Transcript Chapter 13 - Peripheral Nervous System

Chapter 13
The Peripheral Nervous
System and Reflex Activity
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
Overview of PNS
• Modality
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each input is a specific type of sensation: temperature,
pain, pressure, touch, body position, equilibrium,
hearing, vision, smell, taste
individual sensory neurons generally carry only one
modality
• Selectivity of Receptors
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sensory neurons respond strongly to one type of
stimulus and weakly or not at all to other types
some respond accidentally to other types of stimuli –
rubbing one’s eyes mechanically stimulates the eyes’
light receptors in the retina
Components of Sensation
• stimulation - stimulus or change in the environment
• transduction
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requires a sensory receptor cell or organ which
responds to specific stimuli and converts them into
receptor/generator potentials
• impulse generation and conduction
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if a receptor/generator potential (graded potential)
reaches a threshold, then
the neuron’s action potential will be sent to the CNS
• integration
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some region of the CNS must receive and translate
nerve impulses into sensations and perceptions
this generally occurs in the cerebral cortex
Sensory Receptor Classification
1. Classified by the type of stimulus
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Mechanoreceptors
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Thermoreceptors - changes in temperature
Nociceptors - pain due to physical or chemical damage
to nearby tissue
Photoreceptors
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mechanical pressure or stretching
generate action potentials when deformed
light strikes retinal receptor cells
generates action potentials in response to light energy
Chemoreceptors – certain specific chemical molecules
are detected in the mucous fluids of the GI &
respiratory tracts, or in the blood or other body fluids
Sensory Receptor Classification
2. Classified by location
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Exteroceptors
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Interoceptors (visceroceptors)
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located at or near the body’s surface
provide information about the external environment
found in blood vessels, connective tissues, and organs
provide information about the internal environment
Proprioceptors
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located in muscles, tendons, joints and the internal ear
provide information about gravity, body and limb
positions and skeletal muscle movements
Adaptation by Sensory Receptors
• a change in sensitivity to a long-lasting stimulus
• primarily by rapidly-adapting phasic receptors
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pressure, touch, hearing, smell
adapt very quickly, i.e., respond less if the stimulus
remains constant
allows us to shut out background “noise”
• little adaptation by slowly-adapting tonic receptors
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pain, body position, chemicals in the blood or CSF
adapt slowly, continue to respond even when the
stimulus remains constant
continuous input is useful for some modalities because
the body needs to make continuous responses to that
kind of information
Anesthesia
• a partial or complete loss of sensation
• General anesthesia – gas agents act in the CNS
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through poorly understood mechanisms
Local anesthesia – drugs injected near peripheral
nerves inhibit the opening of gated sodium channels,
preventing local transmission of action potentials
Analgesia – reduced perception of pain without loss
of other sensory information or loss of consciousness:
biochemical interference with local stimulus (NSAIDs)
or mimic endogenous endorphins in CNS (opiates)
Paresthesias – abnormal sensations (burning,
tingling, numbness) not related to normal stimulation,
e.g., mechanical pressure on nerves in your leg puts
your foot “to sleep”
Functional Types of Nerves
• mixed nerves – contain both
sensory and motor fibers
• motor (efferent) nerves
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[Note: so-called pure
“motor” nerves do also carry
proprioceptive sensory
signals back to the CNS from
the skeletal muscles, joints,
and tendons being served
by that nerve.]
• sensory (afferent) nerves
Anatomy of Nerves
• bundles/fascicles of
axons & dendrites
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endoneurium – around
individual processes
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perineurium
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around fascicles
individual nerve fibers with
their endoneurium
epineurium - outermost
covering around entire
peripheral nerve
Cranial Nerves
• Twelve pairs of nerves which originate from the brain and exit
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through foramina of the skull
First 2 pairs originate from the forebrain (olfactory, optic)
Remaining 10 pairs originate from the brain stem
What You Should Know
About the Cranial Nerves
• name and number
• general region(s) served
• main functional roles
• modality (sensory, motor*, mixed (m/s))
(* motor nerves carry proprioception sensory information
back to the CNS)
• See Table 13.2 pp. 501- 507 and slides
after end slide in this PPT for details
Spinal Nerves
• 31 pairs of spinal nerves
originate from the spinal
cord
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All are mixed (m/s) nerves
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Thousands of fibers per
spinal nerve
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Each pair serves a particular
region of the body
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Each pair also provides
some service to the region
supplied by the spinal nerve
above it and the spinal
nerve below it (redundancy)
Spinal Nerve Anatomy
• Spinal nerves are
very short, they
divide almost
immediately
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dorsal ramus supplies posterior
body trunk
ventral ramus supplies the rest
of body trunk and
the limbs
meningeal
branch - supplies
the meninges and
blood vessels
within meninges
Dorsal and Ventral Rami of a
Typical Spinal Nerve
Dermatomes
• Areas of skin
innervated by the
cutaneous branches
of each pair of
spinal nerves
• Each pair also
provides some
service to the
region of the spinal
nerve above and
the spinal nerve
below (redundancy)
Reflex Activity
• a reflex is a rapid, predictable, automatic response to
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a stimulus
a reflex is unlearned, unpremeditated, and involuntary
one is conscious of somatic reflexes only after they
occur
reflexes are involved in homeostasis
two fundamental types of reflexes
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somatic reflexes - produce contraction of skeletal
muscle
autonomic (visceral) reflexes
generally, they are not perceived consciously
 produce responses by smooth muscle, cardiac muscle,
glands to adjust conditions of the internal environment
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Components of a Reflex Arc
• 5 Functional Components
1)
2)
3)
receptor - dendrites or other sensory structures respond to
changes in the environment
sensory neuron - conducts an impulse from a receptor to its
axon terminals
integrating center (some region within the CNS)
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4)
5)
simple - monosynaptic (2 cells only: sensory and motor neurons)
complex – polysynaptic (> 2 cells: interneurons involved)
motor neuron - impulses from integrating center to an effector
effector - body part (muscle or gland) which responds to the
motor nerve impulse
Stretch Reflexes
• receptors - muscle spindles
and Golgi tendon organs
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sensory mechanoreceptors
which respond to stretching
increased tension
(stretching) stimulates the
receptors
sends proprioceptive inputs
to spinal cord
• contraction of the skeletal
muscle reduces tension on
the muscle spindle
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lowers the rate of action
potential generation
decreases input to the spinal
cord and higher centers:
cortex and cerebellum
Stretch Reflexes
• Remember that if
a muscle is being
stretched, the
stretch is caused
by the contraction
of its antagonist.
• This sensory
proprioception
information
contributes to
maintaining
proper muscle
tone.
Patellar
Reflex
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monosynaptic
ipsilateral (same side)
segmental (at one level of the spinal cord)
polysynaptic component – for reciprocal
inhibition of the antagonist
Golgi (Deep) Tendon Reflex
• an increase in muscle tension
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activates receptors (Golgi tendon
organ) in the tendon
the muscle relaxes and lengthens in
response to its antagonist’s
contraction
 D-T reflex inhibits the
agonist
 D-T reflex excites the
antagonist
helps to regulate a smooth start and
stop for a contraction
input from the Golgi tendon organs
are sent to the cerebellum and the
cortex
polysynaptic, ipsilateral, and
segmental
Flexor Reflex
• a pull on the limb,
extending it, will trigger
the reflex
• also a painful stimulus – a
burn, pin prick, toe stub,
etc.
• F-R causes an
automatic withdrawal
from the (dangerous)
stimulus
• polysynaptic, ipsilateral,
and segmental
Crossed Extensor Reflex
• flexion of a body part is often balanced by extension of the
same body part on the opposite side of the body
• polysynaptic
• contralateral
• segmental
End Chapter 13
[Note: Summary slides for the
cranial nerves appear after this
slide for your exam 4 review.]
Cranial Nerve I: Olfactory
• Passes through the cribriform
plate of the ethmoid bone
• Fibers run through the olfactory
bulb and terminate in the primary
olfactory cortex
• Functions solely by carrying
afferent impulses for the
sense of smell
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
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
• Parasympathetic cell bodies • Proprioceptor afferents from
are in the ciliary ganglia
extrinsic eye muscles
Cranial Nerve IV: Trochlear
• Fibers emerge from
the dorsal midbrain
and enter the orbits
via the superior
orbital fissures;
innervate the superior
oblique muscle
• Primarily a motor
nerve that directs the
eyeball
• Proprioceptor afferents from
extrinsic eye muscles
Cranial Nerve V: Trigeminal
• Composed of three divisions:
ophthalmic (V1), maxillary
(V2), and mandibular (V3)
• Fibers run from the face to
the pons via the superior
orbital fissure (V1), the
foramen rotundum (V2), and
the foramen ovale (V3)
• Conveys sensory impulses
from various areas of the
face (V1) and (V2), and
supplies motor fibers (V3) for
mastication
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
Cranial Nerve VII:
Facial
• Fibers leave the pons, travel through
the internal acoustic meatus, and
emerge through the stylomastoid
foramen 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
• Sensory function is taste from the
anterior two-thirds of the tongue
Cranial Nerve VIII:
Vestibulocochlear
• Fibers arise from the hearing
and equilibrium apparatus of
the inner ear, pass through
the internal acoustic meatus,
and enter the brainstem at
the pons-medulla border
• Two divisions – cochlear
(hearing) and vestibular
(balance)
• Functions are solely sensory –
equilibrium and hearing
Cranial Nerve IX:
Glossopharyngeal
• Fibers emerge from the
medulla, leave the skull via the
jugular foramen, and run to the
throat
• Nerve IX is a mixed nerve with
motor and sensory functions
• Motor – innervates part of the
tongue and pharynx, and
provides motor fibers to the
parotid salivary gland
• Sensory – fibers conduct taste
and general sensory impulses
from the tongue and pharynx
Cranial Nerve X: Vagus
• The only cranial nerve that
extends beyond the head
and neck
• Fibers emerge from the
medulla via the jugular
foramen
• 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
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
• The spinal root passes
upward into the cranium via
the foramen magnum
• The accessory nerve leaves
the cranium via the jugular
foramen
• Primarily a motor nerve
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Supplies fibers to the larynx,
pharynx, and soft palate
Innervates the trapezius and
sternocleidomastoid, which
move the head and neck
Cranial Nerve XII:
Hypoglossal
• Fibers arise from the
medulla and exit the
skull via the hypoglossal
canal
• Innervates both extrinsic
and intrinsic muscles of
the tongue, which
contribute to swallowing
and speech
End Chapter 13
Cranial Nerve Slides