Motor Units and Proprioceptors

Motor unit anatomy

Alpha motor neuron plus all the innervated muscle fibers.

to increase muscle tension/ force production

More motor units within the muscle become active.

Recruitment threshold of fibers

Type I- low

Type IIa- intermediate

Type IIx- high

Fatigue resistance of fibers

Type I- high

Type IIa- intermediate/ low

Type IIx- low

Force production of mucle fibers

Type I- low

Type IIa- intermediate

Type IIx- high

Power output of muscle fibers

Type I- low

Type IIa- intermediate

Type IIx- high

Capillary density of muscle fibers

Type I- high

Type IIa- intermediate

Type IIx- low

mitochondrial density in muscle fibers

Type I- high

Type IIa- intermediate

Type IIx- low

All-or-none principle

When an alpha motor neuron fires, all innervated fibers fully contract.

Motor unit recruitment order

Lower threshold motor units are recruited before higher threshold ones.

Proprioceptors definition

Specialized sensory receptors providing CNS info to maintain muscle tone and coordinate movement.

proprioceptors changing force-producing capacity via reflexes definition

Rapid, automatic, involuntary bodily responses to stimuli.

muscle proprioceptors

Golgi Tendon Organs (GTO) and Muscle Spindles (intrafusal muscle fibers)

Golgi Tendon Organs (GTO)

Receptors located in tendons giving feedback about muscle tension. Run in series to typical muscle fibers

Muscle spindles

Intrafusal fibers running parallel to muscle fibers giving feedback about length changes.

GTO reflex action

Inhibits alpha motor neurons to cause muscle relaxation and relieve tension, force capabilities decrease

steps of GTO reflex

1. Detects tension applied to a tendon

2. Sensory neurons conduct action potentials to the spinal cord

3. Sensory neurons synapse → inhibitory → interneurons → alpha-motor neuron

4. Inhibition of the alpha motor neurons causes muscle relaxation, relieving the tension applied to the tendon

Muscle spindle reflex action

Large and/or fast stretch of muscle are sense

Activates the agonist muscle and deactivates the antagonist muscle upon rapid stretch.