General Senses

Sensory Input Originates from Receptors:
- Stimulus must excite a receptor, resulting in the generation of an action potential (AP) that must be sent.
Specificity of Receptors:
- Receptors have specificity for stimulus energy, meaning they are designed for a particular type of stimulus.
Receptive Field:
- The stimulus must be applied in a designated receptive field.
Transduction:
- The process through which stimulus energy is converted into an electrical signal called a receptor potential.

Mechanical Receptors:
- Respond to touch, pressure, vibration, and stretch.
Thermoreceptors:
- Sensitive to changes in temperature.
Photoreceptors:
- Respond to light energy (e.g., those found in the retina).
Chemoreceptors:
- Respond to chemicals (e.g., involved in smell and taste, and also related to changes in blood chemistry).
Nociceptors:
- Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Exteroceptors:
- Respond to stimuli arising outside the body and are located at or near the body surface.
- Examples include receptors in the skin for touch, pressure, pain, and temperature and most special sense organs.
Interoceptors (Visceroceptors):
- Respond to stimuli arising within the body, particularly from internal viscera and blood vessels.
- They are sensitive to chemical changes, tissue stretch, and temperature changes.
Proprioceptors:
- Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles.
- They provide information to the brain regarding movements and body position.

Complex Receptors:
- Associated with special senses and are localized collections of cells.
- Include vision, hearing, equilibrium, smell (olfaction), and taste (gustation).
Simple Receptors:
- General senses that are typically tiny and distributed throughout the body.
- Include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.
- Represent cutaneous (skin) sensations.

Tactile (Meissner's) Corpuscles:
- Detect light, discriminative touch.
Lamellar (Pacinian) Corpuscles:
- Respond to deep pressure, stretch, and vibration.
Bulbous Corpuscles:
- Detect deep pressure and stretch.

Structural class	Body location(s)	Stimulus type
Nonencapsulated
Free nerve endings	Most body tissues; especially the epithelia and connective tissues	Primarily pain, heat, and cold
Epithelial tactile complexes	Stratum basale of the epidermis
Hair follicle receptors	Wrapped around hair follicles	Bending of hairs
Encapsulated
Tactile (Meissner's) corpuscles	Dermal papillae of hairless skin	Light pressure, discriminative touch
Bulbous corpuscles	Deep in the dermis, subcutaneous tissue, and joint capsules	Deep pressure and stretch
Lamellar corpuscles	Dermis, subcutaneous tissue, periosteum, tendons, and joint capsules	Deep pressure, stretch, and vibration
Muscle spindles	Skeletal muscles	Muscle stretch (proprioception)
Tendon organs	Tendons	Tendon stretch, tension (proprioception)

Tactile (Meissner) Corpuscles:
- Located in the papillary layer of the dermis.
Lamellar (Pacinian) Corpuscles:
- Located in the reticular layer of the dermis.

Identify Tactile (Meissner) and Lamellar (Pacinian) Corpuscles:
- Use skin model (Ex: 18.1) and slides (Ex: 18.6 & 18.7).
- Questions to ask:
- Where is each receptor located?
- What type of stimulus does each receptor detect?
Reference Chapters 18 & Supplement Exercise for additional information.

Transduction:
- The process by which environmental stimuli are transformed into nerve impulses, which are relayed to the central nervous system (CNS) as sensory input.
Sensation vs. Perception:
- Sensation refers to the awareness of the stimulus, while perception is the conscious interpretation of that stimulus, both of which occur in the brain.
Cortical Mapping:
- Impulses from cutaneous (skin) receptors are relayed to the somatosensory cortex.
- Stimuli from different body regions form a body map, with each body location represented by a specific cortical area.
- This arrangement allows for precise localization of sensations on the body.

Tactile (Meissner's) Corpuscles Stimulation:
- This activity involves determining the two-point threshold, which is the smallest distance at which two points of contact can be felt.
- A smaller distance indicates a higher density of receptors in the receptive field leading to greater discriminatory ability.

Localization Accuracy:
- The brain's ability to determine which area of the skin is being touched.
- A smaller receptive field correlates with greater accuracy in localization, resulting in a smaller error margin.

Tonic Receptors:
- These receptors maintain sensitivity to a stimulus consistently, even with continued exposure.
Phasic Receptors:
- These receptors primarily detect new stimuli or changes in a stimulus.
- The rate of receptor discharge slows down when a stimulus is applied without movement for an extended period.
Adaptation Examples:
- Touch receptors (Tactile/Lamellar Corpuscles) gradually adapt to continued stimulation.
- Temperature receptors (Free Nerve Endings) adapt separately for hot and cold sensations.

Definition:
- Referred pain is experienced in one area of the body while the stimulus originates from another area.
Activity for Referred Pain Testing:
- Free nerve endings are typically the stimulated receptors involved in this phenomenon.

Receptor Anatomy:
- Identify tactile corpuscles and lamellar corpuscles on the skin model and slides.
Receptor Physiology Activities:
- Two-point threshold measurement (using calipers).
- Tactile localization assessment (measuring the error of localization).
- Adaptation of touch receptors (using coins).
- Adaptation of temperature receptors (using bowls of water at different temperatures).
- Referred pain activity (using ice water on the elbow to stimulate the ulnar nerve).