Nervous System The Spinal Cord Nerves 11 22 2013 13 3 Organization of White Matter Divided into Three regions called columns or funiculi Three Columns o Posterior White Columns Lie between posterior gray horns and posterior median sulcus o Anterior White Columns Lie between anterior gray horns and anterior median fissure Anterior White Commissure Area where axons cross from one side of spinal cord to the other Where the anterior white columns are interconnected o Lateral White Columns Located on each side of spinal cord between anterior and posterior columns Tacts or fasciculi o Bundles of axons in the white columns Axons share functional and structural characteristics Uniform diameter myelination and conduction speed o Relay same information in same direction Relay either sensory or motor information depending on the tract o Ascending Tracts o Descending Tracts Carry sensory information to brain Carry motor commands to spinal cord Spinal tracts have very specific functions damage to one produces characteristic loss of sensation or motor control Spinal Cord Summary Spinal cord has a narrow central canal surrounded by gray matter o Contains sensory and motor nuclei Sensory nuclei are dorsal Motor nuclei are ventral Gray Matter White Matter o Covered by a thick layer of white matter o Consists of ascending and descending axons o Organized in columns o Contains axon bundles with specific functions Spinal cord is highly organized o Possible to predict results of injuries to specific areas 13 4 Spinal Nerves and Plexuses Spinal nerves form plexuses that are named according to their level of emergence from the vertebral canal Anatomy of Spinal Nerves Every spinal cord segment is connected to a pair of spinal nerves Every spinal nerve is surrounded by three connective tissue layers that support structures and contain blood vessels Three Connective Tissue Layers o Epineurium Outer layer Dense network of collagen fibers o Perineurium Middle layer Divides nerve into fascicles axon bundles Divides nerve into a series of compartments that contain bundles of axons fascicles o Endoneurium Inner Layer Surrounds individual axons Peripheral Nerves sheaths o Interconnecting branches of spinal nerved surrounded by connective tissue As they branch into the periphery the spinal nerves branch and interconnect form peripheral nerves that innervate body tissue and organs Connective tissue sheaths are continuous with those of the spinal nerves o Form lateral to the intervertebral foramen o At the foramen the dorsal and ventral roots UNITE then branch and form Peripheral Distribution of Spinal Nerves Spinal Nerves pathways to destination Motor Nerves o Dorsal and ventral rami Dorsal Ramus Contains somatic and visceral motor fibers Innervates the back Ventral Ramus Larger branch Innervates ventrolateral structures and limbs Sensory Nerves information Dermatomes o In addition to motor impulses dorsal and ventral rami also carry sensory o Bilateral region of the skin surface o Monitored by a single pair of spinal nerves Each pair services its own dermatome o Clinical Importance Damage or infection of a spinal nerve or dorsal root ganglion will produce a loss of sensation in the corresponding region of the skin Characteristic signs may appear on the skin supplied by that specific nerve Peripheral Neuropathy o Nerve palsies o Regional loss of sensory or motor function Ex Mild form is when foot falls asleep o Due to trauma or compression o The location of the affected dermatomes gives clues to the location of injuries along the spinal cord Information is NOT precise More exact If there is loss of motor control based on the origin and distribution of the peripheral nerves originating at nerve plexuses o Shingles Painful rash caused by chicken pox herpes virus that travels to the dermatome served by the affected sensory nerve Varicella zoster virus VZV Attacks neurons within the dorsal roots of spinal nerves and sensory ganglia of cranial nerves Distribution of rash corresponds to the affected sensory nerves and follows its dermatome Virus stay dormant after chicken pox infection Remains dormant in neurons of anterior gray horns Lies dormant when it reactivates shingles Treatment Large doses of antiviral drugs VZV vaccine Zostavax for use in people 60 years old who have had chicken pox Nerve Plexuses Complex interwoven networks of nerve fibers Formed from blended fibers of ventral rami of adjacent spinal nerves Control skeletal muscles of the neck and limbs Four Major Plexuses of Ventral Rami o Cervical Plexus Supplied head neck upper shoulder Phrenic Nerve Major nerve C3 5 Controls diaphragm a key respiratory muscle Contains the ventral rami of spinal nerves C1 C5 Innervates the neck diaphragm superior part of chest o Brachial Plexus Supplies pectoral girdle and upper limbs Contributions from the ventral rami of spinal nerves C5 T1 Major Nerves Musculocutaneous nerve Median nerve Ulnar nerve Axillary nerve Radial nerve o Lumbar Plexus Supplies anterolateral abdominal wall genitals and lower limbs Major Nerves Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve o Sacral Plexus Supplies buttocks perineum and lower limbs Major Nerves Sciatic nerve Fibular Nerve Tibial Nerve Pudenal Nerve 13 5 Neuronal Pools Neuronal pools are functional groups of interconnected neurons Functional Organization of Neurons Sensory Neurons o Deliver information to CNS o About 10 million Motor Neurons o Deliver commands to peripheral effectors o About million Interneurons o Interpret plan and coordinate signals in and out o About 20 billion Neuronal Pools Functional groups of interconnected neurons interneurons Each with limited input sources and output destination o Can contain both excitatory and inhibitory neurons May stimulate or depress parts of brain or spinal cord o Affects the interpretation of sensory information or the coordination of motor Either scattered or localized commands o Scattered o Localized Involving neurons in several regions of the brain With neurons restricted to one specific location in the brain or spinal cord in Neuronal Pools Neural Circuits Five Patters of o Divergence Spreads stimulation to many neurons or neuronal pool sin CNS Visual info goes to visual cortex and postural balance areas The spread of information from one neuron to several neurons or from one pool to multiple pools Spreading stimulation to MULTIPLE neurons or neuronal pools in the CNS Permits the broad distribution of a
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