Introduction

A living organism consists of a very large number of interdependent fluid compartments. These compartments are bounded by lipid barriers (membranes) and they contain mainly water and exist in an environment that is mostly water. Communication and exchanges between these compartments are carried out by molecules or ions that are able to move through the water and the lipid phases. In order to study these compartmental exchanges it is thus necessary to examine the way in which molecules and ions move through homogeneous liquid phases. We shall consider homogeneous liquid phases because the membrane lipids are in a liquid state and can be treated in the same way as water – that is, as a homogeneous liquid phase (see Chapter 5). Let us first start by an examination of the way in which ions are formed and then go on to analyse the way in which they move through aqueous solutions under the influence of an electric field.

Formation of ions

Ions are formed when neutral molecules are dissociated or when salts are dissolved. To dissolve a salt, or dissociate a molecule, requires a medium with a high dielectric constant (see Appendix 24 where dielectric constants are discussed) which is then able to weaken ionic bonds. An example of an ionic bond is the bond that holds Na+ and Cl ions together in a NaCl crystal. Methane, for example (CH4), is held together by covalent bonds. Ionic and covalent bonds are extreme cases; most chemical bonds are a mixture of the two types.

Introduction

The activity of different cells, tissues and organs in an animal is coordinated by means of specialized communication systems and these systems may be either chemical or electrical. Hormones and neurotransmitters are chemical communicators, that is, they act at a point different from that at which they were produced and released. Electrical communication is by means of excitable cells and the message is carried by action potentials. Because each excitable cell usually produces action potentials of fixed waveforms, information is transmitted by the number of action potentials per unit time (frequency), rather than by the shape of the waveforms. In this chapter we will discuss how these electrical signals are transmitted from one excitable cell to another (Synaptic Transmission) and this is often by a chemical communicator.

The neuromuscular junction

The experimental model used probably most frequently to study synaptic transmission is the nerve-muscle preparation. Electrical stimulation of a nerve results in action potentials being recorded by microelectrodes that are inserted in the muscle fibre. The nerve fibre when it reaches the surface of the muscle fibre splits into a number of different branches, and a recording microelectrode in the muscle fibre in the vicinity of one of these branches shows a slightly unusual action potential in that the action potential has a small hump on its rising edge (see Figure 9.1).

Regulation of composition and volume of biological compartments

Living organisms consist of a large number of interdependent fluid compartments (see Chapter 2) and these compartments are bounded by membranes which perform at least two major functions. The first of these is to act as a physical barrier so as to impede the free movement of particles between adjacent compartments. These particles consist of intracellular organelles (for example, mitochondria), macromolecules that exist both intra- and extracellularly (for example, proteins) and small polar molecules (such as water) and ions. The second is that membranes, since they are not passive and impermeable, are able to regulate the volume and composition of the intracellular environment. This regulation means that the composition and volume of the intracellular and extracellular compartments are maintained at constant values despite fluctuations in the external environment.

Membranes regulate the composition of compartments by being selectively permeable. They are also able to utilize free energy (that is stored either in ATP high-energy bonds or in concentration gradients) to transport ions and molecules against electrochemical or chemical potential gradients (see Chapter 3 for discussion of these gradients. For a discussion of free energy see Appendix 25).

Coupling mechanisms in membrane transport

This so-called ‘uphill’ transport against gradients is possible because membranes contain specialized molecular complexes which are able to couple chemical reactions (for example, ATP ⇋ ADP + Pi + free energy) to ion fluxes.

Non-uniform distribution of currents and voltages along the axon

In the last chapter we assumed that the membrane potential of excitable cells was at all times uniform. In nature this is generally not the case and, in an axon for example, the potential (and the membrane current) are different for each instant in time, along its length. If measurements are made with microelectrodes positioned at equal intervals along an axon (Figure 8.1a), and if a constant current is injected into the middle of an axon (at x = 0), we find that after a steady-state has been obtained the membrane potential decays with distance (Figure 8.1b). The voltage drop between any two consecutive electrodes is also seen to decay (Figure 8.1c). Since the resistance (per unit length) of axoplasm is constant, this means that the current that flows through any given cross-section of the axoplasm must also decay with distance. If the membrane properties are uniform (with distance) then we can conclude that the current that flows across the membrane must also fall with distance. This is pictorially represented in Figure 8.2 where the spaces between the current lines become wider at greater distances from the current source.

Three-dimensional analysis of current distribution along the axon

A better representation of the current distribution along an axon is given in Figure 8.3, which shows the current pattern described in Figure 8.2 that corresponds to only a small wedge-shaped segment of the axon.

Introduction

In Chapter 6 we described how electrical potentials may be recorded across membranes. In the absence of any externally applied currents, or spontaneous activity, these potentials take the form of steady-state membrane potentials. In excitable cells, and as a result of transient changes in the membrane properties, we can also record transient changes of the membrane potential. Some of those changes result in reversal of the polarity of the membrane potential. In this chapter we will examine the basis of this electrical activity which is the action potential. The action potential is a transient change in transmembrane electrical potential.

Although action potentials can be generated in different ways (and they may have a variety of different waveforms) the underlying electrical event is always a change in membrane conductances. The purpose of this chapter is to analyse this conductance change, so that we are able to understand action potential generation in any excitable cell.

Propagated and local action potentials

Action potentials can be recorded either as propagating electrical waves, or as electrical events that take place at a specific point in a cell membrane (local action potential). In the inset of Figure 7.1 an action potential propagates from left to right after electrical stimulation. Records obtained from the shaded area of the inset are displayed in the rest of the figure.

In this chapter I will focus primarily on the social group as a context for the individual and stress that this context both influences and is influenced by the individual. My goal is to suggest that this bidirectional relation between the individual and the social context within which the individual is embedded may be a key instance of the plasticity of human processes.

Of course, it is possible to focus initially on the group, and not first on the individual; within this more macroscopic emphasis, one may discuss the group as a system of interacting individuals all of whom possess plastic processes and flexibility in influencing and being influenced by one another. In fact, I have discussed elsewhere the nature of, and relation between, these individual and group foci (R. Lerner, 1979, in press) and it is of use to present here some of the key features of these earlier discussions.

A model of individual–context relations

To summarize these other presentations, it is useful to consider the diagram presented in Figure 2. Employing T. C. Schneirla's (1957) terms, this figure illustrates what I described in Chapter 1 as a probabilistic–epigenetic conception of development; in the figure I use the term maturation (“Mat.”) to represent endogenous organism changes and the term experience (“Exp.”) to denote all stimulative influences acting on the organism over the course of its life span.

Plasticity is a concept whose significance in developmental scholarship waxes and wanes, as the past decades of this century bear witness. For example, the term was a major entry in the 1902 edition of J. M. Baldwin and E. P. Poulton's Dictionary of Philosophy and Psychology. It was defined as “that property of living substance or of an organism whereby it alters its form under changed conditions of life” (Vol. 2, p. 302). At that time and under the recent influence of Darwinian thought, the discussion of plasticity focused on its origins and role in evolution. In the thirties and forties, plasticity (or modifiability) was again a center of attention. There was Lashley's conception of brain plasticity as well as the early-childhood studies of the detrimental impact of environmental deprivation and, correspondingly, the possible benefits of compensatory programs.

The 1970s and 1980s have seen renewed interest in the notion of plasticity. Research, rather than being restricted to childhood, is now being done, for example, in psychological gerontology, where it has been found that aging is not a fixed general process of decline, but rather that the older organism retains considerable potential for variability and plasticity. Similarly, in continuation of Lashley's earlier ideas, groundbreaking work is now being done in developmental psychobiology (e.g., Gollin, 1981) that underscores the need to study and articulate models that explicitly focus on variations in development.

The preceding chapters have, in my view, indicated that there is plasticity at each of the levels of analysis I have considered. Often, this plasticity derives from processes at one level of analysis influencing and being influenced by other levels' processes. Thus, key processes at any level may be appropriate targets of intervention as a consequence of this plasticity. And because of embeddedness of one level in another, one can modify processes at one level (e.g., cognitive–behavioral ones) to reach a target at another (e.g., those pertinent to neurotransmitters). Such reciprocity provides a compelling rationale of interdisciplinary research and intervention.

Accordingly, there is reason to be optimistic that in the future lie further paths to understanding how to better actualize humans' potential plasticity. But this optimism must be tempered by the recognition that along these paths lie two problematic sets of issues: First, throughout this book I have reviewed evidence that the organism's “reservoir” of plasticity decreases across life; that is, while plasticity is ubiquitous across life, the domains or band within which plasticity may be evidenced or actualized decrease. Second, there are several theoretical, substantive, and technological problems that must be resolved in order to further the ability of science to enhance human plasticity. These two sets of issues constitute significant limitations on both plasticity per se and knowledge about it. Let us treat each issue separately.

Plasticity as an ever-present but declining phenomenon

An organism's plasticity does not remain at a constant level across its life span.

Human development, or more accurately child development, was often studied in the first several decades of this century within university institutes – for example, at Iowa, Minnesota, and Berkeley. Although these centers originally took a multidisciplinary approach, the pluralistic perspective began to erode by the 1950s and was eventually replaced by a single view of development, the psychological (L. P. Lipsitt, pers. comm., December 1979; D. S. Palermo, pers. comm., August 1980; see also Hartup, 1978). The 1970s were marked by calls for a return to interdisciplinary integration (e.g., Brim & Kagan, 1980; Bronfenbrenner, 1977; Burgess & Huston, 1979; Hill & Mattessich, 1979; Lerner & Spanier, 1978; Petrinovich, 1979; Riley, 1979; Sarbin, 1977), calls motivated primarily by instances of failure to confirm some key hypotheses of developmental psychological analysis.

A case in point involves attempts to use a biological model of growth, one based on an idealized maturational (organismic) conception of development, to account for data from the adult and aged years (Baltes, et al., 1980; Baltes & Schaie, 1973). According to this type of organismic conception, the adult and aged years are periods of decline. However, much of the data regarding age changes (e.g., in intellectual performance) during these life periods turned out to be inconsistent with this paradigm. For example, pertinent data sets revealed increasingly greater differences in individual change: As people grow older, the differences between them apparently increase (Baltes, 1979a; Baltes & Schaie, 1974, 1976; Schaie, Labouvie, & Buech, 1973).

The life-span view of human development emphasizes the potential for change across life. As such, plasticity is a key idea in this perspective. Plasticity is seen to arise because the levels of analysis involved in human life are not only themselves comprised of plastic processes but also because each level is reciprocally interactive with (embedded in) all others. Change at one level promotes changes at the others.

In this book I have reviewed theory and data from levels of analysis ranging from the genetic to the intergenerational, social network, and cultural. Although this review is not exhaustive, I believe that the evidence I discussed allows several conclusions to be made. First, at all levels of analysis, the variables considered are part of other processes, and means to manipulate the nature of these variables, or at least change their impact or outcome, are often available. Physical therapy with cerebral palsy children may not result in motor improvement above a “just noticeable difference.” However, the parents of children receiving such therapy may be made more optimistic, may feel less guilty or anxious, and may therefore enter into more positive interactions with their child; the child's feelings about him- or herself may in turn be positively altered, and this change will encourage the continuation of the parents' positive interactions with the child. Although the physical features of the child's functioning may still remain largely unaltered, because the child is embedded in a positive reciprocal relation with his or her context, the intervention may have a quite useful, albeit indirect (and perhaps unintended) effect.