1.6.2.1 - Components of the Stretch Reflex
Description
This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
1.6.2.1 - Components of the Stretch Reflex
The stretch reflex has both a dynamic component and a static component.
The static component of the stretch reflex persists as long as the muscle
is being stretched. The dynamic component of the stretch reflex (which can
be very powerful) lasts for only a moment and is in response to the initial
sudden increase in muscle length. The reason that the stretch reflex has
two components is because there are actually two kinds of intrafusal muscle
fibers: "nuclear chain fibers", which are responsible for the static
component; and "nuclear bag fibers", which are responsible for the dynamic
component.
Nuclear chain fibers are long and thin, and lengthen steadily when
stretched. When these fibers are stretched, the stretch reflex nerves
increase their firing rates (signaling) as their length steadily increases.
This is the static component of the stretch reflex.
Nuclear bag fibers bulge out at the middle, where they are the most
elastic. The stretch-sensing nerve ending for these fibers is wrapped
around this middle area, which lengthens rapidly when the fiber is
stretched. The outer-middle areas, in contrast, act like they are filled
with viscous fluid; they resist fast stretching, then gradually extend
under prolonged tension. So, when a fast stretch is demanded of these
fibers, the middle takes most of the stretch at first; then, as the
outer-middle parts extend, the middle can shorten somewhat. So the nerve
that senses stretching in these fibers fires rapidly with the onset of a
fast stretch, then slows as the middle section of the fiber is allowed to
shorten again. This is the dynamic component of the stretch reflex: a
strong signal to contract at the onset of a rapid increase in muscle
length, followed by slightly "higher than normal" signaling which gradually
decreases as the rate of change of the muscle length decreases.
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