structure less than 2 cm in diameter composed of gray and white matter that is located in the upper two-thirds of the vertebral canal and is surrounded by the bony vertebral column. It is the central processing and relay station (1) receiving input via peripheral nerves from the body and via descending tracts from the brain and (2) projecting output via the peripheral nerves to the body and via the ascending tracts to the brain.
ANATOMIC ORGANIZATION The spinal cord extends from the foramen magnum at the base of the skull to a cone-shaped termination, the conus medullaris, usu-ally located at the caudal level of the first lumbar centrum. The nonneural filum terminale continues caudally as a filament from the conus medullaris to its attachment in the coccyx (see Fig. 5.1).
Meningeal Coverings
The spinal cord is surrounded by three men-inges, which are continuous with those encap-sulating the brain (Chap. 5). All three meninges invest the nerve roots emerging from the spinal cord and are continuous with the connective tissue sheath of the peripheral nerves.
The vascular pia mater is intimately attached to the spinal cord and its roots. The dura and nonvascular arachnoid extends
cau-dally to the sacral-5 vertebral level, where they merge with the filum terminale to form the coccygeal ligament or filum of the dura. The subarachnoid space, which is filled with cere-brospinal fluid (CSF) and blood vessels sur-rounds the spinal cord and is called the spinal or lumbar cistern between the conus medullaris and the sacral-2 level (see Fig. 5.1). The roots of the lumbar and sacral spinal nerves “float”
within the CSF of this cistern. To avoid injury to the spinal cord during removal of CSF, spinal taps into the cistern are made in the lower lumbar region.
The dura mater and the capillary-thin sub-dural space (not containing CSF) surround the arachnoid and merge with the filum terminale at the sacral-2 level. The spinal cord is sus-pended by a series of 20–22 paired lateral septa of pia mater surrounding the cord extending laterally as flanges through the arachnoid to the dura mater, called the denticulate ligaments.
The pointed attachment to the dura mater is between two successive spinal nerves. The lig-aments are oriented rostrocaudally in a frontal plane between the dorsal and ventral roots.
Between the dura mater (equivalent to inner dura mater surrounding the brain) and the periosteum of the vertebral column (equivalent to outer dura mater surrounding the brain) is the epidural space containing venous plexuses and fat. The epidural space caudal to the sacral-2 level is the site for injection of anesthetics used to modify sensory input (e.g., saddle block for painless childbirth).
The Spinal Cord
Anatomic Organization
Spinal Roots and Peripheral Nerves Laminae of the Spinal Cord
Pathways and Tracts
129
Blood Supply
The variably sized spinal arteries are branches of the vertebral, cervical, thoracic, and lumbar arteries. Each artery passes through an intervertebral foramen and divides into an anterior and a posterior spinal root (radicular arteries), which form an anastomotic plexus on the surface of the spinal cord (see Fig. 12.2).
Venous drainage is via a venous plexus, and veins roughly parallel the arterial tree. The large spinovertebral venous plexus is continu-ous rostrally with that surrounding the brain.
Venous pressure within these veins and CSF pressure can become elevated when the out-flow of venous blood into the systemic circula-tion is impeded, as happens when the pressure in the thoracic and abdominal cavities increases while one is lifting a heavy object or coughing.
SPINAL ROOTS AND PERIPHERAL NERVES
The spinal cord receives input and projects output via nerve fibers in the spinal rootlets and roots, spinal nerves, and their branches (see Figs. 7.1 and 7.2). Nerve fibers emerge from the spinal cord in an uninterrupted series of dorsal and ventral rootlets that join to form 31 pairs of dorsal and ventral roots. In the vicinity of each intervertebral foramen, a dorsal root and a ventral root join to form a spinal nerve, which supplies the innervation of a segment of the body. In all, there are 8 pairs of cervical (C), 12 pairs of thoracic (T), 5 pairs of lumbar (L), 5 pairs of sacral (S), and 1 pair of coccygeal (Co) roots and nerves (see Fig. 7.1 and Table 7.1). Cervical-1 and coccygeal-1 usually have only ventral roots.
Spinal cord segments and their nerves are named after their corresponding vertebrae. Cer-vical nerves C1 to C7 are numbered for the ver-tebra just caudal to the foramen through which they pass. In humans, because there are only seven cervical vertebrae, C8 and all other spinal nerves are numbered for the vertebra just
rostral. Because the spinal cord is much shorter than the bony vertebral column, the lumbar and sacral nerves develop long roots, which extend as the cauda equina (horse’s tail) within the spinal cistern (see Figs. 5.1 and 7.1).
The cord is enlarged in those segments that innervate the upper extremities (called the cer-vical [brachial] enlargement, which extends from C5 to T1 spinal levels) and in those seg-ments that innervate the lower extremities (called the lumbosacral enlargement, which extends from L1 to S2).
Functional Components of Spinal Nerves Each spinal nerve contains nerve fibers clas-sified into one of four functional components, namely (1) general somatic afferent, (2) gen-eral viscgen-eral afferent, (3) gengen-eral somatic efferent, and (4) general visceral efferent. Com-ponents that are distributed throughout the body are designated general, whereas those that innervate the body wall and extremities are somatic and those that innervate the viscera are visceral. Furthermore, sensory fibers are affer-ent and motor fibers are efferaffer-ent.
Dorsal Roots
The dorsal (sensory) roots consist of affer-ent fibers that convey input via spinal nerves from the sensory receptors in the body to the spinal cord (see Fig. 7.2). The cell bodies of these neurons are located in the dorsal root ganglia within the intervertebral foramina.
130 The Human Nervous System
Table 7.1 Interspace
Spinal between Process vertebral
of vertebra bodiesa Spinal Cord Segment
C1 C1–2
C6 C6 T1
T10 T10 L1
T12 T12 S1
T12–L1All sacral and coccygeal levels
S2 or S3Caudal terminations of subarachnoid space CoccyxTermination of filum terminale
aNamed from centrum of vertebra above interspace.
Figure 7.1: Topographic relations of the spinal cord segments, spinous processes, and bodies of the vertebrae, intervertebral foramina, and spinal nerves. Refer to Table 7.1. Spinal nerves from cord segments C1–C7 emerge from the spinal canal through the intervertebral foramen immedi-ately above their corresponding vertebrae. Because there are only seven cervical vertebrae, fibers from C8 and all other segments emerge below the corresponding vertebrae.
Despite the term, these ganglia contain no synapses. The fibers of the dorsal root of each spinal nerve supply the sensory innervation to a skin segment known as a dermatome (see Fig.
7.3 and Table 7.2). There is usually no C1 or Co1 dermatome. Adjacent dermatomes over-lap, so that the loss of one dorsal root results in diminished sensation, not a complete loss, in that dermatome (Chap. 9).
The general afferent fibers are classified into (1) general somatic afferent (GSA) fibers, con-veying information from sensors in the extrem-ities and body wall, and (2) general visceral afferent (GVA) fibers, conveying information from the viscera (e.g., circulatory system).
A classification of sensory receptors and their probable functional roles are presented in Table 7.3. Further details concerning sensa-tions, functional significance, reflexes, and pathways associated with these receptors are discussed in subsequent chapters. A
classifica-tion of afferent fibers by conducclassifica-tion velocity recognizes groups I, II, III, and IV fibers (Chap. 8). Furthermore, group I is subdivided into group Ia, for nerve impulses from the pri-mary sensory endings of muscle spindles, and group Ib, for impulses from Golgi tendon organs (see Table 7.4). Group II fibers transmit impulses from encapsulated skin and joint receptors (e.g., Meissner’s and Pacinian cor-puscles), monitoring touch, pressure, tempera-ture, and joint movements, and secondary sensory endings of muscle spindles. Groups III and IV fibers transmit impulses arising from unencapsulated endings, mediating pain, touch, and pressure. In all cases, fibers of greater diameter have a higher conduction velocity than thinner fibers, and myelinated fibers are faster than unmyelinated ones.
A second classification of fibers by conduc-tion velocity into A, B, and C is also employed, for both sensory and motor fibers. The A and B
132 The Human Nervous System
Figure 7.2: Neurons of a reflex arc of the somatic nervous system on the left, and a visceral reflex arc of the sympathetic nervous system on the right. The spinal somatic reflex arcs are described in Chapter 8, and the spinal visceral arcs are described in Chapter 20.
fibers are myelinated and the C fibers are unmyelinated. The A fibers are further divided by conduction velocity (hence, fiber size) into alpha, beta, gamma, and delta (see Table 7.4).
Ventral Roots
The ventral (motor) roots consist of efferent fibers that convey output from the spinal cord.
There are two functional components: (1) gen-eral somatic efferent (GSE) fibers, which inner-vate voluntary striated muscles, and (2) general visceral efferent (GVE) fibers, which convey influences to the involuntary smooth muscles, cardiac muscle, and glands (see Table 7.4).
Figure 7.3: Dermatomal (segmental) innervation of the skin. Refer to Table 7.2. The trigeminal nerve has three dermatomes: ophthalmic division (V1), maxillary division (V2), and mandibular division (V3).
Table 7.2 Dorsal
spinal root Body region innervateda
C2 Occiput
C4 Neck and upper shoulder
T1 Upper thorax and inner side of arm
T4 Nipple zone
T10 Umbilical girdle zone L1 Inguinal region
L4 Great toe, lateral thigh, and medial leg S3 Medial thigh
S5 Perianal region
aDermatome and region to which radicular parts is referred.
A B
These fibers are axons of (1) alpha moto-neurons (lower motomoto-neurons; Chap. 11) that convey impulses to the motor end plates of vol-untary muscle fibers, (2) gamma motoneurons that convey impulses to the motor endings of intrafusal fibers of muscle spindles (Chap. 8), and (3) preganglionic (lightly myelinated) autonomic neurons that synapse with postgan-glionic (unmyelinated) neurons (Chap. 20).
Gamma motoneurons are also known as fusimotor neurons (innervate the muscles of the fusiform-shaped spindles).
The muscles innervated by motoneurons from a single spinal cord segment constitute a myotome. Like dermatomes, there is myotomal overlap. The spinal cord innervation of a few muscles and regions is shown in Table 7.5. A single alpha motoneuron together with the muscle fibers it innervates constitutes a motor unit. Such units vary widely in the number of muscle fibers they contain, ranging from units innervating 3 to 8 muscle fibers in the small finely controlled extraocular muscles of the eye or those in the larynx important for speech, to units containing as many as 2000 muscle fibers in postural muscles (e.g., soleus in the leg).
Individual muscle fibers are innervated by one motor end plate, referred to as a neuromuscular junction, usually located near the middle of the cell. The muscle fibers of a motor unit inter-mingle with those from other motor units.
In general, there is a continuum of attributes that form three functional types of muscle fiber, namely (1) slow-twitch fibers, (2) fast-fatigable-twitch fibers, and (3) fast-fatigue-resistant twitch fibers. Each motor unit comprises only one type. The slow-twitch fibers are the signa-tures of “red meat” muscles that have (1) a rich extracellular matrix of blood capillaries, (2) numerous mitochondria with high levels of oxidative enzymes, and (3) a large amount of myoglobin that helps absorb and store oxygen from the blood. These are slow-twitch fibers because the force they produce in response to an action potential rises and falls relatively slowly. Their fatigue resistance results from a reliance on oxidation catabolism, whereby glu-cose and oxygen from the blood is available to
134 The Human Nervous System
Table 7.3: Classification of Sensory Receptors (Probable Functional Roles)
I. Receptors of general sensibility (exteroceptive) A. Endings in epidermis
1. Free nerve endings (tactile, pain, thermal sense)
2. Terminal disks of Merkel (tactile) 3. Nerve (peritrichial) endings in hair follicle
(tactile, movement detector) B. Endings in connective tissues (skin and
connective tissue throughout body) 1. Free nerve endings (pain, thermal sense) 2. Encapsulated nerve endings
a. End bulbs of Ruffini (touch-pressure, position sense, kinesthesia)
b. Corpuscles of Meissner (tactile, flutter sense)
c. Corpuscles of Pacini (vibratory sense, touch-pressure)
C. Endings in muscles, tendons, and joints (proprioceptive)
1. Neuromuscular spindles (stretch receptors)
2. Golgi tendon organs, neurotendinous endings (tension receptors)
3. End bulbs of Ruffini in joint capsule (touch-pressure, position sense, kinesthesia)
4. Corpuscles of Pacini (touch-pressure, vibratory sense, kinesthesia) 5. Free nerve endings (pain) II. Receptors of special senses
A. Bipolar neurons, of olfactory mucosa (olfaction)
B. Taste buds (gustatory sense) C. Rods and cones in retina (vision)
D. Hair cells in spiral organ of Corti (audition) E. Hair cells in semicircular canals, saccule, and
utricle (equilibrium, vestibular sense) III. Special receptors in viscera (interoceptive)
A. Pressoreceptors in carotid sinus and aortic arch (monitor arterial pressure)
B. Chemoreceptors in carotid and aortic bodies and in or on surface of medulla (monitor arterial oxygen and carbon dioxide levels) C. Chemoreceptors probably located in
supraoptic nucleus of hypothalamus (monitor osmolarity of blood)
D. Free nerve endings in viscera (pain, fullness) E. Receptors in lungs (respiratory and cough
reflexes)
regenerate ATP, which fuels the contractile process. The two subtypes of fast-twitch fibers comprise most of the “white meat” muscles, characterized by a more rapid rise and fall of their force response. They also tend to have a different form of myosin that possesses cross-bridges and produce force more effectively in rapid shortening velocities. One subtype of the fast-fatigable-twitch fibers relies almost exclu-sively on anaerobic catabolism and has rela-tively large stores of glycogen. This provides energy to rapidly rephosphorylate ADP, as glycogen is converted to lactic acid. This source is depleted fairly quickly and full recov-ery could take hours. The second subtype of fast-fatigue-resistant fibers combines relatively fast-twitch dynamics and contractile velocity with enough capability to resist fatigue for minutes to hours. World-class sprinters and
high jumpers, requiring quick starts and short bursts of speed, could have up to 85% fast-twitch fibers in certain muscles. Fibers of the slow motor units contract and relax relatively slowly and tend to fatigue less rapidly. World-class long-distance runners, especially mara-thoners, could have up to 90% slow-twitch Table 7.5
Ventral
spinal root Muscles innervated
C5–6 Biceps brachii (flexes elbow) C6–8 Triceps brachii (extends elbow) T1–8 Thoracic musculature
T6–12 Abdominal musculature L2–4 Quadriceps femoris (knee jerk,
patellar tendon reflex) L5–S1–2 Gastrocnemius (ankle jerk,
Achilles tendon reflex) Table 7.4: Classification of Nerve Fibers
Conduction
Fibers Diameters velocity (m/s) Role/receptors innervated Sensorya
Ia (A-α) 12–20 70–120 Primary afferents of muscle spindle
Ib (A-α) 12–20 70–120 Golgi tendon organ
Touch and pressure receptors
II (A-β) 5–14 30–70 Secondary afferents of muscle spindle
Touch, pressure, and pacinian vibratory sense receptors
III (A-δ) 2–7 12–30 Touch and pressure receptors
Pain and temperature receptors
IV (C) 0.5–1 0.5–2 Pain and temperature receptors
Unmyelinated fibers Motorb
Alplia (A-α) 12–20 15–120 Alpha motoneurons innervating extrafusal muscle fibers in laminaa
Gamma (A-γ) 2–10 10–45 Gamma motoneurons innervating intrafusal
muscle fibers in laminaa
Preganglionic autonomic >3 3–15 Lightly myelinated preganglionic autonomic
fibers (B) fibers
Postganglionic autonomic 1 2 Unmyelinated postganglionic autonomic fibers fibers (C)
aThe fibers of I, II, and III are myelinated and those of IV are unmyelinated. The fibers of I and II are associated with mechanoreceptors and those of III with mechanoreceptors in hair skin.
bCell bodies of alpha and gamma motoneurons are located in lamina IX.
cA lower motoneuron (lower motor neuron, alpha motoneuron, gamma motoneuron) is a motor neuron with its cell body in the CNS and an axon that innervates voluntary (striated, skeletal) muscle fibers.
fibers in certain muscles. They display a great capacity for running through fatigue.
LAMINAE OF THE SPINAL CORD The spinal cord is divided into gray matter (cell bodies, dendrites, axons, and glial cells) and white matter (myelinated and unmyeli-nated axons and glial cells). The nerve fibers within the gray matter are oriented in the trans-verse plane, whereas those of the white matter
are oriented in the longitudinal plane parallel to the neuraxis. The gray matter has been parceled anatomically, primarily on the basis of the microscopic appearance, into nuclei that con-form to a laminar pattern (Rexed’s laminae) (see Figs. 7.4, 7.5, and Table 7.6). The gray matter is also divided into a posterior horn (laminae I–VI), an intermediate zone (lamina VII and X), and an anterior horn (laminae VIII and IX). The white matter is divided into three columns (funiculi): posterior, lateral, and ante-rior (see Fig. 7.5).
136 The Human Nervous System
Figure 7.4: The sensory fibers of the dorsal root and the laminae of the spinal cord in which each type terminates. The heavily myelinated A alpha fibers from neuromuscular spindles and Golgi tendon organs terminate in laminae VI, VII, and IX (Chap. 8). The myelinated A beta fibers from cutaneous mechanoreceptors terminate in laminae III–VI (Chap. 10). The thinly myelinated and unmyelinated A delta and C fibers from nociceptors terminate in laminae I–V (Chap. 9).
(Adapted from Brodal.)
PATHWAYS AND TRACTS Sensory signals originating in sensory receptors in the body and limbs are transmitted through the spinal cord to the brain along sen-sory pathways. Motor commands from the higher centers in the brain descend through the spinal cord along motor pathways. Within the white matter of the spinal cord, the sensory fibers of the pathways form groups called ascending tracts or fasciculi, and fibers of the motor pathways form groups referred to as descending tracts (see Fig. 7.6). The functional significance and location of these pathways form a basis of neurologic diagnosis. Lesions within or impinging upon the nervous system often are revealed by alterations in sensory
Figure 7.5: Section through a cervical level of the spinal cord to illustrate some subdivisions of the gray matter and white matter. The white matter is composed of three funiculi (columns). The gray matter is divided into two horns and an intermediate zone. Division of the gray matter into Rexed’s laminae is shown on the right.
Table 7.6
Lamina Corresponding nucleus
I Posteromarginal nucleus
II Substantia gelatinosa
III and IV Proper sensory nucleus (nucleus proprius)
V Zone anterior to lamina IV
VI Zone at base of posterior horn VII Zona intermedia (includes
intermediomedial and
intermediolateral nuclei, dorsal nucleus of Clarke, and sacral autonomic nuclei)
VIII Zone in anterior horn (restricted to medial aspect in cervical and lumbosacral enlargements) IX Medial nuclear column and lateral
nuclear column
perceptions, balance, movement, or reflex activity, and the site often can be pinpointed by the examiner who has a thorough knowledge of these tracts and the associated roots of the spinal nerves.
Interposed between the afferent neurons of the dorsal roots and the efferent neurons of the ventral roots within the gray matter of the spinal cord are spinal interneurons that send their axons (crossed and uncrossed) to higher and lower segmental levels for the execution of various spinal intersegmental reflexes.
These axons (1) ascend and descend to form columns in the white matter adjacent to the gray matter (black band in Fig. 7.6), (2) orig-inate and termorig-inate within the spinal cord, and
(3) functional connect the various spinal lev-els. The columns are called the spinospinal fasciculi or fasciculi proprii of the spinal cord.
The axons of these fasciculi are linked with the spinoreticular tract, which goes to the brainstem as the spinoreticulothalamic path-way (Chap. 22).
Regional differences are present at various levels of the spinal cord (see Fig. 7.7). The amount of gray matter at any spinal level is primarily related to the richness of the periph-eral innervation. Hence, the gray matter is largest in the spinal segments of the cervical and lumbosacral enlargements innervating the upper and lower extremities; such large struc-tures require a massive innervation. The
tho-138 The Human Nervous System
Figure 7.6: The spinal cord tracts. The ascending tracts are represented as plain outlines on the right, the descending tracts as stippled outlines on the left, and the intrinsic spinal tracts (composed of descending and/or ascending fibers) as solid outlines. The representation of the tracts is arbi-trarily drawn. The lamination of the posterior columns and lateral spinothalamic tracts is indicated:
C, cervical; T, thoracic; L, lumbar; S, sacral.
racic and upper lumbar levels have relatively small amounts of gray matter: They innervate the thoracic and abdominal regions.
The absolute number of nerve fibers in the white matter increases at each successive higher spinal level. Stated otherwise, the white matter of a spinal level caudal to another level
has fewer fibers. The difference occurs because (1) additional fibers of the ascending sensory pathways join the white matter at each succes-sive higher level and (2) fibers of the descend-ing motor pathways from the brain leave the white matter before terminating in the gray matter at each successive level.
Figure 7.7: Representative sections from several levels of the adult human spinal cord. (A) High cervical level; (B) cervical enlargement level; (C) midthoracic level; (D) low thoracic level;
(E) lumbar level. All photographs of these Weigert-stained sections are at the same magnification.
(Courtesy of Dr. Joyce Shriver, Mount Sinai School of Medicine.)
SUGGESTED READINGS
Abdel-Maguid TE, Bowsher D. 1984. Interneurons and proprioneurons in the adult human spinal grey matter and in the general somatic and vis-ceral afferent cranial nerve nuclei. J. Anat.
139(Pt. 1):9-19.
Bowsher D, Abdel-Maguid TE. Superficial dorsal horn of the adult human spinal cord. Neuro-surgery. 1984;15:893-899.
Boyd IA, Davey MR. Composition of Peripheral Nerves. Edinburgh: Livingstone; 1968.
Brodal A. Neurological Anatomy in Relation to Clinical Medicine, 3rd ed. New York: Oxford University Press; 1981.
Brown MC, Hopkins WG, Keynes RJ, Hopkins WG.
Essentials of Neural Development. New York:
Cambridge University Press; 1991.
Coggeshall RE. Law of separation of function of the spinal roots. Physiol. Rev. 1980;60:716-755.
Crock HV. Atlas of Vascular Anatomy of the Skeleton and Spinal Cord. London: Martin Dunitz; 1996.
Crock HV, Yoshizawa H. The Blood Supply of the Vertebral Column and Spinal Cord in Man. New York: Springer-Verlag; 1977.
Crock HV, Yamagishi M, Crock MC. The Conus Medullaris and Cauda Equina in Man: An Atlas of the Arteries and Veins. New York: Springer-Verlag; 1986.
Hopkins WG, Brown MC. Development of Nerve Cells and Their Connections. New York: Cam-bridge University Press; 1984.
Landon DN. The Peripheral Nerve. New York:
Wiley; 1976.
Schiebel A. The organization of the spinal cord. In:
Davidoff RA ed. Handbook of the Spinal Cord.
New York: Dekker; 1984: vol. 2.
140 The Human Nervous System
The spinal cord contains the local neuronal circuits that coordinate somatic reflexes. These circuits are participants in the complex volun-tary movements of the body that are governed by the higher centers of the brain. One expres-sion of somatic motor function is the effortless ease with which humans carry out the most dexterous of motor activities without a con-scious awareness of joint movements and the accompanying muscle contractions synchro-nized with the required “relaxing” of antago-nistic muscles. The quality of sequential combinations of movements is dependent on the continuous flow of visual, somatosensory, and postural (vestibular and joint senses) infor-mation that results in seemingly immediate integrated responsive actions. Although we might be consciously aware of making deci-sions regarding execution of the movements to accomplish the goal, we are unaware of the details that are instrumental in creating the motions, as they seem to take place automati-cally.
Somatic reflexes are the automatic stereo-typic motor responses by voluntary muscles to adequate sensory stimuli. From a vast array of external and internal stimuli bombarding the
body, selections are made by sensory receptors within the skin, voluntary muscles, tendons, and joints. From them, a continuous flow of sensory messages is transmitted via spinal and cranial sensory nerves to the spinal cord and brainstem for processing in order to achieve response goals. These are realized by informa-tion conveyed via alpha motoneurons to the extrafusal muscle fibers and via gamma motoneurons to the intrafusal muscle fibers (see later). The alpha and gamma motoneurons, called lower motoneurons, comprise the final common pathway that controls skeletal muscle activities expressed as reflex, postural, rhyth-mic, and voluntary movements.
The motor apparatus consists of a mechani-cal arrangement of muscles, bones, and joints organized as levers. Each movement at a joint involves the interplay between agonist muscles and antagonist muscles (including accessory muscles for adjustments). The agonist muscles execute the prime movement and the antagonist muscles counterbalance agonists. The antago-nist muscles are involved in decelerating and stabilizing the movement. The spinal reflex responses, such as the withdrawal of the upper limb when the finger contacts a hot stove, are