Fibers of Spinal Nerves Merge Together Then Branch Out Again Forming a Complex Network Is Called a

Chapter three: Beefcake 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 string is the most of import structure between the torso and the encephalon. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the starting time or second lumbar vertebrae. Information technology is a vital link betwixt the brain and the trunk, and from the torso to the brain. The spinal cord is forty to 50 cm long and 1 cm to 1.5 cm in diameter. Two sequent rows of nerve roots emerge on each of its sides. These nerve roots join distally to form 31 pairs of spinal nerves. The spinal string is a cylindrical construction 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 (Due south), (Figure 3.1), each of which is comprised of several segments. The spinal nerve contains motor and sensory nervus fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome (see beneath and Effigy 3.five).

3.two Full general Features

Similar cross-sectional structures at all spinal cord levels (Figure 3.1).
It carries sensory information (sensations) from the body and some from the head to the central 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 polish muscles that mediate voluntary and involuntary reflexes.
It contains neurons whose descending axons mediate autonomic control for most of the visceral functions.
It is of great clinical importance because it is a major site of traumatic injury and the locus for many affliction processes.

Although the spinal cord constitutes only nearly two% of the key nervous organization (CNS), its functions are vital. Knowledge of spinal string functional anatomy makes it possible to diagnose the nature and location of cord damage and many string diseases.

3.iii Segmental and Longitudinal Organization

The spinal cord is divided into iv unlike regions: the cervical, thoracic, lumbar and sacral regions (Figure 3.one). The different cord regions can be visually distinguished from one another. 2 enlargements of the spinal cord tin be visualized: The cervical enlargement, which extends betwixt C3 to T1; and the lumbar enlargements which extends betwixt L1 to S2 (Effigy 3.ane).

The cord is segmentally organized. At that place are 31 segments, defined by 31 pairs of nerves exiting the cord. These nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve (Figure iii.two). Dorsal and ventral roots enter and get out the vertebral cavalcade respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment.

Figure 3.ii
Drawing of the eight, 12, 5, v and ane 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 (Figure 3.3). The spinal cord is attached to the dura past a series of lateral denticulate ligaments emanating from the pial folds.

Figure 3.3
The three spinal string meninges. The asperous ligament, the dorsal root ganglion (A), and an enlarged cartoon of the meninges (B).

During the initial third calendar month of embryonic development, the spinal cord extends the entire length of the vertebral canal and both abound at about the same rate. Equally evolution continues, the body and the vertebral column go along to grow at a much greater charge per unit than the spinal cord proper. This results in deportation of the lower parts of the spinal string with relation to the vertebrae cavalcade. The outcome of this uneven growth is that the adult spinal cord extends to the level of the offset or 2nd 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 within the vertebral canal, results in the lumbar, sacral, and coccygeal roots extending to their appropriate vertebral levels (Figure 3.ii).

All spinal nerves, except the first, exit beneath their corresponding vertebrae. In the cervical segments, in that location are 7 cervical vertebrae and 8 cervical nerves (Figure iii.2). C1-C7 nerves exit above their vertebrae whereas the C8 nerve exits beneath the C7 vertebra. It leaves between the C7 vertebra and the offset thoracic vertebra. Therefore, each subsequent nerve leaves the string below the corresponding vertebra. In the thoracic and upper lumbar regions, the difference betwixt the vertebrae and cord level is three segments. Therefore, the root filaments of spinal string segments accept to travel longer distances to reach the corresponding intervertebral foramen from which the spinal nerves sally. The lumbosacral roots are known as the cauda equina (Figure 3.2).

Each spinal nerve is equanimous of nerve fibers that are related to the region of the muscles and pare that develops from ane body somite (segment). A spinal segment is defined by dorsal roots inbound and ventral roots exiting the cord, (i.e., a spinal cord department that gives ascent to ane spinal nerve is considered as a segment.) (Figure 3.4).

Figure 3.iv
(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 area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion. If a nerve is cut, 1 loses sensation from that dermatome. Because each segment of the cord innervates a different region of the trunk, dermatomes tin be precisely mapped on the body surface, and loss of sensation in a dermatome can indicate the exact level of spinal cord harm in clinical assessment of injury (Figure 3.5). It is important to consider that there is some overlap between neighboring dermatomes. Because sensory information from the body is relayed to the CNS through the dorsal roots, the axons originating from dorsal root ganglion cells are classified as primary sensory afferents, and the dorsal root'southward neurons are the first lodge (one°) sensory neuron. Most axons in the ventral roots ascend from motor neurons in the ventral horn of the spinal cord and innervate skeletal musculus. Others ascend 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 form mixed afferent and efferent spinal nerves, which merge to form peripheral nerves. Knowledge of the segmental innervation of the cutaneous area and the muscles is essential to diagnose the site of an injury.

Effigy 3.v
Innervation arising from single dorsal root ganglion supplied specific skin area (a dermatome). The numbers refer to the spinal segments by which each nerve is named C = cervical; T = thoracic; L = lumbar; S = sacral spinal cord segments (dermatome).

three.4 Internal Structure of the Spinal String

A transverse section of the developed spinal cord shows white matter in the periphery, gray affair inside, and a tiny central culvert filled with CSF at its eye. Surrounding the culvert is a single layer of cells, the ependymal layer. Surrounding the ependymal layer is the greyness affair – a region containing cell bodies – shaped similar the letter "H" or a "butterfly". The ii "wings" of the butterfly are connected beyond the midline past the dorsal gray commissure and below the white commissure (Figure 3.6). The shape and size of the gray affair varies co-ordinate to spinal cord level. At the lower levels, the ratio between gray affair and white matter is greater than in higher levels, mainly because lower levels contain less ascending and descending nervus fibers. (Figure 3.ane and Figure iii.6).

Effigy three.6
Spinal cord section showing the white and the gray thing in four spinal cord levels.

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

The dorsal horn is found at all spinal cord levels and is comprised of sensory nuclei that receive and process incoming somatosensory data. From there, ascending projections sally to transmit the sensory information to the midbrain and diencephalon. The intermediate cavalcade and the lateral horn incorporate autonomic neurons innervating visceral and pelvic organs. The ventral horn comprises motor neurons that innervate skeletal muscle.

At all the levels of the spinal cord, nerve cells in the greyness 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 affair into the ventral spinal roots or the fiber tracts of the white thing. The axons and dendrites of the Golgi blazon Ii cells are largely confined to the neighboring neurons in the gray matter.

A more than recent nomenclature of neurons inside the gray affair is based on function. These cells are located at all levels of the spinal cord and are grouped into iii main categories: root cells, column or tract cells and propriospinal cells.

The root cells are situated in the ventral and lateral grey horns and vary greatly in size. The well-nigh 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: one) somatic efferent root neurons, which innervate the skeletal musculature; and two) the visceral efferent root neurons, besides 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 bars entirely within the CNS. The axons of the column cells grade longitudinal ascending tracts that ascend in the white columns and terminate upon neurons located rostrally in the brain stem, cerebellum or diencephalon. Some column cells transport their axons up and downwardly the cord to terminate in gray matter close to their origin and are known as intersegmental association column cells. Other column cell axons terminate inside the segment in which they originate and are called intrasegmental association column cells. Still other cavalcade cells send their axons beyond the midline to terminate in gray matter close to their origin and are called commissure association column cells.

The propriospinal cells are spinal interneurons whose axons exercise not leave the spinal cord proper. Propriospinal cells account for most 90% of spinal neurons. Some of these fibers also are institute around the margin of the grey matter of the string and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.

three.5 Spinal Cord Nuclei and Laminae

Spinal neurons are organized into nuclei and laminae.

3.6 Nuclei

The prominent nuclear groups of cell columns within the spinal string from dorsal to ventral are the marginal zone, substantia gelatinosa, nucleus proprius, dorsal nucleus of Clarke, intermediolateral nucleus and the lower motor neuron nuclei.

Effigy 3.7
Spinal cord nuclei and laminae.

Marginal zone nucleus or posterior marginalis, is establish at all spinal cord levels as a thin layer of column/tract cells (cavalcade 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 data to the diencephalon (Figure 3.vii).

Substantia gelatinosa is institute at all levels of the spinal cord. Located in the dorsal cap-similar portion of the head of the dorsal horn, it relays pain, temperature and mechanical (light touch) data and consists mainly of column cells (intersegmental cavalcade cells). These column cells synapse in cell at Rexed layers Iv to VII, whose axons contribute to the ventral (anterior) and lateral spinal thalamic tracts. The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus.

Nucleus proprius is located below the substantia gelatinosa in the caput and neck of the dorsal horn. This prison cell group, sometimes called the chief 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, as well as to spinal cerebellar tracts. The axons originating in nucleus proprius project to the thalamus via the spinothalamic tract and to the cerebellum via the ventral spinocerebellar tract (VSCT).

Dorsal nucleus of Clarke is a cell column located in the mid-portion of the base form of the dorsal horn. The axons from these cells pass uncrossed to the lateral funiculus and form the dorsal (posterior) spinocerebellar tract (DSCT), which subserve unconscious proprioception from muscle spindles and Golgi tendon organs to the cerebellum, and some of them innervate spinal interneurons. The dorsal nucleus of Clarke is found simply in segments C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar tract (CCT).

Intermediolateral nucleus is located in the intermediate zone betwixt 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 form the lateral horn. A large proportion of its cells are root cells which transport axons into the ventral spinal roots via the white rami to reach the sympathetic tract equally preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons (Figure 3.vii).

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

iii.7 Rexed Laminae

The distribution of cells and fibers within the gray thing of the spinal cord exhibits a blueprint of lamination. The cellular blueprint of each lamina is equanimous 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 than accurately to role than the previous classification scheme which was based on major nuclear groups (Effigy 3.7).

Laminae I to IV, in general, are concerned with exteroceptive awareness 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 equally a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-Ix comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia.

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

Rexed lamina Two – Composed of tightly packed interneurons. This layer corresponds to the substantia gelatinosa and responds to noxious stimuli while others reply to non-noxious stimuli. The bulk of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells also as descending dorsolateral fasciculus (DLF) fibers. They send axons to Rexed laminae 3 and Iv (fasciculus proprius). Loftier concentrations of substance P and opiate receptors take been identified in Rexed lamina Ii. The lamina is believed to be of import for the modulation of sensory input, with the effect of determining which pattern of incoming data volition produce sensations that will exist interpreted past the brain as beingness painful.

Rexed lamina Iii – Equanimous of variable jail cell size, axons of these neurons bifurcate several times and course a dense plexus. Cells in this layer receive axodendritic synapses from Aβ fibers entering dorsal root fibers. Information technology contains dendrites of cells from laminae IV, V and VI. Near of the neurons in lamina Iii function equally propriospinal/interneuron cells.

Rexed lamina IV – The thickest of the first four laminae. Cells in this layer receive Aß axons which acquit predominantly non-noxious information. In addition, dendrites of neurons in lamina 4 radiate to lamina Ii, and respond to stimuli such equally light bear upon. The sick-divers 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 Two. The neurons in this lamina receive monosynaptic data from Aß, Advertising and C axons which besides conduct 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 VI – Is a wide layer which is best developed in the cervical and lumbar enlargements. Lamina 6 divides too into medial and lateral parts. Grouping Ia afferent axons from muscle spindles cease in the medial part at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the small neurons are interneurons participating in spinal reflexes, while descending brainstem pathways project to the lateral zone of Rexed layer Vi.

Rexed lamina 7 – This lamina occupies a big heterogeneous region. This region is also known as the zona intermedia (or intermediolateral nucleus). Its shape and boundaries vary along the length of the cord. Lamina VII neurons receive data from Rexed lamina Two to Half-dozen as well as visceral afferent fibers, and they serve as an intermediary relay in manual of visceral motor neurons impulses. The dorsal nucleus of Clarke forms a prominent round oval prison cell cavalcade from C8 to L3. The big cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar tract (DSCT). Cells in laminae Five to VII, which do not class a detached nucleus, give rise to uncrossed fibers that course the ventral spinocerebellar tract (VSCT). Cells in the lateral horn of the string in segments T1 and L3 give rise to preganglionic sympathetic fibers to innervate postganglionic cells located in the sympathetic ganglia outside the cord. Lateral horn neurons at segments S2 to S4 give rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia.

Rexed lamina Eight – Includes an area at the base of the ventral horn, but its shape differs at various cord levels. In the cord enlargements, the lamina occupies only the medial part of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate motor action, most probably via 1000 motor neurons which innervate the intrafusal muscle fibers.

Rexed lamina 9 – Equanimous of several distinct groups of large a motor neurons and small-scale γ and β motor neurons embedded within this layer. Its size and shape differ at various cord levels. In the cord enlargements the number of α motor neurons increment and they form numerous groups. The α motor neurons are large and multipolar cells and requite rise to ventral root fibers to supply extrafusal skeletal musculus fibers, while the small γ motor neurons requite ascent to the intrafusal musculus fibers. The α motor neurons are somatotopically organized.

Rexed lamina 10 – Neurons in Rexed lamina X environment the central culvert and occupy the commissural lateral area of the gray commissure, which likewise contains decussating axons.

In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are concerned primarily with proprioceptive sensation and act as a relay betwixt the periphery to the midbrain and the cerebellum. Laminae 8 and Ix grade the final motor pathway to initiate and modulate motor activeness via α, β and γ motor neurons, which innervate striated muscle. All visceral motor neurons are located in lamina 7 and innervate neurons in autonomic ganglia.

3.viii White Matter

Surrounding the gray matter is white affair containing myelinated and unmyelinated nervus fibers. These fibers behave data upwards (ascending) or downwards (descending) the cord. The white matter is divided into the dorsal (or posterior) cavalcade (or funiculus), lateral column and ventral (or anterior) column (Effigy 3.eight). The inductive 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: ane) long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2) long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord greyness thing, and three) shorter nerve fibers interconnecting various spinal string levels such as the fibers responsible for the coordination of flexor reflexes. Ascending tracts are plant in all columns whereas descending tracts are found only in the lateral and the anterior columns.

Effigy 3.eight
The spinal cord white thing and its three columns, and the topographical location of the main ascending spinal cord 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 draw a large group of nerve fibers which are located in a given area (e.grand., posterior funiculus). Inside a funiculus, groups of fibers from various origins, which share common features, are sometimes arranged in smaller bundles of axons chosen fasciculus, (due east.g., fasciculus proprius [Figure three.eight]). Fasciculus is primarily a morphological term whereas tracts and pathways are as well terms applied to nerve fiber bundles which have a functional connotation. A tract is a grouping of nerve fibers which usually has the aforementioned origin, destination, and class and besides has similar functions. The tract name is derived from their origin and their termination (i.e., 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 cord and ends in the thalamus). A pathway usually refers to the entire neuronal circuit responsible for a specific function, and information technology includes all the nuclei and tracts which are associated with that function. For example, the spinothalamic pathway includes the cell bodies of origin (in the dorsal root ganglia), their axons as they projection through the dorsal roots, synapses in the spinal string, and projections of second and 3rd social club neurons beyond the white commissure, which ascend to the thalamus in the spinothalamic tracts.

iii.9 Spinal String Tracts

The spinal cord white matter contains ascending and descending tracts.

Ascending tracts (Effigy three.8). The nerve fibers incorporate the ascending tract emerge from the first social club (1°) neuron located in the dorsal root ganglion (DRG). The ascending tracts transmit sensory data from the sensory receptors to college levels of the CNS. The ascending gracile and cuneate fasciculi occupying the dorsal column, and sometimes are named the dorsal funiculus. These fibers acquit information related to tactile, two signal discrimination of simultaneously applied pressure, vibration, position, and movement 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 acquit unconscious proprioception information from muscles and joints of the lower extremity to the cerebellum. In the ventral column (funiculus) there are four prominent tracts: 1) the paleospinothalamic tract (or anterior spinothalamic tract) is located which carry pain, temperature, and information associated with touch on to the brain stem nuclei and to the diencephalon, two) the spinoolivary tract carries information from Golgi tendon organs to the cerebellum, 3) the spinoreticular tract, and 4) the spinotectal tract. Intersegmental nerve fibers traveling for several segments (2 to four) and are located every bit a thin layer around the gray affair is known as fasciculus proprius, spinospinal or archispinothalamic tract. Information technology carries hurting information to the brain stem and diencephalon.

Descending tracts (Figure 3.9). The descending tracts originate from dissimilar cortical areas and from brain stalk nuclei. The descending pathway carry information associated with maintenance of motor activities such as posture, balance, muscle tone, and visceral and somatic reflex activeness. These include the lateral corticospinal tract and the rubrospinal tracts located in the lateral column (funiculus). These tracts carry information associated with voluntary move. Other tracts such every bit the reticulospinal vestibulospinal and the inductive corticospinal tract mediate balance and postural movements (Figure iii.nine). Lissauer's tract, which is wedged between the dorsal horn and the surface of the spinal cord carry the descending fibers of the dorsolateral funiculus (DFL), which regulate incoming pain awareness at the spinal level, and intersegmental fibers. Additional details near ascending and descending tracts are described in the adjacent few chapters.

Figure 3.nine
The main descending spinal cord tracts.

iii.10 Dorsal Root

Figure 3.10
Spinal cord section with its ventral and dorsal root fibers and ganglion.

Information from the peel, skeletal musculus and joints is relayed to the spinal cord past 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, before reaching the spinal cord, bifurcates into ascending and descending branches entering several segments below and above their ain segment. The ascending dorsal root fibers and the descending ventral root fibers from and to detached torso areas grade a spinal nerve (Figure 3.10). There are 31 paired spinal nerves. The dorsal root fibers segregate into lateral and medial divisions. The lateral partition contains most of the unmyelinated and small myelinated axons conveying pain and temperature information to be terminated in the Rexed laminae I, II, and IV of the gray affair. The medial segmentation of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from skin, muscles and joints; it enters the dorsal/posterior column/funiculus and arise 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-social club (1°) sensory neurons synapse in the medulla oblongata on the 2nd social club (2°) neurons (in nucleus gracilis or nucleus cuneatus). In entering the spinal cord, all fibers send collaterals to dissimilar Rexed lamina.

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

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

3.11 Ventral Root

Ventral root fibers are the axons of motor and visceral efferent fibers and emerge from poorly divers ventral lateral sulcus equally ventral rootlets. The ventral rootlets from detached spinal cord section unite and course the ventral root, which incorporate motor nervus axons from motor and visceral motor neurons. The α motor nerve axons innervate the extrafusal muscle fibers while the small γ motor neuron axons innervate the intrafusal muscle fibers located inside the muscle spindles. The visceral neurons send preganglionic fibers to innervate the visceral organs. All these fibers bring together the dorsal root fibers distal to the dorsal root ganglion to form the spinal nerve (Effigy iii.10).

3.12 Spinal Nerve Roots

The spinal nerve roots are formed past the union of dorsal and ventral roots within the intervertebral foramen, resulting in a mixed nerve joined together and forming the spinal nerve (Figure 3.x). Spinal nervus rami include the dorsal primary nerves (ramus), which innervates the skin and muscles of the back, and the ventral primary nerves (ramus), which innervates the ventral lateral muscles and peel of the trunk, extremities and visceral organs. The ventral and dorsal roots also provide the anchorage and fixation of the spinal cord to the vertebral cauda.

three.13 Claret Supply of the Spinal Cord

The arterial blood supply to the spinal string in the upper cervical regions is derived from ii branches of the vertebral arteries, the inductive spinal artery and the posterior spinal arteries (Figure 3.12). At the level of medulla, the paired anterior spinal arteries bring together to form a single avenue that lies in the anterior median cleft of the spinal cord. The posterior spinal arteries are paired and form an anastomotic chain over the posterior aspect of the spinal cord. A plexus of small arteries, the arterial vasocorona, on the surface of the cord constitutes an anastomotic connection betwixt the inductive and posterior spinal arteries. This arrangement provides uninterrupted blood supplies along the entire length of the spinal cord.

Figure 3.12
The spinal cord arterial circulation.

At spinal cord regions below upper cervical levels, the anterior and posterior spinal arteries narrow and form an anastomotic network with radicular arteries. The radicular arteries are branches of the cervical, body, intercostal & iliac arteries. The radicular arteries supply most of the lower levels of the spinal cord. At that place are approximately 6 to 8 pairs of radicular arteries supplying the anterior and posterior spinal cord (Figure 3.12).

Test Your Knowledge

Question ane
A
B
C
D
East

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

E. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern This reply is Wrong.

The spinal cord does not occupy 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

E. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

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

The spinal cord has 7 (7) cervical segments.

C. Contains the 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

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

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

D. Ends at the conus medullaris

E. Has no arachnoid membrane

The spinal string...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments

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

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

E. Has no arachnoid membrane

The spinal string...

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

E. Has no arachnoid membrane This respond is Incorrect.

Arachnoid membrane covers the spinal string.

Question 2
A
B
C
D
E

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

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

A. Corticospinal This answer is INCORRECT.

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Anterior spinocerebellar

Eastward. Dorsal spinocerebellar

Which of the post-obit tracts crosses at the spinal string level of entry?

A. Corticospinal

B. Ventral spinothalamic This answer is Right!

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

C. Ventral spinocerebellar

D. Anterior spinocerebellar

E. Dorsal spinocerebellar

Which of the post-obit tracts crosses at the spinal string level of entry?

A. Corticospinal

B. Ventral spinothalamic

C. Ventral spinocerebellar This answer is Wrong.

D. Anterior 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

D. Anterior spinocerebellar This answer is Wrong.

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. Inductive spinocerebellar

Eastward. Dorsal spinocerebellar This respond is Wrong.

Question 3
A
B
C
D
E

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

A. Vertebral arteries

B. Posterior spinal arteries

C. Inductive spinal artery

D. Basilar artery

East. Posterior communicating avenue

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

A. Vertebral arteries This answer is INCORRECT.

B. Posterior spinal arteries

C. Anterior spinal artery

D. Basilar avenue

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 artery

E. Posterior communicating avenue

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

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal artery This respond is CORRECT!

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

D. Basilar avenue

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 avenue

D. Basilar artery This respond is INCORRECT.

Due east. Posterior communicating artery

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

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal avenue

D. Basilar artery

E. Posterior communicating avenue This answer is Wrong.

Question 4
A
B
C
D
Due east

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

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region

E. Coccygeal region

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

A. Sacral region This reply is INCORRECT.

B. Thoracic region

C. Lumbar region

D. Cervical region

Eastward. Coccygeal region

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

A. Sacral region

B. Thoracic region This answer is Wrong.

C. Lumbar region

D. Cervical region

Eastward. Coccygeal region

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

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region This answer is CORRECT!

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 nearly 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