Pumped Out

Muscle contraction is chemical. The hand reaches for a pocket in the sheer rock face. “Contract!” says the electrical impulse from the brain. The motor nerve and the muscle meet at the neuromuscular junction. Acetylcholine, a neurotransmitter transfers this message across the junction and is then destroyed by the enzyme acetylcholinesterase. The muscle membrane has received the message, however, and the fibers begin to release calcium. Calcium comes from the sarcoplasmic reticulum inside each muscle cell. Calcium enables the very basic proteins of the muscle cells –actin, the sheath, and myosin, a rod inside the sheath– to slide towards one another. These sliding filaments are called sarcomeres. The sarcomere shortens. The muscles contract. The hand grabs the hold.

Muscle relaxation is chemical. The hand needs to let go. The brain says, “Relax!” Acetylcholine transfers the message from the nerve end to the muscle just to be destroyed by acetylcholinesterase. Again, calcium enters the sarcoplasmic reticulum in the muscle membranes, and the actin and myosin slide away from one another. The sarcomere lengthens. The muscles relax. The hand opens to move to a higher hold in the sheer rock face.

Muscle contraction and relaxation are chemical reactions that use up energy. A very specific type of energy called adenosine triphosphate or ATP is needed. Hands clench, muscles contract. Hands open, muscles relax. Before the climber, who owns these working muscles, tires, highly oxygenated blood is being pumped to the action site. Oxygen allows the muscles to produce ATP by burning fuels such as glycogen, glucose and body fat. This is a clean process called aerobic metabolism. The climber, who owns these working muscles, tires over time and repetition. Blood being pumped to the site contains less and less oxygen. If the muscles are using more oxygen than the blood is pumping to the site, fuels can still be broken down for ATP production, but a residue called lactic acid is left. This is called anaerobic metabolism. Hand and forearm muscles begin to tire. They begin to ache and throb. The lactic acid swells the muscle and oxygen level is further depleted. The hand clutches but only for a few seconds before the muscle stops working. The hand opens up.

The climber, who owns the hands that have opened up and let go of the rock, falls. Back on the ground, while massaging and kneading the aching forearm and hand muscles, allowing blood circulation to clean up the mess of lactic acid, the climber says, “Man, I just couldn’t hold onto anything anymore. I was so pumped out!”

Bibliography
Herlihy, Barbara and Maebius, Nancy K. “Chapter 9: Muscular System.” The Human Body in Health and Illness. 2nd ed. Elsevier Science: Philadelphia, PA, 2003, pp 139-161.

Posted on June 7th, 2008