Biology · Nervous Systems, Senses & Movement
Synapses, neurotransmitters and neural integration
A receiving neuron can have two excitatory inputs yet fail to generate an action potential when another input alters the combined response. Naming one neurotransmitter is therefore less informative than locating its receptor, effect and timing.
Cells, Membranes & Organisation supplies boundaries, ion gradients and transport. Gene Regulation & Cell Signalling introduces receptors and regulatory relationships; Homeostasis, Nutrition & Biological Organisation supplies coordinated physiology and energy accounts. This chapter reintroduces voltage and neural vocabulary before using them. Physics’s Electricity and Magnetism and Optics and Photonics are optional return paths for electrical and lens relations; Chemistry’s Solutions, Concentration & Solubility supports the distinction between ion amount and distribution.
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Trace selected chemical and electrical communication routes and reason about graded inputs under explicitly stated integration rules.
A chemical message changes the receiving cell
A synapse is a specialised site of communication. In the selected chemical synapse, an arriving axonal event changes terminal voltage, opens calcium channels and allows Ca2+ to enter. That increase promotes vesicle fusion and transmitter release. The transmitter then reaches receptors across the extracellular cleft.
The receptor can alter ion-channel activity directly or through intracellular signalling. In the fast selected route, ion movement across the receiving membrane produces a graded postsynaptic potential. The neurotransmitter does not need to enter through the conducting pore itself. Transmitter release, receptor binding and postsynaptic ion current are different events.
Electrical synapses instead connect cellular interiors through gap-junction channels, allowing ionic current between cells. Some permit substantial signalling in both directions; others favour one direction. The electrical panel is one symmetric teaching connection, not a statement that every such junction has equal coupling.
Chemical and electrical synaptic routes
Read the diagram in words
In the chemical panel, terminal calcium entry promotes vesicular release. Transmitter binds a receiving receptor, which changes channel activity; receiving ion movement is a separate route. The electrical panel permits direct current in both directions through its selected junction.
| Compartment | Assigned x coordinate | Assigned y coordinate | Assigned width | Assigned height |
|---|---|---|---|---|
| Presynaptic terminal | 0 | 0 | 10 | 2 |
| Extracellular synaptic cleft | 0 | 2 | 10 | 1 |
| Postsynaptic cell | 0 | 3 | 10 | 2 |
| Displayed structure | Assigned x coordinate | Assigned y coordinate |
|---|---|---|
| Arriving axonal event | 0.5 | 1 |
| Extracellular Ca2+ | 2 | 2.5 |
| Terminal Ca2+ rise | 2 | 1 |
| Transmitter in vesicle | 4 | 1 |
| Released transmitter | 4 | 2.5 |
| Postsynaptic receptor | 5 | 3 |
| Selected ion channel | 8 | 3 |
| Selected extracellular ions | 8 | 2.5 |
| Selected postsynaptic ions | 8 | 4 |
| Graded postsynaptic response | 5 | 4 |
| From | To | Selected relationship | Direction | Passage through | Selected condition | Incoming symbol | Released symbol |
|---|---|---|---|---|---|---|---|
| Arriving axonal event | Terminal Ca2+ rise | Terminal voltage permits selected calcium entry | Directed route | — | — | — | — |
| Extracellular Ca2+ | Terminal Ca2+ rise | Ca2+ crosses the terminal membrane | Directed route | (2, 2) | — | — | — |
| Terminal Ca2+ rise | Transmitter in vesicle | Calcium promotes release | Directed route | — | — | — | — |
| Transmitter in vesicle | Released transmitter | Transmitter released to the cleft | Directed route | — | — | — | — |
| Released transmitter | Postsynaptic receptor | Transmitter reaches and binds receptor | Directed route | — | — | — | — |
| Postsynaptic receptor | Selected ion channel | Binding changes selected channel activity | Directed route | — | — | — | — |
| Selected extracellular ions | Selected postsynaptic ions | Selected ion movement across the receiving membrane | Directed route | (8, 3) | — | — | — |
| Selected ion channel | Graded postsynaptic response | Changed conductance contributes to the graded response | Directed route | — | — | — | — |
| Compartment | Assigned x coordinate | Assigned y coordinate | Assigned width | Assigned height |
|---|---|---|---|---|
| Cell 1 interior | 0 | 0 | 3 | 4 |
| Cell 2 interior | 4 | 0 | 3 | 4 |
| Assigned x coordinate | Assigned y coordinate | Assigned width | Assigned height |
|---|---|---|---|
| 3 | 1.6 | 1 | 0.8 |
| Displayed structure | Assigned x coordinate | Assigned y coordinate |
|---|---|---|
| Cell 1 | 1.5 | 2 |
| Cell 2 | 5.5 | 2 |
| From | To | Selected relationship | Direction | Passage through | Selected condition | Incoming symbol | Released symbol |
|---|---|---|---|---|---|---|---|
| Cell 1 | Cell 2 | Current can pass from cell 1 to cell 2 | Directed route | — | — | — | — |
| Cell 2 | Cell 1 | Current can pass from cell 2 to cell 1 | Directed route | — | — | — | — |
- Model conditions
- One fast chemical route and one symmetric electrical junction. No transmitter identity, sign of postsynaptic effect, delay, current or universal junction symmetry assigned.
Excitatory and inhibitory describe an effect in context
An excitatory effect increases the receiving neuron's likelihood of firing under the relevant conditions; an inhibitory effect reduces it. The effect depends on receptor type, ion gradients, membrane state and circuit location. One chemical name cannot guarantee the same outcome at every target.
An inhibitory conductance need not make voltage more negative. It can reduce the influence of another input by changing the membrane's electrical conductance, sometimes called shunting inhibition. Our arithmetic example uses a negative voltage contribution for one selected inhibitory input; that is a simplified effect, not a definition of every inhibitory synapse.
Postsynaptic potentials are graded and generally decay with time and distance. Inputs arriving close enough together can interact. Spatial summation combines selected inputs from different locations; temporal summation combines successive contributions that still overlap. An arriving spike and its receiving graded potential are not interchangeable units.
Use the assigned endpoint, not a universal addition law
The constructed integration point begins at −65 mV with threshold −50 mV. Each displayed excitatory input contributes +8 mV at the selected endpoint, and one inhibitory input contributes −6 mV. The rule adds those assigned endpoint effects without modelling conductance changes, dendritic geometry or feedback from an emitted spike.
Two coincident excitatory effects give −49 mV and meet the rule's threshold. Adding the inhibitory effect gives −55 mV and does not. A later second excitatory event can have a smaller first-event contribution remaining; if that remainder is +2 mV, the total is −55 mV rather than the coincident −49.
Real neurons integrate conductances and currents in space and time, with thresholds and effects that can vary. The displayed arithmetic isolates the importance of coincidence and opposition. It does not identify an optimal stimulus, predict a person's response or imply that a neuron's emitted action potentials have different sizes according to this sum.
Inputs at the same integration endpoint
Read the diagram in words
From −65 mV, one +8 contribution gives −57; two +8 contributions give −49; adding −6 gives −55. A residual +2 and a new +8 also give −55. Only the −49 endpoint meets the assigned −50 threshold.
| Minimum | Maximum | Axis ticks | Unit |
|---|---|---|---|
| -70 | -40 | -70, -65, -60, -55, -50, -45, -40 | mV |
| Assigned case | Starting voltage (mV) | Selected excitatory input 1 (mV) | Selected excitatory input 2 (mV) | Selected inhibitory input (mV) | Endpoint voltage (mV) | Assigned threshold | Threshold comparison |
|---|---|---|---|---|---|---|---|
| One excitatory contribution | -65 | 8 | 0 | 0 | -57 | -50 | Threshold not met |
| Two coincident excitatory contributions | -65 | 8 | 8 | 0 | -49 | -50 | Threshold met |
| Two excitatory contributions plus inhibition | -65 | 8 | 8 | -6 | -55 | -50 | Threshold not met |
| Second event after the first contribution decays | -65 | 2 | 8 | 0 | -55 | -50 | Threshold not met |
| Contribution at this endpoint | Contribution at this endpoint (mV) |
|---|---|
| Selected excitatory input 1 | 8 |
| Selected excitatory input 2 | 8 |
| Selected inhibitory input | -6 |
- Selected relation
- Endpoint voltage = baseline + assigned contributions
- Selected condition
- The negative contribution is one selected inhibitory case
- Model conditions
- Original instantaneous additive endpoint comparison. Not a conductance-based neuron model or a rule for every inhibitory effect.
Worked example
Include every contribution at the stated moment
With both excitatory inputs and the selected inhibitory input present, the model gives −65+8+8−6=−55 mV. That remains 5 mV below the assigned threshold. With the inhibitory contribution absent, −65+8+8=−49 mV crosses it.
For separated events, use the specified decayed first contribution: −65+2+8=−55 mV. Two event names alone are insufficient. Their values at the same integration endpoint, not their original sizes at different moments, determine the model result.
Check your understanding
Does neurotransmitter release mean that the transmitter passes through the postsynaptic ion channel?
Show reasoning
No. It reaches and binds a receptor; the selected receptor changes ion conductance. Transmitter movement and the resulting ion current are distinct routes.
What does the three-input endpoint give?
Show reasoning
−65+8+8−6=−55 mV, below the assigned −50 mV threshold. The selected rule therefore gives no spike at that endpoint.
Must an inhibitory effect always hyperpolarise the receiving cell?
Show reasoning
No. Inhibition can reduce responsiveness by changing conductance without requiring a more negative voltage. The negative contribution here is one simplified assigned case.
What to carry forward
Trace the communication route, then preserve receptor context, locations and timing when interpreting integration.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e35.1 Neurons and Glial Cells35.2 How Neurons Communicate36.1 Sensory Processes36.3 Taste and Smell36.4 Hearing and Vestibular Sensation36.5 Vision38.1 Types of Skeletal Systems38.4 Muscle Contraction and Locomotion
- Neuroscience · second editionD. Purves, G. J. Augustine, D. Fitzpatrick and colleagues, editorsNeuroscience textbook · 2001Electrical Signals of Nerve CellsSynaptic Transmission