Both voltage-gated and ligand-gated ion channels are large protein molecules, as is the sodium pump Na,K-ATPase. In recent years the primary structure of a number of them has been determined, and by combining this information with the biophysical evidence major advances have been made in our understanding of how they work at the molecular and sub-molecular levels.

cDNA sequencing studies

A protein consists of a long chain built up of twenty different amino acids (Table 5.1), folded on itself in a rather complicated way. Its properties depend critically on the arrangement of the folds, which is determined by the exact order in which its constituent amino acids are strung together. This in turn is specified by the sequence of the nucleotide bases that make up the DNA molecules which constitute the genetic material of the cell. There are only four different bases, and each of the twenty amino acids corresponds according to a universally obeyed triplet code to a specific group of three of them. The information embodied in the base sequence of a DNA molecule is transcribed on to an intermediary messenger RNA, and is then translated during the synthesis of the protein to yield the correct sequence of amino acids. Rapid sequencing methods for nucleotides were perfected by Sanger and his colleagues, and modern recombinant DNA technology makes possible the cloning of DNA so that the quantity required for the determination can be prepared from a single gene.

Skeletal muscles are innervated by motor nerves. Excitation of the motor nerve is followed by excitation and contraction of the muscle. Thus excitation of one cell, the nerve axon, produces excitation of another cell which it contacts, the muscle fibre. The region of contact between the two cells is called the neuromuscular junction. The process of the transmission of excitation from the nerve cell to the muscle cell is called neuromuscular transmission. This chapter is concerned with how this process occurs.

Regions at which transfer of electrical information between a nerve cell and another cell (which may or may not be another nerve cell) occurs are known as synapses, and the process of information transfer is called synaptic transmission. Neuromuscular transmission is just one form of synaptic transmission; we shall examine the properties of some other synapses in the following chapter.

The neuromuscular junction

Each motor axon branches so as to supply an appreciable number of muscle fibres. Fig. 7.1 shows the arrangement in most of the muscle fibres in the frog. Each axon branch loses its myelin sheath where it contacts the muscle cell and splits up into a number of fine terminals which run for a short distance along its surface. The region of the muscle fibre with which the terminals make contact is known as the end-plate. Structures and events occurring in the axon are called presynaptic whereas those occurring in the muscle cell are called post-synaptic.

Dr David Aidley died suddenly but peacefully at home on August 24th 2000.

David was a gifted teacher: generations of students at the University of East Anglia benefited from his broad knowledge and relaxed style. A much wider audience knew David through his books, which over a thirty-year period have provided information, guidance and inspiration to students and their teachers in many parts of the world. The Physiology of Excitable Cells was first published in 1971 and is currently available in a fourth edition. Nerve and Muscle (with Richard Keynes) first appeared in 1981 and the third edition was to appear in proof the week he died. Ion Channels: molecules in action (with Peter Stanfield) was published in 1996. Each book represents an exemplary example of lucid prose and a clear grasp of the subject matter.

David was the perfect author; his books were delivered on time, in good order and each found a ready audience. He was, for these and many other reasons, a delight to work with, and we join his many friends in lamenting his untimely death and extending our condolences to his wife Jessica and their family.

Muscle cells have become adapted to a variety of different functions during their evolution, so that the details of the contractile process and its control are not always identical with those in vertebrate skeletal muscles. In this chapter we examine the properties of mammalian heart and smooth muscles.

Cardiac muscle

Mammalian heart muscle consists of a large number of branching uninucleate cells connected to each other at their ends by intercalated discs (Fig. 11.1). Electron micrography shows that the intercalated discs consist largely of accumulations of dense material on the insides of the two cell membranes; these apparently serve to fix the cells together and allow the filaments of the contractile apparatus in one cell to pull on those of the next one in the line. Gap junction channels (p. 116) are also present in the intercalated discs and these allow electrical currents to flow from one cell to another.

The contractile apparatus is much the same as in skeletal muscles, with thick myosin and thin actin filaments aligned transversely so that the muscle cells as a whole are cross-striated in appearance. As in a skeletal muscle fibre, the interior of the cell also contains mitochondria, sarcoplasmic reticulum and the transverse tubules of the T system.

The cardiac action potential

Intracellular recordings from heart muscle fibres were first made using isolated bundles of Purkinje fibres from dogs. The Purkinje fibres form a specialized conducting system which serves to carry excitation through the ventricle.

The functioning of the nervous system depends largely on the interactions between its constituent nerve cells, and these interactions take place at synapses. In most cases synaptic transmission is chemical in nature, so that, as in neuromuscular transmission, the presynaptic cell releases a chemical transmitter substance which produces a response in the postsynaptic cell. There are a few examples of electrically transmitting synapses, which we shall consider briefly at the end of this chapter.

Acetylcholine is only one of a range of different neurotransmitters. Fig. 8.1 shows some of the variety found in the central nervous system. For a long time it was thought that any one cell would only release one neurotransmitter, but several cases where two of them are released at the same time are now known.

Different chemically transmitting synapses differ in the details of their anatomy, but some features are common to all of them. In the presynaptic terminal the transmitter substance is packaged in synaptic vesicles. The pre- and postsynaptic cells are separated by a synaptic cleft into which the contents of the vesicles are discharged. There are specific receptors for the neurotransmitter on the postsynaptic membrane.

Just as with the neuromuscular junction, our knowledge of how synapses work was greatly affected by the invention of the intracellular microelectrode. Much of the fundamental work with this technique was performed by J. C. Eccles and his colleagues on the spinal motoneurons of the cat, so it is with these that we shall begin our account of synapses between neurons.

The impedance change during the spike

An important landmark in the development of theories about the mechanism of conduction was the demonstration by Cole and Curtis in 1939 that the passage of an impulse in the squid giant axon was accompanied by a substantial drop in the electrical impedance of its membrane. The axon was mounted in a trough between two plate electrodes connected in one arm of a Wheatstone bridge circuit (Fig. 4.1) for the measurement of resistance and capacitance in parallel. The output of the bridge was displayed on a cathoderay oscilloscope, and Rv and Cv were adjusted to give a balance, and therefore zero output, with the axon at rest. When the axon was stimulated at one end, the bridge went briefly out of balance (Fig. 4.2) with a time course very similar to that of the action potential. The change was shown to be due entirely to a reduction in the resistance of the membrane from a resting value of about 1000 ohm cm2 to an active one in the neighbourhood of 25 ohm cm2. The membrane capacitance of about 1 μF/cm2 did not alter measurably.

The sodium hypothesis

Cole and Curtis's results were not wholly unexpected, because it had long been supposed that there was some kind of collapse in the selectivity of the membrane towards K+ ions during the impulse.

People are judged by their actions, and these actions are coordinated by nerve cells and carried out by muscle cells. So an understanding of nerve and muscle is fundamental to our knowledge of how the human body functions.

This book provides an introductory account of how nerve and muscle cells work, suitable for students taking university courses in physiology, cell biology or preclinical medicine. It aims to give a straightforward exposition of the fundamentals of the subject, including particularly some of the experimental evidence upon which our conclusions are based. This edition includes new material reflecting the exciting discoveries that continue to be made in the field. So there is up-to-date detail on topics such as the ion channels involved in electrical activity and the molecular mechanisms of muscular contraction.

Structure of the cell membrane

All living cells are surrounded by a plasma membrane composed of lipids and proteins, whose main function is to control the passage of substances into and out of the cell. In general, the role of the lipids is to furnish a continuous matrix that is impermeable even to the smallest ions, in which proteins are embedded to provide selective pathways for the transport of ions and organic molecules both down and against the prevailing gradients of chemical activity. The ease with which a molecule can cross a cell membrane depends to some extent on its size, but more importantly on its charge and lipid solubility. Hence the lipid matrix can exclude completely all large water-soluble molecules and also small charged molecules and ions, but is permeable to water and small uncharged molecules like urea. The nature of the transport pathways is dependent on the specific function of the cell under consideration. In the case of nerve and muscle, the pathways that are functionally important in connection with the conduction mechanism are (1) the voltage-sensitive sodium and potassium channels peculiar to electrically excitable membranes, (2) the ligandgated channels at synapses that transfer excitation onwards from the nerve terminal, and (3) the ubiquitous sodium pump which is responsible in all types of cell for the extrusion of sodium ions from the interior.

Electrophysiological recording methods

Although the nervous impulse is accompanied by effects that can under specially favourable conditions be detected with radioactive tracers, or by optical and thermal techniques, electrical recording methods normally provide much the most sensitive and convenient approach. A brief account is therefore necessary of some of the technical problems that arise in making good measurements both of steady electrical potentials and rapidly changing ones.

In order to record the potential difference between two points, electrodes connected to a suitable amplifier and recording system must be placed at each of them. If the investigation is only concerned with action potentials, fine platinum or tungsten wires can serve as electrodes, but any bare metal surface has the disadvantage of becoming polarized by the passage of electric current into or out of the solution with which it is in contact. When, therefore, the magnitude of the steady potential at the electrode tip is to be measured, non-polarizable or reversible electrodes must be used, for which the unavoidable contact potential between the metal and the solution is both small and constant. The simplest type of reversible electrode is provided by coating a silver wire electrolytically with silver chloride, but for the most accurate measurements calomel (mercury/mercuric chloride) half-cells are best employed.

When the potential inside a cell is to be recorded, the electrode has to be very well insulated except at its tip, and so fine that it can penetrate the cell membrane with a minimum of damage and without giving rise to electrical leaks.

SUMMARY

Since the discovery that neurons connect to each other via synapses, it has been hypothesized that experience leads to modifications in these connections, and that memory is embodied in these changes. Application of the cellular-connection approach to the study of the plasticity of defense responses to sensory cues has proved to be a particularly fruitful means of studying the relation of learning to changes in synaptic transmission – first in Aplysia, and, more recently, in studies of classical fear conditioning in the rodent. The discovery of artificial means of inducing neural plasticity (longterm potentiation, LTP) has added an important tool for the examination of plasticity mechanisms in specific pathways identified through the successful application of the cellular-connection approach. The demonstration that sensory pathways to the amygdala critical for fear conditioning are susceptible to LTP induction has led to examination of the mechanisms underlying LTP in the amygdala, the ability of these mechanisms to modulate sensory transmission, and their relation to the learning-induced changes in sensoryevoked neural activity that accompany fear conditioning.

Introduction

Learning refers to the acquisition of new information about the world and memory to the storage of that information over time. In his Croonian lecture to the Royal Society in 1894, Ramon y Cajal suggested that learning involves alterations in the strength of connections between neurons, and that these alterations in synaptic strength might persist and underlie memory storage. Although subsequent demonstrations that certain synapses are capable of undergoing functional modification were not uncommon (e.g., Eccles, 1964), such changes were short lasting and not clearly related to the acquisition and storage of information.

SUMMARY

Although our understanding of the relationship between hippocampal long-term potentiation (LTP) and learning is limited, a fresh approach presented here bears promise for resolving this important issue. Blockade of LTP frequently leads to blockade of hippocampus-dependent learning, but there are examples of intact learning in the absence of LTP, and the function of LTP during learning is poorly understood. This lack of understanding may partly stem from the preoccupation with tetanus-induced LTP in experimental designs, since such long-lasting high-frequency activity has never been demonstrated to occur in vivo. A closer understanding of memory related plasticity in the brain requires detailed study of the neuronal activity patterns occurring when an animal learns. Replication of natural patterns of activity in an in vitro preparation suggests that bursting activity in hippocampal pyramidal neurons may play a specific role during encoding of new memories. These data have led to the development of a novel model for information processing in cortical networks, in which separate processing modes may coexist.

Introduction

The question of whether long-term potentiation (LTP) is the basis for learning and memory is one of the most exciting and controversial issues in cognitive neuroscience. Previous attempts to link LTP and memory seem to have gone up the wrong path since, in several respects, we are as confused about this important question today as we were 30 years ago. One reason why we went astray might be that in our enthusiasm to use novel cellular and molecular techniques to explore the mechanisms underlying LTP, we forgot what we were actually trying to understand, namely the mechanism of memory.

SUMMARY

Long-term potentiation (LTP) continues to be the most intensely studied model of neuroplasticity, and is viewed by many as a substrate of learning. Other potential roles of LTP, however, have not received widespread discussion in the field. In this chapter, we will evaluate the LTP phenomena, discuss the putative connection to learning, and expand the discussion to include the participation of LTP and kindling in some forms of neuronal pathology. The available evidence appears to support a role for LTP in a continuum of events reflecting different levels of neural activity ranging from long-term depression (LTD) through LTP and kindling and culminating in cellular degeneration. In place of LTP as the substrate of learning at a neuronal level, we outline a model in which multiple levels of neural organization exert mutual control such that activity and modifiability at any level are controlled by those levels above and below.

Introduction

It is now over a quarter of a century since long-term potentiation (LTP) was first described (Bliss and Lomo, 1973). Initially it was viewed, rightly with great excitement, as an interesting phenomenon possibly linked to learning. Now it is frequently declared in unqualified terms to be the “cellular basis of learning and/or memory” (e.g., “LTP is a learning mechanism,” Fanselow, 1997) and its study virtually dominates the field of neuronal plasticity. Those new to the neurosciences would be forgiven for assuming that this transition occurred because the relationship between LTP and learning or memory had been experimentally demonstrated.

SUMMARY

Advanced aging is usually accompanied by declines in cognitive function, particularly in the areas of learning and memory. The presence of these behavioral deficits makes aged animals a good system in which to explore basic tenets of the hypothesis that long-term potentiation (LTP) is important for learning and memory. Here we review the existing literature describing studies of hippocampal LTP in aged rodents. These studies indicate that age-related decrements in LTP are often, but not always, reported; discrepancies between studies appear to be due to the stimulation protocol used or type of LTP being measured. Correlations between hippocampal LTP status and spatial learning ability in individual aged animals support the notion of an LTP/learning relationship, as do the results of interventional studies demonstrating that drug-related behavioral improvements also reduce LTP deficits. However, these promising results need verification and expansion. We conclude that, due to the complexity of the linkage between its physiologic and behavioral functions, the hippocampal system may not be appropriate for validating the hypothesis that LTP is critical for learning and memory. Nevertheless, studies of hippocampal LTP, particularly in aged animals, may provide an excellent framework for approaches to develop therapeutic interventions for age-related cognitive impairments.

Introduction

For many neuroscientists, long-term potentiation (LTP) has become a mantra for describing changes in the brain that are logically part of a memory encoding mechanism. However, problems arise when this idea is expressed as some formal hypothesis, the simplest examples of which are that inducing LTP should produce learning, or that preventing LTP should prevent learning.

SUMMARY

Synapse-specific increases and decreases in synaptic strength that depend on the activity of the presynaptic and postsynaptic neuron are central to modern theories of how networks in the brain operate in setting up sensory representations of the world, in memory, and in producing appropriate motor responses. The goals of this chapter are to show how different features of these synaptic modifications are crucial to the operation of different types of network, and to the operation of several different brain systems. The types of network considered will be three that are fundamental to brain function, namely pattern associators, autoassociators, and competitive networks. Each performs a different type of operation for the brain. Then the ways in which these types of synaptic modification are implicated in the operation of the hippocampus and related cortical areas in memory (see the section The Primate Hippocampus), and the cerebral neocortex in visual object recognition (see the section Synaptic Modification Rules) and shortterm memory (see the section Short-Term Memory) will be described. The points made apply to any synapse-specific modification process in the brain, regardless of whether that process happens to be long-term potentiation/depression (LTP/LTD). More formal descriptions of the operation of some of the networks introduced here are provided by Rolls and Treves (1998), and by Hertz et al. (1991). Ways in which these architectures may be specified genetically are suggested by Rolls and Stringer (2000b).

Pattern Associators

A fundamental operation of most nervous systems is to learn to associate a first stimulus with a second that occurs at about the same time, and to retrieve the second stimulus when the first is presented.