Download Lecture # 20: The Spinal Cord and Spinal Nerves

Lecture # 20: The Spinal Cord
and Spinal Nerves
Objectives:
1- Describe the major structural features and
functions of the spinal cord.
2- Name the three coverings of the central
nervous system and give the characteristics,
location, and function of each.
3- Identify ventral root, dorsal root, and
dorsal root ganglion and give the
composition of each.
4- Discuss white matter, gray matter, tracts,
and nuclei.
5- Differentiate among motor nerves, sensory
nerves, mixed nerves, cranial nerves, and
spinal nerves.
6- Describe the anatomy of a reflex arc.
(Chapter 13)
Functions of the Spinal Cord
Conduction:
Bundles of fibers passing
information up and down
spinal cord (ascending
and descending tracts) ,
connecting different levels
of the trunk with each
other and with the brain.
Locomotion:
Walking
involves
repetitive,
coordinated actions of several
muscle groups. Central pattern
generators are pools of neurons in
the cord providing control of
flexors and extensors that cause
alternating movements of the
lower limbs.
Reflexes:
They are involuntary, stereotyped responses to stimuli,
such as the withdrawal of a
foot from pain involves
brain, spinal cord and
peripheral nerves.
Surface Anatomy of the Spinal Cord
Foramen
magnum
Cervical
enlargement
Medullary cone
Cauda equina
Lumbar
enlargement
L1 – L2
Terminal filum
It attaches the
spinal cord to
the coccyx and
prevents vertical
movements
Meninges of the Spinal Cord
Epidural space (Space for the epidural anesthesia)
Dura mater
Subdural space
Arachnoid mater
Subarachnoid space (It contains cerebrospinal fluid))
Pia mater
Pia mater
Arachnoid mater
Dura mater
Cross Sectional Anatomy of the Spinal Cord
Gray matter It contains the bodies and dendrites of neurons. Therefore, it
contains little myelin.
It is composed of bundles of axons called tracts. Therefore, it
White matter
contains abundance of myelin.
They carry sensory Posterior (dorsal)
information
root of spinal nerve
Posterior root
ganglion
They contain the
bodies of sensory
neurons
Posterior They contain bodies of
gray horns sensory somatic neurons and
sensory visceral neurons
Lateral gray horns
They contain bodies of
visceral motor neurons
Spinal nerve
They carry both sensory
and motor information
Anterior (ventral)
root of spinal nerve
They carry motor information
Anterior They contain bodies of
gray horns somatic motor neurons
Cerebral
cortex
Somato-sensory neurons
Visceral-sensory neurons
Visceral-motor neurons
Somato-motor neurons
Cerebral
cortex
Thalamus Hypothalamus
SS
VS
VM
SM
Somatic sensory fiber
Somatic motor fiber
Visceral sensory fiber
Visceral motor fiber
White Matter and Spinal Tracts
Posterior (dorsal)
columns or funiculi
Lateral columns
or funiculi
Ascending tracts
(They carry sensory information
up the cord)
Anterior (ventral)
columns or funiculi
Ascending
tracts or
fasciculi
Descending
tracts or
fasciculi
Descending tracts
(They carry motor information up
the cord)
Some Ascending Pathways
Gracile Fasciculus and Cuneate Fasciculus
They carry signals for vibration,
visceral pain, deep and discriminative
touch, and conscious proprioception
(sensations of limbs and trunk
positions and movements).
Spinothalamic Tract
It carries signals for somatic pain,
pressure, temperature, light touch,
tickle, and itch.
Spinocerebellar Tracts
They carry unconscious
proprioceptive signals from limbs
and trunk up to the cerebellum.
Postcentral
gyrus of
cerebrum
It contains the primary
somatosensory cortex
for touch, pain and
temperature.
Thalamus
Cerebellum
Spinocereberall Tracts
Cerebellum
Medulla
oblongata
Spinal cord
Cerebrum
Pons
Medulla
oblongata
Cerebellum
It coordinates rapid,
automatic adjustments
of postural muscles,
and programs and finetunes motor commands
to produce smooth
movements.
Axon of first order
neuron (in posterior
root)
They carry unconscious
proprioceptive signals from
limbs and trunk up to the
cerebellum (contralateral
and ipsilateral).
Axons of second order
neurons
Body of second order
neuron (in posterior
gray horn)
Body of first order
neuron (in posterior
root ganglion)
Spinal cord
Descending Tracts
Motor homunculus of the
precentral gyrus
(Body parts are in
proportion to the
amount of motor
cortex dedicated to
their control)
Corticospinal Tracts
They carry motor signals from the
precentral gyrus of the cerebral
cortex for precise, finely
coordinated limb movements
(contralateral)
Pyramidal cells
(Upper motor
neurons)
Precentral gyrus
of cerebrum
(motor cortex that controls precise
coordinated limb movements)
Postcentral gyrus
of cerebrum
(somatosensory or
somesthetic cortex)
Medulla
oblongata
Anterior corticospinal tracts
(decussation in spinal cord)
Lower motor neurons
To skeletal muscles
Midbrain
Decussation in medulla
oblongata
Lateral corticospinal
tracts
(roughly 85% of the axons
undergo decussation at the
medulla oblongata))
Spinal cord To skeletal muscles
Some Pathways of the Central Nervous System
Tract
Spinothalamic
Tract
(Sensory,
ascending)
Origin
Somatic receptors
for pain, pressure,
temperature, light
touch.
Spinocereberall Proprioceptors in
muscles, tendons,
Tracts
(Sensory,
ascending)
Corticospinal
Tracts
(Motor
descending)
and joints
Precentral gyrus of
cerebrum (motor
cortex)
1rst order
neuron
2nd order
neuron
3rd order Destination
neuron
Posterior
root
ganglion
Posterior Thalamus
gray
horn
Somatosensory
(somesthetic)
cortex in
postcentral
gyrus of brain
Posterior
root
ganglion
Posterior
gray
horn
Cerebellum
(cerebellar
cortex)
Upper motor
neurons
Pyramidal cells of
precentral gyrus
of cerebrum
(motor cortex)
Not
present
Lower motor
neurons
Motor neurons of
anterior horn of
spinal cord
Skeletal
muscles
Decussation
In spinal
cord
Posterior
tracts: none
Anterior
tracts: in
spinal cord
Function
It carries sensory signals for
somatic pain, pressure,
temperature, light touch, tickle,
and itch (contralateral).
They carry unconscious
proprioceptive signals from
limbs and trunk up to the
cerebellum (contralateral and
ipsilateral)
They carry motor signals from
the precentral gyrus of the
cerebral cortex for precise,
finely coordinated limb
Anterior tracts:
movements (contralateral)
Lateraltracts:
in medulla
oblongata
in spinal cord
General Anatomy of Nerves and Ganglia
Sensory pathway
Motor pathway
Posterior root ganglion
Somatosensory
neurons
Sensory pathway
Spinal nerve
Posterior root
Epineurium
Loose connective tissue
that surrounds one nerve
Blood vessels
Anterior root
Motor pathway
General Anatomy of Nerves
Rootlets
Posterior root
Posterior root
ganglion
Anterior root
Up to 20 layers of overlapping
Perineurium squamous cells that surrounds
each fascicle
Unmyelinated nerve fiber
Myelinated nerve fiber
Spinal nerve
Myelin
Endoneurium
Loose connective tissue
that surrounds each fiber
Fascicle
Bundle of nerve fibers
Epineurium
Loose connective tissue
that surrounds each nerve
Spinal Nerves
Proximal Branches:
Posterior root (sensory)
Anterior root (motor)
Spinal nerve
They are mixed nerves, carrying
sensory and motor information.
They emerge through the intervertebral foramina.
Distal Branches:
Posterior ramus
It innervates muscles and joints in the
region of spine and skin of back.
Anterior or ventral ramus
Shortly after they exit the vertebral
column, they branch and combine to
form large PLEXUSES.
They innervate anterior and lateral
skin, and muscles of trunk and limbs.
Meningeal ramus
It innervates meninges, vertebrae and
spinal ligaments.
The Cervical Plexus
C1
Hypoglossal
nerve (XII)
C2
Lesser occipital nerve
C3
Great auricular nerve
Transverse cervical nerve
C4
Ansa cervicalis:
Anterior root
Posterior root
Roots
C5
Supraclavicular nerves
Phrenic nerve
Innervates the diaphragm
The Brachial Plexus
Posterior scapular nerve
C5
Lateral cord
Posterior cord
Medial cord
Suprascapular nerve
C6
Axillary nerve
Musculocutaneous
nerve
Lateral cord
C7
Posterior cord
Median nerve
Medial cord
C8
Radial nerve
T1
Musculocutaneous
nerve
Axillary nerve
Radial nerve
Long thoracic
nerve
Roots
Median nerve
Ulna
Ulnar nerve
Median nerve
Radial nerve
Radius
Trunks
Anterior divisions
Posterior divisions
Cords
Ulnar nerve
Superficial branch
of ulnar nerve
Digital branch
of ulnar nerve
Digital branch
of median nerve
The Lumbar Plexus
Hip bone
Sacrum
Roots
Femoral nerve
Anterior divisions
L1
Pudendal nerve
Posterior divisions
Sciatic nerve
Femur
L2
Iliohypogastric
nerve
Anterior view
Ilioinguinal nerve
From lumbar plexus
L3
Genitofemoral nerve
Tibial nerve
From sacral plexus
Obturator nerve
Common fibular nerve
Superficial fibular nerve
L4
Lateral femoral cutaneous nerve
Deep fibular nerve
L5
Femoral nerve
Obturator nerve
Lumbosacral trunk
Fibula
Tibia
Tibial nerve
Medial plantar nerve
Lateral plantar nerve
Posterior view
Sacral and Coccygeal Plexuses
Lumbosacral
trunk
L4
Roots
Anterior divisions
Posterior divisions
L5
S1
S2
Superior gluteal nerve
Inferior gluteal nerve
S3
S4
S5
Sciatic nerve:
Common fibular nerve
Tibial nerve
Co1
Posterior cutaneous
nerve
Pudendal nerve
Spinal Reflexes
Events in a Reflex Arc
Action potential
Receptor
potential
The Muscle Spindle
The muscle spindles inform the brain of muscle length and body movement
This enables the brain to send motor commands back
to the muscles that control coordinated movement,
corrective reflexes, muscle tone, and posture
Muscle spindles are proprioceptors: specialized
sense organs to monitor the position and
movement of the body parts
The Stretch Reflex
4
3
Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
The stretch reflexes maintain the muscles at the suitable
length, and help to maintain equilibrium and posture.
6
Primary afferent
neuron excited
+
_
7
5
2
Muscle spindle
is stimulated
1
Extensor muscle
stretched
Alpha motor neuron
stimulates extensor
muscle to contract
Extensor
Flexor
8
Flexor muscle
(antagonist) relaxes
Primary afferent
neuron stimulates
inhibitory interneuron
Interneuron inhibit
alpha motor neuron
to flexor muscle