Muscle contraction begins when a nerve impulse reaches the synaptic terminals of a motor neuron that synapses with a muscle fiber. The motor neuron releases an excitatory neurotransmitter, which then diffuses across the neuromuscular junction. The neurotransmitter then encounters receptors on the surface of the muscle fiber, and stimulates the muscle fiber to contract. The process of muscle contraction can be broken into seven steps.
1. Acetylcholine (an exitatory neurotransmitter) is released from the motor neuron, and diffuses across the neuromuscular junction. This generates an action potential in the sarcolemma – the layer surrounding the muscle fiber.
2. The action potential travels deep into the muscle cell via transverse tubules (T-tubules) lined with the same material as the sarcolemma.
3. The action potential reaches the sarcoplasmic reticulum – a structure responsible for the storage of calcium ions in muscle tissue. This action potential causes calcium (Ca2+) ions to be released from the sarcoplasmic reticulum. These ions then diffuse throughout the muscle fiber (in and around myofibrils / myofilaments).
4. Calcium (Ca2+) ions bind to troponin (part of tropomyosin – a protein that surrounds the thin filaments of the sarcomere) and exposes a binding site for myosin. This enables myosin ‘heads’ to bind to the actin.
5. The binding of myosin to actin causes the thin filaments to move inwards towards the H-zone.
6. Cytosolic Ca2+ is taken up once again by the sarcoplasmic reticulum once the action potential has ceased.
7. Without Ca2+, tropomyosin once again blocks myosin binding sites on the thin filaments. Contraction therefore ends, and the muscle fiber relaxes.
Muscle contraction, much like other biological processes, uses energy from adenosine triphosphate (ATP) molecules to do useful work. Muscles require a constant supply of ATP in order to contract. However, ATP is not stable enough to serve as a long-term energy store. Instead, muscles convert sugars (such as glucose) into ATP dynamically, as required. This process can be broken into several stages.
1. Oxygen (O2) molecules and glucose are brought to muscles via the blood stream. Alternately, O2 can be provided by myoglobin – an oxygen store in muscles that resembles hemoglobin. As well, glucose may be obtained from glycogen stores in the muscle fiber.
2. Via the process of glycolysis, glucose is converted into two pyruvate molecules, two reduced nicotinamide adenine dinucleotide molecules (NADH + H+), and two ATP molecules.
3. If there is little oxygen available, ATP production ends with the conversion of pyruvate into lactic acid, which causes muscle soreness after overexertion.
4. If sufficient oxygen is available, pyruvate enters the mitochondrion, and is converted via a decarboxylation reaction into acetyl-coenzyme A, carbon dioxide gas, and NADH + H+. Acetyl-CoA then enters the Krebs cycle, which, in conjunction with the electron transport chain, produce 34 more ATP molecules.
If there is an excess of ATP in a relaxed muscle, it is used to build up creatine phosphate – an energy store – from creatine and inorganic phosphate.
