Anatomy and Physiology 2e · The Nervous System and Nervous Tissue
Communication Between Neurons
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In 30 seconds
An action potential is a neuron's private electrical message, but neurons do not touch — they are separated by a tiny gap called the Synaptic cleft The narrow gap between presynaptic and postsynaptic membranes Full entry →. Communication Between Neurons is the study of how a signal crosses that gap and influences the next cell. The junction where this happens is the Synapse The functional junction between neurons Full entry →, and the process is synaptic transmission.
Two broad types of synapses exist. Electrical synapses connect cells directly through gap junctions, letting ions flow from one cell to the next almost instantly — useful where synchronized, rapid firing matters, such as in some smooth and cardiac muscle. Chemical synapses are far more common in the nervous system. An arriving action potential triggers the release of chemical messengers called neurotransmitters, which diffuse across the cleft and bind receptors on the postsynaptic cell, changing its membrane potential. The effect depends on which receptor is engaged: the postsynaptic cell may be nudged toward firing (excitatory) or away from it (inhibitory), and the final decision is made by summing all the inputs it receives at once.
Why this matters
The synapse is where the nervous system makes its decisions — and where most psychoactive and neurological drugs act. Antidepressants, anesthetics, and many pain medications work by altering Neurotransmitter A chemical messenger released by a neuron Full entry → release, Reuptake Pumping neurotransmitter back into the terminal Full entry →, or receptor binding, so understanding transmission is essential for anyone entering nursing, medicine, or pharmacology. Synaptic dysfunction is also central to disorders from depression and anxiety to Parkinson disease and myasthenia gravis (impaired signaling at the neuromuscular junction). On exams, synaptic transmission is a perennial favorite: expect EPSPs versus IPSPs, Summation Adding postsynaptic potentials in time or space Full entry →, and the sequence of events from action potential to neurotransmitter release.
The college version
Core Concepts
Anatomy of a chemical synapse
- Presynaptic terminal — the axon's swollen end, holding synaptic vesicles packed with neurotransmitter.
- Synaptic cleft — the narrow fluid-filled gap (commonly taught as ~20–40 nanometers) across which the signal travels.
- Postsynaptic membrane — the receiving cell's membrane, studded with receptors.
The postsynaptic "cell" may be a neuron, a muscle fiber (neuromuscular junction), or a gland cell.
The steps of chemical transmission
- An action potential arrives and depolarizes the presynaptic terminal.
- Depolarization opens voltage-gated Ca²⁺ channels.
- Calcium enters, triggering vesicles to fuse with the membrane (Exocytosis Vesicle fusion releasing contents outside the cell Full entry →), releasing neurotransmitter into the cleft.
- Neurotransmitter diffuses across and binds postsynaptic receptors.
- Binding opens or closes ion channels, changing the postsynaptic membrane potential.
- The signal is terminated: neurotransmitter is removed so the effect does not last forever.
The whole sequence takes well under a millisecond — fast enough for reflexes that must happen in a blink.
Receptors: the message decoders
The same neurotransmitter can produce different effects depending on the receptor:
- Ionotropic receptors are themselves ion channels (ligand-gated). Binding opens the channel directly — immediate, brief effects.
- Metabotropic receptors are not channels; binding triggers intracellular signaling cascades (often via G proteins) that indirectly alter channels or gene expression — slower but longer-lasting and amplifiable.
This is why one molecule can do many jobs: the nervous system reuses messengers with different receptors in different circuits.
EPSPs, IPSPs, and summation
Binding produces small, local voltage changes called postsynaptic potentials:
- An EPSP Excitatory postsynaptic potential — a depolarizing nudge Full entry → (excitatory postsynaptic potential) depolarizes the membrane toward threshold — firing becomes more likely.
- An IPSP Inhibitory postsynaptic potential — a hyperpolarizing nudge Full entry → (inhibitory postsynaptic potential) hyperpolarizes (or stabilizes) the membrane away from threshold — firing becomes less likely.
One EPSP is far too small to trigger an action potential alone, so neurons integrate inputs:
- Temporal summation: one presynaptic neuron fires rapidly, and successive EPSPs add before the first decays.
- Spatial summation: several presynaptic neurons fire together, and their EPSPs/IPSPs add across the membrane.
If the summed potential reaches threshold at the axon hillock (the trigger zone), an action potential fires. The neuron is a tiny voting machine: excitatory inputs vote "fire," inhibitory inputs vote "don't," and the tally at the trigger zone decides.
Clearing the signal
- Reuptake: transporters pump neurotransmitter back into the presynaptic terminal for recycling.
- Enzymatic degradation: enzymes in the cleft break the neurotransmitter into inactive pieces.
- Diffusion: neurotransmitter drifts away.
Drugs that block reuptake (many antidepressants) or block degradation (some treatments for myasthenia gravis) work by prolonging neurotransmitter action.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Synapse | Synaptic cleft | The synapse is the whole junction; the cleft is just the gap |
| EPSP | Action potential | An EPSP is small, graded, local, decaying; an action potential is large, all-or-none, propagating — EPSPs may trigger one, they are not one |
| Neurotransmitter | Hormone | Neurotransmitters act locally across a cleft in milliseconds; hormones travel in blood, acting broadly over seconds to hours |
| Temporal summation | Spatial summation | Temporal = same input, repeated over time; spatial = many inputs, simultaneous across the membrane |
| "Excitatory neurotransmitter" | "Always excites" | The effect depends on the receptor, not the molecule |
| Reuptake | Degradation | Reuptake = transporter pumps it back; degradation = enzymes destroy it in the cleft |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Neurons are like kids playing "telephone," but they can't whisper directly — there's a gap between them. So the first kid writes the message on a paper ball and throws it across. The second kid reads it and decides what to do: get excited and pass it along, or stay quiet. Lots of notes can arrive at once, and the kid only shouts onward if enough notes pile up (or one really fast stream). After reading, the notes go in the recycle bin so the message doesn't echo forever.
Worked example
Consider the knee-jerk (patellar) reflex:
- A tap stretches the muscle, exciting a sensory neuron.
- Its action potentials reach the spinal cord and release an excitatory neurotransmitter (acetylcholine is the classic example) onto a motor neuron.
- The motor neuron receives an EPSP; if the summed EPSPs reach threshold, it fires and the quadriceps contracts — the kick.
- Simultaneously, the sensory neuron excites an inhibitory interneuron, which releases an inhibitory neurotransmitter onto the opposing (hamstring) motor neuron. That neuron receives an IPSP and stays quiet, so the hamstring relaxes rather than fighting the quadriceps.
The circuit shows every key idea: Ca²⁺-triggered release, EPSPs and IPSPs, summation at the trigger zone, and receptor-dependent effects — and why inhibition matters: coordinated movement requires some muscles to be told "don't fire."
Key takeaways
- Order of events: action potential → Ca²⁺ entry → vesicle exocytosis → neurotransmitter release → receptor binding → postsynaptic potential → termination.
- Calcium is the trigger for release — no Ca²⁺ entry, no release.
- EPSP depolarizes toward threshold; IPSP moves away. Both are graded, local, and rapidly decaying.
- Temporal summation = one input, rapid repeats; spatial summation = many inputs, simultaneous. The axon hillock decides.
- Ionotropic receptors = fast, direct; metabotropic receptors = slower, longer-lasting, cascades.
- Signal termination is essential; drugs exploit reuptake and degradation to change synaptic strength.
- The receptor, not the neurotransmitter, determines excitation vs. inhibition.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
List, in order, the steps of chemical synaptic transmission.
Show answer
Action potential arrives → Ca²⁺ channels open → Ca²⁺ triggers exocytosis → neurotransmitter diffuses across the cleft → binds receptors, changing membrane potential → signal terminated by reuptake, degradation, or diffusion.
What ion must enter the presynaptic terminal for neurotransmitter release?
Show answer
Calcium (Ca²⁺); its entry is the trigger for vesicle fusion and release.
A neuron receives five simultaneous EPSPs from five different presynaptic neurons. What kind of summation is this?
Show answer
Spatial summation — multiple inputs arriving simultaneously from different neurons.
A drug prevents the enzyme that breaks down acetylcholine in the cleft. What happens to the postsynaptic effect?
Show answer
The acetylcholine stays in the cleft longer, so the postsynaptic effect is prolonged and strengthened (the logic behind certain myasthenia gravis treatments).
Why can the same neurotransmitter excite one synapse and inhibit another?
Show answer
Because the effect is determined by the receptor and the ion channels it controls, not by the molecule: a receptor that opens Na⁺ channels is excitatory; one that opens Cl⁻ or K⁺ channels is inhibitory.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Synapse
- The functional junction between neurons
- Synaptic cleft
- The narrow gap between presynaptic and postsynaptic membranes
- Neurotransmitter
- A chemical messenger released by a neuron
- Synaptic vesicle
- A sac that stores and releases neurotransmitter
- Exocytosis
- Vesicle fusion releasing contents outside the cell
- Ionotropic receptor
- A receptor that is itself an ion channel
- Metabotropic receptor
- A receptor that triggers intracellular signaling cascades
- EPSP
- Excitatory postsynaptic potential — a depolarizing nudge
- IPSP
- Inhibitory postsynaptic potential — a hyperpolarizing nudge
- Summation
- Adding postsynaptic potentials in time or space
- Reuptake
- Pumping neurotransmitter back into the terminal
Sources & references
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