Sensory Physiology: General Properties and Sensory Receptors

General Properties of Sensory Systems

Overview of Sensory Systems

Sensory systems allow the body to detect and respond to various internal and external stimuli. These systems are divided into special senses and somatic senses, each with distinct modalities and mechanisms.

• Special senses: vision, hearing, taste, smell, equilibrium

• Somatic senses: touch, temperature, pain, itch, proprioception

• Stimulus as physical energy: Detected by sensory receptors

• Receptor acts as a transducer: Converts stimulus energy into intracellular signals, usually by changing membrane potential

• Threshold: Minimum stimulus required to generate an action potential sent to the CNS

• Integration in CNS: Sensory information is processed in the cerebral cortex or acted upon subconsciously

• Complexity of receptors: Ranges from simple (single sensory neuron) to complex (multicellular sense organs)

Information Processing by the Sensory Division

Sensory information can be processed consciously or subconsciously, depending on the type and source of the stimulus.

Stimulus Processing Usually Conscious	Special Senses	Somatic Senses
Vision	Touch
Hearing	Temperature
Taste	Pain
Smell	Itch
Equilibrium	Proprioception
Stimulus Processing Usually Subconscious	Somatic Stimuli	Visceral Stimuli
Muscle length and tension	Blood pressure
Proprioception	Distension of gastrointestinal tract
	Blood glucose concentration
	Internal body temperature
	Osmolarity of body fluids
	Lung inflation
	pH of cerebrospinal fluid
	pH and oxygen content of blood

Sensory Receptors

Types and Structure of Sensory Receptors

Sensory receptors are specialized cells or structures that detect specific forms of energy from the environment. They can be classified based on their structure and the type of stimulus they detect.

• Neural receptors:

  • Naked (free) nerve endings: Detect pain, temperature, and some touch sensations

  • Complex neural receptors: Encased in connective tissue capsules; may be myelinated or unmyelinated

• Non-neural receptors for special senses: Highly specialized cells associated with sensory neurons (e.g., hair cells in the ear)

• Accessory structures: Enhance the function of sensory systems (e.g., lamellar corpuscles in skin)

Major Groups of Sensory Receptors

Receptors are categorized by the type of stimulus they respond to:

• Chemoreceptors: Respond to chemical ligands (e.g., taste, smell, oxygen, pH, glucose)

• Mechanoreceptors: Respond to mechanical energy (e.g., pressure, vibration, acceleration, sound)

• Thermoreceptors: Respond to varying degrees of heat

• Photoreceptors: Respond to photons of light (vision)

Table: Types of Sensory Receptors

Type of Receptor	Examples of Stimuli
Chemoreceptors	Oxygen, pH, various organic molecules such as glucose
Mechanoreceptors	Pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, sound
Photoreceptors	Photons of light
Thermoreceptors	Varying degrees of heat

Structure of Sensory Receptors (Figure Reference)

• Simple receptors: Free nerve endings, unmyelinated axons

• Complex receptors: Encapsulated nerve endings, myelinated axons

• Special sense receptors: Specialized receptor cells synapsing with sensory neurons

Key Terms

• Transducer: A device or cell that converts one form of energy into another

• Threshold: The minimum level of stimulus required to activate a receptor

• Proprioceptors: Sensory receptors that detect body position and movement

Example: Mechanoreceptors in Skin

Pacinian corpuscles are mechanoreceptors found in the skin that detect vibration and pressure. Merkel receptors detect steady pressure and texture.

Additional info:

• Receptors may adapt to constant stimuli, reducing their response over time (adaptation).

• Receptor potentials are graded and may lead to action potentials if threshold is reached.