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