Download Peripheral Nervous System (PNS) PNS – all neural structures

Peripheral Nervous System (PNS)
PNS – all neural structures outside the brain and
spinal cord
Includes sensory receptors, peripheral nerves,
associated ganglia, and motor endings
Provides links to and from the external
environment
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Sensory Receptors
Structures specialized to respond to stimuli
Activation of sensory receptors results in graded
potentials that trigger nerve impulses along the
afferent PNS fibers to the CNS
The realization of these stimuli, sensation and
perception, occur in the brain
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Receptor 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
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Receptor Class by Location: Exteroceptors
Respond to stimuli arising outside the body
Found near the body surface
Sensitive to touch, pressure, pain, and temperature
Include the special sense organs
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Receptor Class by Location: Interoceptors
(Visceroceptos)
Respond to stimuli arising within the body
Found in internal viscera and blood vessels
Sensitive to chemical changes, stretch, and
temperature changes
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Receptor Class by Location: Proprioceptors
Respond to degree of stretch of the organs they
occupy
Found in skeletal muscles, tendons, joints,
ligaments, and connective tissue coverings of
bones and muscles
Constantly “advise” the brain of one’s movements
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Receptor Classification by Structural
Complexity
Receptors are structurally classified as either
simple or complex
Most receptors are simple:
Are modified dendritic endings of sensory neurons
encapsulated and unencapsulated varieties
Complex receptors are special sense organs
associated with special senses (vision, hearing,
equilibrium, smell, taste)
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Simple Receptors: Unencapsulated
Free dendritic nerve endings
Respond chiefly to temperature and pain
Cold receptors found superficially (10-40°C)
Heat receptors found deeper (32-48°C)
Outside of these ranges, nociceptors are
activated and pain is perceived
Nociceptors also respond to pinch and chemicals
released from damaged tissue
E.g. slap a sunburn or ungloved hands
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Simple Receptors: Unencapsulated
Abundant in epithelia and connective tissues
Unmyelinated
Distal ends have knob-like swellings
Merkel Cells (tactile; form Merkel discs) lying in the
deeper dermal layers. Function as light touch receptors
Hair follicle receptors (free nerve endings wrapped around
hair follicles) are light touch receptors that detect bending
hair (e.g. detection of a landing mosquito)
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Simple Receptors: Encapsulated Dendritic Endings
Enclosed in a connective tissue capsule
All are mechanoreceptors
Meissner’s corpuscles (tactile corpuscles): found beneath the epidermis
and are numerous in sensitive, hairless skin, e.g. fingers & soles of feet
Pacinian corpuscles (lamellated corpuscles): found deep in the dermis
and subcutaneous tissue underlying the skin
Mechanoreceptors stimulated by deep pressure
Stimulated only wihen pressure is first applied, e.g. sense vibrations
(on/off)
Huge receptor!! Up to 3 mm long and 1.5 mm wide!
Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles
Joint kinesthetic receptors
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Simple Receptors: Encapsulated Dendritic Endings
Ruffin endings: found in the dermis, subcutaneous tissue, and joint capsules.
Respond to deep, continuous pressure
Muscle spindles:
Proprioceptors found thru out the perimysium of skeletal muscle.
They detect muscle stretch and initiate a reflex that resists the stretch
Joint kinesthetic receptors:
Proprioceptors that monitor stretch in the anterior capsules that enclose the
synovial joints
Provide info. on joint position and motion
Golgi tendon organs:
Proprioceptors located in tendons close to the skeletal muscle insertion.
Activated by compression when tendon fibers are stretched with muscle
contraction.
When golgi tendon organs are activated, the contracting muscle is inhibited
causing it to relax.
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From Sensation to Perception
Survival depends upon sensation and perception
Sensation is the awareness of changes in the
internal and external environment
Perception is the conscious interpretation of those
stimuli
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Organization of the Somatosensory System
Input comes from exteroceptors, proprioceptors,
and interoceptors
The three main levels of neural integration in the
somatosensory system are:
Receptor level – the sensory receptors
Circuit level – ascending pathways
Perceptual level – neuronal circuits in the cerebral
cortex
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Figure 13.2
Processing at the Receptor Lever
In order for a stimulus to excite a receptor:
The receptor must have specificity for the stimulus energy
(e.g. light, chemicals, touch, etc..)
The receptor’s receptive field must be stimulated.
The smaller the field, the better the brain can
localize it
Stimulus energy must be converted into a graded potential
(transduction)
A generator potential in the associated sensory neuron must
reach threshold
Voltage gated Na+ channels on the axon are
opened and nerve impulse propagates toward
CNS
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Adaptation of Sensory Receptors
Adaptation occurs when sensory receptors are
subjected to an unchanging stimulus
Phasic receptors:
Fast adapting burst of impulses at onset and
offset
Report changes in environment
Tonic receptors
Provide sustained response with no adaptation
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Processing at the Circuit Level
The task here is to deliver impulses to the appropriate
region of the cerebral cortex for stimulus localization and
perception
Ascending Sensory Pathways: Chains of three neurons
conduct sensory impulses upward to the brain
First-order neurons – soma reside in dorsal root or
cranial ganglia, and conduct impulses from the skin to
the spinal cord or brain stem
Second-order neurons – soma reside in the dorsal horn
of the spinal cord or medullary nuclei and transmit
impulses to the thalamus or cerebellum
Third-order neurons – located in the thalamus and
conduct impulses to the somatosensory cortex of the
cerebrum
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Processing at the Perceptual Level
Interpretation of sensory input occurs at the
cerebral cortex
The ability to identify a sensation depends on the
location of the target neurons in the cortex
regardless of what stimulated the neurons
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Main Aspects of Sensory Perception
Perceptual detection – detecting that a stimulus has occurred
Magnitude estimation – how much of a stimulus is acting
Spatial discrimination – identifying the site or pattern of the stimulus
(e.g. the 2-point descrimination test: back vs. hand)
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. familiar
face)
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Structure of a Nerve
Nerve – cordlike organ of the PNS
consisting of peripheral axons enclosed by
connective tissue
Connective tissue coverings include:
Endoneurium – loose connective
tissue that surrounds axons
Perineurium – coarse connective
tissue that bundles fibers into
fascicles
Epineurium – tough fibrous sheath
around a nerve
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Classification of Nerves
Sensory and motor divisions
Sensory (afferent) – carry impulse to the CNS
Motor (efferent) – carry impulses from CNS
Mixed – sensory and motor fibers carry impulses
to and from CNS; most common type of nerve
Ganglia: a collection of neuron cell bodies
associated with nerves in the PNS
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Regeneration of Nerve Fibers
Rengeneration is most effective, in general, only over short
distances
Damage to nerve tissue is serious because mature neurons
are amitotic
If the soma of a damaged nerve remains intact, damage can
be repaired
Regeneration involves coordinated activity among:
Macrophages – remove debris
Schwann cells – form regeneration tube and secrete growth
factors
Axons – regenerate damaged part
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Regeneration of Nerve Fibers
-Axon and myelin sheath distal to injury disintegrate due to lack of nutrients from
the cell body (Wallerian degeneration)
-Neurilemma remains intact
-Schwann cells proliferate and migrate to injury site and form a regeneration tube
that guide axon sprouts
-Cell body changes; chromatophilic substance breaks up and massive protein
synthesis begins (cell body swells)
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Figure 13.4
Spinal Nerves
Thirty-one pairs of mixed nerves arise from the
spinal cord and supply all parts of the body except
the head
They are named according to their point of issue
8 cervical (C1-C8) (Only 7 cervical vertebrae!)
12 thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 Coccygeal (C0)
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Spinal Nerves
Cervical nerves: exit superior to
the vertebrae
C8 emerges inferior to C7
(between C7 and T1)
T, L, S and coccygeal all exit
inferior to the same numbered
vertebrae
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Figure 13.6
Spinal Nerves: Roots
Ventral roots: motor (efferent) fibers
Dorsal roots: sensory (afferent) fibers
They unite to form a spinal nerve before emerging from the vertebral column
via a intervertebral foramina
After emerging from the foramina, the spinal nerve divides into small dorsal
& larger ventral rami
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Innervation of Specific Body Regions
Spinal nerve rami supply the entire somatic region
of the body from the neck down
Dorsal rami supply the posterior body trunk
Ventral rami supply the anterior body trunk and
limbs and are thicker than dorsal rami
Roots: each root is either sensory or motor
(medial)
Rami: each rami is sensory and motor (lateral)
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Ventral Rami
T2-T12
All ventral rami branch and join
one another lateral to the vertebral
column forming the nerve plexuses
(which primarily serve the limbs)
Only ventral rami form plexuses
Extreme branching of nerves
results in each muscle being
supplied by more than one spinal
nerve
The strategy is to offset damage via
redundant innervation and prevent
paralysis
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Spinal Nerve Innervation: Back, Anterolateral
Thorax, and Abdominal Wall
The back is innervated by dorsal rami via several branches
The thorax is innervated by ventral rami T1-T12 as intercostal nerves
Intercostal nerves supply muscles of the ribs, anterolateral thorax, and
abdominal wall.
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Cervical Plexus
Nerve Plexus combine sets of spinal nerves that serve the same area of
the body into one large grouped nerve.
The cervical plexus is formed by ventral rami of
C1-C4 and is located deep to the sternocleidomastoid muscle
Most branches are cutaneous nerves of the neck, ear, back of head, and
shoulders
The most important nerve of this plexus is the phrenic nerve which innervates
the diaphram
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Brachial Plexus and Upper Limb
Gives rise to nerves that innervate the upper limbs
Formed by C5-C8 and T1 (C4 and T2 may also contribute to
this plexus)
There are four major branches of this plexus
Roots – five ventral rami (C5-T1)
Trunks – upper, middle, and lower, which form divisions
Divisions – anterior and posterior serve the front and back
of the limb
Cords – lateral, medial, and posterior fiber bundles
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Brachial Plexus
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Figure 13.9a
Brachial Plexus: Nerves
Axillary – innervates the deltoid and teres minor
Musculocutaneous – sends fibers to the biceps
brachii and brachialis
Median – branches to most of the flexor muscles of
arm
Ulnar – supplies the flexor carpi ulnaris and part of
the flexor digitorum profundus
Radial – innervates essentially all extensor muscles
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Brachial Plexus: Distribution of Nerves
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Figure 13.9c
Lumbar Plexus and Lower Limb
Arises from L1-L4
Lies within the psoas major
muscle
Major branches descend and
innervate the anterior and
medial thigh
Femoral nerve: anterior
thigh muscles (quads), skin
of anterior thigh, surface of
leg from knee to foot
Obturator nerve: adductor
muscles
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Sacral Plexus
Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and
the perineum
The major nerve is the sciatic, the longest and thickest nerve of the body
The sciatic is actually composed of two nerves: the tibial and the common
fibular (peroneal) nerves
Supplies entire lower limb (except antero-medial thigh)
Tibial nerve: calf muscles and skin, sole of foot
Sural nerve: posterolateral leg skin
Medial/lateral plantar nerves: foot
Fibular nerve: knee joint, skin of lateral calf, dorsum of foot, muscles of
anterolateral leg
Superior/inferior gluteal nerves: gluteal muscles and tensor fascia latae
Pudendal nerve: skin of perineum (ano-genital region)
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Sacral Plexus
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Figure 13.11
Innervation of the Skin: 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
Lumbar (anterior) vs. Sacral (posterior)
Cervical (upper limbs) vs. Thoracic (trunk)
Innervation of Joints:
Question: Which nerves serve which synovial
joints?
Answer: They obey Hilton’s Law which is…
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Don’t you go changing, baby!”
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Figure 13.12
Dermatomes
Actually, Hilton’s Law states,
“Any nerve serving a muscle that produces
movement at a joint also innervates the joint and
the skin over the joint.”
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Motor Endings
Motor endings are PNS elements that activate
effectors by releasing neurotransmitters at:
Neuromuscular junctions
Varicosities at smooth muscle and glands
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Innervation of Skeletal Muscle
Takes place at a neuromusclular junction
Acetylcholine is the neurotransmitter that diffuses across the synaptic cleft
ACh binds to receptors resulting in:
Movement of Na+ and K+ across the membrane
Depolarization of the interior of the muscle cell
An end-plate potential that triggers an action potential
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Innervation of Visceral Muscle and Glands
Junctions of autonomic motor endings and their effectors (smooth, cardiac
muscle and glands)
Autonomic motor axons branch repeatedly, each branch forming synapses
with effector cells via variocosities (“string of beads”)
Contain either Ach or norepinephrine that act on 2nd messenger systems
Thus, they have a slower response time
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Levels of Motor Control
The cerebellum and the basal nuclei are the
ultimate planners and coordinators of complex
motor activities
The three levels of motor control are
Segmental level
Projection level
Precommand level
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Hierarchy of Motor Control
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Figure 13.13
Segmental Level
Lowest level of motor control
Consists of spinal cord circuits
Activates ventral horn neurons in a group of cord
segments causing them to stimulate specific groups
of muscles
Central Pattern Generators (CPGs) are circuits that
stimulate often repeated activities (walking,
breathing, swallowing) without significant input
from the cerebral cortex
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Projection Level
The projection level consists of:
Cortical motor areas that produce the direct
(pyramidal) system
Brain stem motor areas that oversee the indirect
(multineuronal) system
Axons here produce discrete voluntary movements
of skeletal muscles
Projection motor pathways convey info. to lower
motor neurons and send feedback to higher
command levels
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Precommand Level
Cerebellar and basal nuclei systems that:
Regulate motor activity
Precisely start or stop movements
Coordinate movements with posture
Block unwanted movements
Monitor muscle tone
Key center is cerebellum (the target for ascending
proprioceptors, tactile, equilibrium, vision)
Lacks direct connections with the spinal cord.
Acts on motor pathways on the motor cortex via the
thalamus
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Basal Nuclei
Receive inputs from all cortical areas and send
their output back to premotor and prefrontal
cortical areas via the thalamus
Involved in complex aspects of motor control
At rest, the basal nuclei inhibits motor centers of
the brain
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Reflexes
A reflex is a rapid, predictable motor response to a
stimulus
Reflexes may:
Be inborn (intrinsic) or learned (acquired)
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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Reflexes
Reflexes are classified functionally as somatic
reflexes if they activate skeletal muscle, or
autonomic (visceral) reflexes if they activate
visceral effectors of smooth, cardiac muscle or
glands
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Spinal Reflexes
Do Not Need A Brain!
The brain, however, is advised and can facilitate,
inhibit, and adapt the reflex
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Stretch and Deep Tendon Reflexes
For skeletal muscles to perform normally, they
need 2 types of information:
1. Tension in the muscle (and tendon)
2. Length of the muscle
Supplied by the golgi tendon organs
Supplied by the muscle spindles
Both play a role in spinal reflexes, both are
proprioceptors
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Muscle Spindles
Are composed of 3-10 intrafusal muscle fibers that lack myofilaments in their
central regions, are noncontractile, and serve as receptive surfaces
Muscle spindles are wrapped with two types of afferent endings: primary
sensory endings of type 1a fibers and secondary sensory endings of type 2 fibers
These regions are innervated by gamma (γ) efferent fibers
Note: contractile muscle fibers are extrafusal fibers and are innervated by alpha
(α) efferent fibers
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Operation of the Muscle Spindle
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Figure 13.17
Stretch Reflex
Stretching the muscle activates the muscle spindle
Excited γ motor neurons of the spindle cause the
stretched muscle to contract
Afferent impulses from the spindle result in
inhibition of the antagonist muscle
Example: patellar reflex
Tapping the patellar tendon stretches the
quadriceps and starts the reflex action
The quadriceps contract and the antagonistic
hamstrings relax
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Stretch Reflex
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Figure 13.16
Stretch Reflex
The stretch reflex makes sure that the muscle maintains a
certain length
E.g. knee jerk reflex keeps our legs from buckling!
All stretch reflexes are monosynaptic and ipsilateral.
Important things to know:
1. KJR proves that the sensory and motor connections
between that muscle and the spinal cord are intact
2. The vigor of the response indicates the degree of
excitability of the spinal cord
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Golgi Tendon Reflex
The opposite of the stretch reflex
Contracting the muscle activates the Golgi tendon
organs
Afferent Golgi tendon neurons are stimulated,
neurons inhibit the contracting muscle, and the
antagonistic muscle is activated
As a result, the contracting muscle relaxes and the
antagonist contracts
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Golgi Tendon Reflex
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Figure 13.18
Golgi Tendon Reflex
The Golgi Tendon Reflex protects the muscle from
excessively heavy loads by causing the muscle to
relax and drop the load.
First, as a load is placed on the muscle, the afferent
neuron from the Golgi tendon organ fires into the
CNS
Second, the motor neuron from the spinal cord is
inhibited via an IPSP (Inhibitory postsynaptic
potential) and muscle relaxes.
As a result, you drop the load.
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Flexor and Crossed Extensor Reflexes
The flexor reflex is initiated by a painful stimulus
(actual or perceived) that causes automatic
withdrawal of the threatened body part
The crossed extensor reflex has two parts
The stimulated side is withdrawn
The contralateral side is extended
Let’s demonstrate!
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Crossed Extensor Reflex
Interneurons
+
+
–
+
+
–
Afferent
fiber
Efferent
fibers
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
s
xe
e
l
F
Flexor
inhibited
Arm movements
Extensor
stimulated
s
end
t
x
E
Key:
+ Excitatory synapse
– Inhibitory synapse
Right arm
(site of stimulus)
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Left arm (site of
reciprocal activation)
Figure 13.19
Superficial Reflexes
Initiated by gentle cutaneous stimulation
Example:
Plantar reflex is initiated by stimulating the lateral
aspect of the sole of the foot
The response is downward flexion of the toes
Indirectly tests for proper corticospinal tract
functioning
Babinski’s sign: abnormal plantar reflex indicating
corticospinal damage where the great toe
dorsiflexes and the smaller toes fan laterally (seen
in infants up to 1 year of age)
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KU Game Day: The Undefeated Lady Cougars!
Wed. 7 pm
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