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

The Animal Self: Neurobehavioral Correlates 133

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

reflex arc. Through special receptors in the periphery, a sensory neuron
transduces a stimulus (a change in environment) into a nerve impulse
and transmits it from the nerve ending toward the soma and then to
the central nervous system. Some receptors are sensitive to mechanical
forces such as touch, pressure, stretching, and gravity (position sense);
others are sensitive to thermal vibration, sound waves, and electromag-
netic waves (light); yet others to molecular conformation as in taste and
odor. A motor neuron, on the other hand, transmits impulses from the
central nervous system to muscles for movement and locomotion, or to
glands for hormonal secretion or waste product excretion (such as sweat).
A sensory neuron connected directly to a motor neuron forms a monos-
ynaptic arc, as the one involved in the knee jerk reflex. Polysynaptic arcs
are formed when one or more neurons (the inter-neurons) interpose
between the sensory and motor components. These associative neurons
increase in number as the nervous system grows in complexity, so much
so that in the human brain most neurons play associative roles. The out-
come is an enormously complex communication network.
It should be noted that neurons in the central nervous system do
not just serve as a passive conduit for nerve impulses. Many engage in
spontaneous background activity in the absence of a stimulus, a phe-
nomenon believed to be related to the state of consciousness, as well as
the circadian rhythms of the body.
The message traversing across a synapse is mediated by molecules
called neurotransmitters. This chemical relay is much slower compared
to the propagation of an action potential. Furthermore, synaptic trans-
mission is not stereotyped, being subject to multiple factors affecting
speed and efficiency. The resulting delay across synapses provides end-
less opportunities for the fine-tuning of inter-neuronal messages.
In a nutshell, synaptic transmission works as follows. When a nerve
impulse arrives at the pre-synaptic ending, it triggers an influx of calcium
that releases the neurotransmitter from the synaptic vesicle into the syn-
aptic cleft, where the transmitter can act on the post-synaptic receptors
(Fig. 7.4). Whether neurotransmission elicits an action potential in the