If you stand within a door frame and extend both of your arms straight out to to

Question

If you stand within a door frame and extend both of your arms straight out to touch the frame and continually press against it, then you are maintaining a constant muscle contraction. Meanwhile, the gamma moto-neurons are stimulating the spindles and increasing their sensitivity, without affecting muscle spindle length. Therefore, when you step out of the door frame, gravity pulls your arms down causing the stretch and subsequent activation of the spindle leading to spindle activation of muscle contraction.

Mechanistically, what actually happened to your arms as you walked away from the doorway with your arms relaxed? Make sure that you describe the sequence of events in order, using the terms alpha motor neuron, central nervous system, contraction, extrafusal fibers, gravity, intrafusal fibers, sensory neuron, and stretch. Note: The terms are not listed in the correct sequence!

Explanation / Answer

The upward lift experienced in arms, after conducting the doorway experiment as mentioned in the question is called as Kohnstamm Phenomenon. It is a sustained involuntary contraction of a muscle after a prolonged voluntary contraction.

The key factor in the Kohnstamm phenomenon is changes in the stiffness and slackness of intrafusal muscle fibres.The Kohnstamm aftercontraction of the muscle occurs for the following reason:-

Under normal conditions when arms are moved, the alternating stretching and shortening movements of largely inactive muscle lead to development of slack in muscle fibres, including intrafusal fibres. As a result, the levels of maintained spindle activity remain low. However, the Kohnstamm induction involves static contraction of muscles at short length, resulting in the taking up of slack in the intrafusal fibers. Dduring the voluntary, isometric induction contraction, stable actin and myosin cross bridges are formed in intrafusal muscle fibers. Relaxation causes the arm to be slowly brought back to a longer muscle length. However, stable cross bridges in intrafusal fibers remain as it is, maintaining them in a state of relative shortness (compared to their state following alternative contraction histories such as if the arm was moving normally). This relative shortness in intrafusal muscle fibers causes muscle spindles to be stretched and to send afferent signal. This spindle signalling causes muscular contraction via spinal reflexes. This reflexive response constitutes the aftercontraction.

Brain activation is observed during the aftercontraction and aftervibration period. The brain activation involved motor-related areas; usually responsible for voluntary motor command (primary sensory and motor cortices, premotor cortex, anterior and posterior cingulate gyrus) and sensory motor integration structures, such as the posterior parietal cortex. The brain activation associated with the involuntary post-effects is of cerebellar vermis activation. Brain activation during voluntary contraction showed is due to supplementary motor area activation. The after contraction originates from asymmetric proprioceptive inputs, but it also involves a central network where the motor and somatosensory areas and the cerebellum.

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