Sensory Contributions to Movement Control - Muscle Receptors

Vocabulary practice flashcards covering sensory contributions to motor control, open- versus closed-loop architectures, sensory transduction fundamentals, and the innervation and mechanics of muscle spindles and Golgi tendon organs.

Open Loop Control

A movement control system in which motor commands are pre-programmed and executed without modifications from sensory feedback after being initiated; it is vulnerable to command errors and unexpected environmental disturbances.

Closed Loop Control

A movement control system that continuously uses sensory feedback to compare the actual state against the desired state via a comparator, making adjustments via an error signal; it is sensitive to loop time delays and can cause overshoot during rapid movements.

Adequate Stimulus

The particular form of physical energy to which a sensory receptor is most responsive and requires the lowest threshold of intensity to elicit action potentials.

Receptor Potential

A graded electrical potential change in a sensory receptor membrane produced when an adequate stimulus opens or closes ion channels; it initiates action potentials along the sensory axon if it surpasses threshold.

Mechanoreceptors

A category of sensory receptors responsive to mechanical physical changes such as stretch, pressure, or displacement; this group includes both muscle spindle afferents and Golgi tendon organ afferents.

Golgi Tendon Organ (GTO)

A encapsulated proprioceptor located at the junction of tendons and muscle fibers, arranged in series with extrafusal muscle fibers to act primarily as tension and force sensors in skeletal muscle.

Group Ib Afferents

Sensory nerve fibers innervating the Golgi tendon organ whose free nerve endings intertwine with collagen fibers and discharge when muscle tension pulls the tendon and pinches the nerve endings.

Muscle Spindle

An encapsulated proprioceptor situated in parallel with extrafusal muscle fibers that monitors changes in muscle length as well as the velocity (rate) of muscle stretch.

Intrafusal Fibers

Specialized, non-force-producing muscle fibers enclosed within the connective tissue capsule of a muscle spindle, comprising nuclear bag fibers ($\text{bag}_1$ and $\text{bag}_2$) and nuclear chain fibers.

Extrafusal Fibers

Standard skeletal muscle fibers that produce mechanical contractile force to drive body movements, innervated exclusively by alpha ($\alpha$) motor neurons.

Group Ia Afferents (Primary Afferents)

Large-diameter sensory axons that enter the muscle spindle capsule to form annulospiral terminals around all three types of intrafusal fibers ($\text{bag}_1$, $\text{bag}_2$, and chain fibers), providing both length and dynamic velocity information.

Group II Afferents (Secondary Afferents)

Smaller-diameter muscle spindle sensory axons terminating predominantly on nuclear $\text{bag}_2$ and chain fibers that faithfully signal static muscle length without a dynamic velocity response.

Gamma ($\gamma$) Motor Neurons (Fusimotor Neurons)

Motor neurons that innervate the contractile polar ends of intrafusal fibers to regulate the stretch sensitivity of the muscle spindle without exerting direct force on the skeleton.

Dynamic Gamma ($\gamma_D$) Motor Neurons

Fusimotor neurons that selectively innervate nuclear $\text{bag}_1$ intrafusal fibers, enhancing the dynamic (velocity) sensitivity of Group Ia afferents during muscle stretch.

Static Gamma ($\gamma_S$) Motor Neurons

Fusimotor neurons that innervate nuclear $\text{bag}_2$ and chain fibers, increasing baseline resting discharge rates and improving the static length sensitivity of the spindle.

Alpha-Gamma ($\alpha\text{-}\gamma$) Coactivation

The concurrent activation of alpha and gamma motor neurons during voluntary muscle contraction, keeping intrafusal fibers taut so muscle spindles continue providing length feedback during muscle shortening.