Spinal Cord PDF

SPINAL CORD Spinal cord The spinal cord is continuous with the medulla oblongata near the foramen magnum at the base of the skull. It is cylindrical in shape, and occupies the vertebral canal of the vertebral column to the intervertebral disc between L1 & L2 in an adult. Numerous as...

SPINAL CORD Spinal cord The spinal cord is continuous with the medulla oblongata near the foramen magnum at the base of the skull. It is cylindrical in shape, and occupies the vertebral canal of the vertebral column to the intervertebral disc between L1 & L2 in an adult. Numerous ascending and descending tracts pass through the spinal cord and connect with the brain to convey sensory (afferent) and motor (efferent) information.  External features The anterior and posterior surfaces of the spinal cord have several longitudinally running fissures and sulci. Along the midline on the anterior surface of the spinal cord is a deep separation, the anterior median fissure. Posteriorly the spinal cord has a shallower separation, the posterior median sulcus, which is flanked on either side by a posterolateral sulcus. Emerging from the spinal cord are a series of rootlets, which coalesce to form anterior and posterior roots at the corresponding cord segment. These anterior and posterior roots converge to form 31 pairs of spinal nerves along the length of the spinal cord and arranged as:  8 cervical spinal nerves  12 thoracic spinal nerves  5 lumbar spinal nerves  5 sacral spinal nerves  1 coccygeal spinal nerve. Along the length of the spinal cord two regions are enlarged to accommodate the numerous neurons innervating the upper and lower extremities. 1. The cervical enlargement extends from C5 to T1 and innervates the upper extremities. 2. The lumbar enlargement extends from L2 to S3 and innervates the lower extremities.  Spinal meninges Like the brain, the spinal cord is surrounded by three concentric meninges: the dura mater, arachnoid mater, and pia mater. The spinal dura mater is continuous with the inner meningeal layer of the cranial dura mater and extends inferiorly to the posterior surface of the vertebral body of S2. It is separated from the bony vertebral canal by the epidural/extradural space. In addition, unlike in the cranial cavity, the underlying arachnoid mater is not tightly adherent to the dura mater, and instead has a potential space called the subdural space. Although the arachnoid mater of the spinal cord has a less adherent relationship with the dura mater than in the cranial cavity, the overall structure of the arachnoid mater is the same. The subarachnoid space extends inferiorly to the level of the 2nd sacral vertebra. The innermost pia mater is a highly vascular layer that is adherent to the surface of the spinal cord. Midway between the anterior and posterior roots the pia mater forms a flat continuous sheet, the denticulate ligament. to attach onto the dura mater. These delicate attachments stabilize the position of the spinal cord within the central area of the subarachnoid space.  Vascular supply to the spinal cord Arterial supply The spinal cord is supplied by three longitudinally running vessels and several segmental branches. The longitudinally running vessels are the anterior spinal artery and two posterior spinal arteries. 1. The posterior spinal arteries originate in the cranial cavity as branches of the vertebral artery. These arteries descend along the length of the posterior spinal cord on the posterolateral sulcus. 2. The single anterior spinal artery originates within the cranial cavity from the union of two contributing branches from the vertebral arteries. The anterior spinal artery descends along the length of the anterior spinal cord on the anterior median fissure. 3. Reinforcing vascular supply to these longitudinally running vessels is provided by eight to ten radicular (segmental) arteries. Venous drainage of the spinal cord occurs through a series of longitudinally running channels that connect with the anterior and posterior spinal veins on the surface of the cord.  Internal features of the spinal cord A cross-section of the spinal cord reveals an inner H-shaped gray matter consisting of neuronal cell bodies and an outer white matter composed of myelinated neuronal axons. The ventral or anterior horns of gray matter contain cell bodies of motor neurons, whereas the dorsal or posterior horns contain cell bodies receiving sensory information. An enlargement of the lateral portion of the gray matter, can be seen in the T1 to L2 and S2 to S4 region of the spinal cord. The region from T1 to L2 enlarges to accommodate the preganglionic cell bodies of the sympathetic nervous system and The region from S2 to S4 enlarges to accommodate the preganglionic cell bodies of the parasympathetic nervous system.  White Matter The white matter, is divided into anterior, lateral, and posterior white columns or funiculi. The anterior funiculus on each side lies between the midline and the point of emergence of the anterior nerve roots. The lateral funiculus lies between the emergence of the anterior nerve roots and the entry of the posterior nerve roots. The posterior funiculus lies between the entry of the posterior nerve roots and the midline. Structure As in other regions of the central nervous system, the white matter of the spinal cord consists of a mixture of nerve fibers, neuroglia, and blood vessels. It surrounds the gray matter, and its white color is due to the high proportion of myelinated nerve fibers. Nerve cell groups in the anterior gray horns Most nerve cells are large and multipolar, and their axons pass out in the anterior roots of the spinal nerves as alpha efferents, which innervate skeletal muscles. The smaller nerve cells are also multipolar, and the axons of many of these pass out in the anterior roots of the spinal nerves as gamma efferents, which innervate the intrafusal muscle fibers of neuromuscular spindles. The nerve cells of the anterior gray column are divided into three basic groups or columns: medial, central, and lateral. 1. The medial group is present in most segments of the spinal cord and is responsible for innervating the skeletal muscles of the neck and trunk, including the intercostal and abdominal musculature. 2. The central group is the smallest and is present in some cervical segments. In the cervical part of the cord, some of these nerve cells (segments C3-5) specifically innervate the diaphragm and are collectively referred to as the phrenic nucleus. In the upper five six cervical segments, some of the nerve cells innervate the sternocleidomastoid and trapezius muscles. The axons of these cells form the spinal part of the accessory nerve. 3. The lateral group is present in the cervical and lumbosacral segments of the cord and is responsible for innervating the skeletal muscles of the limbs Nerve cell groups in the posterior gray horns There are four nerve cell groups of the posterior gray column: two that extend throughout the length of the cord and two that are restricted to the thoracic and lumbar segments. The substantia gelatinosa group is situated at the apex of the posterior gray column throughout the length of the spinal cord. It receives afferent fibers concerned with pain and temperature from the posterior root. The nucleus proprius is a group of large nerve cells situated anterior to the substantia gelatinosa throughout the spinal cord. This nucleus constitutes the main bulk of cells present in the posterior gray column and receives fibers from the posterior funiculus that are associated with the senses of pressure and crude touch. The nucleus dorsalis (Clarke’s column) is a group of nerve cells situated at the base of the posterior gray column and extending from the 8th cervical segment caudally to the third or fourth lumbar segment. Most of the cells are large and are associated with proprioceptive endings. The visceral afferent nucleus is a group of nerve cells of medium size situated lateral to the nucleus dorsalis; it extends from the first thoracic to the third lumbar segment of the spinal cord. It is associated with receiving visceral afferent information. Nerve cell groups in the lateral gray columns The lateral group of cells form the small lateral gray column, which extends from the first thoracic to the second or third lumbar segment of the spinal cord. The cells are relatively small and give rise to preganglionic sympathetic fibers. A similar group of cells found in the second, third, and fourth sacral segments of the spinal cord give rise to preganglionic parasympathetic fibers. Gray commissure and central canal In transverse sections of the spinal cord, the anterior and posterior gray columns on each side are connected by a transverse gray commissure. In the center of the gray commissure is situated the central canal. The central canal is present throughout the spinal cord. Superiorly, it is continuous with the central canal of the medulla oblongata, and above this, it opens into the cavity of the fourth ventricle. Inferiorly in the conus medullaris, it expands into the fusiform terminal ventricle. It is filled with cerebrospinal fluid and is lined with the ependyma.  Ascending tracts in the spinal cord Sensory information entering the CNS from peripheral sensory receptors is conducted through a series of neurons that synapse with targets in the spinal cord, cerebral cortex, and other brain structures. The sensory modalities carried in these pathways include pain, temperature, touch, and proprioceptive input. Two somatosensory pathways ascend within the spinal cord to reach the cortex: 1. Posterior column pathway (proprioceptive sensation), which conveys sensations of fine touch, vibration, and proprioception. 2. Anterolateral pathways (extroceptive sensation), which convey sensations of pain, temperature, and crude touch. Both of these pathways transmit information through a series of three order neurons. Posterior column pathway First-order neuronal cells bodies of the posterior column pathway are located in a spinal ganglion. Axons then enter the spinal cord through the posterior root to reach either the gracile tract which carries information from the lower limb and trunk, or the cuneate tract, which carries information from the upper limb and neck. These first-order axons then ascend ipsilaterally to the medulla oblongata and synapse with the second-order neuronal cell bodies within the gracile and cuneate nuclei. Axons of these second-order neurons then cross over as the internal arcuate fibers to form the medial lemniscus in the medulla. These second-order axons ascend through the brainstem to reach the third-order neuronal cell bodies in the postero-lateral ventral nucleus of the thalamus. Axons from the third-order neurons then project through the posterior limb of the internal capsule to reach the primary somatosensory cortex.  Anterolateral pathways The anterolateral pathways are composed of tracts: the spinothalamic, spinoreticular, spinocerebellar tracts, spinotectal, spino-olivary. Lateral Spinothalamic Tract The pain and temprature receptors in the skin and other tissues are free nerve endings. The axons entering the spinal cord from the dorsal root ganglion proceed to the tip of the posterior gray column and travel for a distance of one or two segments of the spinal cord and form the dorso-lateral tract of Lissauer. These fibers of the first-order neuron terminate by synapsing with cells in the posterior gray column, including cells in the substantia gelatinosa of Rolandi. The axons of the second-order neurons cross obliquely to the opposite side in the anterior gray and white commissures within one spinal segment of the cord, ascending in the contralateral white column as the lateral spinothalamic tract. The lateral spinothalamic tract lies medial to the anterior spinocerebellar tract and ascend up in the spinal cord to reach the medulla to join the ventral spinothalamic tract forming the spinal lemniscus. The spinal lemniscus terminates by synapsing in the posterolateral ventral nucleus of the thalamus Anterior Spinothalamic Tract The axons enter the spinal cord from the dorsal root ganglion and proceed to the tip of the posterior where they travel for a distance of one or two segments of the spinal cord, contributing to the dorsolateral tract of Lissauer. These fibers of the first-order neuron terminate by synapsing with cells in the nucleus proprius group in the posterior gray column. The axons of the second-order neuron cross to the opposite side in the anterior gray and white commissures within several spinal segments and ascend in the opposite anterior funiculus as the anterior spinothalamic tract. The anterior spinothalamic tract ascend up in the spinal cord to reach the medulla to join the lateral spinothalamic tract forming the spinal lemniscus.. The fibers of the anterior spinothalamic tract terminate by synapsing with the third-order neuron in posterolateral ventral nucleus of the thalamus. Muscle Joint Sense Pathways to the Cerebellum Posterior Spinocerebellar Tract The axons entering the spinal cord from the dorsal root ganglion enter the posterior gray column and terminate by synapsing on the second-order neurons at the base of the posterior gray column. These neurons are the dorsal nucleus (Clarke’s nucleus). The axons of the second-order neurons enter the posterior part of the lateral funiculus on the same side and ascend as the posterior spinocerebellar tract to the medulla oblongata. Here, the tract joins the inferior cerebellar peduncle and terminates in the cerebellar cortex. The posterior spinocerebellar fibers receive muscle joint information of the trunk and lower limbs (proprioceptive sensation). This information concerning tension of muscle tendons and the movements of muscles and joints is used by the cerebellum in the coordination of limb movements and the maintenance of posture. Anterior Spinocerebellar Tract The axons entering the spinal cord from the dorsal root ganglion terminate by synapsing with the second-order neurons in the nucleus dorsalis scattered at the base of the posterior gray column. The majority of the axons of the second-order neurons cross to the opposite side and ascend as the anterior spinocerebellar tract in the lateral funiculus. The fibers, ascend through the medulla oblongata and pons, enter the cerebellum through the superior cerebellar peduncle and terminate in the cerebellar cortex. The anterior spinocerebellar tract conveys muscle joint information of the trunk and the upper and lower limbs. Spinoreticular Tract The axons enter the spinal cord from the dorsal root ganglion and terminate on unknown second-order neurons in the gray matter. The axons from these second order neurons ascend the spinal cord as the spinoreticular tract in the lateral funiculus mixed with the lateral spinothalamic tract. The fibers terminate by synapsing with neurons of the reticular formation in the medulla oblongata, pons, and midbrain. The spinoreticular tract provides an afferent pathway for the reticular formation, which plays an important role in influencing levels of consciousness. In addition, it carries burning sensation Spinotectal Tract The axons enter the spinal cord from the dorsal root ganglion and travel to the gray matter where they synapse on unknown second-order neurons. The axons of the second-order neurons cross the median plane and ascend as the spinotectal tract in the lateral funiculus close to the lateral spinothalamic tract. After passing through the medulla oblongata and pons, they terminate by synapsing with neurons in the superior colliculus of the midbrain. This pathway provides afferent information for spinovisual reflexes and brings about movements of the eyes and head toward the source of the stimulation. Spino-olivary Tract The axons enter the spinal cord from the dorsal root ganglion and terminate on unknown second- order neurons in the posterior gray column. The axons from the second-order neurons cross the midline and ascend as the spino-olivary tract in the lateral funiculus. The axons end by synapsing on third-order neurons in the inferior olivary nuclei in the medulla oblongata. The axons of the third- order neurons cross the midline and enter the cerebellum through the inferior cerebellar peduncle. The spino-olivary tract conveys information to the cerebellum from cutaneous and proprioceptive organs.  Visceral Sensory Tracts Sensations that arise in viscera located in the thorax and abdomen enter the spinal cord through the posterior roots. The cell bodies of the first-order neuron are situated in the dorsal root ganglia. The peripheral processes of these cells receive nerve impulses from pain and stretch receptor endings in the viscera. The central processes, having entered the spinal cord, synapse with second-order neurons in the gray matter, (visceral afferent nucleus) in the posterior gray columns. The axons of the second-order neurons join the spinothalamic tracts and ascend to terminate on the third- order neurons in the ventral posterolateral nucleus of the thalamus.  Descending tracts in the spinal cord Descending tracts through the spinal cord are involved in voluntary movements. These pathways originate from the cerebral cortex and brainstem. The tracts in each of these systems are composed of: 1. Upper motor neuron with cell bodies located in the cerebral cortex or brainstem. 2. Lower motor neuron with cell bodies located in the spinal cord gray matter. It is formed of tracts of the medial and lateral motor systems.  Lateral motor systems Tracts of the lateral motor system include the lateral corticospinal tract and rubrospinal tract. Both are located in the lateral funiculus and synapse on the anterior horn gray. Lateral corticospinal tract Clinically, the most important tract because it is responsible for controlling movement of the upper and lower extremities. Cell bodies of upper motor neurons forming this tract are located in the primary motor cortex. most of the axons decussate to the contralateral side to form the lateral corticospinal tract. The remaining axons will stay ipsilateral and form the anterior corticospinal tract, a tract included in the medial motor systems. After decussating and forming the lateral corticospinal tract, the axons descend through the spinal cord to synapse on the lateral portion of the anterior horn cells.  Rubrospinal tract The other lateral motor system pathway. Cell bodies of upper motor neurons begin in the red nucleus of the midbrain. After leaving the red nucleus, the axons cross the midline and descend as the rubrospinal tract through the brainstem and lateral funiculus of the spinal cord. These axons only descend to cervical regions of the spinal cord, and axons synapse with interneurons in the anterior gray horn to facilitate flexor muscle activity and inhibit extensor muscle activity of the upper limb.  Medial motor systems Tracts of the medial motor system regulate axial or truncal muscles involved in maintaining posture, balance. The four tracts of the medial motor system includes; the anterior corticospinal tract, tectospinal tract, vestibulospinal tract and reticulospinal tract. Anterior corticospinal tract It is formed by the remaining descending upper motor neurons that did not decussate in the medulla and descends through the anterior funiculus of the spinal cord to the level of the upper thoracic region. These axons project bilaterally to synapse on the medial portion of the anterior horn cells. Axons of these lower motor neurons then exit the spinal cord through the anterior root.  Tectospinal tract The axons arise from cell bodies located in the superior colliculus of the dorsal midbrain. These axons decussate after leaving the nucleus to form the tectospinal tract. It continues through the brainstem and cervical regions of the spinal cord near the anterior median fissure. Within the cervical spinal cord, axons project bilaterally to synapse on the anterior horn cells. As the superior colliculus receives visual input, the tectospinal tract modulates reflex postural movements in response to visual stimuli (spino-visual reflexes) Vestibulospinal tract Axons arise from vestibular nuclei located in the pons and medulla oblongata. The medial vestibular nucleus gives rise to the medial vestibulospinal tract, which projects bilaterally to thoracic regions of the spinal cord, and The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract, which descends ipsilaterally through the entire length of the spinal cord to synapse on the anterior horn cells. Given that the vestibular nuclei receive sensory input from the inner ear and cerebellum, this tract facilitates activity of extensor/antigravity muscles and inhibits activity of flexor muscles to maintain balance and an upright posture.  Reticulospinal tract Axons arise from the reticular formation in the pons and medulla oblongata. The axons of the pontine and medullary reticulospinal tracts descend ipsilaterally through the length of the spinal cord in the anterior funiculus and synapse in the anterior horn gray matter. They are believed to function in regulating voluntary movements in reflex activity.  Descending autonomic fibers The higher centers of the central nervous system associated with the control of autonomic activity are situated in the cerebral cortex, hypothalamus, and reticular formation. The descending autonomic tracts exist and form part of the reticulospinal tract. The fibers arise from neurons in the higher centers and cross the midline in the brainstem. They descend in the lateral funiculus of the spinal cord and terminate by synapsing on the autonomic motor cells in the lateral gray columns in the thoracic and upper lumbar (sympathetic outflow) and midsacral (parasympathetic) levels of the spinal cord.  Intersegmental tracts Short ascending and descending tracts that originate and end within the spinal cord exist in the anterior, lateral, and posterior funiculus. The function of these pathways is to interconnect the neurons of different segmental levels, and the pathways are particularly important in intersegmental spinal reflexes.