University:
Camosun College
Course:
BIOL 252 | Pathophysiology for Nursing 1
Academic year:

2024
Views:

58

Pages:

3
Author:

Aysha A.

Mechanism of Proprioceptive Afferent Stretching of Muscle Spindle: When muscles (m/s) are stretched, extrafusal fibers are also stretched. This stretching leads to the stretching of the muscle spindle capsule, which in turn stretches the intrafusal fibers. The center of nuclear bag fibers and nuclear chain fibers becomes deformed, leading to the generation of action potentials (AP) in Type Ia and II fibers. These APs travel to the posterior horn of the spinal cord (SC) via the dorsal pathway to the sensory cortex and via the spinocerebellar tract to the cerebellum. Shortening of Muscle Spindle: When extrafusal fibers are shortened, the capsule of the muscle spindle also shortens, leading to the shortening of nuclear bag fibers and nuclear chain fibers. The deformation is reduced, leading to no action potential generation in Type Ia and II fibers. This results in no excitation of the intrafusal fibers, which means no input to the SC, and the tracking of muscle position is lost. Activation and Contraction: Alpha (Aα) motor neurons and gamma (Aγ) neurons simultaneously send signals. These signals excite both intrafusal and extrafusal neurons, leading to muscle contraction and shortening. Muscle Contraction (Active): Aα motor neurons excite extrafusal cells, leading to muscle contraction and shortening. Aγ motor neurons excite intrafusal cells (contracting alone), which causes contraction of the rib in opposite directions, resulting in stretching and deformation of the center of intrafusal fibers. This leads to AP generation in Type Ia and II fibers, which travel through the dorsal column and spinocerebellar tract. Thus, proprioceptive awareness is conveyed to the cerebrum and cerebellum. Muscle Relaxation (Passive): When spontaneous elongation of extrafusal fibers occurs, it leads to the elongation of intrafusal fibers, followed by muscle contraction. Proprioceptor Circuit: Involves the stretch reflex, monosynaptic reflex, and deep tendon reflex. Response to Tendon Force (Jerk): When force is applied to a tendon (e.g., jerk), muscle spindle fibers are activated. Impulses travel via Type Ia and II fibers to the posterior grey horn, triggering interneurons. Excitation of Aα: Interneurons activate the dorsal column pathway and spinocerebellar tract. APs from Aα neurons travel to the muscles at the site of the jerk and excite extrafusal fibers, leading to muscle contraction and shortening. Innervation and Control of Muscles: UMN (Upper Motor Neuron): Originates from the motor cortex or other suprasegmental sources and travels to the spinal cord (SC). LMN (Lower Motor Neuron): Originates from the spinal cord and innervates muscles directly.