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 • Modified region of muscle fiber at NMJ • Mitochondria & nuclei are abundant at motor end plate.
Figure 9.8a. General NMJ
Motor Unit
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 • triggers an influx of calcium into axon 3. Calcium triggers the release of ACh from vesicles into synaptic cleft.
Stimulus for Contraction Sequence of Actions‌Continued 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