Neurophysiology III: Reflex Arcs PDF

Neurophysiology III: Reflex Arcs ANSC 3080 G. Bedecarrats Learning Objectives Explain the different levels of control of body movement Describe the five basic components of a reflex arc Understand the function of the reflex arc using the case of t...

Neurophysiology III: Reflex Arcs ANSC 3080 G. Bedecarrats Learning Objectives Explain the different levels of control of body movement Describe the five basic components of a reflex arc Understand the function of the reflex arc using the case of the skeletal muscle (somatic arc reflex) 2-month-old calf, gradual onset of hind leg “lameness” Examination  No withdrawal responses in hind legs  Weakness, poor muscle tone  Loss of proprioception  Front legs – normal reflexes, strength and proprioception Where’s the problem? 1  CNS Control of Movement Three levels of control  Higher centers – cerebral cortex (conscious)  Middle level  Sensorimotor cortex  Cerebellum (monitors execution)  Basal ganglia (semi-voluntary movements)  Lower level – Spinal cord from which motor neurons exit (site of somatic reflex arcs) Tensions in muscles and tendons, and angles of joints send back sensory information to CNS (proprioception) Components Involved in the Control of Skeletal Muscles (beyond reflex)  Dark pink = components involved in planning and programming of movements  Light pink = components responsible for the (smooth) execution of movements  Somatic reflex arc involves local integration in spinal cord Concept of a Reflex  An automatic or unconscious response of effectors organs (muscle or gland) to a stimulus  Involuntary  unvarying response  Contains five fundamental components  Sensory cells / receptors  Sensory nerve fibers  Coordinating center (CNS)  Motor nerve fibers  Effectors (muscle or gland cells)  Malfunction of any one component leads to the alteration in the response 2 Somatic (skeletal muscle) Reflex Arcs 1. Receptors:  Transduce the environmental energy  Skin – heat, cold, pressure  Muscle (spindle) – stretch  Tendon (Golgi) - tension  Convert energy into action potentials (AP)  Frequency of APs proportional to intensity of energy transduced (frequency coding) – enables CNS to detect intensity 2. Sensory (afferent) nerve  Conduct AP from the receptor  Cell body in ganglion outside of spinal cord  Enter spinal cord via dorsal roots 3. Coordinating center = CNS synapses  Monosynaptic (muscle spindle reflex)  Multisynaptic (elaborate reflexes) 4. Motor (efferent) nerve  Carries APs from CNS to target (effectors) organs  Via ventral roots  Cell body within the CNS 5. Target organ response  Skeletal muscle e.g. knee jerk (patella) reflex  Multiple targets (scratching) 3  Stretch and Tendon Reflex  Changes in muscle length detected by muscle spindles  Modified muscle fibers = spindle or intrafusal fibers  Middle segment acts as stretch receptor by pinching the bear end of the sensory neuron (act as receptor) sensory afferent nerve spinal cord  Sensory nerve makes excitatory synaptic contact with motor neurons serving the same muscle (extrafusal  fibers) muscle shortening  Changes in tendon length detected by Golgi organ  Senses tension in tendons  Muscle contracts pulls tendon squeezes bare end of neuron (receptors) sensory nerve contacts with an inhibitory interneuron in spinal cord inhibits motor neurons serving the same muscle 4   Skeletal muscles organized in antagonistic muscle groups  Reflex will target both a) Resting position b) Biceps stretched by added weight AP in sensory neuron enters spinal cord activates biceps motor nerve and, via interneuron inhibits triceps motor nerve c) Biceps contract, triceps relax posture restored Knee Jerk Reflex Same protective reflex concept to prevent tendon injury Quadriceps muscle contraction in response to “tapping” of the patella ligament  Brief leg extension  Requires relaxation of opposing flexor muscles (via activation of inhibitory interneurons) FYI Number of motor and sensory nerve fibers in a typical skeletal muscle 5 Calf with Neurologic Abnormalities Cord compression abscess of vertebral body (L4) 6