• COGNITIVE INFLUENCES ON MOTIVE AROUSAL

Several theorists, notably Weiner (1980a) and Heckhausen (1980), have stressed the great importance of cognitive factors in the motivation-action sequence. As Weiner (1979) puts it, “Comprehension Stands with hedonism as among the primary sources of motivation.” He feels that in motivation theory too much emphasis has been placed on affective arousal and not enough on the understanding the person has of what is happening during a motivation-action sequence, which determines whether affective arousal occurs or not.

A great many empirical studies have been carried out to clarify the relationship between cognition and motivation, but before reviewing them it is worth reexamining Figure 6.1, which identifies the key factors in a motivation-action sequence. Arousal demands (cues) typically contact an incentive, which, if it relates to an existing motive disposition, leads to an aroused motive or motivation to act. When, how, and whether this motivation gets converted into action is influenced by skills, cognitions (values), and opportunities, which determine whether a particular kind of behavior occurs or not.

As noted previously, Weiner, Atkinson, and others use the term motivation to describe the final excitatory potential for an act (the impulse to a given act) after it has been influenced by expectations and values, whereas we use the term motivation in the more restricted sense to refer to an aroused motive before it is influenced by expectations and values that shape preferences for particular acts.

Motivation has always fascinated people and will continue to fascinate them so long as there are people around to wonder why human beings and animals behave as they do. Nearly everyone develops an explicit or implicit theory of motivation. We think we know why our parents are sometimes disagreeable and try to continue to control what we do: they want the pleasure of continuing to dominate us. Or we think we know why our girlfriend or boyfriend has abandoned us: he or she prefers to be with someone with greater prestige or more possessions than we have. Or we think we know why we cannot seem to study very hard: we have a low need to achieve. Authors, philosophers, economists, politicians, and the people next door have all operated in terms of theories of motivation. Shakespeare vividly portrayed the lust for power in MacBeth and the longings of love in his Sonnets. Plato explored the nature of love in the Symposium. Economists think in terms of the desire to acquire possessions, and they write of the importance of the profit motive. Political observers from Machiavelli to the present have stressed the importance of the desire for power in human affairs.

• THE MEANING OF LOVE

People appear to have a basic need or desire to be with other people, just as most animals prefer to be with other members of their species. Part of the need is sexual in origin and biologically adaptive, because the two sexes must get together in order to reproduce the species. The need to affiliate with others includes sexual contacts, but it is much broader, including various types of emotional interpersonal attachments that may grow out of natural contact incentives as outlined in Chapter 4. What has always Struck observers about this need is how important it is to life and health, how pervasive it is, and how it appears in many different forms. The word love is commonly used to describe various types of affiliative ties, and everyone agrees that it is important to satisfy the love need, yet no one is quite sure, in the words of a popular song, “What is this thing called love?” Before we review modern psychology's attempt to answer this question, it will be helpful to turn first to an ancient treatment of the topic in Plato's Symposium. The Speakers at this banquet, as reported by Plato, managed to mention most of the important themes that have characterized discussions of the psychology of love ever since.

This book is not a textbook of biophysics, cell biology or the electrophysiology of excitable cells, as there are already a number of excellent books available which deal with these subjects. The book instead is an attempt to describe the origins and derivations of the principles upon which these other books are based.

To understand and apply the principles of excitability requires a knowledge of subjects as diverse as physiology, physics, mathematics, statistics, signal and system analysis. It is a difficult task to obtain this knowledge because the jargon in other fields is often obscure, mathematical proofs are frequently abstruse and generally many original manuscripts have to be consulted. We can both testify to the frustrations that accompany such efforts and this has therefore been written in an attempt to enable the reader to acquire more easily this knowledge. Half of the book is appendices which deal with many of the key concepts from a fairly basic level.

We have assumed that the reader has only a modest mathematical background (about G.C.E. ‘O’ level) and most formulae are derived from first principles. For people with mathematical ability this approach may be somewhat tedious but we make no apologies for this. We consider it necessary that most of the steps in the derivation of an important equation are left in. Far too often have we struggled to follow mathematical proofs that are presented by an author in two lines which in reality take pages to derive.

Introduction

In the previous chapter we showed that acetylcholine (Ach) is released in discrete packets (quanta) in an all-or-none way from the presynaptic nerve terminal. In this chapter we derive a statistical model of quantal release that occurs in synaptic transmission. Since the release of transmitter is probabilistic, the postsynaptic membrane potential randomly fluctuates around a mean value. These fluctuations, or membrane noise, will be analysed in some detail in the latter part of the chapter and used as an example of the way in which membrane noise can be studied more generally.

A probabilistic model of quantal release

It was seen in the last chapter that the actual number of quanta released is not an exact constant and, in fact, the number changes in a random way with every action potential that invades the nerve terminal. (The average number of released quanta per action potential depends on factors such as the calcium or the magnesium concentrations in the bathing fluid. Under normal conditions the average number of quanta released is around 1000. If the calcium is replaced by magnesium the average number of quanta released per action potential may be quite small. It is this situation that we shall be analysing.) Since the exact number of quanta released is not constant, this means that it is possible (although unlikely) that some action potentials may not release any quanta, while others will release one, or two, or more, quanta.

Diffusion of non-electrolytes in solution

In Chapter 2 we discussed the movement of ions in solution under the influence of an electric field. In this chapter we will first consider the movement of non-electrolytes down concentration gradients and then the movements of electrolytes subject to the joint effect of an electric field and a concentration gradient.

Figure 3.1a shows a slab of a solid non-electrolyte, interfaced to a cuboid of solvent of length l and of unit cross-sectional area (1 cm2).

The solid non-electrolyte might, for example, be sugar and sugar molecules can be imagined to be dissolving from the face of the sugar slab into the solvent. The solvent is unstirred and assumed not to react with the non-electrolyte. At time t, after the sugar slab has come into contact with the solvent, we obtain a concentration profile of the type shown in Figure 3.1b (t). This profile is due to solute molecules which move from a high concentration (the face of the sugar slab) to the lower concentration in the solvent. At different times (t1, t2, …, t) different concentration profiles will be obtained.

Fick's First Law

A quantitative treatment of these concentration profiles was first carried out by Fick (who adapted the problem previously solved by Fourier for the conduction of heat through a slab) and assumed that the rate at which a solute flows through a plane of area A at right angles to the flow, is proportional to this area A.

APPENDIX 1. Units and numbers

When electrophysiologists measure currents, voltages cr concentrations they measure physical quantities that are the same physical quantities that a physicist or chemist might deal with. In order to characterize precisely these entities it is necessary to specify two things:

1. (1) The quality of the units, and

2. (2) The numerical size of the units used.

Dimensions

We shall deal first with the quality of physical units and this quality is called the dimension. Natural scientists have agreed on a number of different systems of units. The two most widely used systems are the c.g.s. system in which the fundamental units are:

1. (L) length – centimetre

2. (M) mass – gram

3. (T) time – second

and then there is the MKS system with the fundamental units of:

1. (L) length – metre

2. (M) mass – kilogram

3. (T) time – second

The latter is the so-called rationalized MKS system, more closely related to the International System of Units (SI), which is coming steadily into universal use.

In Table 1 we give some physical units that are used in the two systems. More detailed information is available in Quantities, Units, and Symbols, a report by the Symbols Committee of the Royal Society, 2nd edn, 1975, and in Units of Measurement, Preprint from the Geigy Scientific Tables, 7th edn, 1968 (Basle, Switzerland: Geigy).

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.