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