《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 2/ Anatomy of the Spinal Cord and Brain SNS neurons located in sympathetic chain ganglia sensations are relayed to laminae VIl and X in the TI adjacent to the ventral primary ramus where the through L2 and S2 through S4 spinal segments via the preganglionic SNS axon left the CNS. Then the post- axons of dorsal root ganglion neurons. These visceral ganglionic SNS axon reenters the ventral primary afferent axons contain a wide variety of peptidergic ramus through a gray ramus communicans(found on neurotransmitters that synapse on, and demarcate the all ventral primary rami) to innervate sweat glands, position of, the preganglionic SNS and PSNS neurons arrector pili muscles (i. e, smooth muscle fibers Visceral sensations also enter the CNs via CN iX and attached to the base of hairs ) and vascular smooth X and carry afferent information from the carotid muscle in skeletal muscles and skin. Second. some body and sinus, the thorax, foregut, and midgut preganglionic SNS axons, once inside the sympa- 2.1.3. SPINAL CORD SEXUAL DIMORPHISM thetic chain, can ascend or descend in the sympathetic chain which extends from the base of the skull to the The lumbosacral spinal segments have been found coccyx, before synapsing on postganglionic SNS ne o be sexually dimorphic (i.e, to differ between the rons within the sympathetic chain ganglia whose sexes). Sex differences in afferent input and neuron axons innervate the head and the upper and lower numbers appear to be restricted to the reproductive extremities. The postganglionic SNS axon leaves the organs and their associated musculature. The num- sympathetic chain via a gray ramus communicans bers of motor neurons that innervate the muscles surrounding erectile tissue are more numerous in before entering the spinal nerve for peripheral distri- males than in females, as these muscles are larger bution. Those postganglionic SNS axons destined for the head leave the superior cervical ganglion of the In males. These neurons, in either sex, are found in into the sympathetic neurons that innervate the cremaster muscle. This chain. These preganglionic axons leave the sympa- them in response to changes in temperature. The thetic chain, without synapsing, and form cardiac or female cremaster muscle, much smaller in size, sur splanchnic nerves that synapse on SNS postganglionic rounds the round ligament of the uterus as it passes neurons found in SNS ganglia that lie on the aorta through the inguinal canal and enters the labia or viscera. In humans, the ratio of preganglionic to majora postganglionic SNS neurons is more than 1: 100 indi Although not yet shown for humans, male rats have cating the divergence of the CNs command to the a larger number of lumbar preganglionic SNS neurons periphery. This helps to explain why SNS actions are for the control of reproductive organs. Also in rats widespread in the body, whereas the PSNS control there is a dramatic sexual dimorphism for the number of viscera is precisely controlled because fewer post- of peptidergic afferents in laminae VII and X sur ganglionic PSNS neurons are controlled by a single rounding lumbar SNS and sacral PSNS preganglionic preganglionic PSNS neuron. neurons with males having larger numbers of these Parasympathetic preganglionic neurons reside on peptide-containing afferents than do females. For at the white-gray border of lamina VII in spinal seg- least one of these peptides, galanin, its amount rises ments $2 through $4. Unlike the TI through L2 and falls with the rat' s estrous cycle. Rats also display a spinal segments, there is no discernible intermediolat- sexual dimorphism for spinothalamic neurons in eral cell column Preganglionic PSNS axons leave the lumbar laminae VIl and X. Males have more of these spinal cord via the ventral roots and enter the S2 peptidergic, somatic and visceral sensation relayin through S4 ventral primary rami of the lumbosacral neurons than do females. Not all sexual dimorphisms plexus. Once inside the pelvis, the axons leave the favor males having greater numbers of neurons or ventral primary rami as pelvic splanchnic nerves that afferent inputs. There is a population of laminae VIl synapse on postganglionic PSNS neurons found next and X neurons in the female lumbar spinal cord to or within pelvic organs, erectile tissue, and hindgut that produces dynorphin, a pain-suppressing peptide (i.e, descending and sigmoid colon, rectum, and anal just before and during parturition. Male rats lack this population of dynorphin-producing neurons Just as there are sharp, precisely localized A similar population of lumbar pain-suppressing neu- somatic sensations arising from skin, skeletal muscle, rons may be present in human females, as the ability to and bone, there are dull, diffusely localized visceral withstand pain increases dramatically starting about sensations arising from the internal organs. visceral 1l days before birth

SNS neurons located in sympathetic chain ganglia adjacent to the ventral primary ramus where the preganglionic SNS axon left the CNS. Then the post￾ganglionic SNS axon reenters the ventral primary ramus through a gray ramus communicans (found on all ventral primary rami) to innervate sweat glands, arrector pili muscles (i.e., smooth muscle fibers attached to the base of hairs), and vascular smooth muscle in skeletal muscles and skin. Second, some preganglionic SNS axons, once inside the sympa￾thetic chain, can ascend or descend in the sympathetic chain, which extends from the base of the skull to the coccyx, before synapsing on postganglionic SNS neu￾rons within the sympathetic chain ganglia whose axons innervate the head and the upper and lower extremities. The postganglionic SNS axon leaves the sympathetic chain via a gray ramus communicans before entering the spinal nerve for peripheral distri￾bution. Those postganglionic SNS axons destined for the head leave the superior cervical ganglion of the sympathetic chain via the carotid nerve. Finally, other preganglionic SNS neurons, whose function is the control of viscera, send axons into the sympathetic chain. These preganglionic axons leave the sympa￾thetic chain, without synapsing, and form cardiac or splanchnic nerves that synapse on SNS postganglionic neurons found in SNS ganglia that lie on the aorta or viscera. In humans, the ratio of preganglionic to postganglionic SNS neurons is more than 1:100 indi￾cating the divergence of the CNS command to the periphery. This helps to explain why SNS actions are widespread in the body, whereas the PSNS control of viscera is precisely controlled because fewer post￾ganglionic PSNS neurons are controlled by a single preganglionic PSNS neuron. Parasympathetic preganglionic neurons reside on the white-gray border of lamina VII in spinal seg￾ments S2 through S4. Unlike the T1 through L2 spinal segments, there is no discernible intermediolat￾eral cell column. Preganglionic PSNS axons leave the spinal cord via the ventral roots and enter the S2 through S4 ventral primary rami of the lumbosacral plexus. Once inside the pelvis, the axons leave the ventral primary rami as pelvic splanchnic nerves that synapse on postganglionic PSNS neurons found next to or within pelvic organs, erectile tissue, and hindgut (i.e., descending and sigmoid colon, rectum, and anal canal). Just as there are sharp, precisely localized somatic sensations arising from skin, skeletal muscle, and bone, there are dull, diffusely localized visceral sensations arising from the internal organs. Visceral sensations are relayed to laminae VII and X in the T1 through L2 and S2 through S4 spinal segments via the axons of dorsal root ganglion neurons. These visceral afferent axons contain a wide variety of peptidergic neurotransmitters that synapse on, and demarcate the position of, the preganglionic SNS and PSNS neurons. Visceral sensations also enter the CNS via CN IX and X and carry afferent information from the carotid body and sinus, the thorax, foregut, and midgut. 2.1.3. SPINAL CORD SEXUAL DIMORPHISM The lumbosacral spinal segments have been found to be sexually dimorphic (i.e., to differ between the sexes). Sex differences in afferent input and neuron numbers appear to be restricted to the reproductive organs and their associated musculature. The num￾bers of motor neurons that innervate the muscles surrounding erectile tissue are more numerous in males than in females, as these muscles are larger in males. These neurons, in either sex, are found in the S2 through S4 spinal segments in Onuf’s nucleus. Males also have greater numbers of L1 and L2 motor neurons that innervate the cremaster muscle. This muscle surrounds the testes and raises and lowers them in response to changes in temperature. The female cremaster muscle, much smaller in size, sur￾rounds the round ligament of the uterus as it passes through the inguinal canal and enters the labia majora. Although not yet shown for humans, male rats have a larger number of lumbar preganglionic SNS neurons for the control of reproductive organs. Also in rats, there is a dramatic sexual dimorphism for the number of peptidergic afferents in laminae VII and X sur￾rounding lumbar SNS and sacral PSNS preganglionic neurons with males having larger numbers of these peptide-containing afferents than do females. For at least one of these peptides, galanin, its amount rises and falls with the rat’s estrous cycle. Rats also display a sexual dimorphism for spinothalamic neurons in lumbar laminae VII and X. Males have more of these peptidergic, somatic and visceral sensation relaying neurons than do females. Not all sexual dimorphisms favor males having greater numbers of neurons or afferent inputs. There is a population of laminae VII and X neurons in the female lumbar spinal cord that produces dynorphin, a pain-suppressing peptide, just before and during parturition. Male rats lack this population of dynorphin-producing neurons. A similar population of lumbar pain-suppressing neu￾rons may be present in human females, as the ability to withstand pain increases dramatically starting about 11 days before birth. Chapter 2 / Anatomy of the Spinal Cord and Brain 29

Newton In all the instances mentioned in this section, the artery throughout its length receives anastomotic stablishment of sex differences are under the control branches from segmental vessels that enter interver of the hormonal milieu present during the perinatal tebral foramina with the ventral and dorsal root period. The sex differences in afferent input and spi- fibers. In cervical levels, segmental arteries arise nothalamic neuron number may have future clinical from sources: vertebral arteries, the ascending cervi relevance, as many of the peptidergic neurotransmit- cal branch of the inferior thyroid artery, and the deep ters contained in these structures are involved in sup- cervical branch of the costocervical trunk. At thor- pressing nociception (i.e, pain). Because there are acic levels, posterior intercostal arteries arising from known sex differences in the human response to nox- the descending aorta supply segmental arteries to the ious stimuli, pain-suppressing pharmaceuticals can spinal cord. At lumbar levels, lumbar arteries origi be developed that take into account the inherent sex nating from the abdominal aorta supply blood, and differences in spinal neuropeptides that are under the sacral arteries from internal iliac arteries supply the control of gonadal hormones whose titers vary lowest levels of the spinal cord and cauda equina hroughout life. Each segmental artery enters an intervertebral fora- men to give rise to small posterior and anterior radi- 2.2. Blood Supply of the Spinal Cord cular arteries that follow and supply the dorsal and ventral roots(Fig. 4). Periodically, the segmental The spinal cord receives its blood supply from arteries give rise to anterior medullary arteries and three longitudinal arteries, which are supplemented from segmental vessels along the length of the spinal posterior medullary arteries, which anastomose with the anterior and posterior spinal arteries. Typically, cord(Fig 3 and Fig 4). Extensive anastomoses occur there are three medullary arteries supplying cervical between the longitudinal arteries and the segmental arteries. The anterior spinal artery is the principal spinal cord levels, two supplying thoracic levels, and artery of the anterior two-thirds of each spinal cord two for the lumbar spinal cord segment. As such, it gives off numerous sulcal The great anterior medullary artery (of Adamkie branches(also called central branches)that course wIcz), which arises from a posterior intercostal artery within the anterior median fissure to enter the spinal cord. The smaller, paired posterior spinal arteries are noticeable because of its large size. This artery responsible for the remaining posterior third of the responsible for supplementing the blood supply to the lumbar enlargement. This major contributor to spinal cord, mainly the dorsal columns. The anterior the blood supply of the anterior spinal artery is of spinal artery originates as a common trunk from the clinical importance when the abdominal aorta union of the paired vertebral arteries as they pass clamped for surgery. During surgery, the possibility arises that lumbosacral spinal segments will become The posterior spinal arteries arise either from the infarcted if the anterior spinal artery does not receive vertebral arteries or the posterior inferior cerebellar arteries, which are typically branches of the vertebral enough additional blood from other more caudal segmental arteries to compensate for the temporar arteries. The anterior spinal artery runs the entire loss of blood supply to lumbosacral segments from length of the anterior median fissure (Fig. 3), the great anterior medullary artery Posterior medul although it reaches its largest diameter in the cervical and upper thoracic levels and then begins to diminish lary arteries are smaller and more numerous than are the anterior medullary arteries, with an average of in size as it descends further down the spinal cord. three to five for each spinal cord region. These The posterior spinal arteries run the entire length of the spinal cord and are located in the posterolateral arteries supplement the blood supply to the dorsal sulci. The anterior and posterior spinal arteries ana- third of the spinal cord stomose with each other along the length of the spinal cord via the arterial vasocorona(Fig. 4). The vascular 2.3. Venous Drainage of the Spinal Cord supply to the spinal cord is most attenuated between The veins that drain the spinal cord begin as capil the T3 and T9 levels, and these spinal cord segments laries within it. These coalesce into intramedullary are most vulnerable to ischemia veins, which drain into the more superficial intradural Like any long tubular structure in the body, the (pial) veins located within the pia mater. The venous spinal cord receives multiple arterial tributaries that pattern is variable, but most consistently you will find augment the blood supply provided by the three the following: the anterior median vein, located in the aforementioned spinal arteries. The anterior spinal anterior median fissure, the posterior median vein

In all the instances mentioned in this section, the establishment of sex differences are under the control of the hormonal milieu present during the perinatal period. The sex differences in afferent input and spi￾nothalamic neuron number may have future clinical relevance, as many of the peptidergic neurotransmit￾ters contained in these structures are involved in sup￾pressing nociception (i.e., pain). Because there are known sex differences in the human response to nox￾ious stimuli, pain-suppressing pharmaceuticals can be developed that take into account the inherent sex differences in spinal neuropeptides that are under the control of gonadal hormones whose titers vary throughout life. 2.2. Blood Supply of the Spinal Cord The spinal cord receives its blood supply from three longitudinal arteries, which are supplemented from segmental vessels along the length of the spinal cord (Fig. 3 and Fig. 4). Extensive anastomoses occur between the longitudinal arteries and the segmental arteries. The anterior spinal artery is the principal artery of the anterior two-thirds of each spinal cord segment. As such, it gives off numerous sulcal branches (also called central branches) that course within the anterior median fissure to enter the spinal cord. The smaller, paired posterior spinal arteries are responsible for the remaining posterior third of the spinal cord, mainly the dorsal columns. The anterior spinal artery originates as a common trunk from the union of the paired vertebral arteries as they pass through the foramen magnum into the cranial cavity. The posterior spinal arteries arise either from the vertebral arteries or the posterior inferior cerebellar arteries, which are typically branches of the vertebral arteries. The anterior spinal artery runs the entire length of the anterior median fissure (Fig. 3), although it reaches its largest diameter in the cervical and upper thoracic levels and then begins to diminish in size as it descends further down the spinal cord. The posterior spinal arteries run the entire length of the spinal cord and are located in the posterolateral sulci. The anterior and posterior spinal arteries ana￾stomose with each other along the length of the spinal cord via the arterial vasocorona (Fig. 4). The vascular supply to the spinal cord is most attenuated between the T3 and T9 levels, and these spinal cord segments are most vulnerable to ischemia. Like any long tubular structure in the body, the spinal cord receives multiple arterial tributaries that augment the blood supply provided by the three aforementioned spinal arteries. The anterior spinal artery throughout its length receives anastomotic branches from segmental vessels that enter interver￾tebral foramina with the ventral and dorsal root fibers. In cervical levels, segmental arteries arise from sources: vertebral arteries, the ascending cervi￾cal branch of the inferior thyroid artery, and the deep cervical branch of the costocervical trunk. At thor￾acic levels, posterior intercostal arteries arising from the descending aorta supply segmental arteries to the spinal cord. At lumbar levels, lumbar arteries origi￾nating from the abdominal aorta supply blood, and sacral arteries from internal iliac arteries supply the lowest levels of the spinal cord and cauda equina. Each segmental artery enters an intervertebral fora￾men to give rise to small posterior and anterior radi￾cular arteries that follow and supply the dorsal and ventral roots (Fig. 4). Periodically, the segmental arteries give rise to anterior medullary arteries and posterior medullary arteries, which anastomose with the anterior and posterior spinal arteries. Typically, there are three medullary arteries supplying cervical spinal cord levels, two supplying thoracic levels, and two for the lumbar spinal cord. The great anterior medullary artery (of Adamkie￾wicz), which arises from a posterior intercostal artery located around the 8th to 11th thoracic level, is noticeable because of its large size. This artery is responsible for supplementing the blood supply to the lumbar enlargement. This major contributor to the blood supply of the anterior spinal artery is of clinical importance when the abdominal aorta is clamped for surgery. During surgery, the possibility arises that lumbosacral spinal segments will become infarcted if the anterior spinal artery does not receive enough additional blood from other more caudal segmental arteries to compensate for the temporary loss of blood supply to lumbosacral segments from the great anterior medullary artery. Posterior medul￾lary arteries are smaller and more numerous than are the anterior medullary arteries, with an average of three to five for each spinal cord region. These arteries supplement the blood supply to the dorsal third of the spinal cord. 2.3. Venous Drainage of the Spinal Cord The veins that drain the spinal cord begin as capil￾laries within it. These coalesce into intramedullary veins, which drain into the more superficial intradural (pial) veins located within the pia mater. The venous pattern is variable, but most consistently you will find the following: the anterior median vein, located in the anterior median fissure, the posterior median vein 30 Newton

Chapter 2/ Anatomy of the Spinal Cord and Brain located in the posterior median sulcus, anterolateral continuous with the epineurium of the spinal nerve. veins located near the exit of ventral roots, and poster- At the caudal end of the vertebral canal, the dura ior lateral veins located in or near the entrance of the mater ends as the blind-ended dural sac. the ara- dorsal roots. Each of these valveless veins freely com- chnoid membrane(spider-like), the second meningeal municates with its neighbors forming large anasto- layer, is immediately deep to the dura mater and motic channels along the surface of the spinal cord. consists of a fine network of connective tissue fibers At the base of the skull, the intradural veins unite to that extend to the surface of the spinal cord. It fol- form several trunks that drain into the posterior lows the contours of the dura mater and is present in inferior cerebellar veins and vertebral veins of the the dural sleeves and the dural sac. The pia mater cranial cavity. The intradural veins also communicate ("delicate mother") is the deepest and thinnest with the internal vertebral venous plexus that is con- meningeal layer and is typically adherent to the sur tained within the epidural fat found in the epidural face of the spinal cord, faithfully following its con space of the vertebral canal along its entire length. tour. It is a vascular layer containing arteries The internal vertebral plexus can be divided into an supplying the spinal cord and veins that drain the anterior internal vertebral venous plexus, located spinal cord. The denticulate ligaments are irregularly between the vertebral body and the dural sac, and a found, sawtooth-like lateral extensions of the pia posterior internal vertebral venous plexus between the mater that extend from the side of the spinal cord vertebral arch and the dural sac. The internal verteb- between the dorsal and ventral roots to anchor to the ral venous plexus drains more superficially into the overlying dura mater/arachnoid membrane(Fig 3) external vertebral venous plexus surrounding the ver- Tough and fibrous in nature, there are 20 to 22 pairs tebral column. The external vertebral plexus consists of denticulate ligaments. Extending caudally from of an anterior external vertebral plexus around thethe conus medullaris. in the midst of the cauda vertebral bodies and the posterior external vertebral equina, the pia mater leaves the spinal cord, sur plexus lying on the surface of the vertebral arch. Both rounds a few vestigial neurons. and forms a fine the anterior and posterior external vertebral plexuses filament the filum terminale. Within the dural sac it anastomose with each other and drain into the sys- is known as the filum terminale interna. Once it passes temic segmental veins, including the deep cervical through the dura sac and becomes enveloped in dura veins, intercostal veins, lumbar veins, and lateral mater, it becomes the filum terminale externa(also sacral veins called coccygeal ligament) that exits the sacral hiatus The internal vertebral plexus communicates freely of the vertebral canal and anchors to the dorsum of the with the basilar plexus of the dural venous sinuses, coccyx whereas the external vertebral plexus communicates freely with the pelvic plexus of veins. Clinically, these ciated with the meninges of the spinal cord. (A poten infectious material or metastases to travel up and or death. ) The epidural space is superficial to the dura down the length of the vertebral column from the mater and contains significant amounts of fat that pelvis into the cranium. For example, prostate cancer spreading via this route will lodge and grow within protect the spinal cord. It also contains the internal the marrow of the vertebral bodies and can eventually vertebral plexus of veins, which drains blood from the spread to the bones of the cranium spinal cord to the more superficial external vertebral venous plexus. The subdural space is a potential space that lies deep to the dura mater but superficial to the 2.4. Meninges of the Spinal Cord closely adherent arachnoid membrane. In life,cere The spinal cord is covered by three connective brospinal fluid(CSF) pressure keeps the arachnoid tissue layers known as the meninges(Fig. 1). The pressed against the inner surface of the dural sac, thus most superficial layer is the dura mater ("tough closing this space. After death, the loss of CSF pressure mother"). The spinal dura mater begins at the fora- causes the arachnoid to collapse upon the surface of the men magnum and extends caudally within the ver- spinal cord revealing the subdural space in the cadaver tebral canal as a sac to the second sacral vertebra. At The subarachnoid space separates the arachnoid mem each intervertebral foramen, the dura mater extends brane from the deeper pia mater. Cerebrospinal fluid. as a dural sleeve to cover the dorsal roots, ventral produced by the choroid plexus of the ventricular sys- of the intervertebral foramen where it becomes allows the spinal cord to float within this space e and roots, and spinal ganglia. It ends at the external edge tem, is contained within the subarachnoid spac

located in the posterior median sulcus, anterolateral veinslocated near the exit of ventral roots, and poster￾ior lateral veins located in or near the entrance of the dorsal roots. Each of these valveless veins freely com￾municates with its neighbors forming large anasto￾motic channels along the surface of the spinal cord. At the base of the skull, the intradural veins unite to form several trunks that drain into the posterior inferior cerebellar veins and vertebral veins of the cranial cavity. The intradural veins also communicate with the internal vertebral venous plexus that is con￾tained within the epidural fat found in the epidural space of the vertebral canal along its entire length. The internal vertebral plexus can be divided into an anterior internal vertebral venous plexus, located between the vertebral body and the dural sac, and a posterior internal vertebral venous plexus between the vertebral arch and the dural sac. The internal verteb￾ral venous plexus drains more superficially into the external vertebral venous plexus surrounding the ver￾tebral column. The external vertebral plexus consists of an anterior external vertebral plexus around the vertebral bodies and the posterior external vertebral plexus lying on the surface of the vertebral arch. Both the anterior and posterior external vertebral plexuses anastomose with each other and drain into the sys￾temic segmental veins, including the deep cervical veins, intercostal veins, lumbar veins, and lateral sacral veins. The internal vertebral plexus communicates freely with the basilar plexus of the dural venous sinuses, whereas the external vertebral plexus communicates freely with the pelvic plexus of veins. Clinically, these extensive valveless anastomoses allow for passage of infectious material or metastases to travel up and down the length of the vertebral column from the pelvis into the cranium. For example, prostate cancer spreading via this route will lodge and grow within the marrow of the vertebral bodies and can eventually spread to the bones of the cranium. 2.4. Meninges of the Spinal Cord The spinal cord is covered by three connective tissue layers known as the meninges (Fig. 1). The most superficial layer is the dura mater (‘‘tough mother’’). The spinal dura mater begins at the fora￾men magnum and extends caudally within the ver￾tebral canal as a sac to the second sacral vertebra. At each intervertebral foramen, the dura mater extends as a dural sleeve to cover the dorsal roots, ventral roots, and spinal ganglia. It ends at the external edge of the intervertebral foramen where it becomes continuous with the epineurium of the spinal nerve. At the caudal end of the vertebral canal, the dura mater ends as the blind-ended dural sac. The ara￾chnoid membrane (spider-like), the second meningeal layer, is immediately deep to the dura mater and consists of a fine network of connective tissue fibers that extend to the surface of the spinal cord. It fol￾lows the contours of the dura mater and is present in the dural sleeves and the dural sac. The pia mater (‘‘delicate mother’’) is the deepest and thinnest meningeal layer and is typically adherent to the sur￾face of the spinal cord, faithfully following its con￾tour. It is a vascular layer containing arteries supplying the spinal cord and veins that drain the spinal cord. The denticulate ligaments are irregularly found, sawtooth-like lateral extensions of the pia mater that extend from the side of the spinal cord between the dorsal and ventral roots to anchor to the overlying dura mater/arachnoid membrane (Fig. 3). Tough and fibrous in nature, there are 20 to 22 pairs of denticulate ligaments. Extending caudally from the conus medullaris, in the midst of the cauda equina, the pia mater leaves the spinal cord, sur￾rounds a few vestigial neurons, and forms a fine filament, the filum terminale. Within the dural sac it is known as the filum terminale interna. Once it passes through the dura sac and becomes enveloped in dura mater, it becomes the filum terminale externa (also called coccygeal ligament) that exits the sacral hiatus of the vertebral canal and anchors to the dorsum of the coccyx. Several spaces, either real or potential, are asso￾ciated with the meninges of the spinal cord. (A poten￾tial space is only present due to pathologic conditions or death.) The epidural space is superficial to the dura mater and contains significant amounts of fat that protect the spinal cord. It also contains the internal vertebral plexus of veins, which drains blood from the spinal cord to the more superficial external vertebral venous plexus. The subdural space is a potential space that lies deep to the dura mater but superficial to the closely adherent arachnoid membrane. In life, cere￾brospinal fluid (CSF) pressure keeps the arachnoid pressed against the inner surface of the dural sac, thus closing this space. After death, the loss of CSF pressure causes the arachnoid to collapse upon the surface of the spinal cord revealing the subdural space in the cadaver. The subarachnoid space separates the arachnoid mem￾brane from the deeper pia mater. Cerebrospinal fluid, produced by the choroid plexus of the ventricular sys￾tem, is contained within the subarachnoid space and allows the spinal cord to float within this space. Chapter 2 / Anatomy of the Spinal Cord and Brain 31

Newton The epidural and subarachnoid spaces are clini- pass from the olfactory epithelium of the nasal cavity cally useful. A spinal tap(lumbar puncture) is used into the paired olfactory bulbs. The olfactory bulbs to remove CSF from the lumbar cistern of the dural are directly connected to the temporal cortex by the sac. In this procedure, a needle is inserted through posteriorly running olfactory tracts skin, epaxial muscles, and the elastic ligamentum fla- Posterior to the lesser wing of the sphenoid is the yum located between adjacent vertebral laminae. The middle cranial fossa. It extends caudally to the pet insertion of the needle is usually between the L3 and rous ridge of the temporal bone. The temporal cor L4 laminae. Once through ligamentum flavum, the tex's inferior surface rests on the tegmen tympani of needle passes through the epidural space, dura mater, the temporal bones, and the temporal poles are ind the closely adherent arachnoid membrane and tucked into the concavities of the greater wing of the into the subarachnoid space containing CSF. The sphenoid bone located beneath the overhanging les insertion of the needle into the lumbar cistern below ser wing. In the midline, the depression of the sella the L3 vertebral body ensures that the adult spinal turcica of the sphenoid bone houses the pituitary and cord, which ends at the L1-2 intervertebral disk, will infundibulum of the hypothalamus. Cranial nerves II not be penetrated and damaged by the needle. The to VI exit the skull through the superior orbital fis dorsal and ventral spinal roots of the cauda equina sure and several foramina in the middle cranial fossa are easily pushed aside by the needle and are not(Fig. 6 and Fig. 7) damaged. An epidural block is performed when The posterior cranial fossa is located posterior to anesthesia of the lower body is needed. It is most the petrous ridge of the temporal bone and is bounded frequently performed to relieve the pain of childbirth. by the mastoid process of the temporal bone laterally The anesthetic is delivered into the epidural space with the occipital bone forming the majority of the that is traversed by the dorsal and ventral roots of rest of boundaries. Its floor contains the foramen spinal nerves. The anesthetic agent rapidly diffuses magnum, which allows the seamless continuation of through the epidural fat and into the thinly myeli- the brain stem with the spinal cord. The brain stem nated pain fibers thereby temporarily inactivating and the cerebellum are contained in the posterior cranial fossa. Its roof is formed by the tentorium 3. BRAIN The brain develops from several regions of the ros tral neural tube that bulge laterally as neurogenesis Anterior proceeds. There is a forebrain(prosencephalon), giving Cranial rise to the cerebral cortex, hypothalamus, and thala- Fossa mus, the midbrain(mesencephalon), which remains as the adult midbrain, and the hindbrain(rhombencepha lon), giving rise to the pons, cerebellum, and medulla oblongata. The spinal cord develops from the remain- der of the neural tube. The medulla oblongata, pons, Cranial and midbrain are collectively described as the brain Fossa stem The brain is located and protected in the cranial cavity. The cranial floor is divided into three horizon tal shelves or fossae. from rostral to caudal, which are Cranial successively lower(Fig. 5 and Fig. 6). The anterior Fossa cranial fossa is composed of the crista galli and cribri form plate of the ethmoid bone, the lesser wing of the sphenoid bone, and the orbital part of the frontal bones. The orbital surface. so named because it forms the roof of the orbit, supports the orbital sur- face of the frontal cortex. The ethmoid bone on either Fig. 5. Floor of the cranial cavity demonstrating the bound side of the midline crista galli is perforated (cribri- aries of the anterior cr form plate)to allow the olfactory nerves(CN I) to posterior cranial fossa nial fossa, middle cranial fossa,and

The epidural and subarachnoid spaces are clini￾cally useful. A spinal tap (lumbar puncture) is used to remove CSF from the lumbar cistern of the dural sac. In this procedure, a needle is inserted through skin, epaxial muscles, and the elastic ligamentum fla￾vum located between adjacent vertebral laminae. The insertion of the needle is usually between the L3 and L4 laminae. Once through ligamentum flavum, the needle passes through the epidural space, dura mater, and the closely adherent arachnoid membrane, and into the subarachnoid space containing CSF. The insertion of the needle into the lumbar cistern below the L3 vertebral body ensures that the adult spinal cord, which ends at the L1-2 intervertebral disk, will not be penetrated and damaged by the needle. The dorsal and ventral spinal roots of the cauda equina are easily pushed aside by the needle and are not damaged. An epidural block is performed when anesthesia of the lower body is needed. It is most frequently performed to relieve the pain of childbirth. The anesthetic is delivered into the epidural space that is traversed by the dorsal and ventral roots of spinal nerves. The anesthetic agent rapidly diffuses through the epidural fat and into the thinly myeli￾nated pain fibers thereby temporarily inactivating them. 3. BRAIN The brain develops from several regions of the ros￾tral neural tube that bulge laterally as neurogenesis proceeds. There is a forebrain (prosencephalon), giving rise to the cerebral cortex, hypothalamus, and thala￾mus, the midbrain (mesencephalon), which remains as the adult midbrain, and the hindbrain (rhombencepha￾lon), giving rise to the pons, cerebellum, and medulla oblongata. The spinal cord develops from the remain￾der of the neural tube. The medulla oblongata, pons, and midbrain are collectively described as the brain stem. The brain is located and protected in the cranial cavity. The cranial floor is divided into three horizon￾tal shelves or fossae, from rostral to caudal, which are successively lower (Fig. 5 and Fig. 6). The anterior cranial fossa is composed of the crista galli and cribri￾form plate of the ethmoid bone, the lesser wing of the sphenoid bone, and the orbital part of the frontal bones. The orbital surface, so named because it forms the roof of the orbit, supports the orbital sur￾face of the frontal cortex. The ethmoid bone on either side of the midline crista galli is perforated (cribri￾form plate) to allow the olfactory nerves (CN I) to pass from the olfactory epithelium of the nasal cavity into the paired olfactory bulbs. The olfactory bulbs are directly connected to the temporal cortex by the posteriorly running olfactory tracts. Posterior to the lesser wing of the sphenoid is the middle cranial fossa. It extends caudally to the pet￾rous ridge of the temporal bone. The temporal cor￾tex’s inferior surface rests on the tegmen tympani of the temporal bones, and the temporal poles are tucked into the concavities of the greater wing of the sphenoid bone located beneath the overhanging les￾ser wing. In the midline, the depression of the sella turcica of the sphenoid bone houses the pituitary and infundibulum of the hypothalamus. Cranial nerves II to VI exit the skull through the superior orbital fis￾sure and several foraminae in the middle cranial fossa (Fig. 6 and Fig. 7). The posterior cranial fossa is located posterior to the petrous ridge of the temporal bone and is bounded by the mastoid process of the temporal bone laterally with the occipital bone forming the majority of the rest of boundaries. Its floor contains the foramen magnum, which allows the seamless continuation of the brain stem with the spinal cord. The brain stem and the cerebellum are contained in the posterior cranial fossa. Its roof is formed by the tentorium Fig. 5. Floor of the cranial cavity demonstrating the bound￾aries of the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa. 32 Newton

Chapter 2/ Anatomy of the Spinal Cord and Brain 33 CP ST FO IAM FM HC Fig. 7. Anterior view of the skull showing the right orbit. The superior orbital fissure(arrow) provides a conduit for the nuI abducens, and the ophthalmic division of the trigeminal. The nferior orbital fissure is seen in the floor of the orbit, and par of the nasal cavity is visible on the right Fig. 6. The interior of the skull base with foraminae labeled that provide entrance or exit of cranial nerves. The olfactory nerves enter the skull through the cribriform plate(CP). The the most caudal portion of the brain stem and is ptic nerve enters via the optic canal (OC). The oculomotor, continuous with the spinal cord at the foramen mag trochlear, ophthalmic division of the trigeminal, and abdu- num(Fig 8, Fig 9, Fig. 10, and Fig. 11). Its anterior cens nerves exit the skull through the superior orbital fissure surface contains two prominent ridges along its (white arrow) that is hidden by the overhanging lesser of the sphenoid bone. The maxillary and mandibular divisions length. The most medial pair of ridges are the pyra of the trigeminal nerve exit the skull through the foramen mids formed by the corticospinal tracts, and the more rotundum(FR) and the foramen ovale(FO), respectively. lateral ridges are the olives, formed by the inferior The facial and vestibulocochlear nerves exit through the inter- olivary nuclei. Each pyramid is separated from the nal acoustic meatus(IAM). The vagus and glossopharyngeal other by an anterior median fissure and from the more nerves exit the skull though the jugular foramen (F). The lateral olive by the anterior lateral sulcus. The anterior hypoglossal nerve exits via the hypoglossal canal (HC). The lateral sulcus contains the hypoglossal nerve(CN (FM)and then turns and exits through the jugular foramen. XIm) fibers. The olive is bounded laterally by the The pituitary sits within the sella turcica (ST)of the sphenoid posterior lateral sulcus, which contains the fibers of the glossopharyngeal nerve(CN IX) and the vagus nerve(CN X). The posterior surface of the medulla contains the tubercle of the nucleus gracilis medially cerebelli of the dura mater. Cranial nerves VII to XII and the tubercle of the nucleus cuneatus laterally ( Fig 9). It opens into a diamond-shaped open region exit the skull through several foramina in the poster- known as the rhomboid fossa, which forms the floor of ior cranial fossa(Fig. 6) the fourth ventricle. The medulla oblongata is con- tinuous rostrally with the pons. At the pons-medulla 3. 1. Medulla oblongata junction, the abducens nerve(CN V)arises medially The medulla oblongata, formed from the myelen- and the facial(CN VID) and vestibulocochlear(CN cephalon and located in the posterior cranial fossa, is vIlI) nerves originate further laterally

cerebelli of the dura mater. Cranial nerves VII to XII exit the skull through several foraminae in the poster￾ior cranial fossa (Fig. 6). 3.1. Medulla Oblongata The medulla oblongata, formed from the myelen￾cephalon and located in the posterior cranial fossa, is the most caudal portion of the brain stem and is continuous with the spinal cord at the foramen mag￾num (Fig. 8, Fig. 9, Fig. 10, and Fig. 11). Its anterior surface contains two prominent ridges along its length. The most medial pair of ridges are the pyra￾mids formed by the corticospinal tracts, and the more lateral ridges are the olives, formed by the inferior olivary nuclei. Each pyramid is separated from the other by an anterior median fissure and from the more lateral olive by the anterior lateral sulcus. The anterior lateral sulcus contains the hypoglossal nerve (CN XII) fibers. The olive is bounded laterally by the posterior lateral sulcus, which contains the fibers of the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X). The posterior surface of the medulla contains the tubercle of the nucleus gracilis medially and the tubercle of the nucleus cuneatus laterally (Fig. 9). It opens into a diamond-shaped open region known as the rhomboid fossa, which forms the floor of the fourth ventricle. The medulla oblongata is con￾tinuous rostrally with the pons. At the pons-medulla junction, the abducens nerve (CN VI) arises medially, and the facial (CN VII) and vestibulocochlear (CN VIII) nerves originate further laterally. Fig. 6. The interior of the skull base with foraminae labeled that provide entrance or exit of cranial nerves. The olfactory nerves enter the skull through the cribriform plate (CP). The optic nerve enters via the optic canal (OC). The oculomotor, trochlear, ophthalmic division of the trigeminal, and abdu￾cens nerves exit the skull through the superior orbital fissure (white arrow) that is hidden by the overhanging lesser wing of the sphenoid bone. The maxillary and mandibular divisions of the trigeminal nerve exit the skull through the foramen rotundum (FR) and the foramen ovale (FO), respectively. The facial and vestibulocochlear nerves exit through the inter￾nal acoustic meatus (IAM). The vagus and glossopharyngeal nerves exit the skull though the jugular foramen (JF). The hypoglossal nerve exits via the hypoglossal canal (HC). The accessory nerve enters the skull through the foramen magnum (FM) and then turns and exits through the jugular foramen. The pituitary sits within the sella turcica (ST) of the sphenoid bone. Fig. 7. Anterior view of the skull showing the right orbit. The superior orbital fissure (arrow) provides a conduit for the passage of a number of cranial nerves: oculomotor, trochlear, abducens, and the ophthalmic division of the trigeminal. The inferior orbital fissure is seen in the floor of the orbit, and part of the nasal cavity is visible on the right. Chapter 2 / Anatomy of the Spinal Cord and Brain 33