parts of the central nervous system are organized in successive platelike stages.
A cell in one stage receives many excitatory and inhibitory inputs from the previous stage and sends outputs to many cells at the next stage. The primary input to the nervous system is from receptors in the eyes, ears, skin, and so on, which translate outside information such as light, heat, or sound into electrical nerve signals. The output is contraction of muscles or secretions from gland cells.
TYPICAL NEURAL PATHWAY
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
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.
Although the wiring diagrams for the many subdivisions of the central
nervous system vary greatly in detail, most tend to be based on the relatively simple general plan schematized in the diagram on this page. The diagram
is a caricature, not to be taken literally, and subject to qualifications
that I will soon discuss. On the left of the figure I show the receptors, an array of
informationtransducing nerves each subserving one kind of sensation such as touch,
vibration, or light. We can think of these receptors as the first stage in
some sensory pathway. Fibers from the receptors make synaptic contacts with a second
array of nerve cells, the second stage in our diagram; these in turn make
contact with a third stage, and so on. "Stage" is not a technical or widely
applied neuroanatomical term, but we will find it useful.
Sometimes three or four of these stages are assembled together in a
larger unit, which I will call a structure, for want of any better or widely
accepted term. These structures are the aggregations of cells, usually plates
that I mentioned in Chapter i. When a structure is a plate, each of
the stages forming it may be a discrete layer of cells in the plate. A good example
is the retina, which has three layers of cells and, loosely speaking, three
stages. When several stages are grouped to form a larger structure, the nerve fibers
entering from the previous structure and those leaving to go to the next are
generally grouped together into bundles, called tracts.
You will notice in the diagram how common divergence and convergence are: how almost as a rule the axon from a cell in a given stage splits
on arriving at the next stage and ends on several or many cells, and conversely,
a cell at any stage except the first receives synaptic inputs from a few or many cells
in the previous stage.
We obviously need to amend and qualify this simplified diagram, but
at least we have a model to qualify. We must first recognize that at the input
end we have not just one but many sensory systems—vision, touch, taste,
smell, and hearing—and that each system has its own sets of stages in the
brain. When and where in the brain the various sets of stages are brought together,
if indeed they are brought together, is still not clear.
In tracing one system such as the visual or auditory from the receptors further into the brain, we may find that it splits into separate subdivisions.
In the case of vision, these subsystems might deal separately with eye
movements, pupillary constriction, form, movement, depth, or color. Thus
the whole system diverges into separate subpathways. Moreover, the subpaths may be many, and may differ widely in their lengths. On a gross scale,
some paths have many structures along the way and others few. At a finer
level, an axon from one stage may not go to the next stage in the series but instead
may skip that stage and even the next; it may go all the way to the motor
(You can think of the skipping of stages in neuroanatomy as analogous
to what can happen in genealogy. The present English sovereign is not related
to William the Conqueror by a unique number of generations: the number of "greats" modifying the grandfather is indeterminate because
of intermarriage between nephews and aunts and even more questionable events.)
When the path from input to output is very short, we call it a reflex.
In the visual system, the constriction of the pupil in response to light is
an example of a reflex, in which the number of synapses is probably about six. In
the most extreme case, the axon from a receptor ends directly on a motor neuron,
so that we have, from input to output, only three cells: receptor, motor
and muscle fiber, and just two synapses, in what we call a monosynaptic
reflex arc. (Perhaps the person who coined the term did not consider the nervemuscle junction a real synapse, or could not count to two.) That short
path is activated when the doctor taps your knee with a hammer and your knee jumps. John Nicholls used to tell his classes at Harvard Medical School
that there are two reasons for testing this reflex: to stall for time, and
to see if you have syphilis.
At the output end, we find not only various sets of body muscles that
we can voluntarily control, in the trunk, limbs, eyes, and tongue, but also
sets that subserve the less voluntary or involuntary housekeeping functions, such
as making our stomachs churn, our water pass or bowels move, and our sphincters (between these events) hold orifices closed.
We also need to qualify our model with respect to direction of information flow. The prevailing direction in our diagram on page 24 is obviously
from left to right, from input to output, but in almost every case in which
information is transferred from one stage to the next, reciprocal connections
feed information back from the second stage to the first. (We can sometimes
guess what such feedback might be useful for, but in almost no case do we
have incisive understanding.) Finally, even within a given stage we often
find a rich network of connections between neighboring cells of the same order.
Thus to say that a structure contains a specific number of stages is almost
always an oversimplification.
When I began working in neurology in the early 1950s, this basic plan
of the nervous system was well understood. But in those days no one had any
clear idea how to interpret this bucket-brigade-like handing on of information
from one stage to the next. Today we know far more about the ways in which
the information is transformed in some parts of the brain; in other parts
we still know almost nothing. The remaining chapters of this book are devoted
to the visual system, the one we understand best today. I will next try to
give a preview of a few of the things we know about that system.
We can now adapt our earlier diagram on
page 6 to fit the special case of the visual pathway. As shown in the illustration at the top
of the facing page, the receptors and the next two stages are contained in the retina.
The receptors are the rods and cones; the optic nerve, carrying the retina's
entire output, is a bundle of axons of the third-stage retinal cells, called
retinal ganglion cells. Between the receptors and the ganglion cells are intermediate
the most important of which are the bipolar cells. The optic nerve proceeds
to a way station deep in the brain, the lateral geniculate body. After only
one set of synapses, the lateral geniculate sends its output to the striate cortex,
which contains three or four stages.
You can think of each of the columns in the diagram above as a plate
of cells in cross section. For example, if we were to stand at the right of the
page and look to the left, we would see all the cells in a layer in face-on view.
Each of the columns of cells in the figure represents a two-dimensional array
of cells, as shown for the rods and cones in the diagram to the side.