LESSON 16 THE SPINAL CORD AND NERVES Organization of White matter Posterior white columns lie between posterior gray horns and posterior Anterior white columns lie between anterior gray horns and anterior median sulcus median fissure o Anterior white commissure area where axons cross from one side of spinal cord to the other Lateral white columns located on each side of spinal cord between anterior and posterior columns Tracts or fasciculi are bundles of axons in the white columns o Relay same information in same direction o Ascending tracts carry information to brain o Descending tracts carry motor commands to spinal cord Spinal Cord Summary Spinal cord has a narrow central canal surrounded by gray matter o Containing sensory and motor nuclei Sensory nuclei are dorsal Motor nuclei are ventral o Covered by a thick layer of white matter Gray matter 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 possible to predict results of injuries to specific areas 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 of Spinal Nerves Epineurium outer layer o Dense network of collagen fibers Perineurium middle layer o Divides nerve into fascicles axon bundles Endoneurium inner layer o Surrounds individual axons Peripheral Nerves Interconnecting branches of spinal nerves surrounded by connective tissue sheathes Peripheral Distribution of Spinal Nerves Spinal nerves o Form lateral to the intervertebral foramen at the foramen the dorsal and ventral roots unite then branch and form 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 o In addition to motor impulses dorsal and ventral rami also carry Sensory nerves sensory information Dermatomes o Bilateral region of skin o Monitored by specific pair of spinal nerves Peripheral Neuropathy Regional loss of sensory or motor function Due to trauma or compression Shingles painful rash caused by chicken pox herpes virus that travels to the dermatome served by the affected sensory nerve Virus stays dormant in neurons of the anterior gray horns after chicken pox infection o Virus remains dormant in neurons of anterior gray horns Nerve Plexuses Complex interwoven networks of nerve fibers Control skeletal muscles of the neck and limbs Formed from blended fibers of ventral rami of adjacent spinal nerves The Four Major Plexuses of Ventral Rami Cervical plexus supplies head neck upper shoulder major nerve phrenic nerve C3 5 controls diaphragm Brachial plexus supplies pectoral girdle upper limbs Lumbar plexus supplies anterolateral abdominal wall genitals lower limbs Sacral plexus supplies buttocks perineum lower limbs Functional Organization of Neurons Sensory neurons Deliver information to CNS o About 10 million o About million o About 20 billion Motor neurons Deliver commands to peripheral effectors Interneurons Interpret plan and coordinate signals in and out Neuronal Pools Functional groups of interconnected neurons interneurons Each with limited input sources and output destinations May stimulate or depress parts of brain or spinal cord Neural Circuits in Neuronal Pools Five Patterns of Divergence Convergence Serial Processing Parallel processing Reverberation o Spreads stimulation to many neurons or neuronal pools in CNS visual info goes to visual cortex and postural balance areas o Brings input from many sources to single neuron subconscious and conscious control of breathing o Moves information in single line relay of pain info o Moves same information along several paths simultaneously step onto nail you withdraw foot shift weight feel pain and scream o Positive feedback mechanism o Functions until inhibited o May help maintain consciousness muscle coordination and normal breathing LESSON 17 SPINAL REFLEXES Reflexes Automatic responses coordinated with spinal cord through interconnected sensory neurons motor neurons and interneurons o Produces both simple and complex reflexes o Preserves homeostasis by making rapid adjustments in organs organ o Each time a particular reflex is activated produces the same motor systems function response Neural Reflexes Rapid automatic responses to specific stimuli Basic building block of neural function One Reflex arc the wiring of a single reflex neural reflex produces one motor response o Beginning at receptor o Ending at peripheral effector o Generally opposes original stimulus negative feedback Five Steps in a Neural Reflex 1 Arrival of stimulus activation of receptor a Physical or chemical changes 2 Activation of sensory neuron a Graded depolarization leads to action potential generation 3 Information processing by postsynaptic cell a Triggered by neurotransmitters 4 Activation of motor neuron a Action potential generated in motor neurons 5 Response to peripheral effector a Triggered by neurotransmitters cause skeletal muscle contraction Four Classifications of Reflexes By early Development o Innate reflexes formed before birth Basic neural reflexes objects with eyes o Acquired reflexes learned motor patterns Rapid automatic Ex Withdrawal from pain chewing suckling tracking Ex Riding a bike pressing on brakes when someone By type of Motor Response o Nature of resulting motor response cuts you off in traffic Somatic reflexes involuntary control of muscular system Includes Superficial reflexes of skin mucous membranes Ex Plantar reflex Stretch or deep tendon reflexes E g patellar or knee jerk reflex Visceral reflexes autonomic reflexes control systems other than muscular system e g Consensual light reflex By Complexity of Neural Circuit o Monosynaptic reflex only 1 synapse in reflex arc Sensory neuron synapses directly onto motor neuron o Polysynaptic reflex more than 1 synapse in reflex arc At least one interneuron between sensory neuron and motor neuron By Site of Information Processing o Spinal reflexes occur in spinal cord o Cranial reflexes occur in brain Spinal Reflexes Spinal reflexes range in increasing order of complexity From simplest to most complex o Monosynaptic reflexes o Polysynaptic reflexes o Intersegmental reflex arcs Many segments interact
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