SYNAPTIC TRANSMISSION: Ionotropic Receptors

In the previous module we saw that it is the binding of the neurotransmitter to its receptor on the postsynaptic membrane that produces an effect on the postsynaptic neurone. There are two major classes of receptors and the first of these are known as ionotropic receptors.

Ionotropic receptors are examples of the ligand-gated ion channels that we heard about in the excitable tissues lesson earlier in semester. When a neurotransmitter binds to an ionotropic receptor (see opposite) the channel opens and allows ions to diffuse across the membrane according to their concentration gradient.

The net effect of opening of these ion channels is either depolarisation of the postsynaptic neurone (if it is an excitatory synapse) or hyperpolarisation (if it is an inhibitory synapse).

Whether a synapse is excitatory or inhibitory is determined entirely by the receptor for the neurotransmitter (not the neurotransmitter itself). For example it is possible for the same neurotransmitter to produce depolarisation when it binds to one type of receptor and hyperpolarisation when it binds to another.

In the following sections we will examine in more detail the nature of ionotropic receptors in these two different types of synapse.

Excitatory Synapses

At an excitatory synapse, an action potential in the presynaptic neurone produces a transient depolarisation of the postsynaptic neurone after the 0.5 ms synaptic delay. Although this is a type of depolarising graded potential, when we are talking about synapses we refer to it more specifically as an excitatory postsynaptic potential (EPSP).

The diagram opposite shows the membrane potentials of a presynaptic and postsynaptic neurone that are connected by an excitatory chemical synapse. An action potential in the presynaptic neurone is followed by an EPSP in the postsynaptic neurone.

The EPSP is produced by binding of the neurotransmitter to ionotropic receptors on the postsynaptic membrane.

This opens the ion channel.

In the specific instances of the glutamate and nicotinic acetylcholine receptor this appears to be a non-selective cation channel that permits the simultaneous movement of both K⁺ and Na⁺ across the membrane.

As a consequence Na⁺moves into the cell whilst K⁺ moves out of the cell down their respective concentration gradients.

However the movement of Na⁺ into the cell is helped by the electrical gradient (it is attracted by the negative resting membrane potential) so more Na⁺ ions enter the cell than K⁺ leave the cell.

So the net effect is an influx of positive charge that is directly responsible for the transient depolarisation of the EPSP.

Because the EPSP takes the membrane potential closer to threshold, we think about it increasing the excitability of the postsynaptic neurone.

Inhibitory Synapses

At an inhibitory synapse, an action potential in the presynaptic neurone produces a transient hyperpolarisation of the postsynaptic neurone. This hyperpolarising graded potential is referred to more specifically as an inhibitory postsynaptic potential (IPSP) when we are talking about synaptic transmission.

The diagram opposite shows the membrane potential of a presynaptic and postsynaptic neurone that are connected by an inhibitory chemical synapse. An action potential in the presynaptic neurone is followed by an IPSP in the postsynaptic neurone.

The IPSP is produced by binding of the neurotransmitter to ionotropic receptors on the postsynaptic membrane.

This opens the ion channel.

The IPSP is most commonly caused by neurotransmitters (such as glycine and gamma-aminobutyric acid), that open ligand-gated Cl^- channels.

The concentration gradient for Cl^- favours the movement on this anion into the cell.

So as a consequence of these channels opening there is a net influx of negative charge that results in hyperpolarisation of the postsynaptic neurone.

Because the IPSP takes the membrane potential away from threshold we think about it decreasing the excitability of the postsynaptic neurone or inhibiting it.

So depending on the type of channel to which they are connected, ionotropic receptors can either excite or inhibit postsynaptic neurones. Thus neurotransmitters are one source of the excitatory and inhibitory stimuli that were referred to in somewhat vague terms when describing the graded potentials in the excitable tissue lesson.

One very important feature of ionotropic receptors is that because the potential differences that they elicit are produced directly by the movements of ions, their effect are very rapid in onset (synaptic delay of 0.5 ms) and fairly limited in duration (a few 10’s of ms). As we will see, this is quite different from the metabotropic receptors that are considered in the next part of this lesson