Synapses are tiny communication points where one neuron passes a signal to another neuron, muscle cell, or gland cell. They matter because every thought, reflex, memory, sensation, and movement depends on signals crossing these gaps accurately. In a chemical synapse, an electrical signal in the presynaptic neuron is converted into a chemical message, then converted back into an electrical or cellular response in the postsynaptic cell.
When an action potential reaches the axon terminal, voltage-gated calcium channels open and Ca2+ enters the presynaptic terminal. Calcium causes synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft by exocytosis. Neurotransmitters bind to specific receptors on the postsynaptic membrane, producing excitatory or inhibitory effects depending on the receptor and ion flow.
The signal is then stopped by reuptake, enzymatic breakdown, or diffusion away from the synapse.
Key Facts
- A synapse is the junction where a neuron communicates with another cell.
- An action potential reaching the axon terminal opens voltage-gated Ca2+ channels.
- Ca2+ influx triggers vesicle fusion and neurotransmitter release by exocytosis.
- Neurotransmitters cross the synaptic cleft and bind to specific postsynaptic receptors.
- Excitatory postsynaptic potentials make firing more likely, while inhibitory postsynaptic potentials make firing less likely.
- Net postsynaptic effect = total excitation - total inhibition.
Vocabulary
- Synapse
- A synapse is the specialized junction where a neuron sends a signal to another neuron or target cell.
- Neurotransmitter
- A neurotransmitter is a chemical messenger released by a neuron that binds to receptors on a target cell.
- Synaptic cleft
- The synaptic cleft is the narrow gap between the presynaptic terminal and the postsynaptic membrane.
- Receptor
- A receptor is a protein that binds a specific neurotransmitter and triggers a response in the postsynaptic cell.
- Reuptake
- Reuptake is the process in which neurotransmitters are transported back into the presynaptic neuron for reuse or breakdown.
Common Mistakes to Avoid
- Thinking neurotransmitters are electrical signals. Neurotransmitters are chemical messengers, while action potentials are electrical changes along the neuron membrane.
- Assuming every neurotransmitter is always excitatory or always inhibitory. The effect depends on the receptor type and the ions or pathways it controls.
- Forgetting the role of calcium ions. Ca2+ entry into the presynaptic terminal is the trigger that causes vesicles to release neurotransmitter.
- Ignoring how the signal stops. Neurotransmitters must be removed by reuptake, enzyme breakdown, or diffusion so the postsynaptic cell does not keep responding.
Practice Questions
- 1 A presynaptic terminal contains 80 vesicles, and an action potential causes 25% of them to release neurotransmitter. How many vesicles release their contents?
- 2 A neurotransmitter concentration in the synaptic cleft falls from 120 units to 30 units after reuptake and breakdown. What percent of the original neurotransmitter remains?
- 3 A postsynaptic neuron receives three excitatory inputs and two inhibitory inputs at nearly the same time. Explain how the neuron decides whether it is likely to fire an action potential.