Question

When your brain decides to flex your biceps and the motor neurons to the appropriate muscle has/been excited, what is the chain of eventd that intiate excitation to the actual contraction of the muscle. Be as detailed as possible and assume beginning with the motor neuron releasing ACh.

Answer 1

The junction between somatic motor neuron (neuron that carry signals to the skeletal muscle) and skeletal muscle fiber is called neuromuscular junction. The neurotransmitter released at neuromuscular junction from the motor neuron is Acetylcholine. A region of sarcolemma, the cell membrane of the skeletal muscle fiber opposite to the synaptic bulb of somatic motor neuron is called motor end plate. This motor end plate has several acetylcholine receptors called nicotinic receptors to which the acetylcholine bind.

The Excitation to the actual contraction of the muscle occur in the following sequence.

• Arrival of nerve action potential at the synaptic bulb of the somatic motor neuron open the voltage-gated calcium channels in the synaptic bulb result in influx of Ca2+ ions into the synaptic bulb.
• This Ca2+ ion influx facilitate fusion of synaptic vesicle (contain acetylcholine) with presynaptic membrane of somatic motor neuron result in release of acetylcholine to the synaptic cleft (small space between end of the motor neuron and muscle fiber). This process is known as exocytosis.
• The acetylcholine diffuse toward the motor end plate and binds to the nicotinic receptor present in the motor end plate.This results in opening of cation channels in the Ach receptors leads to influx of Na+ into the muscle fiber which makes inside of the muscle fiber more positively charged.
• This positive membrane potential generate an action potential in the muscle fiber and travels through sarcolemma into the transverse tube and leads to opening of Ca2+ channels in the sarcoplasmic reticulum. The influx of Ca2+ ions into the sarcoplasm occur and the myosin binding sites are exposed to actin by the binding of Ca2+ ions on the troponin present in the thin filament.
• The binding od actin to the myosin heads results in the pulling of thin filaments towards the center of sarcomere and leads to muscle contraction.