Eye, Brain, and Vision
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Many 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.

                       A 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 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.
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 or globs, 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 neuron.
(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 neuron, 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.


                               THE VISUAL PATHWAY
     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 cells, 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.

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

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