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Chapter 5

NROC64H3 Chapter Notes - Chapter 5: Golgi Tendon Organ, Muscle Spindle, Myocyte

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Matthias Niemeier

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Sehar Qureshi
NROC64 Sensorimotor Systems
Chapter 5 "Proprioception"
Muscle Proprioceptors
Two distinct kinds of proprioceptors are found in voluntary muscles, muscle spindles that respond
to muscle length and rate of change of length, and Golgi tendon organs that signal muscle tension
or force
The spindles are in parallel with the main contractile elements in the muscle, so that their
stretching is simply a measure of the degree of stretch x of the muscle itself, the tendon organs
are situated in the muscle tendons, in series with the contractile elements and the load, so that
their stretch is proportional to the tension T exerted by the muscle
Muscle spindles are found in practically all the striated muscles of the body
Each consists of a fluid-filled spindle-shaped capsule some 2–4 mm long and a few hundred
micro metres in diameter, whose ends are attached to the exterior sheaths of neighbouring muscle
Inside are intrafusal fibres, each having contractile ends, and a region in the middle that is not
contractile but contains the nuclei
Two main types of intrafusal fibre are found, differing in the way in which these nuclei are
Nuclear chain fibres are thinner, and their nuclei are lined up in a row along the central portion
like peas in a pod; nuclear bag fibres have a pronounced bulge in the middle in which the nuclei
are bunched together
Two kinds of afferent or sensory fibre innervate the spindle: the larger, primary fibres, belonging
to group Ia, send branches to the central portions of both types of fibre and have annulospiral
endings; the smaller secondary fibres are of group II and branch to the endings mainly on the
nuclear chain fibres, more peripherally than the Ia endings
The secondary fibres are in a sense simpler: their signals are more or less directly proportional to
the degree of stretch of the spindle at any moment; they are essentially non-adapting or static
The Ia fibres, by contrast, are dynamic and show very pronounced adaptation
They respond partly in proportion to muscle length but mostly in proportion to its rate of change,
or velocity: this means that they are in effect predicting the future length of the muscle
Adaptation may be due to energy filtering in which static information is wholly or partly thrown
away before it even reaches the transducer element itself; or there may be membrane adaptation,
in which even a steady conductance at the ending results in a fall-off in firing frequency
In muscle spindles it appears that nearly all is of the energy-filtering kind
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