The thought of stepping up to reach stimulates the frontal lobe of the brain which controls voluntary muscle movements. An action potential is carried by upper motor neurons down descending nerve tracts of the central nervous system to synapse with lower motor neurons in the spinal cord (Seeley, Stephens and Tate, 2007, p. 222). The interneurons then synapse with the spinal nerves of the peripheral nervous system which will then carry the action potential to specific skeletal muscles.
The target muscles are the hamstring muscles which includes the biceps femoris, semitendinosus and semimembranosus as well as the rectus femoris. Hamstring muscles contract to flex the leg at the knee joint, a hinge joint, by pulling on the head of the fibula and the lateral condyle of the tibia while the rectus femoris flexes the thigh at the hip joint, a ball-and-socket joint (Tortora and Derrickson, 2006, p. 389).
At the synapse involving the neuron and muscle fibers of the hamstring muscles, also called the neuromuscular junction, the nerve impulse causes the synaptic vesicles to release Acetylcholine, a neurotransmitter, which diffuse in the synaptic cleft towards the muscle membrane activating the Acetylcholine receptors, which are membrane channels, to open. Sodium ions then enter the muscle fiber making the cell more positively charged and triggering a muscle action potential that propagates into the T-tubules of the muscle (Tortora and Derrickson, 2006, pp.
302-303). This results in the release of calcium ions stored in the sarcoplastic reticulum subsequently causing contraction in the muscles. Part Two: The intent of reaching up above your head is the stimulus that creates the action potential in the neurons of the central nervous system which are transmitted to the appropriate spinal nerves in the peripheral nervous division and then to the muscles which are the effector organs as described in Part I.
The target muscles are the biceps brachii which flexes the arm at the shoulder joint (ball and socket joint) and the flexor carpi radialis which flexes the hand at the wrist joint (condyloid joint) (Tortora and Derrickson, 2006, pp. 370-371). After the events that occur at the neuromuscular junction where the action potential from the nerve fiber is transmitted to the muscle fiber, the process of contraction involves a specific series of events. An enzyme in the myosin (thick filament) heads in the sarcomeres of the muscle fibers breaks down Adenosine Triphosphate, the basic energy unit, into Adenosine Diphosphate via hydrolysis.
This catabolic activity energizes the myosin head and as tropomyosin unblocks the myosin binding sites in actin (thin filament), the myosin heads now bind to actin forming crossbridges, releasing the hydrolyzed phosphates in the process (Tortora and Derrickson, 2006, p. 299). The conversion of chemical energy from ATP into mechanical energy results in a power stroke or movement. This occurs as the crossbridges rotate in order to release the ADP from where they are bound.
The force of rotation slides the actin past the myosin or the thin filament past the thick filament at both ends towards the M line (Tortora and Derrickson, 2006, p. 299). The sarcomeres then shorten which shorten the muscle fiber and consequently the whole muscle. List of
References
Seeley, R. R. , Trent, D. S. and Tate, P. (2007). Essentials of Anatomy and Physiology (6th ed). New York: McGraw-Hill Companies, Inc. Tortora, G. J. and Derrickson, B. (2006). Principles of Anatomy and Physiology (11th ed). New Jersey: John Wiley and Sons Inc.