Ligand-Gated Ion Channels Turn a Neurotransmitter Into a Change in Membrane Potential
Some transmembrane receptors for neurotransmitters are ligand-gated ion channels, also called ionotropic receptors.
When a neurotransmitter binds, the channel opens and specific ions flow across the membrane.
This ion flow changes the membrane potential of the target cell, either depolarising or hyperpolarising it.
Analogy
A neurotransmitter works like a key that flips a switch on the cell surface.
The binding is small, but it opens a channel and lets a flood of ions cross the membrane.
These transmembrane receptors carry signals between neurons and other cells, such as muscle fibres.
Definition
Membrane potential
Membrane potential is the difference in electrical charge between the inside and outside of a cell.
Acetylcholine at the Nicotinic Receptor Opens a Channel and Depolarises the Cell
Acetylcholine is a neurotransmitter that carries signals across synapses, including the junction between a neuron and a muscle fibre.
The nicotinic acetylcholine receptor is a transmembrane protein that acts as both a receptor and an ion channel.
Acetylcholine is released into the synaptic cleft and binds to the receptor on the postsynaptic membrane.
This binding changes the shape of the receptor, opening the ion channel inside the protein.
Positively charged sodium ions ($\text{Na}^+$) flow into the cell through the open channel.
This $\text{Na}^+$ influx makes the inside of the cell less negative, a depolarisation of the membrane potential.
Definition
Depolarization
A change in the membrane potential of the presynaptic neuron, making it more positive.
Note
The nicotinic acetylcholine receptor is a ligand-gated ion channel that opens only when its specific ligand, acetylcholine, binds.
Excitatory and Inhibitory Neurotransmitters
Excitatory neurotransmitters open channels that let positive ions in, which depolarises the cell and makes it more likely to fire.
Acetylcholine at the nicotinic receptor is excitatory because it drives $\text{Na}^+$ into the cell.
Inhibitory neurotransmitters open channels that let negative ions in or positive ions out, which hyperpolarises the cell and makes it less likely to fire.
GABA is inhibitory because it opens channels that let chloride ions ($\text{Cl}^-$) into the cell.
Tip
Depolarisation means the membrane potential becomes less negative, or more positive.
Hyperpolarisation means the membrane potential becomes more negative.
A Change in Membrane Potential Is What Passes the Signal On
At rest the inside of the cell is more negative than the outside.
When ligand-gated channels open, ions flow and shift this balance.
If depolarisation is large enough, it can trigger an action potential that travels along the neuron or muscle fibre.
Full action potential propagation belongs to neural signalling, covered in C2.2.
Example
Touch a hot surface and sensory neurons release neurotransmitters onto motor neurons.
The resulting depolarisation makes your muscles contract and pull your hand away.
Where This Fits Among Receptor Types
Ligand-gated ion channels are fast because the receptor is itself the channel.
Other neurotransmitter receptors act more slowly through G proteins, covered in C2.1.9.
Active recall
What happens to a ligand-gated ion channel when a neurotransmitter binds?
Why does $\text{Na}^+$ influx at the nicotinic acetylcholine receptor cause depolarisation?
How does an inhibitory neurotransmitter change the membrane potential compared with an excitatory one?
What is the difference between depolarisation and hyperpolarisation?
