Transcript 12 - FacultyWeb

Chapter 12
The Central Nervous
System
Part D
Shilla Chakrabarty, Ph.D.
Copyright © 2010 Pearson Education, Inc.
The Spinal Cord: Embryonic Development
Dorsal root ganglion: sensory
neurons from neural crest
• Develops from caudal portion of
embryonic neural tube
• By week 6 two clusters of
neuroblasts that have migrated
from the original neural tube
can be recognized:
 Alar plate—will become
interneurons; axons form
white matter of cord
Alar plate:
interneurons
White
matter
Basal plate:
motor neurons
 Basal plate—will become
motor neurons; axons will
grow to effectors
• Neural crest cells that come to
lie alongside the cord form the
dorsal root ganglia sensory
neurons; axons grow into the
dorsal aspect of the cord
Copyright © 2010 Pearson Education, Inc.
Neural tube
cells
Central
cavity
Spinal Cord
• Location
 Enclosed in the vertebral column, begins from the
foramen magnum of skull
 Ends as conus medullaris at L1 or L2 vertebra just inferior
to the ribs
• Functions
 Provides two-way communication to and from the brain
 Contains spinal reflex centers
Copyright © 2010 Pearson Education, Inc.
Spinal Cord: Protection
• Protected by bone, meninges, and CSF
• Single layer of dura mater is not attached to bony walls of
vertebral column
• Cushion of fat and a network of veins in the epidural
space between the vertebrae and spinal dura mater
• CSF in subarachnoid space
• Denticulate ligaments: extensions of pia mater that
secure cord to dura mater
• Filum terminale: fibrous extension from conus medullaris;
anchors the spinal cord to the coccyx
Copyright © 2010 Pearson Education, Inc.
T12
Ligamentum
flavum
Lumbar puncture
needle entering
subarachnoid
space
L5
L4
Supraspinous
ligament
L5
Filum
terminale
S1
Intervertebral
disc
Copyright © 2010 Pearson Education, Inc.
Arachnoid
matter
Dura
mater
Cauda equina
in subarachnoid
space
Spinal Cord
• About the width of a thumb for most of its length, but has
enlargements in cervical and lumbar regions
• Spinal nerves
 31 pairs attach to the cord by paired roots
• Cervical and lumbar enlargements
 The nerves serving the upper and lower limbs emerge
here
• Cauda equina
 The collection of nerve roots at the inferior end of the
vertebral canal that resemble a horse’s tail
Copyright © 2010 Pearson Education, Inc.
Cross-Sectional Anatomy
• Two lengthwise grooves divide cord into right and left
halves
• Ventral (anterior) median fissure
• Dorsal (posterior) median sulcus
• Gray commissure—connects masses of gray matter;
encloses central canal
Copyright © 2010 Pearson Education, Inc.
Cross-Sectional Anatomy
• Two lengthwise grooves divide cord into right and left halves
• Ventral (anterior) median fissure
• Dorsal (posterior) median sulcus
• Gray commissure—connects masses of gray matter; encloses central canal
Epidural space
(contains fat)
Subdural space
Subarachnoid
space
(contains CSF)
Pia mater
Arachnoid Spinal
mater
meninges
Dura mater
Bone of
vertebra
Dorsal root
ganglion
Body
of vertebra
(a) Cross section of spinal cord and vertebra
Copyright © 2010 Pearson Education, Inc.
Dorsal median sulcus
Dorsal funiculus
White
Ventral funiculus
columns Lateral funiculus
Dorsal root
ganglion
Gray
commissure
Dorsal horn Gray
Ventral horn matter
Lateral horn
Spinal nerve
Dorsal root
(fans out into
dorsal rootlets)
Ventral root
(derived from several
ventral rootlets)
Central canal
Ventral median
fissure
Pia mater
Arachnoid mater
Spinal dura mater
(b) The spinal cord and its meningeal coverings
Copyright © 2010 Pearson Education, Inc.
Figure 12.31b
Gray Matter
• Dorsal horns—interneurons that receive somatic and visceral sensory input
• Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots
• Lateral horns (only in thoracic and lumbar regions) – autonomic or sympathetic neurons
• Dorsal root (spinal) ganglia—contain cell bodies of sensory neurons
Dorsal root (sensory)
Dorsal root ganglion
Dorsal horn (interneurons)
Somatic
sensory
neuron
Visceral
sensory
neuron
Visceral
motor
neuron
Somatic
motor neuron
Spinal nerve
Ventral root
(motor)
Ventral horn
(motor neurons)
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Copyright © 2010 Pearson Education, Inc.
Somatic motor neurons
White Matter
• Composed of myelinated and unmyeinated nerve fibers
• Fibers allow communication between different parts of the spinal cord and
between the cord and brain
• Fibers run in three directions:
• Ascending- up to higher centers (sensory)
• Descending- down to cord from brain, or within the cord to lower levels
(motor tracts)
• Transverse tracts- cross from one side to the other (commissural fibers)
• White matter on each side is divided into three white columns or
funiculi, named according to their position as dorsal (posterior),
lateral, and ventral (anterior) funiculi
• Each funiculus contains several fiber tracts
• Each spinal tract is composed of axons with similar functions
Copyright © 2010 Pearson Education, Inc.
Pathway Generalizations
1. Decussation: Most pathways decussate (cross over)from one side of
the CNS to the other
2. Relay: Most pathways consist of a chain of two or three neurons (a
relay) that contribute to successive tracts of the pathway
3. Somatotopy: Most pathways exhibit somatotopy, a precise spatial
relationship among tract fibers that reflect orderly mapping of the body
4. Symmetry: All pathways are paired symmetrically (one on each side of
the spinal cord or brain)
Copyright © 2010 Pearson Education, Inc.
Ascending tracts
Fasciculus gracilis
Dorsal
white Fasciculus cuneatus
column
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic tract
Ventral spinothalamic
tract
Copyright © 2010 Pearson Education, Inc.
Descending tracts
Ventral white
commissure
Lateral
reticulospinal tract
Lateral
corticospinal tract
Rubrospinal
tract
Medial
reticulospinal
tract
Ventral corticospinal
tract
Vestibulospinal tract
Tectospinal tract
Figure 12.33
Ascending Pathways
• Conduct sensory impulses upward, through chains of three neurons
 First-order neurons: Cell bodies in a ganglion; conduct impulses from cutaneous
receptors and proprioceptors to spinal cord or brain stem; branches diffusely and
synapse with second-order neuron
 Second-order neurons – Interneurons with cell bodies in dorsal horn of spinal cord
or medullary nuclei; axons extend to thalamus or cerebellum where they synapse
 Third-order neuron – Interneuron with cell body in thalamus; axon extends to
somatosensory cortex
Transmitting Somatosensory Information To Sensory Cortex:
 Two pathways transmit somatosensory information to the sensory cortex
via the thalamus for conscious interpretation:
 Dorsal column-medial lemniscal pathways
 Spinothalamic pathways
These pathways collectively provide discriminative touch and conscious
proprioception
 Third pathway, spinocerebellar pathway, terminates in cerebellum and
does not contribute to sensory perception
Copyright © 2010 Pearson Education, Inc.
Dorsal
spinocerebellar
tract (axons of
second-order
neurons)
Medial lemniscus (tract)
(axons of second-order neurons)
Nucleus gracilis
Nucleus cuneatus
Medulla oblongata
Fasciculus cuneatus
(axon of first-order sensory neuron)
Axon of
first-order
neuron
Muscle spindle
(proprioceptor)
(a) Spinocerebellar
pathway
Copyright © 2010 Pearson Education, Inc.
Joint stretch
receptor
(proprioceptor)
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Lumbar spinal cord
Dorsal column–medial
lemniscal pathway
Touch
receptor
Figure 12.34a (2 of 2)
Primary
somatosensory
cortex
Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(a) Spinocerebellar
pathway
Copyright © 2010 Pearson Education, Inc.
Dorsal column–medial
lemniscal pathway
Figure 12.34a (1 of 2)
Anterolateral Pathways
• Formed by lateral and ventral spinothalamic tracts whose fibers cross over in the spinal cord
• Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tract
Medulla oblongata
Lateral
spinothalamic
tract (axons of
second-order
neurons)
Pain receptors
Cervical spinal cord
Lumbar spinal cord
Axons of first-order
neurons
Temperature
receptors
(b) Spinothalamic pathway
Copyright © 2010 Pearson Education, Inc.
Primary
somatosensory
cortex
Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(b) Spinothalamic pathway
Copyright © 2010 Pearson Education, Inc.
Figure 12.34b (1 of 2)
Spinocerebellar Tracts
• Last pair or ascending pathways: ventral and dorsal
tracts that terminate in the cerebellum
• These pathways do not contribute to conscious
sensation
• Convey information about muscle or tendon stretch to
the cerebellum
• Cerebellum uses this information to coordinate skeletal
muscle activity
Copyright © 2010 Pearson Education, Inc.
Dorsal
spinocerebellar
tract (axons of
second-order
neurons)
Medial lemniscus (tract)
(axons of second-order neurons)
Nucleus gracilis
Nucleus cuneatus
Medulla oblongata
Fasciculus cuneatus
(axon of first-order sensory neuron)
Axon of
first-order
neuron
Muscle spindle
(proprioceptor)
(a) Spinocerebellar
pathway
Copyright © 2010 Pearson Education, Inc.
Joint stretch
receptor
(proprioceptor)
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Lumbar spinal cord
Dorsal column–medial
lemniscal pathway
Touch
receptor
Figure 12.34a (2 of 2)
Primary
somatosensory
cortex
Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(a) Spinocerebellar
pathway
Copyright © 2010 Pearson Education, Inc.
Dorsal column–medial
lemniscal pathway
Figure 12.34a (1 of 2)
Descending Pathways and Tracts
• Deliver efferent impulses from the brain to the spinal cord
 Direct pathways—pyramidal tracts that regulate fast and fine (skilled
movements)
 Indirect pathways— complex multineuronal pathways that regulate
muscles for coarse movements; muscles for head, neck and eye
movements, and axial muscles for balance and posture
Involve two neurons:
1.
2.
Upper motor neurons:

Pyramidal cells in primary motor cortex (precentral gyrus)

Axons synapse with interneurons or ventral horn motor neurons
Lower motor neurons:

Ventral horn motor neurons

Innervate skeletal muscles
Copyright © 2010 Pearson Education, Inc.
Spinal Cord Trauma
Any localized damage to spinal cord or its roots leads to
some functional loss
• Functional losses
 Parasthesias: Sensory loss
 Paralysis: Loss of motor function
Copyright © 2010 Pearson Education, Inc.
Spinal Cord Trauma: Paralysis
• Flaccid paralysis—severe damage to the ventral root or
ventral horn cells
 Impulses do not reach muscles; there is no voluntary
or involuntary control of muscles
 Without stimulation, muscles atrophy
• Spastic paralysis—damage to upper motor neurons of
the primary motor cortex
 Spinal neurons remain intact; muscles are stimulated
irregularly by reflex activity
 No voluntary control of muscles
Copyright © 2010 Pearson Education, Inc.
Spinal Cord Trauma
• Transection
 Cross sectioning of the spinal cord at any level
 Results in total motor and sensory loss in regions
inferior to the cut
Paraplegia—transection between T1 and L1
Quadriplegia—transection in the cervical region
NOTE: Anyone with traumatic spinal cord injury must be
watched for symptoms of spinal shock, a transient
period of functional loss that follows the injury
Copyright © 2010 Pearson Education, Inc.
Poliomyelitis
• Destruction of ventral horn motor neurons by the
poliovirus
• Early symptoms include fever, headache, muscle pain and
weakness, and loss of some somatic reflexes
• Later paralysis develops and muscles atrophy
• Death may occur due to paralysis of respiratory muscles
or cardiac arrest
• Survivors often develop postpolio syndrome many years
later, as neurons are lost
Copyright © 2010 Pearson Education, Inc.
Amyotrophic Lateral Sclerosis (ALS)
• Also called Lou Gehrig’s disease is a devastating
neuromuscular condition
• Involves progressive destruction of ventral horn motor
neurons and fibers of the pyramidal tract
• Symptoms—loss of the ability to speak, swallow, and
breathe
• Death typically occurs within five years
• Linked to glutamate excitotoxicity which kills neurons,
attack by the immune system, or both
Copyright © 2010 Pearson Education, Inc.
Developmental Aspects of the CNS
• CNS is established during the first month of development
• Gender-specific areas appear in both brain and spinal
cord, depending on presence or absence of fetal
testosterone
• Maternal exposure to radiation, drugs (e.g., alcohol and
opiates), or infection can harm the developing CNS
• Smoking decreases oxygen in the blood, which can lead
to neuron death and fetal brain damage
Copyright © 2010 Pearson Education, Inc.
Developmental Aspects of the CNS
• The hypothalamus is one of the last areas of the CNS to develop
• Visual cortex develops slowly over the first 11 weeks
• Neuromuscular coordination progresses in superior-to-inferior and
proximal-to-distal directions along with myelination
• Growth and maturation of the nervous system continues throughout
childhood and reflect progressive myelination
• The brain reaches its maximum weight in the young adult
Copyright © 2010 Pearson Education, Inc.
Developmental Aspects of the CNS
Age brings some cognitive declines, but these are not
significant in healthy individuals until they reach their 80s
Shrinkage of brain accelerates in old age
Excessive use of alcohol causes signs of senility unrelated
to the aging process
Copyright © 2010 Pearson Education, Inc.