Ch.13 notes - Spinal Cord, Nerves, and Reflexes
Ch.13 notes - Spinal Cord, Nerves, and Reflexes BIOL 3455.001
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This 9 page Class Notes was uploaded by Filza on Thursday September 22, 2016. The Class Notes belongs to BIOL 3455.001 at University of Texas at Dallas taught by Dr. Ramirez in Summer 2015. Since its upload, it has received 14 views. For similar materials see Anatomy and Physiology in Biology at University of Texas at Dallas.
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Date Created: 09/22/16
Ch.13 Notes - Spinal Cord, Spinal Nerves, Spinal Reflexes 13-1: CNS & PNS - brain and spinal cord - CNS; cranial & spinal nerves - PNS - reflexes - quick, autonomic responses triggered by specific stimuli - ANS - visceral functions (visceral effectors - smooth/cardiac muscle, glands, adipose tissue) 13-2: Spinal cord carries sensory & motor info - spinal cord surrounded by 3 meninges - adult spinal cord ends btw L1 & L2 - posterior median sulcus - shallow, longitudinal groove on posterior (dorsal) surface of spinal cord - anterior median fissure - deeper groove along anterior (ventral) surface - the amount of gray matter is greatest in segments of the spinal cord dedicated to the sensory & motor control of the limbs - enlargement - - cervical enlargement - supplies nerves to shoulder & upper limbs - lumbar enlargement - provides innervation to structures of pelvis & lower limbs - conus medularis - region where spinal cord becomes tapered & conical - filum terminale - slender strand of fibrous tissue, extends from inferior tip of conus medullaris - cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), and sacral segments (S1-S5) - spinal cord divided into 31 segments on basis of spinal nerve’s origin - dorsal root ganglia contain cell bodies of sensory neurons - dorsal root bring sensory info into spinal cord - ventral roots contain axons of motor neurons that extend into periphery to control somatic/ visceral effectors - spinal nerves classified as mixed nerves (both sensory & motor fibers) - spinal cord continues to enlarge and elongate until a person is 4 yrs old - after age 4, vertebral column continues to elongate but spinal cord does not - filum terminale & long ventral/dorsal roots resemble horse’s tail (called cauda equina) - spinal meninges - series of specialized membranes surrounding spinal cord; physical stability & shock absorption - spinal meninges consist of 3 layers: dura mater, arachnoid mater, pia mater - at foramen magnum of skull, spinal meninges are continuous w/the cranial meninges - meningitis - inflammation of the meningeal membranes - dura mater is tough/fibrous & forms outermost covering of spinal cord • contains dense collagen fibers • epidural space - btw dura mater & vertebral canal - contains areolar tissue, blood vessels, & adipose tissue • dura mater doesn’t have extensive, firm connections to vertebrae • outer layer of dura mater fuses w/the periosteum of occipital bone around the margins of the foramen magnum • coccygeal ligament - dura mater tapers from a sheath to a dense cord of collagen fibers that blends w/components of the filum terminale to form the coccygeal ligament - arachnoid mater - middle layer • inner surface of dura mater & outer surface of arachnoid mater covered by simple squamous epithelia • arachnoid trabeculae - delicate network of collagen fibers that extends btw the arachnoid membrane & the outer surface of the pia mater • subarachnoid space filled w/CSF • arachnoid mater extends inferiorly as far as filum terminale, & dorsal/ventral roots of cauda equina lie w/in the fluid-filled subarachnoid space • spinal tap (aka lumbar puncture) - withdrawal of CSF involve insertion of needle into subarachnoid space in inferior lumbar region - pia mater - innermost layer - meshwork of elastic/collagen fibers that is firmly bound to the underlying neural tissue • blood vessels servicing spinal cord run along surface of pia mater, w/in the subarachnoid space 13-3: Gray matter & white matter - gray matter integrates info & initiates commands - white matter carries info from place to place - anterior median fissure & posterior median sulcus divide spinal cord into left & right sides - gray matter forms H or butterfly shape & surrounds central canal - gray matter include cell bodies of neurons, neuroglia, & unmyelinated axons - superficial white matter contain large #s of myelinated & unmyelinated axons Organization of gray matter - nuclei - masses of gray matter w/in CNS - sensory nuclei - receive/relay sensory info from peripheral receptors - motor nuclei - issue motor commands to peripheral effectors - frontal section along length of central canal of spinal cord separates sensory (posterior, dorsal) nuclei from motor (anterior, ventral) nuclei - posterior gray horns contain somatic & visceral sensory nuclei - anterior gray horns contain somatic motor nuclei - lateral gray horns - in thoracic/lumbar segments - contain visceral motor nuclei - gray commissures posterior to & anterior to central canal contain axons that cross from one side of the cord to the other before they reach an area in gray matter Organization of white matter - divided into 3 regions called columns, or funiculi - anterior white commissure - axons cross from one side of spinal cord to the other - each column contains tracts whose axons share functional & structural characteristics - tract - fasciculus - bundle of axons in CNS that is somewhat uniform in diameter, myelination, & propagation speed - all axons w/in a tract relay same type of info in same direction - ascending tracts carry sensory info toward brain & descending tracts convey motor commands to spinal cord 13-4: Spinal nerves form plexuses - connective tissue layers surround each spinal nerve - epineurium - outermost layer - consist of dense network of collagen fibers - perineurium - middle layer - divide nerves into series of compartments that contain bundles of axons, or fascicles - endoneurium - innermost layer - surround individual axons - arteries and veins penetrate the epineurium and branch within the perineurium - capillaries leaving the perineurium branch in the endoneurium - they supply axons and Schwann cells of the nerve and the fibroblasts of the connective tissues - if peripheral axon is severed but not displaced, normal function may return • repairs made after an entire peripheral nerve has been damaged are incomplete due to problems w/axon alignment & regrowth - dermatome - bilateral region of skin surface monitored by a single pair of spinal nerves - each pair of spinal nerves supplies its own dermatome - peripheral nerve palsies (or peripheral neuropathies) - regional losses of sensory & motor function resulting from nerve trauma or compression - dorsal & ventral rami, or branches, applies to spinal nerves T1-L2 - as they converge, ventral rami of adjacent spinal nerves blend their fibers, producing series of compound nerve trunks - nerve plexus - complex interwoven network of nerves is called nerve plexus - only the ventral rami form plexuses - 4 major plexuses: cervical, brachial, lumbar, and sacral plexuses - phrenic nerve - major nerve of cervical plexus; left & right phrenic nerve supply diaphragm - brachial nerve innervates the pectoral girdle & upper limb (C5-T1; can also have fibers from C4 and T2) • nerves that form this plexus originate from trunks & cords • trunks - large bundles of axons from several spinal nerves • cords - smaller branches that originate at trunks • axillary artery supplies upper limb lateral cord forms the musculocutaneous nerve • • median nerve form median & ulnar nerve • posterior cord give rise to axillary & radial nerve - lumbar plexus contain axons from T12-L4 • major nerves are genitofemoral nerve, lateral femoral cutaneous nerve, & femoral nerve - sacral plexus contain axons from L4-S4 • major nerves are sciatic nerve & pudendal nerve • sciatic nerve divides into 2 branches: fibular & tibial nerve • sural nerve formed by fibular nerve - innervate lateral portion of foot; used in nerve grafts • reflexes - automatic responses that are motor responses to specific stimuli 13-5: Interneurons are organized into functional groups called neuronal pools - body has 10 million sensory neurons, 1/2 million motor neurons, 20 billion interneurons - interneurons provide interpretation, planning, & coordination of incoming/outgoing signals - neuronal pools - functional groups of interconnected neurons - few hundred to few thousand neuronal pools - each pool has limited # of input sources/output destinations & may contain both excitatory & inhibitory neurons - output of neuronal pool may stimulate or depress activity in other parts of brain/spinal cord - neural circuits - 5 circuit patterns: • divergence - spread of info from 1 neuron to several neurons or 1 pool to multiple pools - permits broad distribution of specific input • convergence - several neurons synapse on single post-synaptic neuron - several patterns of activity in presyn. neuron can have same effect on post-syn neuron - same motor neurons can be subject to both conscious/subconscious control • serial processing - info is relayed in stepwise fashion, from 1 neuron to another or from 1 pool to the next - occurs as sensory info is relayed from one part of the brain to another - ex: pain sensations on way to consciousness make stops at 2 neuronal pools • parallel processing - several neurons/pools process the same info simultaneously - divergence must take place before parallel processing can occur - as result, many responses can occur simultaneously - ex: stepping on sharp object - neurons distribute info to several pools —> result: withdraw foot, shift position, feel pain, etc. • reverberation - collateral branches of axons along circuit extend back toward source of an impulse & further stimulate the pre-synaptic neurons - like a positive feedback loop involving neurons - once rev. circuit activated, it will continue to function until synaptic fatigue or inhibitory stimulus break cycle - can take place in single or interconnected neuronal pools - ex: help maintain consciousness, muscular coordination, breathing - most complex neural processing steps take place in brain & spinal cord - simplest steps occur w/in PNS & spinal cord - reflexes are basic building blocks of neural function 13-6: Reflexes are rapid, automatic responses to stimuli - reflexes maintain homeostasis by making rapid adjustments in the function of organs/systems - neural reflex - sensory fiber delivers info from peripheral receptors to integration center in CNS & motor fibers carry motor commands to peripheral effectors - endocrine reflex - hormones in bloodstream carry commands to peripheral tissues/organs ReflexArc - route followed by nerve impulses to produce reflex - 5 steps in a neural reflex arc: • arrival of a stimulus & activation of a receptor - ex: lean on tack • activation of a sensory neuron - dendrites stretch —> graded depolarization leads to formation/propagation of action potentials along sensory neuron axons - this info reaches spinal cord by dorsal root - 1st & 2nd steps may involve same or diff. cells • info processing in the CNS - excitatory neurotransmitter (NT) arrive at postsynaptic membrane of interneuron - NT produces EPSP, which is integrated w/other stimuli arriving at the postsynaptic cell atm - info processing thus performed by the interneuron - in simplest reflex (ex: patellar stretch reflex), sensory neuron innervates motor neuron directly (monosynaptic reflex) —> motor neuron performs the info processing - complex reflex involve several interneurons & can release inhibitory/excitatory NTs • activation of motor neuron - axons of stimulated motor neurons carry ac. potentials into periphery thru ventral root response of a peripheral effector • - ex: skeletal muscle whose contraction pulls your hand away from tack - reflex response removes or opposes the original stimulus—> negative feedback - polysynaptic reflexes produce more complicated responses bc interneurons can control motor neurons that activate several muscle groups simultaneously - withdrawal reflex moves affected body parts away from stimulus - flexor reflex - example of withdrawal reflex that affects the muscles of a limb - crossed extensor reflex - involves a contralateral reflex arc • motor response to stimulus also occurs on side opposite the stimulus • complements flexor reflex & the 2 occur simultaneously - reflexes classified on the basis of: development, nature of resulting motor response, complexity of the neural circuit involved, site of info processing - innate reflex - result from the connections that form btw neurons during development • ex: removing hand from hot stove; blinking when eyelashes touch • neural connections responsible for basic motor patterns of innate reflex are genetically programmed - acquired reflexes - more complex, learned motor patterns (ex: driver stepping on break) - somatic reflex - provide mechanism for the involuntary control of the muscular system • rapid response that can be modified later; ex: if slip when walking down stairs - superficial reflexes - triggered by stimuli at the skin or mucous membranes - stretch reflexes - triggered by sudden elongation of a tendon, & muscle it’s attached to - deep tendon reflex - aka myotatic reflex - ex: patellar “knee-jerk” reflex - visceral reflex - autonomic reflexes - control the activities of other systems - polysynaptic reflex - complicated & have at least one interneuron btw sensory & motor neuron • longer delay btw stimulus & response 13-7: Spinal Reflexes Vary in Complexity - intersegmental reflex arcs - complicated spinal reflexes - segments interact to produce a coordinated, highly variable motor respons - monosynaptic reflexes control the most rapid, stereotyped (preexisting, repetitive) motor responses to specific stimuli - patellar reflex - action potentials conducted by large, myelinated TypeAfibers - muscle spindles - receptors in stretch reflexes: • stretching of muscle spindles produce burst of activity in sensory neurons • then leads to stimulation of motor neurons that control motor units in stretched muscle • result is rapid muscle shortening—> return muscle spindles to resting length • rate of action potential generation in sensory neurons dec.—> drop in muscle tone to resting levels - intrafusal muscle fibers - small, specialized skeletal muscle fibers - extrafusal muscle fibers - larger skeletal muscle fibers - responsible for resting muscle tone & greater levels of stimulation, for contraction of entire muscle - gamma motor neurons - motor neurons innervating intrafusal fibers; axons are gamma efferents - postural reflex - help us maintain normal upright posture - gamma efferents let CNS adjust the sensitivity of muscle spindles • vital role whenever voluntary contractions change length of muscle - tendon reflex - monitors external tension produced during muscular contraction & prevents tearing/breaking of tendons - reciprocal inhibition - when one set of motor neurons is stimulated, neurons that control antagonistic muscles are inhibited - ipsilateral reflex arcs - sensory stimulus & motor response occur on same side of body - polysynaptic reflexes range from simple to complex reflexes - all polysynaptic reflexes share basic characteristics: • they involve pools of interneurons • intersegmental in distribution • involve reciprocal inhibition • have reverberating circuits, which prolong the reflexive motor response • several reflexes may cooperate to produce a coordinated, controlled response 13-8: The brain can affect spinal cord-based reflexes - descending pathways from brain provide appropriate facilitations, inhibition, fine-tuning of the established patterns - EPSP make neuron more sensitive to other excitatory stimuli - IPSP makes neuron less responsive to excitatory stimuli - when many of the excitatory synapses are chronically active, the postsynaptic neuron can enter a state of generalized facilitation - reinforcement - an enhancement of spinal reflexes - Jendrassik maneuver - method used to overemphasize the patellar reflex • interlock fingers and try to pull hands apart while light tap is applied to patella - plantar reflex - negative Babinski reflex - curling of toes - positive Babinski reflex - aka Babinski sign - fanning of the toes sensory information diagram motor commands diagram Clinical cases - anesthesia - injected into epidural space • in this way drug affect only spinal nerves in immediate area • result is regional pain reliever called epidural anesthesia & block pain sensations • provides sensory anesthesia • spinal anesthesia - single dose in subarachnoid space - temporary muscle paralysis & sensory loss - spread as movement of CSF distribute anesthetic along spinal cord - respiration continues - shingles - same virus that cause chickenpox (VZV) also cause shingles • this herpes virus attack neurons w/in dorsal roots of spinal nerves & sensory ganglia of cranial nerves • produce painful rash & blisters only in affected areas & follows its dermatome • virus remains dormant w/in neurons of anterior gray horns - carpal tunnel syndrome - compression of one or more nerves at wrist cause changes or losses of sensations in areas innervated by the affected nerves