Synapses

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Synapses

A synapse is a junction between two neurones (e.g. a relay neurone and a motor neurone) or between a neurone and an effector (e.g a motor neurone and a muscle cell). The features of synapses are:

Structure

Synaptic cleft

At a synapse, there is a gap between the cells. This gap is called the synaptic cleft.
When an action potential reaches a synapse, it must be transmitted across the synaptic cleft.

Presynaptic neurone

The presynaptic neurone is the neurone before the synapse.
When an action potential reaches the end of the neurone, it is transmitted across the presynaptic membrane to the postsynaptic membrane, or to an effector cell.

Synaptic knob

The end of the axon of the presynaptic neurone is called the synaptic knob.
The synaptic knob is a swelling which contains synaptic vesicles.
The synaptic knob is the location where the nerve impulse is transmitted across the synpatic cleft.
There are also lots of mitochondria in the synaptic knob. This is because lots of energy is needed to synthesise neurotransmitters.

Synaptic vesicles

Synaptic vesicles are vesicles located in the synaptic knob.
The vesicles contain neurotransmitters. When the vesicles fuse with the presynaptic membrane, neurotransmitters are released into the synaptic cleft.

Neurotransmitters

Neurotransmitters are the chemicals that allow an action potential to be transferred across a synapse.
When neurotransmitters are released from the synaptic vesicles into the synaptic cleft, they bind to specific receptors on the postsynaptic membrane.

Postsynaptic membrane

The postsynaptic membrane is the membrane of the postsynaptic neurone or effector cells.
Receptors on the postsynaptic membrane have a complementary shape to the neurotransmitters released from the synaptic knob.
When neurotransmitters bind to their receptors, the action potential continues.
There are only receptors on the postsynaptic membrane. This ensures the nerve impulse only moves in one direction.

Neurotransmitters

Neurotransmitters are the chemicals that allow an action potential to be transmitted across a synapse. They can be excitatory or inhibitory.

Excitatory

Excitatory neurotransmitters generate an action potential in the postsynaptic cell.
When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is depolarised.
- E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the central nervous system (CNS) an action potential is established.

Inhibitory

Inhibitory neurotransmitters prevent an action potential from being generated in the postsynaptic cell.
When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is hyperpolarised.
- E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the heart, potassium ion channels are opened in the membrane. This prevents an action potential from being established.

Summation

Summation is the process where neurotransmitters from multiple neurones are summed together to produce a response. There are two types of summation: spatial and temporal.

Spatial summation

Spatial summation takes place when multiple presynaptic neurones form a junction with a single neurone.
Each presynaptic neurone releases neurotransmitters. Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane.
Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

Temporal summation

Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time.
More neurotransmitter is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane.
Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.