Chapter 3: Anatomy that the Spinal Cord

Nachum Dafny, Ph.D., department of Neurobiology and also Anatomy, McGovern clinical School the evaluation 07 Oct 2020

Figure 3.1 Schematic dorsal and also lateral view of the spinal cord and four cross sections native cervical, thoracic, lumbar and also sacral levels, respectively.

You are watching: A posterior and anterior root of each spinal segment unite to form a

The spinal cord is the most vital structure between the body and also the brain. The spinal cord extends native the foramen magnum where it is constant with the medulla come the level that the first or 2nd lumbar vertebrae. It is a vital link in between the mind and the body, and also from the human body to the brain. The spinal cord is 40 to 50 cm long and also 1 cm to 1.5 centimeter in diameter. 2 consecutive rows the nerve roots arise on each of that is sides. This nerve roots join distally to kind 31 bag of spinal nerves. The spinal cord is a cylindrical framework of worried tissue composed of white and also gray matter, is uniformly organized and also is separated into 4 regions: cervical (C), thoracic (T), lumbar (L) and also sacral (S), (Figure 3.1), every of i beg your pardon is made up of several segments. The spinal nerve consists of motor and sensory nerve fibers to and from all parts of the body. Every spinal cord segment innervates a dermatome (see below and Figure 3.5).

3.2 basic Features

similar cross-sectional frameworks at every spinal cord level (Figure 3.1). that carries sensory information (sensations) from the body and also some native the head come the main nervous system (CNS) via afferent fibers, and it performs the initial processing of this information. motor 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 has neurons who descending axons adjust autonomic manage for many of the visceral functions. the is of an excellent clinical importance because it is a major site that traumatic injury and the locus because that many condition processes.

Although the spinal cord constitutes only about 2% of the main nervous system (CNS), its attributes are vital. Knowledge that spinal cord functional anatomy makes it feasible to diagnose the nature and also location that cord damage and many cord diseases.

3.3 Segmental and Longitudinal Organization

The spinal cord is separated into four various regions: the cervical, thoracic, lumbar and sacral regions (Figure 3.1). The different cord regions deserve to be visually distinguished from one another. Two enlargements of the spinal cord deserve to be visualized: The cervical enlargement, i m sorry extends in between C3 come T1; and also the lumbar enlargements i m sorry extends between L1 come S2 (Figure 3.1).

The cord is segmentally organized. There are 31 segments, identified 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 ventral root enter and leave the vertebral shaft respectively v intervertebral foramen at the vertebral segments equivalent to the spinal segment.

Figure 3.2 illustration of the 8, 12, 5, 5 and also 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 as 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 ar of the cord (Figure 3.3). The spinal cord is attached come the dura through a collection of lateral denticulate ligaments emanating 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 that embryonic development, the spinal cord extends the whole length of the vertebral canal and also both thrive at around the same rate. As development continues, the body and the vertebral column continue to thrive at a much higher rate than the spinal cord proper. This outcomes in displacement the the lower parts that the spinal cord v relation come the vertebrae column. The result of this uneven development is the the adult spinal cord extends come the level that the an initial or 2nd lumbar vertebrae, and the nerves grow to leave through the exact same intervertebral foramina together they did during embryonic development. This development of the nerve roots arising within the vertebral canal, outcomes in the lumbar, sacral, and coccygeal roots prolonging to their appropriate vertebral level (Figure 3.2).

All spinal nerves, except the first, exit listed below their equivalent vertebrae. In the cervical segments, there room 7 cervical vertebrae and also 8 cervical nerves (Figure 3.2). C1-C7 nerves exit over their vertebrae conversely, the C8 nerve exits below the C7 vertebra. That leaves in between the C7 vertebra and the very first thoracic vertebra. Therefore, each subsequent nerve pipeline the cord listed below the matching vertebra. In the thoracic and upper lumbar regions, the difference between the vertebrae and cord level is 3 segments. Therefore, the root filaments of spinal cord segments need to travel longer ranges to reach the equivalent intervertebral foramen indigenous 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 the are pertained to the region of the muscles and skin that establishes from one human body somite (segment). A spinal segment is defined by dorsal roots entering and also ventral roots exiting the cord, (i.e., a spinal cord section that provides rise come one spinal nerve is considered as a segment.) (Figure 3.4).

Figure 3.4 (A) drawing of the spinal cord v its spinal roots. (B) illustration of the spinal vertebrate. (C) section of the spinal cord, the meninges and also the dorsal and also ventral root of 3 segments.

A dermatome is one area of skin supplied by peripheral nerve yarn originating indigenous a single dorsal root ganglion. If a nerve is cut, one loses sensation from that dermatome. Since each segment of the cord innervates a different an ar of the body, dermatomes can be specifically mapped top top the body surface, and loss of emotion in a dermatome have the right to indicate the precise level of spinal cord damage in clinical assessment of injury (Figure 3.5). That is necessary to take into consideration that there is some overlap between neighboring dermatomes. Because sensory information from the body is relayed come the CNS v the dorsal roots, the axons originating indigenous dorsal source ganglion cells are classified as primary sensory afferents, and also the dorsal root"s neurons space the very first order (1°) sensory neuron. Many axons in the ventral roots arise from engine neurons in the ventral horn of the spinal cord and also innervate skeletal muscle. Rather arise indigenous the lateral horn and synapse on autonomic ganglia that innervate visceral organs. The ventral root axons sign up with with the peripheral procedures of the dorsal root ganglion cell to kind mixed afferent and efferent spinal nerves, which unify to type peripheral nerves. Knowledge of the segmental innervation that the cutaneous area and also the muscles is essential to diagnose the website of an injury.

Figure 3.5 Innervation occurring from single dorsal source ganglion supplied particular skin area (a dermatome). The numbers refer to the spinal segments by which each nerve is called C = cervical; T = thoracic; together = lumbar; S = sacral spinal cord segments (dermatome).

3.4 inner Structure the the Spinal Cord

A transverse section of the adult spinal cord reflects white matter in the periphery, gray issue inside, and a tiny main canal filled with CSF in ~ its center. Neighboring the canal is a single layer the cells, the ependymal layer. Bordering the ependymal layer is the gray matter – a an ar containing cabinet bodies – shaped favor the letter “H” or a “butterfly”. The two “wings” of the butterfly room connected throughout the midline by the dorsal gray commissure and also below the white commissure (Figure 3.6). The shape and size of the gray matter varies according to spinal cord level. At the lower levels, the ratio between gray matter and also white matter is better than in higher levels, mainly because lower level contain much less ascending and also descending nerve fibers. (Figure 3.1 and also Figure 3.6).

The gray issue mainly includes the cabinet bodies of neurons and also glia and is separated into four main columns: dorsal horn, intermediate column, lateral horn and ventral horn column. (Figure 3.6).

The dorsal horn is found at every spinal cord levels and also is made up of sensory nuclei that obtain and procedure incoming somatosensory information. From there, ascending projections arise to transmit the sensory information to the midbrain and diencephalon. The intermediate column and also the lateral horn consist of autonomic neurons innervating visceral and also pelvic organs. The ventral horn comprises engine neurons that innervate skeletal muscle.

At all the levels of the spinal cord, nerve cells in the gray substance room multipolar, varying much in their morphology. Countless of them are Golgi kind I and Golgi form II nerve cells. The axons the Golgi kind I space long and also pass the end of the gray matter into the ventral spinal roots or the fiber tracts the the white matter. The axons and dendrites of the Golgi kind II cells are greatly confined to the neighboring neurons in the gray matter.

A much more recent category of neurons in ~ the gray issue is based on function. This cells are situated at all levels of the spinal cord and also are grouped into three key categories: source cells, tower or street cells and propriospinal cells.

The root cells are positioned in the ventral and also lateral gray horns and vary considerably in size. The many prominent features of the root cells are large multipolar elements exceeding 25 µm of your somata. The root cells add their axons come the ventral roots of the spinal nerves and also are grouped right into two significant divisions: 1) somatic efferent root neurons, i beg your pardon innervate the bones musculature; and 2) the visceral efferent root neurons, likewise called preganglionic autonomic axons, which send their axons to various autonomic ganglia.

The column or tract cells and also their procedures are located mainly in the dorsal gray horn and also are confined completely within the CNS. The axons the the tower cells type longitudinal ascending tracts that ascend in the white columns and also terminate upon neurons located rostrally in the brain stem, cerebellum or diencephalon. Some obelisk cells send your axons up and down the cord to terminate in gray issue close to their origin and also are well-known as intersegmental association obelisk cells. Other column cell axons terminate in ~ the segment in which they originate and are called intrasegmental association column cells. Tho other pillar cells send their axons throughout the midline to end in gray matter close to your origin and also are called commissure association tower cells.

The propriospinal cells are spinal interneurons who axons execute not leave the spinal cord proper. Propriospinal cells account for about 90% that spinal neurons. Few of these fibers additionally are found approximately the margin that the gray issue of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.

3.5 Spinal Cord Nuclei and Laminae

Spinal neurons are organized into nuclei and laminae.

3.6 Nuclei

The significant nuclear groups of cell columns within the spinal cord native dorsal to ventral are 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 every spinal cord levels together a slim layer that column/tract cells (column cells) that caps the reminder of the dorsal horn. The axons the its neurons add to the lateral spinothalamic tract which relays pain and also temperature info to the diencephalon (Figure 3.7).

Substantia gelatinosa is found at every levels of the spinal cord. Situated in the dorsal cap-like portion of the head of the dorsal horn, that relays pain, temperature and mechanical (light touch) information and consists mostly of column cells (intersegmental pillar cells). These shaft cells synapse in cabinet at Rexed class IV to VII, who 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 dubbed the cook sensory nucleus, is connected with mechanical and temperature sensations. That is a poorly defined cell shaft which extends v all segments of the spinal cord and also its neurons add to ventral and also lateral spinal thalamic tracts, and also to spinal cerebellar tracts. The axons originating in cell nucleus proprius task 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 of the base form of the dorsal horn. The axons from this cells pass uncrossed to the lateral funiculus and type the dorsal (posterior) spinocerebellar tract (DSCT), which subserve unconscious proprioception indigenous muscle spindles and also Golgi tendon organs come the cerebellum, and also some of them innervate spinal interneurons. The dorsal cell core of Clarke is discovered only in segment C8 to L3 the the spinal cord and also is most influential in reduced thoracic and upper lumbar segments. The homologous dorsal cell core of Clarke in the medulla is the accessory cuneate nucleus, i m sorry is the beginning of the cuneocerebellar street (CCT).

Intermediolateral nucleus is situated in the intermediary zone in between the dorsal and the ventral horns in the spinal cord levels. Extending from C8 to L3, the receives viscerosensory information and also contains preganglionic sorry neurons, which kind the lateral horn. A big proportion of its cells space root cell which send axons into the ventral spinal roots via the white rami to reach the sorry tract together preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus situated at the S2 come S4 levels has preganglionic parasympathetic neurons (Figure 3.7).

Lower motor neuron nuclei are located in the ventral horn of the spinal cord. They contain predominantly motor nuclei consisting of α, β and γ engine neurons and are discovered at every levels of the spinal cord--they room root cells. The a engine neurons room the final common pathway that the engine system, and also 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 pattern of every lamina is composed of miscellaneous sizes or forms of neurons (cytoarchitecture) i beg your pardon led Rexed to propose a brand-new classification based on 10 layers (laminae). This group is useful because it is related more accurately to role than the previous category scheme which to be based on major nuclear groups (Figure 3.7).

Laminae i to IV, in general, are came to with exteroceptive sensation and also comprise the dorsal horn, conversely, laminae V and also VI are concerned primarily through proprioceptive sensations. Lamina VII is identical to the intermediary zone and acts as a relay in between muscle spindle come midbrain and cerebellum, and laminae VIII-IX make up the ventral horn and also contain largely motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounding the main canal and contains neuroglia.

Rexed lamina i – is composed of a slim layer that cells that cap the tip of the dorsal horn with little dendrites and a complicated array that nonmyelinated axons. Cells in lamina ns respond greatly to noxious and also thermal stimuli. Lamina ns cell axons sign up with the contralateral spinothalamic tract; this layer synchronizes to cell nucleus posteromarginalis.

Rexed lamina II – written of tightly pack interneurons. This layer corresponds to the substantia gelatinosa and also responds come noxious stimuli while rather respond come non-noxious stimuli. The majority of neurons in Rexed lamina II axons receive details from sensory dorsal source ganglion cells and also descending dorsolateral fasciculus (DLF) fibers. Castle send axons to Rexed laminae III and IV (fasciculus proprius). High concentrations of substance P and also opiate receptors have been determined in Rexed lamina II. The lamina is thought to be vital for the modulation that sensory input, v the impact of determining which pattern of incoming info will produce sensations that will certainly be taken by the mind as being painful.

Rexed lamina III – composed of variable cabinet size, axons of this neurons bifurcate number of times and type a thick plexus. Cells in this layer get axodendritic synapses from Aβ yarn entering dorsal source fibers. It has dendrites of cell from laminae IV, V and VI. Most of the neurons in lamina III role as propriospinal/interneuron cells.

Rexed lamina IV – The thickest that the first four laminae. Cell in this layer obtain Aß axons which carry predominantly non-noxious information. In addition, dendrites that neurons in lamina IV radiate come lamina II, and respond to stimuli such together light touch. The ill-defined cell nucleus proprius is situated in the head that this layer. Few of the cells job 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 info from Aß, ad and C axons which likewise carry nociceptive details from visceral organs. This lamina covers a broad zone extending throughout the neck that the dorsal horn and is divided into medial and also lateral parts. Countless of the Rexed lamina V cells job to the brain stem and also the thalamus via the contralateral and also ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells.

Rexed lamina by means of – Is a large layer which is best arisen in the cervical and also lumbar enlargements. Lamina using divides also into medial and lateral parts. Group Ia afferent axons indigenous muscle spindles end in the medial component at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the little neurons space interneurons participating in spinal reflexes, while descending brainstem pathways job to the lateral zone that Rexed layer VI.

Rexed lamina VII – This lamina occupies a big heterogeneous region. This region is likewise known together the zona intermedia (or intermediolateral nucleus). The shape and boundaries vary along the length of the cord. Lamina VII neurons receive information from Rexed lamina II come VI as well as visceral afferent fibers, and they serve as an intermediary relay in transmission of visceral engine neurons impulses. The dorsal cell core of Clarke creates a influential round oval cell obelisk from C8 to L3. The large cells offer rise to uncrossed nerve yarn of the dorsal spinocerebellar street (DSCT). Cells in laminae V to VII, which carry out not kind a discrete nucleus, offer rise come uncrossed fibers that kind the ventral spinocerebellar street (VSCT). Cells in the lateral horn the the cord in segment T1 and L3 offer rise to preganglionic sympathetic yarn to innervate postganglionic cells situated in the sorry ganglia exterior the cord. Lateral horn neurons at segment S2 come S4 give rise to preganglionic neurons that 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, however its form differs at miscellaneous cord levels. In the cord enlargements, the lamina occupies only the medial part of the ventral horn, whereby descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate engine activity, most probably via g engine neurons i m sorry innervate the intrafusal muscle fibers.

Rexed lamina IX – written of several distinctive groups of large a motor neurons and tiny γ and also β engine neurons embedded within this layer. Its size and shape different at various cord levels. In the cord enlargements the number of α motor neurons increase and also they kind numerous groups. The α engine neurons are large and multipolar cells and also give climb to ventral source fibers to supply extrafusal bones muscle fibers, if the little γ motor neurons give rise come the intrafusal muscle fibers. The α motor neurons are somatotopically organized.

See more: Molar Heat Of Fusion Of Water, Heat Of Fusion And Vaporization

Rexed lamina X – Neurons in Rexed lamina X surround the main canal and occupy the commissural lateral area the the gray commissure, which additionally contains decussating axons.

In summary, laminae I-IV are pertained to with exteroceptive sensations, whereas laminae V and also VI are came to primarily through proprioceptive sensation and also act as a relay in between the perimeter to the midbrain and also the cerebellum. Laminae VIII and also IX kind the final motor pathway to initiate and also modulate motor task via α, β and γ engine neurons, i beg your pardon innervate striated muscle. All visceral engine neurons are situated in lamina VII and also innervate neurons in autonomic ganglia.

3.8 White Matter

neighboring the gray matter is white matter containing myelinated and unmyelinated nerve fibers. This fibers conduct information up (ascending) or down (descending) the cord. The white issue is divided into the dorsal (or posterior) shaft (or funiculus), lateral column and also ventral (or anterior) tower (Figure 3.8). The anterior white commissure lives in the facility of the spinal cord, and also it has crossing nerve fibers that belong come the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three basic nerve fiber varieties can be distinguished in the spinal cord white matter: 1) lengthy ascending nerve fibers originally from the pillar cells, which make synaptic relations to neurons in assorted brainstem nuclei, cerebellum and also dorsal thalamus, 2) lengthy descending nerve yarn originating from the cerebral cortex and various brainstem nuclei to synapse in ~ the various Rexed great in the spinal cord gray matter, and 3) shorter nerve yarn interconnecting various spinal cord levels such together the yarn responsible for the coordination that flexor reflexes. Ascending tracts are uncovered in every columns conversely, descending tracts are uncovered only in the lateral and also the anterior columns.