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

The Animal Self: Neurobehavioral Correlates 129

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

Fig. 7.1. A typical neuron. The top part depicts the cell body with multiple dendrites;
the middle part, the axon with myelin sheath; the bottom part, the nerve terminals that
synapse with other neurons. Small protrusions from the dendrites are dendritic spines,
the sites of contact with incoming axons. The information flow is from the dendrites to
the cell body and then to the axon.

potential of –70 millivolts to –50 millivolts, measured from the inside), it
automatically triggers a transient, sudden influx of sodium ions, raising
the potential to about +30 millivolt (called the action potential) before
returning to the resting level (Fig. 7.3). The propagation of the action
potential (nerve impulse), once started, is an all-or-none phenomenon,
its amplitude and velocity being fixed for a given axon. Its frequency, on
the other hand, increases with the strength of stimulation. The speed