Ion channels are integral membrane proteins which contain a hydrophilic inner pore through which ions may pass

• This allows ions to either enter or exit a cell according to the concentration gradient (facilitated diffusion)

The channel proteins may be ion-selective (only allows passage to specific ions) and may be gated (can control the timing of ion movement)

• Ion channels are essential to the operation of nerve cells as they are used to establish charge differentials across a membrane (membrane potentials) 

Voltage-gated Ion Channels:

• Voltage-gated ion channels cycle between an open and closed conformation according to the transmembrane voltage
  

• In neurons, voltage-gated sodium channels are used to transport sodium ions into the neuron during depolarisation
  

• Conversely, voltage-gated potassium channels will transport potassium ions out of the neuron during repolarisation
  

Ligand-gated Ion Channels:

• Ligand-gated channels change their conformation in response to the binding of a specific chemical (ligand)
  

• In neurons, acetylcholine is a neurotransmitter released from the nerve cells to stimulate adjacent cells
  

• Muscles contain nicotinic acetylcholine receptors that will trigger the opening of an ion channel when activated
  

• Binding of acetylcholine to these receptors results in the influx of ions into the muscle, triggering a cascade that results in muscular contraction
  

Types of Ion Channels