outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones

Control and Coordination in Mammals

Objective

Outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones.

1. What are Sensory Receptor Cells?

Sensory receptor cells are specialised cells that convert a specific type of environmental stimulus into an electrical signal (a receptor potential). This process is called sensory transduction.

2. Main Types of Sensory Receptors

• Mechanoreceptors – respond to mechanical deformation (touch, pressure, vibration, stretch).
• Thermoreceptors – respond to changes in temperature.
• Photoreceptors – respond to light (rods and cones in the retina).
• Chemoreceptors – respond to chemical substances (taste buds, olfactory epithelium).
• Nociceptors – respond to potentially damaging stimuli (pain).

3. Structure of a Typical Sensory Receptor Cell

Although the morphology varies, most receptor cells share these features:

1. Specialised membrane region that directly interacts with the stimulus.
2. Ion channels that open or close in response to the stimulus.
3. Connection to a sensory neurone (often via a synapse or a direct continuation of the membrane).

4. Mechanism of Stimulus Detection

When a stimulus acts on the receptor cell, it causes a change in the permeability of the cell membrane to specific ions. This generates a graded change in membrane potential called the receptor potential.

The magnitude of the receptor potential is proportional to the intensity of the stimulus.

5. From Receptor Potential to Action Potential

Key steps:

• Stimulus opens stimulus‑gated ion channels → influx of Na⁺ (or Ca²⁺) → depolarisation.
• If the depolarisation reaches the threshold, voltage‑gated Na⁺ channels open.
• An all‑or‑none action potential is generated at the trigger zone (often at the first node of Ranvier of the sensory neurone).
• The action potential propagates along the sensory neurone to the central nervous system (CNS).

6. Example Pathways

6.1 Touch (Mechanoreception)

Pressure on the skin deforms Meissner’s corpuscles → stretch‑sensitive channels open → receptor potential → action potential in the associated dorsal root ganglion neurone → dorsal column‑medial lemniscal pathway.

6.2 \cdot ision (Photoreception)

Photons are absorbed by retinal pigments in rods/cones → conformational change in opsin → closure of Na⁺ channels → hyperpolarisation (inverse of most receptors) → modulation of glutamate release → signal transmitted via bipolar cells to optic nerve.

6.3 Taste (Chemoreception)

Soluble chemicals bind to G‑protein‑coupled receptors on taste buds → second‑messenger cascade → opening of ion channels → depolarisation → action potential in gustatory neurones → cranial nerves \cdot II, IX, X.

7. Summary Table

8. Key Points to Remember

• Receptor cells are the first step in the sensory pathway – they translate physical or chemical energy into an electrical signal.
• The receptor potential is graded; only when it reaches threshold does an action potential fire.
• Different receptors use different ion channels and may produce depolarisation or hyperpolarisation.
• The speed and fidelity of impulse transmission depend on the type of sensory neurone (myelinated vs. unmyelinated) and the pathway to the CNS.