Biology · Nervous Systems, Senses & Movement

Membrane potentials, action potentials and signal conduction

A small region of an axon changes voltage and helps the next region change voltage. The signal can travel much farther than any one sodium ion entering at the first region. Separate the local movement of charge from the advancing pattern.

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.

Back to the Biology map

By the end, you can

Explain voltage across a membrane, selective ion currents and regenerative propagation without treating a signal as an ion travelling the full axon.

Membrane voltage is a difference between two sides

Membrane potential is the electrical potential inside a cell relative to the outside. We use inside minus outside: a value of −70 millivolts means the inside is seventy millivolts lower. It does not mean the cell contains only negative ions. A small charge separation near the membrane can coexist with nearly electrically neutral bulk solutions.

Ions have concentration and electrical gradients. Their combined electrochemical driving conditions, together with open pathways, determine movement. In the selected resting comparison, K+ is more concentrated inside and Na+ outside; at the assigned voltage, opening the displayed potassium route permits net outward movement, while the displayed sodium route permits inward movement.

Ion pumps help maintain gradients over time, and channels provide selected passive routes. A resting membrane potential is not zero biological activity or thermodynamic equilibrium. The model assigns −70 mV and directional conditions, but no concentrations, equilibrium-potential calculation or universal resting value.

Voltage and conditional ion routes

A signed voltage does not mean that the bulk cell contains only one charge sign.

Read the diagram in words

The assigned inside-minus-outside difference is −70 mV. Na+ is relatively higher outside and K+ inside. The selected open routes permit Na+ inward and K+ outward under the stated electrochemical conditions; no numerical concentration-to-voltage calculation is supplied.

Assigned geometry
Compartment	Assigned x coordinate	Assigned y coordinate	Assigned width	Assigned height
Extracellular solution	0	0	10	2
Cell interior	0	2	10	3

Selected membrane: 2

Assigned geometry
Displayed structure	Assigned x coordinate	Assigned y coordinate
Outside Na+	2	1
Inside Na+	2	3
Inside K+	6	3
Outside K+	6	1
ATP-dependent gradient maintenance	9	4

Selected routes and effects
From	To	Selected relationship	Direction	Passage through	Selected condition	Incoming symbol	Released symbol
Outside Na+	Inside Na+	Na+ inward under the stated conditions	Directed route	(2, 2)	—	—	—
Inside K+	Outside K+	K+ outward under the stated conditions	Directed route	(6, 2)	—	—	—

Ion
Ion	Symbol	Inside distribution	Outside distribution
Sodium ions	Na+	Lower selected concentration	Higher selected concentration
Potassium ions	K+	Higher selected concentration	Lower selected concentration

Selected membrane route
Displayed structure	Assigned x coordinate	Assigned y coordinate
Selected open sodium route	2	2
Selected open potassium route	6	2

Inside minus outside: -70 mV

Selected routes and effects
From	To	Selected membrane route	Selected relationship
Outside Na+	Inside Na+	Selected open sodium route	Na+ inward under the stated conditions
Inside K+	Outside K+	Selected open potassium route	K+ outward under the stated conditions

Model conditions: One selected resting comparison with assigned voltage −70 mV, qualitative distributions and stated electrochemical directions. No concentrations, equilibrium potentials or pump-cycle totals assigned.

An action potential is a timed change in conductance

In a selected axon, sufficient depolarisation recruits voltage-dependent channels and produces an action potential. Depolarisation makes the inside less negative relative to the outside. Sodium entry contributes to the rising phase in this example; sodium-channel inactivation and increasing potassium conductance contribute to the falling phase.

Channels change state on different time courses. Continued potassium conductance can produce an after-hyperpolarisation before the voltage returns toward rest. Pumps maintain the underlying gradients but are not a piston that generates the rapid downward stroke of each spike. Other excitable cells can use different channels and waveforms.

A refractory period describes reduced readiness for another action potential after one occurs. Sodium-channel inactivation contributes to an absolute refractory phase; later conditions can require stronger input. It is inaccurate to say every after-hyperpolarised moment forbids all firing, or that every sodium channel in the cell opens simultaneously.

One constructed action-potential record

The undershoot is not a declaration that every such moment absolutely forbids another spike.

Read the diagram in words

At 0–6 ms the assigned samples are −70, −70, −50, +30, −70, −80 and −70 mV. The initial-to-threshold rise is 20 mV and initial-to-peak rise 100 mV. Channel-state explanations are selected qualitative mechanisms, not calculated currents.

Time in the assigned record
Minimum	Maximum	Axis ticks	Unit
0	6	0, 1, 2, 3, 4, 5, 6	ms

Inside-minus-outside voltage
Minimum	Maximum	Axis ticks	Unit
-90	40	-90, -70, -50, 0, 30, 40	mV

Sample time
Sample time (ms)	Voltage (mV)	Selected event
0	-70	Initial rest
1	-70	Before the assigned rise
2	-50	Assigned threshold reached
3	30	Assigned positive peak
4	-70	Falling phase reaches resting value
5	-80	Selected undershoot
6	-70	Return toward rest

Selected phase
Selected phase	Start	End	Unit	Selected mechanism
Depolarising phase	1	3	ms	Increasing selected inward sodium conductance
Repolarisation and undershoot	3	5	ms	Sodium inactivation and selected outward potassium conductance
Recovery toward rest	5	6	ms	Changing channel availability and conductance

Assigned rest: −70 mV: -70
Assigned threshold: −50 mV: -50

Model conditions: Constructed samples and qualitative phases of one selected sodium/potassium-based axonal event. Not a measured waveform, a biophysical differential-equation solution or a universal threshold.

Regeneration and local current cooperate

Local current associated with one active region changes neighbouring membrane voltage. Regenerative channel activity can then sustain the signal along an axon. The ordinary illustrated event proceeds forward because its initiation and the recent state of the membrane provide different conditions ahead and behind; the cell is not a literal one-way metal wire.

In a selected myelinated axon, internodal wrapping alters electrical spread and active regeneration is concentrated at exposed nodes. Saltatory conduction names this arrangement. Current still connects intervening regions; no sodium ion or action potential is magically teleported across an empty space.

The constructed nodal record places nodes at 0, 2 and 4 mm, with event times 0, 1 and 2 ms. Its assigned average propagation speed is 2 mm per ms, equivalent to 2 metres per second. These values are not measurements or a comparison isolating myelin from diameter, temperature and channel organisation.

Regeneration at selected exposed nodes

Regenerated signalling and long-distance movement of one entering ion are different descriptions.

Read the diagram in words

Nodes at 0, 2 and 4 mm have event times 0, 1 and 2 ms. Four millimetres in two milliseconds gives 2 m/s. Wrapping lies between the nodes, with a continuous axonal current route rather than an empty jump.

Distance from first node
Minimum	Maximum	Axis ticks	Unit
0	4	0, 1, 2, 3, 4	mm

Exposed node
Exposed node	Assigned position (mm)	Assigned event time (ms)
Node 1	0	0
Node 2	2	1
Node 3	4	2

Displayed internode
Displayed internode	Start	End	Unit
Selected myelinated internode	0	2	mm
Selected myelinated internode	2	4	mm

Selected routes and effects
From	To	Selected relationship
Node 1	Node 2	Local current precedes regeneration at the next node
Node 2	Node 3	Local current precedes regeneration at the next node

One continuous selected axon: 0–4 mm
Distance (mm): 4
Elapsed time (ms): 2
Propagation speed (m/s): 2

Model conditions: One schematic axon with three assigned positions and event times. No measured myelin effect, diameter, temperature, ionic flux or complete circuit latency assigned.

Worked example

Read signs and calculate a conditional speed

The assigned voltage rises from −70 to −50 mV and later to +30 mV. The first change is +20 mV and the full rise is +100 mV. Both are changes in a signed difference; neither says that one hundred millivolts of sodium travelled along the axon.

The event takes 2 ms to reach the node 4 mm from its starting node: 4/2=2 mm/ms. Converting both units gives 0.004 m/0.002 s=2 m/s. Nothing in that result establishes a species's conduction speed or the time required for a complete sensory-to-motor response.

Check your understanding

Does −70 mV imply that all ions inside are negatively charged?
Show reasoning
No. It is an inside-minus-outside voltage difference arising from a small charge separation; the bulk fluids still contain both positive and negative ions.
What drives the falling phase in the selected action potential?
Show reasoning
Changing channel conductances, including sodium inactivation and potassium outward current, contribute. ATP-dependent pumps maintain gradients over longer operation rather than directly producing each rapid falling phase.
What is the assigned average nodal propagation speed?
Show reasoning
Four millimetres in two milliseconds gives 2 mm/ms, or 2 m/s. This is a constructed propagation record, not an empirical or whole-circuit latency.

What to carry forward

An electrical signal is a changing local membrane state propagated through connected regions, with ion movement and energy use at distinct scales.

References

Resources supporting this lesson’s explanations and examples.

Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e 35.1 Neurons and Glial Cells 35.2 How Neurons Communicate 36.1 Sensory Processes 36.3 Taste and Smell 36.4 Hearing and Vestibular Sensation 36.5 Vision 38.1 Types of Skeletal Systems 38.4 Muscle Contraction and Locomotionopenstax.org
Neuroscience · second editionD. Purves, G. J. Augustine, D. Fitzpatrick and colleagues, editorsNeuroscience textbook · 2001Electrical Signals of Nerve Cells Synaptic Transmissionncbi.nlm.nih.gov
Neuroscience Online · Resting Potentials and Action PotentialsJohn H. Byrne · Department of Neurobiology and Anatomy, McGovern Medical School at UTHealth HoustonInstitutional neuroscience textbook · chapter revised January 2026nba.uth.tmc.eduRead source