1. Chapter 12 The Central Nervous System Part D Shilla Chakrabarty, Ph.D.

2. The Spinal Cord: Embryonic Development
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
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
White
matter
Neural tube
cells
Central
cavity
Alar plate:
interneurons
Dorsal root ganglion: sensory
neurons from neural crest
Basal plate:
motor neurons

3. 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

4. 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

5. Ligamentum flavum Lumbar puncture needle entering subarachnoid space
Supra-
spinous
ligament L5
Filum
terminale S1
Inter-
vertebral
disc
Cauda equina
in subarachnoid
space
Arachnoid
matter
Dura
mater

6. 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

7. 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

8. 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
(a) Cross section of spinal cord and vertebra
Epidural space
(contains fat)
Pia mater
Spinal
meninges
Arachnoid
mater
Dura mater
Bone of
vertebra
Subdural space
Subarachnoid
space
(contains CSF)
Dorsal root
ganglion
Body
of vertebra

9. (b) The spinal cord and its meningeal coverings
Dorsal median sulcus
Gray
commissure
Dorsal funiculus
Dorsal horn
White
columns
Ventral funiculus
Gray
matter
Ventral horn
Lateral funiculus
Lateral horn
Dorsal root
ganglion
Spinal nerve
Central canal
Dorsal root
(fans out into
dorsal rootlets)
Ventral median
fissure
Ventral root
(derived from several
ventral rootlets)
Pia mater
Arachnoid mater
Spinal dura mater
(b) The spinal cord and its meningeal coverings
Figure 12.31b

10. 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
Somatic
sensory
neuron
Dorsal root (sensory)
Dorsal root ganglion
Visceral
motor neuron
Spinal nerve
Ventral root
(motor)
Ventral horn
(motor neurons)
Dorsal horn (interneurons)
motor
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons

11. 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

12. Pathway Generalizations
Decussation: Most pathways decussate (cross over)from one side of the CNS to the other
Relay: Most pathways consist of a chain of two or three neurons (a relay) that contribute to successive tracts of the pathway
Somatotopy: Most pathways exhibit somatotopy, a precise spatial relationship among tract fibers that reflect orderly mapping of the body
Symmetry: All pathways are paired symmetrically (one on each side of the spinal cord or brain)

13. Ventral corticospinal tract Ventral spinothalamic tract
Ascending tracts
Descending tracts
Fasciculus gracilis
Ventral white
commissure
Dorsal
white
column
Fasciculus cuneatus
Lateral
reticulospinal tract
Dorsal
spinocerebellar
tract
Lateral
corticospinal tract
Rubrospinal
tract
Ventral
spinocerebellar
tract
Medial
reticulospinal
tract
Lateral
spinothalamic tract
Ventral corticospinal
tract
Ventral spinothalamic
tract
Vestibulospinal tract
Tectospinal tract
Figure 12.33

14. 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

15. Medial lemniscus (tract) (axons of second-order neurons)
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)
Joint stretch
receptor
(proprioceptor)
Axon of
first-order
neuron
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Muscle spindle
(proprioceptor)
Lumbar spinal cord
Touch
receptor
(a)
Spinocerebellar
pathway
Dorsal column–medial
lemniscal pathway
Figure 12.34a (2 of 2)

16. Primary somatosensory cortex Axons of third-order neurons Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(a)
Spinocerebellar
pathway
Dorsal column–medial
lemniscal pathway
Figure 12.34a (1 of 2)

17. 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
Axons of first-order
neurons
Temperature
receptors
Lateral
spinothalamic
tract (axons of
second-order
neurons)
Pain receptors
Medulla oblongata
Cervical spinal cord
Lumbar spinal cord
(b)
Spinothalamic pathway

18. Primary somatosensory cortex Axons of third-order neurons Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(b)
Spinothalamic pathway
Figure 12.34b (1 of 2)

19. 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

20. Medial lemniscus (tract) (axons of second-order neurons)
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)
Joint stretch
receptor
(proprioceptor)
Axon of
first-order
neuron
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Muscle spindle
(proprioceptor)
Lumbar spinal cord
Touch
receptor
(a)
Spinocerebellar
pathway
Dorsal column–medial
lemniscal pathway
Figure 12.34a (2 of 2)

21. Primary somatosensory cortex Axons of third-order neurons Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(a)
Spinocerebellar
pathway
Dorsal column–medial
lemniscal pathway
Figure 12.34a (1 of 2)

22. 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:
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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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