Copyright © 2010 Pearson Education, Inc. Chapter 12 The Central Nervous System Part D Shilla Chakrabarty, Ph.D

Copyright © 2010 Pearson Education, Inc.

Chapter 12

The Central Nervous System

Part D

Shilla Chakrabarty, Ph.D.


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

Whitematter

Neural tubecells

Centralcavity

Alar plate:interneurons

Dorsal root ganglion: sensoryneurons from neural crest

Basal plate:motor neurons


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


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


Ligamentumflavum

Supra-spinousligament

Lumbar punctureneedle enteringsubarachnoidspace

Filumterminale

Inter-vertebraldisc

T12

L5

Cauda equinain subarachnoidspace

Duramater

L5

L4

S1

Arachnoidmatter


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


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

Spinalmeninges

Arachnoidmater Dura mater

Bone ofvertebra

Subdural space

Subarachnoidspace(contains CSF)

Dorsal rootganglion

Bodyof vertebra

• 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

Cross-Sectional Anatomy

Copyright © 2010 Pearson Education, Inc. Figure 12.31b

(b) The spinal cord and its meningeal coverings

Dorsal funiculus

Dorsal median sulcus

Central canal

Ventral medianfissure

Pia mater

Arachnoid mater

Spinal dura mater

Graycommissure Dorsal horn Gray

matterLateral hornVentral horn

Ventral funiculusLateral funiculus

Whitecolumns

Dorsal rootganglion

Dorsal root(fans out into dorsal rootlets)

Ventral root(derived from severalventral rootlets)

Spinal nerve


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

Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Dorsal horn (interneurons)

Visceralmotorneuron

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

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


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. Figure 12.33

Ascending tracts Descending tracts

Fasciculus gracilisDorsalwhitecolumn

Fasciculus cuneatus

Dorsalspinocerebellar tract

Lateralspinothalamic tract

Ventral spinothalamictract

Ventral whitecommissure

Lateralcorticospinal tract

Lateralreticulospinal tract

Ventral corticospinaltract

Medialreticulospinal tract

Rubrospinaltract

Vestibulospinal tractTectospinal tract

Ventralspinocerebellartract


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. Figure 12.34a (2 of 2)

Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)

Fasciculus gracilis(axon of first-order sensory neuron)

Axon offirst-orderneuronMuscle spindle(proprioceptor)

Joint stretchreceptor(proprioceptor)

Cervical spinal cord

Touchreceptor

Medial lemniscus (tract)(axons of second-order neurons)

Dorsalspinocerebellartract (axons ofsecond-orderneurons)

Nucleus gracilisNucleus cuneatus

Lumbar spinal cord

(a) Spinocerebellarpathway

Dorsal column–mediallemniscal pathway


Primarysomatosensorycortex

Axons of third-orderneurons

Thalamus

Cerebrum

Midbrain

Cerebellum

Pons




Axons of first-orderneurons

Temperaturereceptors

Lateralspinothalamictract (axons ofsecond-orderneurons)

Pain receptors

Medulla oblongata


Lumbar spinal cord

(b) Spinothalamic pathway

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

Copyright © 2010 Pearson Education, Inc. Figure 12.34b (1 of 2)



Thalamus

Cerebrum

Midbrain

Cerebellum

Pons

(b) Spinothalamic pathway


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


Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)

Fasciculus gracilis(axon of first-order sensory neuron)

Axon offirst-orderneuronMuscle spindle(proprioceptor)

Joint stretchreceptor(proprioceptor)


Touchreceptor

Medial lemniscus (tract)(axons of second-order neurons)

Dorsalspinocerebellartract (axons ofsecond-orderneurons)

Nucleus gracilisNucleus cuneatus

Lumbar spinal cord






Thalamus

Cerebrum

Midbrain

Cerebellum

Pons




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. Upper motor neurons:

Pyramidal cells in primary motor cortex (precentral gyrus)

Axons synapse with interneurons or ventral horn motor neurons

2. Lower motor neurons:

Ventral horn motor neurons

Innervate skeletal muscles


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


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


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


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


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


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



• 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



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

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Copyright © 2010 Pearson Education, Inc. Chapter 12 The Central Nervous System Part D Shilla Chakrabarty, Ph.D