Synaptic Transmission

Synaptic Transmission

Synapses are specialized cell-to-cell contacts that allow the information encoded by action potentials to pass to another cell. Synapses occur at the junction between the processes of two neurons or between a neuron and an effector cell (e.g., a muscle or gland).

There are two types of synapses:

1. Electrical synapses.

2. Chemical synapses.

 Electrical synapses occur where two cells are joined by gap junctions, which conduct current from cell-to-cell via nonselective pores. Cardiac muscle is an example of cells that are electrically coupled via gap junctions.

 Chemical synapses involve the release of a chemical transmitter by one cell that acts upon another cell. Action potentials in a presynaptic cell cause the release of the chemical transmitter, which crosses a narrow cleft to interact with specific receptors on a postsynaptic cell.

 Similar features are present at synapses:

1.            Presynaptic axon terminals store neurotransmitter molecules in vesicles.
2.            Action potentials in the axon depolarizes the axon terminals.
3.            Depolarization causes Ca2+ entry into the axon terminal.
4.            Exocytosis of vesicles occurs, causing release of neurotransmitter into the narrow synaptic cleft.
5.            Neurotransmitter interacts with a specific receptor on the postsynaptic membrane, causing a postsynaptic event. Excitatation occurs when neurotransmitters depolarize the postsynaptic membrane due to opening of Na+ or Ca2+ cannels, inhibition is hyperpolarization the postsynaptic membrane due to opening of K+ or Cl- cannels.
6.             Neurotransmitter signaling is terminated by removal from the synaptic cleft through diffusion, enzymatic degradation, or active reuptake of neurotransmitter molecules.

           The neuromuscular junction connects the nervous system to the muscular system via synapses between efferent nerve fibers and muscle fibers. As an action potential reaches the end of a motor neuron, voltage-dependent calcium channels open allowing calcium to enter the neuron. Calcium binds to sensor proteins on synaptic vesicles fusion with plasmamembrane and subsequent neurotransmitter release from the motor neuron into the synaptic cleft. In vertebrates, motor neurons release acetylcholine, a small molecule neurotransmitter, which diffuses through the synaptic cleft and binds nicotinic acetylcholine receptors  on the plasma membrane of the muscle fiber. The binding of acetylcholine to the receptor can depolarize the muscle fiber, causing a cascade that eventually results in muscle contraction.