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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
13
The Peripheral
Nervous System
and Reflex
Activity: Part A
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Peripheral Nervous System (PNS)
• All neural structures outside the CNS
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
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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
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Classification of Receptors
• Based on:
• Stimulus type
• Location
• Structural complexity
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Classification by Stimulus Type
• Mechanoreceptors — respond to touch, pressure,
vibration, stretch, and itch
• Thermoreceptors
temperature
—
sensitive
to
changes
in
• 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)
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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
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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
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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
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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
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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
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Processing at the Receptor Level
• The receptor must have specificity for the stimulus energy
• The receptor’s receptive field must be stimulated
• The smaller the receptive field the greater the ability of the
brain to localize the site
• Stimulus energy must be converted into a graded potential also
called receptor potential or transduction
• A generator potential is a depolarization that leads to action
potential in the afferent fibers
• Associated sensory neuron (neuron of first order close to the
receptor) must reach threshold
• Information is encoded in the frequency of the stimuli – the greater
the frequency, the stronger is the stimulus.
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Adaptation of Sensory Receptors
• Adaptation is the change in sensitivity in the presence of
constant stimulus
• Adaptation occurs when
• Receptor membranes become less responsive, or
• Receptor potentials decline in frequency or stop
• Receptors responding to pressure, touch, and smell adapt
quickly
• Receptors responding slowly include Merkel’s discs, Ruffini’s
corpuscles (light and deep pressure respectively)
• Pain receptors and proprioceptors do not exhibit adaptation
(WHY?)
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Processing at the circuit Level
• The circuit level role is to deliver the impulses to the appropriate region in
the cerebral cortex.
• The ascending tract typically consists of 3 neurons
• First order neurons
• cell bodies in a ganglion (dorsal or cranial)
• Impulses from skin and proprioceptors to spinal cord or brain stem to
a 2nd order neuron
• Second order neuron
• In the dorsal horn of the spinal cord or in the medulary nuclei
• Transmit impulses to thalamus or cerebellum
• Third order neurons
• Cell bodies in the thalamus (no 3rd-order neurons in the cerebellum)
• Transmit signals to the somatosensory cortex of the cerebrum
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Processing at the circuit Level
• Impulses ascend in :
• Non specific pathway that in general transmit pain,
temperature and touch
• Give branches to reticular formation and thalamus on
the way up
• Sends general information that is also involved in
emotional aspects of perception
• Specific ascending pathways involve in more precise
aspect of sensation
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Processing at the Perceptual Level
• Interpretation of sensory input occurs in the
cerebral cortex
• The ability to identify the sensation depends on
the specific location of the target neurons in the
sensory cortex not on the nature of the message
(all messages are action potentials)
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Main Aspects of Sensory Perception
• Perceptual detection – detecting that a stimulus has occurred
and requires summation
• Magnitude estimation – the ability to detect how intense the
stimulus is
• Spatial discrimination – identifying the site or pattern of the
stimulus
• Feature abstraction – used to identify a substance that has
specific texture or shape
• Quality discrimination – the ability to identify submodalities
of a sensation (e.g., sweet or sour tastes)
• Pattern recognition – ability to recognize patterns in stimuli
(e.g., melody, familiar face)
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Motor ending and motor activity
• Motor ending are the PNS elements that activate effectors by
releasing neurotransmitters
• Innervation of skeletal muscle
• Somatic motor fibers innervate voluntary muscles and form
neuromuscular junction.
• At this junction the NT that is released is the Ach
• Innervation of visceral muscle and glands
• Junctions between autonomic motor endings and their effectors –
smooth and cardiac muscles and glands
• Acetylcholine and norepinephrine are used as neurotransmitters
• Tend to response slower than the somatic motor endings
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Levels of Motor Control
• The three levels of motor control are
• Segmental level
• Projection level
• Precommand level
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Segmental Level
• The segmental level is the lowest level of motor hierarchy
• It consists of segmental circuits of the spinal cord
• A segmental circuit activates network of ventral horn
neurons in a certain segment causing the activation of a
specific group of muscles
• These circuits are called central pattern generators (CPGs)
• A central pattern generator is a network of neurons which
is able to exhibit rhythmic behavior in the absence of sensory
input.
• locomotion, breathing, chewing
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Projection Level
• The projection level controls the spinal cord and consists
of:
• Upper motor neurons - Cortical motor areas that
produce the direct (pyramidal) system
• Voluntary movements of skeletal muscles
• Brain stem motor areas that oversee the indirect
system
• Control reflex and CPG-controlled motor actions
• Pass on information to lower motor neurons and send a
copy of this information to higher command levels
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Precommand Level
• The cerebral cortex is the highest level of conscious
motor pathway but it is not the ultimate planner and
coordinator of complex motor activities
• Cerebellum
• Acts on motor pathways through projection areas of
the brain stem
• Acts on the motor cortex via the thalamus
• Basal nuclei
• Inhibit various motor centers under resting conditions
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Structure of a Nerve
• Nerve – cordlike organ of the
PNS consisting of peripheral
axons enclosed by connective
tissue
• Connective
include:
tissue
coverings
• Endoneurium
–
connective
tissue
surrounds axons
loose
that
• Perineurium
–
coarse
connective tissue that bundles
fibers into fascicles
• Epineurium – tough fibrous
sheath around a nerve
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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
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Ganglia
• Contain neuron cell bodies associated with nerves
• Dorsal root ganglia (sensory, somatic) (Chapter 12)
• Autonomic ganglia (motor, visceral) (Chapter 14)
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Cranial Nerve I: Olfactory
• Arises from the olfactory epithelium
• Passes through the cribriform plate of the ethmoid
bone
• The axons will synapse on the olfactory bulb.
• From the olfactory bulbs, Olfactory nerve
continues through the olfactory tracts and can
synapse on the
• 1. Olfactory cortex (Rhinencephalon) for
conscious interpretation
• 2. Hypothalamus (Limbic system) for
emotional response
• 3. Autonomic Nervous System for other
responses (digestive system)
• Functions solely by carrying afferent impulses for
the sense of smell
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Cranial Nerve II: Optic
• Arises from the retina of the eye
• Optic nerves pass through the
optic canals and converge at the
optic chiasm
• They continue to the thalamus
where they synapse
• From there, the optic radiation
fibers run to the visual cortex
• Functions solely by carrying
afferent impulses for vision
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Cranial Nerve III: Oculomotor
• Fibers extend from the ventral
midbrain, pass through the
superior orbital fissure, and
go to the extrinsic eye
muscles
• Functions in raising the
eyelid, directing the eyeball,
constricting the iris, and
controlling lens shape
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Cranial Nerve IV: Trochlear
• Fibers emerge from the
dorsal midbrain and enter
the orbits via the superior
orbital fissures; innervate
the superior oblique
muscle
• A motor nerve that directs
the eyeball
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Cranial Nerve V: Trigeminal
• Fibers run from the face to the pons, to the thalamus
and to the primary somatosensory cortex
• Three divisions:
• ophthalmic (V1),
sensory from face
• maxillary (V2),
• mandibular (V3) supplies motor fibers (V3) for
mastication
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Cranial Nerve V: Trigeminal
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Figure V from Table 13.2
Cranial Nerve VI: Abdcuens
• Fibers leave the inferior pons and enter the orbit via
the superior orbital fissure
• Primarily a motor nerve innervating the lateral rectus
muscle
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Figure VI from Table 13.2
Cranial Nerve III, IV and VI: extrinsic eye muscles control
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http://www.neuroanatomy.wisc.edu/virtualbrain/Images/13N.jpg
Cranial Nerve VII: Facial
• Fibers leave the pons to the lateral
aspect of the face
• Mixed nerve with five major
branches
• Motor functions include facial
expression, and the transmittal of
autonomic impulses to lacrimal and
salivary glands (subconscious)
• Sensory function is taste from the
anterior two-thirds of the tongue
(taste buds to pons, to the thalamus,
to the insula and parietal cortex for
taste perception)
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Cranial Nerve VIII: Vestibulocochlear
• Fibers arise from the hearing and equilibrium apparatus of the inner
ear to enter the brainstem at the pons-medulla border
• Two divisions – cochlear (hearing) and vestibular (balance)
• Functions are solely sensory – equilibrium and hearing
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Cranial Nerve IX: Glossopharyngeal
• Fibers emerge from the medulla
and run to the throat
• Nerve IX is a mixed nerve
• Motor – innervates part of the
tongue and pharynx, and
provides motor fibers to the
parotid salivary gland
(autonomic)
• Sensory – fibers conduct taste
and general sensory impulses
from the tongue and pharynx
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Cranial Nerve X: Vagus
• The only cranial nerve that
extends beyond the head and
neck
• Fibers emerge from the
medulla The vagus is a mixed
nerve
• Most motor fibers are
parasympathetic fibers to the
heart, lungs, and visceral
organs
• Its sensory function is in taste
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Cranial Nerve XI: Accessory
• Formed from a cranial root
emerging from the medulla and a
spinal root arising from the
superior region of the spinal cord
• A motor nerve
• Supplies fibers to the
larynx, pharynx, and soft
palate
• Innervates the trapezius
and sternocleidomastoid,
which move the head and
neck
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Cranial Nerve XII: Hypoglossal
• Fibers arise from the
medulla
• Innervates both
extrinsic and intrinsic
muscles of the tongue,
which contribute to
swallowing and speech
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Spinal Nerves: Rami
• The short spinal nerves branch into three or four mixed,
distal rami
• Small dorsal ramus
• Larger ventral ramus
• Tiny meningeal branch – innervate the meninges
and blood vessels within the vertebral canal
• Rami communicantes at the base of the ventral
rami in the thoracic region that contain autonomic
nerve fibers
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Nerve Plexuses
• All ventral rami except T2-T12 form nerve networks called
plexuses
• Plexuses are found in the cervical, brachial, lumbar, and sacral
regions
• Each resulting branch of a plexus contains fibers from several
spinal nerves
• Fibers travel to the periphery via several different routes
• Each muscle receives a nerve supply from more than one spinal
nerve
• Damage to one spinal segment cannot completely paralyze a
muscle
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Plexus
Main spinal nerves Regions innervated
Major nerves
Cervical
C1-C5
Skin and muscles of head & Phrenic (diaphragm)
neck. Superior chest and
shoulder
Brachial
C5-C8, T1
Shoulder and upper limbs
Axillary
Musculocutaneous
Radial
Median
Ulnar
Lumbar
L1-L4
Antero-lateral abdominal
Femoral
wall, external genitalia, part
of lower limbs
Sacral
L4-L5, S1-S4
Buttocks, perineum, lower
limbs
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Sciatic
Dermatomes
• A dermatome is the area of skin innervated by the
cutaneous branches of a single spinal nerve
• All spinal nerves except C1 participate in dermatomes
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Reflexes
• A reflex is a rapid, predictable motor response to a stimulus
• Reflexes may be:
• inborn (intrinsic)/basic:
• breathing.
• Putting a hand on a hot stove and quickly removing it,
• learned (acquired)
• Pavlov's dogs, every time Pavlov would feed the dogs he
would ring a bell, before long even if there was no food
if he rang the bell the dogs began to salivate.
• Example: driving skills
• Reflexes may
• Involve only peripheral nerves and the spinal cord
• Involve higher brain centers as well
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Reflex Arc
• There are five components of a reflex arc
• Receptor – site of stimulus
• Sensory neuron – transmits the afferent impulse to
the CNS
• Integration center – either monosynaptic or
polysynaptic region within the CNS
• Motor neuron – conducts efferent impulses from
the integration center to an effector
• Effector – muscle fiber or gland that responds to
the efferent impulse
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Spinal Reflexes
• Spinal somatic reflexes
• Integration center is in the spinal cord
• Effectors are skeletal muscle
• Testing of somatic reflexes is important clinically
to assess the condition of the nervous system
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Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is necessary
• Muscle spindles inform the nervous system of the
length of the muscle
• Golgi tendon organs inform the brain as to the
amount of tension in the muscle and tendons
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Stretch Reflexes
• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased
muscle length (stretch)
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