Human Anatomy & Physiology

Spinal Cord and Spinal Nerves

By
Tahira bibi
 Objectives

At the end of this lecture students will be able to:

Describe anatomy of spinal cord

Define nerve

Discuss plexus of nerves

 Overview of Spinal Cord
• Information highway between brain and body
• The spinal cord is about 18 inches (45 centimeters) in length
• Extends through vertebral canal from foramen
magnum to L1
• Each pair of spinal nerves receives sensory
information and issues motor signals to muscles
and glands
• Spinal cord is a component of the Central Nervous
System while the spinal nerves are part of the
Peripheral Nervous System
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 Functions of the Spinal Cord
• Conduction
– bundles of fibers passing information up and down
spinal cord
• Locomotion
– repetitive, coordinated actions of several muscle
groups
– central pattern generators are pools of neurons
providing control of flexors and extensors (walking)
• Reflexes
– involuntary, stereotyped responses to stimuli
(remove hand from hot stove)
– involves brain, spinal cord and peripheral
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nerves
 Anatomy of the Spinal Cord
• Cylinder of nerve tissue within the vertebral
canal (thick as a finger)
– vertebral column grows faster so in an adult the spinal
cord only extends to L1
• 31 pairs of spinal nerves arise from cervical,
thoracic, lumbar and sacral regions of the cord
– each cord segment gives rise to a pair of spinal
nerves
• Cervical and lumbar enlargements
• Medullary cone (conus medullaris) = tapered tip
of cord
• Cauda equinae is L2 to S5 nerve roots
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horse’s
resemble tail
Gross Anatomy of Lower Spinal Cord

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 Meninges of the Spinal Cord
• 3 Fibrous layers enclosing spinal cord
• Dura mater
– tough collagenous membrane surrounded by
epidural space filled with fat and blood vessels
• epidural anesthesia utilized during childbirth
• Arachnoid mater
– layer of simple squamous epithelium lining dura
mater and loose mesh of fibers filled with CSF
(creates subarachnoid space)
• Pia mater
– delicate membrane adherent to spinal cord
– filium terminale and denticulate ligaments anchor
the cord 13-7
Meninges of Vertebra and Spinal Cord

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 Spina Bifida
• Congenital defect in 1 baby out of 1000
• Failure of vertebral arch to close covering spinal
cord
• Folic acid (B vitamin) as part of a healthy diet for all
women of childbearing age reduces risk

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Cross-Sectional Anatomy of the Spinal Cord

• Central area of gray matter shaped like a butterfly and

surrounded by white matter in 3 columns
• Gray matter = neuron cell bodies with little myelin
• White matter = myelinated axons 13-10
 Gray Matter in the Spinal Cord
• Pair of dorsal or posterior horns
– dorsal root of spinal nerve is totally sensory fibers
• Pair of ventral or anterior horns
– ventral root of spinal nerve is totally motor fibers
• Connected by gray commissure punctured by a
central canal continuous above with 4th
ventricle

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 White Matter in the Spinal Cord
• White column = bundles of myelinated axons that carry
signals up and down to and from brainstem
• 3 pairs of columns or funiculi
– dorsal, lateral, and anterior columns
• Each column is filled with named tracts or fasciculi
(fibers with a similar origin, destination and function)

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 Spinal Tracts

• Ascending and descending tract head up or down while

decussation means that the fibers cross sides
• Contralateral means origin and destination are on
opposite sides while ipsilateral means on same side 13-13
 Dorsal Column
Ascending Pathway
• Deep touch,
visceral pain, vibration,
and proprioception
• Fasciculus gracilis and
cuneatus carry
signals from arm and
leg
• Decussation of 2nd
order neuron in medulla
• 3rd order neuron in
thalamus carries signal 13-14

to cerebral cortex
Spinothalamic
Pathway
• Pain, pressure,
temperature, light
touch, tickle and
itch
• Decussation of the
second order
neuron occurs in
spinal cord
• Third order neurons
arise in thalamus
and continue to
cerebral cortex
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 Spinoreticular Tract

• Pain signals from tissue injury

• Decussate in spinal cord and ascend
with spinothalamic fibers
• End in reticular formation (medulla and
pons)
• 3rd and 4th order neurons continue to
thalamus and cerebral cortex

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 Spinocerebellar Pathway

• Proprioceptive signals from limbs and

trunk travel up to the cerebellum
• Second order nerves ascend in
ipsilateral lateral column

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 Corticospinal
Tract
• Precise, coordinated
limb movements
• Two neuron pathway
– upper motor neuron in
cerebral cortex
– lower motor neuron in
spinal cord
• Decussation in
medulla
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 Descending Motor Tracts
• Tectospinal tract (tectum of midbrain)
– reflex turning of head in response to sights and
sounds
• Reticulospinal tract (reticular formation)
– controls limb movements important to maintain
posture and balance
• Vestibulospinal tract (brainstem nuclei)
– postural muscle activity in response to inner ear
signals

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 Poliomyelitis and ALS
• Diseases causing destruction of motor
neurons and skeletal muscle atrophy
• Poliomyelitis caused by poliovirus spread by
fecally contaminated water
– weakness progresses to paralysis and respiratory
arrest
• Amyotrophic lateral sclerosis
– sclerosis of spinal cord due to astrocyte failure to
reabsorb glutamate neurotransmitter
– paralysis and muscle atrophy

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 Anatomy of a Nerve

• A nerve is a bundle of nerve fibers (axons)

• Epineurium covers nerves, perineurium surrounds a
fascicle and endoneurium separates individual nerve
fibers
• Blood vessels penetrate only to the perineurium 13-21
Anatomy of Ganglia in the PNS

• Cluster of neuron cell bodies in nerve in PNS

• Dorsal root ganglion is sensory cell bodies
– fibers pass through without synapsing 13-22
Spinal Nerve Roots and Plexuses

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 The Spinal Nerves
• 31 pairs of spinal nerves (1st cervical above C1)
– mixed nerves exiting at intervertebral foramen
• Proximal branches
– dorsal root is sensory input to spinal cord
– ventral root is motor output of spinal cord
– cauda equina is roots from L2 to C0 of the cord
• Distal branches
– dorsal ramus supplies dorsal body muscle and skin
– ventral ramus to ventral skin and muscles and limbs
– meningeal branch to meninges, vertebrae and
ligaments

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Branches of a Spinal Nerve
• Spinal nerves: 8
cervical, 12 thoracic, 5
lumbar, 5 sacral and 1
coccygeal.

• Each has dorsal and

ventral ramus.

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 Rami of Spinal Nerves

• Notice the branching and merging of

nerves in this example of a plexus 13-26
 Shingles

• Skin eruptions along path of nerve

• Varicella-zoster virus (chicken pox)
remains for life in dorsal root ganglia
• Occurs after age 50 if immune system
is compromised
• No special treatment

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 Nerve Plexuses
• Ventral rami branch and anastomose
repeatedly to form 5 nerve plexuses
– cervical in the neck, C1 to C5
• supplies neck and phrenic nerve to the diaphragm
– brachial in the armpit, C5 to T1
• supplies upper limb and some of shoulder and neck
– lumbar in the low back, L1 to L4
• supplies abdominal wall, anterior thigh and genitalia
– sacral in the pelvis, L4, L5 and S1 to S4
• supplies remainder of lower trunk and lower limb
– coccygeal, S4, S5 and C0

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The Cervical Plexus

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Key facts about the cervical plexus

Lesser occipital nerve, Great auricular nerve, Transverse cervical

Sensory nerve, Supraclavicular nerve

Branches Mnemonic: Let's Go To Sleep

Muscular branches (to sternocleidomastoid, prevertebral and

Motor levator scapulae), Ansa cervicalis, Phrenic nerve

Branches Mnemonic: MAP

Lesser occipital nerve - skin of the neck and the scalp

Innervations posterosuperior to the clavicle
Greater auricular nerve - skin over the parotid gland, the
posterior aspect of the auricle, and an area of skin extending
from the angle of the mandible of the mastoid process
Transverse cervical nerve - skin covering the anterior
triangle of the neck
Supraclavicular nerve - skin over the neck and over the
shoulder
Ansa cervicalis - infrahyoid muscles
Phrenic nerve - diaphragm, mediastinal pleura, pericardium
of the heart
The Brachial Plexus

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Key facts about the brachial plexus

Roots C5, C6, C7, C8, T1

Trunks Superior trunk

Middle trunk
Inferior trunk
Divisions Three anterior divisions
- Anterior division of superior trunk
- Anterior division of middle trunk
- Anterior division of inferior trunk
Three posterior divisions
- Posterior division of superior trunk
- Posterior division of middle trunk
- Posterior division of inferior trunk
Cords Lateral
Medial
Posterior
Terminal branches Musculocutaneous nerve
Axillary nerve
Radial nerve
Median nerve
Ulnar nerve
Innervation Complete sensory and
motor innervation of the
arm
Organization 'Rugby Teams Don't Cover Bruises'
(standing for: Roots, Trunks, Divisions,
Cords, Branches

Lateral cord branches 'Rugby players are Long Legged Movers'

(standing for: Lateral pectoral nerve,
Lateral root of median nerve,
Musculocutaneous nerve)

Medial cord branches 'Rugby

players Make Many Moves Using Muscles'
(standing for: Medial cutaneous brachial
nerve, Medial cutaneous antebrachial
nerve, Medial pectoral nerve, Ulnar nerve,
Median root of median nerve)
Posterior cord branches 'Rugby players are ULTRA competitive'
(standing for: Upper subscapular nerve,
Lower subscapular nerve, Thoracodorsal
nerve, Radial nerve, Axillary nerve)
The Lumbar Plexus

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Function Innervation of the lower anterior abdominal wall and
certain muscles of the thigh

Source L1 - L4

Branches Iliohypogastric and ilioinguinal (L1) - transversus

abdominis, internal oblique muscles
Genitofemoral (L1, L2) - cremaster muscle
Lateral femoral cutaneous (L2, L3) - skin of the lateral
part of the thigh
Obturator (L2, L3, L4) - obturator externus, adductor
longus, adductor brevis, gracilis, pectineus, adductor
magnus
Femoral (L2, L3, L4) - Iliopsoas, pectineus, sartorius,
quadriceps femoris

Mnemonic Interested In Getting Laid On Friday?

The Sacral and Coccygeal Plexuses

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Key facts about the sacral plexus
Origin L4, L5, S1, S2, S3, S4
Branches Anterior branches: Nerve to quadratus femoris, nerve to
obturator internus, pudendal nerve, nerves to levator ani and external
anal sphincter
Posterior branches: Nerve to piriformis , superior gluteal nerve,
inferior gluteal nerve, posterior femoral cutaneous nerve, perforating
cutaneous nerve, pelvic splanchnic nerves
Terminal branch: Sciatic nerve (divides into tibial and common
peroneal nerves)

Supply Motor: tensor fasciae latae, gluteus maximus, gluteus medius,

gluteus minimus, hamstings, adductor magnus, sphincters, levator ani,
muscles of the leg and foot
Sensory: skin over the medial surface of the buttock to the middle
of the calf muscles, skin of the external genitalia, leg and foot
Organs: urinary bladder, large intestine, external genitalia

Mnemonic for the Superior gluteal nerve, inferior gluteal nerve, posterior cutaneous
major branches nerve of thigh, pudendal nerve, sciatic nerve (SIPPS)
(SIIPS)
Cutaneous Innervation and Dermatomes

• Each spinal nerve

receive sensory input
from a specific area of
skin called
dermatome
• Overlap at edges by
50%
– a total loss of sensation
requires anesthesia of 3
successive spinal nerves

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