I. Introduction to Sensory Receptors
Sensory receptors are specialised nerve endings or structures that detect changes in the body's internal or external environment, known as sensory stimuli. Their primary function is to convert these stimuli into nerve impulses, which are then transmitted via the peripheral nervous system (PNS) to the central nervous system (CNS) for processing and interpretation. Neurons, which are electrically excitable cells, use this property to produce and transmit these action potentials. The cell bodies of most peripheral sensory neurons are located in ganglia outside the CNS and originate from the embryonic neural crest.
II. Classification of Sensory Receptors
Sensory receptors can be classified based on the type of stimulus they detect, their location in the body, and their structural complexity.
- By Type of Stimulus Detected:
- Mechanoreceptors: Respond to mechanical forces such as deformation, touch, pressure, vibration, and stretch.
- Thermoreceptors: Detect changes in temperature (heat and cold).
- Photoreceptors: Sensitive to light energy.
- Chemoreceptors: Respond to changes in the chemical composition of interstitial fluid, as well as smell and taste.
- Nociceptors: Sense potentially damaging stimuli, interpreted as pain, including intense heat, cold, pressure, or chemicals.
- By Location in the Body:
- Exteroceptors: Located near the body surface, detecting external stimuli such as touch, pressure, pain, temperature, and the special senses.
- Interoceptors (Visceroceptors): Found within the body's viscera and blood vessels, responding to internal stimuli like chemical changes, tissue stretch, and temperature, leading to sensations such as pain, discomfort, hunger, or thirst.
- Proprioceptors: Situated in skeletal muscles, tendons, joints, ligaments, and muscle sheaths, providing information about body position and movement.
- By Structural Complexity:
- Simple Receptors: These are the majority, consisting of modified dendritic endings of sensory neurons, often unencapsulated (e.g., free nerve endings).
- Complex Receptors: These are less common and include specialised sense organs (collections of cells associated with vision, hearing, smell, and taste) or more complex encapsulated structures.