Muscle Innervation & Motor Unit

Question 0

Dynamic Nuclear fiber

Answer 0

Intrafusal fiber of spindle nuclear bag.
Sensitive to change in length AND rate of change

Ia Afferents

Question 1

Classification of Neuronal Fibers

Answer 1

Myelinated and Not myelinated

Conduction velocity increases with diameter and myelination.
Ia and Ib = largest diameter, myelinated => fastest
IV = unmyelinated, smallest diameter => slowest

Question 2

Static and Nuclear-Chain Fibers

Answer 2

Intrafusal fibers of spindle nuclear bag
Sensitive to changes in length

Group Ia and Group II afferents

Question 3

Motor innervation of intrafusal and extrafusal fibers

Answer 3

Intrafusal = A-gamma (myelinated and small diameter)
Extrafusal = A-alpha (myelinated and large diameter)

Question 4

Role of muscle spindle

Answer 4

• Participating in stretch reflexes
• Providing proprioceptive information to the CNS
• Regulating muscle contraction under the influence of descending motor pathways and afferent feedback

Question 5

Co-activation of Alpha and Gamma motor neurons

Answer 5

Alpha and gamma neurons are simultaneously stimulated, both extrafusal and intrafusal muscle fibers contract.

Despite shortening of the muscle, the spindles remain “loaded” and thus sensitive to stretch.

Question 6

Role of Golgi Tendon

Answer 6

Mediates inhibition of overactive alpha motor neurons via Ib afferents.

Question 7

Renshaw Cells

Answer 7

Inhibitory interneurons in CNS. A-alpha collaterals synapse with these cells

Question 8

Motor unit Types

Answer 8

Type I: Slow-twitch, low tension, fatigue resistant, aerobic, innervated by small lower motor neurons

Type IIA: Fast-twitch, Large tension, fatigue resistant, some aerobic capacity, innervated by large motor neurons

Type IIB: Fast-twitch, large tension, easily fatigued, anaerobic, innervated by large motor neurons (increased # mitochondria)

Question 9

Lower motor neuron syndrome

Answer 9

1. Atrophy of the muscle fibers of the affected motor unit
2. Abolition of the voluntary and reflexive responses leading to:
   o Hypotonia
   o Hyporeflexia (e.g. weak stretch reflex)
   o Fasciculations (aberrant simultaneous twitches)
   o Fibrillations (aberrant electrical responses of individual muscular fibers – may correlate with fasciculations).

**Note that the impairment of reflexive responses can also arise from damage to the dorsal roots carrying the sensory fibers of the myotatic reflex arc.