PHYSICS

16.7. OTHER BIOELECTRIC POTENTIALS
The recording of bioelectric potentials associated with muscle
activity is called electromyography. These potentials may be
measured at the surface of the body near a muscle or directly
from the muscle by penetrating the skin with needle electrodes.
Electroretinography is a recording of the complex pattern of
bioelectric potentials obtained from the retina of the eye, usually
in response to a visual stimulus.
Electrooculography is a measure of variation in the cornealretinal
potential as affected by the position and movement of the
eye.
Electrogastrography is the analysis of muscle activity associated
with the peristaltic movements of the gastrointestinal tract.
Chapter 17. ELECTROBIOLOGY
17.1. ELECTRIC FIELDS AND ANIMALS
a
Fig. 17.1. Certain types of weakly
electric fishes, of which Gymnarchus is
a striking example, are able to locate
objects by sensing changes in the
electric fields: the lines of electric field
diverge from a poor conductor (a) and
converge towards a good conductor (b)
b
along their bodies and
through these maintain a
considerable electrical potential
difference between
the head and tail (fig. 17.2).
The field is pulsed by the
fish, typically in a 25 ms
pulse duration. The skin of
the fish probably acts as
an insulator except in certain
regions near the head
where jelly-filled pits provide
conducting paths for
the electric field. These pits
have some type of sense
organs at their base and
A variety of aquatic organisms can produce and detect electrical
fields. The ability to sense electrical fields has been found in
a number of inveretebrates and aquatic vertebrates, including
several species of fish, sharks and rays. The platypus (Ornithorhynchus
anatinus) is the only mammal that has a sense of electroreception.
Its prey is located in part by detecting their body electricity.
All ocean animals are surrounded by very low-frequency
electric fields. Seven families of fish can deliver appreciable voltage
outside of their bodies. Two species, the electric eel from the
Amazon Basin and the electric catfish of Africa, are strongly
electric and are capable of developing sufficient electric energy
(10 3 W at 10 2 volts) to kill prey. Weakly electric fish (i.e., can
develop ~ 1 or 2 volts) use a specialized sensory guidance system
for environmental navigation via echolocation. They also use
electric echolocation for communication and for species and sex
recognition in their low visibility environments. Certain types of
weakly electric fish, of which Gymnarchus is a striking example,
are able to locate objects by sensing changes in the electric fields
set up by the fish themselves (fig.· 17.1). The fish has stacks of
modified tissue called electric organs located in a regular array
126
Jelly
Sensory~
cell
Ampulla
of Lorenzini
a
Ampullary
organ
b
Tuberous
organ
Fig. 17.2. General structures of electroreceptive organs of fish: a - the
ampullae of Lorenzini; b ­ the ampullary organ; c ­ the tuberous organ
it is these receptors that can detect very small changes in the field
distribution at the head. With its receptors, a fish can detect the
presence of different objects in muddy streams where the use of
their eyes for catching prey or avoiding predators is impossible.
Strongly electric fish (certain freshwater eels and catfish) have
enormous electrical potential, killing prey and warding off
predators by delivering electric shocks of several hundreds volts.
Eels have the unique ability to discharge both weak and strong
electric current. The weak current is used primarily to locate and
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c