Synaptic Transmission: How Neurons Communicate Explained

Synaptic transmission is how a neuron passes a signal to another cell across a synapse. Most synapses in humans are chemical, using messenger molecules called neurotransmitters.

This matters because synapses allow control, learning and modulation that simple electrical continuity cannot. Most medicines for depression, anxiety, Parkinsons disease and pain act at synapses.

Key Concepts

• Presynaptic terminal: Axon end containing synaptic vesicles filled with neurotransmitter.
• Synaptic cleft: Narrow gap, about 20 to 30 nanometres, between presynaptic and postsynaptic membranes.
• Neurotransmitter: Chemical such as acetylcholine, glutamate, dopamine or serotonin that carries the signal across the cleft.
• Postsynaptic receptors: Proteins that bind transmitter and open ion channels or start cell signalling.
• Excitatory and inhibitory signals: Transmitters can depolarise the next cell (EPSP) or make firing less likely (IPSP).
• Reuptake and enzymatic breakdown: Methods that end the signal, by transporter pumps or enzymes such as acetylcholinesterase.

How It Works

1. An action potential arrives at the presynaptic terminal and opens voltage-gated calcium channels.
2. Calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter into the cleft.
3. Transmitter diffuses across and binds receptors on the postsynaptic membrane.
4. Ion channels open or close, producing a graded potential. If threshold is reached at the axon hillock, the postsynaptic neuron fires.
5. The signal is terminated by diffusion away, enzymatic destruction or reuptake into the presynaptic cell for reuse.

Worked Example

Suppose one terminal releases transmitter producing an excitatory postsynaptic potential of +0.5 millivolts. If the neuron rests at -70 millivolts and threshold is -55 millivolts, needed depolarisation = 15 millivolts. Number of simultaneous inputs needed ≈ 15 ÷ 0.5 = 30 active excitatory synapses if no inhibition occurs. This summation explains why single inputs rarely fire a cell alone and why integration of many signals controls decisions.

Common Misconceptions

• Synapses are physical welds between neurons: Wrong. A cleft remains; communication is usually chemical.
• One neurotransmitter always excites: Wrong. Effect depends on receptor type; the same transmitter can excite one target and inhibit another.
• More transmitter is always better: Wrong. Precise removal is essential; blocking breakdown too much causes overstimulation, as with some poisons.

Real-World Applications

Selective serotonin reuptake inhibitors increase serotonin availability in depression treatment. Neuromuscular blockers and botulinum toxin act at muscle synapses. Myasthenia gravis treatments increase acetylcholine action to improve strength.

In short, synaptic transmission converts an electrical impulse into a chemical message and back into an electrical response. This adjustable step is where the nervous system integrates information and where many therapies act.