15.05 Receptors

Triggering an Action Potential

• Stimulus Types: Light, pressure (touch), sound, temperature, and chemicals.
• Receptor Cells:
  • Specialized cells that respond to specific stimuli and initiate action potentials in sensory neurones.
  • Act as transducers, converting various forms of energy (light, sound, etc.) into electrical impulses.
  • Found in sense organs (e.g., light receptors in the eye, sound receptors in the ear).

Receptor Types and Stimuli Conversion

• Light Receptors (rods/cones in the retina): Convert light to electrical impulses.
• Chemoreceptors (taste buds, olfactory cells): Detect chemical potential from taste and smell.
• Pressure/Touch Receptors (Pacinian and Meissner’s corpuscles): Respond to mechanical pressure.
• Temperature Receptors (Ruffini’s endings): Detect heat.
• Stretch Receptors (proprioceptors in muscles): Detect limb positioning.
• Hair Cells (ear): Balance and hearing (respond to sound/movement).

Taste Receptors Example

• Taste Bud Structure: Each taste bud contains 50-100 chemoreceptor cells sensitive to chemicals in food.
• Mechanism:
  • Sodium Ions (salt): Directly enter through channel proteins, causing depolarization.
  • Depolarization: Increases positive charge inside, creating a receptor potential.
  • Threshold Stimulation: If receptor potential exceeds threshold, voltage-gated calcium channels open, triggering neurotransmitter release and stimulating the sensory neurone to carry the impulse to the brain.

Receptor Potential and Threshold

• Receptor Potential: Change in membrane potential due to stimulus.
  • The membrane potential refers to the electrical potential difference across a cell’s membrane. It arises from the uneven distribution of ions, such as sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and other charged molecules, on either side of the cell membrane. This difference in charge creates a voltage across the membrane, typically measured in millivolts (mV).
• Threshold Potential: Minimum receptor potential needed to generate an action potential in the sensory neurone.
• All-or-None Law: Action potentials are only generated if the stimulus exceeds the threshold; intensity does not affect the amplitude but increases the frequency of action potentials
• Note: if we feel a lot of pain, or if we hear a very loud sound etc., that feeling of intense pain is due to how many signals reach our brain within a certain period of time. There are no big or less big signals that corresponds with more or less pain – either a signal is sent, or it’s not. The only difference in signals are how many signal are sent in a given time. More signals indicates more intense pain or feeling.

Figure (Above): the red arrows shows how the membrane potential changes as this touch receptor is deformed (touched). As the membrane polarity changes, it stimulates the area next to it also to open ion channels, which in tern causes the adjacent cell area to become depolarised as well (indicated by the red arrows). This then in turn depolarises the nerve membrane, and the action potential travels down the nerve (indicated by the black arrows).

Figure (Above): All-or-nothing AP (Action potential). The signal must pass the threshold, in order for it to become an AP that is propagated down the nerve.

Strength of Stimulus and Frequency of Impulses

• Weak Stimulus: Produces a small receptor potential, potentially below the threshold—no action potential generated.
• Threshold or Stronger Stimulus:
  • Above Threshold: Initiates an action potential at low frequency.
  • Increasing Intensity: Raises impulse frequency but not amplitude.

Adaptation in Receptor Cells

• Threshold Adjustment: With continuous stimulation, receptor thresholds often increase, requiring a stronger stimulus for subsequent action potentials.
• Advantage: Helps prevent overstimulation, allowing receptors to focus on significant changes in the environment.

Key Terms

• All-or-None Law: Neurones and muscle cells only respond if the stimulus reaches threshold; action potentials have a consistent amplitude.
• Chemoreceptor: Receptor that responds to chemical stimuli (e.g., taste, smell).
• Receptor Potential: Change in membrane potential in response to a stimulus.
• Threshold Potential: Minimum potential change required to trigger an action potential.