Abstract
Initial investigations into the electrophysiology of smooth muscle were
carried out by Bozler (1946) who studied the ureter, uterus and muscle of the
intestine using external electrodes. He recorded action potentials similar
to those which had been recorded from other excitable tissues during normal
movements of the stomach and small intestine, and found that each movement was
usually accompanied by a repetitive discharge. These observations conflicted
with those of Rosenblueth and his collaborators (1936) who found smooth muscle
to be incapable of giving all-or-none responses. Bozler's observations lead
to the conclusion that 'the activity of smooth muscle is due to the discharge
of impulses which are accompanied by action potentials like those found in
striated muscle and nerve', thus bringing these different tissues into line.
He considered that 'the great variability of the movements which is characteristic
for smooth muscle can be explained by such factors as frequency and
number of impulses discharged, rate of conduction, and variations in excitability'.
Thus early in the history of study of smooth muscle, was it recognized that the
original idea of von Euler, Rosenblueth and others, that some essential
qualitative difference must exist between smooth and skeletal muscle to explain
the wide differences in their behaviour, was not supported by the experimental
evidence, and that in fact the different tissues probably shared the same basic
properties.
With the development of the ionic hypothesis of Hodgkin and Huxley (1952),
the old problem of the existence of basic similarities or differences between
smooth muscle and other excitable tissues had to be approached at the cellular
level. This essay is concerned with the study of the electrophysiological
properties of smooth muscle by investigation of the behaviour of single cells
with microelectrode techniques. It aims to show that on this level the trend
has again been towards a recognition of the fact that a merely quantitative
rather than a major qualitative difference would account for the differences
in properties of the different tissues. This change in outlook is due to the
development and improvement of apparatus and recording techniques. With the
various technical advances, better understanding of the peculiar properties of
smooth muscle has resulted, although much investigation still remains to be
carried out, and it is now realised that, if some difference in the permeability
of the cell membrane is assumed, the characteristics of smooth muscle may be
explained on the basis of the ionic theory.
