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Anatomy
and
Physiology
Biology 2401
Chapter-13
The Spinal Cord and Spinal Nerves
Spinal Cord – Location, Protection and Functions
Location:
• Placed in the vertebral canal
• Extends from just above the foramen magnum to between L1 and L2 vertebrae
Functions:
• Center of reflexes – quick response to stimuli
• Integration (summation of inhibitory and excitatory) nerve impulses
• Has tracks for upward and downward travel (to the brain) of sensory and motor
information
Protection:
• Bony protection – Vertebrae that form a column around the spinal cord
• Connective tissue membranes – meninges
– Outer Dura mater
– Middle Arachnoid mater
– Inner Pia mater
Spinal Cord – Meninges and Spaces
Going from outside to inside:
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•
•
•
•
•
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Vertebrae
– cervical, thoracic and first lumbar
Epidural space
– filled with fat
Dura mater
– membrane that forms a jacket
Subdural space
– minimal space, filled with interstitial fluid
Arachnoid mater
– membrane that almost sticks to dura
mater
Subarachnoid space
– filled with Cerebrospinal fluid (CSF)
Pia mater
– membrane that adheres to the spinal
cord
– extends inferiorly (filum terminale) to
anchor spinal cord to coccyx
– extends laterally (denticulate ligaments)
hold spinal cord inside vertebral column
Spinal cord
Spinal Cord – External Aantomy
Cervical and lumbar enlargements
– where the nerves to the upper
and lower extremities come
out
Conus medullaris
– cone-shaped end of spinal
cord
Filum terminale
– thread-like extension of pia
mater
– stabilizes spinal cord in canal
Cauda equina (horse’s tail)
– dorsal & ventral roots of
lowest spinal nerves
Spinal segment
– area of cord from which each
pair of spinal nerves arises
Spinal Cord – Internal Structure
Anterior median fissure:
•
deeper groove on the anterior surface of the spinal cord
Posterior median sulcus:
•
shallow groove on the posterior side of the spinal cord
Roots: bundles of axons that enter or exit the spinal cord
– Posterior root - sensory axons that enter the spinal cord
• Posterior root ganglion – contains the cell body of the sensory neuron
– Anterior root - motor axons that exit the spinal cord
Spinal Cord – Internal Structure
Gray matter:
Shaped like the letter H or a butterfly
• Posterior gray horn – where
sensory fibers enter the spinal
cord
• Anterior gray horn – where
the motor neurons are located
and motor fibers exit the
spinal cord
• Lateral gray horn – where the
association or interneurons
are located
• Gray commissure – connects
the right and the left sides of
the spinal cord
• Central canal – located in the
gray commissure and contains
CSF
Spinal Cord – Internal Structure
White matter:
– surround the gray matter
– divided into columns – anterior,
posterior and lateral columns
– each column contains tracts
• Tracts are bundles of nerve
fibers that have a common
origin or destination and carry
similar information
– Ascending tracts…take
impulses from spinal cord
to the brain
– Descending tracts…bring
impulses from brain to the
spinal cord
Spinal Nerves
31 pairs of spinal nerves:
C1 – C8 cervical nerves
T1 – T12 thoracic nerves
L1 – L5 lumbar nerves
S1 – S5 sacral nerves
Co1 coccygeal nerve
Spinal Nerves
Spinal nerves:
Bundles of sensory & motor fibers plus connective tissue
Formed by the combination of posterior and anterior roots
Arrangement of fibers is similar to muscle fibers in a muscle
• Endoneurium – connective tissue that wraps around each nerve fiber
• Perineurium – connective tissue that surrounds group of nerve fibers forming a fascicle
• Epineurium – connective tissue that covers the entire nerve
Spinal Nerves - Branches
Each spinal nerve forms 4 branches:
• Posterior (dorsal) ramus - supplies to the skin & muscles of the back
• Anterior (ventral) ramus – supplies to the extremities and anterior wall of the
trunk
• Meningeal ramus – supplies to the meninges and vertebrae
• Rami communicantes (autonomic nerves) – supplies to the internal organs
Spinal Nerves - Plexus
Anterior (ventral) rami branches of
spinal nerves form a network by
joining with other anterior rami
branches….plexus
Found in neck, arm, lower back &
sacral regions
Spinal Nerves – Cervical Plexus
• Formed by the anterior/ventral rami of spinal nerves (C1 to C5)
• Supplies nerves to skin and muscles of head, neck & shoulders
• Also forms phrenic nerve that goes to the diaphragm
– damage to spinal cord above C3 causes respiratory arrest
Spinal Nerves – Brachial Plexus
• Formed by the anterior/ventral rami of spinal nerves (C5 to T1)
• Supplies nerves to skin and muscles of neck, shoulders and upper
extremities
Spinal Nerves – No Thoracic Plexus
No plexus is formed by the anterior/ventral rami branches of
thoracic spinal nerves
Become intercostal nerves to innervate intercostal muscles
Spinal Nerves – Lumbar Plexus
• Formed by the anterior/ventral rami of spinal nerves (L1 to L4)
• Supplies to the skin and muscle of abdominal wall, external genitals &
anterior/medial thigh
– Injury to femoral and obturator nerves causes inability to extend leg & loss
of sensation in thigh
Spinal Nerves – Sacral Plexus
• Formed by the anterior/ventral rami of spinal nerves (L4, L5, S1-S4)
• Supplies to the skin and muscle of buttocks and lower extremities
– Injury to sciatic nerves causes pain from buttocks to below the knee
Reflex and Reflex Arcs
Reflex:
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a fast, predictable, automatic
response to changes in the
environment
helps to maintain homeostasis
Reflex arc:
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Specific pathway followed by a
nerve impulse in a reflex
5 components of reflex arc:
– receptor that detects stimulus
– sensory neuron that sends
impulse to CNS
– integrating center, the CNS
– motor neuron that takes
impulse away from CNS
– effector that responds
Reflex and Reflex Arcs - Types
Reflex and Reflex Arcs - Types
Spinal Reflex:
• When integration takes place in the
spinal cord
Cranial Reflex:
• When integration takes place in the
brain
Somatic Reflex:
• Which result in contraction of
skeletal muscles
Autonomic/Visceral Reflex:
• Where the involuntary effectors
respond…smooth or cardiac
muscles or glands
Monosynaptic Reflex Arc:
• A pathway that has only one synapse in CNS
Polysynaptic Reflex Arc:
• A route that has 2 or more synapse in CNS
Ipsilateral Reflex Arc:
• Where the sensory impulse enters the spinal cord on the same side the motor
impulse exits
Contralateral Reflex Arc:
• Where the sensory impulse enters the spinal cord on the opposite side the motor
impulse exits
Clinical Applications - Disorders
Neuritis
– inflammation of nerves
– caused by injury, chemicals or infection
Shingles
– infection of peripheral nerve by chicken pox virus
– causes pain and line of skin rash
Poliomyelitis
– viral infection causing motor neuron
– Paralysis of muscles  possible death from cardiac failure or respiratory arrest
Spinal cord injury
– Due to accidents, falls, sports, or violence
• Monoplegia – paralysis of one limb
• Paraplegia – paralysis of both lower limbs
• Quadriplegia – paralysis of all four limbs
Meningitis
– Inflammation of meninges