Synapses
Where two neurons meet there is a small gap called a A tiny gap at the junction between two nerve cells, which nerve signals must cross.. The plasma membranes of each neuron are in very close contact and are separated by a narrow space called a synaptic cleft.
An electrical impulse cannot directly cross the gap so a different mechanism has to be used.
- An electrical nerve impulse travels along the The long part of a nerve cell along which the electrical signal quickly moves. of the first neuron (presynaptic neuron).
- When the nerve impulse reaches the Small extensions found on the end of dendrons that connect one neuron to another. at the end of the axon, chemical messengers called Chemical involved in passing nerve impulses from one nerve cell to the next across a synapse. are released.
- These chemicals When particles spread out from a region of higher concentration to a region of lower concentration. across the synaptic cleft. The chemicals bind with receptor molecules on the membrane of the second neuron (postsynaptic neuron).
- The receptor molecules on the second neuron can only bind to the specific neurotransmitters released from the first neuron.
- The binding of neurotransmitter to the receptors stimulates the second neuron to transmit an electrical impulse along its axon. The signal therefore has been carried from one neuron to the next.
Neurotransmitters are stored inside vesicles which are found in the presynaptic neuron.
Between impulses the transmitter molecules are rapidly removed from the synaptic cleft to prevent continuous stimulation of postsynaptic neurons.
There are two types of removal:
- Re-uptake 鈥 the neurotransmitter is reabsorbed back into the presynaptic neuron and restored inside a vesicle ready to be used again. For example, noradrenaline.
- Enzyme degradation 鈥 neurotransmitter is broken down by an enzyme into smaller inactive product which are then reabsorbed by the presynaptic neuron and resynthesised into active neurotransmitter.