Biomedical Engineering Reference
In-Depth Information
The tension in the muscles as a whole is influenced by number of fibers con-
tracting at one time, number of fibers per motor unit (motor unit size), number of
motor units recruited (multiple motor unit summation), active tension generated by
contractile elements, and passive tension generated by elastic components.
A.5
ELECTROMYOGRAPHY
The electrical signal associated with the contraction of a muscle is called an elec-
tromyogram or EMG. Electromyography, which is the study of EMG, has revealed
some basic information. Voluntary muscular activity results in an EMG that increases
in magnitude with tension. However, other variables influencing the signal at any
given time are the velocity of shortening or lengthening of the muscle, rate of tension
buildup, fatigue, and reflex activity.
Muscle tissue conducts electrical potentials somewhat similar to axons of the
nervous system. Motor unit action potential (m.u.a.p.) is an electrical signal gener-
ated in the muscle fibers because of the recruitment of a motor unit. Electrodes
placed on the surface of a muscle or inside the muscle tissue will record the algebraic
sum of every m.u.a.p. transmitted along the muscle fibers at that point in time. Those
motor units away from electrode site will result in a smaller m.u.a.p. than those of
similar size near the electrode.
For a given muscle, there can be a variable number of motor units, each controlled
by a motor neuron through special synaptic junctions called motor end plates. An
action potential transmitted down the motor neuron arrives at the motor end plate
and triggers a sequence of electrochemical events. A quantum of ACh is released.
It then crosses the synaptic gap (200-500 Å
wide) and causes a depolarization of
the postsynaptic membrane. Such a depolarization can be recorded by a suitable
microelectrode and is called an end plate potential (EPP). In normal circumstances,
the EPP is large enough to reach a threshold level and an action potential is initiated
in the adjacent muscle fiber membrane.
The beginning of the m.u.a.p. starts at the Z-line of the contractile element by
means of an inward spread of the stimulus along the transverse tubular system. This
results in a release of Ca
rapidly diffuses to
the contractile filaments of actin and myosin where ATP is hydrolyzed to produce
ADP plus heat plus mechanical energy (tension). The mechanical energy manifests
itself as an impulsive force at the cross-bridges of the contractile element.
The depolarization of the transverse tubular system and the sarcoplasmic retic-
ulum results in a depolarization wave along the direction of the muscle fibers. It is
this depolarization wave front and the subsequent repolarization wave that are seen
by the recording electrodes.
Two general types of EMG electrodes have been developed. Surface electrodes
consist of disks of metal—usually silver/silver chloride of about 1 cm in diameter.
These electrodes detect the average activity of superficial muscles and give more
reproducible results than do indwelling types. Indwelling electrodes are required,
however, for the assessment of fine movements or to record from deep muscles. A
needle electrode is a fine hypodermic needle with an insulated conductor located
2+
in the sarcoplasmic reticulum. Ca
2+
