The Development and Structure of Nervous Systems

In previous chapters, we have discussed the open-ended environmental variability that organisms encounter and detect with sight, taste, smell, feel, and so on. In Chapters 15 and 16, we will describe in basic terms what is known about the central problem of how organisms perceive this infinite and constantly changing variety of sensory signals.

As humans, we tend to relate these issues to our own personal experience, leading us to equate perception with consciousness. But the two are highly or perhaps even entirely different. In fact, we only have indirect indicators—at best—of how different the mental experience of other organisms may be from those that we understand. The nature and even the definition of consciousness are still debated and unclear, as is whether nonhuman organisms experience it at all. Many organisms to whom we would not attribute consciousness clearly do have integrated, organized responses to complex environmental input. Is that so different from what humans experience? How would we know one way or the other?

An organism does not have to be complex and multicellular to respond to light, vibration, temperature change, and other environmental signals. Some do so locally, with no centralized processing, and we noted that immune response was like this and that in plants each part can in many ways respond independently of the rest of the plant. Other species, however, have a hierarchical and highly specialized central nervous system (CNS) that serves to create an internal representation of environmental cues and to organize responses. Even plants and single-celled organisms integrate and respond to multiple environmental cues; therefore, a neural mechanism for receiving and responding to unpredictable environmental signals is clearly not required for survival, or even for adapting to changing external conditions. Only a small subset of living organisms does so through a CNS.

Charles Darwin himself, in his 1872 Descent of Man, goes to great lengths to connect human behavioral components with those of other species, to show that in fact we share common origins with other animals. He was appropriately

Genetics and the Logic of Evolution, by Kenneth M. Weiss and Anne V. Buchanan. ISBN 0-471-23805-8 Copyright © 2004 John Wiley & Sons, Inc.

impressed even by ants: "the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin's head. Under this view, the brain of an ant is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of a man." Do we have a rational basis for asserting that simple organisms do not have a form of "awareness" or organismal level percept and that their reactions to inputs are as unknowing as, for example, that of a motion detector that turns on lights in a house?

In organisms with a CNS, sensory input may undergo some processing locally at the site where it is received, but ultimately raw light, sound, taste, touch, and other signals all proceed to the nervous system in essentially the same way, via action potentials, and the signals are decoded by modality in the brain. The pathway the signal travels in the brain determines how it is ultimately perceived, and the pathway is determined by the destination of the particular neuron through which the signal has traveled to reach the brain. Light signals are sent from the eye to the visual centers in the brain, where they are translated into perception of color, shape, motion, and the like. Sound waves are collected in the ear and shunted along to the auditory centers in the brain to be translated into perceived danger signals, music, communication, and so forth. However, percept—whatever it actually turns out to be—is not entirely location dependent because, as we will see, the same kinds of input (e.g., sound, light) can occur in different parts of the brains in different people or even in the same person at different times in his/her life.

In most sensory systems, neurons project their axons topographically, that is, in an orderly fashion that provides a precise spatial "map" or representation of the location of a particular class of receptors on the surface of the body, whether the retina, the olfactory epithelium, the cochlea, or the skin. The axons terminate in distinct sensory centers of the brain, their parametric representation of the physical world still conserved. That is, neurons that are adjacent in the receptor surface terminate adjacent to each other in the sensory area of the brain to which they project. The central processing system detects in detail where the signal came from, and this may represent the external world in a literal sense, as in vision and touch, but need not as in olfaction or hearing. These ensembles are called neural maps.

Signals are processed through multiple levels in the CNS before final perception. In most sensory systems, both parallel and hierarchical (serial, with one step preceding the next in order) processing are involved in the anatomic connections that send signals to the appropriate centers of the brain for interpretation. Auditory systems, for example, can simultaneously process many sounds as well as many aspects of each sound. Light is processed into increasing complexity by higher cortical areas, and different qualitative aspects of light are processed simultaneously. The various sensory areas in the brain have many functional and structural commonalities, and, despite segregation of the sensory areas of the CNS, in the end the brain integrates the different signals to create what is experienced as a unitary but multidimensional representation of the external world.

It might be rather subjective to discuss which organ system is the most complex in its structure or function, but the nervous system would certainly be a candidate, especially in vertebrates where the developmental dynamics are far from a simple hierarchy and the connection between structure and function not always clear. In Chapter 15, we will describe in broad terms the various types of nervous systems known, their architecture, how they work, and some of what is known about the genetics of their development. In Chapter 16, we will discuss different sensory modalities and how they are decoded and perceived in the brain.

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