Lecture 13 Lecture Outline:I Reflexesa. Physiologyb. ClassificationII Skeletal musclea. Muscle spindlesIII Types of reflexesa. Myotatic reflexb. Flexor withdrawalSpinal Cord: two-way communication pathway, integrating center for spinal reflexes (done in gray matter)Reflex: involuntary response to a stimulus, a “pre-wired” response- Reflex arc: all parts of the NS responsible for the reflex- Sensory receptor is activated, stimulus energy converted to electrical signal- Signal sent down afferent pathway to the integration center (spinal cord)- Signal sent down efferent pathway to the effector (usually skeletal muscle)- Polysynaptic reflex: multiple synapses in the integrating center- Monosynaptic reflex: neurons only synapse once while in integrating center (shorter latent period)- Spinal reflex: processing occurs in the spinal cord- Cranial reflex: processing occurs in the brain- Somatic reflexes: control skeletal muscle contractions, include superficial and stretch reflexes- Visceral reflexes: control actions of smooth and cardiac muscles, glands- Control of skeletal muscle innervated by somatic motor neurons originating in ventral horn of SC and cranial nerve motor nuclei (brain stem) called alpha motor neuronso Alpha motor neurons classified as lower motor neurons- Control activity of alpha motor neurons:o Uses temporal and spatial summation of EPSPs and IPSPso Alpha motor neuron: Receives synaptic input from higher CNS areas (motor cortex, brain stem), somatosensory afferents, and proprioceptors in muscle spindles, that it must interpret to determine if action potential is sent to effector- Myotatic (muscle stretch) reflex: basic mechanism of muscle tone (resistance a skeletal muscle offers to being stretched)o Present in all skeletal muscles, strongest in the extensor muscles of the limbs, and backo Counters the influence of gravity, maintaining postureo Structure of muscle spindles:  Group of intrafusal muscle cells contained in connective tissue capsule Function is to send information about how much they are being stretched, muscle length Stimulated to contract by gamma motor neurons Stretching the muscle will cause spindle to send more action potentials Contracting the muscle will cause spindle to send less action potentials Mechanical contraction of spindle fibers can cause skeletal muscle contraction (knee jerk)Bio 312 Frequency of action potentials from spindles communicates length of muscle Agonist muscles flex muscles of a current joint, antagonist muscles flex joints in other directiono Reciprocal inhibition: spinal organization, serves to coordinate actions of agonist and antagonist muscles (very complex circuitry to help control reflexes)o Contraction of intrafusal muscle fibers, causes stretch of spindle fiber Alpha-gamma co-activation: allows communication of spindle fibers to brain throughout entire flexion of muscle, even after muscle spindle has been fully loosened As muscles passively stretch (opposite muscle as flexor) spindle sensitivity will lower to inhibit myotatic reflex of that muscle- Flexor-withdrawal reflex: spinal cord-mediated somatic reflex initiated by diverse group of somatosensory receptors, polysynaptico Strongest when pain receptors activatedo Pain receptor activated and information sent through primary neuron to synapse in spinal cordo Secondary neuron synapses with a tertiary excitatory neuron which causes flexor muscle to stimulateo Will also synapse with a tertiary inhibitory neuron which causes extensor to relaxo Will also activate appropriate muscles in opposite limb to help maintain balanceo Will persist for a while after stimulus is removed to help protect from further damage from that stimulus- Spinal reflexes: all that is required is for components of reflex arc to be intact, even if spinal cord is severed- Myotatic and somatosensory reflexes mediated by isolated SC are often exaggerated due to loss of inhibitory signals from descending tracts (spasticity)- Complex motor patterns can be elicited solely at the spinal cord level due to presence of pre-wired circuitryo Reciprocal inhibitiono Crossed extensor