Somatic motor neurons are part of the voluntary nervous system. Upon nerve firing, they release acetylcholine from the nerve terminal, and the acetylcholine activates sodium/potassium channels.
By activating sodium/potassium channels, acetylcholine allows for sodium influx into the cell, causing depolarization.
The cell reaches an all-or-nothing threshold electric potential value. This means that if the electric potential isn’t high enough nothing happens. When it is high enough, the potential is overcome, and this triggers opening of voltage-gated calcium channels.
Voltage-gated calcium channels open on the cell membrane and allow calcium influx into the cell.
The sarcoplasmic reticulum responds to the calcium influx and depolarization by releasing its internal calcium storage.
This flood of calcium released from the sarcoplasmic reticulum enters myofibrils and binds to filaments to cause contraction.
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