Muscle Contractions

Chapter 9, Section 2

SynapseSynapse: Functional (not physical) junction between an axon of a neuron and another cell

The two cells are separated by a physical space, called the synaptic cleft.

Neurotransmitters are stored within synaptic vesicles of the presynaptic cell and they’re released into the synapse.

Neuromuscular Junction

Neuromuscular Junction (NMJ) refers to the synapse between an axon and a muscle fiber.

Motor End Plate is a highly folded region of muscle fiber at NMJ that contain abundant mitochondria

Figure 9.8a. General NMJ

Motor neurons innervate effectors (muscles or glands)

A motor unit includes a motor neuron and all of the muscle fibers it controls

Figure 9.9 two motor units. The muscle fibers of a motor unit are innervated (controlled) by a single motor neuron.

Motor Unit

1 motor unit may control between 1 and 1000 muscle fibers

Stimulus for Contraction

Acetylcholine (ACh) is the only neurotransmitter that initiates skeletal muscle contraction

1. A nerve impulse (Action Potential) reaches axon terminal

2. The impulse opens calcium channels at the axon terminal • Calcium diffuse into axon

3. The calcium triggers the release of ACh from vesicles into synaptic cleft.

Sequence of Actions

Sequence of Actions…Continued

4. ACh diffuses across synaptic cleft & binds to receptors on motor endplate.

5. ACh opens Na+ channels on muscle

6. Na+ floods into the muscle, initiating a muscle impulse.

7. A muscle impulse (action potential) is propagated across the entire muscle.

Stimulus for Contraction

Stimulus for a muscle impulse. Corresponds to steps 1-7 in the previous slides.

The muscle impulse causes the release of calcium from the SR. Calcium binds to troponin and tropomyosin is repositioned exposing the actin filaments.

8. The muscle impulse diffuses across sarcolemma and down the t-tubules into the cisternae of sarcoplasmic reticula.

9. The sarcoplasmic reticula release their calcium supplies into the sarcoplasm.

10. Calcium binds to troponin and the troponin repositions the tropomyosin, so the myosin can bind to actin.

11. Cross-bridge cycling causes the muscle to contract.

Stimulus for contraction continued…

Excitation-Contraction Coupling

Calcium released from sarcoplasmic reticulum binds to troponin.

Troponin moves tropomyosin, exposing actin filaments to myosin cross-bridges.

myosin heads bind to actin, forming a cross bridge and cross-bridge cycling causes the muscle to contract.

End of Chapter 9, Section 2