Chapter 12
Spinal Cord & Spinal Nerves
AP1 Chapter 12
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Chapter 12 Outline
I. Spinal Cord (SC)
II. Reflexes
III. Interaction with SC reflexes
IV. Structure of peripheral nerves
V. Spinal Nerves
AP1 Chapter 12
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Chapter 12: SC & spinal nerves
CNS
PNS
• Structure
• Structure:
– Nerves & ganglia outside of
the CNS
– Ganglia: accumulation of
cell bodies in the PNS
– Brain
– Spinal Cord
• Fxn:
– Receives sensory info
– Integrates & evaluates the
input it receives
– Stores some information
– Initiates rxns
• Fxn:
AP1 Chapter 12
– Collects information from
numerous sources inside &
outside the body & relays it
through axons of sensory
neurons to the CNS
– Axons of the motor neurons
of the PNS carry info from
the CNS out to the body
regulating the structures
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I. Spinal Cord (CNS!)
CNS division
Major communication link between the
brain & the PNS inferior to the head.
Participates in the integration of info &
prod’s responses thru reflex
mechanisms
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I. Spinal Cord: General Structure
Fig 12.1 pg 412
• SC: extends from the foramen magnum to
L2
• Segments: cervical, thoracic, lumbar, &
sacral
• SC gives rise to 31 pairs of spinal nerves
that exit the vertebral column thru
intervertebral foramen or the sacral foramina
• 2 regions of enlargement
– Cervical enlargement
• Loc where axons that supply the
upper limbs enter & leave the SC
– Lumbosacral enlargement
• Loc where axons supplying the lower
limbs enter or leave SC
• Conus medullaris: SC tapers into a cone-like
region
• Cauda equina: numerous roots extending
inferiorly from LS enlargement that
resemble a horse’s tail
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I. SC: Meninges of the SC
• Meninges: CT coverings around
brain & SC
• Dura Mater:“tough mother”
Fig 12.2
Pg 413
– Most superficial & thickest
– DM forms a “sac”  thecal sac
surrounds SC. Attaches to the rim of
the foramen magnum  C2
– Epidural Space: layer of fat between
the DM & the Vertebral Foramen
(cushion)
• Arachnoid Mater: “Spider web like”
• middle membrane very thin & wispy
– Subdural space: small amount of
serous fluid
• Pia Mater: “Affectionate”
• inner most bound very tightly to
surface of SC
– Subarachnoid space: contains web-like
strands of arachnoid mater+ blood
vessels, & cerebrospinal fluid (CSF)
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I. SC: Meninges of the SC
• SC is held in place by
2 things:
– Denticulate ligaments
• CT septa extending
from the lateral sides of
the SC to the DM
– Filum terminale
• CT strand that anchors
the conus medullaris &
thecal sac to the 1st
coccygeal vertebra,
limiting superior
movement
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I. SC: Cross section of the SC
Figure 12.3 pg 414
• Anterior Median Fissure
• Posterior Median Sulcus
• White matter
– Columns/Funiculi: Anterior (Ventral), posterior (Dorsal), lateral
• Each column is divided into Tracts/Faciculi/Pathways
• Gray Matter:
– Horns: Posterior, Anterior, Lateral (only in areas associated
w/ANS)
•
•
•
•
Commissures (Gray & White)
Central Canal
Ventral Root
Dorsal Root
– Dorsal Root Ganglion
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Cross Section of the Spinal Cord
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I. SC: Cross section of the SC
• White matter
Fig 12.3 pg 415
– Columns are divided into
tracts
– “Blue” ascending tracts that
carry info from the body to the
brain.
– “Pink” descending tracts that
carry info from the brain to the
body.
Axons within a given tract
carry basically the same type
of info (even if there is
overlap).
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Spinal Nerves Origin
Dorsal Root Ganglion
Dorsal Roots
Multiple Dorsal Rootlets
Sensory (Afferent)
Neurons
Can either go to an
interneuron in the
Posterior gray horn OR
pass into WM & ascend
or descend in the SC
Spinal Nerves
MIXED
Ventral Roots
Multiple Ventral Rootlets
Autonomic
Neuron
Somatic Motor
(Efferent) Neurons
Cell bodies are in the lateral
(autonomic) or anterior
(multipolar somatic motor)
gray horns
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II. Reflexes
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II. Reflexes
• Reflex: an automatic
• Basic Structural Unit
– NS Neuron
• Basic Functional Unit
– NS Reflex Arc
– Smallest, simplest
portion capable of
receiving a stimulus &
prod’ing a response
– Can be used as a tool to
learn about NS fxn
response to a stimulus
prod’d by a reflex arc.
It occurs w/o conscious
thought
AP1 Chapter 12
 Somatic Reflexes (pain)
 Autonomic Reflexes
(blood pH)
 Monosynaptic Reflex
(SNMN)
 Polysynaptic Reflex (SN
IN MN)
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II. Reflexes
5 basic components of a reflex arc
1. Sensory receptor
2. Sensory neuron
3. Interneuron
4. Motor Neuron
5. Effector Organ
Fig 12.5
Page 416
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II. Reflexes
• 3 major SC reflexes include:
A. Stretch Reflex
B. Golgi tendon reflex
C. Withdrawal Reflex
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A. Stretch Reflex: Simplest Reflex
• Reflex in which muscles contract in response to
stretching force applied to them. (Knee
jerk/Posture)
• Sensory Receptor Muscle Spindle (MS*)
– When the mallet hits the patellar ligament it stretches
the Quadricepts, the sensory receptor (MS*) conduct
AP’s to the SC where they directly synapse to  a
Motor Neurons
– a motor neuron stimulation  AP to muscle
Contraction initiated to resist stretch (muscle
contracting is the one being stretched)
– Monosynaptic Reflex:
**There is no interneuron involvement**
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A. Stretch Reflex: Simplest Reflex
Perception of muscle stretch
Figure 12.6 page 417
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B. Golgi Tendon Reflex
• “Golgi Tendon Organs” GTO
– “sensory receptor”
– Encapsulated nerve endings that have their “axon
terminals” embedded in the collagen fibers in tendons
attached to muscles.
• When a great amount of tension is applied to the
tendon the sensory neurons of the GTO are
stimulated AP carried to SC and an Inhibitory
Interneuron wh/ are stimulated to release inhibitory
NT’s  These NT’s inhibit the a motor neurons of
the associated muscle causing relaxation.
• **Purpose??** To protect both muscles & tendons
from XSV tension causing damage
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B. Golgi Tendon Reflex
Figure 12.7 page 418
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C. Withdrawal/Flexor Reflex
Removal of a body part from a painful stimulus
1. Reciprocal Innervation
• While an agonist is
contracting automatic
relaxation of the
antagonist occurs
2. Crossed Extensor
• While one leg is pulling
away from the painful
stimulus, the other “noninjured” limb is extending
to transfer the weight.
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C. Withdrawal/Flexure Reflex
• Sensory Receptors Pain receptors (PR)
– PR carry AP’s thru the dorsal root to the SC
where they synapse with excitatory
interneurons  which synapse with a motor
neurons  stimulate (usually flexure)
muscles to remove the limb from the painful
stimulus
– **Collateral branches of sensory neurons
synapse w/ ascending fibers to the brain
conscious awareness of pain!!**
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C. Withdrawal/Flexor Reflex
Figure 12.8 page 419
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C. Withdrawal/Flexure Reflex:
1. Reciprocal Innervation
• Reinforces the Withdrawal Reflex’s efficiency.
• As the pain receptors carry the AP to the SC
they split.
– ½ goes to the agonist muscle to bend the leg
away from the painful stimulus
– ½ goes to collateral branches of the pain
receptors  Collateral branches synapse with
inhibitory interneurons (Reciprocal innervation)
Inhibitory interneurons synapse with the a motor
neurons of the antagonist muscles can cause
them to relax, THUS not opposing the action of
the agonist.
– **Last thing that is needed is fighting muscles**
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C. Withdrawal/Flexure Reflex:
1. Reciprocal Innervation
Figure 12.9 page 419
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C. Withdrawal/Flexure Reflex:
2. Crossed Extensor Reflex
• During flexion of one limb caused by the
withdrawal reflex, the opposite limb is
stimulated to extend
• As the pain receptors carry the AP to the SC
they split.
– ½ goes to the agonist muscle to bend the leg
away from the painful stimulus
– Collateral branches of the pain receptors synapse
with excitatory interneurons that cross thru the
white commissure of the SC to activate a motor
neurons in the opposite leg cause the muscles
in the opposite leg to contract & support the body
weight during the withdrawal reflex.
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C. Withdrawal/Flexure Reflex:
2. Crossed Extensor Reflex
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III. Interactions with SC Reflexes
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III. Interactions with SC Reflexes
Page 421
• Reflexes do not operate as isolated entities w/in the NS
b/c of divergent & convergent pathways.
• Pain: Withdrawal reflex, reciprocal reflex, crossed
extensor reflex, & signal to the brain for pain perception
• NT’s can be inhibitory or stimulatory & can D sensitivity
via EPSP or IPSP mechanisms
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IV. Structure of Peripheral Nerves
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IV. Structure of Peripheral Nerves
• Peripheral Nerves: consist of
axons, Schwann cells, & CT.
• Endoneurium:
Page 421
– Delicate CT layer that
surrounds each axon & its
Schwann cell Sheath
• Perineurium
– Heavier CT layer that
surrounds groups of axons to
form fascicles
• Epineurium
– Outer most DCT layer that
binds the nerve fascicles
together to form the nerve
– Epineurium of the PNS is
continuous with the Dura Mater
of the CNS
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V. Spinal Nerves (PNS!)
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Fig 12.13 pg 422
V. Spinal Nerves
• 31 pairs
– 25 exit thru the intervertebral
foramina
– 1st pair exits between the skull &
C1
– S1-5 exit thru the sacral foramina
 Cervical Nerves Letter:
C1-C8
 Thoracic Nerves
T1-T12
 Lumbar Nerves
L1-L5
 Sacral Nerves
S1-S5
 Coccygeal Nerves
indicative of
where nerve
emerges
#: indicative of
location w/in
each region
(smallest # is
most superior)
C0
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V. Spinal Nerves: Dermatome
• Nerves arising from each
region of the SC &
vertebral column supply
specific regions of the
body.
– Each spinal nerve has a
specific cutaneous sensory
distribution
• Dermatomal Map
– Sensory cutaneous
distribution of spinal nerves
• Dermatome
Figure 12.14 page 423
– Area of the skin supplied
w/sensory innervation by a
pair of spinal nerves
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V. Spinal Nerves: Nerves & fxns of SC
• Head & diaphragm mvmt
– C1-C4
• Neck & shoulder mvmt
– C4-C6
Figure 12.14 page 423
• Upper limb mvmt
– C6-T1
• Rib mvmt in breathing,
vertebral column mvmt, & tone
in postural back muscles
– T1-T12
• Hip mvmt
– T11-L2
• Lower limb mvmt
– L2-S3
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V. Spinal Nerves: Ramus “branch”
Fig 12.15
Pg 424
• These are branches that come off of the spinal
nerves
– Up to 3 branches may come off of the spinal nerve
1. Dorsal Ramus (Rami)
2. Ventral Ramus (Rami)
3. Communicating Ramus (Rami)

Only found from the thoracic to upper lumbar
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Classification of Rami
Communicating
Rami
– Carry axons
associated with
the Sympathetic
Nervous System
of the
Autonomic
Nervous System
Back
Front
Dorsal Rami
Ventral Rami
– Innervate:
1. Most deep
back muscles
of the dorsal
trunk
responsible
mvmt of the
vertebral
column
2. The CT and
skin near the
midline of the
back
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– Distributed in
2 ways:
 Thoracic
Region
 Plexuses
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Ventral Rami Distributed in 2 ways:
1. Thoracic Region
2. Plexuses
 Form intercostal
nerves wh/extend
along the inferior
margin of the rib &
innervate the
intercostal muscles &
skin over the thorax
AP1 Chapter 12
 Nerves that are
organized as braids
produced by
intermingling of the
nerves
 Thus the nerves from
a plexus usually have
axons from >1 spinal
nerve & are from >1
level of the SC
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Ventral Rami: Plexuses
1.
Cervical Plexus
–
2.
Brachial Plexus
–
3.
Fig 12.13
Pg 422
L1-L4
Sacral Plexus
–
5.
C5-T1
Lumbar Plexus
–
4.
C1-C4
L4-S4
Coccygeal Plexus
–
S5-Co
Fig 12.6
Pg 426
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Cervical Plexus (Origin C1-C4)
• Branches innervate
superficial neck muscles
(mvmt) & skin of neck &
posterior portion of the
head (sensory)
• Ansa cervicalis (C1&C3
making loop)
• Phrenic Branches
– (C3C5) derived from both
cervical & brachial plexus
– Descend along neck to enter
thorax sides of
mediastinum Diaphragm 
Contraction  breathing
control
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Brachial Plexus (Origin C5-T1)
• 5 rami (red) join to form
3 trunks (blue) Separate
into 3 divisions (purple &
green) Join to create 3
cords (orange) split to
becomes the 5 major
branches of the upper
limb.
1.
2.
3.
4.
5.
AP1 Chapter 12
Axillary nerve
Radial nerve
Musculocutaneous nerve
Median nerve
Ulnar nerve
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Brachial Plexus: Axillary nerve
• Motor Fxn:
– Lateral rotation of the
arm
– Abducts the arm
(moves away from
trunk)
• Sensory (Cutaneous)
Fxn:
– Inferior Lateral
Shoulder
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Brachial Plexus: Radial nerve
• Motor Fxn:
– Extends & flexes elbow
– Extends, adducts, & abducts
wrist
– Supinates forearm & hand
– Extends fingers
– Abducts & extends thumb
• Sensory Fxn
– Posterior forearm & arm
– Lateral 2/3 of the dorsum of
the hand
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Brachial Plexus:
Musculocutaneous nerve
• Motor Fxn:
– Flexes shoulder &
elbow
– Supinates forearm &
hand
• Sensory Fxn
– Lateral surface of the
forearm
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Brachial Plexus: Ulnar Nerve
• Motor Fxn:
– Flexes wrist, fingers,
metacarpophalangeal
joints
– Extends the
interphalangeal joints
– Controls pinkie mvmt
– Adducts wrist & thumb
• Sensory Fxn
– Medial 1/3 of the hand,
pinkie, & medial ring
finger
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Brachial Plexus: Median Nerve
• Motor Fxn:
– Pronates forearm &
hand
– Abducts wrist
– Controls thumb mvmt
– Flexes wrist & fingers
• Sensory Fxn:
– Lateral 2/3 of the palm &
hand, thumb & index &
middle fingers
– Lateral ½ of the ring
finger
– Dorsal tips of all
aforementioned fingers
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Brachial Plexus: Other
• Motor:
– scapula & arm control
• Sensory
– Innervation of the medial arm & forearm
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Lumbar & Sacral Plexus
(a.k.a. lumbosacral Plexus)
• 4 major nerves that enter the
lower limb
1. Obturator Nerve
•
Innervates medial thigh
2. Femoral Nerve
•
Innervates anterior thigh
3. Tibial Nerve
•
Innervates posterior thigh, leg, & foot
4. Common Fibular Nerve
•
Innervates posterior thigh, anterior &
lateral leg, and foot
5. Other
•
Supply lower back, hip, & lower
abdomen
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AP1 Chapter 12
Lumbosacral Plexus:
Obturator Nerve
• Motor:
– Rotates the thigh medially
– Adducts thigh
– Flexes knee
• Sensory:
– Superior medial portion of the
thigh
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Lumbosacral Plexus:
Femoral Nerve
• Motor:
– Flexes hip & knee
– Extends the knee
• Sensory:
– Anterior & lateral thigh
– Medial leg & foot
(sephnous branch)
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Lumbosacral Plexus:
Tibial Nerve
• Motor:
–
–
–
–
Extends hip
Flexes knee & toes
Plantar flexes foot
Adducts thigh
• Sensory:
– Tibial nerve None
• Branches:
a) Medial & lateral Plantar
nerves
–
–
Motor: flex & adduct toes
Sensory: sole of the foot
b) Sural Nerve (not shown)
–
–
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Motor: None
Sensory: Lateral & posterior
1/3 of leg & lateral foot
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Lumbosacral Plexus:
Common fibular nerve
• Motor:
– Extends the hip
– Flexes knee
• Sensory:
– Lateral surface of the knee
• Branches
– Deep fibular nerve
• Motor: dorsiflexion of the foot &
extension of the toes
• Sensory: Big & second toe
– Superficial fibular nerve
• Motor: Plantar flexs & everts the
foot
• Sensory: Dorsal Anterior 1/3 of
the leg & dorsum
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Lumbosacral Plexus: Other
• Motor:
– Gluteal nerves: act on femur & affect hip
– Pudendal nerves: act on abdomen
• Sensory:
– Innervate the skin of the suprapubic area,
external genitalia, superior medial thigh, &
posterior thigh.
• Pudendal nerves: vital role in sexual stimulation &
response
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Coccygeal Plexus
• Motor:
– Innervation to muscles of the pelvic floor
• Sensory:
– Skin of the coccyx
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