Chapter 10: Sensory Physiology – Structure and Function of Sensory Systems

Chapter 10: Sensory Physiology

Introduction to Sensory Physiology

Sensory physiology explores how the body detects, transduces, and interprets information from the environment and internal milieu. Specialized sensory receptors convert various forms of energy into neural signals, which are then processed by the nervous system to produce perception and guide behavior.

Structure of Sensory Receptors in the Skin

The skin contains multiple types of sensory receptors, each specialized for detecting specific stimuli such as pressure, texture, stretch, and pain. These receptors are distributed throughout the skin and connect to sensory nerves that transmit information to the spinal cord and brain.

• Merkel receptors: Sense steady pressure and texture.

• Meissner's corpuscles: Detect light touch and low-frequency vibration.

• Ruffini endings: Respond to skin stretch.

• Pacinian corpuscles: Detect deep pressure and high-frequency vibration.

• Free nerve endings: Sense pain (nociception), temperature, and crude touch.

• Sensory nerves: Carry signals from these receptors to the spinal cord.

System: Senses External and Internal Environment

Overview

The sensory system is responsible for detecting changes in the external and internal environment and relaying this information to the central nervous system (CNS) for processing. Sensory information may or may not reach conscious perception, depending on how it is processed by the brain.

• External senses: Include vision, hearing, touch, taste, and smell.

• Internal senses: Monitor internal body conditions such as blood pressure, pH, and body position.

Sensory Specificity Establishment

Sensory specificity refers to the ability of receptors to respond preferentially to certain types of stimuli. This is established by several factors:

1. Receptor specificity: Each receptor is tuned to a particular type of stimulus (modality).

2. Threshold: The stimulus must be above a minimum strength to be detected.

3. Stimulus location and modality: The origin and type of stimulus are encoded by the receptor and neural pathway.

4. Location of sensory projection: The brain determines the origin of the signal based on the neural pathway activated.

5. Stimulus intensity and duration: The strength and length of the stimulus are encoded in the frequency and pattern of action potentials projected to the CNS.

Processing of Sensory Information

• Conscious perception: Some sensory information is consciously perceived, while other information is processed subconsciously or ignored (e.g., zoning out).

• Threshold (absolute threshold): The minimum amount of stimulus strength required to perceive a sensation.

• Inhibitory modulation: The CNS can decrease perception by inhibiting sensory pathways, causing some stimuli to fall below the threshold for conscious awareness.

Sensory Receptors

Definition and Function

A sensory receptor is a specialized cell or structure that detects an identifiable stimulus, such as light, chemicals, or mechanical pressure. Each receptor has a modality to which it is most sensitive, known as its adequate stimulus.

• Adequate stimulus: The type of stimulus to which a receptor is most responsive. For example, photoreceptors are most responsive to light but may respond to other stimuli if intense enough.

• Receptor potentials: Graded electrical potentials generated by sensory receptors in response to stimuli. If the receptor potential reaches threshold, it triggers action potentials in the associated sensory neuron.

Sensory Transduction

Sensory transduction is the process by which sensory receptors convert physical or chemical stimuli into electrical signals (action potentials) that the CNS can interpret.

Types of Sensory Receptors Based on Adequate Stimuli

• Mechanoreceptors: Respond to mechanical forces such as pressure, vibration, and stretch.

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to light (electromagnetic energy).

• Chemoreceptors: Detect chemical stimuli (e.g., taste, smell, blood pH).

• Nociceptors: Respond to potentially damaging stimuli, signaling pain.

Summary Table: Types of Sensory Receptors

Receptor Type	Adequate Stimulus	Example Location	Function
Mechanoreceptor	Mechanical pressure, stretch	Skin, ear, blood vessels	Touch, hearing, blood pressure regulation
Thermoreceptor	Temperature changes	Skin, hypothalamus	Detect heat and cold
Photoreceptor	Light (photons)	Retina of the eye	Vision
Chemoreceptor	Chemicals (e.g., O2, CO2, taste molecules)	Nose, tongue, blood vessels	Smell, taste, blood chemistry
Nociceptor	Painful or damaging stimuli	Skin, viscera	Pain perception

Key Concepts and Definitions

• Modality: The type of stimulus detected (e.g., light, sound, pressure).

• Transduction: Conversion of a stimulus into an electrical signal.

• Threshold: The minimum stimulus intensity required to generate a response.

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

• Action potential: An all-or-none electrical signal propagated along neurons.

Example: Mechanoreceptors in the Skin

• Merkel cells: Detect steady pressure and texture, important for fine tactile discrimination.

• Meissner's corpuscles: Sensitive to light touch, especially in hairless skin (e.g., fingertips).

• Pacinian corpuscles: Respond to deep pressure and vibration.

• Ruffini endings: Detect skin stretch, contributing to the perception of object manipulation.

Summary

Sensory physiology is fundamental to understanding how organisms interact with their environment. Specialized receptors detect specific stimuli, transduce them into electrical signals, and relay this information to the CNS for processing and perception. The diversity of sensory receptors allows for the detection of a wide range of environmental and internal cues, ensuring survival and adaptation.