Electromyogram (EMG)

The contraction of the skeletal muscle results in the generation of action potentials in the individual muscle fibres, a record of which is known as electromyogram. This activity is similar to that observed in the cardiac muscle, but in the skeletal muscle, repolarization takes place much more rapidly, the action potential lasting only a few milliseconds.

The electromyography is an instrument which records the electrical activity of the muscles to determine whether a muscle is properly contracting or not. It is also used to determine nerve conduction velocity.

Since most EMG measurements are made to obtain an indication of the amount of activity of a given muscle or a group of muscles rather than of an individual muscle fibre, the EMG pattern is usually a summation of the individual action potentials from the fibres constituting the muscle or muscles being studied.

The electrical activity of the underlying muscle mass can be observed by means of surface electrodes on the skin. However it is usually preferred to record the action potentials from individual motor units for better diagnostic information using needle electrodes.

In voluntary contraction of the skeletal muscle; the muscle potentials range from 50 μV to 5 mV and the duration from 2 to 15 ms. the values vary with the anatomic position of the muscle, the size and location of the electrode. In a relaxed muscle, there are usually no action potentials.

How Muscles Work

Muscles are a series of parallel fiber bundles. They are made of protein. The filament bundles are called myofibrils. In between them, there are deposits of glycogen or starch i.e. carbohydrate fuel. Muscle fibres are really the muscle cells with their nuclei along the outer edge. These bundles are surrounded by a connective tissue like the insulation of an electric cable. A small muscle has a few bundles of fiber, while a large muscle has hundreds of bundles. Each muscle has a large belly and two tapering ends or tendons. Each tendon is attached to a bone.

Skeletal or voluntary muscles are activated by motor nerves in the spinal cord. Each nerve splits into several strands to enter different strands of the muscle fiber. A typical muscle motor unit is shown:

An electrical signal runs down from the brain to the tip of the motor nerve. Between the nerve ending and the muscle, a small amount of chemical called acetyl choline alights on special receptors on the surface of the muscle. The muscle then contracts and remains in this state as long as the acetyl choline is in touch with the receptor. For the muscle to relax enzyme which neutralizes acetyl choline comes into action.

All conscious movements occur when signals from the brain are sent down the spinal cord to stimulate particular motor nerves. The process of contraction is started by the flow of calcium into the muscle cells through micro tubules between the myofibrils. Individual muscles can only contract; they cannot push. For movement in the other direction, another muscle is activated.

Note that, the contraction of the cardiac muscles is not brought about by motor nerves but by the SA node.

Related: Biomedical Signals Acquisition Instruments

A tendon is like an inelastic cord which binds the muscle to a bone. This, movements of bones are brought about by the contraction of a muscle. Reflex movements takes place when motor nerves are activated by signals from the spinal cord.

If you place a hand on a hot object, the sensors will sense the temperature and a signal will go to the brain from the sensory nerves. The brain will send a signal via motor nerves to the muscles of the hand and then the hand will be lifted from the hot object.

The motor nerve can exist only in the polarized or depolarized state. Action potential lasts only for a few milliseconds and can be observed by placing surface electrodes on the skin. For better accuracy, we can insert needle electrodes into the muscle. The set-up and waveform is shown in the figure below:

To study the functioning of a nerve, the person is asked to contract the muscle voluntarily and the muscle potentials are recorded. The potentials range from 50 μV to 2 mV and lasts for 2-10 milliseconds. The values depend upon the location of the electrodes:

You can also read more about: Electrocardiogram

Paralysis

Muscular paralysis can occur due to:

• Lesions in the parts of the nervous system.
• The motor centres in the brain maybe damaged due to haemorrhage.
• The conducting pathways in the spinal cord maybe damaged. In this case, the muscle cannot contract voluntarily, but activity can be provoked by giving an external electrical stimulus.