Chapter 9, Section 2 Muscle Contractions
Synapse •Synapse: Functional (not physical) junction between an axon of a neuron and another cell
• Synaptic cleft: physical space that separates the two cells
•Synaptic vesicle: stores and releases neurotransmitters from axon into synapse
Neuromuscular Junction Neuromuscular Junction (NMJ): Synapse between axon and muscle Motor End Plate â€˘ Modified region of muscle fiber at NMJ â€˘ Mitochondria & nuclei are abundant at motor end plate.
Figure 9.8a. General NMJ
Motor neuron: neuron that controls effectors (muscles or glands) Motor unit: motor neuron and all the muscle fibers it controls
1 motor unit may control between 1 and 1000 muscle fibers
Figure 9.9 two motor units. The muscle fibers of a motor unit are innervated (controlled) by a single motor neuron.
Stimulus for Contraction Acetylcholine (ACh): Neurotransmitter that initiates skeletal muscle contraction
Sequence of Actions 1. Nerve impulse (Action Potential) reaches axon terminal 2. Impulse opens calcium channels at axon terminal â€˘ triggers an influx of calcium into axon 3. Calcium triggers the release of ACh from vesicles into synaptic cleft.
Stimulus for Contraction Sequence of Actionsâ€ŚContinued 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. Muscle impulse (action potential) is propagated across the entire muscle.
Initiation of a muscle impulse at the Neuromuscular Junction (Numbers correspond to steps 1-7 in previous slides).
Stimulus for Contraction: Muscle Impulse 1. Muscle impulse diffuses across sarcolemma 2. Impulse travels down t-tubules and into cisternae of Sarcoplasmic Reticulum 3. Sarcoplasmic Reticulum releases Ca2+ into sarcoplasm, initiating a muscle contraction.
Excitation-Contraction Coupling Troponin-Tropomyosin Complex • Tropomyosin • Blocks binding sites on actin when a muscle is at rest
• Troponin • Ca2+ binds to troponin during muscle contraction. •Troponin moves tropomyosin, which exposes binding sites on actin
Excitation-Contraction Coupling Calcium released from sarcoplasmic reticulum binds to troponin. Troponin moves tropomyosin, exposing actin filaments to myosin cross-bridges. Cross-bridges on myosin link to actin, and pull the actin filaments causing a muscle contraction.
End of Chapter 9, Section 2