Spinal_Cord

SPINAL CORD I. INTRODUCTION: • It is derived from the caudal part of the neural tube. • It is surrounded by three membranes, the meninges—the outer dura mater, the middle arachnoid mater and the inner pia mater. II. EXTERNAL MORPHOLOGY: A. LOCATION OF SPINAL CORD • It extends in adults, from the foramen magnum to the lower border of the L1 vertebra. • In newborns, it extends to the L3 vertebra. • It is continuous with the medulla oblongata at the spinomedullary junction, a plane which is marked by three structures: 1. the forman magnum, 2. the pyramidal decussation, and 3. the emergence of the first cervical nerve. B. RELATIONSHIP OF SPINAL CORD SEGMENTS WITH VERTEBRAL NUMBERS: • Because the spinal cord is shorter than the vertebral column, the spinal cord segments do not correspond numerically with the vertebrae that lie at the same level • The following table will help determine which spinal segment is related to a given vertebral body: VERTEBRAE SPINAL SEGMENT • Cervical vertebrae Cervical nerves +1 (C1-C8 & T1) • T1-T6 vertebrae Upto T6 nerve +2 (T2-T8) • T7-T9 vertebrae Upto T9 nerve +3 (T9-T12) • T10 vertebra L1 & L2 nerves • T11 vertebra L3 & L4 nerves • T12 vertebra L5 nerve • L1 vertebra All remaining nerves (S1-S5 & Co) C. ATTACHMENTS OF SPINAL CORD 1. LIGAMENTUM DENTICULATUM • These are two flattended bands of pia mater that attach to the dura mater with about 20 teeth. 2. FILUM TERMINALE • It is a vertical filament of pia mater extending from the conus medullaris to the back surface of vertebral foramen of coccyx, with which it fuses. 3. SPINAL NERVE ROOTS • Along the entire length of the spinal cord are attached 31 pairs of spinal nerves by the anterior or motor roots and posterior or sensory roots. • Each posterior root possesses a posterior root ganglion that is located in the intervertebral foramen. • Each root is attached to the spinal cord by a series of rootlets, which extend the whole length of the corresponding segment of the spinal cord. • They provide the strongest fixation of the spinal cord to the vertebral canal D. SHAPE OF SPINAL CORD • It is an elongated structure, flattened dorsoventrally, and is approximately 1 cm in diameter. • It has a fusiform cervical enlargement in the region of C5-T1 segments, where it gives origin to brachial plexus for the upper limb. • It has an other fusiform lumbar enlargement in the region of L1-S3 segments, where it gives origin to lumbar and sacral plexuses for the lower limb. • It tapers caudally as the conus medullaris. E. SPINAL NERVES • Along the entire length of the spinal cord are attached 31 pairs of spinal nerves—8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. • The spinal nerves contain both motor and sensory fibers. 1. SPECIAL CONSIDERATIONS • The C1 and the Co nerve usually have no dorsal (sensory) roots and no corresponding dermatomes. • The C1 passes between the atlas and the skull. • The C2 passes between the atlas and the axis. • With the exception of C1, all spinal nerves exit the vertebral canal via intervertebral or sacral foramina. • The C1-7 exit at the upper border of their corresponding vertebrae. • C8 exits at the lower border of C7 vertebra. • T1-Co exit at the lower border of their corresponding vertebrae. 2. FUNCTIONAL COMPONENTS OF SPINAL NERVE FIBERS a. General Somatic afferent (GSA) fibers • They convey sensory input form skin, muscles, bones, and joints to the central nervous system (CNS) b. General visceral afferent (GVA)fivers • They convey sensory input from visceral organs to the CNS. c. General somatic efferent (GSE) fibers • They convey motor output from anterior horn neurons to skeletal muscles. d. General visceral efferent (GVE) fibers • They convey motor output form lateral horn neurons (T1-L2), via paravertebral or prevertebral ganglia, to glands, smooth muscles, and visceral organs (=called sympathetic divisions of the autonomic) • They also convey motor output from the sacral parasympathetic nucleus (S2-S4) to the pelvic viscera via intramural ganglia (=called parasympathetic division of the autonomic nervous system). 3. COMPONENTS AND BRANCHES OF SPINAL NERVES • The spinal nerve is formed by the union of dorsal (anterior or motor) and ventral (posterior or sensory) roots within the corresponding intervetebral foramen, resulting in a mixed nerve. a. Dorsal root • It enters the dorsal horn as dorsal rootlets, conveying sensory input form the body via the dorsal root ganglion. • It joins the ventral root distal to the dorsal root ganglion, within the intervertebral foramen, to form the spinal nerve. b. Dorsal root ganglion • It is located on the dorsal or sensory root, within the intervertebral foramen. • It contains pseduounipolar neurons of neural crest origin, which transmit sensory input (GSA & GVA) form the periphery in its peripheral axons via nerve fibres, to the spinal cord in its central axons via the dorsal roots. c. Ventral root • It emerges as ventral rootlets from the anterior horn, conveying motor output form visceral and somatic motor neurons. • It joins the dorsal roots distal to the dorsal root ganglion and within the intervertebral foramen to form the spinal nerve. d. Cauda equina • It consists of the lumbosacral dorsal and ventral nerve roots (L2-Co) that descend from the spinal cord through the subarachnoid space to exit through their respective intervertebral or sacral foramina. 4. SPINAL NERVE RAMI a. Dorsal primary ramus • It innervates the skin and muscles of the back. b. Ventral primary ramus • It innervates the anterior and lateral muscles and skin of the trunk and extremities. c. Meningeal ramus • It innervates the meninges within the vertebral column. d. Gray communicating rami • They contain unmyelinated postganglionic sympathetic fibers • They are associated with all spinal nerves. e. White communicating rami • They contain myelinated preganglionic sympathetic fibers. • They are found only in thoracolumbar segments of the spinal cord (T-1-L2) F. SURFACE STRUCTURES 1. ANTERIOR MEDIAN FISSURE • It is a deep anterior midline groove. 2. ANTERIOR LATERAL SULCUS • It is a shallow groove from which the ventral rootlets emerge. 3. DORSAL LATERAL SULCUS • It is a shallow groove into which the dorsal rootless enter. 4. DORSAL INTERMEDIATE SULCUS • It is a shallow groove that is continuous with the dorsal intermediate septum that separates the fasciculus gracilis and the fasciculus cuneatus. • It is found between the dorsal lateral and posterior median sulci but only rostral to T6 spinal segment. 5. POSTERIOR MEDIAN SULCUS • It is a shallow dorsal midline groove that is continuous with the dorsal median septum. III. Internal Morphology • In transverse sections, the spinal cord consists of central gray matter and peripheral white matter. A. GRAY MATTER • It is located centrally within the spinal cord. • It is H-shaped in a configuration that varies according to spinal cord level. • The horizontal limb of the H is called the central gray commissure, that has the central canal in its middle. The part of central gray matter that lies anterior to the central canal is called the anterior gray commissure, while the part of central gray matter that lies posterior to the central canal is called the posterior gray commissure. • The vertical limb of the H is divided into three horns on each side. 1. NERVE CELLS IN POSTERIOR GRAY HORN • The posterior gray horn receives and processes sensory input. • It is found at all levels. • It includes the following nuclei: a. Substantia gelatinosa NAME: • Substantia gelatinosa CATEGORY: • GSA LOCATION: • It is situated at the apex of the posterior gray horn. EXTENT: • It is found at all cord levels. AFFERENTS: • It receives its afferents from the receptors associated with crude touch, pain, and temperature sensations. • It also receives input from descending fibres from supraspinal levels. EFFERENTS: It projects its efferents via spinothalamic tracts. b. Nucleus proprius NAME: • Nucleus proprious (the largest of the posterior horn nuclei) CATEGORY: • GSA LOCATION: • It is situated anterior to the substantia gelatinosa in the posterior grey horn. EXTENT: • It is found at all cord levels. AFFERENTS: • It receives its afferents from the posterior white columns which are associated with vibration, pressure, two-point discrimination (fine touch) and conscious proprioception. EFFERENTS: • It projects its efferents to various motor and sensory neuron groups in different spinal cord segments. c. Nucleus dorsalis (of Clarke) NAME: • Nucleus dorsalis CATEGORY: • GSA LOCATION: • It is situated at the base of the posterior grey horn. EXTENT: • It extends from C8 to L3. AFFERENTS: • It receives its afferents from the muscle spindle and Golgi tendon organs which are associated unconscious proprioception. EFFERENTS: • It projects its efferents via spinocerebellar tract. d. Visceral Afferent Nucleus (VAN) NAME: • Visceral afferent nucleus CATEGORY: • GVA LOCATION: • It is situated lateral to the nucleus dorsalis in the posterior grey horn. EXTENT: • It extends from T1 to L2. AFFERENTS: • It receives its afferents from the visceral organs. EFFERENTS: It projects its efferents mostly via spinothalamic tract. 2. NERVE CELLS IN LATERAL GRAY HORN • It is found between the dorsal and ventral horns. • It contains preganglionic sympathetic/parasympathetic fibres. a. Thoraco-lumbar sympathetic nucleus NAME: • Thoraco-lumbar sympathetic nucleus CATEGORY: • GVE LOCATION: • It is situated between the dorsal and ventral horns. EXTENT: • It extends from T1 to L2. AFFERENTS: • It receives its afferents from the descending autonomic fibres. EFFERENTS: • It projects its efferents via preganglionic sympathetic fibres to the visceral organs. b. Sacral parasympathetic nucleus NAME: • Sacral parasympathetic nucleus CATEGORY: • GVE LOCATION: • It is situated between the dorsal and ventral horns. EXTENT: • It extends from S2 to S4. AFFERENTS: • It receives its afferents from the descending autonomic fibres. EFFERENTS: • It projects its efferents via preganglionic parasympathetic fibres to the pelvic viscera. 3. NERVE CELLS IN ANTERIOR GRAY HORN • Most cells are large and multipolar, and their axons pass out in the anterior roots of the spinal nerves as alpha efferents, which innervate extrafusal fibres of the skeletal muscle. • Some cells are small and multipolar, and their axons pass out in the anterior roots of the spinal nerves as gamma efferents, which innervate intrafusal fibres of the skeletal muscle. • For practical purposes, the nerve cells of the anterior grey horn may be divided into three basic groups—medial intermediate and lateral. =MEDIAL GROUP OF ANTERIOR HORN CELLS= • It is present in most segments of the spinal cord. • It is responsible for innervating the skeletal muscles of the neck and trunk, including intercostals and abdominal musculature. • They are controlled by anterior corticospinal tracts, bilaterally =LATERAL GROUP OF ANTERIOR HORN CELLS= • It is present in cervical and lumbosacral segments of the spinal cord. • It is responsible for innervating the skeletal muscles of the lims. • They are controlled by contralateral lateral corticospinal tracts. =INTERMEDIATE GROUP OF ANTERIOR HORN CELLS= a. Spinal Accessory nucleus • It extends from C1 to C6. • It gives rise to the spinal root of the accessory nerve (CN XI). • It innervates the stenocleidomastoid and trapezius. b. Phrenic nucleus • It extends from C3 to C5. • It innervates the diaphragm. B. WHITE MATTER • It consists of bundles of myelinated fibers that surround the central gray matter. • It consists of ascending and descending fiber pathways called tracts. • It is divided bilaterally by sulci into three major division: 1. DORSAL COLUMN • It is located between the dorsal median sulcus and the dorsal lateral sulcus. • It is subdivided above T6 into two fasciculi:. a. Fasciculus gracilis • It is located between the dorsal median sulcus and the dorsal intermediate sulcus. • It is found at all cord levels. b. Fasciculus cuneatus • It is located between the dorsal intermediate sulcus and the dorsal lateral sulcus. • It is found only at the upper thoracic and cervical cord levels C1 – T6. 2. LATERAL COLUMN • It is located between the dorsal lateral and ventral latera sulci. • It contains the following major tracts: a. Anterior spinocerebellar tract b. Posterior spinocerebellar tract c. Lateral spinothalamic tract d. Lateral corticospiinal tract e. Rubrospinal tract 3. VENTRAL COLUMN • It is located between the anterior median fissure and the ventral lateral sulcus. • It contains the following major tracts: a. Olivospinal tract b. Vestibulospinal tract c. Anterior spinothalamic tract d. Anterior corticospinal tract e.Tectospinal tract