The Spinal Cord and Spinal Nerves Review Sheet

Affiliate three: Anatomy of the Spinal String

three.1 Introduction

Effigy 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 well-nigh important structure between the body and the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. Information technology is a vital link betwixt the brain and the body, and from the trunk to the brain. The spinal string is 40 to 50 cm long and 1 cm to 1.5 cm in bore. Two consecutive rows of nerve roots emerge on each of its sides. These nerve roots join distally to form 31 pairs of spinal nerves. The spinal cord is a cylindrical structure of nervous tissue composed of white and greyness matter, is uniformly organized and is divided into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), (Figure 3.1), each of which is comprised of several segments. The spinal nerve contains motor and sensory nerve fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome (see below and Figure 3.v).

3.2 General Features

Like cantankerous-sectional structures at all spinal cord levels (Figure 3.ane).
It carries sensory data (sensations) from the torso and some from the head to the fundamental nervous system (CNS) via afferent fibers, and information technology performs the initial processing of this information.
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 well-nigh of the visceral functions.
It is of nifty clinical importance considering it is a major site of traumatic injury and the locus for many disease processes.

Although the spinal cord constitutes but about 2% of the cardinal nervous arrangement (CNS), its functions are vital. Knowledge of spinal cord functional anatomy makes information technology possible to diagnose the nature and location of string damage and many cord diseases.

3.3 Segmental and Longitudinal Organization

The spinal cord is divided into four unlike regions: the cervical, thoracic, lumbar and sacral regions (Figure 3.1). The different string regions can be visually distinguished from one another. Two enlargements of the spinal cord can exist visualized: The cervical enlargement, which extends betwixt C3 to T1; and the lumbar enlargements which extends betwixt L1 to S2 (Effigy three.one).

The cord is segmentally organized. In that location are 31 segments, defined by 31 pairs of nerves exiting the string. These nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, v sacral, and 1 coccygeal nerve (Figure 3.2). Dorsal and ventral roots enter and go out the vertebral column respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment.

Figure iii.2
Drawing of the 8, 12, 5, 5 and 1 cervical, thoracic, lumbar, sacral and coccygeal spinal nerves and their exit from the vertebrate, respectively.

The cord is sheathed in the same iii 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 attached to the dura past a series of lateral denticulate ligaments emanating from the pial folds.

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

During the initial third month of embryonic development, the spinal cord extends the entire length of the vertebral canal and both grow at nearly the same rate. Every bit development continues, the body and the vertebral column continue 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 column. The outcome of this uneven growth is that the developed spinal cord extends to the level of the first or 2nd lumbar vertebrae, and the fretfulness grow to go out through the same intervertebral foramina equally 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 advisable vertebral levels (Figure 3.2).

All spinal nerves, except the commencement, exit below their corresponding vertebrae. In the cervical segments, there are seven cervical vertebrae and eight cervical nerves (Figure three.2). C1-C7 fretfulness get out above their vertebrae whereas the C8 nerve exits below the C7 vertebra. It leaves between the C7 vertebra and the start thoracic vertebra. Therefore, each subsequent nervus leaves the cord beneath the respective vertebra. In the thoracic and upper lumbar regions, the divergence between the vertebrae and cord level is 3 segments. Therefore, the root filaments of spinal cord segments have to travel longer distances to reach the respective intervertebral foramen from which the spinal fretfulness sally. The lumbosacral roots are known as the cauda equina (Effigy iii.ii).

Each spinal nerve is composed of nervus fibers that are related to the region of the muscles and skin that develops from one body somite (segment). A spinal segment is defined by dorsal roots entering and ventral roots exiting the string, (i.due east., a spinal cord section that gives ascension to i spinal nervus is considered as a segment.) (Figure three.4).

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

A dermatome is an surface area of pare supplied by peripheral nerve fibers originating from a unmarried dorsal root ganglion. If a nerve is cut, one loses awareness from that dermatome. Because each segment of the string innervates a different region of the trunk, dermatomes tin be precisely mapped on the torso surface, and loss of sensation in a dermatome can signal the exact level of spinal string 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 every bit primary sensory afferents, and the dorsal root's neurons are the beginning 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 nerves, which merge to grade peripheral fretfulness. Knowledge of the segmental innervation of the cutaneous expanse and the muscles is essential to diagnose the site of an injury.

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

3.4 Internal Structure of the Spinal String

A transverse department of the adult spinal cord shows white matter in the periphery, grey affair inside, and a tiny central canal filled with CSF at its center. Surrounding the culvert is a single layer of cells, the ependymal layer. Surrounding the ependymal layer is the greyness matter – a region containing prison cell bodies – shaped like the letter of the alphabet "H" or a "butterfly". The two "wings" of the butterfly are connected across the midline past the dorsal gray commissure and below the white commissure (Effigy three.vi). The shape and size of the gray matter varies according to spinal string level. At the lower levels, the ratio betwixt grayness thing and white matter is greater than in higher levels, mainly because lower levels comprise less ascending and descending nerve fibers. (Figure three.1 and Figure 3.vi).

Figure 3.6
Spinal cord section showing the white and the gray matter in four spinal cord levels.

The gray thing mainly contains the jail cell bodies of neurons and glia and is divided into four main columns: dorsal horn, intermediate cavalcade, lateral horn and ventral horn column. (Figure three.6).

The dorsal horn is establish at all spinal string levels and is comprised of sensory nuclei that receive and process incoming somatosensory information. From there, 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 musculus.

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 Two nerve cells. The axons of Golgi blazon I are long and pass out of the gray 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 grey matter.

A more than recent classification of neurons within the gray matter is based on function. These cells are located at all levels of the spinal string and are grouped into three main 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 most 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 2 major divisions: 1) somatic efferent root neurons, which innervate the skeletal musculature; and 2) the visceral efferent root neurons, as well called preganglionic autonomic axons, which send their axons to various autonomic ganglia.

The cavalcade or tract cells and their processes are located mainly in the dorsal greyness horn and are bars entirely within the CNS. The axons of the cavalcade cells form longitudinal ascending tracts that ascend in the white columns and finish upon neurons located rostrally in the encephalon stalk, cerebellum or diencephalon. Some column cells ship their axons upwardly and down the string to stop in gray thing close to their origin and are known every bit intersegmental association cavalcade cells. Other cavalcade cell axons terminate within the segment in which they originate and are called intrasegmental association column cells. Still other column cells send their axons across the midline to terminate in gray thing shut to their origin and are chosen commissure clan column cells.

The propriospinal cells are spinal interneurons whose axons practice non leave the spinal cord proper. Propriospinal cells business relationship for most ninety% of spinal neurons. Some of these fibers besides are found around the margin of the gray matter of the string 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.

iii.6 Nuclei

The prominent nuclear groups of cell columns within 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 three.seven
Spinal cord nuclei and laminae.

Marginal zone nucleus or posterior marginalis, is plant at all spinal cord levels as a thin layer of cavalcade/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 three.7).

Substantia gelatinosa is plant at all levels of the spinal cord. Located in the dorsal cap-like portion of the head of the dorsal horn, it relays pain, temperature and mechanical (lite touch) information and consists mainly of column cells (intersegmental column cells). These column cells synapse in prison cell at Rexed layers Four 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 cell group, sometimes called the primary sensory nucleus, is associated with mechanical and temperature sensations. It is a poorly defined cell cavalcade 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 cell column located in the mid-portion of the base of operations class 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 but in segments C8 to L3 of the spinal string and is nearly prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accompaniment 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 form the lateral horn. A large proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to reach the sympathetic tract as preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons (Figure iii.vii).

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 last common pathway of the motor organisation, and they innervate the visceral and skeletal muscles.

3.vii Rexed Laminae

The distribution of cells and fibers inside 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 suggest a new nomenclature based on 10 layers (laminae). This nomenclature is useful since it is related more than accurately to function than the previous classification scheme which was based on major nuclear groups (Effigy 3.7).

Laminae I to Iv, in full general, are concerned with exteroceptive awareness and comprise the dorsal horn, whereas laminae 5 and Half dozen are concerned primarily with proprioceptive sensations. Lamina VII is equivalent to the intermediate zone and acts every bit a relay between musculus spindle to midbrain and cerebellum, and laminae 8-IX comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the primal culvert 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 assortment 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 II – 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 bulk of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells every bit well as descending dorsolateral fasciculus (DLF) fibers. They transport axons to Rexed laminae Iii and Four (fasciculus proprius). High concentrations of substance P and opiate receptors have been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the effect of determining which pattern of incoming information will produce sensations that will be interpreted by the brain as being painful.

Rexed lamina 3 – Composed of variable jail cell size, axons of these neurons bisect 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, V and Vi. Nigh of the neurons in lamina 3 role equally propriospinal/interneuron cells.

Rexed lamina IV – The thickest of the first four laminae. Cells in this layer receive Aß axons which carry predominantly non-noxious information. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond to stimuli such as low-cal touch. The ill-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 V – Equanimous neurons with their dendrites in lamina 2. The neurons in this lamina receive monosynaptic information from Aß, Ad and C axons which also 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 Five cells project to the encephalon stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells.

Rexed lamina Half-dozen – Is a broad layer which is all-time developed in the cervical and lumbar enlargements. Lamina Six divides as well into medial and lateral parts. Grouping Ia afferent axons from muscle spindles terminate 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 likewise known every bit the zona intermedia (or intermediolateral nucleus). Its shape and boundaries vary forth the length of the cord. Lamina VII neurons receive information from Rexed lamina II to Vi equally 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 circular oval prison cell column from C8 to L3. The big cells give ascension to uncrossed nerve fibers of the dorsal spinocerebellar tract (DSCT). Cells in laminae 5 to Seven, which practise not form a discrete nucleus, requite rise to uncrossed fibers that grade the ventral spinocerebellar tract (VSCT). Cells in the lateral horn of the cord 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 requite rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia.

Rexed lamina 8 – Includes an area at the base of the ventral horn, just its shape differs at various cord levels. In the string enlargements, the lamina occupies only the medial role of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate motor activity, most probably via g motor neurons which innervate the intrafusal muscle fibers.

Rexed lamina Nine – Composed of several distinct groups of big a motor neurons and pocket-size γ and β motor neurons embedded within this layer. Its size and shape differ at various string levels. In the cord enlargements the number of α motor neurons increase and they form numerous groups. The α motor neurons are large and multipolar cells and give 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 Ten surround the central canal and occupy the commissural lateral area of the greyness commissure, which likewise contains decussating axons.

In summary, laminae I-Four are concerned with exteroceptive sensations, whereas laminae V and 6 are concerned primarily with proprioceptive sensation and act every bit a relay between the periphery to the midbrain and the cerebellum. Laminae VIII and IX form the final 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.

iii.8 White Matter

Surrounding the gray affair is white thing containing myelinated and unmyelinated nerve fibers. These fibers deport data up (ascending) or downwardly (descending) the cord. The white matter is divided into the dorsal (or posterior) column (or funiculus), lateral column and ventral (or anterior) column (Effigy 3.viii). The anterior white commissure resides in the centre of the spinal string, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nervus fiber types can be distinguished in the spinal string 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, ii) long descending nerve fibers originating from the cognitive cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal string greyness matter, and 3) shorter nervus fibers interconnecting diverse spinal cord levels such every bit the fibers responsible for the coordination of flexor reflexes. Ascending tracts are plant in all columns whereas descending tracts are found but in the lateral and the anterior columns.

Effigy 3.8
The spinal cord white matter and its iii columns, and the topographical location of the main ascending spinal string tracts.

4 different terms are ofttimes used to depict bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway. Funiculus is a morphological term to depict a large group of nerve fibers which are located in a given area (east.g., posterior funiculus). Inside a funiculus, groups of fibers from diverse origins, which share common features, are sometimes bundled in smaller bundles of axons called fasciculus, (e.m., fasciculus proprius [Effigy iii.8]). 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 group of nerve fibers which usually has the aforementioned origin, destination, and course and likewise 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 cord and ends in the thalamus). A pathway ordinarily refers to the entire neuronal circuit responsible for a specific function, and it includes all the nuclei and tracts which are associated with that function. For case, the spinothalamic pathway includes the jail cell bodies of origin (in the dorsal root ganglia), their axons equally they project through the dorsal roots, synapses in the spinal cord, and projections of second and tertiary order neurons across the white commissure, which arise to the thalamus in the spinothalamic tracts.

three.9 Spinal Cord Tracts

The spinal cord white affair contains ascending and descending tracts.

Ascending tracts (Figure 3.8). The nerve fibers comprise the ascending tract sally from the first club (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 conduct data related to tactile, 2 point bigotry of simultaneously applied pressure, 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 crude touch information from somatic and visceral structures. Nearby laterally, the dorsal and ventral spinocerebellar tracts comport unconscious proprioception information from muscles and joints of the lower extremity to the cerebellum. In the ventral column (funiculus) in that location are four prominent tracts: one) the paleospinothalamic tract (or anterior spinothalamic tract) is located which carry pain, temperature, and information 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, three) the spinoreticular tract, and four) the spinotectal tract. Intersegmental nervus fibers traveling for several segments (2 to 4) and are located as a thin layer around the greyness affair is known equally fasciculus proprius, spinospinal or archispinothalamic tract. It carries pain information to the brain stem and diencephalon.

Descending tracts (Figure 3.9). The descending tracts originate from different cortical areas and from brain stalk nuclei. The descending pathway carry information associated with maintenance of motor activities such equally posture, balance, muscle 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 as the reticulospinal vestibulospinal and the inductive corticospinal tract mediate balance and postural movements (Figure 3.ix). 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. Boosted details about ascending and descending tracts are described in the adjacent few chapters.

Figure 3.9
The principal descending spinal cord tracts.

iii.10 Dorsal Root

Figure 3.x
Spinal cord section with its ventral and dorsal root fibers and ganglion.

Information from the skin, skeletal muscle and joints is relayed to the spinal string 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 inbound several segments below and in a higher place their own segment. The ascending dorsal root fibers and the descending ventral root fibers from and to discrete body areas form a spinal nerve (Figure iii.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 modest myelinated axons carrying pain and temperature data to exist terminated in the Rexed laminae I, Two, and Iv of the gray matter. The medial sectionalization of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from peel, muscles and joints; information technology enters the dorsal/posterior column/funiculus and arise in the dorsal column to be terminated in the ipsilateral nucleus gracilis or nucleus cuneatus at the medulla oblongata region, i.eastward., the axons of the first-order (i°) 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 unlike Rexed lamina.

Axons entering the cord in the sacral region are found in the dorsal cavalcade about the midline and comprise the fasciculus gracilis, whereas axons that enter at higher levels are added in lateral positions and incorporate the fasciculus cuneatus (Effigy 3.xi). This orderly representation is termed "somatotopic representation".

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

3.eleven Ventral Root

Ventral root fibers are the axons of motor and visceral efferent fibers and emerge from poorly defined ventral lateral sulcus as ventral rootlets. The ventral rootlets from discrete spinal cord department unite and form the ventral root, which contain motor nerve 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 within the muscle spindles. The visceral neurons ship 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 (Figure 3.10).

3.12 Spinal Nerve Roots

The spinal nerve roots are formed by the wedlock of dorsal and ventral roots within the intervertebral foramen, resulting in a mixed nerve joined together and forming the spinal nerve (Effigy iii.10). Spinal nerve rami include the dorsal primary fretfulness (ramus), which innervates the peel and muscles of the back, and the ventral master fretfulness (ramus), which innervates the ventral lateral muscles and skin 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.

3.13 Blood Supply of the Spinal String

The arterial blood supply to the spinal string in the upper cervical regions is derived from two branches of the vertebral arteries, the inductive spinal artery and the posterior spinal arteries (Effigy iii.12). At the level of medulla, the paired anterior spinal arteries join to form a single artery that lies in the inductive median fissure of the spinal cord. The posterior spinal arteries are paired and form an anastomotic chain over the posterior attribute of the spinal cord. A plexus of small arteries, the arterial vasocorona, on the surface of the string constitutes an anastomotic connection between the anterior and posterior spinal arteries. This organization provides uninterrupted blood supplies along the entire length of the spinal cord.

Effigy 3.12
The spinal string arterial circulation.

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

Examination Your Knowledge

Question i
A
B
C
D
E

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 answer is Wrong.

The spinal string does non 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

Due east. Has no arachnoid membrane

The spinal cord...

A. Occupies the lumbar cistern

B. Has twelve (12) cervical segments This respond is Incorrect.

The spinal cord has vii (7) cervical segments.

C. Contains the cell bodies of postganglionic sympathetic efferent neurons

D. Ends at the conus medullaris

Eastward. 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 This answer is INCORRECT.

Postganglionic neurons are located in the periphery, non 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 jail cell bodies of postganglionic sympathetic efferent neurons

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

E. Has no arachnoid membrane

The spinal cord...

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

East. Has no arachnoid membrane This answer 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 string level of entry?

A. Corticospinal This reply is INCORRECT.

B. Ventral spinothalamic

C. Ventral spinocerebellar

D. Inductive spinocerebellar

East. Dorsal spinocerebellar

Which of the post-obit tracts crosses at the spinal string 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. 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 respond is Wrong.

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 INCORRECT.

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 INCORRECT.

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. 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 answer is Incorrect.

B. Posterior spinal arteries

C. Inductive spinal avenue

D. Basilar artery

E. Posterior communicating avenue

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

A. Vertebral arteries

B. Posterior spinal arteries This answer is Incorrect.

C. Anterior spinal artery

D. Basilar artery

E. Posterior communicating artery

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

A. Vertebral arteries

B. Posterior spinal arteries

C. Anterior spinal artery This answer is Right!

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

D. Basilar avenue

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 artery

D. Basilar avenue This answer is INCORRECT.

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

E. Posterior communicating avenue This answer is Incorrect.

Question 4
A
B
C
D
E

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

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region

E. Coccygeal region

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

A. Sacral region This respond is INCORRECT.

B. Thoracic region

C. Lumbar region

D. Cervical region

Due east. Coccygeal region

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

A. Sacral region

B. Thoracic region This answer is Wrong.

C. Lumbar region

D. Cervical region

E. Coccygeal region

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

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region This answer is Correct!

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

East. Coccygeal region

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

A. Sacral region

B. Thoracic region

C. Lumbar region

D. Cervical region

E. Coccygeal region This answer is Wrong.

Question 5
A
B
C
D
E