Synapses are the junctions where neurons communicate with other neurons, muscle cells, or gland cells. This cheat sheet helps students connect action potentials to chemical signaling and understand how information moves through the nervous system. It is useful for reviewing nervous system physiology, cell communication, and the biological basis of behavior.
Grade 11-12 students need these ideas for topics such as reflexes, drugs, learning, and neurological disorders.
The core process begins when an action potential reaches the axon terminal and opens voltage-gated calcium channels. Calcium ions trigger vesicles to release neurotransmitters into the synaptic cleft, where they bind to receptors on the postsynaptic membrane. Excitatory signals make a postsynaptic neuron more likely to fire, while inhibitory signals make it less likely to fire.
Signal strength depends on receptor type, neurotransmitter removal, and the combined effects of many synapses.
Key Facts
- A chemical synapse includes the presynaptic terminal, synaptic cleft, and postsynaptic membrane.
- An action potential reaching the axon terminal opens voltage-gated Ca2+ channels, causing Ca2+ to enter the presynaptic neuron.
- Ca2+ influx triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter by exocytosis.
- Neurotransmitters bind to specific receptors, so the effect depends on both the chemical messenger and the receptor type.
- An excitatory postsynaptic potential, or EPSP, depolarizes the postsynaptic membrane and moves membrane potential closer to threshold.
- An inhibitory postsynaptic potential, or IPSP, hyperpolarizes the postsynaptic membrane or stabilizes it below threshold.
- Neurotransmitter action is ended by reuptake, enzymatic breakdown, or diffusion away from the synaptic cleft.
- Temporal summation adds signals that arrive close together in time, while spatial summation adds signals from different synapses.
Vocabulary
- Synapse
- A specialized junction where a neuron communicates with another neuron, muscle cell, or gland cell.
- Neurotransmitter
- A chemical messenger released by a presynaptic neuron that affects a postsynaptic cell.
- Synaptic cleft
- The tiny gap between the presynaptic terminal and the postsynaptic membrane.
- Receptor
- A protein on the postsynaptic membrane that binds a specific neurotransmitter and starts a cellular response.
- Reuptake
- The process in which neurotransmitter molecules are transported back into the presynaptic neuron.
- Summation
- The adding together of postsynaptic potentials to determine whether the neuron reaches threshold.
Common Mistakes to Avoid
- Saying neurotransmitters cross directly through the postsynaptic membrane is wrong because they usually bind to receptors on the membrane instead.
- Forgetting the role of Ca2+ is wrong because calcium entry is the trigger that causes vesicles to release neurotransmitter.
- Assuming every neurotransmitter is always excitatory is wrong because the effect depends on the receptor and can be excitatory or inhibitory.
- Confusing reuptake with receptor binding is wrong because reuptake removes neurotransmitter from the cleft, while receptor binding starts the postsynaptic response.
- Treating one small EPSP as enough to fire an action potential is wrong because the axon hillock usually requires summed input to reach threshold.
Practice Questions
- 1 A neuron has a resting membrane potential of -70 mV and a threshold of -55 mV. If EPSPs of +5 mV, +4 mV, and +6 mV arrive together, does the neuron reach threshold?
- 2 A postsynaptic neuron is at -68 mV. An IPSP changes the membrane potential by -7 mV. What is the new membrane potential?
- 3 List the correct order of events from an action potential arriving at the axon terminal to neurotransmitter removal from the synaptic cleft.
- 4 Explain why the same neurotransmitter can produce different effects in different target cells.
Understanding Synapses & Neurotransmitters
Neurotransmitter release is not an all-or-nothing message in the way an action potential is. A stronger pattern of action potentials usually causes more calcium to enter the terminal over time. This raises the chance that more vesicles will fuse.
Each vesicle releases a small packet, often called a quantum, of transmitter. The receiving cell detects the total effect of many packets. This helps explain why neurons can pass graded information about light intensity, touch pressure, or muscle stretch even though individual action potentials have a fixed size.
Receptors do more than detect a chemical. Some receptors are ion channels that open almost immediately when a transmitter binds. These produce fast changes in membrane voltage and are important in rapid reflex pathways.
Other receptors start chemical changes inside the cell through proteins and second messengers. Their effects begin more slowly but can last longer.
They may alter ion channels, enzyme activity, or gene expression. A single neurotransmitter can therefore excite one cell and inhibit another cell if those cells carry different receptor types.
Summation usually happens near the axon hillock, the region where the neuron decides whether to produce an action potential. Excitatory inputs do not guarantee firing. Inhibitory inputs can cancel or reduce their effect.
The location of a synapse matters too. Inputs close to the axon hillock usually have more influence than equally strong inputs far out on dendrites.
Inhibition near the cell body or axon hillock can be especially powerful because it can prevent excitatory currents from bringing the membrane to threshold. Neural activity is therefore the result of constant calculation across thousands of inputs.
Removal of neurotransmitter is essential for clear signaling. Reuptake transports transmitter back into the releasing neuron or into nearby support cells called glia. The recovered molecules may be reused to make new vesicle contents.
Enzymes break down certain transmitters, producing inactive products. If transmitter remained in the cleft too long, receptors would stay active and the next message would be harder to distinguish.
Many medicines change synaptic signaling by blocking reuptake, blocking receptors, or changing transmitter breakdown. This can help with disease, yet it can also cause side effects because the same transmitter may act in many brain regions and body tissues.
When studying diagrams, track the direction of each event carefully. Calcium enters the presynaptic terminal, while neurotransmitter moves across the cleft and receptors sit on the target cell. Do not assume that a named neurotransmitter has only one effect.
Focus on the receptor and the ions or internal pathway it controls. It also helps to separate a short electrical event from the longer chemical events it triggers. This distinction makes drug effects, synaptic fatigue, learning, and disorders such as epilepsy easier to understand.