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U N I T 1 0
Nervous System
U N D E R S T A N D I N G
The Stretch Reflex
Spinal Reflex Centers.
Afferent
impulses from the Ia sensory fiber
of the muscle spindle are transmit-
ted to the spinal cord, where they
synapse with
α
motor neurons of the
stretched muscle to form a mono-
synaptic reflex arc. They are “mono-
synaptic” because only one synapse
separates the primary sensory input
from the motor neuron output. The
reflex muscle contraction that fol-
lows resists further stretching of the
muscle. As this spinal reflex activ-
ity is occurring, impulses providing
information on muscle length are
transmitted to higher centers in the
brain. It is the coordinated activ-
ity of all the monosynaptic reflexes
supplying the extrafusal fibers in
a skeletal muscle that provides the
muscle tone needed for organized
movement.
Muscle tone is controlled by the stretch reflex, which monitors changes in muscle
length.The activity of the stretch reflex can be divided into three steps: (1) activation
of the stretch receptors, (2) integration of the reflex in the spinal cord, and (3)
regulation of reflex sensitivity by higher centers in the brain.Testing the (4) knee-jerk
reflex provides a means of assessing the stretch reflex.
Stretch Reflex Receptors.
Skeletal muscle is composed of two
types of muscle fibers: a large num-
ber of extrafusal fibers, which con-
trol muscle movement, and a smaller
number of intrafusal fibers, which
control muscle tone. The intrafusal
fibers are encapsulated in sheaths,
forming a muscle spindle that runs
parallel to the extrafusal fibers. Each
intrafusal fiber is innervated by a
large Ia sensory nerve fiber, which
encircles the central noncontractile
portion of the fiber to form the so-
called
annulospiral ending.
Because
the spindles are oriented parallel to
the extrafusal muscle fibers, stretch-
ing of the extrafusal fibers also
stretches the spindle fibers and stim-
ulates the receptive endings of the Ia
afferent neuron.
1
2
Muscle
spindle
Intrafusal
muscle fiber
Extrafusal
muscle fiber
Primary sensory (Ia)
nerve fiber
Homonymous
muscle
Ia afferent
α
motor neuron
Ascending fibers
to brain centers
