Overview
This lecture covers sensory pathways and nerves in the peripheral nervous system, focusing on how sensory information is detected, transmitted, and interpreted from the body to the central nervous system.
Types of Sensory Receptors
- Sensory receptors detect stimuli and send signals to the central nervous system.
- Exteroceptors sense external conditions (e.g., touch), while interoceptors sense internal body conditions.
- Three main receptor types: mechanoreceptors (pressure/touch), chemoreceptors (chemicals like pH, taste), and electromagnetic receptors (light/vision).
Sensory Signal Processing Steps
- Sensory response involves: 1) Stimulation of receptors; 2) Transduction (conversion of stimulus into nerve impulse); 3) Transmission (signal sent along nerves); 4) Interpretation (brain perceives signal).
- Receptor potentials are graded, summing up the stimulus strength to determine if action potential will occur.
Mechanoreceptors: Touch and Pressure
- Mechanoreceptors are activated when their membranes are physically distorted.
- Cutaneous receptors in the skin include nociceptors (pain), thermoreceptors (temperature), and various mechanoreceptors (touch/pressure).
- Nociceptors (pain receptors) are free nerve endings sensitive to damage or noxious chemicals like capsaicin.
Thermoreceptors: Temperature
- Thermoreceptors detect heat or cold, with more cold receptors than warm receptors in the skin.
- These receptors use transient receptor potential ion channels to respond to temperature changes.
Touch Receptors and Types
- Touch mechanoreceptors include Meissner's corpuscles (fine touch, phasic, small receptive fields, superficial), Merkel's discs (pressure, tonic, deeper), and Pacinian corpuscles (deep pressure, large receptive fields, encapsulated).
- Tonic receptors (e.g., Merkel's discs) fire continuously; phasic receptors (e.g., Meissner's corpuscles) fire only when stimulated.
Other Sensory Receptors in Skin, Muscles, and Joints
- Receptors also exist in muscles and joints to sense stretch and tension, preventing injury.
- Proprioceptors provide information about body position in space.
- Muscle spindles and Golgi tendon organs limit excessive muscle force, protecting muscles and tendons.
Nerve Transmission Speed
- Larger axon diameter and myelination increase nerve signal transmission speed.
- Most sensory neurons are myelinated for faster, more efficient signaling.
Key Terms & Definitions
- Exteroceptor — receptor sensing external environment.
- Interoceptor — receptor sensing internal body state.
- Mechanoreceptor — detects mechanical changes (touch, pressure).
- Chemoreceptor — detects chemical stimuli.
- Electromagnetic receptor — detects light/EM signals.
- Transduction — converting a stimulus into a neural signal.
- Receptor potential — graded change in membrane potential in response to a stimulus.
- Nociceptor — pain receptor.
- Thermoreceptor — temperature-sensitive receptor.
- Phasic receptor — responds to stimulus change, not constant stimulation.
- Tonic receptor — responds continuously while stimulus is present.
- Proprioceptor — senses body position and movement.
Action Items / Next Steps
- Review the roles and mechanisms of different skin mechanoreceptors.
- Prepare for the next lecture covering dermatomes and sensory signaling pathways.