how to tell what level you are at when looking at the spinal cord

Affiliate iii: Anatomy of the Spinal Cord

3.i Introduction

Figure three.1
Schematic dorsal and lateral view of the spinal cord and four cross sections from cervical, thoracic, lumbar and sacral levels, respectively.

The spinal cord is the most of import structure between the body and the brain. The spinal string extends from the foramen magnum where information technology is continuous with the medulla to the level of the first or second lumbar vertebrae. Information technology is a vital link between the brain and the body, and from the body to the brain. The spinal cord is forty to 50 cm long and ane cm to 1.5 cm in diameter. 2 consecutive rows of nerve roots emerge on each of its sides. These nervus roots join distally to form 31 pairs of spinal fretfulness. The spinal string is a cylindrical structure of nervous tissue composed of white and gray matter, is uniformly organized and is divided into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (Southward), (Effigy 3.ane), each of which is comprised of several segments. The spinal nervus contains motor and sensory nerve fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome (run into below and Effigy 3.5).

three.two Full general Features

Similar cross-sectional structures at all spinal cord levels (Figure 3.i).
It carries sensory data (sensations) from the body and some from the head to the cardinal nervous system (CNS) via afferent fibers, and it performs the initial processing of this data.
Motor neurons in the ventral horn project their axons into the periphery to innervate skeletal and smooth muscles that mediate voluntary and involuntary reflexes.
Information technology contains neurons whose descending axons mediate autonomic control for most of the visceral functions.
It is of nifty clinical importance because it is a major site of traumatic injury and the locus for many illness processes.

Although the spinal cord constitutes merely about 2% of the central nervous system (CNS), its functions are vital. Knowledge of spinal cord functional anatomy makes it possible to diagnose the nature and location of string damage and many string diseases.

3.iii Segmental and Longitudinal Organization

The spinal cord is divided into iv dissimilar regions: the cervical, thoracic, lumbar and sacral regions (Figure 3.1). The different cord regions can be visually distinguished from i another. Ii enlargements of the spinal cord can be visualized: The cervical enlargement, which extends between C3 to T1; and the lumbar enlargements which extends betwixt L1 to S2 (Figure 3.1).

The string is segmentally organized. There are 31 segments, divers past 31 pairs of nerves exiting the cord. These nerves are divided into 8 cervical, 12 thoracic, five lumbar, 5 sacral, and 1 coccygeal nerve (Figure 3.2). Dorsal and ventral roots enter and leave the vertebral cavalcade respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment.

Figure three.2
Drawing of the 8, 12, 5, five and 1 cervical, thoracic, lumbar, sacral and coccygeal spinal nerves and their get out from the vertebrate, respectively.

The cord is sheathed in the same three meninges as is the brain: the pia, arachnoid and dura. The dura is the tough outer sheath, the arachnoid lies beneath it, and the pia closely adheres to the surface of the cord (Effigy 3.3). The spinal string is fastened to the dura past a series of lateral denticulate ligaments emanating from the pial folds.

Figure three.3
The three spinal cord meninges. The denticulate ligament, the dorsal root ganglion (A), and an enlarged drawing of the meninges (B).

During the initial third month of embryonic development, the spinal string extends the unabridged length of the vertebral canal and both grow at most the same rate. As development continues, the body and the vertebral column continue to abound at a much greater rate than the spinal cord proper. This results in deportation of the lower parts of the spinal cord with relation to the vertebrae column. The effect of this uneven growth is that the adult spinal cord extends to the level of the first or second lumbar vertebrae, and the fretfulness grow to exit through the same intervertebral foramina as they did during embryonic development. This growth of the nerve roots occurring inside the vertebral canal, results in the lumbar, sacral, and coccygeal roots extending to their appropriate vertebral levels (Figure 3.2).

All spinal fretfulness, except the first, exit below their corresponding vertebrae. In the cervical segments, at that place are 7 cervical vertebrae and eight cervical fretfulness (Effigy 3.2). C1-C7 nerves get out higher up their vertebrae whereas the C8 nerve exits below the C7 vertebra. It leaves betwixt the C7 vertebra and the start thoracic vertebra. Therefore, each subsequent nervus leaves the cord below the corresponding 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 have to travel longer distances to attain the corresponding intervertebral foramen from which the spinal nerves emerge. The lumbosacral roots are known as the cauda equina (Effigy 3.two).

Each spinal nerve is composed of nervus fibers that are related to the region of the muscles and skin that develops from ane torso somite (segment). A spinal segment is divers by dorsal roots entering and ventral roots exiting the cord, (i.e., a spinal cord section that gives rise to one spinal nerve is considered every bit a segment.) (Effigy 3.4).

Figure 3.4
(A) Drawing of the spinal cord with its spinal roots. (B) Drawing of the spinal vertebrate. (C) Department of the spinal cord, its meninges and the dorsal and ventral roots of three segments.

A dermatome is an surface area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion. If a nerve is cut, one loses sensation from that dermatome. Considering each segment of the cord innervates a different region of the trunk, dermatomes can be precisely mapped on the body surface, and loss of sensation in a dermatome tin can indicate the exact level of spinal cord harm in clinical assessment of injury (Figure 3.five). It is of import to consider that at that place is some overlap between neighboring dermatomes. Because sensory data from the torso is relayed to the CNS through the dorsal roots, the axons originating from dorsal root ganglion cells are classified as chief sensory afferents, and the dorsal root's neurons are the first order (1°) sensory neuron. Virtually axons in the ventral roots arise from motor neurons in the ventral horn of the spinal cord and innervate skeletal muscle. Others arise from the lateral horn and synapse on autonomic ganglia that innervate visceral organs. The ventral root axons join with the peripheral processes of the dorsal root ganglion cells to class mixed afferent and efferent spinal fretfulness, which merge to form peripheral nerves. Noesis of the segmental innervation of the cutaneous surface area and the muscles is essential to diagnose the site of an injury.

Effigy three.five
Innervation arising from unmarried dorsal root ganglion supplied specific skin area (a dermatome). The numbers refer to the spinal segments past which each nerve is named C = cervical; T = thoracic; L = lumbar; S = sacral spinal string segments (dermatome).

3.4 Internal Structure of the Spinal Cord

A transverse section of the adult spinal cord shows white matter in the periphery, gray matter inside, and a tiny key canal filled with CSF at its center. Surrounding the culvert is a unmarried layer of cells, the ependymal layer. Surrounding the ependymal layer is the gray affair – a region containing cell bodies – shaped like the letter of the alphabet "H" or a "butterfly". The two "wings" of the butterfly are connected across the midline by the dorsal grey commissure and below the white commissure (Figure 3.6). The shape and size of the greyness thing varies according to spinal cord level. At the lower levels, the ratio between grey thing and white thing is greater than in college levels, mainly considering lower levels contain less ascending and descending nerve fibers. (Effigy 3.1 and Figure iii.vi).

Figure 3.six
Spinal cord department showing the white and the gray affair in 4 spinal cord levels.

The gray matter mainly contains the cell bodies of neurons and glia and is divided into four principal columns: dorsal horn, intermediate column, lateral horn and ventral horn column. (Figure 3.6).

The dorsal horn is institute at all spinal cord levels and is comprised of sensory nuclei that receive and procedure incoming somatosensory information. From at that place, ascending projections emerge to transmit the sensory information to the midbrain and diencephalon. The intermediate column and the lateral horn comprise autonomic neurons innervating visceral and pelvic organs. The ventral horn comprises motor neurons that innervate skeletal muscle.

At all the levels of the spinal string, nerve cells in the gray substance are multipolar, varying much in their morphology. Many of them are Golgi type I and Golgi type Ii nerve cells. The axons of Golgi type I are long and laissez passer out of the greyness matter into the ventral spinal roots or the fiber tracts of the white matter. The axons and dendrites of the Golgi type II cells are largely confined to the neighboring neurons in the gray matter.

A more contempo classification of neurons within the gray matter is based on function. These cells are located at all levels of the spinal cord and are grouped into three primary categories: root cells, column or tract cells and propriospinal cells.

The root cells are situated in the ventral and lateral gray horns and vary greatly in size. The virtually prominent features of the root cells are large multipolar elements exceeding 25 µm of their somata. The root cells contribute their axons to the ventral roots of the spinal nerves and are grouped into two major divisions: i) 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 course longitudinal ascending tracts that arise in the white columns and stop upon neurons located rostrally in the brain stalk, cerebellum or diencephalon. Some cavalcade cells transport their axons up and down the cord to cease in greyness thing close to their origin and are known as intersegmental association cavalcade cells. Other column prison cell axons terminate inside the segment in which they originate and are chosen intrasegmental association cavalcade cells. Yet other column cells transport their axons across the midline to terminate in greyness affair 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. Propriospinal cells account for about 90% of spinal neurons. Some of these fibers likewise are plant around the margin of the gray matter of the cord and are collectively chosen the fasciculus proprius or the propriospinal or the archispinothalamic tract.

iii.5 Spinal Cord Nuclei and Laminae

Spinal neurons are organized into nuclei and laminae.

3.vi Nuclei

The prominent nuclear groups of prison cell columns inside 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.

Figure 3.seven
Spinal string nuclei and laminae.

Marginal zone nucleus or posterior marginalis, is constitute at all spinal cord levels every bit a sparse layer of column/tract cells (column cells) that caps the tip of the dorsal horn. The axons of its neurons contribute to the lateral spinothalamic tract which relays pain and temperature information to the diencephalon (Figure 3.7).

Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like portion of the caput of the dorsal horn, information technology 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 4 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 jail cell group, sometimes called the main sensory nucleus, is associated with mechanical and temperature sensations. It is a poorly divers cell column which extends through all segments of the spinal cord and its neurons contribute to ventral and lateral spinal thalamic tracts, equally 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 jail cell column located in the mid-portion of the base grade of the dorsal horn. The axons from these cells laissez passer 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 institute only in segments C8 to L3 of the spinal string and is virtually 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, information technology receives viscerosensory information and contains preganglionic sympathetic neurons, which grade the lateral horn. A big proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to accomplish the sympathetic tract equally preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons (Effigy 3.7).

Lower motor neuron nuclei are located in the ventral horn of the spinal cord. They comprise predominantly motor nuclei consisting of α, β and γ motor neurons and are found at all levels of the spinal cord--they are root cells. The a motor neurons are the concluding mutual pathway of the motor system, and they innervate the visceral and skeletal muscles.

iii.seven Rexed Laminae

The distribution of cells and fibers within the gray matter of the spinal cord exhibits a pattern of lamination. The cellular blueprint 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 3.7).

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 incorporate the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the key canal and contains neuroglia.

Rexed lamina I – Consists of a thin layer of cells that cap the tip of the dorsal horn with pocket-size dendrites and a complex assortment of nonmyelinated axons. Cells in lamina I reply mainly to noxious and thermal stimuli. Lamina I cell axons join the contralateral spinothalamic tract; this layer corresponds to nucleus posteromarginalis.

Rexed lamina 2 – 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 too as descending dorsolateral fasciculus (DLF) fibers. They ship axons to Rexed laminae III and IV (fasciculus proprius). High concentrations of substance P and opiate receptors take been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the outcome of determining which pattern of incoming information will produce sensations that volition be interpreted by the brain every bit beingness painful.

Rexed lamina 3 – Composed of variable prison cell size, axons of these neurons bifurcate several times and class a dense plexus. Cells in this layer receive axodendritic synapses from Aβ fibers entering dorsal root fibers. It contains dendrites of cells from laminae IV, Five and VI. Most of the neurons in lamina III role as propriospinal/interneuron cells.

Rexed lamina IV – The thickest of the first iv laminae. Cells in this layer receive Aß axons which bear predominantly non-noxious information. In add-on, dendrites of neurons in lamina Four radiate to lamina II, and answer to stimuli such as calorie-free impact. The sick-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 5 – Composed neurons with their dendrites in lamina II. The neurons in this lamina receive monosynaptic data from Aß, Ad and C axons which also carry nociceptive information from visceral organs. 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 5 cells projection 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 Six – 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 finish 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 projection to the lateral zone of Rexed layer Vi.

Rexed lamina VII – This lamina occupies a big heterogeneous region. This region is too known as the zona intermedia (or intermediolateral nucleus). Its shape and boundaries vary forth the length of the cord. Lamina VII neurons receive data from Rexed lamina Ii to Half dozen too 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 prison cell column from C8 to L3. The large cells requite ascension to uncrossed nerve fibers of the dorsal spinocerebellar tract (DSCT). Cells in laminae V to 7, which do non course a detached nucleus, give rise to uncrossed fibers that class the ventral spinocerebellar tract (VSCT). Cells in the lateral horn of the cord in segments T1 and L3 give ascension 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 surface area at the base of operations of the ventral horn, only 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 cease. The neurons of lamina VIII modulate motor activity, well-nigh probably via k motor neurons which innervate the intrafusal muscle fibers.

Rexed lamina Nine – Composed of several distinct groups of large a motor neurons and small γ and β motor neurons embedded inside this layer. Its size and shape differ at various cord levels. In the string enlargements the number of α motor neurons increase and they course numerous groups. The α motor neurons are large and multipolar cells and requite rise to ventral root fibers to supply extrafusal skeletal muscle fibers, while the small γ motor neurons give rise to the intrafusal muscle fibers. The α motor neurons are somatotopically organized.

Rexed lamina X – Neurons in Rexed lamina X surround the key culvert 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 Five and VI are concerned primarily with proprioceptive sensation and act equally a relay between the periphery to the midbrain and the cerebellum. Laminae 8 and 9 form the last motor pathway to initiate and modulate motor activity via α, β and γ motor neurons, which innervate striated muscle. All visceral motor neurons are located in lamina Seven and innervate neurons in autonomic ganglia.

3.viii White Matter

Surrounding the greyness thing is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information upward (ascending) or downwardly (descending) the string. The white thing is divided into the dorsal (or posterior) column (or funiculus), lateral column and ventral (or anterior) column (Figure 3.8). 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: i) 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 cognitive cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray affair, and 3) shorter nerve fibers interconnecting various spinal cord levels such equally the fibers responsible for the coordination of flexor reflexes. Ascending tracts are plant in all columns whereas descending tracts are plant only in the lateral and the anterior columns.

Figure three.viii
The spinal cord white affair and its three columns, and the topographical location of the main ascending spinal string tracts.

Four different terms are often used to describe bundles of axons such as those constitute in the white affair: 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 (eastward.chiliad., posterior funiculus). Within a funiculus, groups of fibers from diverse origins, which share mutual features, are sometimes arranged in smaller bundles of axons called fasciculus, (e.thou., fasciculus proprius [Effigy three.eight]). Fasciculus is primarily a morphological term whereas tracts and pathways are also terms applied to nerve fiber bundles which have a functional connotation. A tract is a group of nerve fibers which commonly has the same origin, destination, and course and also has similar functions. The tract name is derived from their origin and their termination (i.eastward., corticospinal tract - a tract that originates in the cortex and terminates in the spinal cord; lateral spinothalamic tract - a tract originated in the lateral spinal string and ends in the thalamus). A pathway normally refers to the entire neuronal circuit responsible for a specific function, and it includes all the nuclei and tracts which are associated with that office. For case, the spinothalamic pathway includes the prison cell bodies of origin (in the dorsal root ganglia), their axons as they projection through the dorsal roots, synapses in the spinal cord, and projections of second and 3rd order neurons across the white commissure, which ascend to the thalamus in the spinothalamic tracts.

3.ix Spinal Cord Tracts

The spinal cord white affair contains ascending and descending tracts.

Ascending tracts (Figure 3.viii). The nerve fibers comprise the ascending tract emerge from the first lodge (1°) neuron located in the dorsal root ganglion (DRG). The ascending tracts transmit sensory information from the sensory receptors to higher levels of the CNS. The ascending gracile and cuneate fasciculi occupying the dorsal column, and sometimes are named the dorsal funiculus. These fibers bear information related to tactile, two betoken discrimination of simultaneously applied pressure level, vibration, position, and move sense and conscious proprioception. In the lateral column (funiculus), the neospinothalamic tract (or lateral spinothalamic tract) is located more anteriorly and laterally, and carries pain, temperature and rough touch information from somatic and visceral structures. Nearby laterally, the dorsal and ventral spinocerebellar tracts carry unconscious proprioception information from muscles and joints of the lower extremity to the cerebellum. In the ventral column (funiculus) there are four prominent tracts: i) the paleospinothalamic tract (or anterior spinothalamic tract) is located which carry pain, temperature, and data associated with touch to the brain stem nuclei and to the diencephalon, 2) the spinoolivary tract carries information from Golgi tendon organs to the cerebellum, 3) the spinoreticular tract, and iv) the spinotectal tract. Intersegmental nerve fibers traveling for several segments (ii to four) and are located as a thin layer around the gray affair is known as fasciculus proprius, spinospinal or archispinothalamic tract. It carries pain data to the brain stem and diencephalon.

Descending tracts (Figure iii.ix). The descending tracts originate from unlike cortical areas and from brain stem nuclei. The descending pathway conduct data associated with maintenance of motor activities such as posture, residual, musculus tone, and visceral and somatic reflex action. These include the lateral corticospinal tract and the rubrospinal tracts located in the lateral cavalcade (funiculus). These tracts carry information associated with voluntary movement. Other tracts such every bit the reticulospinal vestibulospinal and the inductive corticospinal tract mediate remainder and postural movements (Figure 3.9). Lissauer's tract, which is wedged betwixt the dorsal horn and the surface of the spinal string deport the descending fibers of the dorsolateral funiculus (DFL), which regulate incoming pain sensation at the spinal level, and intersegmental fibers. Additional details about ascending and descending tracts are described in the next few chapters.

Effigy 3.ix
The main descending spinal cord tracts.

3.10 Dorsal Root

Effigy 3.x
Spinal string section with its ventral and dorsal root fibers and ganglion.

Information from the skin, skeletal muscle and joints is relayed to the spinal string by sensory cells located in the dorsal root ganglia. The dorsal root fibers are the axons originated from the primary sensory dorsal root ganglion cells. Each ascending dorsal root axon, earlier reaching the spinal cord, bifurcates into ascending and descending branches inbound several segments below and to a higher place their ain segment. The ascending dorsal root fibers and the descending ventral root fibers from and to discrete body areas class a spinal nerve (Effigy 3.10). There are 31 paired spinal nerves. The dorsal root fibers segregate into lateral and medial divisions. The lateral division contains most of the unmyelinated and small myelinated axons carrying pain and temperature information to be terminated in the Rexed laminae I, Two, and IV of the grayness matter. The medial division of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from pare, muscles and joints; it enters the dorsal/posterior column/funiculus and ascend in the dorsal column to exist terminated in the ipsilateral nucleus gracilis or nucleus cuneatus at the medulla oblongata region, i.e., the axons of the first-guild (1°) sensory neurons synapse in the medulla oblongata on the 2nd order (2°) neurons (in nucleus gracilis or nucleus cuneatus). In entering the spinal cord, all fibers send collaterals to different Rexed lamina.

Axons entering the cord in the sacral region are found in the dorsal column near the midline and comprise the fasciculus gracilis, whereas axons that enter at higher levels are added in lateral positions and incorporate the fasciculus cuneatus (Figure 3.11). This orderly representation is termed "somatotopic representation".

Figure 3.11
Somatotopical representation of the spinal thalamic tract and the dorsal column.

three.11 Ventral Root

Ventral root fibers are the axons of motor and visceral efferent fibers and emerge from poorly defined ventral lateral sulcus every bit ventral rootlets. The ventral rootlets from discrete spinal cord section unite and form the ventral root, which comprise motor nerve axons from motor and visceral motor neurons. The α motor nerve axons innervate the extrafusal musculus fibers while the pocket-sized γ motor neuron axons innervate the intrafusal musculus fibers located within the muscle spindles. The visceral neurons send preganglionic fibers to innervate the visceral organs. All these fibers join the dorsal root fibers distal to the dorsal root ganglion to form the spinal nerve (Effigy 3.10).

3.12 Spinal Nerve Roots

The spinal nerve roots are formed by the marriage of dorsal and ventral roots within the intervertebral foramen, resulting in a mixed nervus joined together and forming the spinal nerve (Figure iii.10). Spinal nerve rami include the dorsal primary nerves (ramus), which innervates the skin and muscles of the back, and the ventral master fretfulness (ramus), which innervates the ventral lateral muscles and skin of the torso, extremities and visceral organs. The ventral and dorsal roots also provide the anchorage and fixation of the spinal cord to the vertebral cauda.

3.13 Blood Supply of the Spinal Cord

The arterial claret supply to the spinal cord in the upper cervical regions is derived from two branches of the vertebral arteries, the anterior spinal artery and the posterior spinal arteries (Figure 3.12). At the level of medulla, the paired anterior spinal arteries join to grade a single artery that lies in the anterior median crevice of the spinal string. The posterior spinal arteries are paired and form an anastomotic chain over the posterior attribute of the spinal string. A plexus of small-scale arteries, the arterial vasocorona, on the surface of the cord constitutes an anastomotic connection betwixt the inductive and posterior spinal arteries. This organization provides uninterrupted claret supplies along the entire length of the spinal cord.

Figure three.12
The spinal string arterial circulation.

At spinal cord regions beneath upper cervical levels, the anterior and posterior spinal arteries narrow and course an anastomotic network with radicular arteries. The radicular arteries are branches of the cervical, trunk, intercostal & iliac arteries. The radicular arteries supply almost of the lower levels of the spinal cord. There are approximately half dozen to 8 pairs of radicular arteries supplying the anterior and posterior spinal cord (Figure 3.12).

Test Your Knowledge

Question i
A
B
C
D
E

The spinal string...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments

C. Contains the jail cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris

E. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern This answer is Incorrect.

The spinal cord does non occupy the lumbar cistern.

B. Has twelve (12) cervical segments

C. Contains the jail cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris

East. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments This reply is INCORRECT.

The spinal cord has vii (7) cervical segments.

C. Contains the prison cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris

Eastward. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments

C. Contains the cell bodies of postganglionic sympathetic efferent neurons This answer is Incorrect.

Postganglionic neurons are located in the periphery, not in the spinal cord.

D. Ends at the conus medullaris

E. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments

C. Contains the cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris This answer is Correct!

Eastward. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments

C. Contains the cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris

East. Has no arachnoid membrane This respond is INCORRECT.

Arachnoid membrane covers the spinal cord.

Question 2
A
B
C
D
East

Which of the following tracts crosses at the spinal cord level of entry?

A. Corticospinal

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Anterior spinocerebellar

E. Dorsal spinocerebellar

Which of the following tracts crosses at the spinal cord level of entry?

A. Corticospinal This answer is Wrong.

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Anterior spinocerebellar

E. Dorsal spinocerebellar

Which of the following tracts crosses at the spinal cord level of entry?

A. Corticospinal

B. Ventral spinothalamic This answer is CORRECT!

From these tracts, only the lateral spinothalamic tract crosses at the entry level.

C. Ventral spinocerebellar

D. Inductive spinocerebellar

E. Dorsal spinocerebellar

Which of the following tracts crosses at the spinal string level of entry?

A. Corticospinal

B. Ventral spinothalamic

C. Ventral spinocerebellar This respond is Incorrect.

D. Anterior spinocerebellar

E. Dorsal spinocerebellar

Which of the following tracts crosses at the spinal cord level of entry?

A. Corticospinal

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Anterior spinocerebellar This answer is Wrong.

Eastward. Dorsal spinocerebellar

Which of the following tracts crosses at the spinal cord level of entry?

A. Corticospinal

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Inductive spinocerebellar

E. Dorsal spinocerebellar This answer is Wrong.

Question 3
A
B
C
D
E

The claret supply for the corticospinal tract is derived from the:

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal artery

D. Basilar artery

E. Posterior communicating avenue

The blood supply for the corticospinal tract is derived from the:

A. Vertebral arteries This respond is INCORRECT.

B. Posterior spinal arteries

C. Inductive spinal avenue

D. Basilar artery

E. Posterior communicating avenue

The blood supply for the corticospinal tract is derived from the:

A. Vertebral arteries

B. Posterior spinal arteries This answer is Wrong.

C. Anterior spinal artery

D. Basilar avenue

East. Posterior communicating avenue

The blood supply for the corticospinal tract is derived from the:

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal avenue This respond is Right!

The anterior spinal artery supplies the corticospinal tract and the other tracts in this region.

D. Basilar artery

E. Posterior communicating artery

The blood supply for the corticospinal tract is derived from the:

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal avenue

D. Basilar artery This respond is INCORRECT.

Due east. Posterior communicating artery

The blood supply for the corticospinal tract is derived from the:

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal artery

D. Basilar artery

East. Posterior communicating artery This answer is Incorrect.

Question four
A
B
C
D
E

In the laminar somatotopic organization of the dorsal columns, the nearly lateral fibers stand for:

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region

Eastward. Coccygeal region

In the laminar somatotopic organization of the dorsal columns, the well-nigh lateral fibers represent:

A. Sacral region This answer is INCORRECT.

B. Thoracic region

C. Lumbar region

D. Cervical region

E. Coccygeal region

In the laminar somatotopic organization of the dorsal columns, the nearly lateral fibers represent:

A. Sacral region

B. Thoracic region This respond is Incorrect.

C. Lumbar region

D. Cervical region

E. Coccygeal region

In the laminar somatotopic system of the dorsal columns, the about lateral fibers represent:

A. Sacral region

B. Thoracic region

C. Lumbar region This answer is INCORRECT.

D. Cervical region

E. Coccygeal region

In the laminar somatotopic arrangement of the dorsal columns, the most lateral fibers represent:

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region This reply is Right!

The fibers entering at the lumbar region are located in the lateral portion of the dorsal columns.

Due east. Coccygeal region

In the laminar somatotopic organization of the dorsal columns, the virtually lateral fibers represent:

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region

E. Coccygeal region This answer is INCORRECT.

Question 5
A
B
C
D
E