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LAB 11 - Muscle Physiology


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Muscles are the "engine" that your body uses to propel itself. they turn energy into motion. It would be impossible for you to do anything without your muscles. Absolutely everything that you conceive of with your brain is expressed as muscular motion.

Skeletal Muscle Tissue
Muscle is a very specialized tissue that has both the ability to contract and the ability to conduct electrical impulses. Muscles are classified both functionally as either voluntary or involuntary and structurally as either striated or smooth. From this, there emerges three types of muscles: smooth muscle, skeletal muscle and cardiac muscle.

Skeletal Muscle

Elongated cells
Multiple peripheral nuclei
Visible striations

Cardiac Muscle

Branching cell
Single central nucleus
Visible striations

Smooth Muscle

Spindle shaped cell
Single central nucleus
Lack visible striations

Neuromuscular Junction
Skeletal muscle cells contract as a result of impulses from motor neurons. The place where a motor neuron stimulates a muscle cell is called a neuromuscular junction. In order for skeletal muscle cells to contract each cell must be stimulated by a process of a motor neuron.

Sliding Filament Theory
The theory of how muscle contracts is the sliding filament theory. The contraction of a muscle occurs as the thin filament slide past the thick filaments. The sliding filament theory involves five different molecules plus calcium ions. The five molecules are: myosin, actin, tropomyosin, troponin, and ATP.

The myosin molecules are bundled together to form the thick filament. The head (cross bridge) of the myosin molecule has the ability to move back and forth. The flexing movement of the head provides the power stroke for muscle contraction. The hinge portion of linear tail allows vertical movement so that the cross bridge can bind to actin on the thin filament. The cross bridge has two important binding sites. One site specifically binds ATP, a high energy molecule.

This binding of ATP transfers energy to the myosin cross bridge as ATP is hydrolyzed into ADP and inorganic phosphate. The second binding site on the myosin cross bridge binds to actin.
Actin is the major component of the thin filament. Tropomyosin entwines around the actin and covers the binding sites on the actin subunits and prevents myosin cross bridge binding.
Troponin is attached and spaced periodically along the tropomyosin strand. After an action potential calcium ions are released from the terminal cisternae and bind to troponin. This causes a conformational change in the tropomyosin-troponin complex, "dragging" the tropomyosin strands off the binding site.
The five organic molecules and the calcium ions all work together in a coordinated maneuver to cause the thin filament to slide past the thick filament, and are illustrated here.

Muscle Metabolism

The energy necessary for muscle contraction is provided by ATP. ATP energizes the power stroke of the myosin cross bridge, disconnects the myosin cross bridge from the binding site on actin at the conclusion of a power stroke, and energizes the calcium ion pump. In order to make ATP, the muscle does the following: breaks down creatine phosphate, adding the phosphate to ADP to create ATP, carries out anaerobic respiration by which glucose is broken down to lactic acid and ATP is formed, and carries out aerobic respiration by which glucose, glycogen, fats and amino acids are broken down in the presence of oxygen to produce ATP.

Fun Fact - After death, calcium levels inside the muscle cells rise and the body's level of ATP drops. Inside the muscles, myosin binds to actin and the muscles contract. However, with no ATP to reset the cross bridges and release the myosin, all of the muscles remain contracted and stiff. This state is called rigor mortis.

Contraction of Motor Units

The contraction of a skeletal muscle is the result of the activity of groups of muscle cells called motor units. In skeletal muscle, the cells never contract individually. Rather they contract as groups of muscle cells that are collectively connected to a motor nerve originating in the spinal cord. The combination of the motor nerve cell (neuron) and the muscle cells it innervates is known as the motor unit. The size of the motor units determines the precision of movement that a particular muscle can produce.