Reflex Integration

Motor Unit Output

Three types of motor units: Fast Fatiguable, Fast Fatigue Resistant, and Slow

Property	Fast Fatiguable (Ia)	Fast Fatigue Resistant (Ib)	Slow (II)
Contraction Time	Short	Intermediate	Long
Fatigue Resistance	Low	Intermediate	High
Motor Unit and Muscle Fiber Size	Large	Intermediate	Small
Contraction Force	Large	Intermediate	Small
Firing Pattern	Phasic	Intermediate	Tonic
Motor Neuron Size	Large	Intermediate	Small
Recruitment Threshold ("size principle")	High	Intermediate	Low

Muscle force is a function of:

Force per motor unit (# of muscle fibers per unit)
Frequency of firing per motor unit
# of motor units recruited and acting together

Discreteness of Muscle Action (fine motor motions) is a function of:

size of motor unit (smaller units in fingers)
lateral inhibition via Renshaw cell (firing neurons inhibit less intensely active neurons).

Control of Limb (Joint) Position – There are two ways of bringing a joint to a new position:

(1) Fire only synergistic muscles, shortening (stiffening) the muscle and simultaneously relaxing the antagonists (reciprocal innervation) Þ this mode is energy efficient but subject to rapid fluctuation of muscle output.

(2) Co-contraction of both synergists and antagonists, predominantly synergists so that the joint as a whole is stiffer

this mode is energy wasteful but less subject to fluctuation (less jerkiness)

Muscle Mechanoreceptors and Their Central Actions

(1) Muscle Spindles: length detectors; composed of intrafusal fibers arranged in parallel to the muscle (extrafusal fibers)

Two types:

(a) Nuclear bag fibers (mainly group Ia afferent fibers): phasic response to sudden changes in firing pattern
(b) Nuclear chain fibers (mainly group II afferent fibers): respond slowly to muscle stretch (tonic)

Central Action: length-feedback system: Ia afferent fibers excite motor neurons for synergistic muscles (group II have lesser role); stretch reflex: sudden muscle lengthening produces reflex contraction that counteracts lengthening

(2) Golgi Tendon Organ: force detector; in series arrangement to muscle, within tendon

measures tendon stretch, therefore indicates contractile force (muscle tension)

Central Action: via Ib fibers an interneuron excites an antagonist muscle and inhibits synergist motor neurons.

Contrasting Sensory Actions of Muscle Spindles and Golgi Tendon Organs: Both discharge when a muscle is lengthened

however, when a muscle is actively contracted the spindles ß firing but tendon organs Ý firing

this phenomenon can be explained by the anatomy of the two organs since Golgi tendon organs are in series they will fire whether the muscle is stretched or contracted; the muscle spindle will only fire when it is stretched

Central Regulation of Spindle Sensitivity (Gain)

Gamma motor neurons regulate gain.

Anatomy: innervate the contractile poles of both types of spindle fibers.

Physiology: Gamma firing contracts spindle poles, stretches coiled Ia and II terminals Þ makes more sensitive to stretch

Central and Motor Actions:

Ý excitatory drive on same muscle a motoneurons Þ more contraction of muscle.

prevents contraction from "collapsing" spindle, causing silence (allows the brain to receive constant input despite the fact that the spindle is close to being collapsed)

response to sudden environmental increase or decrease of length or to counteract fatigue.

allows central regulation of muscle length by indirect means.

imbalance of central inputs to g motor neurons leads to Ý gamma drive Þ Ý tone and hyper-reflexia

in voluntary movement, there is co-activation of a and g motor neurons, presumably to fire muscle and maintain length feedback. They work synergistically to modulate the sensitivity of the muscle system (a feedbacks to g ).

Polysynaptic Flexion Reflexes

Flexion Withdrawal Reflex: avoids pain stimuli; requires a , primary sensory, interneurons and propriospinal

nociceptorÞ cutaneous afferent fiberÞ subst gelatinosaÞ stim.flexors of ipsilateral limb, inhib.extensor of ipsilateral limb

accompanying the Flexor Withdrawal Reflex, there is a Crossed Extension Reflex which inhibits the flexors and stimulates the extensors of the contralateral limb

flexion reflex is hyperactive after upper motor neuron injury

Spinal Capacity for Locomotion

evidence could be "central pattern generator" that allows cat to walk despite transection of spinal cord at thoracic level
suppressed in humans