LECTURE 14: SPINAL REFLEXESREQUIRED 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 inits orchestration of voluntary motor tasks or maintenance of a steady motor state

VOLUNTARY TASK COMMANDS MODIFY REFLEX RESPONSES

Differenttasks

result inoppositetricepsreflex

response

When task is “pinch thumb and finger together”, load on

thumbreflexively increases

flexion of index finger.

When task is “tap thumb to finger”,

load on thumbdoes not affect index finger.

CONDITIONED WITHDRAWL REFLEX IS A REFLEX OF MOTOR BEHAVIOR, NOT OF MUSCLES

FINGER ON PAD AUDITORY TONE SHOCK1 sec

FINGER ON PAD AUDITORY TONE NO SHOCK1 sec

FINGER ON PAD AUDITORY TONE NO SHOCK1 sec

PRECONDITIONPRECONDITION

CONDITIONEDCONDITIONED

FINGERFINGERINVERTEDINVERTED

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 contractionof extensor muscles on theopposite limb, as a reflexiveposture-correcting measure

Spinal pathways are polysynaptic,utilizing various spinal interneurons

Opposing effects on counteractingmuscles for a joint referred to as

reciprocal innervation

STRETCH REFLEX ACTS TO RESIST MUSCLE LENGTHENING

Passive muscle stretch stimulatesIa afferent sensory fibers in muscle spindles

Ia afferents synapse directly ontoalpha motor neurons innervating the

stretched muscle as well asmotor neurons innervating synergistic

muscle groups(MONOSYNAPTIC REFLEX ARC)

Same Ia afferents also synapse oninhibitory Ia interneurons that

project to alpha motor neurons thatinnervate 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 promotesmuscle contraction and reciprocal

muscle inhibition using thespinal reflex circuitry

OTHER VOLUNTARY COMMANDS BLOCK INHIBITORY INTERNEURONSTO PROMOTE JOINT RIGIDITY

Joint stiffening is associated withvoluntary commands such as

positioning a hand to catch a ballor setting finger curvatures before

playing a chord on the piano

The cortical commands includeactivation of descending pathways

that provide inhibition ofIa 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 simulataneousalpha motor neuron firing

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

CORTICAL INITIATION OF MOVEMENT ACTIVATES BOTH ALPHA AND GAMMA MOTOR NEURONSALLOWING 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 morealpha motor neuron activity

Reciprocally, a lightening of load reducesIa contribution to overall

alpha motor neuron activity

Therefore, the stretch reflex is usedduring voluntary commandsto 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