Lecture 19: muscle fibers

Types of muscle fibers

• 1. Intrafusal: sensory receptor fibers (only a few fibers in each muscle)

• 2. extrafusal: force-generating fibers that make up the bulk of a muscle

muscle spindle fibers

• sensory receptor for detecting changes in muscle stretch (length and velocity)

  • do not contribute to force generated by the muscle

  • stretch along w/ the extrafusal fibers

  • stretch is what is registered by the receptor

• also call intrafusal muscle fibers (afferent)- 1a

Muscle spindle fibers continued

• after the muscle stretches, the intrafusal fibers need to contract to return to their resting length

  • intrafusal/spindle (efferent)- y gamma LMN

• when contracting a muscle to generate force, extrafusal fibers are activated by a LMNS

  • extrafusal (efferent)- a alpha LMN

Alpha- Gamma Coactivation

• when the CNS instructs a muscle to contract, it not only sends the appropriate signals to the alpha LMN to contract the extrafusal fibers, it also instructs gamma LMN to contract the intrafusal fibers appropriately

• the process regulates muscle tone by ensuring the muscle is always sensitive to stretch

• Low tone: always stretch out (floppy)

• High tone: very tight, sensitive to stretch

Golgi Tendon Organ

• sensory receptor in the tendon that detects changes in muscle force or the load applied to a muscle

  • GTO registers muscle contraction

  • Afferent- 1b

Agonist and Antagonist Relationship

• when 1 muscle (agonist) contracts, the GTO for that muscle registers the contraction is response to the load applied.

• at the same time, the opposing muscle (antagonist) stretches, the muscle spindles register the stretch

• the two muscles maintain a balanced relationship

muscle spindle and GTO

• chart

spinal reflexes

• stretch reflex (monosynaptic)

• reciprocal inhibition reflex (polysynaptic)

• autogenic inhibition reflex (monosynaptic)

• reciprocal excitation reflex (polysynaptic)

• flexor withdrawal reflex (polysynaptic)

additional reflexes

• extensor plantar response (cutaneous reflex)

changes in normal muscle tone (muscle spindles responsible)

• flaccidity- unable to do a muscle contraction

• hypotonicity

• normal tone

• hypertonicity

• spasticity, rigidity- can’t move muscles at all. too much contraction and can’t contract any further