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SECTION II
Physiology of Nerve & Muscle Cells
MONOSYNAPTIC REFLEXES:
THE STRETCH REFLEX
When a skeletal muscle with an intact nerve supply is stretched,
it contracts. This response is called the
stretch reflex.
The stimu-
lus that initiates the reflex is stretch of the muscle, and the re-
sponse is contraction of the muscle being stretched. The sense
organ is a small encapsulated spindlelike or fusiform shaped
structure called the muscle spindle, located within the fleshy part
of the muscle. The impulses originating from the spindle are
transmitted to the CNS by fast sensory fibers that pass directly to
the motor neurons which supply the same muscle. The neu-
rotransmitter at the central synapse is glutamate. The stretch re-
flex is the best known and studied monosynaptic reflex and is
typified by the
knee jerk reflex
(see Clinical Box 9–1).
STRUCTURE OF MUSCLE SPINDLES
Each muscle spindle has three essential elements: (1) a group
of specialized intrafusal muscle fibers with contractile polar
ends and a noncontractile center, (2) large diameter myelinat-
ed afferent nerves (types Ia and II) originating in the central
portion of the intrafusal fibers, and (3) small diameter myeli-
nated efferent nerves supplying the polar contractile regions
of the intrafusal fibers (Figure 9–2A). It is important to under-
stand the relationship of these elements to each other and to
the muscle itself to appreciate the role of this sense organ in
signaling changes in the length of the muscle in which it is lo-
cated. Changes in muscle length are associated with changes in
joint angle; thus muscle spindles provide information on po-
sition (ie,
proprioception
).
The
intrafusal fibers
are positioned in parallel to the
extrafusal fibers
(the regular contractile units of the muscle)
with the ends of the spindle capsule attached to the tendons at
either end of the muscle. Intrafusal fibers do not contribute to
the overall contractile force of the muscle, but rather serve a
pure sensory function. There are two types of intrafusal fibers
in mammalian muscle spindles. The first type contains many
nuclei in a dilated central area and is called a
nuclear bag
fiber
(Figure 9–2B). There are two subtypes of nuclear bag
fibers,
dynamic
and
static.
Typically, there are two or three
nuclear bag fibers per spindle. The second intrafusal fiber
type, the
nuclear chain fiber,
is thinner and shorter and lacks
a definite bag. Each spindle has about five of these fibers.
There are two kinds of sensory endings in each spindle, a
single
primary (group Ia) ending
and up to eight
secondary
(group II) endings.
The Ia afferent fiber wraps around the cen-
ter of the dynamic and static nuclear bag fibers and nuclear
chain fibers. Group II sensory fibers are located adjacent to the
centers of the static nuclear bag and nuclear chain fibers; these
fibers do not innervate the dynamic nuclear bag fibers. Ia affer-
ents are very sensitive to the velocity of the change in muscle
length during a stretch
(dynamic response);
thus they provide
information about the speed of movements and allow for quick
corrective movements. The steady-state (tonic) activity of
group Ia and II afferents provide information on steady-state
length of the muscle
(static response).
The top trace in Figure
9–2C shows the dynamic and static components of activity in a
Ia afferent during muscle stretch. Note that they discharge
most rapidly while the muscle is being stretched (shaded area
of graphs) and less rapidly during sustained stretch.
The spindles have a motor nerve supply of their own. These
nerves are 3–6
μ
m in diameter, constitute about 30% of the
fibers in the ventral roots, and are called
γ
-motor neurons.
There are two types of
γ
-motor neurons:
dynamic,
which sup-
ply the dynamic nuclear bag fibers and
static,
which supply
the static nuclear bag fibers and the nuclear chain fibers. Acti-
vation of dynamic
γ
-motor neurons increases the dynamic
sensitivity of the group Ia endings. Activation of the static
γ- motor neurons increases the tonic level of activity in both
group Ia and II endings, decreases the dynamic sensitivity of
group Ia afferents, and can prevent silencing of Ia afferents
during muscle stretch (Figure 9–2C).
FIGURE 9–1
The reflex arc.
Note that at the receptor and in the CNS a nonpropagated graded response occurs that is proportionate to the
magnitude of the stimulus. The response at the neuromuscular junction is also graded, though under normal conditions it is always large enough
to produce a response in skeletal muscle. On the other hand, in the portions of the arc specialized for transmission (afferent and efferent axons,
muscle membrane), the responses are all-or-none action potentials.
Sense
organAfferent
neuronEfferent
neuronNeuromuscular
junctionSynapse MuscleGenerator
potentialAction
potentialsEPSPs
(and IPSPs)Action
potentialsEndplate
potentialsAction
potentials