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You may have noticed that so far we have talked about a nerve fibre or axon as though the plasma membrane has a continuous and unobstructed outer boundary. But if you look back to Figure 4 you will recall that in a motor neurone the axon is myelinated. It is surrounded by a fatty myelin sheath that only has gaps at the nodes of Ranvier. Not all neurones in humans have myelin sheaths. For example, many of the neurones in the autonomic system that controls unconscious activities are non-myelinated. Non-myelinated neurones conduct impulses in the way described above.

In myelinated neurones, ions can only pass through the plasma

membrane at the nodes. Therefore action potentials can only occur at the nodes, so impulses in effect jump from node to node, as shown in Figure 11. This is called saltatory conduction. (Saltatory is derived from the Latin word for ‘dancing’ – it has nothing to do with salt.) At each node there is a concentration of sodium and potassium ion gated channels. Since the nodes are about 1 mm apart, saltatory conduction has the great advantage of being much faster. Conduction of impulses in myelinated neurones is approximately 50 times faster than in non-myelinated neurones.

The myelin sheath

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For a neurone that can transmit 300 impulses per second, how long is the refractory period?

5

Look at the graph in Figure 10. At what stage does this suggest that the refractory period ends?

Suggest why a refractory period is important for nerve function.

6

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Figure 11 Saltatory conduction in myelinated neurones. Conduction of impulses in these neurones is approximately 50 times faster than in non-myelinated neurones.

myelin sheath axon

node of Ranvier action potential jumps from node to node

resting potential repolarising action potential depolarising resting potential

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Two other factors affect the rate at which nerve fibres conduct impulses – temperature and the diameter of the fibre. The rate of conduction is slowest at low temperatures and in thin fibres.

In mammals temperature rarely makes a significant difference since they normally maintain a fairly constant temperature. In animals whose

temperature varies with the environmental temperature cold conditions can considerably slow reaction times.

The diameter of a fibre makes a difference because thicker fibres can maintain the resting potential more efficiently; a lower proportion of ions leak in or out by diffusion. The myelin sheath in mammals reduces leakage significantly. The fibres can be quite thin and still have a high rate of conduction. This gives mammals an advantage since impulses can be rapidly transmitted long distances, so large animals can respond quickly.

Invertebrates possess only non-myelinated fibres. These are usually quite thin and relatively slow conductors. However, in some circumstances where speed of conduction is particularly vital, some invertebrates have evolved exceptionally large diameter fibres. Much of the early research on nerve conduction was done using giant axons from squids. We saw some of the results in Figure 10. These giant axons have a diameter of nearly 1 mm, over a hundred times greater than other axons in the squid (Figure 12). They can conduct impulses about ten times as fast as normal motor neurones.

The giant axons extend the full length of the main part of the body, with branches to the muscles along the way. When the squid is startled, impulses are conducted at about 35 metres per second along the giant axons. The muscles contract almost instantly, providing the sudden force that squirts water out backwards and jet-propels the squid away from danger. Such violent contraction would not be possible if the impulses travelled slowly along the nerves with the muscles contracting in sequence as the impulses pass. Squids are not the only invertebrates to have a similar adaptation for an escape mechanism. Earthworms for example have large diameter axons running the length of their body. These allow them to contract muscles rapidly and withdraw quickly into a burrow when threatened by a bird.

Figure 12 Common squid. The main part of the body (on the left) has large muscles surrounding a water-filled cavity. When the squid needs to escape from a predator these muscles contract very rapidly and the squid shoots off using a form of jet propulsion.

Suggest why a low temperature reduces the rate of conduction of impulses.

Q 7

Synapses

Synapses

Synapses are the junctions between neurones. Neurones do not actually touch each other. There is always a small gap called the synaptic cleft, which is usually about 20 nm wide. (A nanometre (nm) is a thousandth of a micrometre (μm) and a millionth of a millimetre.) This gap prevents electrical impulses passing directly from one neurone to another. (Note that you should be careful not to refer to impulses crossing synapses.) Instead, communication between neurones is by chemical neurotransmitters that cross the cleft from one neurone to another. This is a slower process than the passage of an impulse along a nerve fibre. Figure 13 shows a synaptic cleft.

A synapse lies between the tiny branching end of a fibre and either a dendrite or the cell body of the next neurone. The branching end is always slightly swollen into a synaptic knob (Figure 14). A motor neurone may have approximately 8000 synapses on its dendrites and another 2000 directly on the cell body. It has been estimated that neurones in the brain have an average of approximately 40 000 synapses and that some have as many as 200 000. The number of possible pathways between neurones is phenomenal, which helps to explain the amazing complexity of the brain (see the chapter introduction, page 150).

So, what happens when an impulse reaches a synapse at the end of an axon? We will describe here what happens in a cholinergic synapse, which uses acetylcholine as a neurotransmitter. Acetylcholine (ACh) is the main neurotransmitter in synapses in the nerves outside the central nervous system, as well as in some parts of the brain. It is also used at the junctions between motor neurones and muscles. These neuromuscular junctions work in much the same way as synapses, as we shall see in Chapter 8.

Many other neurotransmitters are used in the nervous system, but the principles of synaptic transmission are similar in all cases.

When an action potential arrives at the pre-synaptic membrane at the end of a synaptic knob, it stimulates gated calcium ion channels to open.

The concentration of calcium ions (Ca^2⫹) in the fluid of the synaptic cleft is

synaptic cleft

Figure 14 Structure of a synapse.

synaptic knob into cleft in response to an impulse reaching the synaptic knob

acetylcholine receptors post-synaptic membrane post-synaptic neurone

Figure 13 Coloured scanning electron micrograph of a synaptic cleft. The synaptic knob containing vesicles of neurotransmitter is clearly visible.

higher than the concentration inside the synaptic knob. Therefore calcium ions rapidly diffuse through the open channels into the knob. As you can see from Figure 14, inside the knob there are many vesicles containing acetylcholine. These vesicles are tiny droplets of acetylcholine surrounded by a membrane. When calcium ions enter, some of these vesicles move to the membrane of the cleft and fuse with it. The acetylcholine spills out into the cleft and rapidly diffuses across to the membrane on the other side. This post-synaptic membrane has receptor molecules on its surface to which the acetylcholine molecules attach. These receptors are protein molecules.

(Don’t confuse them with receptor cells, such as the rods and cones that we studied in Chapter 6.)

The receptors are gated channels that are opened chemically rather than by a change in voltage. When acetylcholine molecules attach to the receptor molecules the receptors change shape and allow sodium and potassium ions through. As in other parts of the neurone membrane the sodium ions rush in more rapidly than the potassium ions diffuse out.You will recall that the excess of sodium ions entering the neurone depolarises the membrane.

If enough sodium ions enter and the threshold is exceeded, an action potential is set up (page 157). Once an action potential does start it travels all the way along the neurone as an impulse until it reaches the synapses at the other end. Figure 15 illustrates the sequence of the events that occur at a synapse.

1 Action potential opens gated channels. Ca²⁺ions flow in.

2 ACh vesicles move to membrane and fuse with it.

3 ACh released into synaptic cleft and diffuses across it.

4 ACh attaches to receptors and gated Na⁺and K⁺channels

5 Na⁺ions diffuse in faster than K⁺ions diffuse out.

Action potential created in post-synaptic membrane.

receptors

Figure 15 Sequence of events during an impulse transmission at a cholinergic synapse.

You will no doubt have realised that during transmission across a synapse changes take place that must be reversed if it is to keep on transmitting.

Just as the resting potential has to be restored in an axon, the ionic balance and the acetylcholine have to be restored in the synapse. Calcium ions are removed from the synaptic knob by active transport, ensuring that the concentration outside the membrane is always higher. The acetylcholine is removed from the receptors by an enzyme called acetylcholinesterase that

breaks it down to acetate and choline. These diffuse back into the synaptic knob where they are synthesised into acetylcholine once again. Vesicles are refilled, so the acetylcholine is continuously recycled.