Muscular Biopolymers - Engineered Biomimicry - page 141

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 trans-
mitted down the motor neuron arrives at the
motor end plate and triggers a sequence of elec-
trochemical events. A quantum of acetylcholine
(ACh) is released. It then crosses the synaptic gap
(200-500 Å wide) and causes a depolarization of
the postsynaptic membrane. Such a depolariza-
tion can be recorded by a suitable microelectrode
and is called an end plate potential (EPP). In nor-
mal 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 -disc of the contractile element by means of an
inward spread of the stimulus along the trans-
verse tubular system. This results in a release of
Ca 2 + in the SR. Ca 2 + rapidly diffuses to the con-
tractile filaments of actin and myosin where
ATP is hydrolyzed to produce ADP plus heat
plus mechanical energy (tension). The mechani-
cal energy manifests itself as an impulsive force
at the cross-bridges of the contractile element.
The depolarization of the transverse tubular
system and the SR results in a depolarization
wave along the direction of the muscle fibers. It
is this depolarization wave front and the subse-
quent 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 in-
dwelling types. In-dwelling 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 inside and
bared to the muscle tissue at the open end of

the needle. The needle itself forms the other
conductor.
In-dwelling electrodes are influenced by both
waves that actually pass by their conducting
surface and by waves that pass within a few
millimeters of the bare conductor. The same is
true for surface electrodes.
ATP is an important molecule for the life of
living cells. It provides energy for various cellular
activities such as muscular contraction, movement
of chromosomes during cell division, movement
of cytoplasm within cells, transporting substances
across cell membranes, and putting together
larger molecules from smaller ones during syn-
thetic reactions. Structurally, ATP consist of three
phosphate groups attached to an adenosine unit
composed of adenine and five-carbon sugar ribose.
ATP is the energy reserve of living systems.
When a reaction requires energy, ATP can trans-
fer just the right amount, because it contains two
high-energy phosphate bonds. When the termi-
nal phosphate group P is hydrolyzed by addition
of a water molecule, the reaction releases energy.
This energy is used by the cell to power its activi-
ties. The resulting molecule, after removal of the
terminal phosphate groups, is ADP. This reaction
may be represented as follows:

ATP →ADP + P + ENERGY.

(6.1)

The energy supplied by the catabolism of
ATP into ADP is constantly being used by the
cell. Since the supply of ATP at any given time
is limited, a mechanism exists to replenish it.
A phosphate group is added to ADP to manu-
facture more ATP. The reaction may be repre-
sented as follows:

ADP + P + ENERGY → ATP.

(6.2)

The energy required to attach phosphate
groups to ADP to make ATP is provided by
breakdown of glucose in the cellular respiration
process, which has two phases:

1. Anaerobic . In the absence of oxygen, glucose
is partially broken down by the glycolysis