Synapses

The junction that occurs between two neurones

Through chemical transmitters

Acetylcholine

Cholinergic synapses

An action potential arrives at the synapse, depolarising the synaptic knob

• Protein channels specific to calcium ions open
• Calcium ions diffuse into the synaptic knob

Synaptic vesicles carrying acetylcholine fuse with the pre-synaptic membrane and release the neurotransmitter into the synaptic cleft

The acetylcholine diffuses across the cleft and binds with specific protein receptors on the post synaptic membrane due to their complementary tertiary structure

• Protein channels specific to sodium ions open in the post synaptic membrane
• This allows sodium ions to diffuse into the neurone 
• This causes depolarisation of the post synaptic membrane

If depolarisation exceeds the threshold value an action potential is initiated

• Acetylcholine in the cleft is broken down by the enzyme, acetylcholinesterase 
• The products are reabsorbed by the synaptic knob and the acetylcholine is re-synthesised 
• This prevents the acetylcholine from remaining in the cleft and continuing to produce action potentials in the adjoining neurone

The synapse between the motor neurone and muscle

The same sequence of events as in the cholinergic synapse except that the post synaptic membrane is the muscle fibre (sarcolemma) and its depolarisation leads to a sequence of events resulting in muscle contraction

• Vesicles are only present in the presynaptic neurone 
• Synapses can only pass impulses in one direction

The threshold value must be reached

Sufficient neurotransmitters are not released to lead to an action potential in the post synaptic membrane

A number of pre synaptic neurones converge providing a combined amount of neurotransmitter capable of exceeding threshold value

A higher frequency of action potentials along a single pre synaptic neurone may release enough neurotransmitter to exceed threshold value

The protein channel specific to negatively charged chloride ions

• The diffusion of chloride ions into the post synaptic membrane makes the inside of the neurone more negative
• This makes it less likely for threshold value to be reached and action potentials to be produced

• Some have a similar shape to the transmitter and bind to the protein receptors on the post synaptic membrane causing sodium channels to open producing action potentials 
• Some have a similar shape to the transmitter and block the protein receptors on the post synaptic membrane preventing the neurotransmitter from binding and action potentials being generated 
• Some inhibit acetycholinesterase or inhibit the uptake of the transmitter so the transmitter remains in the cleft and continues to produce action potentials