Sensory receptors are the specialized units that allow the human body to communicate with its environment by detecting internal and external changes.

Classification of Receptors

Receptors are classified through several different lenses:

• By Structure: They can be free dendritic endings (e.g., pain and temperature), encapsulated endings (e.g., Meissner’s corpuscles), or specialized modified epithelial cells (e.g., taste buds and hair cells).

• By Functional Category (Adequate Stimulus): This grouping is based on the energy they transduce: mechanoreceptors (mechanical deformation), photoreceptors (light), chemoreceptors (chemical stimuli), thermoreceptors (heat/cold), and nociceptors (pain/tissue damage).

• By Information Source: Exteroceptors respond to stimuli from the external environment (e.g., sight and smell), interoceptors monitor internal conditions (e.g., blood pressure and pH), and proprioceptors provide a sense of body position and movement.

• By Adaptation Rate: Phasic (rapidly adapting) receptors respond with a burst of activity but quickly decrease their firing rate (e.g., Pacinian corpuscles), while tonic (slowly adapting) receptors maintain a relatively constant firing rate as long as the stimulus is applied (e.g., Merkel cells and nociceptors).

Functions of Receptors

The primary function of a receptor is to serve as a biological transducer, converting various forms of environmental energy (mechanical, thermal, or electromagnetic) into electrochemical nerve impulses. Receptors also act as sensory filters; according to the law of specific nerve energies, each receptor is specialized to respond with the lowest threshold to only one modality, known as the adequate stimulus. Additionally, they perform sensory coding, representing four elementary attributes of a stimulus for the brain: modality, location, intensity, and duration.

Properties of Receptors

• Excitability: The ability of neurons and receptor cells to produce and conduct changes in membrane potential in response to stimuli.

• Receptive Fields: The specific area of the body where stimulation results in a change in the firing rate of a sensory neuron.

• Lateral Inhibition: A process where strongly stimulated neurons inhibit neighboring ones to sharpen the borders of a sensation and improve localization.

• Sensory Adaptation: The property where a maintained stimulus of constant strength leads to a decline in the frequency of action potentials.

• Law of Projection: Regardless of where a sensory pathway is stimulated, the brain "projects" the sensation back to the receptor's location (e.g., phantom limb pain).

Mechanism of Receptor Potential Generation

The process of converting a stimulus into an electrical signal is called sensory transduction.

1. Transduction Interaction: An environmental stimulus interacts with the receptor, causing a physical change—such as mechanical deformation of the membrane, a photochemical reaction (in the retina), or chemical binding (in taste or smell).

2. Ion Channel Gating: This interaction causes specific ion channels in the receptor membrane to open or close.

3. Generator Potential: The resulting current flow (usually an influx of Na+) produces a local, graded, non-propagated change in membrane potential called a receptor or generator potential.

4. Action Potential Initiation: If the magnitude of the generator potential reaches a critical threshold (typically ~10 mV), it triggers all-or-none action potentials at a spike generator region, such as the first node of Ranvier.

5. Intensity Coding: Because action potentials are all-or-none, the intensity of the stimulus is coded by the frequency of action potentials generated—the larger the generator potential, the more frequent the spikes.