Teaching About Disinhibition. Roger Tannerthies. Jefferson City, Missouri.
APStracts 6:040S, 1999.
Disinhibition is a concept that is useful to explain many functions of the
nervous system and also other systems of the human body. In psychology it
describes bizarre behaviors of patients who are unable to inhibit their
impulses. Students may hear a definition of disinhibition, but do not realize
what a broad concept it is. Disinhibition is the release of inhibition, a
double negative, which results in increased activity. A good example of
disinhibition happens in exercise, when normal vagal tone that slows heart rate
is withdrawn, thereby increasing heart rate. Sympathetic activity to the heart
also increases with exercise, which raises heart rate directly. A good analogy
to explain disinhibition to students is to consider an automobile at the top of
a hill. Releasing the brake and allowing the car roll down the hill is
disinhibition. Disinhibition explains many phenomena in the nervous system. For
example, the only output from the cerebellar cortex onto subcortical nuclei is
from Purkinje cells that are inhibitory. When Purkinje cells are inhibited by
Basket cells, their inhibitory effect on the subcortical neurons is reduced
(disinhibition), so the subcortical neurons fire faster. This contributes to
sequences of inhibition and disinhibition that the cerebellum uses to coordinate
body movements. Injuries to motor control systems in humans are often
associated with excessive activity rather than to lack of activity, because many
pathological conditions damage neurons involved in normal inhibition. Patients
with Parkinson's disease, Huntington's disease and cerebral palsy have
particular exaggerated movements that are due to disinhibition. The spastic
paralysis (too much activity of antagonistic muscles at rest) of patients with
strokes or spinal injuries is due to the loss of the tonic inhibition from the
brain to the spinal cord, causing hypertonia and hyperreflexia. The role of
disinhibition in spasticity is well illustrated by an example from insect
physiology. When praying mantises mate, the smaller male mounts the back of the
female. During copulatory thrusting the front of the male may slip and have his
head eaten off by the female. If this happens, the copulatory movements become
more vigorous, because tonic inhibitory activity from the head ganglion no
longer suppresses neurons in the thoracic ganglion. Students always remember
this example. Disinhibition is also useful to describe the operation of negative
feedback control systems, which maintain the levels of ions, nutrients and
hormones in the body relatively constant. For example, secretion of trophic
hormones from the anterior pituitary stimulates production of hormones from
endocrine glands elsewhere in the body. When the peripheral endocrine gland
produces too little hormone, then there is less negative feedback to the
anterior pituitary (and hypothalamus). So, more trophic hormone is produced to
try to stimulate the original target endocrine gland. The increased blood level
of trophic hormone is the result of disinhibition. Homeostatic control with
opposing forces to increase or decrease blood levels of various chemicals can be
likened to a damped oscillation. In our bodies plenty of excitation is opposed
by some inhibition, and then this inhibition is partly opposed by disinhibition.
The balance of opposing forces maintains a controlled midlevel range of normal
operation. Excess activity often results from too much disinhibition rather
than from too much facilitation. The final outcome may appear similar, but
understanding the physiology provides more logical explanations of how the
result is achieved.
Received 8 September 1999; accepted in final form 21 September 1999.
APS Manuscript Number S047-9.
Article publication pending Advances in Physiology Education.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 1 November 1999