Initiation of a Muscle Contraction
1) A contraction is initiated in the central nervous system, which is composed of the brain and brainstem. The contraction can either be voluntary or involuntary. We will only be focusing on voluntary contractions, which involve skeletal muscle
2) A motor neuron in the ventral (front part) of the spinal cord is activated, and a new action potential is then generated, which is passed outward in the ventral root of the spinal cord.
3) As the action potential travels down the motor neuron axon it arrives at the pre-synaptic terminal
4) The action potential causes voltage-gated Ca2+ channels to open. The causes Ca2+ to enter into the pre-synaptic terminal
5) The rise of intracellular Ca2+ triggers synaptic vesicles filled with acetylcholine (ACh) to pass readily through pre-synaptic membrane into the synaptic cleft.
6) The ACh vesicles travel across the synaptic cleft, which is only 50 nm wide. The bind to acetylcholine receptors, namely nicotinic acetylchoine receptors (nAChR), the nAChR are found within the sacrolemma, which is the outer surface of a muscle cells.
7) An new action potential is then generated and depolarizes the transverse tubules (t-tubules), which lie along the sacrolemma
8) A depolarization of the T-Tubule causes Ca2+ release from the sacroplasmic reticulum (SR)
9) Ca2+ binds to the tropnon-tropomysion complex in the actin filaments. This releases the inhibition that prevented actin from combining with myosin
10) During muscle action, actin combines with myosin-ATP. Action also activates the enzyme myosin ATPase, which then splits ATP. The energy from this reaction produces myosin crossbridge movement and creates tension.
11) ATP binds to the myosin crossbridge; this breaks the actin-myosin bond and allows the crossbridge to dissociate from actin. The filaments then slide past each other and the muscle shortens.
12) Crossbridge activation continues when Ca2+ concentration remains high enough (due to membrane depolarization) to inhibit the troponin-torprmyson system
- ACh at the neuromuscular junction is broken down by acetylcholinesterase (AChE), and this terminates the stream of action potentials along the muscle fiber surface.
- The SR ceases to release calcium ions, and immediately starts to uptake all the calcium ions that have been released.
- In the absence of calcium ions, a change in the configuration of troponin and tropomyosin then blocks the action of the myosin molecule heads, and contraction ceases.
- Kurt Kuhn - www.FUSIONBodybuilding.com