Q: What is the role of the nervous system in mobility?
The nervous system is responsible for the third force acting on the body: the internal force of human movement (besides gravity and externally applied force). It receives spatial and sensory input and generates motor output. Behaviors, such as motor control, motor skill development, joint positions, and flexibility are learned utilizing sensory pathways in the nervous system that can be triggered by the sensation of a stretch, and motor pathways which respond.
A distinction is made between sensory receptors and mechanoreceptors. Mechanoreceptors are sensory nerve endings which respond to mechanical forces. For instance, it is discussed that the Ruffini corpuscles, one of four types of mechanoreceptors, are triggered through sustained stretches and placement of props system in a restorative yoga class, and thus contribute to the calming of the sympathetic nervous system (Schleip, 2003).
Sensory receptors are nerve endings that sense a stimulus and deliver it to the nervous system to initiate a response. Proprioceptors provide kinesthetic awareness, and enable determination of joint positions and limbs in space. The primary receptors involved in stretching and range of motion are the muscle spindles (sensing information about length) and the golgi tendon organs (GTOs; sensing information about tensile forces).
As an example, muscle spindles are stretched alongside the muscle, detecting a change in muscle fiber length. “When the CNS [Central Nervous System] perceives this change as potentially injurious, the afferent (sensory) muscle spindle initiates a reflex-like response resulting in the efferent (motor) response. The stretched muscle quickly contracts (excitation) to counteract the stretch.” (Mitchell, 2015). Termed reciprocal inhibition, the reflex (e.g. tested in the knee-jerk test) is an instantaneous mechanism that allows for smooth coordinated movements between the stabilizing co-contraction of the muscle groups (Alter, 2004).
This article is the fourth of a six-part biomechanics series in which I share excerpts from a recent paper I wrote for my Advanced Teacher Training with Tatjana Mesar.