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The Spinal Cord and Spinal Nerve Grouping of Neural Tissue White Matter - groups of neurons myelinated by: - Schwann cells in the PNS - Oligodendrocytes in the CNS Gray Matter - nerve cell bodies and dendrites or - unmyelinated axons and neuroglia Nerve - bundle of fibers (axons or dendrites) outside the CNS Ganglia - collections of nerve cell bodies outside the CNS Tract - a bundle of fibers in the CNS Ascending Tract - sensory tracts going to the brain Descending Tract - motor tracts coming from the brain Nucleus - a mass of unmyelinated nerve cell bodies in the CNS Horns - chief areas of gray matter in spinal cord SPINAL CORD Meninges Epidural space - between the dura mater and the wall of the vertebral canal - filled with adipose tissue which serves as padding Dura Mater - runs from the foramen magnum to S2 where it fuses with the filum terminale - continuous with the dura mater of the brain Subdural Space - contains serous fluid Arachnoid Mater - avascular delicate layer continuous with the arachnoid of the brain - adheres to the dura mater Subarachnoid Space - where Cerebral Spinal Fluid flows Pia Mater - adheres to the surface of the brain and spinal cord - Denticulate Ligaments - pieces of pia mater that suspend the spinal cord in the middle of the dural space and protect it against injury General Features of the Spinal Cord Enlargements - regions where the spinal cord is thicker - cervical - lumbar Conus Medullaris - pointed end of the cord near L1 - L2 Filum Terminale - pia mater strand that attaches spinal cord to the bottom of the vertebral canal. Cauda Equina - nerves that arise from the lower portion of the cord occupy the space below L2 Structure of the Spinal Cord in Cross-section Functions of the spinal cord 1. To convey sensory impulses from the PNS to the brain. 2. To conduct motor impulses from the brain to the PNS. 3. Integration of reflexes. Reflex Center - spinal cord is center for some reflex actions - the basic components are: 1. Receptor - responds to a stimulus and initiates a nerve impulse in a sensory neuron dendrite. 2. Sensory Neuron - has the cell body in the dorsal root ganglion (see diagram above) Passes nerve impulse into the spinal cord through the dorsal root to the posterior horn of the gray matter. 3. Center - region of the spinal cord where the incoming sensory information generates an outgoing motor impulse - usually contains internuncial neurons 4. Motor Neuron - transmits impulses to muscle or gland through the ventral root to the spinal nerve. 5. Effector - the organ (gland or muscle) that responds to the impulse from the motor neuron Types of Reflexes - reflexes are fast responses to certain changes in the internal or external environment that allow the body to maintain homeostasis - there are several types of reflexes - Spinal reflexes - carried out by the spinal cord alone - Somatic reflexes - result in the contraction of skeletal muscle - Cranial reflexes - involve brain centers and cranial nerves - Visceral (autonomic) reflexes - cause contraction of smooth or cardiac muscle, or secretion by glands Reflexes of Clinical Significance 1. Patellar Reflex - knee jerk - relates to damage to the 2nd, 3rd, or 4th lumbar vertebra - absent in people with chronic diabetes and neurosyphilis - exaggerated in disease or injury to corticospinal tracts 2. Achilles Tendon - ankle jerk - relates damage to lumbosacral region of the spine - absent in people with chronic diabetes, neurosyphillis, alcoholism, subarachnoid hemorrhages - exaggerated with cervical cord compression, or a lesion of the motor tracts of the first or second sacral segments of the cord 3. Babinski Sign - outer margin of the sole of the foot is stimulated - in children under l 1/2 years of age the great toe is extended with or without fanning of the toes - after 1 1/2 years may indicate corticospinal damage - in adults the response is to curl the toes under 4. Abdominal Reflex - stroke the side of the stomach and the umbilicus (belly-button) deviates laterally to the opposite side - absence indicates damage to corticospinal tracts, lesions of reflex centers in the thoracic area of the cord, or multiple sclerosis Spinal Nerves - 31 pairs of spinal nerves - named according to region and level of the spinal cord from which they emerge - 8 cervical - 12 thoracic - 5 lumbar - 5 sacral - a coccyx - spinal nerves attach to the spinal cord in two places 1. dorsal root - with sensory fibers 2. Ventral root - with motor fibers - therefore each spinal nerve is a mixed nerve with both sensory and motor fibers