The Physiology of Muscle Contractions

What is the best fuel? All muscles require fuel (energy substrate) in order to contract. The preferred fuel source is carbohydrates. Fatty acids (a component of fats) can be used but the process is more time consuming and the steps are more tedious. Proteins are the least preferred fuel source. They are better used for organ structure, blood components and muscle tissue.

Quick Energy for muscle contraction is made by the: Creatine-Phosphate phosphorylation (CrP) system. This system moves the P from CrP to ADP and regenerates ATP. The CrP is found in muscle cells in limited quantity and comes from our diet.

Short-term energy needs are met by breaking down glucose in the cytosol. This process is called glycolysis* (glucose – splitting). In the absence of glucose, it is necessary to first convert stored glycogen into glucose through a step called glycogenolysis (glycogen – splitting) and then proceeding with glycolysis* Glycolysis* is an 11-step event that will produce 2 ATPs as well as a by-product called lactic acid. * Oxygen is not present in the cytosol, so glycolysis is also known as anaerobic respiration.

Wait, I’ve heard of Lactic Acid! Lactic acid is an acid. It changes the pH of the cell and it contributes to muscle soreness and stiffness after exercise.

Energy for Contractions Primary Energy for continuous muscle contraction comes from the ATP that is produced by aerobic respiration. This method only provides energy as long as there is available glucose and O2. Aerobic respiration produces ~ 36 ATP/glucose molecule.

Now that you know how to get ATP, let’s look at why it is important for muscle contraction.

Anatomy of Skeletal Muscle Because of it’s shape, the muscle cell is called a fiber. 1. the cell membrane is called the sarcolemma 2. each fiber is filled with myofibrils Sarcolemma

Myofibrils are composed of contractile units or sections called sarcomeres. 1. each sarcomere is composed of two types of protein filaments known as: myofilaments - thick and thin filaments 2. the myosin composes the thick filaments 3. the actin composes the thin filaments

Anatomy of Skeletal Muscle Each myosin molecule has a rod-like extension that ends in two heads The myosin heads reach out and bond with the actin and form a temporary connection of the actin myofilaments during contraction. This action requires 1 ATP.

When muscles contraction occurs the sarcomere shortens. 1. The actin molecules are pulled toward the center of the sarcomere 2. When multiple sarcomeres are shortened, the entire muscle fiber is shortened.

Relaxed state Contracted state

Contractions and Calcium Muscle contractions require the presence of ATP and calcium ions Normally, troponin blocks the active sites on actin Calcium binds with and alters the position of troponin

By moving troponin out of the way, the active sites on actin is exposed, this gives myosin a place to bond and cause a muscle contraction. The contraction ends when ATP degrades to ADP and magnesium forces the release of calcium.