Outline the process by which stimulation of a motor nerve produces contraction in a skeletal muscle
Answer
The action potential, which develops in response to stimulation of motor nerve, propagates by activating voltage-gated sodium channels along the axon toward the junction. When it reaches the junction, it causes a calcium ion influx through voltage-gated calcium channels. The Ca2+ influx causes vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine out into the extracellular space between the motor neuron terminal and the neuromuscular junction of the skeletal muscle fiber.
The acetylcholine binds to nicotinic type acetylcholine receptor on the neuromuscular junction and opens multiple “acetylcholinegated” channels. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This initiates an action potential at the membrane. . The action potential travels along the muscle fiber membrane and spread to inner portion of the muscle fiber trough T-tubules, depolarizing muscle fiber. Depolarization of muscle fiber causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.
The calcium binds to the troponin C and uncovers the myosine binding sites on actin . Myosin heads bind firmly to actin. Then thin filaments slid on thick filament producing muscle contraction (shortening the sarcomere and the I-band) . Energy for this contraction provide from previous hydrolysis of ATP to ADP and Pi by myoine.
Summary
- Neuron action potential arrives at end of motor neuron
- Calcium ion influx into nerve terminal
- Acetylcholine (ACh) is released form nerve terminal
- ACh binds to nicotinc receptors on motor end plate
- Open acetylcholine-gated channels
- Sodium ions to diffuse to the interior of the muscle fiber membrane
- Initiates an action potential at the membrane
- Muscle action potential sweeps into the T tubules
- Release of Ca from the cisternae of the sarcoplasmic reticulum
- Ca binds to the Troponin C
- Uncover myosin binding site on actine
- Myosin binds to actin (cross bridge is formend)(ADP released from myosin)
- Myosin head pivots (pulling actin)
- Myosin releases from actin (cross bridge is broken)(another ATP binds to myosin)
- Myosin re-extends into "ready" position (ATP--- ADP+Pi)(ADP is bound to myosin)
Labels: Excitable tissue, Excitable tissue MCQs, Excitable tissue SEQs, Physiology SEQ