Transmission of an Impulse

Test yourself

Resting Potential

When a neurone has not been stimulated, it is at resting state.

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

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

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

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

When a resting neurone is stimulated, its membrane experiences a change in potential difference. This change is called repolarisation. The steps are:

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

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

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

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

Hyperpolarisation

There is a short period after depolarisation 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

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

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

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

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

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

$Illustrative background for Refractory period$ $Illustrative background for Refractory period ?? "content$

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.

1Unity & Diversity - Molecules

1.1Water

1.1.1Water & Hydrogen Bonding

1.1.2IB Multiple Choice - Water & Hydrogen Bonding

1.1.3Extended Response - Water

1.2DNA Structure & Replication

1.2.1DNA

1.2.2RNA

1.2.3DNA Replication

1.2.4DNA Replication - Summary

1.2.5IB Multiple Choice - DNA Replication

1.3Transcription & Gene Expression

1.3.1Introduction to Transcription & Protein Synthesis

1.3.2Types of RNA

1.3.3Transcription

1.3.4RNA Processing in Eukaryotes

1.3.5IB Multiple Choice - Transcription

1.4Translation

1.4.1Translation

1.4.2Retroviruses

1.4.3Scientific Practices: Coronavirus

1.4.4IB Multiple Choice - Translation

2Unity & Diversity - Cells

2.1The Origin of Cells

2.1.1Origins - Endosymbiosis

2.2Introduction to Cells

2.2.1Introduction to Cells

2.2.2Cell Size & Surface Area

2.2.3Surface Area to Volume Ratio

2.2.4Eukaryotic vs Prokaryotic Cells

2.2.5Stem Cells

2.2.6Stem Cells in Disease

2.3Ultrastructure of Cells

2.3.1Golgi, Lysosomes & Ribosomes

2.3.2Cell Walls & Vacuole

2.3.3Prokaryotes

2.3.4Binary Fission

2.4Cell Division

2.4.1The Cell Cycle

2.4.2Mitosis

2.4.3Regulation of the Cell Cycle

2.4.4End of Topic Quiz - The Cell Cycle

2.4.5IB Multiple Choice - The Cell Cycle

2.4.6Extended Response - Mitosis

2.5Structure of DNA & RNA

2.5.1DNA & RNA

2.6DNA Replication, Transcription & Translation

2.6.1Protein Synthesis

2.6.2Transcription & Translation

2.6.3End of Topic Test - DNA, Genes & Protein Synthesis

2.6.4Exam-Style Question - Protein Synthesis

2.6.5End of Topic Test - Coronavirus Translation

2.6.6IB Multiple Choice - Transcription

2.6.7IB Multiple Choice - Translation

2.7Cell Respiration

2.7.1Overview of Respiration

2.7.2Anaerobic Respiration

2.7.3End of Topic Test - Anaerobic Respiration

2.7.4Respiration Experiments

2.7.5End of Topic Test - Respiration

2.7.6IB Multiple Choice - Respiration

2.8Photosynthesis

2.8.1Overview of Photosynthesis

2.9Viruses

2.9.1Viruses

3Unity & Diversity - Organisms

3.1Diversity of Organisms

3.1.1Chromosomal Theory of Inheritance

3.1.2Chromosomal Basis of Inherited Disorders

3.1.3IB Multiple Choice - Chromosomal Inheritance

3.2Evidence for Evolution

3.2.1Evidence for Evolution - Fossils & DNA

3.2.2Evidence for Evolution - Anatomy & Geography

3.2.3IB Multiple Choice - Evidence for Evolution

3.2.4Extended Response - DNA & Evolution

3.3Natural Selection

3.3.1Introduction

3.3.2Evolutionary Fitness

3.3.3Natural Selection & Variation

3.3.4End of Topic Quiz - Natural Selection

3.3.5IB Multiple Choice - Natural Selection

3.3.6Speciation

4Unity & Diversity - Ecosystems

4.1Classification

4.1.1Introduction to Biodiversity

4.1.2Keystone Species

4.1.3Courtship & Ancestry

4.1.4Classification

4.2Cladistics

4.2.1Evolutionary Relationships

4.2.2IB Multiple Choice - Species & Taxonomy

4.3Evolution & Speciation

4.3.1Evidence for Evolution - Fossils & DNA

4.3.2Evidence for Evolution - Anatomy & Geography

4.3.3IB Multiple Choice - Evidence for Evolution

4.3.4Extended Response - DNA & Evolution

4.3.5Populations

4.3.6Mutations, Genetic Drift, & Gene Flow

4.3.7Speciation

4.3.8Rate of Speciation

4.3.9Allopatric & Sympatric Speciation

4.4Conservation of Biodiversity

4.4.1Coming Soon!

5Form & Function - Molecules

5.1Carbohydrates & Lipids

5.1.1Monosaccharides

5.1.2Disaccharides & Polysaccharides

5.1.3Triglycerides & Phospholipids

5.1.4Cellulose

5.1.5Starch & Glycogen

5.2Proteins

5.2.1Structure & Functions of Proteins

6Form & Function - Cells

6.1Membranes & Membrane Transport

6.1.1Passive Movement Across Membranes

6.1.2Active Movement Across Membranes

6.1.3Endocytosis & Exocystosis

6.1.4Cell Membrane Structure & Permeability

6.2Organelles & Compartmentalization

6.2.1Eukaryotic Cells & Organelles

6.2.2Subcellular Components & Organelles

6.3Cell Specialization

6.3.1Cell Specialization

7Form & Function - Organisms

7.1Gas Exchange

7.1.1Lung Structure & Alveoli

7.1.2Ventilation

7.1.3Lung Disease

7.1.4Lung Disease Data

7.2Transport

7.2.1The Circulatory System

7.2.2The Heart

7.2.3Blood Vessels

7.2.4Cardiovascular Disease

7.2.5Xylem Structure

7.2.6Cohesion-Tension Theory

7.2.7Investigating Transpiration Rate

7.2.8Phloem Structure

7.2.9Translocation

7.3Muscle & Motility

7.3.1Skeletal Muscle

7.3.2Sliding Filament Theory

7.3.3Contraction

8Form & Function - Ecosystems

8.1Species, Communities & Ecosytems

8.1.1Population Dynamics

8.1.2Communities & Measuring Diversity

8.1.3Relationships Between Populations

8.1.4Overview of Ecosystems

8.1.5Nutrient Cycles

8.1.6Fertilisers

8.1.7Eutrophication

8.1.8Chi-Squared Test

8.2Energy Flow

8.2.1Energy

8.2.2Energy Flow

8.2.3Biomass & Food Chains

8.2.4Heterotrophs & Autotrophs

8.3Carbon Cycle

8.3.1Carbon Cycle

8.4Climate Change

8.4.1Carbon Dioxide

8.4.2The Greenhouse Effect

8.4.3Global Warming

8.4.4Maintaining Biodiversity

9Interaction & Interdependence - Molecules

9.1Enzymes

9.1.1Enzymes

9.1.2Factors Affecting Enzyme Activity

9.1.3Enzyme-Controlled Reactions

9.1.4End of Topic Test - Enzymes

9.2Metabolism

9.2.1Molecules to Metabolism

9.3Cell Respiration

9.3.1Glycolysis

9.3.2Aerobic Respiration

9.4Photosynthesis

9.4.1Light-Dependent Reaction

9.4.2Light-Independent Reaction

9.4.3End of Topic Test - Photosynthesis Reactions

9.4.4Limiting Factors

9.4.5Photosynthesis Experiments

9.4.6End of Topic Test - Photosynthesis

9.4.7End of Topic Test - Photosynthesis

10Interaction & Interdependence - Cells

10.1Chemical Signalling

10.1.1Coming Soon!

10.2Neural Signalling

10.2.1Medical Application

10.2.2Speed of Transmission

10.2.3Synapses

10.2.4End of Topic Test - Nervous Comm&Coord

10.2.5Neuron Structure

10.2.6Types of Synapse

10.2.7End of Topic Test - Neurones & Action Potentials

10.2.8Transmission of an Impulse

10.2.9End of Topic Test - Synapses

10.3Adaptation to Environment

10.3.1Coming Soon!

10.4Ecological Niches

10.4.1Coming Soon!

11Interaction & Interdependence - Organisms

11.1Integration of Body Systems

11.1.1Plant Tropisms

11.2Defence Against Disease

11.2.1Immune System

11.2.2The Immune Response

11.2.3Antibodies

11.2.4Primary & Secondary Response

11.2.5Vaccines

11.2.6HIV

11.2.7End of Topic Test - Immune System

12Interaction & Interdependence - Ecosystems

12.1Populations & Communities

12.1.1Coming Soon!

12.2Transfers of Energy & Matter

12.2.1Coming Soon!

13Continuity & Change - Molecules

13.1DNA Replication

13.1.1DNA Replication

13.2Protein Synthesis

13.2.1Protein Synthesis

13.2.2Transcription & Translation

13.2.3End of Topic Test - DNA, Genes & Protein Synthesis

13.3Mutation & Gene Editing

13.3.1Genes

13.3.2Mutations

13.3.3DNA Technology

13.3.4PCR & Gel Electrophoresis

13.3.5Genetic Modification

13.3.6Cloning

14Continuity & Change - Cells

14.1Gene Expression

14.1.1Introduction to Transcription & Protein Synthesis

14.1.2Types of RNA

14.1.3Transcription

14.1.4RNA Processing in Eukaryotes

14.1.5IB Multiple Choice - Transcription

14.2Water Potential

14.2.1Controlling Blood Water Potential

15Continuity & Change - Organisms

15.1Inheritance

15.1.1Non-Nuclear Inheritance

15.1.2Linked Genes

15.1.3IB Multiple Choice - Non-Mendelian Genetics

15.1.4Extended Response - Inheritance

15.1.5Introduction to Non-Mendelian Inheritance

15.1.6Chi-Squared Test

15.1.7End of Topic Quiz - Inheritance

15.1.8Sex-Linked Genes

15.1.9Grade 4-5 (Scientific Practices) - Inheritance

15.2Homeostasis

15.2.1Overview of Homeostasis

15.2.2Blood Glucose Concentration

15.2.3Controlling Blood Glucose Concentration

15.2.4End of Topic Test - Blood Glucose

15.2.5Type I & Type II Diabetes

15.2.6Human Reproductive Systems

15.2.7Hormonal Control of the Menstrual Cycle

15.2.8Kidneys & Osmoregulation

16Continuity & Change - Ecosystems

16.1Natural Selection

16.1.1Natural Selection

16.2Stability & Change

16.2.1Coming Soon!

16.3Climate Change

16.3.1Carbon Dioxide

16.3.2The Greenhouse Effect

16.3.3Global Warming

16.3.4Maintaining Biodiversity

Jump to other topics