inferior frontal gyus

Inferior frontal sulcus

Frontal pole

Pons

Temporal pole

Medulla

Left cerebral hemisphere

Parieto-occipital sulcus

Occipital pole

Occipital lobe

Preoccipital notch

Vermis of cerebellum

Left cerebellar hemisphere

Color Atlas of Brain

Postcentral gyrus

Central sulcus

Superior frontal gyrus

Parieto-occipital sulcus

Middle frontal gyrus

Inferior frontal gyrus

Lateral sulcus

Superior temporal gyrus Middle temporal gyrus

Preoccipital notch

Right cerebellar hemisphere

Inferior temporal gyrus

Medulla

Central sulcus

Precentral gyrus

Postcentral gyrus

Cingulate gyrus

Parieto-occipital sulcus

Corpus callosum Septum pellucidum

Cuneus Calcarine sulcus Lingual gyrus

Anterior commissure

Temporal lobe

Pons Medulla

Vermis of cerebellum (cut)

Figure CA-3 Top: Right lateral view of the brain. Bottom: Medial view of the right side of the brain following median sagittal section. Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

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Color Atlas of Brain

Longitudinal fissure

Cingulate gyrus Corpus callosum Septum pellucidum

Caudate nucleus

Internal capsule (anterior limb)

Claustrum

Putamen

Cingulate gyrus

Fornix

Corpus callosum

Internal capsule (posterior limb) Thalamus Caudate nucleus

Lateral sulcus

Insular cortex

Putamen

Globus pallidus

Amygdaloid nucleus

Uncus

Mammillary bodies

Figure CA-4 Coronal sections of the brain passing through the temporal pole (top) , the mammillary bodies (bottom) . Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

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Color Atlas of Brain

Corpus callosum (genu) Lateral ventricle (anterior horn)

Caudate nucleus Internal capsule (anterior limb) Globus pallidus

Fornix

Putamen

Third ventricle

Internal capsule (posterior limb)

Thalamus

Corpus callosum (splenium)

Lateral ventricle (posterior horn)

Occipital lobe

Lateral ventricle Corpus callosum

Putamen Internal capsule (posterior limb) Caudate nucleus

Fornix

Thalamus

Globus pallidus

Third ventricle

Lateral ventricular (lateral horn)

Crus cerebri of midbrain

Hippocampus

Pons

Figure CA-5 Top: Horizontal section of the cerebrum showing the lentiform nucleus, the caudate nucleus, the thalamus, and the internal capsule. Bottom: Oblique coronal section of the brain.

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Color Atlas of Brain

Olfactory bulb Olfactory tract

Optic nerve (CN II) Optic chiasma

Oculomotor nerve (CN III) Trochlear nerve (CN IV)

Abducens nerve (CN VI) Trigeminal nerve (CN V)

Vestibulocochlear nerve (CN VIII)

Roots of the glossopharyngeal, vagus, and cranial part of accessory nerves

Hypoglossal nerve roots (CN XII)

Spinal part of accessory nerve (CN XI)

Figure CA-6 Inferior view of the brain showing cranial nerves. The facial nerves cannot be seen.

Color Atlas of Brain

Midbrain

Median sulcus Medial eminence Facial colliculus Sulcus limitans

Hypoglossal triangle Stria medullares Vestibular area in fourth ventricle

Vagal triangle

Sulcus limitans

Medulla

Primary fissure Anterior lobe

Superior aspect of vermis

Middle lobe

Declive

Left cerebellar hemisphere

Middle cerebellar peduncle

Flocculus

Central lobule

Tonsil

Inferior aspect of vermis

Right cerebellar hemisphere

Figure CA-7 Top: Posterior view of the brainstem. The greater part of the cerebellum had been removed to expose the floor of the fourth ventricle. Middle: Superior view of the cerebellum showing the vermis and right and left cerebellar hemispheres. Bottom: Inferior view of the cerebellum showing the vermis and right and left cerebellar hemispheres. Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

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Color Atlas of Brain

Fornix Interventricular foramen (entrance to lateral ventricle)

Cingulate gyrus

Corpus callosum

Corpus callosum (genu)

Corpus callosum (splenium)

Calcarine sulcus Cuneus

Anterior commissure Lamina terminalis

Optic chiasma

Superior medullary velum

Foramen of Magendie

Tuber cinereum

Cerebral aqueduct Fourth ventricle

Mammillary body

Site of third ventricle

Figure CA-8 Enlarged medial view of the right side of the brain following median sagittal section, showing the continuity of the central canal, fourth ventricle, cerebral aqueduct, and the third ventricle and entrance into the lateral ventricle through the interventricular foramen.

7 Cerebellum and Its Connections

CHAPTER OBJECTIVES • Review the structure and functions of the cerebellum

• Describe the afferent and efferent connections of the cerebellum within the central nervous system

The cerebellum is divided into three main lobes: anterior , middle , and flocculonodular lobes . The anterior lobe may be seen on the superior surface of the cerebellum and is separated from the middle lobe by a wide V-shaped fissure called the primary fissure (Figs. 7-2 and 7-3). The middle lobe (sometimes called the posterior lobe), which is the largest part of the cer ebellum, is situated between the primary and uvulo nodular fissures . The flocculonodular lobe is situated posterior to the uvulonodular fissure (see Fig. 7-3). A deep horizontal fissure that is found along the mar gin of the cerebellum separates the superior from the The cerebellum plays a very important role in the control of posture and voluntary movements. It uncon sciously influences the smooth contraction of voluntary muscles and carefully coordinates their actions, together with the relaxation of their antagonists. Students should commit the functions of the connections of the cerebel lum to the remainder of the central nervous system to memory, as this will greatly assist in the retention of the material. In this chapter, great emphasis is placed on the fact that each cerebellar hemisphere controls muscular movements on the same side of the body and that the cerebellum has no direct pathway to the lower motor neurons but exerts its control via the cerebral cortex and the brainstem. dysdiadochokinesia, and the history are characteristic of right-sided cerebellar disease. A computed tomography scan reveals a tumor in the right cerebellar hemisphere. Understanding the structure and the nervous connec tions of the cerebellum and, in particular, knowing that the right cerebellar hemisphere influences voluntary muscle tone on the same side of the body enable the neurologist to make an accurate diagnosis and institute treatment.

GROSS APPEARANCE The cerebellum is situated in the posterior cranial fossa and is covered superiorly by the tentorium cerebelli. It is the largest part of the hindbrain and lies posterior to the fourth ventricle, the pons, and the medulla oblon gata (Fig. 7-1). The cerebellum is somewhat ovoid and constricted in its median part. It consists of two cere bellar hemispheres joined by a narrow median vermis . The cerebellum is connected to the posterior aspect of the brainstem by three symmetrical bundles of nerve fibers: the superior , middle , and inferior cerebellar peduncles (see Figs. 1-12 and 5-16). On examination, she has diminished tone of the mus cles of her right upper limb, as seen when her elbow and wrist joints are passively flexed and extended. Similar evidence is found in the right lower limb. When asked to stretch out her arms in front of her and hold them in position, she demonstrates obvious signs of right-sided tremor. When asked to touch the tip of her nose with her left index finger, she performs the movement without any difficulty, but when she repeats the movement with her right index finger, she either misses her nose or hits it due to the irregularly contracting muscles. When she is asked to quickly pronate and supinate the forearms, the move ments are normal on the left side but jerky and slow on the right side. Mild papilledema of both eyes is found. No other abnormal signs are seen. The right-sided hypotonia, static tremor, and intention tremor associated with voluntary movements, right-sided A 56-year-old female is examined by a neurologist for a variety of symptoms, including an irregular swaying gait and a tendency to drift to the right when walking. Her fam ily recently noticed that she has difficulty in keeping her balance when standing still, and she finds that standing with her feet apart helps her keep her balance.

267

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CHAPTER 7 Cerebellum and Its Connections

Cerebral aqueduct Superior colliculus Inferior colliculus

Midbrain

Superior medullary velum Lingula Central lobule

Cerebral peduncle

Primary fissure

Declive

Oculomotor nerve

Folium

Pons

Horizontal fissure

Cavity of fourth ventricle

Tuber

Root of fourth ventricle and choroid plexus

Cerebellar hemisphere

Medulla oblongata

Nodule

Pyramid

Median aperture in roof of fourth ventricle (inferior medullary velum)

Cortex of cerebellum

Central canal Uvula Tonsil

Figure 7-1 Sagittal section through the brainstem and the vermis of the cerebellum.

Superior aspect of vermis

Anterior lobe

Primary fissure

Middle lobe (posterior lobe)

Declive

Horizontal fissure

Uvula

Middle cerebellar peduncle Tonsil

Flocculus

Biventral lobule

Inferior semilunar lobule

Figure 7-2 Cerebellum. ( A ) Superior view. ( B ) Inferior view.

Pyramid

Tuber

269

Structures

Lingula

Primary fissure

Central lobule Ala

Anterior lobe

Quadrangular lobule

Declive

Horizontal fissure

Lobulus simplex Folium

Superior semilunar lobule

Middle lobe

Tuber

Inferior semilunar lobule

Pyramid

Biventral lobule

Uvula

Flocculonodular lobe

Tonsil Flocculus

Nodule

Uvulonodular fissure

Two cerebellar hemispheres

Pons

Cerebellum

Vermis

Medulla oblongata

Figure 7-3 Flattened view of the cerebellar cortex showing the main cerebellar lobes, lobules, and fissures. ( B ) Relationship between the diagram in ( A ) and the cerebellum.

inferior surfaces but has no morphologic or functional significance (see Figs. 7-2 and 7-3).

A section made through the cerebellum parallel with the median plane divides the folia at right angles, and the cut surface has a branched appearance, called the arbor vitae . The gray matter of the cortex throughout its extent has a uniform structure. It may be divided into three layers: (1) an external layer, the molecular layer ; (2) a middle layer, the Purkinje cell layer ; and (3) an internal layer, the granular layer (Figs. 7-4 and 7-5). Molecular Layer The molecular layer contains two types of neurons: the outer stellate cell and the inner basket cell (see Fig. 7-4). These neurons are scattered among dendritic arborizations and numerous thin axons that run parallel to the long axis of the folia. Neuroglial cells are found between these structures.

STRUCTURES The cerebellum is composed of an outer covering of gray matter called the cortex and inner white matter. Embedded in the white matter of each hemisphere are three masses of gray matter forming the intracerebel lar nuclei . Cerebellar Cortex The cerebellar cortex can be regarded as a large sheet with folds lying in the coronal or transverse plane. Each fold or folium contains a core of white matter covered superficially by gray matter (see Fig. 7-1).

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CHAPTER 7 Cerebellum and Its Connections

Granular cell

Golgi cell

Stellate cell

Cerebellar cortex

Molecular layer

Purkinje cell

Purkinje cell layer

Granular layer

White matter

Climbing fiber

Basket cell

Mossy fiber

Cerebellar nuclei

Purkinje cell axon

Efferent cerebellar fibers

Figure 7-4 Cellular organization of the cerebellar cortex. Note the afferent and efferent fibers.

Purkinje Cell Layer The Purkinje cells are large Golgi type I neurons. They are flask-shaped and arranged in a single layer (see Figs. 7-4 and 7-5). In a plane transverse to the folium, the den drites of these cells are seen to pass into the molecular layer, where they undergo profuse branching. The pri mary and secondary branches are smooth, and subse quent branches are covered by short, thick dendritic spines . The spines form synaptic contacts with the par allel fibers derived from the granule cell axons. At the base of the Purkinje cell, the axon arises and passes through the granular layer to enter the white matter. There, the axon acquires a myelin sheath, and it terminates by synapsing with cells of one of the intrace rebellar nuclei. Collateral branches of the Purkinje axon make synaptic contacts with the dendrites of basket and stellate cells of the molecular layer in the same area or in distant folia. A few of the Purkinje cell axons pass directly to end in the vestibular nuclei of the brainstem.

Granular Layer The granular layer is packed with small cells with densely staining nuclei and scanty cytoplasm. Each cell gives rise to four or five dendrites, which make claw-like endings and have synaptic contact with mossy fiber input (see Fig. 7-4). The axon of each granule cell passes into the molecular layer, where it bifurcates at a T junction, the branches running parallel to the long axis of the cerebellar folium (see Fig. 7-4). These fibers, known as parallel fibers , run at right angles to the dendritic processes of the Purkinje cells. Most of the parallel fibers make synaptic con tacts with the spinous processes of the dendrites of the Purkinje cells. Neuroglial cells are found through out this layer. Scattered throughout the granular layer are Golgi cells. Their dendrites ramify in the molecular layer, and their axons terminate by splitting up into branches that synapse with the dendrites of the gran ular cells (see Fig. 7-5). Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

271

Structures

Figure 7-5 Light micrographs of the cerebellum. ( A ) The cerebellum consists of a core of white matter (W) and superficially located gray matter (G). The gray matter is subdivided into the outer molecular layer (ML), a middle Purkinje cell layer (PL), and the inner granular layer (GL), each layer consisting of high concentration of cell bodies belonging to neurons of specialized morphologies and functions. The less-dense appearance of the molecular layer is because of the sparse arrangement of nerve cell bodies, whereas the darker appearance of the granular layer is a function of the great number of darkly staining nuclei packed closely together. (×14). ( B ) Here, the interface between the outer white matter (W) and a core of gray matter (G) is readily evi dent ( asterisks ). The numerous nuclei ( arrowheads ) present in white matter belong to the various neuroglia, which support the axons traveling up and down the spinal cord. The large neuron cell bodies (CB) in the gray matter possess euchromatic nuclei with dense, dark nucleoli. Blood ves sels (BV), which penetrate deep into the gray matter, are surrounded by processes of neuroglial cells, forming the blood–brain barrier (not visible). Small nuclei ( arrows ) in gray matter belong to the neuroglial cells, whose cytoplasm and cellular processes are not evident. (×132). (Reproduced with permission from Gartner LP. Gartner & Hiatt’s Atlas and Text of Histology . 8th ed. Baltimore, MD: Wolters Kluwer; 2002: Fig. 8-7A,B.) Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

Functional Areas Clinical observations by neurologists and neurosur geons and the experimental use of positron emission tomography have shown that the cerebellar cortex can be divided into three functional areas. The cortex of the vermis influences the movements of the long axis of the body, namely, the neck, shoul ders, thorax, abdomen, and hips (Fig. 7-6). Immediately

lateral to the vermis is a so-called intermediate zone of the cerebellar hemisphere. This area has been shown to control the muscles of the distal parts of the limbs, especially the hands and feet. The lateral zone of each cerebellar hemisphere appears to be concerned with the planning of sequential movements of the entire body and is involved with the conscious assessment of movement errors.

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CHAPTER 7 Cerebellum and Its Connections

The globose nucleus consists of one or more rounded cell groups that lie medial to the emboliform nucleus. The fastigial nucleus lies near the midline in the ver mis and close to the roof of the fourth ventricle; it is larger than the globose nucleus. The intracerebellar nuclei are composed of large, multipolar neurons with simple branching dendrites. The axons form the cerebellar outflow in the superior and inferior cerebellar peduncles. White Matter The small amount of white matter in the vermis closely resembles the trunk and branches of a tree and thus is termed the arbor vitae (see Fig. 7-1). There is a large amount of white matter in each cerebellar hemisphere. The white matter is made up of three groups of fibers: (1) intrinsic, (2) afferent, and (3) efferent. Intrinsic fibers do not leave the cerebellum but con nect different regions of the organ. Some interconnect folia of the cerebellar cortex and vermis on the same side; others connect the two cerebellar hemispheres together. Afferent fibers form the greater part of the white matter and proceed to the cerebellar cortex. They enter the cerebellum mainly through the inferior and middle cerebellar peduncles. Efferent fibers constitute the output of the cer ebellum and commence as the axons of the Purkinje cells of the cerebellar cortex. The great majority of the Purkinje cell axons pass to and synapse with the neu rons of the cerebellar nuclei (fastigial, globose, embo liform, and dentate). The axons of the neurons then leave the cerebellum. A few Purkinje cell axons in the flocculonodular lobe and in parts of the vermis bypass the cerebellar nuclei and leave the cerebellum without synapsing.

Primary fissure

Figure 7-6 Somatosensory projection areas in the cerebellar cortex.

Intracerebellar Nuclei Four masses of gray matter are embedded in the white matter of the cerebellum on each side of the midline (Fig. 7-7). From lateral to medial, these are the dentate , emboliform , globose , and fastigial nuclei . The dentate nucleus is the largest of the cerebellar nuclei. It has the shape of a crumpled bag with the open ing facing medially. The interior of the bag is filled with white matter made up of efferent fibers that leave the nucleus through the opening to form a large part of the superior cerebellar peduncle. The emboliform nucleus is ovoid and situated medial to the dentate nucleus, partially covering its hilus.

Fastigial nucleus Globose nucleus

Emboliform nucleus

Dentate nucleus

Cerebellar hemisphere

Cavity of the fourth ventricle

Figure 7-7 Position of the intracere bellar nuclei.

Pons

273

Cerebellar Cortical Mechanisms

Fibers from the dentate, emboliform, and globose nuclei leave the cerebellum through the superior cere bellar peduncle. Fibers from the fastigial nucleus leave through the inferior cerebellar peduncle. CEREBELLAR CORTICAL MECHANISMS As a result of extensive cytologic and physiologic research, certain basic mechanisms have been attributed to the cerebellar cortex. The climbing and the mossy fibers constitute the two main lines of input to the cortex and are excitatory to the Purkinje cells (Fig. 7-8). Climbing fibers are the terminal fibers of the olivo cerebellar tracts. They are so named because they ascend through the layers of the cortex like a vine. They pass through the granular layer of the cortex and termi nate in the molecular layer by dividing repeatedly. Each climbing fiber wraps around and makes many synaptic contacts with the dendrites of a Purkinje cell. A single Purkinje neuron makes synaptic contact with only one climbing fiber. However, one climbing fiber makes con tact with up to 10 Purkinje neurons. A few side branches leave each climbing fiber and synapse with the stellate cells and basket cells. The mossy fibers are the terminal fibers of all other cerebellar afferent tracts. They have multiple branches

and exert a much more diffuse excitatory effect. A single mossy fiber may stimulate thousands of Purkinje cells through the granule cells. What, then, is the function of the remaining cells of the cerebellar cortex, namely, the stellate, basket, and Golgi cells? Neurophysiologic research, using microelectrodes, indicates that they serve as inhibitory interneurons. They not only limit the area of cortex excited but also probably influence the degree of Purkinje cell excitation produced by the climbing and mossy fiber input. By this means, fluctuat ing inhibitory impulses are transmitted by the Purkinje cells to the intracerebellar nuclei, which, in turn, mod ify muscular activity through the motor control areas of the brainstem and cerebral cortex. Thus, the Purkinje cells form the center of a functional unit of the cerebel lar cortex. Intracerebellar Nuclear Mechanisms The deep cerebellar nuclei receive afferent nervous information from two sources: (1) inhibitory axons from the Purkinje cells of the overlying cortex and (2) excitatory axons that are branches of the afferent climbing and mossy fibers passing to the overlying cortex. In this manner, a given sensory input to the cerebellum sends excitatory information to the nuclei, which, a short time later, receive cortical processed

Golgi cell

Granular cell

Basket cell

Cerebellar cortex

Purkinje cell

Molecular layer

Purkinje cell layer

Granular layer

White matter

Mossy fiber

Figure 7-8 Functional organization of the cerebellar cortex. The arrows indicate the direction taken by the nerve impulses. The red arrows indicate information entering the cerebellum.

Cerebellar nucleus

Climbing fiber

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CHAPTER 7 Cerebellum and Its Connections

Superior cerebellar peduncle

Cerebral peduncle

Middle cerebellar peduncle

Vermis

Optic nerve

Oculomotor nerve

Trochlear nerve

Pons

Trigeminal nerve Vestibulocochlear nerve Abducens nerve

Olive Pyramid

Spinal root of accessory nerve

Inferior cerebellar peduncle

Medulla oblongata

Figure 7-9 Three cerebellar peduncles connecting the cerebellum to the rest of the central nervous system.

inhibitory information from the Purkinje cells. Efferent information from the deep cerebellar nuclei leaves the cerebellum to be distributed to the remainder of the brain and spinal cord. Cerebellar Cortical Neurotransmitters Pharmacologic research has suggested that the excit atory climbing and mossy afferent fibers use glutamate ( γ -aminobutyric acid) as the excitatory transmitter on the dendrites of the Purkinje cells. Further research has indicated that other afferent fibers entering the cortex liberate norepinephrine and serotonin at their endings that possibly modify the action of the glutamate on the Purkinje cells. Cerebellar Peduncles The cerebellum is linked to other parts of the central nervous system by numerous efferent and afferent fibers grouped together on each side into three large bundles, or peduncles (Fig. 7-9). Superior cerebellar peduncles connect the cerebellum to the midbrain, middle cerebel lar peduncles connect the cerebellum to the pons, and inferior cerebellar peduncles connect the cerebellum to the medulla oblongata. CEREBELLAR AFFERENT FIBERS The cerebellum receives major afferent tracts from the cerebral cortex, pons, medulla oblongata, and spi nal cord.

Cerebellar Afferent Fibers from the Cerebral Cortex

The cerebral cortex sends information to the cerebel lum by three pathways: (1) corticopontocerebellar, (2) cerebro-olivocerebellar, and (3) cerebroreticulocer ebellar pathways. Corticopontocerebellar Pathway The corticopontine fibers arise from nerve cells in the frontal, parietal, temporal, and occipital lobes of the cerebral cortex, descend through the corona radiata and internal capsule, and terminate on the pontine nuclei (Fig. 7-10). The pontine nuclei give rise to the transverse fibers of the pons , which cross the midline and enter the opposite cerebellar hemisphere as the middle cerebellar peduncle (see Figs. 5-16 and 5-17). Cerebro-Olivocerebellar Pathway The cortico-olivary fibers arise from nerve cells in the frontal, parietal, temporal, and occipital lobes of the cerebral cortex and descend through the corona radiata and internal capsule to terminate bilaterally on the infe rior olivary nuclei. The inferior olivary nuclei give rise to fibers that cross the midline and enter the opposite cerebellar hemisphere through the inferior cerebellar peduncle. These fibers terminate as the climbing fibers in the cerebellar cortex. Cerebroreticulocerebellar Pathway The corticoreticular fibers arise from nerve cells from many areas of the cerebral cortex, particularly the

275

Cerebellar Afferent Fibers

Corticopontocerebellar pathway

Corticoreticular cerebellar pathway Cortico-olivary cerebellar pathway

Pontine nuclei

Reticular formation

Inferior olivary nucleus

Figure 7-10 Cerebellar afferent fibers from the cerebral cortex. The cerebellar peduncles are shown as ovoid dotted lines .

sensorimotor areas. They descend to terminate in the reticular formation on the same side and on the opposite side in the pons and the medulla. The cells in the retic ular formation give rise to the reticulocerebellar fibers that enter the cerebellar hemisphere on the same side through the inferior and middle cerebellar peduncles. This connection between the cerebrum and the cere bellum is important in the control of voluntary movement. Information regarding the initiation of movement in the cerebral cortex is probably transmitted to the cerebellum so that the movement can be monitored, and appropriate adjustments in the muscle activity can be made. Cerebellar Afferent Fibers from the Spinal Cord The spinal cord sends information to the cerebellum from somatosensory receptors by three pathways:

Anterior Spinocerebellar Tract Axons entering the spinal cord from the posterior root ganglion terminate by synapsing with the neurons in the nucleus dorsalis (Clarke column) at the base of the pos terior gray column. Most of the axons of these neurons cross to the opposite side and ascend as the anterior spinocerebellar tract in the contralateral white column; some ascend as the anterior spinocerebellar tract in the lateral white column on the same side (Fig. 7-11). The fibers enter the cerebellum through the superior cer ebellar peduncle and terminate as mossy fibers in the cerebellar cortex. Collateral branches that end in the deep cerebellar nuclei are also given off. Those fibers

276

CHAPTER 7 Cerebellum and Its Connections

Superior cerebellar peduncle

Vestibular nuclei

Middle cerebellar peduncle

Cerebellum

Dentate nucleus

Vestibular nerve

Inferior cerebellar peduncle

Nucleus cuneatus

Medulla oblongata

Anterior spinocerebellar tract Posterior spinocerebellar tract

Anterior spinocerebellar tract

Spinal cord

Figure 7-11 Cerebellar afferent fibers from the spinal cord and internal ear. The cerebellar peduncles are shown as ovoid dotted lines . Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

that cross over to the opposite side in the spinal cord are thought to cross back within the cerebellum. The anterior spinocerebellar tract is found at all segments of the spinal cord, and its fibers convey mus cle joint information from the muscle spindles, tendon organs, and joint receptors of the upper and lower limbs. The cerebellum likely receives information from the skin and superficial fascia by this tract. Posterior Spinocerebellar Tract The axons entering the spinal cord from the posterior root ganglion enter the posterior gray column and

terminate by synapsing on the neurons at the base of the posterior gray column. These neurons are known collectively as the nucleus dorsalis (Clarke column). The axons of these neurons enter the posterolateral part of the lateral white column on the same side and ascend as the posterior spinocerebellar tract to the medulla oblongata. Here, the tract enters the cerebellum through the inferior cerebellar peduncle and terminates as mossy fibers in the cerebellar cortex. Collateral branches that end in the deep cerebellar nuclei are also given off. The posterior spinocerebellar tract receives muscle joint information from the muscle spindles, tendon organs, and joint receptors of the trunk and lower limbs.

277

Cerebellar Efferent Fibers

CEREBELLAR EFFERENT FIBERS The entire output of the cerebellar cortex is through the axons of the Purkinje cells. Most of the axons of the Purkinje cells end by synapsing on the neurons of the deep cerebellar nuclei (see Fig. 7-4). The axons of the neu rons that form the cerebellar nuclei constitute the effer ent outflow from the cerebellum. A few Purkinje cell axons pass directly out of the cerebellum to the lateral vestibu lar nucleus. The efferent fibers from the cerebellum con nect with the red nucleus, thalamus, vestibular complex, and reticular formation. Globose-Emboliform-Rubral Pathway Axons of neurons in the globose and emboliform nuclei travel through the superior cerebellar peduncle and cross the midline to the opposite side in the decussa tion of the superior cerebellar peduncles (Fig. 7-12). The fibers end by synapsing with cells of the contralat eral red nucleus, which give rise to axons of the rubro spinal tract . Thus, this pathway crosses twice, once in the decussation of the superior cerebellar peduncle and again in the rubrospinal tract close to its origin. By this means, the globose and emboliform nuclei influence motor activity on the same side of the body. Dentatothalamic Pathway Axons of neurons in the dentate nucleus travel through the superior cerebellar peduncle and cross the midline to the opposite side in the decussation of the superior cerebellar peduncle . The fibers end by synapsing with cells in the contralateral ventrolateral nucleus of the thalamus . The axons of the thalamic neurons ascend

Cuneocerebellar Tract These fibers originate in the nucleus cuneatus of the medulla oblongata and enter the cerebellar hemisphere on the same side through the inferior cerebellar pedun cle (see Fig. 7-10). The fibers terminate as mossy fibers in the cerebellar cortex. Collateral branches that end in the deep cerebellar nuclei are also given off. The cuneo cerebellar tract receives muscle joint information from the muscle spindles, tendon organs, and joint receptors of the upper limb and upper part of the thorax. The vestibular nerve receives information from the inner ear concerning motion from the semicircular canals and position relative to gravity from the utricle and saccule. The vestibular nerve sends many afferent fibers directly to the cerebellum through the inferior cerebellar peduncle on the same side. Other vestibular afferent fibers pass first to the vestibular nuclei in the brainstem, where they synapse and are relayed to the cerebellum (see Fig. 7-11). They enter the cerebellum through the inferior cerebellar peduncle on the same side. All the afferent fibers from the inner ear termi nate as mossy fibers in the flocculonodular lobe of the cerebellum. Other Afferent Fibers In addition, the cerebellum receives small bundles of afferent fibers from the red nucleus and the tectum. The afferent cerebellar pathways are summarized in Table 7-1. Cerebellar Afferent Fibers from the Vestibular Nerve

Table 7-1

Afferent Cerebellar Pathways

Pathway

Function

Origin

Destination

Corticopontocerebellar

Conveys control from cerebral cortex Conveys control from cerebral cortex

Frontal, parietal, temporal, and occipital lobes Frontal, parietal, temporal, and occipital lobes

Via pontine nuclei and mossy fibers to cerebellar cortex Via inferior olivary nuclei and climbing fibers to cerebellar cortex

Cerebro-olivocerebellar

Cerebroreticulocerebellar Conveys control from cerebral cortex Anterior spinocerebellar Conveys information from muscles and joints Posterior spinocerebellar Conveys information from muscles and joints

Sensorimotor areas

Via reticular formation

Muscle spindles, tendon organs, and joint receptors Muscle spindles, tendon organs, and joint receptors Muscle spindles, tendon organs, and joint receptors

Via mossy fibers to cerebellar cortex Via mossy fibers to cerebellar cortex Via mossy fibers to cerebellar cortex

Cuneocerebellar

Conveys information from muscles and joints of upper limb Conveys information of head position and movement Conveys information from midbrain

Vestibular nerve

Utricle, saccule, and semicir cular canals

Via mossy fibers to cortex of flocculonodular lobe

Other afferents

Red nucleus and tectum Cerebellar cortex

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CHAPTER 7 Cerebellum and Its Connections

Cerebral cortex

Corticospinal fibers

Thalamus

Dentatothalamic pathway

Red nucleus

Globose-emboliform rubral pathway

Rubrospinal tract

Superior cerebellar peduncle

Cerebellar cortex

Deep cerebellar nuclei

Middle cerebellar peduncle

Fastigial vestibular pathway

Inferior cerebellar peduncle

Fastigial reticular pathway

Reticular formation

Vestibular nucleus

Corticospinal tract

Vestibulospinal tract

Decussation of pyramid

Corticospinal tract

Rubrospinal tract Reticulospinal tract

Figure 7-12 Cerebellar efferent fibers. The cerebellar peduncles are shown as ovoid dotted lines .

projecting on the neurons of the lateral vestibular nucleus on both sides. Remember that some Purkinje cell axons project directly to the lateral vestibular nucleus. The neurons of the lateral vestibular nucleus form the vestibulospinal tract . The fastigial nucleus exerts a facilitatory influence mainly on the ipsilateral extensor muscle tone. Fastigial Reticular Pathway The axons of neurons in the fastigial nucleus travel through the inferior cerebellar peduncle and end by syn apsing with neurons of the reticular formation. Axons of these neurons influence spinal segmental motor activity through the reticulospinal tract. The efferent cerebellar pathways are summarized in Table 7-2.

through the internal capsule and corona radiata and terminate in the primary motor area of the cerebral cor tex. By this pathway, the dentate nucleus can influence motor activity by acting on the motor neurons of the opposite cerebral cortex; impulses from the motor cor tex are transmitted to spinal segmental levels through the corticospinal tract. Remember that most of the fibers of the corticospinal tract cross to the opposite side in the decussation of the pyramids or later at the spinal segmental levels. Thus, the dentate nucleus coor dinates muscle activity on the same side of the body. Fastigial Vestibular Pathway The axons of neurons in the fastigial nucleus travel through the inferior cerebellar peduncle and end by

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