How neurones work

A neurone is a terminally differentiated and highly specialised cell. Its appearance is unmistakable. Thin branching projections of various lengths grow like a dense copse of trees from the cell body. (The cell body is where the nucleus, mitochondria and other customary structural components are housed.) These projections are called dendrites. Their job is to pick up chemical or electrical signals, usually from other neurones. The signals picked up by the dendrites change the electrical potential in the cell body. We shall explain how this happens shortly.

Fig. 16-1: a neuron, showing cell body, dendrites, axon and axon terminus. (See also chapter 5.)

One projection, the axon, has the opposite job. It conveys electrical impulses outwards from the neurone's cell body, not into it as the dendrites do. The axon arises at a small swelling known as the axon hillock. It ends in a terminal arborisation, rather as a big river might end in a delta. The terminal arborisation might comprise thousands of branches. Each branch ends in a terminus. The termini might connect with an effector, such as a muscle cell or a secretory cell in a gland; thus, an impulse travelling along the axon could make the animal twitch a limb or roll up into a ball, or secrete a noxious chemical. Alternatively, the axon termini might "connect" via synapses with the dendrites of other neurones.

cell body cell body

Fig. 16-1: a neuron, showing cell body, dendrites, axon and axon terminus. (See also chapter 5.)

In most cells, including neurones, the inside is electrically negative compared to the outside. The potential difference is about 0.05 - 0.1 volts40. In neurones, this difference is called the resting potential. Signals picked up by the dendrites alter the permeability of the membrane to certain ions, changing the resting potential locally. All these local changes are transmitted to the cell body, which adds them together. If the net effect is to decrease the cell body's resting potential sufficiently, then the axon hillock membrane becomes more permeable to ions, and for a few thousandths of a second the inside becomes positive relative to the outside. We say that the axon hillock membrane has been depolarised. A current immediately flows between this depolarised region and neighbouring parts of the axon, causing the latter to depolarise in turn. This effect is progressive; a wave of depolarisation travels along the axon. This wave of depolarisation is called an action potential.

Some stimuli tend to depolarise a dendrite membrane, lowering the cell body's resting potential and thereby making it more likely that the axon hillock will depolarise and cause an action potential. Other stimuli have the opposite effect: they make the resting potential bigger, not smaller - they hyperpolarise rather than depolarise - and therefore tend to prevent an action potential travelling along its axon.

After an action potential has left the axon hillock, a few milliseconds elapse before another one can be initiated. This brief delay is called the refractory period. Nerve conduction is therefore not a steadily flowing electric current; rather, it comprises a series of very short impulses. The number of action potentials per second can vary from zero (when the neurone is inactive) to a maximum value that depends on the detailed structure of the neurone.

Synapses

A tiny gap, just a few nanometres across, separates the terminus of each axon branch from the next neurone. This gap is a synapse. Neurones "talk" to each other across synapses. When an action potential reaches the axon terminus it causes the release of a small package of a special chemical substance, a neurotransmitter. The neurotransmitter crosses the synapse and binds to receptors on the dendrites or cell body of the next (postsynaptic)

40 This effect has quite a simple explanation, which was found by Donnan early in the 20th century. The main component of the membrane potential is the "Donnan potential" - a simple physico-chemical phenomenon. Nothing specifically biological (or magical) is involved.

neurone. When a receptor is occupied, the local membrane is either slightly depolarised or slightly hyperpolarised, so the electrical potential in the postsynapic cell body is changed, making it either more or less likely that an action potential will travel along the axon.

Picture a neurone (N) with two dendrites, each of which forms a synapse with an axon terminus from a different neurone. If one of these presynaptic neurones hyperpolarises its dendrite and the other depolarises its dendrite, then the response of neurone N will depend on the sum of the two inputs. The faster the action potentials in the depolarising axon are compared to those in the hyperpolarising one, the more likely N is to be activated, i.e. to transmit action potentials along its own axon. Thus, the rate at which a neurone "fires" depends on the sum of its current inputs. In a real neurone in a mammal's brain there might be ten thousand or more inputs rather than two; but the principle is the same.

How are the neurotransmitter packages41 assembled in the axon termini? We mentioned this briefly in chapter 5. Neurotransmitters are made in the cell body and are packaged inside small membrane-bound vesicles. These vesicles are taken to the ends of the axon by a motor-driven process using fibres of the cytoskeleton, which run all the way along the axon like railway lines. An action potential makes some of these vesicles fuse with the axon terminus membrane, releasing their contents into the synapse. The empty vesicle is carried back to the cell body along the cytoskeletal fibres, to be reloaded with fresh neurotransmitter and returned to the terminus for further

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