Transmission of an Impulse

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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.

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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 Biology

2Molecular Biology

2.1Water

2.1.1Water & Hydrogen Bonding

2.1.2IB Multiple Choice - Water & Hydrogen Bonding

2.1.3Extended Response - Water

2.2Carbohydrates & Lipids

2.2.1Monosaccharides

2.2.2Disaccharides & Polysaccharides

2.2.3Triglycerides & Phospholipids

2.2.4Cellulose

2.2.5Starch & Glycogen

2.3Proteins

2.3.1Structure & Functions of Proteins

2.4Enzymes

2.4.1Enzymes

2.4.2Factors Affecting Enzyme Activity

2.4.3Enzyme-Controlled Reactions

2.4.4End of Topic Test - Enzymes

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

3Genetics

4Ecology

4.1Species, Communities & Ecosytems

4.1.1Population Dynamics

4.1.2Communities & Measuring Diversity

4.1.3Relationships Between Populations

4.1.4Overview of Ecosystems

4.1.5Nutrient Cycles

4.1.6Fertilisers

4.1.7Eutrophication

4.1.8Chi-Squared Test

4.2Energy Flow

4.2.1Energy

4.2.2Energy Flow

4.2.3Biomass & Food Chains

4.2.4Heterotrophs & Autotrophs

4.3Carbon Cycle

4.3.1Carbon Cycle

4.4Climate Change

4.4.1Carbon Dioxide

4.4.2The Greenhouse Effect

4.4.3Global Warming

4.4.4Maintaining Biodiversity

5Evolution & Biodiversity

5.1Evidence for Evolution

5.1.1Evidence for Evolution - Fossils & DNA

5.1.2Evidence for Evolution - Anatomy & Geography

5.1.3IB Multiple Choice - Evidence for Evolution

5.1.4Extended Response - DNA & Evolution

5.2Natural Selection

5.2.1Introduction

5.2.2Evolutionary Fitness

5.2.3Natural Selection & Variation

5.2.4End of Topic Quiz - Natural Selection

5.2.5IB Multiple Choice - Natural Selection

5.2.6Speciation

5.3Classification of Biodiversity

5.3.1Introduction to Biodiversity

5.3.2Keystone Species

5.3.3Courtship & Ancestry

5.3.4Classification

5.4Cladistics

5.4.1Evolutionary Relationships

5.4.2IB Multiple Choice - Species & Taxonomy

6Human Physiology

6.1Digestion & Absorption

6.1.1Overview of Digestion

6.1.2Digestion in Mammals

6.1.3Absorption

6.1.4End of Topic Test - Substance Exchange & Digestion

6.1.5End of Topic Test - Substance Ex & Digestion

6.2The Blood System

6.2.1The Circulatory System

6.2.2The Heart

6.2.3Blood Vessels

6.2.4Cardiovascular Disease

6.3Defence Against Infectious Disease

6.3.1Immune System

6.3.2The Immune Response

6.3.3Antibodies

6.3.4Primary & Secondary Response

6.3.5Vaccines

6.3.6HIV

6.3.7End of Topic Test - Immune System

6.3.8Exam-Style Question - Immune System

6.3.9End of Topic Test - Immune System

6.4Gas Exchange

6.4.1Lung Structure & Alveoli

6.4.2Ventilation

6.4.3Lung Disease

6.4.4Lung Disease Data

6.5Neurons & Synapses

6.5.1Neuron Structure

6.5.2Transmission of an Impulse

6.5.3Speed of Transmission

6.5.4End of Topic Test - Neurones & Action Potentials

6.5.5Synapses

6.5.6Types of Synapse

6.5.7Medical Application

6.5.8End of Topic Test - Synapses

6.5.9End of Topic Test - Nervous Comm&Coord

6.6Hormones, HomeoStasis & Reproduction

6.6.1Overview of Homeostasis

6.6.2Blood Glucose Concentration

6.6.3Controlling Blood Glucose Concentration

6.6.4End of Topic Test - Blood Glucose

6.6.5Type I & Type II Diabetes

6.6.6Human Reproductive Systems

6.6.7Hormonal Control of the Menstrual Cycle

7AHL: Nucleic Acids

7.1DNA Structure & Replication

7.1.1DNA

7.1.2RNA

7.1.3DNA Replication

7.1.4DNA Replication - Summary

7.1.5IB Multiple Choice - DNA Replication

7.2Transcription & Gene Expression

7.2.1Introduction to Transcription & Protein Synthesis

7.2.2Types of RNA

7.2.3Transcription

7.2.4RNA Processing in Eukaryotes

7.2.5IB Multiple Choice - Transcription

7.3Translation

7.3.1Translation

7.3.2Retroviruses

7.3.3Scientific Practices: Coronavirus

7.3.4IB Multiple Choice - Translation

8AHL: Metabolism, Cell Respiration & Photosynthesis

8.1Metabolism

8.1.1Molecules to Metabolism

8.2Cell Respiration

8.2.1Glycolysis

8.2.2Aerobic Respiration

8.3Photosynthesis

8.3.1Light-Dependent Reaction

8.3.2Light-Independent Reaction

8.3.3End of Topic Test - Photosynthesis Reactions

8.3.4Limiting Factors

8.3.5Photosynthesis Experiments

8.3.6End of Topic Test - Photosynthesis

8.3.7End of Topic Test - Photosynthesis

9AHL: Plant Biology

9.1Transport in the Xylem of Plants

9.1.1Structure

9.1.2Cohesion-Tension Theory

9.1.3Investigating Transpiration Rate

9.2Transport in the Phloem of Plants

9.2.1Structure

9.2.2Translocation

9.3Growth in Plants

9.3.1Plant Tropisms

10AHL: Genetics & Evolution

10.1Meiosis

10.1.1Stages of Meiosis

10.2Inheritance

10.2.1Linked Genes

10.2.2Sex-Linked Genes

10.2.3Non-Nuclear Inheritance

10.2.4Chi-Squared Test

10.2.5End of Topic Quiz - Inheritance

10.2.6IB Multiple Choice - Non-Mendelian Genetics

10.2.7Introduction to Non-Mendelian Inheritance

10.2.8Extended Response - Inheritance

10.2.9Grade 4-5 (Scientific Practices) - Inheritance

10.3Gene Pools & Speciation

10.3.1Populations

10.3.2Mutations, Genetic Drift, & Gene Flow

10.3.3Speciation

10.3.4Rate of Speciation

10.3.5Allopatric & Sympatric Speciation

11AHL: Animal Physiology

11.1Antibody Production & Vaccination

11.1.1Immune System

11.1.2The Immune Response

11.1.3Antibodies

11.1.4Primary & Secondary Response

11.1.5Vaccines

11.2Movement

11.2.1Skeletal Muscle

11.2.2Sliding Filament Theory

11.2.3Contraction

11.3The Kidney & Osmoregulation

11.3.1Kidneys & Osmoregulation

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