15.1.5
Action Potentials
Resting Potential
Resting Potential
When a neurone has not been stimulated, it is at resting state.


Potential difference
Potential difference
- At resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.
- This is because there are more positive ions outside the cell than inside.
- The difference in charge is called a potential difference.


Sodium-potassium pumps
Sodium-potassium pumps
- The resting potential is maintained by sodium-potassium pumps in the neurone membrane.
- Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.
- This leads to a build-up of positive ions outside the cell.


Potassium ion channels
Potassium ion channels
- There are potassium ion channels in the neurone membrane. This means it is permeable to K+ ions.
- When K+ ions are transported into neurones, they can diffuse back out.
- The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.


Resting potential
Resting potential
- Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane.
- This potential difference is called the resting potential.
- The neurone is said to be polarised.
- Resting potential is about −70mV.
Depolarisation of the Neurone Cell Membrane
Depolarisation of the Neurone Cell Membrane
When a resting neurone is stimulated, its membrane experiences a change in potential difference. This change is called depolarisation. The steps are:


Stimulation
Stimulation
- Na+ ion channels in the cell membrane open when a neurone is stimulated.
- Na+ ions flood into the neurone.
- The potential difference across the membrane changes to become more positive inside the neurone.


Depolarisation
Depolarisation
- If the potential difference increases above the threshold value (about −55mV) then the membrane will become depolarised.
- More sodium channels open and there is a sharp increase in potential difference to about +30mV.


All-or-nothing
All-or-nothing
- Depolarisation is an all-or-nothing response.
- If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same.
- If the stimulus is stronger, action potentials will be produced more frequently but their size will not increase.


Repolarisation
Repolarisation
- After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.
- K+ ions are transported out of the neurone and the potential difference becomes more negative.
- This is called repolarisation.


Hyperpolarisation
Hyperpolarisation
- There is a short period after repolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.
- This is called hyperpolarisation.
- Hyperpolarisation prevents the neurone from being restimulated instantly. This is called the refractory period.


Resting potential
Resting potential
- After the refractory period, the potassium ion channels close and the membrane returns to its resting potential.
- The process where a neurone is depolarised and returns to resting potential is called an action potential.


Action Potential
Action Potential
The stages involved in the depolarisation of a neurone membrane are called an action potential. Action potentials move along the neurone in a wave.


Sodium ions
Sodium ions
- When an action potential is generated, there are more Na+ ions inside the neurone than outside.
- Some of these Na+ ions diffuse sideways along the neurone axon.


Sodium ion channels
Sodium ion channels
- The presence of Na+ ions creates a change in potential difference further along the neurone membrane.
- If this reaches the threshold value, sodium ion channels at this part of the membrane open.
- Na+ ions diffuse into the neurone.
- This part of the neurone now becomes depolarised.


Wave of depolarisation
Wave of depolarisation
- Na+ diffuse all along the neurone in this way.
- Depolarisation takes place along the neurone membrane. This creates a wave of depolarisation.


Refractory period
Refractory period
- The period of hyperpolarisation in an action potential is called the refractory period.
- The ion channels are recovering during the refractory period. This means an action potential cannot be stimulated again instantly.
- This ensures that the wave of depolarisation travels in one direction.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Unlock your full potential with Seneca Premium
Unlimited access to 10,000+ open-ended exam questions
Mini-mock exams based on your study history
Unlock 800+ premium courses & e-books