The
spinal cord is an elongated cylindrical structure, about 45 centimetres (18
inches) long, that extends from the medulla oblongata of the hindbrain to a
level between the first and second lumbar vertebrae of the backbone. The
terminal part of the spinal cord is called the conus medullaris. Associated
with local regions of the spinal cord and imposing upon it an external
segmentation are 31 pairs of spinal nerves, each of which receives and
furnishes one dorsal and one ventral root. On this basis the spinal cord is
divided into the following nerves: 8 cervical (C), 12 thoracic (T), 5 lumbar
(L), 5 sacral (S), and 1 coccygeal. Spinal nerve roots emerge via
intervertebral foramina; lumbar and sacral spinal roots, descending for some
distance within the subarachnoid space before reaching the appropriate
foramina, produce a group of nerve roots about the conus medullaris known as
the cauda equina. Two enlargements of the spinal cord are evident: (1) a
cervical enlargement (C5 through T1), which provides nerve supply for the
upper extremity, and (2) a lumbosacral enlargement (L1 through S2), which
supplies the lower extremity.
A cross section of the spinal cord reveals long
tracts of myelinated nerve fibers (known as white
matter) arranged around the periphery of a symmetrical, butterfly-shaped
cellular matrix of gray matter (see figure to right).
The gray matter forms three pairs of horns
throughout most of the spinal cord: (1) the dorsal horns, composed of
sensory neurons; (2) the lateral horns, well defined in thoracic segments
and composed of visceral neurons; and (3) the ventral horns, especially
large in the cord enlargements and composed of motor neurons.
The
white matter forming the ascending and descending spinal tracts falls in
three paired funiculi, or sectors: the dorsal or posterior funiculi, lying
between the dorsal horns; the lateral funiculi, lying on each side of the
spinal cord between the dorsal-root entry zones and the emergence of the
ventral roots; and the ventral funiculi, lying between the ventral median
sulcus and each ventral-root zone.
The
cellular gray matter has a “cytoarchitectural lamination” in which nine
laminae are customarily indicated by Roman numerals (see figure, above).
Laminae I to V, forming the dorsal horns, receive sensory input. Lamina VII
forms the intermediate zone at the base of all horns. Lamina IX is composed
of clusters of large (alpha) motor neurons, whose axons innervate striated
muscle, and small (gamma) motor neurons, which innervate contractile
elements of the muscle spindle. Axons of both alpha and gamma motor neurons
emerge via the ventral roots. Laminae VII and VIII have variable
configurations, and lamina VI is present only in the cervical and
lumbosacral enlargements. In addition, cells surrounding the central canal
of the spinal cord form an area often referred to as lamina X.
All primary sensory neurons that enter the
spinal cord originate in ganglia that are located in the intervertebral
foramina. Peripheral processes of the nerve cells in these ganglia convey
sensation from various receptors, and central processes of the same cells
enter the spinal cord dorsolaterally as bundles of nerve filaments. Fibers
conveying specific forms of sensation follow separate pathways. Impulses
concerned with pain and noxious stimuli largely end upon cells in parts of
laminae I and II, while impulses associated with tactile sense end in lamina
IV or on processes of cells in that lamina. Signals from stretch receptors
(i.e., muscle spindles and tendon organs) end upon cells in parts of laminae
V, VI, and VII; collaterals of these fibers involved in the stretch reflex
project into lamina IX.
Virtually all parts of the spinal gray contain interneurons, which connect
various cell groups. Many interneurons have short axons distributed locally,
but some have axons that extend for several spinal segments. Some
interneurons may modulate or change the character of signals, while others
play key roles in transmission and in patterned reflexes.
Sensory tracts ascending in the white matter of the
spinal cord arise either from cells of spinal ganglia or from intrinsic
neurons within the gray matter that receive primary sensory input.
The largest ascending tracts, the fasciculi gracilis
and cuneatus, arise from spinal ganglion cells and ascend in the dorsal
funiculus to the medulla. The fasciculus gracilis receives fibers from
ganglia below thoracic 6, while spinal ganglia from higher segments of the
spinal cord project fibers into the fasciculus cuneatus. The fasciculi
terminate upon large nuclear masses (the nuclei gracilis and cuneatus) in
the medulla. Cells of these nuclei give rise to fibers that cross completely
and form the medial lemniscus; the medial lemniscus in turn projects to the
ventrobasal nuclear complex of the thalamus. In this way, the dorsal
column/medial lemniscal system conveys signals associated with tactile,
pressure, and kinesthetic (or positional) sense to sensory areas of the
cerebral cortex.
Fibers concerned with pain, thermal sense, and light
touch enter the lateral-root entry zone and then ascend or descend near the
periphery of the spinal cord before entering superficial laminae of the
dorsal horn—largely parts of laminae I, IV, and V. Cells in these laminae
then give rise to fibers of the two spinothalamic tracts. Those crossing in
the ventral white commissure (ventral to the central canal) form the lateral
spinothalamic tract, which, ascending in the ventral part of the lateral
funiculus, conveys signals related to pain and thermal sense. The anterior
spinothalamic tract arises from fibers that cross the midline in the same
fashion but ascend more anteriorly in the spinal cord; these convey impulses
related to light touch. At medullary levels the two spinothalamic tracts
tend to merge and cannot be distinguished as separate entities. Many of the
fibers, or collaterals, of the spinothalamic tracts end upon cell groups in
the reticular formation, while the principal tracts convey sensory impulses
to relay nuclei in the thalamus.
Impulses from stretch receptors are carried by
large-diameter fibers that synapse upon cells in deep laminae of the dorsal
horn or in lamina VII. The posterior spinocerebellar tract arises from the
dorsal nucleus of Clarke and ascends peripherally in the dorsal part of the
lateral funiculus. The anterior spinocerebellar tract ascends on the ventral
margin of the lateral funiculus. Both tracts transmit signals to portions of
the anterior lobe of the cerebellum and are involved in mechanisms that
automatically regulate muscle tone without reaching consciousness.
Tracts descending to the spinal cord are concerned
with voluntary motor function, muscle tone, reflexes and equilibrium,
visceral innervation, and modulation of ascending sensory signals. The
largest and most important, the corticospinal tract, originates in broad
regions of the
cerebral cortex. Smaller descending tracts, which include the
rubrospinal tract, the vestibulospinal tract, and the reticulospinal tract,
originate in discrete and diffuse nuclei in the midbrain, pons, and medulla.
Most of these brain-stem nuclei themselves receive input from the cerebral
cortex, the cerebellar cortex, deep nuclei of the cerebellum, or some
combination of these.
In addition, autonomic tracts, which descend from
various nuclei in the brain stem to preganglionic sympathetic and
parasympathetic neurons in the spinal cord, constitute a vital link between
the centres that regulate visceral functions and the nerve cells that
actually effect changes.
Universally regarded as the single most important
tract concerned with skilled voluntary activity, the corticospinal tract
originates from pyramid-shaped cells in the premotor, primary motor, and
primary sensory cortex. Containing about one million fibers, it forms a
significant part of the posterior limb of the internal capsule and is a
major constituent of the crus cerebri in the midbrain. As the fibers emerge
from the pons, they form compact bundles on the ventral surface of the
medulla, known as the medullary pyramids. In the lower medulla about 90
percent of the fibers of the corticospinal tract decussate and descend in
the dorsal part of the lateral funiculus of the spinal cord. Of the fibers
that do not cross in the medulla, approximately 8 percent cross in cervical
spinal segments. As the tract descends, fibers and collaterals are given off
at all segmental levels, synapsing upon interneurons in lamina VII and upon
motor neurons in lamina IX. Approximately 50 percent of the corticospinal
fibers terminate within cervical segments.
The rubrospinal tract arises from cells in the
caudal part of the red nucleus, an encapsulated cell group in the midbrain
tegmentum. Fibers of this tract decussate at midbrain levels, descend in the
lateral funiculus of the spinal cord (overlapping ventral parts of the
corticospinal tract), enter the spinal gray, and terminate on interneurons
in lamina VII. Through these crossed rubrospinal projections, the red
nucleus exerts a facilitating influence on flexor alpha motor neurons and a
reciprocal inhibiting influence on extensor alpha motor neurons. Because
cells of the red nucleus receive input from the motor cortex (via
corticorubral projections) and from globose and emboliform nuclei of the
cerebellum (via the superior cerebellar peduncle), the rubrospinal tract
effectively brings flexor muscle tone under the control of these two regions
of the brain.
The vestibulospinal tract originates from cells of
the lateral vestibular nucleus, which lies in the floor of the fourth
ventricle. Fibers of this tract descend the length of the spinal cord in the
ventral and lateral funiculi without crossing, enter laminae VIII and IX of
the anterior horn, and end upon both alpha and gamma motor neurons, which
innervate ordinary muscle fibers and fibers of the muscle spindle (see below
Functions of the human nervous
system: Movement). Cells of the lateral vestibular nucleus receive
facilitating impulses from labyrinthine receptors in the utricle and from
fastigial nuclei in the cerebellum. In addition, inhibitory influences upon
these cells are conveyed by direct projections from Purkinje cells in the
anterior lobe of the cerebellum. Thus, the vestibulospinal tract mediates
the influences of the vestibular end organ and the cerebellum upon extensor
muscle tone.
A smaller number of vestibular projections, originating from the medial and
inferior vestibular nuclei, descend ipsilaterally in the medial longitudinal
fasciculus only to cervical levels. These fibers exert excitatory and
inhibitory effects upon cervical motor neurons.
The reticulospinal tracts arise from relatively
large but restricted regions of the reticular formation of the pons and
medulla—the same cells that project ascending processes to intralaminar
thalamic nuclei and play an important role in maintaining alertness and the
conscious state. The pontine reticulospinal tract arises from aggregations
of cells in the pontine reticular formation, descends ipsilaterally as the
largest component of the medial longitudinal fasciculus, and terminates
among cells in laminae VII and VIII. Fibers of this tract exert facilitating
influences upon voluntary movements, muscle tone, and a variety of spinal
reflexes. The medullary reticulospinal tract, originating from reticular
neurons on both sides of the median raphe, descends in the ventral part of
the lateral funiculus and terminates at all spinal levels upon cells in
laminae VII and IX. The medullary reticulospinal tract inhibits the same
motor activities that are facilitated by the pontine reticulospinal tract.
Both tracts receive input from regions of the motor cortex.
Descending fiber systems concerned with visceral and
autonomic activities emanate from collections of cells at various levels of
the brain stem. For example, hypothalamic nuclei project to visceral nuclei
in both the medulla and spinal cord; in the spinal cord these direct
hypothalamospinal projections terminate upon cells of the intermediolateral
cell column in thoracic, lumbar, and sacral segments. Preganglionic
parasympathetic neurons originating in the oculomotor nuclear complex in the
midbrain project not only to the ciliary ganglion but also directly to
spinal levels. Some of these fibers reach lumbar segments of the spinal
cord, most of them terminating in parts of laminae I and V. Pigmented cells
in an area of the rostral pons known as the isthmus form a blackish blue
collection visible in gross brain sections; known as the locus ceruleus,
these cells are rich in norepinephrine and distribute this neurotransmitter
widely to all regions of the brain and spinal cord. Fibers from the locus
ceruleus descend to spinal levels without crossing and are distributed to
terminals in the anterior horn, the intermediate zone, and the dorsal horn.
Other noradrenergic cell groups in the pons, near the motor nucleus of the
facial nerve, project uncrossed noradrenergic fibers that terminate in the
intermediolateral cell column (that is, lamina VII of the lateral horn).
Postganglionic sympathetic neurons associated with this system have direct
effects upon the cardiovascular system. Cells in the nucleus of the solitary
tract project crossed fibers to the phrenic nerve nucleus (in cervical
segments 3 through 5), the intermediate zone, and the anterior horn at
thoracic levels; these innervate respiratory muscles.
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