5.1 Receptors Are Transducers

What is a Receptor?

A receptor is a specialized cell, tissue, or organ that detects changes (stimuli) in the internal or external environment. Receptors act as biological transducers — they convert one form of energy (mechanical, thermal, chemical, electromagnetic) into electrochemical energy in the form of a nerve impulse.

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Receptors as Transducers

When a stimulus acts on a receptor:

1. The receptor membrane becomes more permeable to ions.
2. A generator potential (receptor potential) is produced — a local, graded, non-propagated change in membrane potential.
3. If the generator potential reaches the threshold, it triggers an action potential in the associated sensory neuron.
4. The action potential travels along the sensory nerve to the CNS.

Generator Potential vs. Action Potential

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Encoding Stimulus Intensity

Although each action potential has a fixed amplitude (all-or-none law), the intensity of a stimulus is encoded by:

• Frequency of action potentials (temporal summation) — stronger stimulus → more frequent impulses.
• Number of receptors activated (spatial summation) — stronger stimulus → more receptors fire.

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Sensory Adaptation

Sensory adaptation is the decrease in receptor sensitivity during prolonged or continuous stimulation, resulting in a lower frequency of nerve impulses.

• Fast-adapting receptors (e.g., touch, smell): adapt quickly — useful for detecting changes.
• Slow-adapting receptors (e.g., pain, proprioceptors): adapt slowly — useful for monitoring ongoing conditions.

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Classification of Receptors

By Source of Stimulus

By Type of Stimulus

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Structure and Function of Specific Receptors

1. Olfactory Receptors (Smell)

• Located in the olfactory epithelium of the nasal cavity.
• Each receptor is a bipolar neuron with hair-like cilia projecting into the mucus layer.
• Mechanism: Airborne odorant molecules dissolve in mucus → bind to receptor proteins on cilia → generator potential produced → action potential sent via the olfactory nerve (Cranial Nerve I) to the olfactory bulb in the brain.
• Olfactory receptors are fast-adapting (explain why persistent smells are no longer noticed).

2. Gustatory Receptors (Taste)

• Located in taste buds on the tongue (and soft palate, epiglottis).
• Taste buds contain gustatory receptor cells with microvilli (taste hairs) projecting through a taste pore.
• Five basic tastes: Sweet, Sour, Salty, Bitter, Umami.
• Mechanism: Dissolved chemicals (tastants) bind to receptor proteins on microvilli → generator potential → action potentials transmitted via Cranial Nerves VII, IX, and X to the brain.

3. Touch and Pain Receptors (Cutaneous Receptors)

The skin contains several types of mechanoreceptors and nociceptors:

Pain (Nociception):

• Detected by free nerve endings (nociceptors) distributed throughout the skin, muscles, and organs.
• Stimulated by tissue damage, extreme temperature, or chemicals (e.g., bradykinin, prostaglandins).
• Pain signals travel via fast A-delta fibres (sharp, localized pain) and slow C fibres (dull, aching pain).

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Receptors in Homeostasis

Receptors are the first element of the three-component homeostatic mechanism:

1. Receptor — detects a deviation from the set point (e.g., a thermoreceptor detects a drop in body temperature).
2. Control Center (e.g., hypothalamus, brain) — receives information from the receptor, compares it to the set point, and sends corrective instructions.
3. Effector (muscles or glands) — carries out the response to restore homeostasis (e.g., shivering to generate heat).

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