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THE GENERATION OF SPECIFIC RESPONSES TO STRETCH, PAIN OR
VISCERAL SENSATIONS.
THE STRETCH REFLEX
The stretch reflex is a monosynaptic, postural reflex that among the others works against the
gravity force. First of all it is important to mention that muscles are attached to tendons which
hold them to the bone. At the attachment of the muscles to tendons there is a muscle spindle
which is very sensitive to stretch so it is the receptor of the reflex arc of stretch reflex. Muscle
spindles are composed of 3-12 intrafusal fibers, of which there are three types: dynamic nuclear
bag fibers, static nuclear bag fibers, nuclear chain fibers and the axons of sensory neurons. Type
Ia fibers connect to both nuclear bag fibers and nuclear chain fibers. The Ia sensory fiber fires
when the muscle is stretching. Its cell bodies can be found in the dorsal root ganglia of the
spinal cord, and make synpases directly on alpha motor neurons that innervate the same muscle.
This activation of the Ia afferent causes the monosynaptic activation of the alpha motor neuron
that causes the muscle to contract. Alpha motor neurons are lower motor neurons that travel
from the spinal cord to the muscle and they innervate extrafusal muscle fibers of skeletal
muscle. Axons of gamma motor neurons also terminate in muscle spindles; they make synapses
at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the
sensory afferents. These motor neurons both travel from the spinal cord to the muscle and back
in a continuous loop. Conscious movement comes from impulses in the brain travelling down
the spinal cord, over this loop, and then back to the brain for processing. However, the stretch
reflex skips the brain portion of the trip and follows the simple loop from muscle to spinal cord
and back, making it very rapid sequence. When the central nervous system sends out signals to
alpha neurons to fire, signals are also sent to gamma motor neurons to do the same. This process
maintains the tautness of muscle spindles and is called alpha gamma co-activation. Without
gamma motor neurons, muscle spindles would be very loose as the muscle contracts more.
THE WITHDRAWAL REFLEX
is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, causing
stimulation of sensory-, and motor neurons. We have all experienced this reflex after
accidentally touching a hot stove or a sharp object, as we withdraw our hand even before we
consciously experience the sensation of pain. This quick reflex removes the limb from the
damaging stimulus more quickly than if the pain signal had to travel up to the brain, be brought
to conscious awareness, and then trigger a decision to withdraw the limb.
THE VISCERAL REFLEX
The operation of visceral organs is modulated by the autonomic nervous system. Sensory
visceral afferents convey information to the centers of the CNS from receptors distributed in
the organs. The brain stem and the spinal cord regulate the motor activity of the organs via
viscero-motor efferents. A stimulus activates sensory receptors in the viscera. Then primary
visceral afferents carry the information to the dorsal horn and after that the information transfer
to interneurons. Sensory stimulus is generated in organ. Primary visceral afferents carry the
information to the dorsal horn. There the transfer of information occurs to interneurons.
Interneurons transfer the input to visceral somatomotor neurons. Preganglionic efferent is
projected to the prevertebral ganglion causing the innervation of ganglionic cells. Ganglion neurons are
projected to executive structure (smooth muscle), which causes contraction state of smooth muscles to
change.