Human Physiology HSCI 3020: Peripheral Nervous System and Sensory Afferents

Introduction to Human Physiology

Peripheral Nervous System

• Focus: Tactile and visceral afferents

• Definition of Afferents: These are mechanoreceptors that innovate tactile receptors which respond to stimuli such as pressure and touch.

Mechanoreceptors

• Mechanoreceptors allow us to perceive our environment through various modalities.

• They are essential in creating perceptions which enhance our acuity and localization of different environmental stimuli.

Types of Receptors

1. Haptic Receptors

   • Types discussed:

     • Hair receptors

     • Merkel's discs

     • Pacini corpuscles

     • Ruffini endings

     • Meissner's corpuscles

2. Receptor Characteristics:

   • Discs vs. Corpuscles: The distinction is significant in their structural makeup and sensory responsibilities.

   Merkel's Discs

   • Function: Sense light touch and texture; activated by sustained touch.

   • Applications: Reading Braille—these discs have a sensory field and respond to touch and texture.

   • Mediated by: Large myelinated nerve fibers.

   Meissner's Corpuscles

   • Function: Detect light touch and fluttering sensations (i.e., tickling).

   • Structural Insight: Resemble an onion structure; deeper under the skin.

   • Sensation Mechanism: Reduce sensitivity after sustained pressure until relief occurs, illustrating tissue disruption sensitivity.

   Pacini Corpuscles

   • Function: Sensitive to deep pressure and vibration.

   • Distinct Characteristic: Mediate the sensation of deep vibration, offering distinct responses compared to lighter pressures of Meissner's corpuscles.

   Ruffini Endings

   • Function: Responsible for sensing deep sustained pressure and skin stretch.

Sensory Modalities and Perception

• Insight: Different types of receptors create varied sensory modalities, contributing to overall sensory perception.

• Tactile perception comprises light, sustained touch and deep pressure, among others.

Sensory Adaptation

• Definition: The process where sensation diminishes over time when an external stimulus remains constant.

  • Example: Feeling a pressure when it starts (like placing a quarter on an arm) but not sensing it continuously unless the pressure is released.

• Mechanism of Adaptation:

  • Involves sensory receptors adjusting to ongoing stimuli. For example, a Pacini corpuscle registers pressure only while layers are moving, feeling the pressure upon initiation or release, but not when stationary.

• Contrast Clarification: Differentiate between adaptation (peripheral adjustment) and habituation (central nervous system process, potentially outside this discussion).

Visceral Stimuli

• Definition: Information sourced from the viscera or internal organs, often operating beneath conscious awareness.

• Examples:

  • Conscious Sensations: Hunger, fullness.

  • Unconscious Processes: Gastric motility, blood pressure, blood glucose regulation, blood oxygen levels, and osmolarity.

Homeostatic Control

• Control Mechanism: Involves maintaining tight regulation of physiological parameters through peripheral afferent signals relaying information to the central nervous system.

• Example of Controlled Variables:

  • Blood Pressure, Oxygen, Osmolarity

• Pathway of Sensory Information:

  • Sensor Types:

    • Baroreceptors (sensing pressure changes)

    • Chemoreceptors (sensing chemical levels)

    • Osmoreceptors (sensing fluid balance)

  • Integration: Signals processed by the central nervous system to elicit appropriate responses that maintain homeostatic set points.

Conclusion

• Summary of Learning:

  • Tactile and visceral afferents play crucial roles in our sensory perception of both the external environment and internal states.

  • Understanding adaptation versus habituation is key to comprehending sensory processing.

• Next Lecture Focus: Sensory acuity and pain will be discussed in the following session, building upon the knowledge of the current lecture.