AP1 Chapter 12 1

Chapter 12

Spinal Cord & Spinal Nerves

AP1 Chapter 12 2

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 3

Chapter 12: SC & spinal nervesCNS

• Structure:– Brain – Spinal Cord

• Fxn:– Receives sensory info– Integrates & evaluates the

input it receives– Stores some information – Initiates rxns

PNS• Structure

– Nerves & ganglia outside of the CNS

– Ganglia: accumulation of cell bodies in the PNS

• Fxn:– 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

AP1 Chapter 12 4

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

I. Spinal Cord: General Structure• 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 5

Fig 12.1 pg 412

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I. SC: Meninges of the SC• Meninges: CT coverings around brain

& SC• Dura Mater:“tough mother”

– 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)

Fig 12.2Pg 413

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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

• 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

Figure 12.3 pg 414

AP1 Chapter 12 9

Cross Section of the Spinal Cord

AP1 Chapter 12 10

I. SC: Cross section of the SC• White matter

– 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). Fig 12.3 pg 415

Sp

inal

Ner

ves

Ori

gin

11

Multiple Dorsal Rootlets

Dorsal Roots

Ventral RootsDorsal Root Ganglion

Sensory (Afferent) Neurons

Somatic Motor (Efferent) Neurons

Spinal NervesMIXED

Autonomic Neuron

Multiple Ventral Rootlets

Can either go to an interneuron in the

Posterior gray horn OR pass into WM & ascend

or descend in the SC

Cell bodies are in the lateral (autonomic) or anterior

(multipolar somatic motor) gray horns

AP1 Chapter 12 12

II. Reflexes

AP1 Chapter 12 13

II. Reflexes• 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

• Reflex: an automatic response to a stimulus prod’d by a reflex arc. It occurs w/o conscious thought

Somatic Reflexes (pain) Autonomic Reflexes

(blood pH)

Monosynaptic Reflex (SNMN)

Polysynaptic Reflex (SN IN MN)

AP1 Chapter 12 14

II. R

efle

xes 5 basic components of a reflex arc

1. Sensory receptor

2. Sensory neuron

3. Interneuron

4. Motor Neuron

5. Effector Organ

Fig 12.5Page 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**

A. Stretch Reflex: Simplest Reflex

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Perception of muscle stretchFigure 12.6 page 417

AP1 Chapter 12 18

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 ReflexFigure 12.7 page 418

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C. Withdrawal/Flexor ReflexRemoval 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 “non-injured” 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**

AP1 Chapter 12 24

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

• 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

Page 421

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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:

– 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 AP1 Chapter 12

Page 421

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V. Spinal Nerves (PNS!)

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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

C1-C8

Thoracic NervesT1-T12

Lumbar NervesL1-L5

Sacral NervesS1-S5

Coccygeal NervesC0

Fig 12.13 pg 422

Letter: indicative of where nerve emerges#: indicative of location w/in each region (smallest # is most superior)

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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– Area of the skin supplied

w/sensory innervation by a pair of spinal nervesFigure 12.14 page 423

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V. Spinal Nerves: Nerves & fxns of SC• Head & diaphragm mvmt

– C1-C4

• Neck & shoulder mvmt– C4-C6

• 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

Figure 12.14 page 423

V. S

pina

l Ner

ves:

Ram

us

“bra

nch

”

• 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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Fig 12.15Pg 424

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Ventral Rami– Distributed in

2 ways: Thoracic

Region Plexuses

Dorsal 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

Classification of Rami

Communicating Rami

– Carry axons associated with the Sympathetic Nervous System of the Autonomic Nervous System

Back Front

AP1 Chapter 12 37

Ventral Rami Distributed in 2 ways:

1. Thoracic Region Form intercostal

nerves wh/extend along the inferior margin of the rib & innervate the intercostal muscles & skin over the thorax

2. Plexuses 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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Ven

tral

Ram

i: P

lexu

ses

1. Cervical Plexus– C1-C4

2. Brachial Plexus– C5-T1

3. Lumbar Plexus– L1-L4

4. Sacral Plexus– L4-S4

5. Coccygeal Plexus– S5-Co

Fig 12.13Pg 422

Fig 12.6Pg 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– (C3C5) derived from both

cervical & brachial plexus– Descend along neck to enter

thorax sides of mediastinum Diaphragm Contraction breathing control

AP1 Chapter 12 40

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. Axillary nerve

2. Radial nerve

3. Musculocutaneous nerve

4. Median nerve

5. 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

AP1 Chapter 12 45

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

AP1 Chapter 12 46

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 limb1. 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

abdomenAP1 Chapter 12

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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)– Motor: None– Sensory: Lateral & posterior 1/3

of leg & lateral foot

AP1 Chapter 12 51

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

AP1 Chapter 12 53

Coccygeal Plexus

• Motor:– Innervation to muscles of the pelvic floor

• Sensory:– Skin of the coccyx