Chapter 3: Anatomy that the Spinal Cord
Nachum Dafny, Ph.D., room of Neurobiology and Anatomy, The UT medical School at Houston Reviewed and revised 07 Oct 2020
Figure 3.1 Schematic dorsal and lateral check out of the spinal cord and also four overcome sections native cervical, thoracic, lumbar and also sacral levels, respectively.
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The spinal cord is the most important structure between the body and the brain. The spinal cord extends indigenous the foramen magnum whereby it is consistent with the medulla come the level of the very first or 2nd lumbar vertebrae. It is a vital link between the brain and the body, and from the human body to the brain. The spinal cord is 40 to 50 cm long and also 1 cm to 1.5 cm in diameter. 2 consecutive rows of nerve roots emerge on every of its sides. This nerve roots sign up with distally to form 31 pairs of spinal nerves. The spinal cord is a cylindrical framework of concerned tissue written of white and also gray matter, is uniformly organized and also is split into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), (Figure 3.1), each of which is made up of several segments. The spinal nerve has motor and sensory nerve fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome (see below and Figure 3.5).
3.2 basic Featuressimilar cross-sectional structures at all spinal cord level (Figure 3.1). the carries sensory information (sensations) native the body and also some native the head to the main nervous device (CNS) via afferent fibers, and also it performs the initial processing of this information. engine neurons in the ventral horn project their axons into the periphery to innervate skeletal and also smooth muscles the mediate voluntary and involuntary reflexes. It contains neurons who descending axons mediate autonomic manage for most of the visceral functions. it is of an excellent clinical importance since it is a major site the traumatic injury and also the locus for many condition processes.
Although the spinal cord constitutes only about 2% of the central nervous device (CNS), its features are vital. Knowledge of spinal cord functional anatomy provides it possible to diagnose the nature and also location of cord damage and also many cord diseases.
3.3 Segmental and also Longitudinal Organization
The spinal cord is separated into four various regions: the cervical, thoracic, lumbar and sacral areas (Figure 3.1). The various cord regions have the right to be visually differentiated from one another. 2 enlargements the the spinal cord deserve to be visualized: The cervical enlargement, i m sorry extends in between C3 to T1; and the lumbar enlargements i beg your pardon extends between L1 come S2 (Figure 3.1).
The cord is segmentally organized. There space 31 segments, characterized by 31 pairs of nerves exiting the cord. These nerves are split into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve (Figure 3.2). Dorsal and also ventral roots enter and leave the vertebral obelisk respectively v intervertebral foramen in ~ the vertebral segments matching to the spinal segment.
Figure 3.2 illustration of the 8, 12, 5, 5 and 1 cervical, thoracic, lumbar, sacral and coccygeal spinal nerves and their leave from the vertebrate, respectively.
The cord is sheathed in the very same three meninges together is the brain: the pia, arachnoid and also dura. The dura is the hard outer sheath, the arachnoid lies in ~ it, and also the pia closely adheres to the surface of the cord (Figure 3.3). The spinal cord is attached to the dura by a collection of lateral denticulate ligaments create from the pial folds.
Figure 3.3 The 3 spinal cord meninges. The denticulate ligament, the dorsal source ganglion (A), and also an enlarged drawing of the meninges (B).
During the initial third month of embryonic development, the spinal cord extend the whole length the the vertebral canal and also both prosper at about the exact same rate. As advance continues, the body and also the vertebral column proceed to thrive at a much better rate 보다 the spinal cord proper. This outcomes in displacement of the lower parts of the spinal cord v relation come the vertebrae column. The result of this uneven growth is that the adult spinal cord extends to the level the the very first or 2nd lumbar vertebrae, and the nerves thrive to exit through the exact same intervertebral foramina together they did during embryonic development. This expansion of the nerve roots arising within the vertebral canal, outcomes in the lumbar, sacral, and also coccygeal roots expanding to their suitable vertebral levels (Figure 3.2).
All spinal nerves, other than the first, exit below their equivalent vertebrae. In the cervical segments, there are 7 cervical vertebrae and 8 cervical nerves (Figure 3.2). C1-C7 nerves exit above their vertebrae vice versa, the C8 nerve exits listed below the C7 vertebra. The leaves between the C7 vertebra and the very first thoracic vertebra. Therefore, each subsequent nerve leaves the cord listed below the equivalent vertebra. In the thoracic and also upper lumbar regions, the difference in between the vertebrae and also cord level is three segments. Therefore, the source filaments the spinal cord segments have to travel longer distances to with the equivalent intervertebral foramen native which the spinal nerves emerge. The lumbosacral root are known as the cauda equina (Figure 3.2).
Each spinal nerve is composed of nerve fibers that are concerned the region of the muscles and skin that creates from one human body somite (segment). A spinal segment is defined by dorsal roots entering and also ventral root exiting the cord, (i.e., a spinal cord section that provides rise come one spinal nerve is thought about as a segment.) (Figure 3.4).
Figure 3.4 (A) illustration of the spinal cord through its spinal roots. (B) illustration of the spinal vertebrate. (C) ar of the spinal cord, that is meninges and also the dorsal and ventral root of three segments.
A dermatome is one area that skin provided by peripheral nerve fibers originating from a single dorsal source ganglion. If a nerve is cut, one loses sensation from the dermatome. Due to the fact that each segment of the cord innervates a different region of the body, dermatomes deserve to be specifically mapped top top the body surface, and also loss of sensation in a dermatome deserve to indicate the specific level of spinal cord damages in clinical assessment of injury (Figure 3.5). That is important to take into consideration that there is part overlap in between neighboring dermatomes. Due to the fact that sensory details from the human body is relayed come the CNS v the dorsal roots, the axons originating native dorsal root ganglion cells are classified as major sensory afferents, and also the dorsal root"s neurons are the first order (1°) sensory neuron. Most axons in the ventral roots arise from motor neurons in the ventral horn that the spinal cord and also innervate skeletal muscle. Rather arise native the lateral horn and also synapse ~ above autonomic ganglia that innervate visceral organs. The ventral source axons join with the peripheral procedures of the dorsal source ganglion cell to kind mixed afferent and efferent spinal nerves, which merge to form peripheral nerves. Knowledge of the segmental innervation the the cutaneous area and the muscles is crucial to diagnose the site of an injury.
Figure 3.5 Innervation occurring from solitary dorsal root ganglion supplied details skin area (a dermatome). The numbers refer to the spinal segments through which each nerve is named C = cervical; T = thoracic; l = lumbar; S = sacral spinal cord segments (dermatome).
3.4 interior Structure of the Spinal Cord
A transverse ar of the adult spinal cord mirrors white matter in the periphery, gray issue inside, and also a tiny main canal filled through CSF at its center. Neighboring the canal is a single layer of cells, the ependymal layer. Bordering the ependymal class is the gray matter – a an ar containing cabinet bodies – shaped choose the letter “H” or a “butterfly”. The 2 “wings” the the butterfly room connected across the midline by the dorsal gray commissure and also below the white commissure (Figure 3.6). The shape and size that the gray issue varies follow to spinal cord level. At the reduced levels, the ratio in between gray matter and also white issue is higher than in higher levels, mainly due to the fact that lower level contain less ascending and also descending nerve fibers. (Figure 3.1 and Figure 3.6).
The gray issue mainly includes the cabinet bodies that neurons and also glia and also is split into 4 main columns: dorsal horn, intermediate column, lateral horn and ventral horn column. (Figure 3.6).
The dorsal horn is discovered at every spinal cord levels and also is comprised of sensory nuclei that obtain and process incoming somatosensory information. From there, ascending projections emerge to transmit the sensory info to the midbrain and also diencephalon. The intermediary column and also the lateral horn make up autonomic neurons innervating visceral and also pelvic organs. The ventral horn comprises motor neurons that innervate bones muscle.
At every the levels of the spinal cord, nerve cells in the gray substance space multipolar, varying lot in their morphology. Plenty of of them are Golgi kind I and also Golgi kind II nerve cells. The axons of Golgi form I are long and pass the end of the gray matter right into the ventral spinal root or the fiber tracts of the white matter. The axons and also dendrites the the Golgi type II cells are mainly confined come the neighboring neurons in the gray matter.
A much more recent group of neurons within the gray issue is based upon function. This cells are located at all levels of the spinal cord and also are grouped right into three key categories: source cells, obelisk or tract cells and propriospinal cells.
The source cells are situated in the ventral and lateral gray horns and also vary greatly in size. The most prominent functions of the root cells are large multipolar elements exceeding 25 µm of their somata. The source cells contribute their axons to the ventral roots of the spinal nerves and also are grouped right into two significant divisions: 1) somatic efferent source neurons, which innervate the skeleton musculature; and also 2) the visceral efferent root neurons, likewise called preganglionic autonomic axons, i m sorry send your axons to various autonomic ganglia.
The column or tract cells and also their processes are located mainly in the dorsal gray horn and are confined completely within the CNS. The axons the the pillar cells kind longitudinal ascending tracts the ascend in the white columns and also terminate upon neurons situated rostrally in the brain stem, cerebellum or diencephalon. Some column cells send your axons up and also down the cord to terminate in gray issue close to your origin and also are well-known as intersegmental association shaft cells. Other column cell axons terminate in ~ the segment in which castle originate and also are called intrasegmental association shaft cells. Tho other shaft cells send your axons throughout the midline to end in gray issue close to their origin and also are referred to as commissure association column cells.
The propriospinal cells are spinal interneurons who axons perform not leave the spinal cord proper. Propriospinal cell account for about 90% the spinal neurons. Several of these fibers also are found about the margin that the gray matter of the cord and are jointly called the fasciculus proprius or the propriospinal or the archispinothalamic tract.
3.5 Spinal Cord Nuclei and also Laminae
Spinal neurons are organized right into nuclei and also laminae.
The significant nuclear teams of cabinet columns within the spinal cord indigenous dorsal to ventral space the marginal zone, substantia gelatinosa, cell core proprius, dorsal nucleus of Clarke, intermediolateral nucleus and the reduced motor neuron nuclei.
Marginal zone cell core or posterior marginalis, is uncovered at all spinal cord levels as a slim layer that column/tract cells (column cells) that caps the guideline of the dorsal horn. The axons of its neurons contribute to the lateral spinothalamic street which relays pain and also temperature info to the diencephalon (Figure 3.7).
Substantia gelatinosa is uncovered at every levels that the spinal cord. Situated in the dorsal cap-like section of the head that the dorsal horn, it relays pain, temperature and mechanical (light touch) information and consists mainly of tower cells (intersegmental pillar cells). These obelisk cells synapse in cell at Rexed layers IV come VII, who axons add 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 listed below the substantia gelatinosa in the head and neck of the dorsal horn. This cabinet group, sometimes called the cook sensory nucleus, is connected with mechanical and also temperature sensations. That is a poorly identified cell tower which extends with all segments of the spinal cord and also its neurons add to ventral and also lateral spinal thalamic tracts, and to spinal cerebellar tracts. The axons originating in cell nucleus proprius project to the thalamus via the spinothalamic tract and to the cerebellum via the ventral spinocerebellar street (VSCT).
Dorsal cell core of Clarke is a cabinet column situated in the mid-portion that the base type of the dorsal horn. The axons from these cells happen uncrossed to the lateral funiculus and type the dorsal (posterior) spinocerebellar street (DSCT), i beg your pardon subserve unconscious proprioception from muscle spindles and also Golgi tendon organs to the cerebellum, and also some of them innervate spinal interneurons. The dorsal nucleus of Clarke is uncovered only in segments C8 come L3 of the spinal cord and is most influential in reduced thoracic and upper lumbar segments. The homologous dorsal cell nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar street (CCT).
Intermediolateral cell core is located in the intermediary zone in between the dorsal and the ventral horns in the spinal cord levels. Prolonging from C8 come L3, the receives viscerosensory information and contains preganglionic forgiveness neurons, which type the lateral horn. A large proportion of its cells space root cells which send axons into the ventral spinal root via the white rami to with the sympathetic tract together preganglionic fibers. Similarly, cabinet columns in the intermediolateral nucleus situated at the S2 come S4 levels contains preganglionic parasympathetic neurons (Figure 3.7).
Lower motor neuron nuclei are located in the ventral horn the the spinal cord. Lock contain primarily motor nuclei consist of of α, β and also γ motor neurons and are uncovered at every levels the the spinal cord--they space root cells. The a motor neurons are the final usual pathway the the motor system, and also they innervate the visceral and also skeletal muscles.
3.7 Rexed Laminae
The distribution of cells and fibers within the gray issue of the spinal cord exhibits a pattern of lamination. The cellular pattern of each lamina is created of miscellaneous sizes or shapes of neurons (cytoarchitecture) i m sorry led Rexed to suggest a brand-new classification based upon 10 layers (laminae). This classification is useful since it is related much more accurately to function than the previous classification scheme which was based on major nuclear groups (Figure 3.7).
Laminae i to IV, in general, are involved with exteroceptive sensation and comprise the dorsal horn, conversely, laminae V and also VI are pertained to primarily v proprioceptive sensations. Lamina VII is equivalent to the intermediary zone and acts together a relay between muscle spindle come midbrain and also cerebellum, and laminae VIII-IX consist of the ventral horn and also contain mostly motor neurons. The axons of these neurons innervate mostly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia.
Rexed lamina ns – is composed of a thin layer the cells that cap the pointer of the dorsal horn with little dendrites and a complicated array of nonmyelinated axons. Cells in lamina ns respond mostly to noxious and thermal stimuli. Lamina ns cell axons join the contralateral spinothalamic tract; this layer corresponds to cell nucleus posteromarginalis.
Rexed lamina II – written of tightly pack interneurons. This layer synchronizes to the substantia gelatinosa and responds come noxious stimuli while others respond to non-noxious stimuli. The bulk of neurons in Rexed lamina II axons receive information from sensory dorsal source ganglion cells and also descending dorsolateral fasciculus (DLF) fibers. Castle send axons come Rexed laminae III and also IV (fasciculus proprius). High concentrations of substance P and opiate receptors have actually been identified in Rexed lamina II. The lamina is thought to be vital for the modulation the sensory input, through the effect of determining which pattern of incoming information will produce sensations that will be construed by the brain as gift painful.
Rexed lamina III – created of variable cabinet size, axons of this neurons bifurcate several times and kind a thick plexus. Cells in this layer obtain axodendritic synapses from Aβ yarn entering dorsal root fibers. It contains dendrites of cell from laminae IV, V and also VI. Many of the neurons in lamina III role as propriospinal/interneuron cells.
Rexed lamina IV – The thickest the the first four laminae. Cell in this layer receive Aß axons which lug predominantly non-noxious information. In addition, dendrites of neurons in lamina IV radiate come lamina II, and also respond come stimuli such together light touch. The ill-defined cell core proprius is located in the head that this layer. Some of the cells task to the thalamus via the contralateral and ipsilateral spinothalamic tract.
Rexed lamina V – composed neurons with their dendrites in lamina II. The neurons in this lamina receive monosynaptic details from Aß, ad and C axons which also carry nociceptive details from visceral organs. This lamina covers a vast zone extending across the neck of the dorsal horn and is separated into medial and also lateral parts. Countless of the Rexed lamina V cells project to the brain stem and also the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and also rubrospinal fibers synapse top top its cells.
Rexed lamina using – Is a large layer i beg your pardon is best occurred in the cervical and also lumbar enlargements. Lamina by means of divides additionally into medial and also lateral parts. Group Ia afferent axons native muscle spindles terminate in the medial component at the C8 to L3 segmental levels and are the resource of the ipsilateral spinocerebellar pathways. Numerous of the small neurons are interneurons participating in spinal reflexes, if descending brainstem pathways task to the lateral zone that Rexed layer VI.
Rexed lamina VII – This lamina occupies a big heterogeneous region. This region is additionally known together the zona intermedia (or intermediolateral nucleus). The shape and also boundaries differ along the length of the cord. Lamina VII neurons receive information from Rexed lamina II to VI and visceral afferent fibers, and they serve as one intermediary relay in transmission of visceral motor neurons impulses. The dorsal nucleus of Clarke creates a significant round oval cell column from C8 to L3. The huge cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar street (DSCT). Cells in laminae V come VII, which do not kind a discrete nucleus, give rise to uncrossed fibers that kind the ventral spinocerebellar tract (VSCT). Cell in the lateral horn that the cord in segment T1 and also L3 provide rise come preganglionic sympathetic yarn to innervate postganglionic cells located in the sorry ganglia outside the cord. Lateral horn neurons at segment S2 come S4 offer rise come preganglionic neurons of the sacral parasympathetic yarn to innervate postganglionic cells situated in peripheral ganglia.
Rexed lamina VIII – has an area at the base of the ventral horn, but its form differs at assorted cord levels. In the cord enlargements, the lamina occupies only the medial component of the ventral horn, wherein descending vestibulospinal and also reticulospinal yarn terminate. The neurons that lamina VIII modulate engine activity, most more than likely via g engine neurons i m sorry innervate the intrafusal muscle fibers.
Rexed lamina IX – written of several distinct groups of huge a motor neurons and little γ and β motor neurons installed within this layer. That size and also shape differ at assorted cord levels. In the cord enlargements the variety of α engine neurons increase and also they form numerous groups. The α motor neurons are big and multipolar cells and also give increase to ventral source fibers to it is provided extrafusal skeleton muscle fibers, when the tiny γ engine neurons give rise to the intrafusal muscle fibers. The α motor neurons room somatotopically organized.
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Rexed lamina X – Neurons in Rexed lamina X surround the central canal and occupy the commissural lateral area the the gray commissure, which also contains decussating axons.
In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are pertained to primarily v proprioceptive sensation and act as a relay in between the perimeter to the midbrain and the cerebellum. Laminae VIII and IX type the last motor pathway to initiate and also modulate motor activity via α, β and also γ motor neurons, i beg your pardon innervate striated muscle. Every visceral motor neurons are located in lamina VII and also innervate neurons in autonomic ganglia.
3.8 White Matter
bordering the gray issue is white issue containing myelinated and unmyelinated nerve fibers. This fibers conduct details up (ascending) or down (descending) the cord. The white issue is split into the dorsal (or posterior) obelisk (or funiculus), lateral column and ventral (or anterior) pillar (Figure 3.8). The anterior white commissure lives in the facility 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 varieties can be differentiated in the spinal cord white matter: 1) lengthy ascending nerve fibers initially from the pillar cells, which do synaptic relationships to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2) long descending nerve fibers originating native the cerebral cortex and also various brainstem nuclei come synapse in ~ the different Rexed layers in the spinal cord gray matter, and 3) much shorter nerve yarn interconnecting miscellaneous spinal cord level such as the fibers responsible because that the coordination of flexor reflexes. Ascending tracts are discovered in all columns whereas descending tracts are uncovered only in the lateral and the anterior columns.