Nervous and Muscular Systems

Nerve Impulses Action Potential When a dendrite or cell body is stimulated, the permeability of the neuron’s membrane changes suddenly. The membrane potential reverses and the neuron’s cell body becomes more positively charged than the exterior of a cell. This reverse in polarity begins an action potential.

Synaptic Transmission

Neuromuscular Junction Each skeletal muscle fiber has a nerve ending that controls its activity. Synaptic vesicles hold acetylcholine, a neruotransmitter that binds to receptors on the muscle

Neuromuscular Junction When acetylcholine binds to the receptors, the muscle cell releases calcium ions from where they are stored Calcium begins the process of muscular contraction

Actin and Myosin Proteins

Muscle Structure Since actin filaments are the main protein in contraction and relaxation, actin filaments are anchored at their endpoints and create a structure called the Z band. The region from one Z line to the next is called a sarcomere. This unit is the basic unit of contraction for skeletal and cardiac muscle.

When calcium is not present:

Muscular Contraction When calcium is released, it allows the myosin filaments’ heads attach to points on the actin filaments.

Muscular Contraction The myosin heads then bend inward, pulling the actin with them.

Muscular Contraction The myosin heads then let the actin go, bend back into their original position, attach to a new point on the actin filament, and pull again.

Muscular Contraction The action just described shortens the sarcomere. ATP is required to detach the myosin heads so that the sarcomere can continue to contract. Because myosin heads must attach and detach a number of times during a single muscle contraction, muscle cells must have a continuous supply of ATP.

Contraction of a Muscle Section 3 Muscular System Chapter 45 Contraction of a Muscle

Muscle Fatigue Muscle cells store glycogen, which provides muscles with energy, and have several mitochondrion for cell respiration. During prolonged and vigorous exertion and when glucose has been used up, fat molecules are utilized for energy. When energy availability fails to keep pace with its use, muscle fatigue sets in. Muscle fatigue is the physiological inability of a muscle to contract, and is a result of a relative depletion of ATP.

Muscle Fatigue Oxygen Debt When the level of oxygen coming into the body is not enough to meet the demands of energy production, an oxygen debt is established. Oxygen is needed for cellular respiration, so when ATP is needed, but oxygen is limited, lactic acid is produced during fermentation