128 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”7.1 Neurons: Building Blocks of the Nervous System
A single-celled organism such as an amoeba moves around in search of
food. Such simple locomotion needs internal coordination of the con-
tractile elements, so that when one end extends forward the opposite
end retracts. Now, imagine a hypothetical two-cell organism. If the two
cells pull in opposite directions, locomotion would be impossible and
the animal would starve. The two cells need to communicate and coor-
dinate, probably by ion fluxes across plasma membranes. But visualize
a multicellular animal with hundreds and thousands of cells. Clearly,
communication by contact one cell at a time would be extremely slow
and cumbersome. In this situation evolution came up with specialized,
dedicated cells for rapid coordination over some distance. These are the
neurons, the building blocks of the nervous system. Although through-
out evolution the nervous system varies in complexity, from the simple
nerve net of a coelenterate to the billions of specifically interconnected
neurons of the human brain, individual nerve cells share many common
features and their functions are basically the same.
Figures. 7.1 and 7.2 show a typical neuron and its connections.
A neuron consists of three major parts: (1) a cell body or soma that con-
tains the nucleus and other organelles; (2) numerous short, branching
processes called dendrites that receive messages from other neurons;
(3) a single long axon that conducts messages away from the soma. An
axon sends out branches at the end of its course that terminate as nerve
endings, forming connections (synapses) with the dendrites and cell
bodies of other neurons. Neuronal messages traverse down the axon in
the form of action potentials, which are waves of electrical perturba-
tions caused by ionic fluxes across the plasma membrane (inward flow of
sodium followed by outward flow of potassium). This is made possible by
a unique property of the axonal plasma membrane called “excitability”
(also present in muscle cells). Like other plasma membranes, the axonal
membrane is a capacitor that separates charges across its two surfaces,
forming an electrical potential. But, unlike other membranes, once the
axonal membrane potential reaches a certain threshold (from a resting