Download Spinal Nerves

PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
Ninth Edition
College
Human Anatomy & Physiology
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part A
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Peripheral Nervous System (PNS)
• Provides links from and to world outside
body
• All neural structures outside brain
– Sensory receptors
– Peripheral nerves and associated ganglia
– Efferent motor endings
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Figure 13.1 Place of the PNS in the structural organization of the nervous system.
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Sensory Receptors
• Specialized to respond to changes in
environment (stimuli)
• Activation results in graded potentials that
trigger nerve impulses
• Sensation (awareness of stimulus) and
perception (interpretation of meaning of
stimulus) occur in brain
© 2013 Pearson Education, Inc.
Classification of Receptors
• Based on
– Type of stimulus they detect
– Location in body
– Structural complexity
© 2013 Pearson Education, Inc.
Classification by Stimulus Type
• Mechanoreceptors—respond to touch,
pressure, vibration, and stretch
• 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)
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Classification by Location
• Exteroceptors
– Respond to stimuli arising outside body
– Receptors in skin for touch, pressure, pain,
and temperature
– Most special sense organs
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Classification by Location
• Interoceptors (visceroceptors)
– Respond to stimuli arising in internal viscera
and blood vessels
– Sensitive to chemical changes, tissue stretch,
and temperature changes
– Sometimes cause discomfort but usually
unaware of their workings
© 2013 Pearson Education, Inc.
Classification by Location
• Proprioceptors
– Respond to stretch in skeletal muscles,
tendons, joints, ligaments, and connective
tissue coverings of bones and muscles
– Inform brain of one's movements
© 2013 Pearson Education, Inc.
Classification by Receptor Structure
• Simple receptors for general senses
– Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle
sense
– Modified dendritic endings of sensory neurons
• Receptors for special senses
– Vision, hearing, equilibrium, smell, and taste
(Chapter 15)
© 2013 Pearson Education, Inc.
Simple Receptors of the General Senses
• Thermoreceptors
– Cold receptors (10–40ºC); in superficial
dermis
– Heat receptors (32–48ºC); in deeper dermis
– Outside those temperature ranges 
nociceptors activated  pain
© 2013 Pearson Education, Inc.
Unencapsulated Dendritic Endings
• Nociceptors
– Player in detection – vanilloid receptor
• Ion channel opened by heat, low pH, chemicals,
e.g., capsaicin (red peppers)
– Respond to:
• Pinching, chemicals from damaged tissue,
capsaicin
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Other Nonencapsulated Dendritic Endings
• Light touch receptors
– Tactile (Merkel) discs
– Hair follicle receptors
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Table 13.1 General Sensory Receptors Classified by Structure and Function (1 of 3)
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Table 13.1 General Sensory Receptors Classified by Structure and Function (2 of 3)
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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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Adaptation of Sensory Receptors
• Adaptation is change in sensitivity in
presence of constant stimulus
– Receptor membranes become less
responsive
– Receptor potentials decline in frequency or
stop
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Figure 13.3 Map of referred pain.
Lungs and
diaphragm
Heart
Gallbladder
Appendix
Liver
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
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Structure of a Nerve
• Cordlike organ of PNS
• Bundle of myelinated and nonmyelinated
peripheral axons enclosed by connective
tissue
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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
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Figure 13.4a Structure of a nerve.
Endoneurium
Perineurium
Nerve
fibers
Blood
vessel
Fascicle
Epineurium
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Figure 13.4b Structure of a nerve.
Axon
Myelin sheath
Endoneurium
Perineurium
Epineurium
Fascicle
Blood
vessels
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Classification of Nerves
• Most nerves are mixtures of afferent and
efferent fibers and somatic and autonomic
(visceral) fibers
• Classified according to direction transmit
impulses
– Mixed nerves – both sensory and motor
fibers; impulses both to and from CNS
– Sensory (afferent) nerves – impulses only
toward CNS
– Motor (efferent) nerves – impulses only away
from CNS
© 2013 Pearson Education, Inc.
Classification of Nerves
• Pure sensory (afferent) or motor (efferent)
nerves are rare; most mixed
• Types of fibers in mixed nerves:
– Somatic afferent
– Somatic efferent
– Visceral afferent
– Visceral efferent
• Peripheral nerves classified as cranial or
spinal nerves
© 2013 Pearson Education, Inc.
Ganglia
• Contain neuron cell bodies associated with
nerves in PNS
– Ganglia associated with afferent nerve fibers
contain cell bodies of sensory neurons
• Dorsal root ganglia (sensory, somatic)
(Chapter 12)
– Ganglia associated with efferent nerve fibers
contain autonomic motor neurons
• Autonomic ganglia (motor, visceral) (Chapter 14)
© 2013 Pearson Education, Inc.
Regeneration of Nerve Fibers
• Mature neurons are amitotic but if soma of
damaged nerve is intact, peripheral axon may
regenerate
• If peripheral axon damaged
– Axon fragments (Wallerian degeneration); spreads
distally from injury
– Macrophages clean dead axon; myelin sheath intact
– Axon filaments grow through regeneration tube
– Axon regenerates; new myelin sheath forms
• Greater distance between severed ends-less
chance of regeneration
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Regeneration of Nerve Fibers
• Most CNS fibers never regenerate
• CNS oligodendrocytes bear growth-inhibiting
proteins that prevent CNS fiber regeneration
• Astrocytes at injury site form scar tissue
containing chondroitin sulfate that blocks axonal
regrowth
• Treatment
– Neutralizing growth inhibitors, blocking receptors for
inhibitory proteins, destroying chondroitin sulfate
promising
© 2013 Pearson Education, Inc.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (1 of 4)
Endoneurium
Schwann cells
Droplets
of myelin
Fragmented
axon
Site of nerve damage
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1 The axon
becomes
fragmented at
the injury site.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (2 of 4)
Schwann cell
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Macrophage
2 Macrophages
clean out the dead
axon distal to the
injury.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (3 of 4)
Aligning Schwann cells
form regeneration tube
Fine axon sprouts
or filaments
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3 Axon sprouts,
or filaments, grow
through a
regeneration tube
formed by
Schwann cells.
Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (4 of 4)
Schwann cell
Single enlarging
axon filament
© 2013 Pearson Education, Inc.
New myelin
sheath forming
4 The axon
regenerates and a
new myelin sheath
forms.
PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
Ninth Edition
College
Human Anatomy & Physiology
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part B
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Cranial Nerves
• Twelve pairs of nerves associated with brain
– Two attach to forebrain; rest with brain stem
• Most mixed nerves; two pairs purely sensory
• Each numbered (I through XII) and named from
rostral to caudal
"On occasion, our trusty truck acts funny—very
good vehicle anyhow"
"Oh once one takes the anatomy final, very
good vacations are heavenly"
© 2013 Pearson Education, Inc.
Figure 13.6a Location and function of cranial nerves.
Filaments of
olfactory nerve (I)
Frontal lobe
Olfactory bulb
Olfactory tract
Optic nerve (II)
Optic chiasma
Temporal lobe
Optic tract
Oculomotor
nerve (III)
Trochlear
nerve (IV)
Infundibulum
Trigeminal
nerve (V)
Abducens
nerve (VI)
Cerebellum
Medulla oblongata
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Facial nerve (VII)
Vestibulocochlear
nerve (VIII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Figure 13.6b Location and function of cranial nerves.
Cranial nerves
I – VI
I
II
III
IV
V
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
VI Abducens
Sensory
function
Motor
function
PS*
fibers
Yes (smell)
Yes (vision)
No
No
Yes (general
sensation)
No
No
No
Yes
Yes
Yes
No
No
Yes
No
No
Yes
No
Cranial nerves
VII – XII
VII Facial
VIII Vestibulocochlear
IX
X
XI
XII
Glossopharyngeal
Vagus
Accessory
Hypoglossal
Sensory
function
Motor
function
PS*
fibers
Yes (taste)
Yes (hearing
and balance)
Yes (taste)
Yes (taste)
No
No
Yes
Some
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
*PS = parasympathetic
© 2013 Pearson Education, Inc.
Composition of Cranial Nerves
• Some mixed nerves contain both somatic and
autonomic fibers
– Most motor neuron cell bodies in ventral gray matter
of brain stem
– Some autonomic motor neurons in ganglia
• To remember primary functions of cranial nerves
as sensory, motor, both:
– "Some say marry money, but my brother believes
(it’s) bad business (to) marry money."
© 2013 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
Ninth Edition
College
Human Anatomy & Physiology
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part C
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Spinal Nerves
• 31 pairs of mixed nerves named for point
of issue from spinal cord
– Supply all body parts but head and part of
neck
– 8 cervical (C1–C8)
– 12 thoracic (T1–T12)
– 5 Lumbar (L1–L5)
– 5 Sacral (S1–S5)
– 1 Coccygeal (C0)
© 2013 Pearson Education, Inc.
Spinal Nerves
• 31 pairs of mixed nerves named for point
of issue from spinal cord
– Supply all body parts but head and part of
neck
– 8 cervical (C1–C8)
– 12 thoracic (T1–T12)
– 5 Lumbar (L1–L5)
– 5 Sacral (S1–S5)
– 1 Coccygeal (C0)
© 2013 Pearson Education, Inc.
Spinal Nerves
• Only 7 cervical vertebrae, yet 8 pairs
cervical spinal nerves
– 7 exit vertebral canal superior to vertebrae for
which named
– 1 exits canal inferior to C7
• Other vertebrae exit inferior to vertebra for
which named
© 2013 Pearson Education, Inc.
Figure 13.7 Spinal nerves.
Cervical plexus
Brachial plexus
Cervical
nerves
C1 – C8
Cervical
enlargement
Intercostal
nerves
Thoracic
nerves
T1 – T12
Lumbar
enlargement
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Lumbar plexus
Lumbar
nerves
L1 – L5
Sacral plexus
Sacral
nerves
S1 – S5
Cauda equina
Coccygeal
nerve
Co1
Figure 13.9 The cervical plexus.
Ventral rami
Segmental
branches
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Ventral
rami:
C1
Greater auricular
nerve
C2
Transverse
cervical nerve
C3
Ansa cervicalis
C4
Accessory nerve (XI)
Phrenic nerve
Supraclavicular
nerves
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C5
Figure 13.10a The brachial plexus.
Anterior
divisions
Posterior
divisions
Trunks
Roots
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
Roots (ventral rami):
C4
C5
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial
Medial pectoral
Lateral pectoral
Axillary
Upper subscapular
Musculocutaneous
Lower subscapular
Radial
Thoracodorsal
Median
Ulnar
Roots (rami C5–T1), trunks, divisions, and cords
© 2013 Pearson Education, Inc.
Medial cutaneous
nerves of the arm
and forearm
Figure 13.10b The brachial plexus.
Major terminal
branches
(peripheral nerves)
Cords
Divisions
Trunks
Anterior
Musculocutaneous
Lateral
Median
Medial
Ulnar
Upper
Posterior
Anterior
Roots
(ventral
rami)
C5
C6
Middle
C7
Posterior
Radial
Posterior
C8
Anterior
Lower
Axillary
Posterior
T1
Flowchart summarizing relationships within the brachial plexus
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Brachial Plexus: Five Important Nerves
• Axillary—innervates deltoid, teres minor, and skin and
joint capsule of shoulder
• Musculocutaneous—innervates biceps brachii and
brachialis, coracobrachialis, and skin of lateral forearm
• Median—innervates skin, most flexors, forearm
pronators, wrist and finger flexors, thumb opposition
muscles
• Ulnar—supplies flexor carpi ulnaris, part of flexor
digitorum profundus, most intrinsic hand muscles, skin of
medial aspect of hand, wrist/finger flexion
• Radial—innervates essentially all extensor muscles,
supinators, and posterior skin of limb
© 2013 Pearson Education, Inc.
Figure 13.10c The brachial plexus.
Axillary nerve
Humerus
Radial nerve
Musculocutaneous nerve
Ulna
Radius
Ulnar nerve
Median nerve
Radial nerve (superficial branch)
Dorsal branch of ulnar nerve
Superficial branch of ulnar nerve
Digital branch of ulnar nerve
Muscular branch
Median nerve
Digital branch
The major nerves of the upper limb
© 2013 Pearson Education, Inc.
Figure 13.10d The brachial plexus.
Musculocutaneous
nerve
Lateral cord
Posterior cord
Axillary nerve
Medial cord
Radial nerve
Median nerve
Ulnar nerve
Cadaver photo
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Table 13.4 Branches of the Brachial Plexus
© 2013 Pearson Education, Inc.
Lumbar Plexus
• Arises from L1–L4
• Innervates thigh, abdominal wall, and
psoas muscle
• Femoral nerve—innervates quadriceps
and skin of anterior thigh and medial
surface of leg
• Obturator nerve—passes through
obturator foramen to innervate adductor
muscles
© 2013 Pearson Education, Inc.
Figure 13.11 The lumbar plexus.
Ventral rami
Ventral
rami:
L1
L2
Iliohypogastric
Ilioinguinal
Iliohypogastric
Femoral
Ilioinguinal
Lateral
femoral
cutaneous
Genitofemoral
Lateral femoral
cutaneous
L3
Obturator
L4
Anterior
femoral
cutaneous
Saphenous
Obturator
Femoral
L5
Lumbosacral
trunk
Ventral rami and major branches of the lumbar plexus
Distribution of the major nerves from the
lumbar plexus to the lower limb
© 2013 Pearson Education, Inc.
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic
structures, and perineum
• Sciatic nerve
– Longest and thickest nerve of body
– Innervates hamstring muscles, adductor
magnus, and most muscles in leg and foot
– Composed of two nerves: tibial and common
fibular
© 2013 Pearson Education, Inc.
Figure 13.12a The sacral plexus.
Ventral rami
Superior
gluteal
Ventral
rami:
L4
L5
Lumbosacral
trunk
Inferior
gluteal
Common
fibular
Tibial
Posterior
femoral
cutaneous
Pudendal
Sciatic
S1
S2
S3
S4
S5
Co1
Ventral rami and major branches of the sacral plexus
© 2013 Pearson Education, Inc.
Figure 13.12b The sacral plexus.
Superior gluteal
Inferior gluteal
Pudendal
Sciatic
Posterior femoral
cutaneous
Common fibular
Tibial
Sural (cut)
Deep fibular
Superficial fibular
Plantar branches
© 2013 Pearson Education, Inc.
Distribution of the major nerves from
the sacral plexus to the lower limb
Figure 13.12c The sacral plexus.
Gluteus maximus
Piriformis
Inferior gluteal
nerve
Common fibular
nerve
Tibial nerve
Pudendal nerve
Posterior femoral
cutaneous nerve
Sciatic nerve
Cadaver photo
© 2013 Pearson Education, Inc.
Table 13.6 Branches of the Sacral Plexus
© 2013 Pearson Education, Inc.
Innervation of Skin: Dermatomes
• Dermatome - area of skin innervated by
cutaneous branches of single spinal nerve
• All spinal nerves except C1 participate in
dermatomes
• Extent of spinal cord injuries ascertained
by affected dermatomes
• Most dermatomes overlap, so destruction
of a single spinal nerve will not cause
complete numbness
© 2013 Pearson Education, Inc.
Figure 13.13 Map of dermatomes.
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C2
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T2
C5
C6
C6
C7
L1
C8
L2
T12
S2
S3
T2
C5
C6
L1
C8
L2
S1
L4
S2
S3
S4
S5
C6
C7
C6
C7
C8
C8
L2
S2
S2
S1
L1
L3
L5
L4
T11
T12
L1
L3
L5
C7
C6
S1
L3
C5
L2
L5
L4
L3
L5
L5
L4
S1
S1
L4
L5
Anterior view
© 2013 Pearson Education, Inc.
Posterior view
L4
L5
S1
Innervation of Joints
• To remember which nerves serve which
synovial joint
– Hilton's law: Any nerve serving a muscle that
produces movement at joint also innervates
joint and skin over joint
© 2013 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
Ninth Edition
College
Human Anatomy & Physiology
CHAPTER
13
The Peripheral
Nervous
System and
Reflex Activity:
Part D
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Peripheral Motor Endings
• PNS elements that activate effectors by
releasing neurotransmitters
© 2013 Pearson Education, Inc.
Review of Innervation of Skeletal Muscle
• Takes place at neuromuscular junction
• Neurotransmitter acetylcholine (ACh)
released when nerve impulse reaches
axon terminal
• ACh binds to receptors, resulting in:
– Movement of Na+ and K+ across membrane
– Depolarization of muscle cell
– An end plate potential, which triggers an
action potential  muscle contraction
© 2013 Pearson Education, Inc.
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Fusing synaptic
vesicles
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
ACh
4 ACh diffuses across the synaptic
cleft and binds to its receptors on
the sarcolemma.
5 ACh binding opens ion
channels in the receptors that
allow simultaneous passage of
Na+ into the muscle fiber and K+
out of the muscle fiber. More Na+
ions enter than K+ ions exit,
which produces a local change
in the membrane potential called
the end plate potential.
© 2013 Pearson Education, Inc.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
Synaptic
cleft
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Postsynaptic
membrane
ion channel opens;
ions pass.
ACh
Acetylcholinesterase
Degraded ACh
Ion channel closes;
ions cannot pass.
Slide 1
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Fusing synaptic
vesicles
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
ACh
4 ACh diffuses across the synaptic
cleft and binds to its receptors on
the sarcolemma.
5 ACh binding opens ion
channels in the receptors that
allow simultaneous passage of
Na+ into the muscle fiber and K+
out of the muscle fiber. More Na+
ions enter than K+ ions exit,
which produces a local change
in the membrane potential called
the end plate potential.
© 2013 Pearson Education, Inc.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
Synaptic
cleft
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Postsynaptic
membrane
ion channel opens;
ions pass.
ACh
Acetylcholinesterase
Degraded ACh
Ion channel closes;
ions cannot pass.
Slide 8
Reflexes
• Inborn (intrinsic) reflex - rapid, involuntary,
predictable motor response to stimulus
– Example – maintain posture, control visceral
activities
– Can be modified by learning and conscious
effort
• Learned (acquired) reflexes result from
practice or repetition,
– Example – driving skills
© 2013 Pearson Education, Inc.
Reflex Arc
• Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent
impulses to CNS
3. Integration center—either monosynaptic or
polysynaptic region within CNS
4. Motor neuron—conducts efferent impulses
from integration center to effector organ
5. Effector—muscle fiber or gland cell that
responds to efferent impulses by contracting
or secreting
© 2013 Pearson Education, Inc.
Figure 13.15 The five basic components of all reflex arcs.
Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross scetion)
© 2013 Pearson Education, Inc.
Reflexes
• Functional classification
– Somatic reflexes
• Activate skeletal muscle
– Autonomic (visceral) reflexes
• Activate visceral effectors (smooth or cardiac
muscle or glands)
© 2013 Pearson Education, Inc.
Spinal Reflexes
• Spinal somatic reflexes
– Integration center in spinal cord
– Effectors are skeletal muscle
• Testing of somatic reflexes important
clinically to assess condition of nervous
system
– If exaggerated, distorted, or absent 
degeneration/pathology of specific nervous
system regions
© 2013 Pearson Education, Inc.
Stretch and Tendon Reflexes
• To smoothly coordinate skeletal muscle
nervous system must receive
proprioceptor input regarding
– Length of muscle
• From muscle spindles
– Amount of tension in muscle
• From tendon organs
© 2013 Pearson Education, Inc.
The Stretch Reflex
• Maintains muscle tone in large postural
muscles, and adjusts it reflexively
– Causes muscle contraction in response to
increased muscle length (stretch)
© 2013 Pearson Education, Inc.
Stretch Reflexes
• How stretch reflex works
– Stretch activates muscle spindle
– Sensory neurons synapse directly with 
motor neurons in spinal cord
–  motor neurons cause stretched muscle to
contract
• All stretch reflexes are monosynaptic and
ipsilateral
© 2013 Pearson Education, Inc.
Stretch Reflexes
• Reciprocal inhibition also occurs—IIa
fibers synapse with interneurons that
inhibit  motor neurons of antagonistic
muscles
– Example: In patellar reflex, stretched muscle
(quadriceps) contracts and antagonists
(hamstrings) relax
© 2013 Pearson Education, Inc.
Stretch Reflexes
• Positive reflex reactions indicate
– Sensory and motor connections between
muscle and spinal cord intact
– Strength of response indicates degree of
spinal cord excitability
• Hypoactive or absent if peripheral nerve
damage or ventral horn injury
• Hyperactive if lesions of corticospinal tract
© 2013 Pearson Education, Inc.
Reflexes
• Inborn (intrinsic) reflex - rapid, involuntary,
predictable motor response to stimulus
– Example – maintain posture, control visceral
activities
– Can be modified by learning and conscious
effort
• Learned (acquired) reflexes result from
practice or repetition,
– Example – driving skills
© 2013 Pearson Education, Inc.
Reflex Arc
• Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent
impulses to CNS
3. Integration center—either monosynaptic or
polysynaptic region within CNS
4. Motor neuron—conducts efferent impulses
from integration center to effector organ
5. Effector—muscle fiber or gland cell that
responds to efferent impulses by contracting
or secreting
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Figure 13.15 The five basic components of all reflex arcs.
Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross scetion)
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Reflexes
• Functional classification
– Somatic reflexes
• Activate skeletal muscle
– Autonomic (visceral) reflexes
• Activate visceral effectors (smooth or cardiac
muscle or glands)
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Spinal Reflexes
• Spinal somatic reflexes
– Integration center in spinal cord
– Effectors are skeletal muscle
• Testing of somatic reflexes important
clinically to assess condition of nervous
system
– If exaggerated, distorted, or absent 
degeneration/pathology of specific nervous
system regions
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The Stretch Reflex
• Maintains muscle tone in large postural
muscles, and adjusts it reflexively
– Causes muscle contraction in response to
increased muscle length (stretch)
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Stretch Reflexes
• How stretch reflex works
– Stretch activates muscle spindle
– Sensory neurons synapse directly with 
motor neurons in spinal cord
–  motor neurons cause stretched muscle to
contract
• All stretch reflexes are monosynaptic and
ipsilateral
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Stretch Reflexes
• Reciprocal inhibition also occurs—IIa
fibers synapse with interneurons that
inhibit  motor neurons of antagonistic
muscles
– Example: In patellar reflex, stretched muscle
(quadriceps) contracts and antagonists
(hamstrings) relax
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Stretch Reflexes
• Positive reflex reactions indicate
– Sensory and motor connections between
muscle and spinal cord intact
– Strength of response indicates degree of
spinal cord excitability
• Hypoactive or absent if peripheral nerve
damage or ventral horn injury
• Hyperactive if lesions of corticospinal tract
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