Sensory System Overview

Introduction to the Sensory System

The sensory system enables humans to perceive and respond to both external and internal stimuli, allowing us to experience the world and maintain homeostasis.

• External information: Examples include the sound of a dog barking, visual cues, and environmental temperature.

• Internal information: Includes changes such as a sudden shift in blood pressure or internal pain.

Cells That Detect Stimuli

Types of Sensory Receptors

Receptors are specialized areas of sensory neurons that detect specific types of stimuli. Each type of receptor is adapted to respond to a particular modality.

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

• Pain receptors (nociceptors): Respond to tissue injury, chemicals, or hypoxia.

• Thermoreceptors: Sense changes in temperature.

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

• Photoreceptors: Detect light (found in the retina of the eye).

Components of Sensation

Four Essential Components

Sensation involves a sequence of events that allow the body to detect and interpret stimuli.

1. Stimulus: A change in the environment that activates a receptor.

2. Receptor: Specialized cell or structure that detects the stimulus.

3. Sensory nerve: Transmits the signal from the receptor to the brain.

4. Brain: Interprets the signal, resulting in conscious perception.

Characteristics of Sensation

Projection and Adaptation

Sensations have two key characteristics that influence how stimuli are perceived and processed.

• Projection: The brain refers the sensation back to its source, allowing localization of the stimulus. For example, pain from an injury is felt at the site of injury, and phantom limb pain occurs when the brain projects sensation to a missing limb.

• Adaptation: With continuous stimulation, sensory receptors become less responsive. Some receptors, such as those for smell and temperature, adapt quickly, while pain receptors do not adapt.

General Senses

Five General Senses

The general senses are distributed throughout the body and provide information about the environment and body status.

• Pain

• Touch

• Pressure

• Temperature

• Proprioception (sense of body position and movement)

Illustration of General Senses

General senses can be visualized as follows:

• Pain: Detected by nociceptors in skin and internal tissues.

• Touch: Mechanoreceptors in the skin respond to light contact.

• Pressure: Mechanoreceptors in deeper tissues respond to sustained force.

• Temperature: Thermoreceptors detect heat and cold.

• Proprioception: Receptors in muscles and joints provide information about limb position.

Pain Receptors (Nociceptors)

Structure and Function

Pain receptors, or nociceptors, are free nerve endings that respond to potentially damaging stimuli.

• Stimulated by tissue injury, chemicals, or tissue hypoxia.

• Widely distributed throughout the skin, viscera, and other internal tissues.

• Do not adapt to continuous stimulation, ensuring persistent awareness of harmful conditions.

Summary Table: Types of Sensory Receptors

Key Concepts and Applications

• Example: When you touch a hot surface, thermoreceptors and nociceptors are activated, sending signals to the brain for interpretation as heat and pain.

• Clinical Application: Understanding projection helps explain phenomena such as referred pain and phantom limb pain in patients.

Additional info: These notes summarize the foundational concepts of the sensory system, including receptor types, sensation components, and general senses, as covered in a college-level Anatomy & Physiology course.