THE SPINAL CORD
Development of the Spinal Nerves
The spinal cord has 31 pairs of spinal nerves. These are attached at regular intervals corresponding to the paired somites and to the paired nodules of the neural crest. Each spinal nerve is similar in developmental sequence, structure and fundamental plan. Each derives from the dorsal and ventral roots
Formation of the Dorsal Roots
Each paired nodule of neural crest produces neuroblasts for a dorsal root ganglion. Each neuroblast in the dorsal root ganglion produces a process that bifurcate into peripheral and central branches. The central branch pierces the dorsolateral aspect of the spinal cord, forming the dorsal root. Upon entering the spinal cord, the dorsal root axon characteristically branches. The branches may run up or down the cord, but at the level of the entry, the axon synapses variously on dorsal horn neurons, spinal interneurons and ventral horn motorneurons. The peripheral branch extends to a receptor in the skin or viscera.
Formation of the Ventral Roots
Neuroblasts in the ventral horn gray matter differentiate and produce axons that exit from the ventrolateral aspect of the spinal cord. Two types of axons enter the ventral rootes -axons destined for skeleteal muscles and axons destined for autonomic ganglia. The axons going to skeletal muscles issue from motorneurons in the ventral horns. They travel directly to the muscle without further synapses. The autonomic axons issue from neurons in the intermediate horn. These autonomic axons do not run directly to their glands or smooth muscles. Instead, the autonomic axons synapse upon a peripheral neuron in a para or prevertebral ganglion of the sympathetic nervous system. The peripheral neuron then innervates the effector. Thus the autonomic pathway of the PNS involves two neurons; the skeletal muscle pathway involves only one. The preganglionic neuron runs to the ganglion by a small ramus (r. comminicans albus) from the peripheral nerve trunk. The postganglionic axon rejoin the trunk by another ramus (r. comminicans griseus).
Development and Innervation of Somites
Somites are mesodermal derivatives, develop as a series of regular, paired lumps on each side of the neural tube. Somites produces the somatic structures of the body. Their mesoderm differentiates into dermatomes, myotomes and sclerotomes. The dermatome produces the dermis, the deep layer of the skin beneath of the epidermis. The epidermis derives from the surface ectoderm. The myotome differentiates into skeletal muscle. The scelorotome differentiates into the skeleton and related connective tissue. The somites extend from the caudal end of the spinal cord to the midbrain level. Each somite nerve innervates all of the tissues derived from its original somite and only those tissues. It innervates the dermis derived from a particular somite's dermatome, the muscles derived from the somite's myotome, and the bone derived from its scelorotome. This rule holds even when the somite derivatives migrate and undergo extensive transformations in the arm and leg regions. Figure 4A shows the transformation of the dermatomes. Only the thoracic region retains the original somite simplicity since it is unaltered by face, arm or leg growth. Opposite each somite, a single paravertebral autonomic ganglion forms, but some ganglia coalesce in the cervical region. The single-somite-single-ganglion arrangement is confined roughly to the thoracic region.
Formation of Somatic Nerve Plexuses
Figure 3 shows that a spinal nerve trunk upon entering a plexus contain axons from only one spinal nerve serving only one spinal segment. The axons of the individual nerve trunks intermingle in the plexus, but each axon retains its own identity and does not anastomose with axons of another segment. The peripheral nerves issuing from a somatic plexus may contain axons from more than one nerve trunk or spinal segment. Nerve trunks form three plexuses along the spinal cord: the cervical, brachial and lumbosacral. The brachial and lumbosacral plexuses are the largest because the somite derivatives, undergo the greatest redistribution in the limb buds, which forms the arms and legs. No plexuses occur in the throracic region, where the somites retain their original serial simplicity. Figs. 5-6 show the segmental and peripheral innervation of the skin.
In summary: 1) One spinal nerve innervates the dermatome, myotome, and sclerotome derived from one somite; 2) Wherever the somite derivative migrates during embryogenesis, it retains its original somite nerve; 3) The most extensive rearrangement of the somites is in the head, arms, and legs. The thorax retains the simple serial somite plan, undisturbed by somite rearrangements; 4) The somatic nerve plexuses redistribute the axons from the spinal nerve trunks into convenient pathways to the migrated somite derivatives of the head, arms and legs.
Relationship of Spinal Roots, Nerves, and the Spinal Cord to Vertebral Levels
The average adult has 31 to 32 pairs of spinal nerves, each one corresponding to an embryonic somite. The spinal nerves are numbered in relation to the vertebrae. There are 8 pairs of cervical nerves, 12 thoracic, 5 lumbar, 5 sacral, and 1-2 coccygeal. There are only 7 cervical vertebrae but 8 cervical nerves because cervical nerve 1 (C1) comes out rostral to the first cervical vertebra and cervical nerve 8 (C8) comes out caudal to the seventh cervical vertebra.
Because the vertebral column elongates faster during gestation than the spinal cord, the caudal tip of the cord, which originally lay opposite the cocyx, comes to lie opposite the first lumbar vertebra. Because of this relative elevation (ascensus), the more caudal a nerve root the further it must run to reach its intervertebral foramen and the greater its downward angulation. Since the tip of the cord lies at L1, a physician can insert a needle into the subarachnoid space at L4/L5 or L5/S1 to obtain CSF for diagnostic analysis without fear puncturing the cord. The nerve roots will move aside and generally are undamaged by the needle (Fig. 7).
Gross Anatomy of the Spinal Cord
The spinal cord is a cylindrical elongated part of the central nervous system. It extends from the level of the foramen magnum to the body of the first lumbar vertebra, an average length of 43 cm. Rostrally, the spinal cord continues uninterruptedly into the medulla oblongata. The level of the foramen magnum arbitrarily divides the medulla and the cord. Caudally, the spinal cord ends at the conus medullaris. The tip of the conus medullaris extends to the sacrum as a thin strand, the filum terminale, composed only of glia. After the ascensus, dorsal and ventral spinal nerve roots angle downward on either side of the filum terminale, extending from the lumbosacral cord to their original vertebral foramina. This groups of roots is called the cauda equina. The spinal cord varies in diameter from about 1cm -1.5 cm. The thoracic region is the narrowest. The spinal cord has two gross enlargements, the cervical and the lumbosacral, to accomodate the extra neurons that innervate the limbs (Fig. 8).
Cross Sectional Anatomy of the Spinal Cord. Gray and White Matter
When cut transversely, the spinal cord consists of an outer zone of white matter and a central, H-shaped region of grey matter. The arms of the H, extending dorsally and ventrally, are called the dorsal horn and ventral horn. The grey matter is organized into nuclei and laminae and extends as a column through the length of the spinal cord. For the laminar and nuclear pattern of the spinal gray matter and longitudinal extent of nuclei in the spinal cord see Fig. 9. The spinal gray matter contains three main types of neurons (somaotmotor, visceromotor, sensory and interneurons: see below).
The white matter of the cord contains axons running longitudinally. Some of these axons convey signals from the cord to higher levels of the CNS, others from higher levels to the cord. Finally, a large proportion of the fibers serve cooperation between the segments of the cord. Since the first two groups of axons become successively more numerous in the rostral direction, the proportion of white to gray matter increases from caudal to rostral. The white matter is divided into funiculi, or columns.
Distribution of the Dorsal Root Axons in the Cord
Afferent fibers from the receptors follow the peripheral nerves toward the CNS. The sensory fibers of the spinal nerves have their perikarya in the dorsal root ganglia. Likewise, the sensory fibers in the cranial nerves have their perikarya in ganglia close to the brain stem. The ganglion cells are pseudounipolar and send one long process peripherally, ending freely or in encapsulated sense organs. The central process enters the cord and then divides into an ascending and a descending branch. These branches give off several collaterals ventrally to the gray matter of the cord. The different kinds of sensory receptors are supplied with axons of characteristic thickness. Impulses from low-threshold mechanoreceptors are, for example, conducted in the thick myelinated fibers (A-alfa [Ia, Ib] and A-beta [II). These large fibers that constitute the medial division of dorsal roots, mediate sensory modalities consisting of touch, perception of texture, perception of form, and modality termed proprioception, which gives a sense of where the- body parts are (position sense) and of tension of joints and muscles. They divide into ascending and descending branches and terminate in lamina III-VI (A-Beta) and L VI-VII, IX (A-alfa fibers).
Impulses from cold receptors are conducted in thin myelinated fibers (A delta), whereas
unmyelinated (C) fibers conduct from heat receptors. Impulses from nociceptors
are conducted in A delta and C fibers. These fibers
constitute the lateral division of dorsal roots. In the spinal cord, the
termination of A delta and C fibers are almost
completely separated from those of the A- alfa and A-beta fibers.
These fibers accumulate at the apex of the dorsal horn and they form the
dorsolateral tract of Lissauer. A-delta fiber terminate primarily in Lamina I
Efferent Fibers via the Ventral Root Innervate Muscles and Glands
The motor neurons have large, multipolar perikarya and are in the ventral horn proper. The dendrites extend for a considerable distance in the gray matter. The axons leave the cord through the ventral root, follow the spinal nerves, and end in skeletal muscles. These neurons are also called alfa motorneurons and are the largest in the spinal cord and among the largest in the CNS. They are located in Rexed Lamina IX. The smaller gamma motorneurons send gamma-sized axons in the peripheral nerves and innervate the intrafusal fibers in the muscle spindles. The alpha motorneurons sends a collateral axon to an interneuron, the Renshaw cell, which sends an inhibitory syanpase back to the alpha motorneurons.
There is also another group of neurons that sends its axons out of the cord through the ventral root. These supply smooth muscles and glands with motor signals, and belong to the autonomic nervous system. The autonomic system controls the vascular smooth muscles and visceral organs throughout the body. The cell bodies lie in the lateral horn. These neurons form the intermediolateral column (T1-L2) and constitutes the sympathetic part of the autonomic NS. A corresponding, smaller group of neurons is present in the sacral cord (S2-S4) and belongs to the parasympathetic part of the autonomic NS (see below).
The Spinal Cord Consists of Cooperating Subunits that are Controlled by Descending Pathways from Higher Brain Centers
Many of the functional tasks of the spinal cord are under strict control and supervision from higher levels of the CNS. This control is mediated by fibres from the brainstem and the cerebral cortex, which descend in the white matter of the cord and terminate in the gray matter.
Arrangement of the spinal pathways
a)Law of the peripheral position of long fibers
b)Law of lamination by level of entry or body topography
c)Law of separation of sensory pathways by sensory modalities
Myotatic Stretch (Proprioceptive) Reflexes: determines muscle length
The most famous stretch reflex is the quadriceps reflex (knee jerk reflex), produced by tapping the patellar tendon, which in turn stretches the quadriceps. The reflex is initiated by special muscle receptors called muscle spindles, which are sensitive to stretch. Muscle spindles are composed of 8-10 modified muscle fibers called intrafusal fibers arranged in parallel with the ordinary (extrafusal) fibers that make up the bulk of the muscle. Sensory fibers (Ia) are coiled around the central part of the spindle. Streching the muscle deforms the intrafusal muscle fibers, which lead to increased activity of the sensory fibers that innervate each spindle. The impulses are transmitted through Ia afferent fibers to the spinal cord, where the fibers establish synaptic contact with alpha motor neurons, which in turn produce contraction of quadriceps and extension of the leg at the knee. At the same time as the quadriceps contracts there is a reciprocal inhibition of the antagonistic muscles, the flexors of the knee. The inhibition of the flexors is mediated by polysynaptic reflex arcs, and since the motor neurons for the flexors are located in more caudal segments than the motor neurons for quadriceps, the inhibitory reflex is intersegmental, in contrast with the stretch reflex, which is intrasegmental (reciprocal innervation).
Borrowing a concept from engineering, the stretch reflex arc can be viewed as a negative feedback loop that tends to maintain muscle length at a constant value. The desired muscle length is specified by the activity of descending pathways that influence the motor neuron pool. Deviations from the desired length are detected by the muscle spindles; thus increases or decreases in the stretch of the intrafusal fibers change the level of activity in the sensory fibers that innervate the spindles. These changes, in turn, lead to appropriate adjustments in the activity of the alpha motor neurons, returning the muscle to the desired length.
The gain is adjusted by changing the level of activation of the gamma motor neurons. These small gamma motor neurons are interspersed among the alpha motor neurons in the ventral horn of the spinal cord. An increase in the activity of gamma motor neurons produces an increase in the amount of tension in the intrafusal fibers. Although the intrafusal fibers are much too sparse to generate a net increase in muscle tension, contraction of the intrafusal fibers increases the sensitivity of Ia sensory fibers to muscle stretch. The same stretch can then produce a larger amount of Ia afferent activity, which causes an increase in the activity of the alpha motor neurons that innervate the extrafusal muscle fibers.
The Inverse Myotatic Reflex: limits the muscle tension
Another sensory structure that is important in the reflex regulation of motor unit activity is the Golgi tendon organ. Golgi tendon organs are encapsulated endings located at the junction of the muscle and tendon. Each tendon organ is related to a single group Ib sensory axon (the Ib axons are slightly smaller than the Ia axons that innervate the muscle spindles). In contrast to the parallel arrangement of extrafusal muscle fibers and spindles, Golgi tendon organs are in series with the muscle fibers. When a muscle is passively stretched, most of the change in length occurs in the muscle fibers, since they are more elastic than the fibrils of the tendon. When a muscle actively contracts, however, the force acts directly on the tendon, leading to an increase in the tension of the collagen fibrils in the tendon organ and compression of the intertwined sensory receptors. As a result, Golgi tendon organs are sensitive to increase in muscle tension that arise from muscle contraction and, unlike spindles, are much less sensitive to passive stretch.
The Ib axons from Golgi tendon organs contact inhibitory interneurons in the spinal cord (called Ib inhibitory interneurons) that synapse, in turn with the alpha motor neurons that innervate the same muscle. The Golgi tendon circuit is thus a negative feedback system that regulates muscle tension, decreasing the activation of muscles when exceptionally large forces are generated. This reflex circuit also operates at reduced levels of muscle force, counteracting small changes in muscle tension by increasing or decreasing the inhibition of alpha motor neurons. Under these conditions, the Golgi tendon system tends to maintain a steady level of muscle force, counteracting effects such as fatique, which diminishes muscle force. If the muscle spindle system is viewed as a feedback system that monitors and maintains muscle length, then the Golgi tendon system is a feedback system that monitors and maintains muscle force. Like the muscle spindle system, the Golgi tendon organ system is not a closed loop. Ib inhibitory interneurons also receive synaptic inputs from a variety of other sources, including cutaneous receptors, joint receptors, muscle spindles, and descending pathways. Together these inputs regulate the responsiveness of Ib interneurons to activity arising in Golgi tendon organs.
Although there are stretch reflexes in all muscles, they are especially prominent in antigravity muscles, where they form the basis for postural reflexes. Stretching of a muscle does not necessarily elicit a reflex contraction. Many factors influence whether there will be a response, such as the velocity of stretching, how long the stretch is, whether the muscle is active when being stretched, and whether- a reflex contraction is functionally appropriate. Short (30 msec) and long-latency stretch reflex. Fig. 21 shows a theoretical possibility of how the same muscle pair may work synergistically or antagonistically in various situations.
Some stretch reflexes are routinely tested in neurologic examinations. The most commonly tested stretch reflexes have the following segmental reflex center:
1) The biceps brachialis reflex :flexion of the elbow by tapping the biceps tendon (C5-C6).
2) The brachioradial reflex :flexion of the elbow and supination of the forearm by tapping the styloid process of the radius (C5-C6)
3) The triceps brachialis reflex: extension of the elbow through a tap on the triceps tendon. (C6-C7)
4) Quadriceps (patellar tendon) reflex: extension of the knee by tapping the ligamentum patellae L2-L4)
5) Triceps surae (Achilles tendon) reflex: plantar flexion of the foot by tapping the Achilles tendon (L5-S2).
Flexion and Crossed Extensor (Withdrawal) Reflex
The po1ysynaptic flexor reflex serves important protective functions. One of its purposes is to achieve a rapid withdrawal of a limb in response to painful cutaneous stimuli. To maintain position the flexor withdrawal reflex is usually accompanied by extension of the opposite limb through action of the crossed extensor reflex. Receptors: free nerve endings in the skin. Afferent arch: Adelta and C fibers which terminate in the marginal zone (Lissauer) and in the dorsal part of the central gray matter. Central mechanism: the central processes of the primary sensory neurons synapse with interneurons and funicular neurons that in turn innervate ipsilateral flexor and crossed extensor muscles.
Like the other reflex pathways, interneurons in the flexion reflex pathway receive converging inputs from several different sources, including cutaneous receptors, other spinal cord interneurons and descending pathways. Although the functional significance of this complex pattern of connectivity is uncertain, changes in the character of the reflex following damage to descending pathways provide a clue. Under normal conditions, a noxious stimulus is required to evoke the flexion reflex; following damage to descending pathways, however, other types of stimulation, such as moderate squeezing of a limb, can produce the same response. Thus, the descending projections to the cord may function, at least in part, to gate the responsiveness of interneurons in the flexion reflex pathway to a variety of sensory inputs.
Receptors: skin (mainly pain and temperature) and visceroceptors. Afferent arch: primary neurons are in the spinal ganglion. The fibers are medium-thin myelinated and unmyelinated. Central mechanism: Between the afferent and the efferent neurons there is always one or several interneuron. Efferent branch: the efferent arch of the reflex consist of two neurons: one is located in the lateral horn (n. intermediolateralis:between T1-L3) the second in the para- or prevertebral sympathetic chain. The postganglionar axons returning to the spinal nerve function as sudomotor, vasomotor or piloarector. Those neurons which synapse in the prevertebral chain innervate though their postganglionic processes the viscera.
Referred Pain. Hyperesthesia (Fig. 27). Recording from spinothalamic cells in the spinal cord has shown that many can be activated by nociceptive stimuli applied to visceral organs and to the skin. Higher centers, however, impulses arriving from a particular spinothalamic cell always interpreting as coming from the skin. When signals arise for the first time in the heart, they are misinterpreted as coming from the skin. This phenomenon, commonly experienced with diseases of visceral organs, is called referred pain. Infarction of the heart, for example, is usually accompanied by pain localized to the left arm, diseases of the gallbladder may manifest themselves with pain below the right shoulder blade. Convergence on spinothalamic cells may also explain the phenomenon of hyperesthesia - that is, a region of skin becomes abnormally sensitive, such that even light touch may provoke pain. This is commonly observed with diseases of visceral organs. Thus, impulses from the visceral organ excite the spinothalamic cell so that less excitation from the skin is necessary to fire the cell.
Sensory Neurons in the Cord Give Rise to Ascending Pathways to Higher Brain Centers
The second main type of spinal neurons sends axons to higher levels of the CNS. Their perikarya are mainly located in the dorsal horn and in the transition zone between the dorsal and ventral horn. Their job is to inform the brain of the activities of the spinal cord, and especially about what is going on in the body. The dorsal root fibers form synaptic contacts -in part directly, in part indirectly via interneurons -with neurons in the spinal cord, sending their axons to various parts of the brain. Such axons, destined for a common target in the brain, are grouped together in the spinal white matter, forming ascending tracts.