Chapter 13 - Body Senses: An Overview

Body senses differ from other senses (vision, hearing) because they monitor multiple stimuli.
- Vision monitors light.
- Hearing monitors sound waves.
Body senses involve six types of stimuli:
- Tactile Stimulation: Mechanoreceptors, thermoreceptors.
- Temperature: Thermoreceptors.
- Pain: Nociceptors.
- Proprioception: Monitors body position using various receptors.
- Equilibrioception: Balance and spatial orientation using semicircular canals (discussed in Chapter 10).
- Haptic Perception: Combines several body senses to actively explore objects via touch.
The body senses are more diverse and intricate than vision or hearing.

Brief introduction to various senses.
Transmission of sensory information from peripheral receptors to the spinal cord and the somatosensory cortex.
Episode Breakdown:
- Episode 1: Tactile Perception.
- Episode 2: Proprioception.
- Episode 3: Nociception.
- Episode 4: Thermoreception.
- Episodes 5 & 6: Connection between body and brain.
- Episode 7: Haptic Perception.
- Episode 8: Equilibrioception.

Tactile perception involves mechanoreceptors in the skin.
Tactile perception is based on mechanical deformation of the skin, including:
- Indentation
- Vibration
- Stretching
Skin Types:
- Hairy Skin: Contains hair cells.
- Glabrous Skin: Lacks hair cells.
Skin Layers:
- Epidermis: Outermost layer; elastic and regenerating.
  - Provides a barrier, repels moisture, and prevents water loss.
  - Produces dead skin cells.
- Dermis: Inner layer containing various structures.
  - Includes hair follicles (in hairy skin), nerves, sweat glands, blood vessels, and sebaceous glands.
    - Sebaceous glands secrete sebum to lubricate and waterproof the epidermis.
  - Supports the epidermis with collagen and elastin.
  - Contains mechanoreceptors like Merkel cells, Pacinian corpuscles, Meissner corpuscles, and Ruffini cells.

Mechanoreceptors in the dermis transduce mechanical deformations into electrical signals sent to the brain.
Function similarly to photoreceptors or hair cells by converting external stimuli (vibrations, indentations, stretching) into signals that the brain can interpret.
Signals are sent to the somatosensory cortex (discussed in Episode 5).
Four Main Types of Mechanoreceptors:
- Merkel's Discs
- Ruffini's Corpuscles
- Meissner's Corpuscles
- Pacinian Corpuscles
Free nerve endings will be discussed after mechanoreceptors.

Merkel (SA1) and Meissner (FA1) are Type 1 fibers with small receptive fields.
- Densely arranged near the skin surface.
- Respond to deformations with fine spatial detail.
Ruffini and Pacinian cells are Type 2 fibers with large receptive fields.
- More sparsely distributed and located deeper in the dermis.
- Have lower spatial resolution compared to Type 1 fibers.
Type 1:
- Small receptive fields.
- Near the surface.
- High spatial resolution.
Type 2:
- Large receptive fields.
- Deeper in the dermis.
- Lower spatial resolution.

Merkel and Ruffini cells: Slow-adapting (SA) fibers.
- Begin firing at the onset of stimulation and continue throughout its duration.
- Decrease firing rate only when the stimulus is removed.
Meissner and Pacinian cells: Fast-adapting (FA) fibers.
- Fire at the onset and offset of the stimulus.
- Respond to changes in stimulation.
Real-world function:
- All mechanoreceptors work together to provide information about touch.
- Multiple receptors likely firing simultaneously during activities.

Scenario: Person reaches, grips a box, lifts it, holds it, puts it down, releases grip, and removes hand.
Fast-adapting fibers (Meissner's and Pacinian) increase action potential rate at the onset and offset of stimulus, remaining relatively inactive while holding the box.
Slow-adapting fibers (Merkel and Ruffini) fire continuously while holding the box and stop firing when the box is put down.

Involved in pleasant touch.
Unmyelinated: Transmit information slowly.
Present only in hairy skin and not in glabrous skin.
Respond best to slow, gentle touch at a neutral (body) temperature.
Researchers discovered this through experiments:
- Tactile sensation at three temperatures (cool, neutral, warm).
  - Neutral is body temperature around $98.6^\circ$ Fahrenheit.
- Five velocities of a computer-controlled probe on the forearm (0.3 cm/s to 30 cm/s).
Findings:
- C tactile mechanoreceptors responded most strongly to the slowest speed and neutral temperature
- Other mechanoreceptors increased firing rate with increasing speed, irrespective of temperature.