Download a.Nerve Regeneration

THE PERIPHERAL NERVOUS SYSTEM
AND
REFLEX ACTIVITY
THE PERIPHERAL NERVOUS SYSTEM
• PNS: includes all neural structures
outside the brain and spinal cord, that
is, the sensory receptors, peripheral
nerves, and their associated ganglia, and
efferent motor endings
PNS in the Structural Organization
of the Nervous system
SENSORY RECEPTORS
AND
SENSATION
SENSORY RECEPTORS
• Sensory receptors are specialized to
respond to changes in their
environment called stimuli
– Sensation: awareness of the stimulus (PNS)
– Perception: interpretation of the meaning of
the stimulus (CNS: brain)
SENSORY RECEPTORS
• Sensory receptors may be classified 3
ways:
– Receptors my be classified according to the
activating stimulus
– Receptors may be classified based on their
location or the location of the activating
stimulus
– Receptors may be classified based on their
overall structural complexity
SENSORY RECEPTORS
• Classification based on Stimulus Type:
overstimulation of anyone of the following receptors
can result in pain:
– Mechanoreceptors: generates nerve impulses when stimulated
by forces (touch, pressure, vibration, stretch, itch)
– Thermoreceptors: sensitive to temperature changes
– Photoreceptors: respond to light energy (e.g. retina of the eye)
– Chemoreceptors: respond to chemicals in solution (molecules
smelled or tasted, or changes in blood chemistry)
– Nociceptors: respond to potentially damaging stimuli that result
in pain
SENSORY RECEPTORS
• Classification based on Location:
– Exteroceptors:
• Sensitive to stimuli arising outside the body
• Receptors near or at the body surface
– Interoceptors:
• Respond to stimuli arising from within the body
• Internal viscera and blood vessels
– Propriceptors:
• Respond to internal stimuli from musculoskeletal organs
– Skeletal muscle, joints, tendons, ligaments, connective tissue
coverings of bone and muscle
• Monitors degree of stretch of the organs
SENSORY RECEPTORS
• Classification based on Complexity:
– Simple:
• Modified dendritic endings of sensory neurons
– Found in the skin, mucous membranes, muscles, and
connective tissues
• Monitor most types of general sensory information
– Complex:
• Actually sense organs
– Localized collection of cells working together to
accomplish a specific receptive process (vision, hearing,
smell, and taste)
SENSORY RECEPTORS
• Anatomically:
– Classified as free dendritic endings or
encapsulated dendritic endings
SENSORY RECEPTORS
• Free Dendritic Endings:
– Naked
– Present nearly everywhere in the body, but abundant in
epithelia and connective tissue
– Most unmyelinated
– Small diameters
– Free, or naked, nerve endings are present everywhere in the
body and respond primarily to pain and temperature
– Merkel discs: deep layers of skin epidermis
• Function as light touch receptors
– Root Hair Plexuses: light touch receptors that detect bending of
hairs
– Itch receptors: seemed to be stimulated by chemicals
(histamine and bradykinin) present at inflamed sites
SENSORY RECEPTORS
•
•
Encapsulated Dendritic Endings: consist of one or more fiber terminals of
sensory neurons enclosed in a connective tissue capsule
Virtually all are mechanoreceptors
–
Meissner’s corpuscles are receptors for discriminatory and light touch in hairless areas of
the body
•
•
–
Krause’s end bulbs: variation of Meissner’s corpuscles
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•
–
Neuromuscular spindles
Golgi tendon organs are stimulated when the associated muscle stretches the tendon
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Dermis
Muscle spindles detect when a muscle is being stretched and initiate a reflex that resists
the stretch
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–
Deep in epidermis
Respond only when pressure is first applied
Ruffini’s corpuscles respond to deep and continuous pressure
•
–
Abundant in mucous membranes
Mucocutaneous corpuscles
Pacinian, or lamellated, corpuscles, are stimulated when deep pressure is first applied
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•
–
Tactile corpuscles
Skin
Proprioceptors
Joint kinesthetic receptors monitor the stretch in the articular capsules of synovial joints
OVERVIEW: FROM SENSATION TO
PERCEPTION
• The somatosensory system: the part of the
sensory system serving the body wall and limbs,
involve the receptor level, the circuit level, and
the perceptual level
– Processing at the receptor level involves a stimulus
that must excite a receptor in order for sensation to
occur
– Processing at the circuit level is involved with
delivery of impulses to the appropriate region of the
cerebral cortex for stimulus localization and
perception
– Processing at the perceptual level involves
interpretation of sensory input in the cerebral cortex
SOMATOSENSORY SYSTEM
TRANSMISSION LINES:
NERVES
AND
THEIR STRUCTURE
AND
REPAIR
NERVES
AND
ASSOCIATED GANGLIA
• A nerve is a cordlike organ consisting of parallel
bundles of peripheral axons (some myelinated and
some not) enclosed by connective tissue wrappings:
– Most mixed: both sensory and motor
– Some are only sensory
– Some are only motor
• Ganglia are collections of neuron cell bodies
associated with nerves in the PNS:
– Ganglia associated with afferent nerve fibers contain cell
bodies of sensory neurons
– Ganglia associated with efferent nerve fibers mostly contain
cell bodies of autonomic motor neurons
NERVE STRUCTURE
NERVE REGENERATION
• Mature neurons do not divide
• If damage to a neuron occurs to the axon and the cell body
remains intact, cut or compressed axons can regenerate:
– Post-trauma axon regrowth is never exactly the same as what existed
before the injury
– Much of the functional recovery after nerve injury involves retraining the
nervous system to respond appropriately so that stimulus and response
are coordinated
– Unlike peripheral nerve fibers, most of those within the CNS never
regenerate under normal circumstances (damage to the brain or spinal
cord has been viewed as irreversible)
• The key to CNS regeneration will most likely be found in the
hippocampus, a brain region important in learning and memory
because this area produces significant numbers of new
neurons throughout life
NERVE REGENERATION
NERVE REGENERATION
• a.Nerve
Regeneration:
– Peripheral axon has
been severed or
crushed
– Separated ends seal
themselves off
– Substances being
transported along the
axon begin to
accumulate in the
sealed ends
NERVE REGENERATION
• b.Nerve Regeneration:
– Within a few hours, the
axon and its myelin sheath
distal to the site of injury
begin to disintegrate
because they cannot
receive nutrients (Wallerian
degeneration)
• Spreads distally from the
injury site, completely
fragmenting the axon
– Macrophages
phagocytize
disintegrating myelin and
axonal debris
NERVE REGENERATION
• c.Nerve
Regeneration:
– Once the debris has
been disposed of,
surviving Schwann
cells proliferate
– Axonal growth takes
place
NERVE REGENERATION
• d.Nerve
Regeneration:
– The Schwann cells
protect, support, and
remyelinate the
regenerating axons
NERVE REGENERATION
• The greater the distance between the severed nerve
endings, the less the chance of recovery because
adjacent tissues block growth by protruding into the
larger gaps
• Neurosurgeons align cut nerve endings surgically to
enhance the chance of successful regeneration
(silicon tubes filled with biodegradable collagen)
• The post-trauma axon regrowth is never exactly the
same as what existed before the injury– pinpoint
accuracy in nerve fibers (realignment) is impossible
• Much of the functional recovery after nerve injury
involves retraining the nervous system to respond
appropriately so that stimulus and responses are
coordinated
CRANIAL NERVES
CRANIAL NERVES
• Twelve pairs of cranial nerves are associated with
the brain and pass through various foramina of the
skull
• The first two pairs attach to the forebrain; the rest
originate from the brain stem
• Other than the vagus nerves, which extend well into
the abdomen, the cranial nerves serve only head and
neck structures
• Names reveal the most important structures they serve
or their primary functions
• Traditionally they are numbered using Roman
Numerals
CRANIAL NERVES
•
•
•
•
•
Twelve pairs of cranial nerves are
associated with the brain and pass
through various foramina of the
skull
The first two pairs attach to the
forebrain; the rest originate from
the brain stem
Other than the vagus nerves,
which extend well into the
abdomen, the cranial nerves serve
only head and neck structures
Names reveal the most
important structures they serve
or their primary functions
Traditionally they are numbered
using Roman numerals
CRANIAL NERVES
•
•
•
•
•
•
•
•
•
•
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•
I: Olfactory nerves are responsible for smell
II: Optic nerves are responsible for vision
III: Oculomotor nerves play a role in eye movement
IV: Trochlear nerves play a role in eye movement
V: Trigeminal nerves are general sensory nerves of the face
VI: Abducens nerves play a role in eye movement
VII: Facial nerves function as the chief motor nerves of the face
VIII: Vestibulocochlear nerves are responsible for hearing and equilibrium
IX: Glossopharyngeal nerves innervate part of the tongue and pharynx
X: Vagus nerves innervate the heart, lungs, and the abdominal organs
XI: Accessory nerves move structures associated with the head and neck
XII: Hypoglossal nerves are mixed nerves that arise from the medulla and
serve the tongue
CRANIAL NERVES
•
•
•
•
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•
•
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•
I: Olfactory nerves are responsible for smell
II: Optic nerves are responsible for vision
III: Oculomotor nerves play a role in eye
movement
IV: Trochlear nerves play a role in eye
movement
V: Trigeminal nerves are general sensory
nerves of the face
VI: Abducens nerves play a role in eye
movement
VII: Facial nerves function as the chief motor
nerves of the face
VIII: Vestibulocochlear nerves are
responsible for hearing and equilibrium
IX: Glossopharyngeal nerves innervate part
of the tongue and pharynx
X: Vagus nerves innervate the heart, lungs,
and the abdominal organs
XI: Accessory nerves move structures
associated with the head and neck
XII: Hypoglossal nerves are mixed nerves
that arise from the medulla and serve the
tongue
CRANIAL NERVES
• On occasion, our trusty truck acts funny–
very good vehicle anyhow
CRANIAL NERVES
• On
occasion,
our trusty
truck acts
funny– very
good vehicle
anyhow
•
•
•
•
•
•
•
•
•
•
•
•
I: Olfactory nerves are responsible for smell
II: Optic nerves are responsible for vision
III: Oculomotor nerves play a role in eye
movement
IV: Trochlear nerves play a role in eye
movement
V: Trigeminal nerves are general sensory
nerves of the face
VI: Abducens nerves play a role in eye
movement
VII: Facial nerves function as the chief motor
nerves of the face
VIII: Vestibulocochlear nerves are
responsible for hearing and equilibrium
IX: Glossopharyngeal nerves innervate part
of the tongue and pharynx
X: Vagus nerves innervate the heart, lungs,
and the abdominal organs
XI: Accessory nerves move structures
associated with the head and neck
XII: Hypoglossal nerves are mixed nerves
that arise from the medulla and serve the
tongue
CRANIAL NERVES
CRANIAL NERVES
•
•
•
•
I
II III
IV
V VI VII
Some say marry money, but my brother
VIII
IX X
XI
XII
says (its) bad business (to) marry money
CRANIAL NERVES
•
•
•
•
I
II III
IV
V VI VII
Some say marry money, but my brother
VIII
IX X
XI
XII
says (its) bad business (to) marry money
SPINAL NERVES
•
•
•
Thirty-one pairs of mixed spinal nerves arise from the spinal cord and
serve the entire body except the head and some areas of the neck
All are mixed nerves
Named according to their point of issue from the spinal cord
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–
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•
8 pairs of cervical spinal nerves (C1 – C8)
12 pairs of thoracic nerves (T1 – T12)
5 pairs of lumbar nerves (L1 – L5)
5 pairs of sacral nerves (S1 – S5)
1 pair of tiny coccygeal nerves ( C0)
Notice that there are 8 pairs of cervical nerves but only 7 cervical
vertebrae
– First 7 pairs of cervical nerves exit the vertebral canal superior to the vertebrae
for which they are named (C8 emerges inferior to the 7th cervical vertebra–
between C7 and T1)
All the remaining spinal nerves leave the vertebral column inferior to the
same-numbered vertebra
SPINAL NERVES
SPINAL NERVES
• Innervation of Specific Body Regions:
– Each spinal nerve connects to the spinal cord by a dorsal root and
a ventral root
– Rami lie distal to and are lateral branches of the spinal nerves that carry
both motor and sensory fibers
– The back is innervated by the dorsal rami with each rami innervating
the muscle in line with the point of origin from the spindle column
– The cervical plexus is formed by the ventral rami of the first four
cervical nerves
– The brachial plexus is situated partly in the neck and partly in the axilla
and gives rise to virtually all the nerves that innervate the upper limb
– Only in the thorax are the ventral rami arranged in a simple segmental
pattern corresponding to that of the dorsi rami
– The area of skin innervated by the cutaneous branches of a single
nerve is called a dermatone
– Hinton’s law states that any nerve serving a muscle that produces
movement at a joint also innervates the joint and the skin over the joint
FORMATION OF SPINAL NERVES
SPINAL NERVES
• Innervation of
Specific Body
Regions:
– Each spinal nerve
connects to the spinal
cord by a dorsal root
and a ventral root
– Rami lie distal to and
are lateral branches of
the spinal nerves that
carry both motor and
sensory fibers
SPINAL NERVES
•
•
•
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•
•
The cervical plexus is formed by the
ventral rami of the first four cervical nerves
The brachial plexus is situated partly in the
neck and partly in the axilla and gives rise to
virtually all the nerves that innervate the
upper limb
Only in the thorax are the ventral rami
arranged in a simple segmental pattern
corresponding to that of the dorsi rami
The sacral and lumbar plexuses overlap
and because many fibers of the lumber
plexus contribute to the sacral plexus via the
lumbosacral trunk, the two plexuses are
often referred to as the lumbosacral plexus
The area of skin innervated by the
cutaneous branches of a single nerve is
called a dermatone
Hinton’s law states that any nerve serving a
muscle that produces movement at a joint
also innervates the joint and the skin over
the joint
FORMATION OF SPINAL NERVES
• Formation of one
pair of spinal nerves
by the union of the
ventral and dorsal
roots of the spinal
cord
FORMATION OF SPINAL NERVES
• Spinal nerve is very
short
• Roots lie medial to and
form the spinal nerves
– Each root is strictly
sensory or motor in
function
• Ramus (branch of a
nerve, artery, vein, or
bone) lie distal to and are
lateral branches of the
spinal nerves
– Like spinal nerves, carry
both sensory and motor
fibers
RAMI DISTRIBUTION OF THE
SPINAL NERVE
RAMI DISTRIBUTION OF THE
SPINAL NERVE
• Cross-sectional view of the left
side of the body at the level of
the thorax, showing the
distribution of the dorsal and
ventral rami of the spinal nerve
• Notice the rami
communicantes branches of
the spinal nerve
• Spinal nerve is very short
– Emerging from its foramen it
divides into the dorsal, ventral,
and meningeal branches
(reenters the vertebral canal to
innervate the meninges and
blood vessels within)
PLEXUS
• A network of converging and diverging
nerve fibers, blood vessels, or
lymphatics
CERVICAL PLEXUS
CERVICAL PLEXUS
• Single most important nerve
from this plexus is the
phrenic nerve
– Input from C3 and C4
– Supplies both motor and
sensory fibers to the
diaphragm which is a major
muscle in the breathing
– Irritation of this nerve causes
spasms resulting in hiccups
– If severed, the diaphragm is
paralyzed and respiration
arrest occurs
• Kept alive by mechanical
respirators
BRACHIAL PLEXUS
BRACHIAL PLEXUS
•
•
•
Gives rise to virtually all the nerves
that innervate the upper limb
Rami (roots) gives rise to trunks
then divisions then cords (Really
Tired Drink Coffee)
Cords are named for their
relationship to the axillary artery
(lateral / posterior / medial)
–
The three cords gives rise to the
main nerves of the upper limb:
•
•
•
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•
Axillary (deltoid/teres
major/skin/shoulder joint)
Musculocutaneous (biceps
brachii/brachialis/skin)
Median (skin/most flexors/lateral palm)
Ulnar (medial hand/intrinsic hand/finger
and wrist flexion)
Radial: largest branch (extensors)
–
–
Nerve rest against the medial
epicondyle of the humerus (funny bone)
» Striking results in tingling of the
little finger
Compression and ischemia (decrease
blood supply) can result in temporary
paralysis
BRACHIAL PLEXUS
LUMBAR PLEXUS
LUMBAR PLEXUS
• Overlaps with sacral
plexuses
• Innervates abdominal
and psoas (loins: lower
part of the back and sides
between the ribs and
pelvis) muscles
• Innervates anterior
(quadriceps) / medial
(adductors) / posterior
thigh
SACRAL PLEXUS
SACRAL PLEXUS
•
•
Serves the buttock, lower limbs,
pelvic, and perineum (structures
between the anus and external
genitalia)
Sciatic nerve: largest branch
– Supplies nearly the entire lower
limb except the anteromedial
thigh
– Actually two nerves (tibial and
fibular) wrapped in a common
sheath
– Motor branches to the hamstring
muscles (all thigh extensors and
knee flexors) and to the adductor
magnus
•
Pudendal nerve: perineum and
external genitalia (clitoris and
penis)
– Voluntary control of urination
and erection
DERMATOMES
• Dermatomes: area of skin innervated by the
cutaneous branches of a single spinal nerve
• All spinal nerves except C1 participate in
dermatomes
• Regions not cleanly separated as map indicates
• Considerable overlapping:
– Thus, destruction of a single spinal nerve will not
result in complete numbness in a specific area
DERMATOMES
DERMATOMES
• Dermatomes: area of skin
innervated by the cutaneous
branches of a single spinal
nerve
• All spinal nerves except C1
participate in dermatomes
• Regions not cleanly separated
as map indicates
• Considerable overlapping:
– Thus, destruction of a single
spinal nerve will not result
in complete numbness in a
specific area
DERMATOMES
• Lumbar nerves supply
most of the anterior
surfaces of the thigh and
legs
• Sacral nerves serve
most of the posterior
surfaces of the lower
limbs
• In the limbs, the overlap
is less complete and
some skin regions are
innervated by just one
spinal nerve
MOTOR ENDINGS
AND
MOTOR ACTIVITY
PERIPHERAL MOTOR ENDINGS
• Peripheral motor endings are the PNS
element that activates effectors by releasing
neurotransmitters
• The terminals of the somatic motor fibers that
innervate voluntary muscles form elaborate
neuromuscular junctions with their effector cells
and they release the neurotransmitter
acetylcholine
• The junctions between autonomic motor
endings and the visceral effectors involve
varicosities and release either acetylcholine or
epinephrine as their neurotransmitter
PNS in the Structural Organization
of the Nervous system
PERIPHERAL MOTOR ENDINGS
• The terminals of the somatic
motor fibers that innervate
voluntary muscles form
elaborate neuromuscular
junctions with their effector
cells and they release the
neurotransmitter
acetylcholine
• The junctions between
autonomic motor endings
and the visceral effectors
involve varicosities and
release either acetylcholine
or epinephrine as their
neurotransmitter
MOTOR CONTROL
• Motor Control
– Three levels:
• Segmental
• Projection
• Programs/Instruction
LEVELS OF MOTOR CONTROL
• The segmental level is the lowest level on the motor
control hierarchy and consists of the spinal cord circuits
(stimulating a specific group of muscle fibers)
– Called central pattern generators (CRGs)
• The projection level: the spinal cord has direct control
– Control reflex and fixed-action motor actions and produce
discrete voluntary movements of skeletal muscles
• The precommand level is made up of the cerebellum
and the basal nuclei and is the highest level of the motor
system hierarchy
LEVELS OF MOTOR CONTROL
LEVEL OF MOTOR CONTROL
• Segmental Level:
– Lowest level of
hierarchy
– Consists of segmental
circuits of the spinal
cord
– Spinal cord neurons
arranged in
reverberating circuits
– Inherited, not learned
LEVEL OF MOTOR CONTROL
• Projection Level:
– Consists of cortical
motor areas that
produce the direct
pyramidal system
– Axons of these
neurons project to the
spinal cord
– Houses command
neurons that modify
and control the
segmental apparatus
LEVEL OF MOTOR CONTROL
• Programs and
Instructions Level:
– Cerebellum and basal
nuclei
– Regulate motor movement
(start/stop, coordination,
block unwanted, and
monitor)
– Sensory and motor
integrated
– Cerebellum: lacks direct
connection to the spinal
cord
• Acts through the
projection areas of the
thalamus and brain stem
REFLEX ACTIVITY
THE REFLEX ARC
• Reflexes are unlearned, rapid,
predictable motor responses to a
stimulus, and occur over highly specific
neural pathways called reflex arc
THE REFLEX ARC
COMPONENTS of a REFLEX ARC
• 1. Receptor: site of stimulus
action
• 2. Sensory Neuron: transmits
the afferent impulse to the
CNS
• 3. Integration Center:
– Always within the CNS
– Synapse between the sensory
and motor neurons
• 4. Motor Neuron: conducts
efferent impulses from the
integration center to an effector
organ
• 5. Effector: muscle fiber or
gland cell that responds to the
efferent impulse
SPINAL REFLEXES
• Spinal reflexes are somatic (body skeletal muscle)
reflexes mediated by the spinal cord (many not
involving the brain)
– In the stretch reflex the muscle spindle is stretched and excited
by either an external stretch or an internal stretch
– The Golgi tendon reflex produces muscle relaxation and
lengthening in response to contraction
– The flexor, or withdrawal, reflex is initiated by a painful
stimulus and causes automatic withdrawal of the threatened
body part from the stimulus
– They crossed extensor reflex is a complex spinal reflex
consisting of an ipsilateral withdrawal reflex (affecting the same
side of the body) and a contralateral extensor reflex (affecting
the opposite side of the body)
– Superficial reflexes are elicited by gentle cutaneous stimulation
Anatomy of the Spindle and Golgi
Tendon Organ
Operation of the Muscle Spindle
STRETCH REFLEX
STRETCH REFLEX
• 1. Stretching the muscle
activates a muscle spindle
(encapsulated receptor found
in skeletal muscle that is
sensitive to stretch)
• 2. Impulses transmitted by
afferent fibers from muscle
spindle to motor neurons in the
spinal cord causes the
stretched muscle to contract
• 3.Impulses transmitted by
afferent fibers from muscle
spindle to interneurons in
the spinal cord result in
reciprocal inhibition of the
antagonist muscle
STRETCH REFLEX
• Patellar Reflex:
– Tapping the patellar tendon
excites muscle spindles in
the quadriceps muscle
– Afferent impulses travel to
the spinal cord, where
synapses occur with motor
neurons and interneurons
– Motor neurons send
impulses to the quadriceps
resulting in contraction and
inhibition of its antagonist
(hamstring), which causes
extension of the knee and
forward movement of the
foot
PATELLAR REFLEX
• Knee-jerk
• Results in information:
– Proves that the sensory and motor
connections between that muscle and the
spinal cord are intact
– The vigor of the motor response indicates
the degree of excitability of the spinal cord
STRETCH REFLEX
• Absent:
– Diabetes
– Neurosyphilis
– Coma
• Hyperactive:
– Lesions of corticospinal tract which reduces
the inhibitory effect of the brain
• Polio (inflammation of the gray matter of brain and
spinal cord)
• Stroke (sudden loss of neurological function)
DEEP GOLGI TENDON REFLEX
DEEP GOLGI TENDON REFLEX
• Golgi tendon organ:
– proprioceptors
located in tendons,
close to the point of
skeletal muscle
insertion
– Important to smooth
onset and termination
of muscle contraction
DEEP GOLGI TENDON REFLEX
•
•
•
•
•
Help ensure smooth onset and
termination of muscle
contraction involving rapid
switching between flexion and
extension such as in running:
1. Golgi tendon receptors in the
contracted muscles tendon are
stimulated
2. Afferent impulse to the spinal
cord
3. Information sent to the
cerebellum where muscle tension
is adjusted
4. Simultaneously, motor neurons
in spinal cord circuits supplying
the contracting muscles are
inhibited and antagonist muscles
are activated (Reciprocal
Activation)
FLEXOR REFLEX
• Flexor reflex initiated by a painful
stimulus (actual or perceived)
– Automatic withdrawal of the threatened body
part from the stimulus
– Protective reflexes important to our survival
– They override the spinal pathways and
prevent any other reflexes from using them at
the same time
CROSSED EXTENSOR REFLEX
• Complex spinal reflex consisting of an
ipsilateral (same side) withdrawal reflex
and a contralateral (opposite side)
extensor reflex
CROSSED EXTENSOR REFLEX
CROSSED EXTENSOR REFLEX
• Example illustrated when a
stranger suddenly grabbed
your arm
• Incoming afferent fibers
synapse with interneurons
that control the flexor
withdrawal response on the
same side of the body and
with other interneurons that
control the extensor
muscles on the opposite
side
PAIN TRANSMISSIOM
DEVELOPMENTAL ASPECTS
OF
THE PERIPHERAL NERVOUS SYTEM
• The Spinal Nerves branch from the developing
spinal cord and adjacent neural crest and exit
between the forming vertebrate
– Each nerve becomes associated with the adjacent
muscle mass
• Cranial Nerves innervate muscles of the head in
a similar way
• Sensory Receptors atrophy to some degree with
age, and there is a decrease in muscle tone in
the face and neck, reflexes occur a bit more
slowly