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13
The Peripheral Nervous System and Reflex Activity:
Part A
Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
Sensory Receptors
• Specialized to respond to changes in their environment (stimuli)
• Activation results in graded potentials that trigger nerve
impulses
• Sensation (awareness of stimulus) and perception
(interpretation of the meaning of the stimulus) occur in the brain
Classification of Receptors
• Based on:
• Stimulus type
• Location
• Structural complexity
Classification by Stimulus Type
• Mechanoreceptors—respond to touch, pressure, vibration, stretch, and
itch
• Thermoreceptors—sensitive to changes in temperature
• Photoreceptors—respond to light energy (e.g., retina)
• Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in
blood chemistry)
• Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold,
excessive pressure, inflammatory chemicals)
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
Classification by Location
2.Interoceptors (visceroceptors)
• Respond to stimuli arising in internal viscera and blood vessels
• Sensitive to chemical changes, tissue stretch, and temperature
changes
Classification by Location
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
• Thermoreceptors
• Cold receptors (10–40ºC); in superficial dermis
• Heat receptors (32–48ºC); in deeper dermis
Unencapsulated Dendritic Endings
• Nociceptors
• Respond to:
• Pinching
• Chemicals from damaged tissue
• Temperatures outside the range of thermoreceptors
• Capsaicin
Unencapsulated Dendritic Endings
• Light touch receptors
• Tactile (Merkel) discs
• Hair follicle receptors
Encapsulated Dendritic Endings
• All are mechanoreceptors
•
•
•
•
•
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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
From Sensation to Perception
• Survival depends upon 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
Sensory Integration
• 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
Processing at the Receptor Level
• Receptors have specificity for stimulus energy
• Stimulus must be applied in a receptive field
• Transduction occurs
• Stimulus energy is converted into a graded potential called a
receptor potential
Processing at the Receptor Level
• In general sense receptors, the receptor potential and
generator potential are the same thing
stimulus

receptor/generator potential in afferent neuron

action potential at first node of Ranvier
Processing at the Receptor Level
• In special sense organs:
stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)
Adaptation of Sensory Receptors
• Adaptation is a change in sensitivity in the presence of a
constant stimulus
• Receptor membranes become less responsive
• Receptor potentials decline in frequency or stop
Adaptation of Sensory Receptors
• Phasic (fast-adapting) receptors signal the beginning or end of
a stimulus
• Examples: receptors for pressure, touch, and smell
• Tonic receptors adapt slowly or not at all
• Examples: nociceptors and most proprioceptors
Processing at the Circuit Level
• Pathways of three neurons conduct sensory impulses upward to the
appropriate brain regions
• First-order neurons
• Conduct impulses from the receptor level to the second-order neurons in the
CNS
• Second-order neurons
• Transmit impulses to the thalamus or cerebellum
• Third-order neurons
• Conduct impulses from the thalamus to the somatosensory cortex
(perceptual level)
Processing at the Perceptual Level
• Identification of the sensation depends on the specific location of the
target neurons in the sensory cortex
• Aspects of sensory perception:
• Perceptual detection—ability to detect a stimulus (requires summation of
impulses)
• Magnitude estimation—intensity is coded in the frequency of impulses
• Spatial discrimination—identifying the site or pattern of the stimulus (studied
by the two-point discrimination test)
Main Aspects of Sensory Perception
• Feature abstraction—identification of more complex aspects
and several stimulus properties
• Quality discrimination—the ability to identify submodalities of a
sensation (e.g., sweet or sour tastes)
• Pattern recognition—recognition of familiar or significant
patterns in stimuli (e.g., the melody in a piece of music)
Perception of Pain
• Warns of actual or impending tissue damage
• Stimuli include extreme pressure and temperature, histamine,
K+, ATP, acids, and bradykinin
• Impulses travel on fibers that release neurotransmitters
glutamate and substance P
• Some pain impulses are blocked by inhibitory endogenous
opioids
Structure of a Nerve
• Cordlike organ of the PNS
• Bundle of myelinated and unmyelinated peripheral axons
enclosed by connective tissue
Structure of a Nerve
• Connective tissue coverings include:
• Endoneurium—loose connective tissue that encloses axons and
their myelin sheaths
• Perineurium—coarse connective tissue that bundles fibers into
fascicles
• Epineurium—tough fibrous sheath around a nerve
Classification of Nerves
• Most nerves are mixtures of afferent and efferent fibers and somatic and
autonomic (visceral) fibers
• Pure sensory (afferent) or motor (efferent) nerves are rare
• Types of fibers in mixed nerves:
• Somatic afferent and somatic efferent
• Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal nerves
Ganglia
• Contain neuron cell bodies associated with nerves
• Dorsal root ganglia (sensory, somatic) (Chapter 12)
• Autonomic ganglia (motor, visceral) (Chapter 14)
Regeneration of Nerve Fibers
• Mature neurons are amitotic
• If the soma of a damaged nerve is intact, axon will regenerate
• Involves coordinated activity among:
• Macrophages—remove debris
• Schwann cells—form regeneration tube and secrete growth factors
• Axons—regenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS
fiber regeneration