Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Neurobehavioral Correlates 135

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

For each transmitter there are several types of receptors with sub-
tle differences in response. Although each neuron produces and releases
only one type of transmitter, it can receive multiple incoming transmitters,
some with opposing effects. This convergence of influences makes the
outcome difficult to predict. Some transmitter receptors are ion channels,
allowing the influx of ions such as sodium, calcium, or chloride (ionotropic
receptors). Others activate intracellular second messengers (mainly cyclic
AMP) through G-protein-coupled receptors (metabotropic receptors).
Neuromodulators are small peptides that do not directly alter
post-synaptic electrical potential but indirectly influence the action of
transmitters through intracellular second messengers.^4 Many neuro-
modulators coexist with the major transmitters in the same neuron, and
the distinction between the two is not always clear-cut. All in all, there
are about thirty neurotransmitters and neuromodulators combined.
Other than neurons, glial cells, which do not conduct action poten-
tials, are present in the nervous system; they comprise 90% of cells in
the brain. These are: oligodendrocytes (known as Schwann cells in the
peripheral nerves), which, by forming myelin around axons, serve as
insulation to speed up nerve conduction; astrocytes, which aid in the
metabolism of neurons, in the processing of neurotransmitters, and in
the maintenance of the internal environment of the brain by controlling
the blood-brain barrier; and microglia, which possess immune and scav-
enger functions.

7.2 From Nerve Net to Brain: Evolution
of the Nervous System

The nervous system increased in complexity as evolution proceeded,
from simple nerve nets to ganglia and finally to the brains of higher ani-
mals. The following provides a glimpse of this biological procession over
hundreds of millions of years.
The simplest form of nervous system appeared in the small fresh
water animal called hydra (Fig. 7.5A). Hydra has a tubular body a few