The root cells contribute their axons to the ventral roots of the spinal nerves and are grouped into two major divisions: 1 somatic efferent root neurons, which innervate the skeletal musculature; and 2 the visceral efferent root neurons, also called preganglionic autonomic axons, which send their axons to various autonomic ganglia.
The column or tract cells and their processes are located mainly in the dorsal gray horn and are confined entirely within the CNS. The axons of the column cells form longitudinal ascending tracts that ascend in the white columns and terminate upon neurons located rostrally in the brain stem, cerebellum or diencephalon.
Some column cells send their axons up and down the cord to terminate in gray matter close to their origin and are known as intersegmental association column cells.
Other column cell axons terminate within the segment in which they originate and are called intrasegmental association column cells. Still other column cells send their axons across the midline to terminate in gray matter close to their origin and are called commissure association column cells.
The propriospinal cells are spinal interneurons whose axons do not leave the spinal cord proper. Some of these fibers also are found around the margin of the gray matter of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.
The prominent nuclear groups of cell columns within the spinal cord from dorsal to ventral are the marginal zone, substantia gelatinosa, nucleus proprius, dorsal nucleus of Clarke, intermediolateral nucleus and the lower motor neuron nuclei.
The axons of its neurons contribute to the lateral spinothalamic tract which relays pain and temperature information to the diencephalon Figure 3. Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like portion of the head of the dorsal horn, it relays pain, temperature and mechanical light touch information and consists mainly of column cells intersegmental column cells.
These column cells synapse in cell at Rexed layers IV to VII, whose axons contribute to the ventral anterior and lateral spinal thalamic tracts. The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus. Nucleus proprius is located below the substantia gelatinosa in the head and neck of the dorsal horn. This cell group, sometimes called the chief sensory nucleus, is associated with mechanical and temperature sensations.
It is a poorly defined cell column which extends through all segments of the spinal cord and its neurons contribute to ventral and lateral spinal thalamic tracts, as well as to spinal cerebellar tracts. The axons originating in nucleus proprius project to the thalamus via the spinothalamic tract and to the cerebellum via the ventral spinocerebellar tract VSCT. Dorsal nucleus of Clarke is a cell column located in the mid-portion of the base form of the dorsal horn.
The axons from these cells pass uncrossed to the lateral funiculus and form the dorsal posterior spinocerebellar tract DSCT , which subserve unconscious proprioception from muscle spindles and Golgi tendon organs to the cerebellum, and some of them innervate spinal interneurons.
The dorsal nucleus of Clarke is found only in segments C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar tract CCT.
Intermediolateral nucleus is located in the intermediate zone between the dorsal and the ventral horns in the spinal cord levels. Extending from C8 to L3, it receives viscerosensory information and contains preganglionic sympathetic neurons, which form the lateral horn. A large proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to reach the sympathetic tract as preganglionic fibers.
Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons Figure 3. Lower motor neuron nuclei are located in the ventral horn of the spinal cord. The a motor neurons are the final common pathway of the motor system, and they innervate the visceral and skeletal muscles. The distribution of cells and fibers within the gray matter of the spinal cord exhibits a pattern of lamination.
The cellular pattern of each lamina is composed of various sizes or shapes of neurons cytoarchitecture which led Rexed to propose a new classification based on 10 layers laminae. This classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups Figure 3.
Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn, whereas laminae V and VI are concerned primarily with proprioceptive sensations. Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-IX comprise the ventral horn and contain mainly motor neurons.
The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia. Rexed lamina I — Consists of a thin layer of cells that cap the tip of the dorsal horn with small dendrites and a complex array of nonmyelinated axons.
Cells in lamina I respond mainly to noxious and thermal stimuli. Lamina I cell axons join the contralateral spinothalamic tract; this layer corresponds to nucleus posteromarginalis. Rexed lamina II — Composed of tightly packed interneurons. This layer corresponds to the substantia gelatinosa and responds to noxious stimuli while others respond to non-noxious stimuli. The majority of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells as well as descending dorsolateral fasciculus DLF fibers.
High concentrations of substance P and opiate receptors have been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the effect of determining which pattern of incoming information will produce sensations that will be interpreted by the brain as being painful.
Rexed lamina III — Composed of variable cell size, axons of these neurons bifurcate several times and form a dense plexus. Rexed lamina IV — The thickest of the first four laminae. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond to stimuli such as light touch.
The ill-defined nucleus proprius is located in the head of this layer. Some of the cells project to the thalamus via the contralateral and ipsilateral spinothalamic tract.
Rexed lamina V — Composed neurons with their dendrites in lamina II. This lamina covers a broad zone extending across the neck of the dorsal horn and is divided into medial and lateral parts. Many of the Rexed lamina V cells project to the brain stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells. Rexed lamina VI — Is a broad layer which is best developed in the cervical and lumbar enlargements.
Lamina VI divides also into medial and lateral parts. Group Ia afferent axons from muscle spindles terminate in the medial part at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the small neurons are interneurons participating in spinal reflexes, while descending brainstem pathways project to the lateral zone of Rexed layer VI.
Rexed lamina VII — This lamina occupies a large heterogeneous region. This region is also known as the zona intermedia or intermediolateral nucleus. Its shape and boundaries vary along the length of the cord.
Lamina VII neurons receive information from Rexed lamina II to VI as well as visceral afferent fibers, and they serve as an intermediary relay in transmission of visceral motor neurons impulses. The dorsal nucleus of Clarke forms a prominent round oval cell column from C8 to L3.
The large cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar tract DSCT. Cells in the lateral horn of the cord in segments T1 and L3 give rise to preganglionic sympathetic fibers to innervate postganglionic cells located in the sympathetic ganglia outside the cord. Lateral horn neurons at segments S2 to S4 give rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia.
Rexed lamina VIII — Includes an area at the base of the ventral horn, but its shape differs at various cord levels. In the cord enlargements, the lamina occupies only the medial part of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate.
The neurons of lamina VIII modulate motor activity, most probably via g motor neurons which innervate the intrafusal muscle fibers. Its size and shape differ at various cord levels. Rexed lamina X — Neurons in Rexed lamina X surround the central canal and occupy the commissural lateral area of the gray commissure, which also contains decussating axons.
In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are concerned primarily with proprioceptive sensation and act as a relay between the periphery to the midbrain and the cerebellum.
All visceral motor neurons are located in lamina VII and innervate neurons in autonomic ganglia. Surrounding the gray matter is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information up ascending or down descending the cord.
The white matter is divided into the dorsal or posterior column or funiculus , lateral column and ventral or anterior column Figure 3. The anterior white commissure resides in the center of the spinal cord, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nerve fiber types can be distinguished in the spinal cord white matter: 1 long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2 long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter, and 3 shorter nerve fibers interconnecting various spinal cord levels such as the fibers responsible for the coordination of flexor reflexes.
Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and the anterior columns. Four different terms are often used to describe bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway. Funiculus is a morphological term to describe a large group of nerve fibers which are located in a given area e.
Within a funiculus, groups of fibers from diverse origins, which share common features, are sometimes arranged in smaller bundles of axons called fasciculus, e. Fasciculus is primarily a morphological term whereas tracts and pathways are also terms applied to nerve fiber bundles which have a functional connotation. Nerve plexuses are composed of afferent and efferent fibers that arise from the merging of the anterior rami of spinal nerves and blood vessels.
There are five spinal nerve plexuses—except in the thoracic region—as well as other forms of autonomic plexuses, many of which are a part of the enteric nervous system. The cervical plexus is formed by the ventral rami of the upper four cervical nerves and the upper part of fifth cervical ventral ramus. The network of rami is located deep within the neck. The brachial plexus is formed by the ventral rami of C5—C8 and the T1 spinal nerves, and lower and upper halves of the C4 and T2 spinal nerves.
The plexus extends toward the armpit axilla. The lumbar plexus is formed by the ventral rami of L1—L5 spinal nerves with a contribution of T12 form the lumbar plexus. This plexus lies within the psoas major muscle. The sacral plexus is formed by the ventral rami of L4-S3, with parts of the L4 and S4 spinal nerves. It is located on the posterior wall of the pelvic cavity.
The coccygeal plexus serves a small region over the coccyx and originates from S4, S5, and Co1 spinal nerves. It is interconnected with the lower part of sacral plexus. Brachial plexus : Cervical C5—C8 and thoracic T1 nerves comprise the brachial plexus, which is a nerve plexus that provides sensory and motor function to the shoulders and upper limbs. Lumbar plexus : The lumbar plexus is comprised of the ventral rami of the lumbar spinal nerves L1—L5 and a contribution from thoracic nerve T The posterior green and anterior yellow divisions of the lumbar plexus are shown in the diagram.
The intercostal nerves are part of the somatic nervous system and arise from anterior divisions rami anteriores, ventral divisions of the thoracic spinal nerves T1 to T The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum. Intercostal nerves : An image of the intercostal brachial nerves.
They differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation. These are limited in their distribution to the parietes wall of the thorax. The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic, are confined to the walls of the thorax and are named thoracic intercostal nerves.
Near the sternum, they cross in front of the internal mammary artery and transversus thoracis muscle, pierce the intercostales interni, the anterior intercostal membranes, and pectoralis major, and supply the integument of the front of the thorax and over the mamma, forming the anterior cutaneous branches of the thorax.
The branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus. The seventh intercostal nerve terminates at the xyphoid process, at the lower end of the sternum. The anterior divisions of the seventh, eighth, ninth, tenth, and eleventh thoracic intercostal nerves are continued anteriorly from the intercostal spaces into the abdominal wall; hence they are named thoraco-abdominal nerves or thoracicoabdominal intercostal nerves.
Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system. This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura. This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region.
Damage to the visceral pleura is experienced as an unlocalized ache. A dermatome is an area of skin that is supplied by a single spinal nerve, and a myotome is a group of muscles that a single spinal nerve root innervates. A dermatome is an area of skin that is supplied by a single spinal nerve. There are eight cervical nerves, twelve thoracic nerves, five lumbar nerves and five sacral nerves. Each of these nerves relays sensation, including pain, from a particular region of the skin to the brain.
Dermatomes : Dermatomes are areas of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Dermatomes and the associated major cutaneous nerves are shown here in a ventral view.
Along the thorax and abdomen, the dermatomes are like a stack of discs, with each section supplied by a different spinal nerve. Along the arms and the legs, the pattern is different. The dermatomes run longitudinally along the limbs, so that each half of the limb has a different dermatome. Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints.
Dermatomes have clinical significance, especially in the diagnosis of certain diseases. Symptoms that follow a dermatome, such as pain or a rash, may indicate a pathology that involves the related nerve root. Examples include dysfunction of the spine or a viral infection. Viruses that remain dormant in nerve ganglia, such as the varicella zoster virus that causes both chickenpox and shingles, often cause either pain, rash, or both in a pattern defined by a dermatome. Shingles rash : The shingles rash appears across a dermatome.
In this patient, one of the dermatomes in the arm is affected, restricting the rash to the length of the back of the arm. Shingles is one of the only diseases that causes a rash in a dermatomal pattern, and as such, this is its defining symptom. The rash of shingles is almost always restricted to a specific dermatome, such as on the chest, leg, or arm caused by the residual varicella zoster virus infection of the nerve that supplies that area of skin. Shingles typically appears years or decades after recovery from chickenpox.
A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome. The testing of myotomes provides the clinician with information about the level in the spine where a lesion may be present.
During testing, the clinician looks for muscle weakness of a particular group of muscles. Results may indicate lesions to the spinal cord nerve root, or intervertebral disc herniation that presses on the spinal nerve roots.
The peripheral nervous system PNS consists of the nerves and ganglia outside of the brain and spinal cord. Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood —brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
Spinal nerve : This diagram indicates the formation of a typical spinal nerve from the dorsal and ventral roots. Numbers indicate the types of nerve fibers: 1 somatic efferent, 2 somatic afferent, 3—5 sympathetic efferent, 6—7 sympathetic afferent. The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body.
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs C1—C8 , 12 thoracic pairs T1—T12 , 5 lumbar pairs L1—L5 , 5 sacral pairs S1—S5 , and 1 coccygeal pair.
The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic.
For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus. The spinal cord is one of the two major components of the central nervous system the other being the brain ; its proper functioning is absolutely essential to a healthy nervous system.
The spinal cord contains motor neurons that innervate skeletal muscle and allow for movement as well as motor tracts that carry directives for motor movement down from the brain. The spinal cord also receives all of the sensory information from the periphery of our bodies, and contains pathways by which that sensory information is passed along to the brain. Motor neurons leave the cord in collections of nerves called ventral rootlets, which then coalesce to form a ventral root. Sensory information is carried by sensory neurons in dorsal roots , which enter the cord in small bundles called dorsal rootlets.
The cell bodies for these sensory neurons are clustered together in a structure called the dorsal root ganglion , which is found alongside the spinal cord. The ventral root and dorsal root come together just beyond the dorsal root ganglion moving away from the cord to form a spinal nerve. Spinal nerves travel to the periphery of the body; there are 31 pairs of spinal nerves in total.
Each area of the spinal cord from which a spinal nerve leaves is considered a segment and there are 31 segments in the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
The spinal cord terminates in a cone-shaped structure called the conus medullaris , which is usually found at around the first or second lumbar vertebrae L1-L2. However, the spinal cord like the brain is surrounded by protective membranes known as the meninges , and the meningeal layers known as the dura mater and arachnoid mater continue for several more segments to about the second sacral vertebrae beyond the end of the cord itself.
Because this extension of the meningeal covering of the cord--sometimes referred to as the dural sheath --continues past the end the cord, it creates a cerebrospinal fluid -filled cavity known as the lumbar cistern where there is no cord present.
Additionally, although the conus medullaris is found at around L2, there are still several pairs of spinal nerves that must travel to the lower half of the body from the final segments of the cord. These nerves travel through the lumbar cistern; the straggly collection of fibers here is referred to as the cauda equina because it resembles a horse's tail. Cerebrospinal fluid is often taken from the lumbar cistern if it needs to be sampled for testing e.
This procedure is known as a lumbar puncture or spinal tap; it is done from the lumbar cistern because there is little risk of damaging the spinal cord by inserting a needle there since the cord is not present at that level of the vertebral canal. The spinal cord is attached to the end of the dural sheath by a thin extension of the pia mater known as the filum terminale.
The filum terminale also extends from the end of the dural sheath to attach the spinal cord to the tailbone. In both cases, the filum terminale helps to anchor the cord in place. When you look at the spinal cord in cross-section at any level you will see what some describe as an H-shaped or a butterfly-shaped area of grey matter surrounded by white matter. The grey matter consists of cell bodies of motor and sensory neurons, and is divided into three regions.
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