APORTACIONES TURÍSTICAS, ECONÓMICAS Y EMPLEO DE CANTUR A CANTABRIA

Individual synaptic potentials are too small to activate voltage-dependent sodium channels so they do not trigger action potentials. Instead they are passively conducted over the nerve cell membrane, getting smaller with both time and distance as they spread. This decay of small potentials is determined solely by the physics of the neuron. Generally, the smaller the diameter of a neuron, axon or dendrite along which a potential is spreading, the shorter the distance over which it will decay, the faster this will happen, and the slower the potential is conducted. This is crucial in determining how neurons integrate their inputs and hence how information is processed in the nervous system. In addition it accounts for why action potentials, which do not decay in time and distance, are needed for long-distance transmission. Synaptic potentials decay to zero within a few millimeters in most neurites, so cannot carry information any great distances. However, some short interneurons (e.g. those in the retina) do not fire nerve impulses, but rely on synaptic potentials for transmission along neurites.

Many thousands of synapses are formed on a neuron, both excitatory and inhibitory. At any given time a subset of these will be activated to generate epsps and ipsps. A special property of these graded potentials is that they summate, or add together. If a sufficient number of epsps are produced, in summing they will drive the axon hillock membrane potential across the threshold for triggering action potentials and the neuron will fire. The axon hillock is crucial because, being the region of a neuron with the highest density of voltage-dependent sodium channels, it has the lowest threshold. If at any Neurons as

decision-making devices

Key Notes

Small potentials (e.g. synaptic potentials) decay with time and distance. This behavior underlies how individual nerve cells treat their inputs and hence all information processing in the nervous system. Postsynaptic potentials (psps) generated on a neuron, both excitatory and inhibitory, add together (summate). If the result of this summation is that the axon hillock membrane potential is driven beyond threshold, the neuron will fire. So, whether or not a neuron will fire at any moment depends on how many excitatory and inhibitory synapses are active, and where they are located. It is by integrating synaptic inputs in this way that neurons act as computational devices.

The summation of psps generated at slightly different times is temporal summation. The summation of potentials arriving on different parts of the neuron is spatial summation. The geometry of a neuron determines the size and time course of synaptic potentials as they spread, and hence the extent of summation that occurs. If summation results in a sufficiently large depolarization of the axon trigger zone a nerve cell will fire.

Related topic Postsynaptic events (C3)

Neurons as decision-making

devices

instant insufficient excitatory synapses are activated, or a high level of excitatory synaptic input is more than offset by the generation of ipsps from inhibitory input, then the axon hillock will not be driven across the threshold and the cell will not fire. So, neurons are decision-making devices. The decision – to fire or not – is actually taken by the axon hillock on the basis of whether the sum total of epsps and ipsps causes its membrane potential to go more positive than the firing threshold. It is this operation that constitutes information processing by individual neurons. In engineering terms, a synapse converts digital signals (action potentials) into analog ones (postsynaptic potentials). The neuron then integrates all its analog signals over a short time and compares the result of that integration with a given threshold to decide whether to fire. When it does fire the output is digital.

Experiments on pyramidal cells show that about 100 excitatory synapses, on average, must be activated at the same time to trigger an action potential. However, the efficacy with which a synapse can influence firing depends on its position. Because postsynaptic potentials decay as they spread passively towards the axon hillock, a synapse far out on a distal dendrite will have less effect than one closer to the cell body. In this context it is noteworthy that on pyramidal cells there are only about 250 inhibitory synapses on the cell body but 10 000 or so excitatory axodendritic synapses. The relative strength of a synapse in contributing to a neuron’s output is its weighting. This need not be a fixed property but may change with time.

Summation If an afferent neuron fires a series of action potentials in quick succession (a

volley), then the earliest psps generated in the postsynaptic cell will not have time to decay before the next psps arrive. Hence successive psps summate over time. This is referred to as temporal summation. If sufficient, temporal summa- tion will cause the postsynaptic cell to reach firing threshold.

The summing of postsynaptic potentials generated at separate points on the neuron surface is called spatial summation (Fig. 1). If a sufficient number of excitatory synapses are activated in relation to inhibitory ones the cell will fire.

Although temporal and spatial summation are described as separate processes, both occur together as a neuron is stimulated and it is their combined effect which dictates whether it will fire. The precise details of how summation works depend on a neuron’s geometry because this determines exactly how all the synaptic potentials set up on the cell decay as they spread towards the axon hillock. The frequency with which a cell fires, and how long it fires, is deter- mined by the amplitude and duration of the depolarization of the axon hillock membrane.

C4 – Neural integration 63

(a) (b)

2 mV

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Fig. 1. Spatial summation. In each case the upper trace is the summed response of the two lower epsps generated at synapses: (a) a long way apart, (b) close together.