Sensation, Perception, and Sensory Receptors: Study Notes for Anatomy & Physiology II

Overview of A&P II Topics

• Senses (general, special)

• Endocrine System

• Cardiovascular System (blood, heart, & vessels)

• Immunity

• Respiratory System

• Digestive System (part 1 & 2)

• Metabolism/Nutrition

• Urinary System

• Reproduction

• Development

Sensation vs. Perception

Introduction

The human nervous system enables us to detect and interpret stimuli from our environment. Sensation and perception are two fundamental processes that allow us to experience the world.

• Sensation: The process of detecting stimuli through sensory receptors. It involves four key steps: stimulation, transduction, conduction, and translation.

• Perception: The conscious awareness and interpretation of sensory information.

• Example: Feeling the warmth of the sun on your skin (sensation) and recognizing it as sunlight (perception).

Sensation, Motor Output, & Integration

Processes Involved

Sensation requires a series of steps to convert external stimuli into meaningful information for the brain.

• Stimulation: A stimulus activates a sensory receptor.

• Transduction: The stimulus is converted into an electrical signal.

• Conduction: The electrical signal is transmitted to the central nervous system (CNS).

• Translation: The CNS interprets the signal, resulting in perception.

Motor output refers to the response generated by the CNS, such as muscle movement or gland secretion.

Sensory Transduction

Conversion of Stimulus to Electrical Signal

Sensory transduction is the process by which sensory receptors convert external stimuli into electrical signals that can be interpreted by the nervous system.

• Receptor potentials: Graded electrical changes produced when a stimulus is detected, often due to the influx of sodium ions ($Na^+$).

• Action potentials: Generated when receptor potentials reach a threshold, allowing the message to be conducted to the CNS.

• Adaptation: The phenomenon where a stimulus remains but action potentials stop. There are two types: rapid and slow adaptation.

Example: Touching a hot surface produces a receptor potential, which may lead to an action potential if the stimulus is strong enough.

Sensory Receptors – Classification

Types and Structures

Sensory receptors are specialized cells or structures that detect specific types of stimuli. They can be classified based on microscopic structure, location, and the type of stimulus detected.

• Microscopic structure: Free nerve endings, encapsulated endings, or separate cells.

• Location: Exteroceptors (detect external stimuli) and interoceptors (detect internal stimuli).

• Type of stimulus detected: Mechanoreceptors (mechanical), thermoreceptors (temperature), nociceptors (pain), photoreceptors (light), chemoreceptors (chemicals).

Example: Mechanoreceptors in the skin detect pressure and vibration.

Mechanoreceptors – Classification

Types of Mechanoreceptors

Mechanoreceptors are sensory receptors that respond to mechanical forces such as touch, pressure, stretch, and vibration.

• Merkel cell fibers: Slow-adapting; detect fine touch.

• Meissner (tactile) corpuscles: Rapid-adapting; detect fine touch.

• Ruffini endings (corpuscles): Slow-adapting; detect stretch and movement.

• Pacinian (lamellated) corpuscles: Rapid-adapting; detect vibration and pressure.

Example: Pacinian corpuscles allow us to feel vibrations from a phone.

Additional Mechanoreceptors

• Hair follicle receptors: Free nerve endings at the base of hair follicles; detect hair movement.

• Proprioceptors: Located in muscles, tendons, and joints; detect body position and movement (the "kinesthetic sense").

Example: Muscle spindles help maintain posture by sensing muscle stretch.

Sensory Neurons – Speed & Receptive Field

Conduction Speed and Discrimination

Sensory neurons transmit information at different speeds depending on their structure and function.

• Fast conduction: Neurons with large axon diameter and heavy myelination; associated with proprioception.

• Slow conduction: Neurons with small axon diameter and little myelination; associated with pain and temperature.

• Touch discrimination: The ability to identify the type and source of touch stimuli increases with higher density of small-field receptors.

Dermatomes and Pain Perception

Mapping Sensory Pathways and Pain Types

Dermatomes and nociceptors play key roles in mapping sensory input and detecting pain.

• Dermatomes: Regions of skin innervated by specific spinal nerves; used to map sensory pathways.

• Nociceptors (pain receptors): Provide information about tissue damage and disease symptoms.

• Pain localization: Pain can be somatic (superficial or deep), visceral, or referred. Pain may be fast or slow depending on the type of fibers involved.

Table: Types of Sensory Receptors

Receptor Type	Stimulus Detected	Location	Adaptation
Merkel cell fibers	Fine touch	Skin	Slow
Meissner corpuscles	Fine touch	Skin (dermal papillae)	Rapid
Ruffini endings	Stretch & movement	Skin	Slow
Pacinian corpuscles	Vibration & pressure	Deep skin layers	Rapid
Hair follicle receptors	Hair movement	Base of hair follicle	Rapid
Proprioceptors	Body position & movement	Muscles, tendons, joints	Variable

Key Equations

• Action Potential Generation:

$\text{If } V_{m} \geq V_{\text{threshold}}, \text{ then action potential is generated}$

• Receptor Potential:

$\text{Receptor potential} \propto \text{stimulus intensity}$

Summary

• Sensation and perception are essential for interpreting environmental stimuli.

• Sensory transduction converts stimuli into electrical signals for the CNS.

• Sensory receptors are classified by structure, location, and stimulus type.

• Mechanoreceptors detect mechanical changes; proprioceptors sense body position.

• Sensory neurons vary in conduction speed and receptive field size, affecting discrimination and pain perception.

• Dermatomes and nociceptors help map sensory pathways and localize pain.