Motor Functions of Spinal Cord Flashcards
reciprocal inhibition
when a stretch reflex excites one muscle it often simultaneously inhibits the antagonist muscles, the neuronal circuit that causes this reciprocal relation is called reciprocal innervation
positive supportive reaction
pressure on the footpad of a decerebrate animal causes the limb to extend against the pressure applied to the foot
cord righting reflex
when a spinal animal is laid on its side, it will make uncoordinated movements to try to raise itself to standing position
crossed extensor reflex
about 0.2 to 0.5 sec after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend
magnet reaction
in positive support reactions, pressure on one side causes extension in that direction.
the oscillation back and forth between flexor and extensor muscles in rhythmic stepping movements results from?
mutually reciprocal inhibition circuits within the matrix of the cord itself, oscillating between neurons controlling agonist and antagonist muscles
stumble reflex
if the top of the foot encounters an obstruction during forward thrust, the forward thrust will temporarily stop then in rapid sequence the foot will be lifted higher and proceed to be placed over the obstruction.
mark time reflex
if a well healed spinal animal is held up and its legs are allowed to dangle freely the stretch on the limbs occasionally elicits diagonal stepping between the forelimbs and hindlimbs
galloping reflex
when almost equal stretch or pressure stimuli are applied to the limbs on both sides of the body at the same time unequal stimulation elicits diagonal walking reflex
scratch reflex
it is initiated by itch or tickle sensation. it involves two functions:
- position sense
- to and fro scratching movement
mass reflex
in a spinal animal the spinal cord suddenly becomes excessively active causing mass discharge in large portions of the spinal cord
usual stimulus of mass reflex
strong pain stimulus to skin
excessive filling of a viscus
what are the effects of mass reflex?
- major portions of bodys skeletal muscles go into flexor spasm
- colon and bladder evacuate
- the arterial pressure rises to maximal values
- body breaks out into profuse sweating
mass reflex mechanism is similar to that of
epileptic seizures
spinal shock
when the spinal cord is suddenly transected in the upper neck at first essentially all cord functions including the cord reflexes immediately become depressed to the point of complete silence
after-effects of spinal shock
- arterial blood press falls drastically
- all skeletal muscle reflexes are blocked during initial stages of shock (first to return is the stretch reflex)
- sacral reflexes for bladder and colon emptying are blocked in the first few weeks
types of neurons in each spinal cord segment
sensory relay neurons
anterior motor neurons
interneurons
anterior motor neurons
- located in anterior gray matter
- are of two types alpha motor neurons, which give rise to type A alpha motor nerve fibers which go to extrafusal fibers and gamma motor neurons, which give rise to type A gamma motor nerve fibers which go to intrafusal fibers of skeletal muscle
motor innervation of extrafusal skeletal muscle
alpha efferent fibers
Renshaw Cells
- located in anterior horns of spinal cord in association with motor neurons.
- almost immediately after the anterior motor neuron axon leaves the body of the neuron collateral branches from the axon pass to the renshaw cells
- they are inhibitory, thus stimulation of each motor neuron inhibits adjacent motor neurons (lateral inhibition)
- renshaw cells help in focus and sharpening of signals
more than half of the nerve fibers that ascend and descend in the spinal cord are:
propriospinal fibers, they aid in multisegmental reflexes including reflexes that coordinate simultaneous movements in forelimbs and hindlimbs
muscle sensory receptors
- muscle spindles (in belly) send info about length/ change in length
- golgi tendon organs (in tendons) send info about tendon tension/ change in tension
motor innervation of intrafusal skeletal muscle
gamma efferent fibers
central receptor area of muscle spindle has:
primary and secondary afferent endings
primary afferent endings
aka annulospiral ending
type la
17 micrometer
transmits sensory signals at a velocity of 70-120m/s
excited by both the nuclear bag and nuclear chain fibers
secondary afferent endings
type ll
8 micrometers
spreads like branches on a bush
excited by nuclear chain fibers only
types of muscle spindle intrafusal fibres:
nuclear bag muscle fiber(1-3 in each spindle)
nuclear chain fibers (3-9 in each spindle)
static response
when the receptor portion of the muscle spindle is stretched slowly the number of impulses transmitted from both the primary and secondary endings increase almost directly in proportion to the degree of stretchung and continue to transmit these impulses for a few seconds
the gamma efferent system is controlled
- primarily by bulboreticular facilitatory region (involved in antigravity contractions)
- secondarily by impulses transmitted to bulboreticular area from the cerebellum basal ganglia and cerebral cortex
clonus
muscle jerks oscillate
transmission of impulses from the tendon organ into the cns is done by:
type lb fibers
lengthening reaction
when tension on the muscle and therefore on the tendon becomes extreme, the inhibitory effect of the golgi tendon organ can be so great that it leads to a sudden reaction in the spinal cord that causes instantaneous relaxation of the entire muscle
dorsal spinocerebellar tracts carry
instantaneous information from both the muscle spindles and the golgi tendon organs directly to the cerebellum at 120m/s (fastest in brain/spinal cord)
pathway for flexor reflex (pain/nociceptive)
i. diverging circuits
ii. reciprocal inhibition circuits
iii. circuits for discharge
