Chapter 13. The Nature of Somatic Reflexes

Transcription

1 Chapter 13 The Nature of Somatic Reflexes

2 Nature of Reflexes (1 of 3) A reflex is an involuntary responses initiated by a sensory input resulting in a change in a gland or muscle tissue occur without our intent and often even without our awareness Four important properties of a reflex reflexes require stimulation not spontaneous actions, but responses to sensory input reflexes are quick involve few if any interneurons and minimum synaptic delay reflexes are involuntary occur without intent and difficult to suppress automatic response reflexes are stereotyped occur essentially the same way every time

3 Nature of Reflexes (2 of 3) Spinal Cord Reflex (Reflex arc) Simple reflex arc between sensor in muscle, spinal cord, and skeletal muscle Mediated by somatic nervous system This type of reflex does not involve the brain! Mono synaptic reflex arc is the simplest / only one synapse see video Spinal Reflex Stretch Reflex mediated by somatic nervous system Same sensor but this requires involvement of higher brain centers Involves groups of muscles Example of action in posture and dancing skeletal muscle reflexes mediated by the spinal cord, brainstem, cerebellum, and/or cerebrum ANS and Conditioned reflexes (learned responses) You can condition your pet to their feeding You keep the dog food in a kitchen cabinet Your dog learns if you go to the cabinet then you are going to put food in the dog s bowl The dog will start to salivate when you go to the cabinet These are visceral reflex associated with the ANS and the reflex has been conditioned

4 Nature of Reflexes (3 of 3) pathway of reflex arc somatic receptors in skin, muscles, or tendons afferent nerve fibers carry information from receptors to posterior horn of spinal cord or the brainstem integrating center a point of synaptic contact between neurons in the gray matter of the spinal cord or brainstem determines whether the efferent neurons issue a signal to the muscles efferent nerve fibers carry motor impulses to skeletal muscle skeletal muscles the somatic effectors carry out the response

5 The Muscle Spindle stretch receptors embedded in skeletal muscles Number of muscle spindles in an organ will vary Higher concentration in muscles with fine control Hands may have 100 muscle spindles per gram but gastrocnemius only 10 per gram. muscle spindles inform the brain of muscle length, body movement, and create mechanism for brain to adjust tone of muscle enables brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture intrafusal fibers muscle fibers (two type) within spindle / adjust tone of spindle as muscle length changes (two type) nuclear chain fibers nuclear bag fibers Extrafusal fibers muscle fibers outside spindle / responsible for the contraction of the organ Sensory fibers (afferent) from muscle spindle Primary afferent large / fast/ very sensitive to small changes in muscle length and sudden body movement Secondary afferent fibers intermediate sized fibers / slower conduction speed / less responsive to the rate of muscle shortening Motor neurons Gamma motor neurons from anterior horn to the intrafusal fibers / slow fibers / adjust the length of the intrafusal fibers as the extrafusal fibers change length Alpha motor neurons from anterior horn to extrafusal fibers / respond to signals from brain and spinal refexes

6 Muscle Spindle Bone Peripheral nerve (motor and sensory nerve fibers) Tendon Secondary afferent fiber Muscle spindle Skeletal muscle Extrafusal muscle fibers Connective tissue sheath (cut open) Intrafusal muscle fibers: Nuclear chain fiber Nuclear bag fiber Motor nerve fibers: Gamma Alpha Sensory nerve fibers: Primary Secondary

7 The Tendon Reflex (Only the Spinal Component of the Stretch Reflex) Reflex contraction of a muscle when its tendon is tapped Knee-jerk (patellar) reflex This is a monosynaptic tendon reflex one synapse between the afferent and efferent neurons testing somatic reflexes helps diagnose many diseases View Video The Stretch Reflex

8 Patellar Tendon Reflex Arc Spinal Cord Reflex To brain 6 Primary afferent neuron stimulates inhibitory interneuron 4 Primary afferent neuron stimulates alpha motor neuron to extensor muscle 7 Interneuron inhibits alpha motor neuron to flexor muscle 3 Primary afferent neuron excited 5 Alpha motor neuron stimulates extensor muscle to contract 2 Muscle spindle stimulated 1 Extensor muscle stretched 8 Flexor muscle (antagonist) relaxes

9 The Stretch Reflex Stretch (Myotatic) Reflex when a muscle is stretched, it fights back contracts slightly to increased tone making it stiffer than unstretched muscle helps maintain equilibrium and posture head starts to tip forward as you fall asleep muscles contract to raise the head Often involves groups of muscles stabilize joints by balancing tension in extensors and flexors smoothing muscle actions Mediated primarily by the brain Requires afferent nerve signals to higher brain centers efferent nerve signals returning to the muscle to adjust muscle tension This is not strictly a spinal reflex

10 The Stretch Reflex Stretch Reflex Requires muscle spindles Sense organs inside a muscle organ Embedded in the perimysium near the tendons Stretch stimulates muscle spindle to send afferent signals sense organs positioned within muscle organ proprioceptors specialized sense organs to monitor the position and movement of the body parts View Video on Web site / The Stretch Reflex

11 The Flexor (Withdrawal) Reflexes 2 Sensory neuron activates multiple interneurons requires contraction of the flexors and relaxation of the extensors in the same limb 3 Ipsilateral motor neurons to flexor excited 5 Contralateral motor neurons to extensor excited flexor reflex the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus 4 1 Ipsilateral flexor contracts Stepping on glass stimulates pain receptors in right foot 6 Contralateral extensor contracts reciprocal inhibition reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonistic muscles Afferent fiber may synapse on several inter-neurons which then synapse on efferent fibers going to primary movers, synergist, and/or antagonistic muscles. ithdrawal of right leg (flexor reflex) Extension of left leg (crossed extension reflex) View Video Flexor Reflex

12 The Crossed Extensor Reflex 2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sensory neuron activates multiple interneurons polysynaptic reflex arc - pathway in which signals travel over many synapses on their way back to the muscle Cross extensor reflex 3 Ipsilateral motor neurons to flexor excited 5 Contralateral motor neurons to extensor excited the contraction of extensor muscles in the limb opposite of the one that is withdrawn maintains balance by extending other leg 4 Ipsilateral flexor contracts ipsilateral reflex arc one in which the sensory input and the motor output are on the same sides of the spinal cord flexor reflex 6 Contralateral extensor contracts contralateral reflex arc one in which the input and output are on opposite sides crossed extension reflex ithdrawal of right leg (flexor reflex) 1 Stepping on glass stimulates pain receptors in right foot Extension of left leg (crossed extension reflex) intersegmental reflex one in which the input and output occur at different levels (segments) of the spinal cord View Video Crossed Extensor Reflex

13 The Tendon Reflex tendon organs (Golgi Tendon Organ) proprioceptors in a tendon near its junction with a muscle 1mm long, nerve fibers entwined in collagen fibers of the tendon Sends to the spinal cord that provide the CNS with feedback on the degree of muscle tension at the joint tendon reflex in response to excessive tension on the tendon inhibits muscle from contracting too strongly moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone Balance contraction force between different fascicles Nerve fibers Tendon organ Tendon bundles Muscle fibers

14 Visceral Reflexes These are reflex arcs which involve the visceral motor fibers Parasympathetic nerve fibers Sympathetic nerve fibers These are reflex arcs that regulate essential homeostatic reflexes like temperature regulation and blood pressure. We will exam these reflexes in Chapter 15