AQA A Level Biology

Revision Notes

6.2.3 Nerve Impulses

Action Potentials

  • The surface of the tongue is covered in many small bumps known as papillae
  • The surface of each papilla is covered in many taste buds
  • Each taste bud contains many receptor cells known as chemoreceptors
    • These chemoreceptors are sensitive to chemicals in food and drinks
  • Each chemoreceptor is covered with receptor proteins
    • Different receptor proteins detect different chemicals

An example of the sequence of events that results in an action potential in a sensory neurone

  • Chemoreceptors in the taste buds that detect salt (sodium chloride) respond directly to sodium ions
  • If salt is present in the food (dissolved in saliva) being eaten or the liquid being drunk:
    • Sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptor cells
    • This leads to depolarisation of the chemoreceptor cell membrane
    • The increase in positive charge inside the cell is known as the receptor potential
    • If there is sufficient stimulation by sodium ions and sufficient depolarisation of the membrane, the receptor potential becomes large enough to stimulate voltage-gated calcium ion channel proteins to open
    • As a result, calcium ions enter the cytoplasm of the chemoreceptor cell and stimulate exocytosis of vesicles containing neurotransmitter from the basal membrane of the chemoreceptor
    • The neurotransmitter stimulates an action potential in the sensory neurone
    • The sensory neurone then transmits an impulse to the brain

Chemoreceptors in the tongue (1), downloadable AS & A Level Biology revision notesChemoreceptors in the tongue (2)_2, downloadable AS & A Level Biology revision notes

Tasting salt – an example of the sequence of events that results in an action potential in a sensory neurone

  • When receptors (such as chemoreceptors) are stimulated, they are depolarised
  • If the stimulus is very weak or below a certain threshold, the receptor cells won’t be sufficiently depolarised and the sensory neurone will not be activated to send impulses
  • If the stimulus is strong enough to increase the receptor potential above the threshold potential then the receptor will stimulate the sensory neurone to send impulses
  • This is an example of the all-or-nothing principle
    • An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
  • Rather than staying constant, threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones

Receptor Potentials (2), downloadable AS & A Level Biology revision notesReceptor Potentials (1), downloadable AS & A Level Biology revision notes

The receptor potential increases as the strength of the stimulus increases. As the strength of stimulus increases beyond the threshold, the frequency (not amplitude) of impulses increases.

Exam Tip

Some receptors, like the chemoreceptors described above, are specialised cells that detect a specific type of stimulus and affect the sensory neurone’s electrical activity. Other receptors are just the ends of the sensory neurones (for example, many types of touch receptors).

Nerve Impulses

  • Neurones transmit electrical impulses, which travel extremely quickly along the neurone cell surface membrane from one end of the neurone to the other
  • Unlike a normal electric current, these impulses are not a flow of electrons
  • These impulses, known as action potentials, occur via very brief changes in the distribution of electrical charge across the cell surface membrane
    • Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon

Resting potential

  • In a resting axon (one that is not transmitting impulses), the inside of the axon always has a slightly negative electrical potential compared to outside the axon
  • This potential difference is usually about -70mV (ie. the inside of the axon has an electrical potential about 70mV lower than the outside)
  • This is called the resting potential

Action potentials

  • There are channel proteins in the axon membrane that allow sodium ions or potassium ions to pass through
  • These open and close depending on the electrical potential (or voltage) across the axon membrane and are known as voltage-gated channel proteins (they are closed when the axon membrane is at its resting potential)
  • When an action potential is stimulated (eg. by a receptor cell) in a neurone, the following steps occur:
    • Voltage-gated channel proteins in the axon membrane open
    • Sodium ions pass into the axon down the electrochemical gradient (there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
    • This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – a process known as depolarisation
    • This triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
    • This is an example of positive feedback (a small initial depolarisation leads to greater and greater levels of depolarisation)
    • If the potential difference reaches around -50mV (known as the threshold value), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of around +30mV
    • An action potential is generated
    • The depolarisation of the membrane at the site of the first action potential causes current to flow to the next section of the axon membrane, depolarising it and causing sodium ion voltage-gated channel proteins to open
    • This triggers the production of another action potential in this section of the axon membrane and the process continues
    • In the body, this allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane

Transmission of a nerve impulse (1), downloadable AS & A Level Biology revision notesTransmission of a nerve impulse (2), downloadable AS & A Level Biology revision notes

How an impulse is transmitted in one direction along the axon of a neurone

Repolarisation and the refractory period

  • Very shortly (about 1 ms) after an action potential in a section of axon membrane is generated, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon
  • Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
  • This returns the potential difference to normal (about -70mV) – a process known as repolarisation
    • There is actually a short period of hyperpolarisation. This is when the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential
  • The potassium ion voltage-gated channel proteins then close and the sodium ion channel proteins in this section of membrane become responsive to depolarisation again
    • Until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive
    • This is known as the refractory period

Action potential graph, downloadable AS & A Level Biology revision notes

The five stages of an action potential: stimulus, depolarisation, repolarisation, hyperpolarisation and return to resting state

Exam Tip

During the refractory period, a section of the axon is unresponsive. This is very important as it ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential. This makes the action potentials discrete events and means the impulse can only travel in one direction. This is essential for the successful and efficient transmission of nerve impulses along neurones.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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