Atlas MCAT Course
Systems & Physiology → Synaptic Transmission
Synaptic Transmission
Turning electrical signals into computation.
01The idea
Action potentials travel within a neuron. They cannot jump gaps or choose targets on their own. To build circuits, the nervous system needs directionality, selectivity, and modulation. This is the role of the synapse.
Transmission becomes computation.
THREAD · Signal vs. response
02Build the model
AElectrical → chemical → electrical
At most synapses, signaling follows a three-step sequence. Each step adds control.
BPresynaptic events: calcium is the trigger
When an action potential reaches the terminal, the membrane depolarizes, voltage-gated Ca²⁺ channels open, and Ca²⁺ enters down its steep electrochemical gradient. This calcium influx triggers vesicle fusion and causes neurotransmitter release.
CNeurotransmitter release: probabilistic, not guaranteed
Vesicle fusion is not automatic: it depends on Ca²⁺ concentration and is regulated by specialized proteins. This introduces variability, modulation, and plasticity.
DPostsynaptic receptors: interpreting the message
Neurotransmitters bind to receptors on the postsynaptic membrane. Two broad receptor classes:
| Ionotropic receptors | Metabotropic receptors |
|---|---|
| Ligand-gated ion channels | G-protein coupled |
| Fast responses | Slower responses |
| Direct electrical effects | Indirect, amplified effects |
EExcitation vs. inhibition: direction matters
Excitatory postsynaptic potentials (EPSPs) depolarize the membrane and increase the probability of firing. Inhibitory postsynaptic potentials (IPSPs) hyperpolarize or stabilize the membrane and decrease the probability of firing.
FGraded potentials: analog signals
Unlike action potentials, synaptic potentials are graded: their magnitude varies with stimulus strength, and they decay with distance. This allows fine control and integration of multiple inputs.
GSummation: how neurons decide
A neuron fires based on the net effect of all synaptic inputs. In temporal summation, repeated inputs over time add together. In spatial summation, inputs from different synapses combine.
HThe axon hillock: decision point
The axon hillock has a high density of voltage-gated Na⁺ channels and serves as the integration zone. If summed inputs reach threshold, an action potential fires; if they don’t, there is no signal. This converts analog input into digital output.
ITermination: why signals must stop
Neurotransmitter action ends via reuptake, enzymatic breakdown, or diffusion away. This ensures temporal precision and prevents runaway signaling.
JSpeed vs. precision: synaptic tradeoffs
Chemical synapses are slower than electrical coupling, but allow amplification and regulation. Electrical synapses are rare, extremely fast, and less flexible.
03See the connections
The lesson names these links itself. Read the map left to right.
Presynaptic
- AP → Ca²⁺ entry
- Vesicle release (probabilistic)
Postsynaptic
- Receptor decides the effect
- EPSPs and IPSPs (graded)
Decision
- Summation at the axon hillock
- Termination: reuptake, breakdown, diffusion
Next: neural circuits.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A drug blocks voltage-gated Ca²⁺ channels at the axon terminal. What happens?
Think first.
The same neurotransmitter excites one cell and inhibits another. How is that possible?
Think first.
One strong excitatory synapse is active, but many inhibitory synapses are active at the same time. Does the neuron fire?
Think first.
A drug blocks neurotransmitter reuptake. What happens to signaling at that synapse?
05MCAT lens
Predict the direction
Blocking Ca²⁺ channels → no neurotransmitter release
Increasing inhibitory input → reduced firing probability
Increasing excitatory frequency → temporal summation
Blocking reuptake → prolonged signaling
No memorization required — just logic.
06Common traps
Trap 1: Strong synapse guarantees firing
Misconception
Net input matters.
Correct model
Integration is cumulative.
Trap 2: Neurotransmitter identity defines effect
Misconception
Receptor determines outcome.
Correct model
Same signal, different meaning.
Trap 3: Synaptic signals are all-or-none
Misconception
They are graded.
Correct model
Only action potentials are digital.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
How action potentials trigger neurotransmitter release
Check against 02 · A and B: depolarization opens Ca²⁺ channels.
Why Ca²⁺ is essential
Check against 02 · B: calcium is the permission signal.
How excitation and inhibition differ
Check against 02 · D and E: receptor type sets EPSP or IPSP.
How neurons integrate information
Check against 02 · F through H: graded inputs summed at the hillock.
If something feels unclear, the missing idea is usually thisSynapses turn signals into decisions by integrating many graded inputs.
08Remember
REMEMBER · if you keep only a few things
- Electrical → chemical → electrical; each step adds control.
- Ca²⁺ entry is the permission signal for release.
- Receptors, not transmitters, decide excitation vs inhibition.
- Synaptic potentials are graded; only APs are digital.
- The axon hillock sums inputs against threshold.
- Reuptake, breakdown, and diffusion end the signal.
09Forward hook
Now that you understand single-neuron signaling, the next question is:
How do neurons organize into functional circuits that control behavior and physiology?
That leads directly to neural circuits, reflex arcs, and central vs peripheral organization.
Next lessonNeural Circuits→