Sensory Receptors: Structure, Function, and Classification

Sensory Receptors

Introduction

Sensory receptors are specialized structures that detect changes in the environment and convert these stimuli into electrical signals, allowing the nervous system to interpret and respond to internal and external conditions. Understanding their classification, function, and adaptation is essential for comprehending how the body perceives and processes sensory information.

Types of Sensation

Special Senses

• Vision

• Olfaction (smell)

• Gustation (taste)

• Audition (hearing)

• Equilibrium (balance)

General Senses

• Somatic Senses: Touch, temperature, pain

• Proprioception: Body movement and position

Classification of Sensory Receptors

Structural Classification

• Free nerve endings: Dendrites of a neuron, often unencapsulated, detect pain and temperature.

• Encapsulated nerve endings: Dendrites enclosed in connective tissue, detect pressure and touch.

• Special receptor cells: Non-neuronal cells that synapse with sensory neurons (e.g., hair cells in the ear).

Stimulus Classification

• Chemoreceptors: Detect specific chemicals (e.g., taste, smell, blood pH).

• Mechanoreceptors: Respond to pressure, stretch, vibration, acceleration, and sound.

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to photons of light (e.g., rods and cones in the retina).

• Nociceptors: Detect irritation, pain, or tissue damage.

Location Classification

• Exteroceptors: Respond to external stimuli (e.g., cutaneous receptors, special senses receptors).

• Interoceptors (Visceroceptors): Respond to internal stimuli (e.g., within organs).

• Proprioceptors: Detect body movement and position (e.g., muscle spindles, tendon organs).

Table: Criteria for Classifying Receptors

Classification	Description	Examples
General senses	Distributed throughout the body; structurally simple	Tactile (touch) receptors
Special senses	Located only in the head; structurally complex	Receptors for vision, taste, hearing, equilibrium
Exteroceptors	Detect external stimuli	Receptors in skin, mucous membranes
Interoceptors	Detect internal stimuli	Receptors in blood vessels, internal organs
Proprioceptors	Detect body and limb movements	Muscle spindles, tendon organs
Chemoreceptors	Detect chemical changes	Taste receptors, blood CO2 receptors
Mechanoreceptors	Detect mechanical forces	Tactile receptors, baroreceptors
Thermoreceptors	Detect temperature changes	Temperature receptors in skin
Photoreceptors	Detect light	Rods and cones in retina
Nociceptors	Detect pain	Pain receptors in almost all organs

Transduction and Receptor Potentials

Transduction

• Transduction is the process by which sensory receptors convert a stimulus into an electrical signal.

• Stimulus opens or closes ion channels in the receptor membrane, usually causing a net influx of Na+ (or other cations) and depolarizing the membrane.

Receptor Potential

• A receptor potential is a graded potential generated in a sensory receptor.

• It can initiate an action potential (AP) in a sensory neuron or alter neurotransmitter (NT) release from a special receptor cell.

• Depolarization or hyperpolarization can facilitate or inhibit AP generation in the sensory neuron.

Sensation Properties

Modality

• Each receptor is most sensitive to one type of stimulus (its modality).

• Example: Mechanoreceptors in the carotid artery detect stretch, not pH or oxygen content.

Projection

• The brain determines the location of a stimulus based on which receptors are activated and where their sensory neurons terminate in the cerebrum.

Intensity

• Determined by the number of receptors activated and the frequency of action potentials.

• All action potentials are the same size; intensity is encoded by frequency, not amplitude.

Duration

• A longer stimulus produces more action potentials.

• Stimulus duration affects receptor adaptation.

Receptor Adaptation

Tonic Receptors

• Respond to stimuli that require constant monitoring.

• Produce electrical signals as long as the stimulus continues.

• Example: Mechanoreceptors that monitor blood pressure.

Phasic Receptors

• Respond to changes in stimulus.

• Stop producing electrical signals if the stimulus remains constant; sensitivity is reduced.

• Example: Tactile receptors (adapt to touch).

Somatic Senses

• Touch

• Temperature

• Nociception (pain, itch)

• Receptors are located in the skin and viscera.

Temperature Receptors

• Free nerve endings sensitive to cold and warm (relative to body temperature).

• Adaptation occurs between 20–40°C (68–104°F); outside this range, tissue damage may occur and adaptation does not happen.

Nociceptors

• Free nerve endings with large receptive fields, responsive to chemical, temperature, and mechanical stimuli.

• Respond to strong or potentially damaging stimuli.

• Chemicals released by tissue damage lower the threshold for activation (sensitization).

• Some chemicals (e.g., histamine) cause itch rather than pain.

Pain and Referred Pain

• Pain is a subjective experience, varying between individuals.

• Referred pain occurs when sensory neurons from different locations converge on the same pathway to the brain. The brain interprets pain as coming from somatic receptors, which are more frequently the source of sensory input.

• Example: Pain from a heart attack may be felt in the left arm or shoulder.

Summary Table: Types and Properties of Sensory Receptors

Receptor Type	Stimulus Detected	Location	Example
Chemoreceptor	Chemicals	Blood vessels, nose, tongue	Olfactory cells, taste buds
Mechanoreceptor	Pressure, vibration, stretch	Skin, muscles, ears	Tactile corpuscles, baroreceptors
Thermoreceptor	Temperature	Skin	Cold and warm receptors
Photoreceptor	Light	Retina	Rods and cones
Nociceptor	Pain, tissue damage	Skin, most organs	Free nerve endings

Key Equations

• Frequency coding of intensity:

$\text{Frequency of APs} \propto \text{Stimulus Intensity}$

• Receptor potential and action potential relationship:

$\text{If receptor potential} > \text{threshold} \rightarrow \text{action potential generated}$

Additional info:

• Referred pain is clinically important for diagnosing internal organ issues, as pain may be perceived at a site distant from the actual source.

• Adaptation allows the nervous system to ignore constant, unchanging stimuli, focusing attention on new or changing information.