Physiology - Lecture 24: Contraction and relaxation of skeletal muscle fibers

What composes the motor unit?

1. one motor neuron

2. the muscle fibers that it innervates

What significance is there to the number of fibers innervated by a single motor neuron?

• The fewer the number of fibers/neuron the finer the movement

  • larynx: 2-3 fibers

  • eye movements: 10 -20 fibers/neuron

  • biceps and gastrocnemius: 2000 - 3000 fibers/neuron

What is the neuromuscular junction (NMJ)?

• a specialized form of a chemical synapse between an alpha motor neuron terminal and the muscle fiber it innervates

• are made at terminal nerve endings at the midpoint of a muscle fiber

Mow many NMJ does a muscle fiber have?

one

What is the function of synaptic bulbs?

contain synaptic vesicles which contains molecules of the neurotransmitter ACh

How is a muscle action potential produced?

1. ACh is released from synaptic vesicle

2. ACh binds to ACh receptor

3. Muscle action potential is produced

4. ACh is broken down

Where do the synaptic vesicles release the ACh?

The active zone (presynaptic membrane opposite to the junctional folds on the sarcolemma)

What is the purpose of the junctional folds on the sarcolemma?

Increase postsynaptic surface area where the ACh ligand receptors (Na+ & K+ channels) are mostly located

Where can acetylcholinesterase be found?

Basal lamina & postsynaptic membrane

What is the function of acetylcholinesterase?

ATP breaking enzyme

What happens in the neuromuscular junction?

1. Action potential reaches the axonal bulb

2. Ca2+ channels open

3. exocytosis, which releases ACh

4. ACh binds to receptors which causes depolarization of the postsynaptic membrane (sarcolemma)

What is the receptor that ACh binds to?

nicotinic cholinergic receptors

What happens when Ca2+ binds to troponin?

Changes the shape of the troponin-tropomyosin complex and uncovers the myosin binding sites on actin which allows cross-bridge binding. Binding of actin and myosin triggers a power stroke that pulls thin filament inward during contraction

How is the myosin head energized?

ATP → ADP + Pi

What happens when the Pi group is released?

triggers the power stroke of contraction

What is the latent period?

the period from the stimulus to the beginning of contraction

What is the refractory period?

the period when a muscle fiber can’t respond to another action potential