SYNAPSES

INTRODUCTION: There are two types of connections between electrically active cells, chemical synapses and electrical synapses. In a chemical synapse, a chemical signal namely, a neurotransmitter is released from one cell and it affects the other cell. In an electrical synapse, there is a direct connection between the two cells so that ions can pass directly from one cell to the next. If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Chemical synapses involve the transmission of chemical information from one cell to the next. An example of a chemical synapse is the neuromuscular junction (NMJ).

CHARACTERISTICS OF A SYNAPSE: In the nervous system, there are many more synapses that are essentially the same as the NMJ. All synapses have common characteristics, which can be summarized in this list:

1) Presynaptic element.

2) Neurotransmitter (packaged in vesicles)

3) Synaptic cleft

4) Receptor proteins

5) Postsynaptic element

6) Neurotransmitter elimination or re-uptake

For the NMJ, these characteristics are as follows: the presynaptic element is the motor neuron's axon terminals, the neurotransmitter is acetylcholine, the synaptic cleft is the space between the cells where the neurotransmitter diffuses, the receptor protein is the nicotinic acetylcholine receptor, the postsynaptic element is the sarcolemma of the muscle cell, and the neurotransmitter is eliminated by acetylcholinesterase. Other synapses are similar to this, and the specifics are different, but they all contain the same characteristics.

NEUROTRANSMITTER RELEASE: When an action potential reaches the axon terminals, voltage-gated Ca²⁺ channels in the membrane of the synaptic end bulb open. The concentration of Ca²⁺ increases inside the end bulb, and the Ca²⁺ ion associates with proteins in the outer surface of neurotransmitter vesicles. The Ca²⁺ facilitates the merging of the vesicle with the presynaptic membrane so that the neurotransmitter is released through exocytosis into the small gap between the cells, known as the synaptic cleft. Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.

RELATED;

1.  NEUROTRANSMITTER SYSTEMS  

2.  DIVISIONS OF THE NERVOUS SYTEM

3.  DOPAMINE

REFERENCES