15.1.5 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 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

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

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