A TYPICAL NEURAL PATHWAY
This scanning electron 
microscope picture shows a 
neuromuscular junction in a 
frog. The slender nerve fiber 
curls down over two muscle 
fibers, with the synapse at the 
lower left of the picture.

    Now that we know something 
about impulses, synapses, 
excitation, and inhibition, we 
can begin to ask how nerve 
cells are assembled into larger 
structures. We can think of the 
central nervous system--the 
brain and spinal cord--as 
consisting of a box with an 
input and an output. The input 
exerts its effects on special 
nerve cells called receptors, 
cells modified to respond to 
what we can loosely term 
"outside information" rather 
than to synaptic inputs from 
other nerve cells. This 
information can take the form 
of light to our eyes; of 
mechanical deformation to our 
skin, eardrums, or 
semicircular canals; or of 
chemicals, as in our sense of 
smell or taste. In all these 
cases, the effect of the stimulus 
is to produce in the receptors 
an electrical signal and 
consequently a modification in 
the rate of neurotransmitter 
release at their axon terminals.

    (You should not be confused 
by the double meaning of 
receptor; it initially meant a 
cell specialized to react to 
sensory stimuli but was later 
applied also to protein 
molecules specialized to react 
to neurotransmitters.)

    At the other end of the 
nervous system we have the 
output: the motor neurons, 
nerves that are exceptional in 
that their axons end not on 
other nerve cells but on muscle 
cells. All the output of our 
nervous system takes the form 
of muscle contractions, with 
the minor exception of nerves 
that end on gland cells. This is 
the way, indeed the only way, 
we can exert an influence on 
our environment. Eliminate an 
animal's muscles and you cut it 
off completely from the rest of 
the world; equally, eliminate 
the input and you cut off all 
outside influences, again 
virtually converting the animal 
into a vegetable. An animal is, 
by one possible definition, an 
organism that reacts to outside 
events and that influences the 
outside world by its actions.

    The central nervous system, 
lying between input cells and 
output cells, is the machinery 
that allows us to perceive, 
react, and remember--and it 
must be responsible, in the 
end, for our consciousness, 
consciences, and souls. One of 
the main goals in neurobiology 
is to learn what takes place 
along the way--how the 
information arriving at a 
certain group of cells is 
transformed and then sent on, 
and how the transformations 
make sense in terms of the 
successful functioning of the 
animal.