Muscle Contraction

Muscle Movement

• Muscle fiber must be stimulated:– By an electrical signal called muscle action potential (AP)– Delivered by motor neuron

• Motor neuron + muscle fibers = motor unit• Simulation of 1 motor neuron causes all the

muscle fibers in that motor unit to act at the same time.

– Precise movements = many motor units controlling few muscles fibers

– Powerful movements = few motor units controlling many muscle fibers.

Axon of Motor NeuronAction Potential

Nucleus Axon Terminal of NMJ

Sarcolemma of Muscle Fiber

Myofibril

NMJ

Vesicle

Mitochondrion

Synaptic Cleft

Sarcolemma

#1Nerve impulse (AP) arrives at axon terminal

#2Calcium ions (Ca2+) released into axon terminal

Ca2+Ca2+

#3Ca2+ causes synaptic vesicles to release Acetylcholine (ACh) via exocytosis

Steps at NMJ

#4ACh diffuses across synaptic cleft and binds to receptors in the sarcolemma

#5Binding of ACh to its receptors opens ion channels. - Na+ moves into muscle fiber (more positive)- K+ moves out of muscle fiber (more negative)Creates a new AP

#6ACh effects are stopped by the enzyme acetylcholinesterase (AChE) -- Breaks down remaining ACh in the synaptic cleft

Steps at NMJ

Na+

K+

Na+

Steps for E-C Coupling Excitation-Contraction

#5 from NMJBinding of ACh to its receptors opens ion channels. - Na+ moves into muscle fiber (more positive)- K+ moves out of muscle fiber (more negative)Creates a new AP

Vocabulary

Sarcoplasmic Reticulum (SR)Network of tubules that surround each myofibril

T (Transverse) TubulesTunnel like extensions of the sarcolemma, run between terminal cisternae of the SR, and allow for AP to reach deep regions of muscle cells.

Terminal Cisternae of SRNetwork of tubules that surround each myofibril and store calcium ions

Steps for E-C Coupling Excitation-Contraction

#1Action potential is spread alongthe sarcolemma and down the T tubules

#2Causes the terminal cisternae of the SR to release stored calcium ions into the sarcoplasm.-- within 1 millisecond!

#2Causes the terminal cisternae of the SR to release stored calcium ions into the sarcoplasm.-- within 1 millisecond!

#3Calcium binds to troponin andremoves the blocking action oftropomyosin. When Ca2+ binds,troponin changes shape, exposingbinding sites for myosin on the actin myofilaments.

#4Myosin binding to actin forms cross bridges and results in the sliding of the actin filaments towards the center of the sarcomere (power stroke) = muscle contraction.

Steps for E-C Coupling & Muscle Contraction