Nerve cells and synapses .1 Nerve cell structure

In document [Chris Barnard] Animal Behavior Mechanism org (Page 127-130)

3 Physiological mechanisms and behaviour

3.1 Nervous systems and behaviour

3.1.1 Nerve cells and synapses .1 Nerve cell structure

True nervous systems are found only in multicellular animals. Here they form a tissue of discrete, self-contained nerve cells or neurons. Like any other type of animal cell, neurons comprise an intricate system of cell organelles surrounded by a cell membrane (Fig. 3.2a).

Unlike other animal cells, however, they are specialised for transmitting electrical mess-ages from one part of the body to another, a specialisation that is reflected in both their structure and their physiology.

A neuron has three basic structural components. The main body of the cell, or soma, is a broad, expanded structure housing the nucleus. Extending from it are two types of cytoplasm-filled process called axons and dendrites. Axons carry electrical impulses away from the soma and pass them to other neurons or to muscle cells, while dendrites receive impulses from other neurons and direct them to the soma. All three components are usually surrounded by glial cells which, though not derived from nerve tissue, come to form a more or less complex sheath around the axon. In invertebrates, the glial cell membranes may form a loose, multilayered sheath in which there is still room for cyto-plasm between the layers (tunicated axon). In vertebrates, the sheath is bound more tightly so that no gaps are left. The glial cells are now known as Schwann cells and are arranged along the axon in a characteristic way. Each Schwann cell covers about 2 mm of axon. Between neighbouring cells, there is a small gap, known as a node of Ranvier, where the membrane of the axon is exposed to the extracellular medium. Axons with this punctuated Schwann cell sheath arrangement are called myelinated or medullated axons, and the formation of the myelin sheath greatly enhances the speed and quality of impulse conduction.

3.1 n Nervous systems and behaviour x 97 Figure 3.2 (a) A motor neuron connecting

with a muscle fibre (see text). (b) The rela-tionship between ion flow and membrane potential during an action potential in a non-myelinated axon. In squid neurons, the concentration of potassium (K++) ions is some 20 times greater inside the cell than outside, whereas sodium (Na++) ions are about 10 times more concentrated outside.

Diffusion of ions towards equilibrium is counteracted both by the low permeability of the cell membrane to Na++ and by a metabolic pump which transfers the ions against their gradients. The net result of this ionic imbalance is a negative resting poten-tial across the membrane of some −−60 to

−−70 mV, depending on the neuron. When the neuron is stimulated, the membrane suddenly becomes highly permeable to Na+at the site of stimulation and there is a massive influx of Na++which results in a sharp depolarisation to around ++40 mV known as an action potential. The formation of an action potential at one part of the membrane stimulates an increase in Na++permeability in the adjacent part, and a wave of depolarisa-tion courses down the axon. As soon as the action potential has passed a given point, the resting potential is then restored by a metabolic pump. Modified from Barnard (1983) after Adrian (1974).

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3.1.1.2 Nerve cell function

The basis of the neuron’s ability to conduct electrical impulses (action potentials) is the distribution of electrically charged ions inside and outside the cell (see Fig. 3.2b). The rate of conduction of the action potential depends on how good a conductor the axon is. One way to increase conductivity is to make the diameter of the axon larger, as has occurred in the ‘giant’ axons of squid and earthworms (Annelida, Oligochaeta).

However, increasing the size of axons is costly in both materials and space. Insects and vertebrates have solved the conduction problem by insulating their axons instead. Their myelinated (see above) axons allow the flow of current only through certain areas (the nodes of Ranvier [Fig. 3.2a]). Consequently, action potentials are conducted from node to node extremely rapidly.

In most neurons, action potentials are all-or-nothing events, though in some arthropod sensory receptors and certain other specialised cells, where short-distance communica-tion is required, informacommunica-tion can be coded as graded potentials. Neurons are limited in the kind of information they can transmit because they have only two states: ‘on’ or

‘off’. Transmitted information is therefore digital. Different grades of information can be transmitted by neurons, but only through changes in the frequency of depolarisations (Fig. 3.2b), not by changes in their intensity. Increasing the strength of the stimulus will simply generate more depolarisations, not bigger ones. The mechanical reason for this comes back to the nature of depolarisation, which depends on a fixed influx of sodium ions before the neuron reverts to its negative resting potential. The functional reason seems to be to preserve the integrity of the information as it is transmitted down sometimes lengthy axons. The simple signal ‘on’ can be transmitted reliably from one end of the axon to the other, almost regardless of length. A graded signal, however, is susceptible to the vagaries of signal decay over long distances: what starts out as 90% ‘on’ at one end of the axon might dwindle to 30% ‘on’ by the time it gets to the other end.

3.1.1.3 Communication between neurons

The membrane boundaries of neurons are complete, and the contact between cells that is essential to the transmission of impulses is accomplished by close juxtaposition rather than the formation of a continuous syncytium (collection of cells without cell walls). The region of juxtaposition is known as a synapse, a term that is also applied to neuronal junctions with sensory receptors and muscle fibres. In most cases, transmission across the synapse occurs through the medium of transmitter substances known as neurotransmitters, although electrical communication is known where the juxtaposition is very close (e.g. in invertebrate ‘giant’ axons [3.1.3.1]). Various neurotransmitters are known, including acetylcholine (an excitatory transmitter at vertebrate neuromuscular junctions and in the central nervous systems of both vertebrates and invertebrates), adrenalin and noradrenalin, dopamine, glutamate, γ-aminobutyric acid (GABA) and 5-hydroxytryptomine (serotonin). Neurotransmitters have important consequences for behaviour and show some differences between vertebrate and invertebrate nervous systems. Apart from the general excitatory or inhibitory properties of acetylcholine and GABA, neurotransmitters such as β-endorphin (one of the opioid group), dopamine and serotonin have important effects on many things, including mood, pain perception, sleep, attention and learning. Synapses facilitate complex cross-connections between neural pathways and are a crucial feature in the evolution of behavioural coordination and integration.

However, not all communication between neurons is by chemical transmission across synapses. Non-synaptic interactions are also recognised. Recent evidence, for example, suggests that nitric oxide can establish non-synaptic communication between gluta-matergic neurons and surrounding cells where transmission is mediated by monoamine.

These interactions appear to be important in the function of regions of the brain con-cerned with learning, memory and the coordination of movement (Kiss & Vizi 2001).

In document [Chris Barnard] Animal Behavior Mechanism org (Page 127-130)