Concepts: Nerve impulse, its conduction and synaptic transmission

2.1 The resting state of the membrane

A neuron that is not conducting an impulse is said to be resting, and in this state the membrane is polarised. The outside surface of the axonal membrane carries a positive charge relative to the inside, and the difference in charge across the membrane is the resting potential. The membrane is much more permeable to potassium ions than to sodium ions, the potassium ions tend to leave the cell, and the inside therefore becomes negatively charged. The sodium-potassium pump, which is a form of ATPase, then moves three sodium ions out of the cell for every two potassium ions it moves in, and this unequal pumping keeps the ion distribution unbalanced. The result is a steady membrane potential of about minus seventy millivolts in a typical neuron.

2.2 The action potential

A stimulus that is strong enough changes the permeability of the membrane at the point of stimulation. The sodium channels open, sodium ions rush into the axon, and the inside of the membrane becomes positive while the outside becomes negative. This change is depolarisation, and it produces the action potential which is the nerve impulse. The point of stimulation is now said to be active, and it is followed by repolarisation: the sodium channels close, the potassium channels open and potassium ions move out, so the membrane returns to its polarised state. For a short time after the impulse has passed, the region is less excitable and cannot be stimulated again, and this period is the refractory period. The impulse therefore travels as a wave of depolarisation followed by repolarisation, and it always moves away from the point of stimulation because the region just behind it is refractory.

2.2.1 Conduction along an axon

The action potential does not stay at one spot. The local current that it creates stimulates the next patch of the membrane, and the process repeats along the axon, so the impulse travels the length of the fibre. In an unmyelinated axon the impulse travels continuously, whereas in a myelinated axon the myelin insulates the segments between the nodes and the impulse jumps from one node of Ranvier to the next. This jumping conduction, called saltatory conduction, is faster than continuous conduction and it saves energy, and it is one reason a myelinated nerve carries signals at high speed. The diameter of the axon also matters, because a wider axon offers less resistance to the current.

2.3 Transmission across a synapse

The axon ends in a synaptic knob that faces another neuron or a muscle or gland cell, and the junction is a synapse. There are two kinds. At an electrical synapse the membranes of the two cells are very close and the current passes directly from one to the other through small connecting channels, so the transmission is very fast. Such synapses are rare in mammals. At a chemical synapse the two membranes are separated by a small gap, the synaptic cleft, and the signal is carried across the gap by a chemical.

In the chemical synapse the synaptic knob contains vesicles filled with neurotransmitters. When the impulse arrives at the knob, calcium ions enter the terminal and cause the vesicles to fuse with the membrane and release their transmitter into the cleft by exocytosis. The transmitter molecules diffuse across the cleft and bind to specific receptors on the post-synaptic membrane. The binding opens ion channels and a new electrical change starts in the next cell. The transmitter is then removed, either by being broken down by an enzyme or by being taken back into the presynaptic knob, so that the synapse is ready for the next signal. Acetylcholine is one of the neurotransmitters used in this way. Because the chemical step takes a little time, a chemical synapse is slower than an electrical one, but it can be regulated and it can be excitatory or inhibitory depending on the transmitter and the receptor.

2.3.1 Why a synapse is more than a junction

A chemical synapse gives the nervous system a place to control the signal rather than simply pass it on. The amount of transmitter released can be increased or decreased by other neurons that end on the same synaptic knob, so the same incoming impulse can produce a strong or a weak response in the next cell. The transmitter may also be excitatory in one synapse and inhibitory in another, depending on which receptor is present. Because of this, synapses are the places where signals are summed, filtered and gated, and they are the reason the nervous system can make decisions instead of merely carrying every impulse along in the same way.