Overview
This lecture explains how the sense of touch works, describing tactile receptors, their function, and how touch signals are transmitted and processed in the nervous system.
Tactile Receptors and Stimuli
- Touch (tactile sense) is the perception of physical contact with the skin.
- Mechanical stimuli (pressure, stretch, vibration, movement) deform tactile receptors.
- Deformation opens ion channels, leading to cell depolarization.
- If depolarization reaches threshold, action potentials are generated and sent to the brain.
Types and Functions of Tactile Receptors
- Tactile receptors are nerve endings located in skin and deeper tissues.
- Receptors can be encapsulated (with connective tissue) or unencapsulated.
- Rapidly-adapting receptors respond to changes but quickly stop firing with continuous stimulus; they detect movement and changes.
- Slowly-adapting receptors keep firing during constant stimulation and detect steady pressure, texture, edges, and shapes.
- Some receptors are sensitive to light touch, others to heavy pressure.
Receptive Fields and Sensitivity
- Each sensory neuron covers a receptive field; touch anywhere in this field sends a single signal.
- On the back, receptive fields are large, so two points may feel like one touch.
- On the fingers, receptive fields are small, allowing two close points to be distinguished.
- Small receptive fields = higher sensitivity and detection of lower vibration frequencies; large receptive fields = lower sensitivity, detect higher frequencies.
Touch Signal Pathway
- Touch pathway has three neurons:
- First-order neurons (touch receptors) send signals through sensory fibers into the spinal cord.
- Second-order neurons (in spinal cord or medulla) cross to the opposite side and ascend to the thalamus.
- Third-order neurons carry information from thalamus to sensory cortex.
- Face and head signals travel mainly via the trigeminal nerve to brainstem and thalamus.
Sensory Cortex Organization
- The sensory cortex is organized so neighboring areas process touch from adjacent body regions.
- The amount of cortex dedicated to a body part relates to its sensory receptor density.
- Highly sensitive areas (fingers, face) have larger cortical representation than less sensitive areas (trunk).
Key Terms & Definitions
- Tactile receptor — Nerve ending that detects touch or mechanical stimuli.
- Receptive field — Area of skin monitored by a single sensory neuron.
- Rapidly-adapting receptor — Responds quickly to changes but stops firing with constant stimulus.
- Slowly-adapting receptor — Continues firing as long as the stimulus is present.
- Action potential — Electrical signal sent along neurons.
- Thalamus — Brain structure relaying sensory information to the cortex.
- Sensory cortex — Brain area processing touch and other sensory input.
Action Items / Next Steps
- Review the pathway of touch signal transmission.
- Study differences between rapidly- and slowly-adapting receptors.
- Practice identifying receptive field sizes on various body parts.