Sequence of Events Resulting in an Action Potential (Cambridge (CIE) A Level Biology): Revision Note

Sequence of events resulting in an action potential

• 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:

  • Sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptor cells

  • This leads to the 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 neurotransmitters 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

• 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