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The Axon Terminal of the Motor Neuron and the Motor End Plate of the Muscle Fiber

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The Axon Terminal of the Motor Neuron and the Motor End Plate of the Muscle Fiber How does the body initiate a voluntary muscle contraction? *What was the stimulus that allow muscle cells to demonstrate excitability? Neuromuscular Junction Before a skeletal muscle fiber can contract it must be stimulated by an electrical signal called a muscle action potential. This electrical signal is provided by nerve cells called motor neurons. Each motor neuron will stimulate a number of muscle fibers. We refer to the motor neuron and all the muscle fibers it stimulates as a motor unit. Muscle involved in small precise movements ● Example: eye movement muscles ● Each motor unit will control fewer than 10 muscle fibers. Muscle involved in large powerful movements ● Example: Biceps brachii (elbow flexion) ● Each motor unit will control as many as 2000 muscle fibers. Therefore, require fewer motor neurons. Practice Which group of muscles will have few fibers per motor unit? Why? ➔ muscles that bend (flex) the knee or ➔ muscles that bend (flex) the fingers? Key parts of the neuromuscular junction Axon - the long process of the motor neuron (carries electrical signal away). Axon terminals - branches of the axon that approach the muscle fiber. Synaptic end bulbs - enlarged endings of the axon terminals. (DO NOT touch muscle fiber.) Synaptic vesicles - packages of Acetylcholine/ACh (neurotransmitters) in the synaptic end bulbs. Motor end plate - region of the sarcolemma near the synaptic end bulb. Synaptic cleft - the space between the axon terminal (synaptic end bulb) and the sarcolemma (motor end plate) Neuromuscular Junction It is the the region formed by the axon terminal of the motor neuron and the motor end plate of the muscle fiber. What Happens at the Neuromuscular junction? Physiology of the NMJ To initiate a muscle contraction the motor neuron excites a skeletal muscle in the following way: (Remember that the neuron and muscle cell will not physically touch one another) Events at the NMJ 1. Release of Acetylcholine. An action potential will travel down the axon of a motor neuron. Once reaching the synaptic end bulb it triggers the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft. ? What structure is the acetylcholine stored in? Events at the NMJ 2. Activation of ACh receptors. ACh diffuses across the synaptic cleft and binds to receptors on the motor end plate. This will open channels for Na+, which will then flow across the membrane. ? What is the motor end plate a part of? Events at the NMJ 3. Generation of muscle action potential. As the sodium ions move into the muscle cell it creates an electrical charge (muscle action potential). The action potential travels across the sarcolemma and through the T- tubules. Events at the NMJ 4. Breakdown of ACh. The enzyme acetylcholinesterase (AChE) rapidly breaks down the ACh in the synaptic cleft. What do you think? What would happen if the release of ACh was blocked? NMJ Pictorial Activity NMJ Animation Physiology of a muscle contraction What happens now that we have a muscle action potential? Remember that for a muscle contraction we need both ATP (energy) and Ca2+ In a relaxed muscle the Ca2+ is stored in the sarcoplasmic reticulum. Steps in a muscle contraction 1. Muscle action potential travels across the sarcolemma and t-tubules. This triggers Ca2+ channels in the sarcoplasmic reticulum to open. This allows Ca2+ to move into the sarcoplasm. 2. Ca2+ binds to troponin molecules in the thin myofilament. This causes troponin to change shape, moving the troponin- tropomyosin proteins and exposing the myosin binding site on actin. 3. ATP attaches to the myosin head. This provides the myosin head with energy. 4. The high energy myosin head attach to the myosin- binding sites on actin (called crossbridges). 5. Then the myosin head rotates or swivels. This is referred to as the power stroke. This slides the thin filament over the thick filament into the H zone. 6. ATP binds to the myosin heads allowing them to detach from the actin and reenergize the myosin head. The myosin head is now ready to combine with a myosin- binding site further along the thin myofilament. Steps 4-6 will continue to repeat as long as ATP and Ca2+ are available. As a muscle contracts there will be some myosin heads attached to the actin while others will be detached and preparing to bind again. What do you think What would happen if calcium ion was available but no ATP? Muscle contraction animation Homework In paragraph form, describe the entire process of events involved in a muscle contraction. Start with the action potential traveling down the axon of a motor neuron and end with the release of the myosin head from the actin..
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