• A cell that responds to a stimulus is called a receptor cell
• Receptor cells are transducers – they convert energy in one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
• Receptor cells are often found in sense organs (eg. light receptor cells are found in the eye)
  • Some receptors, such as light receptors in the eye and chemoreceptors in the taste buds, are specialised cells that detect a specific type of stimulus and influence the electrical activity of a sensory neurone
  • Other receptors, such as some kinds of touch receptors, are just the ends of the sensory neurones themselves
• When receptors cells are stimulated they are depolarised
  • If the stimulus is very weak, the cells are not sufficiently depolarised and the sensory neurone is not activated to send impulses
  • If the stimulus is strong enough, the sensory neurone is activated and transmits impulses to the CNS

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

• 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
• 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

• 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-none law
  • 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