《医学生理学 Medical Physiology》参考教材资料（PDF电子书籍）Neuroscience in Medicine, Third edition, 2008

Chapter 1/Cytology and Organization of Cell Types 23 Ross MH, Pawlina W. Histology: a text and atlas, Sth ed. Tsankova N, Renthal w, Kumar A, Nestler EJ. Epi Baltimore: Lippincott Williams wilkins, 2006 tion in psychiatric disorders. Nat Rev Neurosci 2007: 8: 355-367 Silva AJ, Stevens CF, Tonegawa S, Wang Y. Deficient hippo- Tuomanen E Entry of pathogens into the central nervous system. campal long-term potentiation in a calcium-calmodulin FEMS Microbiol Rev 1996: 18: 289-299. kinase lI mutant mice. Science 1992. 257: 201-206 Wente SR Gatekeepers of the nucleus. Science 2000: 288: 1374-1377 Stevens CF. Bristling with teeth. Curr Biol 1991: 1: 369-371 Waggoner D. Mechanisms of disease: epigenesis. Semin Pediat Steward O, Banke<'y intracellular transport of RNA in neu- Wolff JR. Quantitative aspects of astroglia. Comptes Rendu du Vie Getting the message from the gene to the Neurol2007;14:7-14. synapse: sorting an rons. Trends Neurosci 1992: 15: 180 Congres International de Neuropathologic 1970: 31: 327. Suntharalingam M, Wente SR Peering through the pore: nuclear Yuste R, Bonhoeffer T. Morphological changes in dendritic pore complex structure, assembly, and function. Developmen- ines associated with long-term synaptic plasticity. Ann tal Cell2003:4:775-789 Rev neurosci2001;24:1071-1089

Ross MH, Pawlina W. Histology: a text and atlas, 5th ed. Baltimore: Lippincott Williams & Wilkins, 2006. Silva AJ, Stevens CF, Tonegawa S, Wang Y. Deficient hippo￾campal long-term potentiation in a calcium-calmodulin kinase II mutant mice. Science 1992;257:201–206. Stevens CF. Bristling with teeth. Curr Biol 1991;1:369–371. Steward O, Banker GA. Getting the message from the gene to the synapse: sorting and intracellular transport of RNA in neu￾rons. Trends Neurosci 1992;15:180. Suntharalingam M, Wente SR. Peering through the pore: nuclear pore complex structure, assembly, and function. Developmen￾tal Cell 2003;4:775–789. Tsankova N, Renthal W, Kumar A, Nestler EJ. Epigenetic regula￾tion in psychiatric disorders. Nat Rev Neurosci 2007;8:355–367. Tuomanen E. Entry of pathogens into the central nervous system. FEMS Microbiol Rev 1996;18:289–299. Wente SR. Gatekeepers of the nucleus. Science 2000;288:1374–1377. Waggoner D. Mechanisms of disease: epigenesis. Semin Pediatr Neurol 2007;14:7–14. Wolff JR. Quantitative aspects of astroglia. Comptes Rendu du Vie Congres International de Neuropathologic 1970;31:327. Yuste R, Bonhoeffer T. Morphological changes in dendritic spines associated with long-term synaptic plasticity. Ann Rev Neurosci 2001;24:1071–1089. Chapter 1 / Cytology and Organization of Cell Types 23

Anatomy of the Spinal cord and brain Bruce w. Newton CoNTeNTS INTRODUCTION SPINAL CORD EXTERNAL ANATOMY SELECTED READINGS 1 INTRODUCTION segments composed of 8 cervical, 12 thoracic, 5 lum- bar,5 sacral, and I coccygeal segment. Each spinal The central nervous system( CNS)is divided into a cord segment has an associated pair of spinal nerves rostral brain and a caudal spinal cord. The brain is that originate as a series of fine rootlets. Each spinal nerve is formed from dorsal root fibers, which are located in the vertebral canal that is formed by the 31 sensory axons whose cell bodies reside in dorsal root vertebral foramina from the individual vertebra. The Anglia located outside the CNs, and ventral root two are continuous with one another at the foramen fibers, which are motor axons originating from ven- magnum of the occipital bone. Any neural structure tral horn motor neurons in the spinal cord gray mat- chat lies outside the pia mater covering of the CNs is ter(Fig. 1, Fig. 2, and Fig 3). At spinal cervical levels considered part of the peripheral nervous system. Cl through C4, axons forming the accessory nerve Therefore, the 12 pairs of cranial nerves, which arise (cranial nerve [CN] XI)originate from the lateral side from the brain, and the 31 pairs of spinal nerves of the spinal cord intermediate between the dorsal originating from the spinal cord with their associated and ventral root fibers before ascending and entering ganglia are, by convention, part of the peripheral the skull via the foramen magnum. In contrast with nervous system. Both the brain and spinal cord are the number of spinal cord segments, the vertebral organized into gray matter, where the neuronal cell column, which surrounds and protects the spina bodies are located and white matter, which contains cord has 7 cervical vertebrae. 12 thoracic vertebrae the long myelinated tracts of the CNS. Spinal cord 5 lumbar vertebrae, 5 sacral vertebrae typically fused gray matter is centrally located and surrounded by into a single sacrum, and 3 to 4 coccygeal vertebrae white matter, whereas the opposite occurs in the cer- fused into a common coccyx. During development, ebral cortex the nascent embryonic spinal cord extends the length of the vertebral column, but subsequent in utero dif- 2. SPINAL CORD EXTERNAL ANATOMY ferential growth of the vertebral column versus the pinal cord results in the infant spinal cord ending at The spinal cord, continuous with the brain's the caudal aspect of the third lumbar vertebra Addi medulla oblongata, is a long cylinder beginning at tional growth results in the adult spinal cord ending the foramen magnum and extending, in adults, to at the level of the intervertebral disk between the the intervertebral disk between the first and second first and second lumbar vertebrae. The remainder of lumbar vertebrae. It is divided into 31 repeating the vertebral canal below the level of the second lumbar vertebra contains the obliquely oriented dor- From: Neuroscience in Medicine, Edited by: P. M. Conn sal and ventral roots traveling to their proper point e Humana Press. Totowa NJ 2008 of exit from the vertebral canal at the appropriate

2 Anatomy of the Spinal Cord and Brain Bruce W. Newton CONTENTS INTRODUCTION SPINAL CORD EXTERNAL ANATOMY BRAIN SELECTED READINGS 1. INTRODUCTION The central nervous system (CNS) is divided into a rostral brain and a caudal spinal cord. The brain is contained in the cranial cavity, and the spinal cord is located in the vertebral canal that is formed by the 31 vertebral foraminae from the individual vertebra. The two are continuous with one another at the foramen magnum of the occipital bone. Any neural structure that lies outside the pia mater covering of the CNS is considered part of the peripheral nervous system. Therefore, the 12 pairs of cranial nerves, which arise from the brain, and the 31 pairs of spinal nerves originating from the spinal cord with their associated ganglia are, by convention, part of the peripheral nervous system. Both the brain and spinal cord are organized into gray matter, where the neuronal cell bodies are located and white matter, which contains the long myelinated tracts of the CNS. Spinal cord gray matter is centrally located and surrounded by white matter, whereas the opposite occurs in the cer￾ebral cortex. 2. SPINAL CORD EXTERNAL ANATOMY The spinal cord, continuous with the brain’s medulla oblongata, is a long cylinder beginning at the foramen magnum and extending, in adults, to the intervertebral disk between the first and second lumbar vertebrae. It is divided into 31 repeating segments composed of 8 cervical, 12 thoracic, 5 lum￾bar, 5 sacral, and 1 coccygeal segment. Each spinal cord segment has an associated pair of spinal nerves that originate as a series of fine rootlets. Each spinal nerve is formed from dorsal root fibers, which are sensory axons whose cell bodies reside in dorsal root ganglia located outside the CNS, and ventral root fibers, which are motor axons originating from ven￾tral horn motor neurons in the spinal cord gray mat￾ter (Fig. 1, Fig. 2, and Fig. 3). At spinal cervical levels C1 through C4, axons forming the accessory nerve (cranial nerve [CN] XI) originate from the lateral side of the spinal cord intermediate between the dorsal and ventral root fibers before ascending and entering the skull via the foramen magnum. In contrast with the number of spinal cord segments, the vertebral column, which surrounds and protects the spinal cord, has 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae typically fused into a single sacrum, and 3 to 4 coccygeal vertebrae fused into a common coccyx. During development, the nascent embryonic spinal cord extends the length of the vertebral column, but subsequent in utero dif￾ferential growth of the vertebral column versus the spinal cord results in the infant spinal cord ending at the caudal aspect of the third lumbar vertebra. Addi￾tional growth results in the adult spinal cord ending at the level of the intervertebral disk between the first and second lumbar vertebrae. The remainder of the vertebral canal below the level of the second lumbar vertebra contains the obliquely oriented dor￾sal and ventral roots traveling to their proper point of exit from the vertebral canal at the appropriate From: Neuroscience in Medicine, Edited by: P. M. Conn Humana Press, Totowa, NJ 2008 25

Newton PLS AM、PMs DH DM DRG IML VR AMF DS Fig. 1. Schematic diagram of a cross-section of the spinal cord. The gray matter occupies the central region of the spinal cord The ventral horn(VH)and dorsal horn(DH)are found in all spinal segments; the intermediolateral gray horn(IML) is found in he TI through L2 segments. The anterior median fissure(AMF)extends through the white matter to the gray matter. The posterior median sulcus(PMS)and posterolateral sulcus(PLS)are located on the posterior side of the white matter. Dorsal root (R) axons that enter the dorsal horn originate from cell bodies in the dorsal root ganglion(DRG). The ventral root (Vr)is composed of axons from motor neurons in the ventral horn. The dura mater(DM) covers the spinal cord and extends to the intervertebral foramen as a dural sleeve (DS)that merges with the epineurium. The arachnoid membrane( Am) lies deep to the dura mater and has fibrous strands that extend to the pia mater on the surface of the spinal cord. Gray ramus Sweat mus Spinal nerve Ventral motor root Lateral column communicans Fig. 2. Components of spinal nerves. A typical somatic motor and sensory pathway of a spinal nerve is shown on the left: the final common sympathetic nervous system pathway is shown on the right On the left, somatic and visceral sensory axons(small dashes) travel through the dorsal or ventral rami and the spinal nerve before reaching their pseudounipolar cell body of origin in the dorsal root ganglion. The axons then form the dorsal sensory roots and enter the dorsal gray horn or dorsal white matter Axons of somatic motor neurons(large dashes), located in the ventral gray horn, leave the CNS to form the ventral motor roots The motor axons then enter the spinal nerve and diverge into a dorsal or ventral ramus to travel to their skeletal muscle target On the right, preganglionic sympathetic axons (heavy solid line) leave the CNS through the ventral motor roots. The axons then enter the spinal nerve and travel through a white ramus communicans to enter the chain ganglion. From there, preganglionic sympathetic axons pass out of the chain ganglion via a variety of routes (heavy lines with arrows) to reach the postsynaptic sympathetic neurons(not shown). Some preganglionic sympathetic axons synapse on postganglionic sympathetic neurons in the chain ganglion. If so, the postganglionic axon(light solid line)leaves the chain ganglion through the gray ramus commu- nicans to enter a ventral or dorsal ramus to reach its peripheral target. Connections for spinal reflexes are shown in the gray matter and are discussed in the chapter of this volume dealing with the organization of the spinal cord

Fig. 1. Schematic diagram of a cross-section of the spinal cord. The gray matter occupies the central region of the spinal cord. The ventral horn (VH) and dorsal horn (DH) are found in all spinal segments; the intermediolateral gray horn (IML) is found in the T1 through L2 segments. The anterior median fissure (AMF) extends through the white matter to the gray matter. The posterior median sulcus (PMS) and posterolateral sulcus (PLS) are located on the posterior side of the white matter. Dorsal root (DR) axons that enter the dorsal horn originate from cell bodies in the dorsal root ganglion (DRG). The ventral root (VR) is composed of axons from motor neurons in the ventral horn. The dura mater (DM) covers the spinal cord and extends to the intervertebral foramen as a dural sleeve (DS) that merges with the epineurium. The arachnoid membrane (AM) lies deep to the dura mater and has fibrous strands that extend to the pia mater on the surface of the spinal cord. Fig. 2. Components of spinal nerves. A typical somatic motor and sensory pathway of a spinal nerve is shown on the left; the final common sympathetic nervous system pathway is shown on the right. On the left, somatic and visceral sensory axons (small dashes) travel through the dorsal or ventral rami and the spinal nerve before reaching their pseudounipolar cell body of origin in the dorsal root ganglion. The axons then form the dorsal sensory roots and enter the dorsal gray horn or dorsal white matter. Axons of somatic motor neurons (large dashes), located in the ventral gray horn, leave the CNS to form the ventral motor roots. The motor axons then enter the spinal nerve and diverge into a dorsal or ventral ramus to travel to their skeletal muscle target. On the right, preganglionic sympathetic axons (heavy solid line) leave the CNS through the ventral motor roots. The axons then enter the spinal nerve and travel through a white ramus communicans to enter the chain ganglion. From there, preganglionic sympathetic axons pass out of the chain ganglion via a variety of routes (heavy lines with arrows) to reach the postsynaptic sympathetic neurons (not shown). Some preganglionic sympathetic axons synapse on postganglionic sympathetic neurons in the chain ganglion. If so, the postganglionic axon (light solid line) leaves the chain ganglion through the gray ramus commu￾nicans to enter a ventral or dorsal ramus to reach its peripheral target. Connections for spinal reflexes are shown in the gray matter and are discussed in the chapter of this volume dealing with the organization of the spinal cord. 26 Newton

Chapter 2/ Anatomy of the Spinal Cord and Brain The spinal cords anterior(ventral) surface has a deep anterior median fissure along its entire length which typically contains the anterior spinal artery (Fig. I and Fig. 3). The anterior median fissure DL extends deeply into the spinal cord, dividing the anterior half of the spinal cord into two separate cylinders. On its posterior(dorsal)side, several long itudinal depressions are visible: a midline posterior median sulcus and two posterolateral sulci on either DM Side of the posterior median sulcus. The dorsal roots of the individual spinal nerves originate from the posterolateral sulcus. A posterior intermediate sulcus, intermediate between the dorsal median sulcus and he posterolateral sulcus, is present beginning at vr the upper thoracic and cervical spinal levels. Its for- mation results from the location of two separate ascending sensory tracts, the fasciculus gracilis and fasciculus cuneatus, from the lower and upper limbs respectively(Fig. I and Fig. 4) ASA 2.1. Spinal Cord Internal Anatomy The spinal cord is divided into an outer layer of white matter and an inner core of gray matter(Fig. 1 and Fig. 4). The white matter is composed of long- itudinally oriented myelinated sensory and motor fiber tracts that ascend and descend the length of the spinal cord( Fig. 4). The gray matter takes the shape of (M)has been cut to reveal the ventral rootlets (vr) that an"H and is composed of paired dorsal and ventral comprise one ventral root of the spinal cord. The anterior gray horns that run the length of the spinal cord pinal artery(ASA)lies in the anterior median fissure. The From the first thoracic to the second lumbar spinal ventral roots have been removed from the right side of one segments, the gray matter contains a small additional egment to reveal the denticulate ligament(DL), which is a horn(cell column), known as the intermediolateral tooth-like pia mater extension from the surface of the spinal cell column. intermediate between the ventral horn cord to the dura mater and dorsal horn Contained within the horizontal interconnecting gray matter is the central canal, the part of the ventricular system of the CNS that is most representative of the embryonic neurocele of the intervertebral foramina. This collection of dorsal and neural tube ventral roots. contained within the lumbar cistern of he dural sac, is called the cauda equina because of its 2.1.1. SPINAL GRAY MATTER ANATOMY resemblance to a horses tail According to the cytoarchitecture, the spinal gray The spinal cord has two noticeable swellings along matter is divided into 10 regions(I through X)as its length: a cervical enlargement at lower cervical to described by Rexed. The dorsal gray horns are sen- upper thoracic levels(C4 through TI) and a lumbar sory in nature and receive somatic and visceral affer enlargement at lumbar to upper sacral levels (LI ent inputs. The ventral gray horns have a motor hrough SI). These swellings are formed by the larger function and contain the motor neurons whose number of motor and sensory neurons needed for the axons innervate skeletal muscles. The motor neurons innervation of the upper and lower extremities com- are arranged in columns oriented in a rostrocaudal pared with the thoracic, abdominal, and sacral fashion with each column innervating a skeletal mus- regions. At its termination, the spinal cord narrows cle or group of functionally related skeletal muscles into the conus medullaris, representing a reduction in For example, motor neurons that innervate the quad- neuronal cell bodies and myelinated tracts riceps femoris muscles of the anterior thigh are

intervertebral foramina. This collection of dorsal and ventral roots, contained within the lumbar cistern of the dural sac, is called the cauda equina because of its resemblance to a horses’ tail. The spinal cord has two noticeable swellings along its length: a cervical enlargement at lower cervical to upper thoracic levels (C4 through T1) and a lumbar enlargement at lumbar to upper sacral levels (L1 through S1). These swellings are formed by the larger number of motor and sensory neurons needed for the innervation of the upper and lower extremities com￾pared with the thoracic, abdominal, and sacral regions. At its termination, the spinal cord narrows into the conus medullaris, representing a reduction in neuronal cell bodies and myelinated tracts. The spinal cord’s anterior (ventral) surface has a deep anterior median fissure along its entire length, which typically contains the anterior spinal artery (Fig. 1 and Fig. 3). The anterior median fissure extends deeply into the spinal cord, dividing the anterior half of the spinal cord into two separate cylinders. On its posterior (dorsal) side, several long￾itudinal depressions are visible: a midline posterior median sulcus and two posterolateral sulci on either side of the posterior median sulcus. The dorsal roots of the individual spinal nerves originate from the posterolateral sulcus. A posterior intermediate sulcus, intermediate between the dorsal median sulcus and the posterolateral sulcus, is present beginning at the upper thoracic and cervical spinal levels. Its for￾mation results from the location of two separate ascending sensory tracts, the fasciculus gracilis and fasciculus cuneatus, from the lower and upper limbs, respectively (Fig. 1 and Fig. 4). 2.1. Spinal Cord Internal Anatomy The spinal cord is divided into an outer layer of white matter and an inner core of gray matter (Fig. 1 and Fig. 4). The white matter is composed of long￾itudinally oriented myelinated sensory and motor fiber tracts that ascend and descend the length of the spinal cord (Fig. 4). The gray mattertakes the shape of an ‘‘H’’ and is composed of paired dorsal and ventral gray horns that run the length of the spinal cord. From the first thoracic to the second lumbar spinal segments, the gray matter contains a small additional horn (cell column), known as the intermediolateral cell column, intermediate between the ventral horn and dorsal horn. Contained within the horizontal, interconnecting gray matter is the central canal, the part of the ventricular system of the CNS that is most representative of the embryonic neurocele of the neural tube. 2.1.1. SPINAL GRAY MATTER ANATOMY According to the cytoarchitecture, the spinal gray matter is divided into 10 regions (I through X) as described by Rexed. The dorsal gray horns are sen￾sory in nature and receive somatic and visceral affer￾ent inputs. The ventral gray horns have a motor function and contain the motor neurons whose axons innervate skeletal muscles. The motor neurons are arranged in columns oriented in a rostrocaudal fashion with each column innervating a skeletal mus￾cle or group of functionally related skeletal muscles. For example, motor neurons that innervate the quad￾riceps femoris muscles of the anterior thigh are Fig. 3. Anterior surface of the spinal cord. The dura mater (DM) has been cut to reveal the ventral rootlets (VR) that comprise one ventral root of the spinal cord. The anterior spinal artery (ASA) lies in the anterior median fissure. The ventral roots have been removed from the right side of one segment to reveal the denticulate ligament (DL), which is a tooth-like pia mater extension from the surface of the spinal cord to the dura mater. Chapter 2 / Anatomy of the Spinal Cord and Brain 27

Newton Fasciculus gracilis onor spinal artory Postenor spinal medullary artery Lateral corticospinal trad Arterial Anterior spinocerebellar trac Medial longitudinal fasciculus a Antenor spinal modul ary artery Sulcal artery Anterior spinal artery Fig. 4. The arterial supply (right side) and internal anatomy of the spinal cord. Both the anterior and posterior spinal arteries receive contributions from anterior and posterior spinal medullary arteries that arise from segmental arteries. An arterial vasocorona(AVC)creates anastomoses between the anterior and posterior spinal arteries. The gray matter has a posterior horn (PH; also called dorsal horn), an anterior horn(AH; also called ventral horn), and an intermediate zone(IZ). The central canal (CenC)and the anterior white commissure(AwCom)are labeled. The major white matter tracts are named The somatotopic arrangement of axons within two tracts is seen on the right side of the schematic: N, neck; A, arm; T, trunk; L, leg: S, sacral region. (Used with permission, from Neuroanatomy An Atlas of Structures, Sections, and Systems, 7th ed, by Duane Haines, New York: Lippincott Williams and Wilkins) this column, individual clusters of motor neurons glionic (also called postsynaptic). All preganglion located in the L3 and L4 spinal cord segments Within cell body located in the periphery is called postgan- innervate just one of the four muscles comprising SNS neurons are found in thoracolumbar spinal the quadriceps femoris. Motor neuron columns cord segments TI through L2. The parasympathetic bit a rostrocaudal somatotopy so that as we de division, as the name suggests, lies on either side in the spinal cord, the motor neuron columns of the SNS. The preganglionic PSNS neurons are vate muscles located successively lower in the body. located in craniosacral positions and are associated There is also a mediolateral somatotopy, such that with cranial nerves Ill, VIl, IX, and X and sacral motor neurons innervating distal limb musculature spinal cord segments S2 through $4 re more lateral than those motor neurons innervat- The majority of sNs preganglionic neurons are ng more proximal limb muscles. Superimposed on found in the intermediolateral cell column while this mediolateral arrangement, motor neurons that approximately 15% are distributed more medially in innervate limb flexors lie dorsal to those that inner- a ladder- like pattern that extends through lamina VIl vate extensor muscles to lamina x dorsal to the central canal this more medial distribution of preganglionic SNS neurons is 2.1.2. THE SPINAL AUTONOMIC NERVOUS SYSTEM of clinical significance because syringomyelia(a cavi A portion of the intermediate region of the spinal tation of the spinal cord that starts adjacent to the gray matter (Rexed laminae VII and X) contains central canal) that extends into the thoracic or upper preganglionic (also called presynaptic) neurons of lumbar spinal segments can destroy these medially the autonomic nervous system(ANS). The ANS has located preganglionic SNS neurons and compromise two divisions: the sympathetic nervous system(SNS) ANS function and the parasympathetic nervous system(PSNS). Preganglionic SNS axons exit the CNs via the Both the SNS and PSNS use a two-neuron chain as ventral roots. After entering the spinal nerve, the the final common pathway to innervate their periph- axon enters the ventral primary ramus and then eral targets: smooth muscle, cardiac muscle, and passes through the white ramus communicans found glands.(Note: The ANS never innervates striated on ventral primary rami TI through L2(Fig. 2) muscle fibers. The ANS neuronal cell body located this point the preganglionic axon can do one of sev- in the CNS is called preganglionic and the neuronal eral things. First, it can synapse on postganglionic

located in the L3 and L4 spinal cord segments. Within this column, individual clusters of motor neurons innervate just one of the four muscles comprising the quadriceps femoris. Motor neuron columns exhi￾bit a rostrocaudal somatotopy so that as we descend in the spinal cord, the motor neuron columns inner￾vate muscles located successively lower in the body. There is also a mediolateral somatotopy, such that motor neurons innervating distal limb musculature are more lateral than those motor neurons innervat￾ing more proximal limb muscles. Superimposed on this mediolateral arrangement, motor neurons that innervate limb flexors lie dorsal to those that inner￾vate extensor muscles. 2.1.2. THE SPINAL AUTONOMIC NERVOUS SYSTEM A portion of the intermediate region of the spinal gray matter (Rexed laminae VII and X) contains preganglionic (also called presynaptic) neurons of the autonomic nervous system (ANS). The ANS has two divisions: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). Both the SNS and PSNS use a two-neuron chain as the final common pathway to innervate their periph￾eral targets: smooth muscle, cardiac muscle, and glands. (Note: The ANS never innervates striated muscle fibers.) The ANS neuronal cell body located in the CNS is called preganglionic and the neuronal cell body located in the periphery is called postgan￾glionic (also called postsynaptic). All preganglionic SNS neurons are found in thoracolumbar spinal cord segments T1 through L2. The parasympathetic division, as the name suggests, lies on either side of the SNS. The preganglionic PSNS neurons are located in craniosacral positions and are associated with cranial nerves III, VII, IX, and X and sacral spinal cord segments S2 through S4. The majority of SNS preganglionic neurons are found in the intermediolateral cell column while approximately 15% are distributed more medially in a ladder-like pattern that extends through lamina VII to lamina X dorsal to the central canal. This more medial distribution of preganglionic SNS neurons is of clinical significance because syringomyelia (a cavi￾tation of the spinal cord that starts adjacent to the central canal) that extends into the thoracic or upper lumbar spinal segments can destroy these medially located preganglionic SNS neurons and compromise ANS function. Preganglionic SNS axons exit the CNS via the ventral roots. After entering the spinal nerve, the axon enters the ventral primary ramus and then passes through the white ramus communicans found on ventral primary rami T1 through L2 (Fig. 2). At this point the preganglionic axon can do one of sev￾eral things. First, it can synapse on postganglionic Fig. 4. The arterial supply (right side) and internal anatomy of the spinal cord. Both the anterior and posterior spinal arteries receive contributions from anterior and posterior spinal medullary arteries that arise from segmental arteries. An arterial vasocorona (AVC) creates anastomoses between the anterior and posterior spinal arteries. The gray matter has a posterior horn (PH; also called dorsal horn), an anterior horn (AH; also called ventral horn), and an intermediate zone (IZ). The central canal (CenC) and the anterior white commissure (AWCom) are labeled. The major white matter tracts are named. The somatotopic arrangement of axons within two tracts is seen on the right side of the schematic: N, neck; A, arm; T, trunk; L, leg; S, sacral region. (Used with permission, from Neuroanatomy An Atlas of Structures, Sections, and Systems, 7th ed., by Duane Haines, New York: Lippincott Williams and Wilkins.) 28 Newton