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The Spinal Cord and Spinal
Bio 137 Anatomy & Physiology I
Spinal Cord Protection
• Bones
– Spinal cavity (Vertebral Column)
• Meninges – below bony coverings
– Dura mater
– Arachnoid mater
– Pia mater
• Cerebrospinal Fluid
Meninges of the Spinal Cord
Spinal Cord Meninges: Dura Mater
• Dura mater continues into vertebral canal as a
sheath that surrounds the spinal cord
• The dura mater of the spinal cord is not
directly attached to the bone of the vertebra
– An epidural space is found between the dural
sheath and bony walls of vertebrae
• This space is filled with loose connective tissue,
vessels and fat
• Provides a cushion for the cord
Meninges of Brain & Cord
CSF Circulation
• CSF circulates in
lateral ventricles to
3rd and 4th ventricles
of brain and to
central canal of
spinal cord
• CSF is also found in
subarachnoid spaces
External Cord Anatomy
The spinal cord begins as a continuation of the
medulla oblongata (the most inferior portion of the
brain stem)
– Extends from the
foramen magnum of
the occipital bone
– Terminates as the
conus medullaris
between L1 - L2.
External Cord Anatomy
The spinal cord has two enlargements, one in
the cervical area from C4–T1, and another in the
lumbar area between
 The cervical enlargement
correlates with the sensory
input and motor output to the
upper extremities.
 The lumbar enlargement handles
motor output and sensory input
to and from the legs.
External Cord Anatomy
Arising from the conus medullaris is the filum
terminale, an extension of the pia mater that
extends inferiorly and
blends with the arachnoid
and dura to anchor the spinal
cord to the coccyx.
The cauda equina or “horses
tail” are the roots of the
lower spinal nerves that angle
down alongside the filum terminale.
Spinal nerves
below conus
descend and
form cauda
PNS: Spinal Nerves
• 31 segments of the spinal cord
– Each gives rise to a pair of spinal nerves, which branch to various
body parts
• All are Mixed nerves
8 cervical (numbered C1 to C8)
12 thoracic (T1 to T12)
5 lumbar (L1 to L5)
5 sacral (S1 to S5)
1 coccygeal (Co)
• Please keep in mind that these numbers refer to the spinal
Lumbar Puncture
 A needle inserted into the subarachnoid space for the
purpose of withdrawing CSF (for diagnosis or to reduce
pressure) or to introduce a drug or contrast agent is called
a lumbar puncture.
 CSF is often collected to diagnose meningitis or some other
disease of the CNS.
 Agents injected into the subarachnoid space include drugs
such as antibiotics, chemotherapeutic agents, or analgesics,
or contrast media for radiographic procedures.
• The pressure of CSF in the SAS can also be measured during a
lumbar puncture.
Lumbar Puncture
The site used for most lumbar punctures is between
the 3rd and 4th (or 4th and 5th) lumbar vertebrae below the termination of the actual cord in the
region of the cauda equina. With the needle in the
SAS, CSF can be sampled.
 Anesthetics can also
be given in this way,
but using 1/10 the
dose required for
epidural anesthesia.
Spinal Cord Structure
Cross Section
Cross Section of Spinal Cord
• Anterior median fissure and Posterior median sulcus
divide cord into left and right halves
Spinal Cord Cross Section
White matter surrounds ‘butterfly’ of gray matter
• Posterior horns
• Lateral horns
• Anterior horns
• Gray commissure
– Surrounds central canal
• Composed of Interneuronal
cell bodies and some motor
neuron cell bodies
• Posterior column (funiculi)
• Lateral column
• Anterior column
• Composed of bundles of
myelinated axons that
comprise major nerve
pathways called nerve tracts
Spinal Cord Model (cross-section)
Spinal Cord Structures
• Dorsal Root Ganglion
– Cell bodies of sensory neurons (unipolar)
• Dorsal Root
– Axons of sensory neurons
• Ventral Root
– Axons of motor neurons
• Fusion of dorsal root & ventral root forms a spinal
Spinal Cord Model (cross-section) Photo from
Internal Cord Anatomy
 A tract is a bundle of neuronal
axons that are all located in a
specific area of the cord and all
traveling to the same place
(higher or lower in the brain or
• Sensory (Ascending) Tracts
– Sensory impulses from body to
• Motor (Descending) Tracts
– Motor impulses from brain to
Spinal Cord Injury
• If damage is to ascending tracts, lose sensory
function below injury
• If descending tracts are damaged, lose motor
function below injury
• A reflex is a fast, involuntary response to a stimulus.
• In a spinal reflex the integration takes place in the
spinal cord, not the brain.
 A reflex arc is a pathway that a nerve impulse follows to
produce a reflex.
 Components of a reflex arc:
 Sensory receptor and a sensory neuron
 Integrating center inside the cord
 A motor neuron
 An effector
 Usually some sort of
muscle or a gland which
makes something move
or secrete (“involuntarily”).
Reflex Arc
Withdrawal Reflex
• At least a 3-neuron pathway
•Protective, prevents or limits tissue damage
Knee-Jerk Reflex
2-neuron pathway that maintains an
upright position
Spinal Nerves: Nerve Plexus
• Spinal nerves exit out intervertebral foramina
and immediately branch
• Anterior rami (branches) of spinal nerves
(except T2-T11) combine to form a nerve plexus
• Fibers (anterior branches) of various spinal nerves are
sorted and recombined to feed an area of the body
• Particular body part receives ‘nerves’ consisting
of nerve fibers from different starting points of
origin in cord
Nerve Plexuses
The cervical plexus, formed by the anterior rami of C1C5, serves the head, neck, and diaphragm.
 The phrenic nerves arise from the cervical plexus to supply the
major muscle of
respiration (“C3,4,5 keep
the diaphragm alive”).
Brachial Plexus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•Formed by anterior
branches C5-C8 and T1
•Lies deep within shoulders
•There are five (5)
•Supply muscles of the
arms and hands and
some shoulder, chest
and back muscles
Ventral rami: C5, C6, C7, C8, T1
Trunks: upper, middle, lower
Anterior divisions
Posterior divisions
scapular n.
Suprascapular n.
Lateral pectoral n.
Medial pectoral n.
Lower subscapular n.
Thoracodorsal n.
Musculocutaneous n.
Axillary n.
Median n.
Ulnar n.
Axillary n.
Radial n.
Median n.
Musculocutaneous n.
Radial n.
Ulnar n.
Nerve Plexuses
Some of the major nerves that arise from the
brachial plexuses are the:
musculocutaneous nerve
axillary nerve
radial nerve
median nerve
ulnar nerve
long thoracic nerve
Nerve Plexuses
The ulnar nerve is the largest unprotected (by
muscle or bone) nerve in the human body. It
emerges from the medial and lateral cords of
the brachial plexus to supply the medial half of
the hand. Striking the medial epicondyle of the
humerus where the nerve is exposed is referred
to as bumping one’s “funny bone”.
 Damage to the nerve leads to abnormal sensations
in the 4–5th fingers and an inability to abduct
or adduct the little and ring fingers.
Nerve Plexuses
The lumbar plexus is formed by the anterior
rami of L1-L4 to supply the anterolateral
abdominal wall, external genitalia, and part of
the lower limbs.
 The femoral and obturator nerves
come from the lumbar plexus.
Nerve Plexuses
The sacral plexus is formed by the anterior rami
of L4-L5 and S1-S4. It supplies the buttocks,
perineum, and part of the
lower limbs.
 It gives rise to the largest
nerve in the body, the
sciatic nerve.
Nerve Plexuses
The coccygeal plexus is formed by the anterior
rami of S4-S5 and the coccygeal nerves. It is a
small plexus from which the anococcygeal nerve
exits to supply a small area of skin in the
coccygeal region.
Spinal Cord versus spinal nerve
• Which is worse?
• Why?