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
and lateral
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
SPINAL REFLEXES
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.