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
PRECONDITION
FINGER ON PAD
AUDITORY TONE
FINGER ON PAD
AUDITORY TONE
1 sec
SHOCK
CONDITIONED
FINGER
INVERTED
FINGER ON PAD
AUDITORY TONE
1 sec
1 sec
NO SHOCK
NO SHOCK
CUTANEOUS REFLEXES PROVIDE PROTECTIVE AND POSTURAL FUNCTIONS
Painful cutaneous stimulus
transmitted by Ad 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