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Anatomy & Physiology I
Lecture 12
Chapter 13: The Peripheral Nervous
System and Reflex Activity
The Peripheral Nervous System
• Provides links from and to world outside body
• All neural structures outside brain
– Sensory receptors
– Peripheral nerves and associated ganglia
– Efferent motor endings
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 – the awareness of stimulus
Classification of Receptors
• Based on
– Type of stimulus they detect
– Location in body
– Structural complexity
Stimulus Classification
• Mechanoreceptors—respond to touch,
pressure, vibration, and stretch
• Thermoreceptors—sensitive to changes in
temperature
• Photoreceptors—respond to light energy
– retina
Stimulus Classification
• Chemoreceptors—respond to chemicals
– smell, taste, changes in blood chemistry
• Nociceptors—sensitive to pain-causing stimuli
– extreme heat or cold, excessive pressure,
inflammatory chemicals
Classified by Location
• Exteroceptors
– Respond to stimuli arising outside body
– Receptors in skin for touch, pressure, pain, and
temperature
– Most special sense organs
Classified by Location
• Interoceptors (visceroceptors)
– Respond to stimuli arising in internal viscera and
blood vessels
– Sensitive to chemical changes, tissue stretch, and
temperature changes
Classified 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
Sensory Integration
• Survival depends upon sensation and
perception
• Sensation - the awareness of changes in the
internal and external environment
• Perception - the conscious interpretation of
those stimuli
Somatosensory System
• Somatosensory system – part of sensory
system serving body wall and limbs
• Receives inputs from
– Exteroceptors, proprioceptors, and interoceptors
• Input relayed toward head, but processed
along way
Neural Integration
• Levels of neural integration in sensory
systems:
1. Receptor level—sensory receptors
2. Circuit level—processing in ascending
pathways
3. Perceptual level—processing in cortical
sensory areas
Figure 13.2 Three basic levels of neural integration in sensory systems.
3 Perceptual level (processing in cortical
sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
Medulla
(processing in
ascending pathways) Spinal cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
1 Receptor level
(sensory reception and
transmission to CNS)
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Joint
kinesthetic
receptor
Cerebellum
Receptor Level
• Receptors have specificity for stimulus energy
• Stimulus changed to graded potential
• Graded potentials must reach threshold for AP
Processing at Receptor Level
In general sense receptors, graded potential
called generator potential
Stimulus

Generator potential in afferent neuron

Action potential
Circuit Level Processing
• Pathways of three neurons conduct sensory impulses
upward to appropriate cortical regions
• Three sensory neurons
– First-order sensory neurons
– Second-order sensory neurons
– Third-order sensory neurons
Circuit Neurons
• First-order sensory neurons
– Conduct impulses from receptor level to spinal reflexes or
second-order neurons in CNS
• Second-order sensory neurons
– Spinal reflex neuron
– Transmit impulses to third-order sensory neurons
• Third-order sensory neurons
– Conduct impulses from thalamus to the somatosensory cortex
(perceptual level)
Perceptual Level
• Interpretation of sensory input depends on
specific location of target neurons in sensory
cortex
• Aspects of sensory perception:
– Perceptual detection
– Magnitude estimation
– Spatial discrimination
Sensory perception
• Perceptual detection
– ability to detect a stimulus
• Magnitude estimation
– intensity coded in frequency of impulses
• Spatial discrimination
– identifying site or pattern of stimulus
Figure 13.2 Three basic levels of neural integration in sensory systems.
3 Perceptual level (processing in cortical
sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
Medulla
(processing in
ascending pathways) Spinal cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
1 Receptor level
(sensory reception and
transmission to CNS)
© 2013 Pearson Education, Inc.
Joint
kinesthetic
receptor
Cerebellum
Feature abstraction
• Identification of more complex aspects and
several stimulus properties
• Quality discrimination
– ability to identify submodalities of a sensation
– sweet or sour tastes
• Pattern recognition
– recognition of familiar or significant patterns in stimuli
– melody in piece of music as opposed to the individual
notes
Structure of a Nerve
• Cordlike organ of PNS
• Bundle of myelinated and nonmyelinated
peripheral axons enclosed by connective
tissue
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
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
© 2013 Pearson Education, Inc.
Classification of Nerves
• Most nerves are mixtures of afferent and
efferent fibers and somatic and autonomic
(visceral) fibers
• Classified according to direction they transmit
impulses
– Mixed nerves
– Sensory (afferent) nerves
– Motor (efferent) nerves
Classification of Nerves
• 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
Mixed Nerves classified according to
region they innervate
• Types of fibers in mixed nerves:
– Somatic afferent
– Somatic efferent
– Visceral afferent
– Visceral efferent
• All peripheral nerves classified as cranial or
spinal nerves
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) (Ch. 12)
• Ganglia associated with efferent nerve fibers
contain autonomic motor neurons
– Autonomic ganglia (motor, visceral) (Ch. 14)
The Cranial Nerves
• Twelve pairs of nerves associated with brain
– Two attach to forebrain; rest with brain stem
• Each numbered (I through XII) and named from
anterior to posterior
Helpful Reminders
On occasion, our trusty truck acts funny, very
good vehicle anyhow”
Oh once one takes the anatomy final, very good
vacation are heavenly”
Only Owls Observe Them Traveling And Finding
Voldemort Guarding Very Secret Horcruxes
The 12 Cranial Nerves
•
•
•
•
•
•
Olfactory
Optic
Oculomoter
Trochlear
Trigeminal
Abducens
•
•
•
•
•
•
Facial
Vestibulocochlear
Glossopharyngeal
Vagus
Accessory
Hypoglossal
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.
• You have one nose, so the olfactory nerve is CN I, and it controls the sense of
smell and innervates the nose.
• You have two eyes, so the optic nerve is CN II, which functions to produce vision.
• You have six abs, so the abducens nerve is CN VI, which abducts the eye.
• CN VII: A mnemonic for the names of the five branches of the Facial nerve is To
Zanzibar By Motor Car. (Temporal, Zygomatic, Buccal, Mandibular and Cervical)
• The number 8 resembles an ear, so CN VIII, the vestibulocochlear/acoustic nerve
is a sensory nerve for hearing.
• The number 11 can resemble a pair of arms or shoulders, and CN XI innervates
the sternocleidomastoid and trapezius muscles and controls shoulder and neck
movements.
• The tongue licks the wound is a reminder that when the hypoglossal nerve
(CNXII) is damaged, the tongue deviates to the same side of a lesion to the cranial
nerve
– if the tongue deviates to the right, the right side of CNXII is damaged).
I: The Olfactory Nerves
•
•
•
•
Sensory nerves of smell
Run from nasal mucosa to olfactory bulbs
Fibers synapse in olfactory bulbs
Pathway terminates in primary olfactory
cortex
• Purely sensory (olfactory) function
II: The Optic Nerves
• Arise from retinas
• Pass through optic canals, converge and
partially cross over at optic chiasma
• Optic tracts continue to thalamus, where they
synapse
• Optic radiation fibers run to occipital (visual)
cortex
• Purely sensory (visual) function
III: The Oculomotor Nerves
• Fibers extend from ventral midbrain through
superior orbital fissures to four of six extrinsic
eye muscles
• Function in raising eyelid, directing eyeball,
constricting iris (parasympathetic), and
controlling lens shape
IV: The Trochlear Nerves
• Fibers from dorsal midbrain enter orbits via
superior orbital fissures to innervate superior
oblique muscle
• Primarily motor nerve that directs eyeball
– rotation of eye
V: The Trigeminal Nerves
• Largest cranial nerves; fibers extend from pons to face
• Three divisions
– Ophthalmic (V1)
– Maxillary (V2)
– Mandibular (V3)
• Convey sensory impulses from various areas of face (V1)
and (V2)
• Supply motor fibers (V3) for mastication
VI: The Abducens Nerves
• Fibers from inferior pons enter orbits via
superior orbital fissures
• Primarily a motor, innervating lateral rectus
muscle to move eye laterally
VII: The Facial Nerves
• Chief motor nerves of face with 5 major
branches
• Motor functions include facial expression,
parasympathetic impulses to lacrimal and
salivary glands
• Sensory function (taste) from anterior twothirds of tongue
The Facial Nerves
Remembering the Facial Nerve
VIII: The Vestibulocochlear Nerves
• Afferent fibers from hearing receptors
(cochlear division) and equilibrium receptors
(vestibular division)
• Enter brain stem at pons-medulla border
• Mostly sensory function; small motor
component for adjustment of sensitivity of
receptors
IX: The Glossopharyngeal Nerves
• Fibers run to throat
• Motor functions
– innervate part of tongue and pharynx for
swallowing, and provide parasympathetic fibers to
parotid salivary glands
• Sensory functions
– conduct taste and general sensory impulses from
pharynx and posterior tongue
X: The Vagus Nerves
• Only cranial nerves that extend beyond head and
neck region
• Most motor fibers are parasympathetic fibers
that help regulate activities of heart, lungs, and
abdominal viscera
• Sensory fibers carry impulses from thoracic and
abdominal viscera, heart rate, respiratory rate,
and taste buds of posterior tongue and pharynx
XI: The Accessory Nerves
• Formed from ventral rootlets from C1–C5
region of spinal cord (not brain)
• Accessory nerves exit skull to innervate
trapezius and sternocleidomastoid muscles
XII: The Hypoglossal Nerves
• Fibers from medulla exit innervate extrinsic
and intrinsic muscles of tongue that
contribute to swallowing and speech
The 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)
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
Figure 13.7 Spinal nerves.
Cervical plexus
Brachial plexus
Cervical
nerves
C1 – C8
Cervical
enlargement
Intercostal
nerves
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Lumbar
nerves
L1 – L5
Sacral plexus
Sacral
nerves
S1 – S5
Cauda equina
Coccygeal
nerve
Co1
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Spinal Nerves: Roots
• Each spinal nerve connects to spinal cord via
two roots
• Ventral roots
– Contain motor (efferent) fibers from ventral horn
motor neurons
– Fibers innervate skeletal muscles
Spinal Nerves: Roots
• Dorsal roots
– Contain sensory (afferent) fibers from sensory
neurons in dorsal root ganglia and conduct
impulses from peripheral receptors
• Dorsal and ventral roots unite to form spinal
nerves, which emerge from vertebral column
via intervertebral foramina
Figure 13.8a Formation of spinal nerves and rami distribution.
Gray matter
White matter
Ventral root
Dorsal root
Dorsal and ventral
rootlets of spinal
nerve
Dorsal root
ganglion
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal nerve
Spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Anterior view showing spinal cord, associated nerves, and vertebrae.
The dorsal and ventral roots arise medially as rootlets and join laterally to
form the spinal nerve.
© 2013 Pearson Education, Inc.
Spinal Nerves: Rami
• Spinal nerves quite short (~1-2 cm)
• Each branches into mixed rami
– Dorsal ramus
– Ventral ramus (larger)
– Meningeal branch
• reenters vertebral canal, innervates meninges and blood
vessels
– Rami communicantes
• autonomic pathways that join ventral rami in thoracic region
Nerve Plexuses
• All ventral rami except T2–T12 form interlacing
nerve networks called nerve plexuses
– cervical, brachial, lumbar, and sacral
• Ventral rami of T2–T12 as intercostal nerves
supply muscles of ribs, anterolateral thorax,
and abdominal wall
Ventral Plexus vs Dorsal Rami
• Dorsal Rami innervates muscles of the back
• Ventral rami form interlacing nerve networks
called plexuses
Spinal Nerves: Plexuses
• Within plexus fibers criss-cross and
redistribute nerves
– Each branch contains fibers from several spinal
nerves
– Fibers from ventral ramus go to body periphery
via several routes
• Each limb muscle innervated by more than
one spinal nerve
Figure 13.8b Formation of spinal nerves and rami distribution.
Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Intercostal nerve
Dorsal root ganglion
Dorsal root
Ventral root
Sympathetic trunk
ganglion
Branches of intercostal nerve
Lateral cutaneous
Anterior cutaneous
Sternum
Cross section of thorax showing the main roots and branches of a spinal nerve.
© 2013 Pearson Education, Inc.
Cervical Plexus
• Formed by ventral rami of C1–C4
• Most branches form cutaneous nerves
– Innervate skin of neck, ear, back of head, and
shoulders
– Other branches innervate neck muscles
• Phrenic nerve
– Major motor and sensory nerve of diaphragm
(receives fibers from C3–C5)
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
Brachial Plexus and Upper Limb
• Formed by ventral rami of C5–C8 and T1
• Gives rise to nerves that innervate upper limb
Brachial Plexus
• Major branches of this plexus:
– Roots—five ventral rami (C5–T1), which form
– Trunks—upper, middle, and lower, which form
– Divisions—anterior and posterior, which form
– Cords—lateral, medial, and posterior
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
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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
© 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.
5 Important Nerves of Brachial Plexus
•
•
•
•
•
Axillary
Musculocutaneous
Median
Ulnar
Radial
Anterolateral Thorax and Abdominal
Wall
• Ventral rami in thorax (T1-T12)
– Form intercostal nerves that supply intercostal
muscles, muscle and skin of anterolateral thorax,
most abdominal wall
– Give off cutaneous branches to skin along course
• Dorsal rami innervate posterior body trunk
Lumbar Plexus
• Arises from L1–L4
• Innervates thigh, abdominal wall,
• Femoral nerve
– innervates quadriceps and skin of anterior thigh
and medial surface of leg
• Obturator nerve
– innervate adductor muscles
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
© 2013 Pearson Education, Inc.
Distribution of the major nerves from the
lumbar plexus to the lower limb
6 Important Nerves of Lumbar Plexus
•
•
•
•
•
•
Femoral
Obturator
Lateral femoral cutaneous
Iliohypogastric
Ilioinguinal
Genitofemoral
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
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
Distribution of the major nerves from
© 2013 Pearson Education, Inc. the sacral plexus to the lower limb
5 Important Nerves of Sacral Plexus
• Sciatic nerve
– Tibial
– Common fibular
•
•
•
•
Superior gluteal
Inferior gluteal
Posterior femoral cutaneous
Pudendal
Innervation of Skin: Dermatomes
• 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
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
© 2013 Pearson Education,
Inc. view
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
End of Part I
• Part II on Wednesday (6/19)
• Peripheral Motor Endings and the Reflex Arc
(Ch. 15)
• Chapter 14: The Autonomic Nervous System
(ANS)
Today’s Lab
• Lab Exercise 21
– Activity 1 – 3 only (Activity 3 Wednesdays' lecture)
• Lab Exercise 22
– Introduction to the reflex arc (Wednesday)
– Optional Activities 1 - 4
• Review your material. The final is next week!

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