Structure and Function in Neurobiology: A Conceptual Framework and the Localization of Functions

Abstract

A framework of concepts on structure and function of the nervous system is presented, in which we attempt to formulate the intended meanings of some ambiguous statements on structure and function in neurobiology in a set of strictly defined concepts.

Function. The word “function” as generally used has different meanings. Two meanings of the question “What is the function of B?” are particularly relevant for this subject: (1) “What does B do?” and (2) “Why did B evolve?” It is shown that question 1 must be answered before question 2.

Function of brain region S. The question “What does brain region S do?” has a meaning identical to the question “What is the relation between inputs and outputs of S, at specified conditions of S?” The answer to this question is the I/O-function. The I/O-function can be described at different levels, for instance at the molecular or cellular level. When neuroscientists ask “What is the function of brain region S?”, the intended meaning appears to be either “What is the extra-CNS I/O-function of S?,” or “Why did S evolve?” The “extra-CNS I/O-function of CNS subsystem S” is “the I/O-function of S in which input and output elements are outside the CNS.” In many cases, neuroscientists want to know the “behavioral I/O-function of CNS subsystem S,” i.e., the “I/O-function of S in which input and output elements are outside the organism”; they want to know “what is represented by the inputs of S outside the organism, and what are the effects of the outputs of S outside the organism.” Loosely speaking, when a neuroscientist wants to know “the function of brain region S,” he wants to know “the meaning of its neural messages, and their behavioral effects.” Some examples of generally accepted behavioral I/O-functions of clearly sensory and motor parts of the CNS are presented.

Localization of function. The following conclusions are drawn for “localization of function F.” (1) “Function F” must be specified as “I/O-function F'” referring to input and output elements outside the CNS. (2) “Localization of I/O-function F'” is a kind of shorthand for “identification of the neural activity representing the states of these input and output elements.” (3) If neurons exist whose activity only (or mainly) represents this input (“pontifical cells”), and/or only (or mainly) generates this output (“command neurons”) (or, in other words, whose behavioral I/O-function is F'), it might be said metaphorically that “function F is localized in these neurons.” In most cases, the existence of such neurons is still an open question.