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Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
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
Table 13.1
Encapsulated Dendritic Endings
• 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
Table 13.1
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
Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
Figure 13.2
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)
Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
Figure 13.2
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
Endoneurium
Axon
Myelin sheath
Perineurium
Epineurium
Fascicle
Blood
vessels
(b)
Figure 13.3b
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
Endoneurium
Schwann cells
Droplets
of myelin
1 The axon
becomes
fragmented at
the injury site.
Fragmented
axon
Site of nerve damage
Figure 13.4 (1 of 4)
Schwann cell
Macrophage
2 Macrophages
clean out the
dead axon distal
to the injury.
Figure 13.4 (2 of 4)
Aligning Schwann cells
form regeneration tube
3 Axon sprouts,
or filaments,
grow through a
regeneration tube
formed by
Schwann cells.
Fine axon sprouts
or filaments
Figure 13.4 (3 of 4)
Schwann cell
Site of new
myelin sheath
formation
4 The axon
regenerates and
a new myelin
sheath forms.
Single enlarging
axon filament
Figure 13.4 (4 of 4)
Cranial Nerves
• Twelve pairs of nerves associated with the
brain
• Most are mixed in function; two pairs are
purely sensory
• Each nerve is identified by a number
(I through XII) and a name
“On occasion, our trusty truck acts funny—very
good vehicle anyhow”
Frontal lobe
Temporal lobe
Infundibulum
Facial
nerve (VII)
Vestibulocochlear
nerve (VIII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Filaments of
olfactory
nerve (I)
Olfactory bulb
Olfactory tract
Optic nerve
(II)
Optic chiasma
Optic tract
Oculomotor
nerve (III)
Trochlear
nerve (IV)
Trigeminal
nerve (V)
Abducens
nerve (VI)
Cerebellum
Medulla
oblongata
(a)
Figure 13.5 (a)
Cranial nerves
I – VI
I
II
III
IV
V
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
VI Abducens
Cranial nerves
VII – XII
VII Facial
VIII Vestibulocochlear
IX
X
XI
XII
(b)
Glossopharyngeal
Vagus
Accessory
Hypoglossal
Sensory
function
Motor
function
PS*
fibers
Yes (smell)
Yes (vision)
No
No
Yes (general
sensation)
No
No
Yes
Yes
Yes
No
No
Yes
No
No
No
Yes
No
Sensory
function
Motor
function
PS*
fibers
Yes (taste)
Yes (hearing
and balance)
Yes
Some
Yes
No
Yes (taste)
Yes (taste)
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
*PS = parasympathetic
Figure 13.5 (b)
I: The Olfactory Nerves
• Arise from the olfactory receptor cells of nasal
cavity
• Pass through the cribriform plate of the
ethmoid bone
• Fibers synapse in the olfactory bulbs
• Pathway terminates in the primary olfactory
cortex
• Purely sensory (olfactory) function
Table 13.2
II: The Optic Nerves
• Arise from the retinas
• Pass through the optic canals, converge and
partially cross over at the optic chiasma
• Optic tracts continue to the thalamus, where
they synapse
• Optic radiation fibers run to the occipital
(visual) cortex
• Purely sensory (visual) function
Table 13.2
III: The Oculomotor Nerves
• Fibers extend from the ventral midbrain
through the superior orbital fissures to the
extrinsic eye muscles
• Functions in raising the eyelid, directing the
eyeball, constricting the iris
(parasympathetic), and controlling lens shape
Table 13.2
IV: The Trochlear Nerves
• Fibers from the dorsal midbrain enter the
orbits via the superior orbital fissures to
innervate the superior oblique muscle
• Primarily a motor nerve that directs the
eyeball
Table 13.2