Physiology Lab (Phsl 3063, Phsl 3701)

Fall 2012












Lesson Summaries

Review Material


Atlas of Human Cardiac Anatomy


BME Department

Physiology Department

Institute of Technology









Lessons 1 & 2 - EMG I & II

There are three types of muscle in the human body: 1) Skeletal, 2) Cardiac, and 3) Smooth. Skeletal muscle will be addressed here.

Skeletal muscle is a specialized tissue because of its ability to contract, which ultimately allows your body to move. Its ability to contract is due to units within the muscle cells called sarcomeres. Sarcomeres can contract because they contain two filaments, actin and myosin. In the presence of ATP and calcium, these filaments interact to cause contraction of the sarcomere.


When you want to pick something up, your brain sends a signal down the spinal cord to the set of motor neurons that control the muscles in your arm. A motor neuron is a specialized neuron that innervates a specific set of muscle cells; one motor neuron and the muscle cells it innervates are called a motor unit. When a somatic motor neuron is activated, all of the muscle fibers it innervates respond to the neuron’s impulses by generating their own electrical signals that lead to contraction of the activated muscle fibers. The size of the motor unit arrangement of a skeletal muscle (e.g., 1:10, 1:50, or 1:3000) is determined by its function (flexion, extension, etc.) and location in the body. The smaller the size of a muscle’s motor units, the greater the number of neurons needed for control of the muscle, and the greater the degree of control the brain has over the extent of shortening. For example, muscles which move the fingers have very small motor units to allow for precise control, as when operating a computer keyboard. Muscles that maintain posture of the spine have very large motor units, since precise control over the extent of shortening is not necessary.
Physiologically, the degree of skeletal muscle contraction is controlled by:
1. Activating a desired number of motor units within the muscle, and
2. Controlling the frequency of motor neuron impulses in each motor unit.
When an increase in the strength of a muscle’s contraction is necessary to perform a task, the brain increases the number of simultaneously active motor units within the muscle. This process is known as motor unit recruitment.

otor units
Image from

A couple of additional concepts these modules addressed are that of Tonus and Fatigue.

Tonus is the tension your muscle has at rest. Increased TONUS = increased level of readiness.

Fatigue occurs when the muscle force is reduced for a given short your muscle gets tired. Fatigue can occur for a few reasons:

1) Neuromuscular fatigue in which the motor neuron has reduced availability of acetylcholine thereby reducing the end-plate potential and the number of muscle action potentials (APs). Generally, the muscle will fatigue prior to neuromuscular fatigue.
2) There may be less ATP available for crossbridge action. Reducing the number of crossbridges reduces the force of the contraction.
3) Lactic acid accumulation interferes with the coupling of muscle AP with contractile processes.
4) The ionic balance for AP propagation may be affected by the relative lack of ATP for the Na+-K+ ATPase pump so that AP propagation is impaired.







This page was last updated on November 10, 2008