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What is the Peripheral Nervous System (PNS) - and what structures does it include?

All neural structures outside the brain:
1. Sensory receptors
2. Peripheral nerves and associated ganglia
3. Motor endings

Sensory Receptors:
1. Specialized to respond to changes in their environment (stimuli)
2. Activation results in graded potentials that trigger nerve impulses
3. Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

Classification of Receptors Based on:
A. Stimulus type
B. Location
C. Structural complexity

What do Mechanoreceptors, Thermoreceptors, Photoreceptors, Chemoreceptors, and
Nociceptors respond to?

1. Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch
2. Thermoreceptors—sensitive to changes in temperature
3. Photoreceptors—respond to light energy (e.g., retina)
4. Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)
5. Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Differences between Exteroceptors, Interoceptors, Proprioceptors

Classification by Location:
1. Exteroceptors
Respond to stimuli arising outside the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs
2. Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
3. Proprioceptors
Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
Inform the brain of one's movements

Classification by Structural Complexity:
1. Complex receptors (special sense organs)
Vision, hearing, equilibrium, smell, and taste (Chapter 15)
2. Simple receptors for general senses:
Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense
*Unencapsulated (free) or encapsulated dendritic endings

*Unencapsulated Dendritic Endings
Free or naked nerve endings found everywhere in the body but especially in epithelial and connective tissue. Most are unmyelinated, small in diameter with knoblike swellings at the end
Cold receptors (10-40ºC); in superficial dermis
Heat receptors (32-48ºC); in deeper dermis
Respond to:
Chemicals from damaged tissue
Temperatures outside the range of thermoreceptors
-Light touch receptors
Tactile (Merkel) discs
Hair follicle receptors

*Encapsulated Dendritic Endings
They consist of one or more fiber terminals of sensory neurons enclosed in a connective tissue capsule
-All are mechanoreceptors
-Meissner's (tactile) corpuscles—discriminative touch
-Pacinian (lamellated) corpuscles—deep pressure and vibration
-Ruffini endings—deep continuous pressure
-Muscle spindles—muscle stretch
-Golgi tendon organs—stretch in tendons
-Joint kinesthetic receptors—stretch in articular capsules

What is Sensation and Perception?

Sensation: the awareness of changes in the internal and external environment
Perception: the conscious interpretation of those stimuli

Sensory Integration:
- Input comes from exteroceptors, proprioceptors, and interoceptors
- Input is relayed toward the head, but is processed along the way
- Levels of neural integration in sensory systems:
1. Receptor level—the sensor receptors
2. Circuit level—ascending pathways
3. Perceptual level—neuronal circuits in the cerebral cortex


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