Sensory Physiology: General Properties of Sensory Systems

General Properties of Sensory Systems

Overview of Sensory Systems

Sensory systems allow the body to detect and interpret various forms of external and internal 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, which act as transducers converting energy into intracellular signals (usually changes in membrane potential).

• Threshold: Minimum stimulus intensity 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, Hearing, Taste, Smell, Equilibrium	Touch, Temperature, Pain, Itch, Proprioception
Stimulus Processing Usually Subconscious	Somatic Stimuli	Visceral Stimuli
	Muscle length and tension, Proprioception	Blood pressure, 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

Receptors and Forms of Energy

Types of Sensory Receptors

Sensory receptors are specialized to detect specific forms of energy and convert them into neural signals.

• Neural receptors:

  • Naked ("free") nerve endings

  • Complex neural receptors encased in connective tissue capsules (may be myelinated or unmyelinated)

• Non-neural receptors:

  • Specialized cells associated with sensory neurons (e.g., hair cells in the ear)

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

Major Groups of Sensory Receptors

• 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 temperature changes

• Photoreceptors: Respond to light (vision)

Examples of Sensory Receptor Structures

• Simple receptors: Free nerve endings, unmyelinated axons

• Complex receptors: Encapsulated nerve endings, myelinated axons

• Special sense receptors: Specialized cells (e.g., hair cells in the ear) synapsing with sensory neurons

Table: Types of Sensory Receptors

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

Key Terms and Concepts

• Transducer: A device or cell that converts one form of energy into another (e.g., sensory receptor converting physical stimulus into electrical signal).

• Threshold: The minimum level of stimulus required to activate a receptor and generate an action potential.

• Receptor potential: A graded change in membrane potential in response to a stimulus.

• Integration: The process by which the CNS interprets and responds to sensory input.

Example: Mechanoreceptors in the Skin

• Pacinian corpuscles: Detect vibration and pressure.

• Merkel receptors: Detect steady pressure and texture.

Additional info: The complexity and specialization of sensory receptors allow the nervous system to detect and discriminate a wide range of stimuli, contributing to perception and homeostasis.