LECTURE 14: SPINAL REFLEXES REQUIRED READING: Kandel text, Chapter 36 Skeletal motor reflexes are coordinated contractions and relaxations of specific muscles in response to sensory inputs Reflexes are mediated by neural circuits passing through the spinal cord Many motor reflexes are modifiable “subroutines” used by the brain in its orchestration of voluntary motor tasks or maintenance of a steady motor state VOLUNTARY TASK COMMANDS MODIFY REFLEX RESPONSES Different tasks result in opposite triceps reflex response When task is “pinch thumb and finger together”, load on thumb reflexively increases flexion of index finger. When task is “tap thumb to finger”, load on thumb does not affect index finger.

CONDITIONED WITHDRAWL REFLEX IS A REFLEX OF MOTOR BEHAVIOR, NOT OF MUSCLES FINGER ON PADAUDITORY TONESHOCK 1 sec FINGER ON PADAUDITORY TONENO SHOCK 1 sec FINGER ON PADAUDITORY TONENO SHOCK 1 sec PRECONDITION CONDITIONED FINGERINVERTED

CUTANEOUS REFLEXES PROVIDE PROTECTIVE AND POSTURAL FUNCTIONS Painful cutaneous stimulus transmitted by A  afferent fibers induces contraction of flexor muscles in the stimulated limb and inhibition of opposing extensor muscles The stimulus also induces contraction of extensor muscles on the opposite limb, as a reflexive posture-correcting measure Spinal pathways are polysynaptic, utilizing various spinal interneurons Opposing effects on counteracting muscles for a joint referred to as reciprocal innervation

STRETCH REFLEX ACTS TO RESIST MUSCLE LENGTHENING Passive muscle stretch stimulates Ia afferent sensory fibers in muscle spindles Ia afferents synapse directly onto alpha motor neurons innervating the stretched muscle as well as motor neurons innervating synergistic muscle groups (MONOSYNAPTIC REFLEX ARC) Same Ia afferents also synapse on inhibitory Ia interneurons that project to alpha motor neurons that innervate antagonistic muscle groups

VOLUNTARY MOVEMENT COMMANDS USE STRETCH REFLEX TO SIMPLIFY TASK During voluntary movement, specific motor cortex neurons with corticospinal projection are active Corticospinal axons branch and synapse on both alpha motor neurons and opposing Ia interneurons Therefore, cortical command promotes muscle contraction and reciprocal muscle inhibition using the spinal reflex circuitry

OTHER VOLUNTARY COMMANDS BLOCK INHIBITORY INTERNEURONS TO PROMOTE JOINT RIGIDITY Joint stiffening is associated with voluntary commands such as positioning a hand to catch a ball or setting finger curvatures before playing a chord on the piano The cortical commands include activation of descending pathways that provide inhibition of Ia interneurons

MUSCLE SPINDLES HAVE SPECIALIZED SENSORY AND MOTOR COMPONENTS Firing of gamma motor neurons contracts polar aspects of intrafusal fibers, thereby stretching the muscle spindle and stimulating Ia afferent firing….. UNLESS the entire muscle is shortening in parallel due to simulataneous alpha motor neuron firing

PARALLEL ALPHA AND GAMMA MOTOR NEURON ACTIVITY KEEPS MUSCLE SPINDLE AT A SIMILAR STRETCH AND ALLOWING FOR CONTINUED Ia AFFERANT ACTIVITY

CORTICAL INITIATION OF MOVEMENT ACTIVATES BOTH ALPHA AND GAMMA MOTOR NEURONS ALLOWING THE SPINDLE TO CONTINUALLY COMPENSATE FOR LOAD FLUCTUATION During voluntary motor command, spindle tension during contraction is maintained by gamma motor activity. If load increases during task, the spindle stretches, providing more alpha motor neuron activity Reciprocally, a lightening of load reduces Ia contribution to overall alpha motor neuron activity Therefore, the stretch reflex is used during voluntary commands to keep a movement on pace

SPINAL REFLEX STRENGTH CAN BE MODIFIED BY TONIC MOTOR ACTIVITY

GOLGI TENDON ORGAN REFLEX IS REVERSED BY CORTICAL ACTIVITY

NEXT LECTURE: VOLUNTARY MOVEMENT READING: Kandel text, Chapter 38