Synapse Transmission
There are two types of synapses found in your body: electrical and chemical. Electrical synapses allow the direct passage of ions and signaling molecules from cell to cell. In contrast, chemical synapses do not pass the signal directly from the presynaptic cell to the postsynaptic cell. In a chemical synapse, an action potential in the presynaptic neuron leads to the release of a chemical messenger called aneurotransmitter. The neurotransmitter then diffuses across the synapse and binds to receptors on the postsynaptic cell. Binding of the neurotransmitter leads to the production of an electrical signal in the postsynaptic cell. Why does the body have two types of synapses? Each type of synapse has functional advantages and disadvantages. An electrical synapse passes the signal very quickly, which allows groups of cells to act in unison. A chemical synapse takes much longer to transmit the signal from one cell to the next; however, chemical synapses allow neurons to integrate information from multiple presynaptic neurons, determining whether or not the postsynaptic cell will continue to propagate the signal. Neurons respond differently based on information transmitted by multiple chemical synapses. Let’s take a closer look at the structure and function of each type of synapse.
Electrical synapses transmit action potentials via the direct flow of electrical current at gap junctions. Gap junctions are formed when two adjacent cells have transmembrane pores that align. The membranes of the two cells are linked together and the aligned pores form a passage between the cells. Consequently, several types of molecules and ions are allowed to pass between the cells. Due to the direct flow of ions and molecules from one cell to another, electrical synapses allow bidirectional flow of information between cells. Gap junctions are crucial to the functioning of the cardiac myocytes and smooth muscles.
Chemical synapses comprise most of the synapses in your body. In a chemical synapse, a synaptic gapor cleft separates the pre- and the postsynaptic cells. An action potential propagated to the axon terminal results in the secretion of chemical messengers, called neurotransmitters, from the axon terminals. The neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor proteins on the cell membrane of the postsynaptic cell. Binding of the neurotransmitter to the receptors on the postsynaptic cell leads to a transient change in the postsynaptic cell’s membrane potential.
The process of synaptic transmission at a chemical synapse between two neurons follows these steps:
An action potential, propagating along the axon of a presynaptic neuron, arrives at the axon terminal.
The depolarization of the axolemma (the plasma membrane of the axon) at the axon terminal opens Ca2+channels and Ca2+ diffuses into the axon terminal. Ca2+ bind with calmodulin, the ubiquitous intracellular calcium receptor, causing the synaptic vesiclesto migrate to and fuse with the presynaptic membrane. The neurotransmitter is released into the synaptic cleft by the process of exocytosis. The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane. Binding of the neurotransmitters to the postsynaptic receptors causes a response in the postsynaptic cell.
The response can be of two kinds:
1. A neurotransmitter may bind to a receptor that is associated with a specific ion-channel which, when opened, allows for diffusion of an ion through the channel. If Na+ channels are opened, Na+ rapidly diffuses into the postsynaptic cell and depolarizes the membrane towards the threshold for an action potential. If K+ channels are opened, K+ diffuses out of the cell, depressing the membrane polarity below its resting potential (hyperpolarization). If Cl- channels are opened, Cl- moves into the cell leading to hyperpolarization. 2. The neurotransmitter may bind to a transmembrane receptor protein, causing it to activate a G-protein on the inside surface of the postsynaptic membrane. A cascade of events leads to the appearance
of a second messenger (calcium ion, cyclic AMP (cAMP), or IP3) in the cell. Second messengers can have diverse effect on the cell ranging from opening an ion channel to changing cell metabolism to initiating transcription of new proteins.