Proprioceptor Mechanics

what is proprioception?

detection of changes and stimuli caused within the muscle/joint system itself

they detect mechanical changes in muscle and joints

what is kinesthesia?

the ability to detect joint or body position in space and determine the forces acting on the body

concerned with position and movement, effort and heaviness, and perceived timing of movements

utilize proprioceptor feedback as well as sensory feedback from skin and other external sensory systems

what are the types of proprioceptive receptors?

muscle proprioceptors (muscle spindles, Golgi tendon organs)

joint and skin proprioceptors (Pacinian corpuscles, Ruffini endings)

Labyrinthine and neck proprioceptors

what are the extrafusal fibers?

the muscle fibers used to produce muscular contraction

what are the two types of muscle fibers in a muscle spindle?

extrafusal and intrafusal fibers

what are intrafusal fibers?

the muscle fibers comprising the spindle

what are the nuclear bag fibers?

the intrafusal fibers containing a bag of nuclei

what are nuclear chain fibers?

the intrafusal fibers containing multiple nuclei in a row

both have non-contractile centers and contractile ends

the ends are innervated and can contract like normal muscle

what are the afferent fibers?

neurons with action potentials moving away from the spindle (sensory, Ia and II)

what are the efferent fibers?

(gamma fibers) neurons with APs moving toward the spindle (motor)

what are the alpha motor neurons?

motor neurons to extrafusal muscle

what are the gamma motor neurons?

motor neurons to intrafusal muscle

what are the primary endings?

contain afferent fibers only

located at the center of intrafusal fibers

Ia afferents

what are the secondary endings?

located on the periphery on the intrafusal fibers

II afferents

what is the purpose of gamma stimulation?

it enhances either the dynamic or static afferent response and occurs from the gamma motor neurons

gamma motor neurons can turn on before muscle changes length to make the spindles more sensitive to stretching and changes in length

how does gamma stimulation occur?

gamma neurons create a contraction at the ends of the spindle

elongates the middle of the spindle like a stretch

ensures the spindle continues to fire even when stretch is removed

what is the static response of the muscle spindle?

the response to changes in length

the response occurs because stretch elongates the center of the spindle, stimulating the Type I afferent fibers

what changes in spindle firing rate occur during the static response?

when muscle is stretched more, it fires at a proportionally increasing rate

when stretch is stopped or paused, the nerve firing slows to a constant rate

the static response of a muscle stretch in a fixed position

what is the stretch reflex?

a reflex arc caused by firing of Type I afferent fibers

when the stretch is vigorous enough, the alpha motor fibers are stimulated causing the muscle fibers to contract in response to the stretch

a protective mechanism

what is the dynamic response of muscle spindles?

the response to length of muscle

the Type II fibers are stimulated as muscle length changes

however, if muscle is held stationary, the fiber does not decrease its impulses (a good indicator of muscle length)

what occurs with gamma stimulation in the nuclear chain fibers?

with the chain fiber within the spindle, both the primary and secondary endings stretch

this causes an increase in discharge in both endings → as the spindle adapts to the stretch, the primary ending discharge rate decreases and the secondary endings continue to fire

the gamma motor neuron stimulates the contractile components

this has minimal effect on the primary endings but significantly increases the response of the secondary endings

what is the function of the Golgi Tendon Organ (GTO)?

it monitors tension (force) within the muscle

it is stimulated when its collagen is compressed during a stretch or contraction of the muscle

what is the structure of the GTO?

located at the musculoskeletal junction

a collection of collagen fascicles

collagen fascicles are directly connected to the extrafusal fibers of the muscle

what is the innervation of GTO’s?

the afferent fibers are Type Ib neurons

the amount of stimulation is proportional to the deformation of the GTO (proportional to the tension in the muscle)

several muscle fibers will insert into one GTO

what type of tension are GTO’s more sensitive to?

active tension rather than passive tension

what occurs in the GTO during passive stretch?

the tension is evenly distributed across all of the GTO’s - they are in series with the muscle fibers

what occurs in the GTO during active tension?

the GTO’s are in parallel → therefore parallel to the tension generated, thus it is more sensitive to the muscle fibers shortening

what type of reflex do the GTO’s generate?

an inhibitory reflex proportional to the amount of tension in the muscle

the alpha motor neuron discharge to the muscle is reduced, reducing the stretch reflex

often called the inverse stretch reflex

it is thought that GTO activity is a limiting factor in weight training → serves as a protective mechanism so extreme levels of tension in the muscle are avoided

what are Pacinian Corpuscles?

onion shaped receptors that respond to changes in pressure and are sensitive to rapid changes

what are Ruffini endings?

they signal joint position, angular velocity, and joint pressure

they respond to changes as small as 2 degrees of joint motion

what is the function of Labyrinthine proprioceptors?

they are located in the inner ear and detect head motion

what is the function of neck proprioceptors?

they are located in the neck muscles and detect the relationship between the head and body (send opposite signals as Labyrinthine proprioceptors)

what is the purpose of reciprocal inhibition?

it is complementary to the stretch reflex and inhibits antagonist muscles

what is flexor withdrawal?

it is the response to pain stimuli via free nerve endings and produces flexion of the ipsilateral flexors

what is the crossed extensor reflex?

it is a response to pain with contralateral stimulation of extensors

assists with moving away from painful stimulus or supporting weight

what is the extensor thrust reflex?

pressure on the sole of the feet stimulates leg extensor muscles (in infants)

what is the tonic neck reflex?

it causes stimulation of neck extensors and occurs in the direction of head rotation