For muscles to contract, they have to be stimulated by nerve impulses. A motor neuron (nerve cell) can either stimulate a few muscle cells or hundreds, depending on the particular muscle. A motor unit is one neuron and all the skeletal muscle cells it stimulates. When a long, thread like extension of the neuron, the nerve fiber or axon, reaches the muscle, it branches into a number of axon terminals and forms junctions with the sarcolemma of a different muscle cell. These junctions, neuromuscular junctions, have vesicles full of a chemical known as a neurotransmitter. For skeletal muscles the neurotransmitter stimulates acetylcholine (ACh). ACh activates the muscle, crosses the gap, contains a lot of calcium, and is stored in vesicles. The muscle also needs an electric impulse that travels through it called action potential.
The sliding filament theory is one way to answer what causes the filaments to slide. It is the theory of how muscle contracts. It occurs as thin filaments
slide past the thick filaments. This involves five different molecules plus
calcium ions: myosin, actin, tropomyosin, troponin (covers up the actin so the myosin can't randomly grab it), and ATP. The steps of the sliding
filament theory: 1. Nerve impulse arrive at an axon terminal and triggers the release of ACh.2. ACh diffuses across the cleft, binds to its receptors, and triggers a muscle action potential (AP).
3. Acetylcholinesterase (AChE) in synaptic cleft destroys ACh so another muscle action potential does not happen unless more is released from a motor neuron. 4. Muscle AP opens Ca2+ release channels in the sarcoplasmic reticulum (SR). 5. Ca2+ binds to the troponin on the thin filament, exposing the binding site for myosin.6.Contraction. Power strokes use ATP. Myosin heads bind to actin, swivel and release then thin filaments are pulled toward center of
sarcomere. 7. Ca2+ release channels in SR close and Ca2+ active transport pumps use ATP to restore low level of calcium ions in sarcoplasm. 8. Troponin-tropomyosin complex slides back into position where it blocks the mydsin-binding site on actin. 9. Muscle relaxes.
ATP is the only energy source that can be used directly to power muscle
activity so it must be regenerated continuously if contraction is to
continue. There are three ways the body produces energy (ATP) that is used
for muscle contraction. 1. Use of creatine phosphate (CP). It
is found in muscle fibers but not other cell types. CP and ADP result in
transfers of a high energy phosphate group, producing more ATP in a fraction of a second. 2. Aerobic respiration. It occurs in the mitochondria
and when it is happening glucose is broken down completely to carbon
dioxide and water, and some of the energy released as the bonds are broken is captured in the bonds of ATP molecules. 3. Anaerobic glycolysis and lactic acid formation. Glycosis occurs in the cytosol. Glucose is broken down to pyruvic acid and small amounts of energy are captured in ATP bonds. When aerobic pathways cannot keep up with demands of ATP the pyruvic acid generated from glycosis is converted to lactic acid.