| There are three types of receptors in the
body. These receptors are constantly sending information back to
the central nervous system about the success of a motor program or
movement.
The three types of
receptors are:
1. Skin Receptors
2. Muscle Receptors
3. Joint Receptors
The type of
receptors we are most concerned with are muscle receptors.
There are two
types of muscle receptors:
1. Muscle Spindles
2. Golgi Tendon Organs.
MUSCLE SPINDLES
Muscle spindles are located in the belly of the skeletal muscle, encased
in a fusiform capsule. A fusiform capsule is a capsule that is
tapered on each end. A rodlike structure runs through the capsule,
and is divided into thirds. The centermost third is the muscle
spindle. The two outer thirds are called intrafusal muscle
fibers. The entire capsule is encased in extrafusal muscle
fibers.
While extrafusal
muscle fibers (normal muscle fibers) are innervated efferently by alpha
motor neurons, intrafusal muscle fibers are innervated efferently by
gamma motor neurons.
The muscle spindle
is innervated by two different afferent sensory neurons.
1. Primary
2. Secondary
Primary afferent
neurons are sensitive to the amount and velocity of stretch placed on
the muscle spindle. Secondary afferent neurons are sensitive only
to the amount of stretch.
If the muscle
itself is stretched, then the muscle will be stretched as well because
the extrafusal and intrafusal muscle fibers are attached in
series. If the muscle spindle is stretched a threshold amount,
then the primary and secondary afferent fibers will transmit a signal
back to the central nervous system. The CNS intreprets the signal,
and understands that the amount of stretch in the muscle spindle could
potentially cause structural damage to the muscle. Therefore, a
signal is sent down the alpha motor neuron to the skeletal muscle that
encloses the muscle spindle, and the skeletal muscle contracts.
Ultimately, the stretch that could have been damaging to the skeletal
muscle is prevented by contracting the muscle.
GOLGI TENDON
ORGANS
Golgi tendon
organs are located where the tendon attaches to the muscle. Unlike
muscle spindles, they do not sense stretch - they sense excessive force
and induce relaxation of the muscle via inhibition of the alpha motor
neuron. |
