Week 5: Senses - Completed

Introduction to the Senses

• Sensory system processes information to generate awareness of the environment.
• Sensations influence motor activity for interaction with the world.
• Categories of Sensations:
  • General Senses: Touch, pain, temperature, proprioception, vibration, pressure.
  • Special Senses: Vision, taste, smell, hearing, equilibrium.
• Processes differ for general and special senses; special senses processed via cranial nerves.

Somatic and Visceral Senses

• Somatic Sensory Division: Detects general and special senses.
• Visceral Sensory Division: Monitors changes in digestive system and organs.
• Motor Division: Facilitates movement or gland secretion, not sensation.

Receptors and Sensory Functions

• Thermoreceptors: Respond to thermal stimulation (heat or cold); e.g., wrapping hands around a warm cup.
• Dermatomes: Skin regions supplied by spinal nerves for somatic sensations.
• Myotomes: Muscle groups attached to spinal nerves.
• Lines of Cleavage: Parallel to collagen fibers in skin; relevant in healing and surgery.
• Rule of Nines: Tool for assessing total body surface area involved in burns.

Sensation Versus Perception

• Sensation: Activation of sensory receptors by stimuli, resulting in action potentials relayed to CNS.
• Perception: The brain's selection, organization, and interpretation of sensations.
  • Sensation leads to perception but is distinct from it.
  • Not all sensations are perceived.

Sensory Adaptation

• Reduction in Sensitivity: Occurs with constant exposure to a stimulus; beneficial for focusing on other stimuli.
• Phasic Receptors: Fast-adapting receptors that respond quickly to stimuli but shut down on continuous exposure.
• Tonic Receptors: Slowly adapting receptors that provide continuous frequency of action potentials for the duration of stimuli.

Receptor Types

• The conventional belief of five senses is an oversimplification; numerous sensory systems provide nuanced information about the environment.
  • Special Sense Organs: Complex structures (e.g., vision, hearing) requiring specific cells for signal transduction.
  • General Senses: More widely distributed; include free nerve endings and encapsulated receptors.
  • Sensory Transduction: Conversion of stimulus into an action potential.

Categories of Sensory Receptors

• Classification Criteria: Based on distribution, origin of stimuli, and cell types.
• Structure/Classifications:
  • Free nerve endings (pain, temperature, light touch).
  • Encapsulated nerve endings (lamellated corpuscles for pressure).
  • Specialized receptor cells (photoreceptors in the retina).

Functional Classification of Receptors

• Thermoreceptors: Detect temperature changes; internal (hypothalamus) and external (skin).
• Mechanoreceptors: Respond to mechanical changes (touch, pressure, stretch); includes tactile receptors, baroreceptors, and proprioceptors.
• Photoreceptors: Detect light (converts light into electrical signals).
• Chemoreceptors: Respond to chemicals in taste and smell, and body fluids.
• Nociceptors: Sensing pain from tissue damage through various stimuli.

Pain and Nociceptors

• Nociceptors: Free nerve endings sensitive to various painful stimuli, classified into fast (sharp pain) and slow (aching pain).
• Referred Pain: Sensation of pain in locations different from its source due to nerve signal misinterpretation by the brain (e.g., heart attack pain felt in arm).

Tactile Sensations

• Combination of Sensory Modalities: Tactile sensation includes touch, pressure, vibration, itch, and tickle.
• Different Receptors: Free nerve endings, Meissner's corpuscles (light touch), Merkel cells (pressure), Ruffini corpuscles (stretch), Pacinian corpuscles (vibration).

Receptive Fields and Discrimination

• Receptive Field: Area monitored by a receptor cell; smaller fields allow for better localization.
• Two-point Discrimination: Ability to distinguish two points touching the skin; varies by body area.
• Tactile Localization: Ability to determine specific skin regions that have been stimulated; sensory homunculus represents this area distribution in the brain.

Summary of Sensory Systems

• Various receptors enable detection of different stimuli:
  • Nociceptors: Sense tissue damage.
  • Thermoreceptors: Detect temperature changes.
  • Mechanoreceptors: Sense mechanical changes (touch, pressure).
  • Photoreceptors: Sense light.
  • Chemoreceptors: Respond to chemicals.
  • Proprioceptors: Sense movement of muscles and joints.

Conclusion

• Integration of all senses is essential for interaction with the environment and cognitive functioning. Understanding the complexity and specific functions of sensory receptors aids in recognizing their roles in perception and adaptation to stimuli.