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Spinal Cord

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Spinal Cord Chapter 12 - continued The Spinal Cord The spinal cord extends from the foramen magnum to the level of the 1st or 2nd lumbar vertebrae It is enclosed ... – PowerPoint PPT presentation

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Title: Spinal Cord


1
Spinal Cord
  • Chapter 12 - continued

2
The Spinal Cord
  • The spinal cord extends from the foramen magnum
    to the level of the 1st or 2nd lumbar vertebrae
  • It is enclosed within the vertebral column

3
The Spinal Cord
  • The spinal cord is a provides a two way
    conduction pathway to and from the brain
  • It is a major reflex center

4
The Spinal Cord
  • The spinal cord is protected by bone, cerebro-
    spinal fluid, and meninges
  • Dura mater, arachnoid, pia mater

5
The Spinal Cord
  • Between the bony vertebrae and the dural sheath
    is a large epidural space filled with a soft
    padding of fat and a network of veins
  • Cerebrospinal fluid fills the subarachnoid space

6
The Spinal Cord
  • Inferiorly, the dural and subarachnoid membranes
    extend to the level of S2 while the spinal cord
    ends at L1
  • Subarachnoid space beyond L1 is an ideal site for
    a spinal tap

7
The Spinal Cord
  • The spinal cord terminates in a tapering cone
    shaped structure called the conus medullaris

8
The Spinal Cord
  • A fiberous extension of the pia mater, the filum
    terminale extends inferiorly from the conus
    medullaris to attach to the posterior surface of
    the coccyx

9
The Spinal Cord
  • There are 31 pairs of spinal nerves that arise
    from the cord by paired roots and exit from the
    vertebral column via the intervertebral formina
  • Each segment of the spinal cord is defined by a
    pair of spinal nerves that lie just superior to
    their corresponding vertebra

10
The Spinal Cord
  • The spinal cord has obvious enlargements where
    the nerves serving the upper and lower limb arise
  • Cervical enlargement
  • Lumbar enlargement

11
The Spinal Cord
  • Because the cord does not reach the end of the
    vertebral column, the lumbar and sacral spinal
    nerve roots angle sharply downward and travel
    inferiorly before reaching their intervertebral
    foramina
  • This collection of nerve roots at the inferior
    end of the vertebral canal is called the cauda
    equina
  • The arrangement reflects the fact that during
    vertebral column grows more rapidly than does the
    spinal cord

12
Embryonic Development
  • The spinal cord develops from the caudal portion
    of the embryonic neural tube
  • By the end of the 6th week each side of the
    developing cord has two clusters of neuroblasts
    that have migrated outwarded from the neural tube

13
Embryonic Development
  • The two clusters are the dorsal alar plate and a
    ventral basal plate
  • Alar plate neurons become interneurons
  • The basal plate neurons become motor neurons that
    sprout axons that grow out to the effector organs

14
Embryonic Development
  • Axons that emerge from alar plate cells form the
    external white matter of the cord by growing
    outward along the length of the CNS
  • The alar plates expand dorsally and the basal
    plates expand vertically to become the H-shaped
    mass of gray matter

15
Embryonic Development
  • Neural crest cells that come to lie alongside the
    cord form the dorsal root ganglia containing
    sensory nerve cell bodies, which send their axons
    to the dorsal aspect of the brain

Neural crest cells
16
Cross Section of Spinal Cord
  • The spinal cord has two grooves that mark its
    surface
  • Anterior median fissure / Posterior medial sulcus

17
Gray Matter and Spinal Roots
  • These grooves run the length of the cord and
    partially divide it into right and left halves
  • Gray matter inside, the white matter outside

18
Gray Matter and Spinal Roots
  • The gray matter consists of a mixture of neuron
    cell bodies, their unmyelinated processes, and
    neuroglia (support cells)

19
Gray Matter and Spinal Roots
  • The white matter is composed of myelinated and
    unmyelinated nerve fibers that represent
    ascending, descending and transverse pathways

20
Gray Matter and Spinal Roots
  • The gray matter consists of mirror-image lateral
    gray masses connected by a cross-bar of gray
    matter called the gray commissure that encloses
    the central canal

21
Gray Matter and Spinal Roots
  • The two posterior projections of gray matter are
    the posterior (dorsal) horns the anterior pair
    are the anterior (ventral) horns with lateral
    horns in the lumbar and thoracic portions of the
    cord

22
Gray Matter and Spinal Roots
  • The anterior horns house nerve cell bodies of the
    somatic motor neurons
  • These send their axons out via ventral roots of
    the spinal cord to the skeletal muscles

23
Gray Matter and Spinal Roots
  • The amount of ventral gray matter present at a
    given level of the spinal cord reflects the
    amount of skeletal muscle innervated at that
    particular level
  • Thus, the anterior horns are the largest in the
    areas where the innervation for limbs is present
  • Cervical enlargement / arms
  • Lumbar enlargement / legs

24
Gray Matter and Spinal Roots
  • The lateral horn neurons are autonomic
    (sympathetic) motor neurons that serve the
    visceral organs
  • Their axons also leave the cord via the ventral
    root

25
Gray Matter and Spinal Roots
  • Afferent fibers carrying impulses from peripheral
    sensory receptors form the dorsal roots of the
    spinal cord

26
Gray Matter and Spinal Roots
  • The cell bodies of the associated sensory neurons
    are found in an enlarged region of the dorsal
    root called the dorsal root ganglion or spinal
    ganglion

27
Gray Matter and Spinal Roots
  • After entering the cord, the axons take a number
    of routes
  • Some enter the posterior white matter of the cord
    or brain, others synapse with interneurons

28
Gray Matter and Spinal Roots
  • The dorsal and ventral roots are very short and
    fuse laterally to form the spinal nerves which
    are then considered part of the peripheral
    nervous system (PNS)

29
Gray Matter and Spinal Roots
  • The spinal gray matter can be divided further
    according to its neurons relative involvement in
    the innervation of the somatic and visceral
    regions of the body
  • The four zones are somantic sensory (ss)
    visceral sensory (vs) visceral motor (vm)
    somatic motor (sm)

30
White Matter
  • The white matter of the spinal cord is composed
    of myelinated and unmyelinated nerve fibers that
    allow communication between different parts of
    the spinal cord and between the cord and the
    brain

31
White Matter
  • Nerve fibers run in three directions
  • Ascending / up to higher centers (sensory inputs)
  • Descending / down to the cord from the brain or
    from within the cord to lower levels (motor
    outputs)
  • Transversely / across from one side of the cord
    to the other (commissural fibers)

32
White Matter
  • The ascending and descending tracts make up most
    of the white matter of the spinal cord
  • Ascending tracts are shown in blue and labeled at
    left
  • Descending tracts are shown in red and labeled at
    right

33
White Matter
  • The white matter on each side of the column is
    divided into three white columns or funiculi and
    labeled according to their position (posterior,
    lateral, anterior)
  • Each funiculi contains several fiber tracts, and
    each tract is made up of axons with similar
    destinations and functions

34
White Matter
  • All major spinal tracts are actually part of
    multi- neuron pathways that connect the brain to
    the body
  • These ascending and descending pathways contain
    not only spinal cord neurons but also parts of
    peripheral neurons and neurons in the brain

35
White Matter
  • Generalizations about spinal pathways
  • Most pathways cross over from one side of the CNS
    to the other at some point
  • Most consist of a chain of two or three neurons
    that contribute to successive tracts
  • Most exhibit somatotopy, a precise spatial
    relationship among the tract fibers that reflects
    the orderly mapping of the body
  • All pathways and tracts are paired (right and
    left) with a member of the pair on each side of
    the spinal cord or brain

36
Ascending (Sensory) Tracts
  • The ascending pathways conduct sensory impulses
    upward, typically through chains of three
    successive neurons (first-, second, and
    third-order neurons) to various regions of the
    brain
  • Most of the incoming information results from
    stimulation of
  • General sensory receptors
  • Touch / pressure / temperature / pain
  • Stimulation of proprioceptors
  • Muscle stretch / tendon / joint

37
Ascending (Sensory) Tracts
  • In general, sensory information is conveyed along
    six main pathways on each side of the spinal cord
  • Four transmit impulses to the sensory cortex for
    conscious interpretation
  • Fasciculi cuneatus
  • Fasciculi gracilis
  • Lateral spinothalamic tract
  • Anterior spinothalamic tract
  • Two transmit impulses to the cerebellum to
    coordinate muscle activity
  • Anterior spinocerebellur tract
  • Posterior spinocerebellur tract

38
Ascending (Sensory) Tracts
  • Posterior funiculi (dorsal white column)
  • Fasciculi cuneatus
  • Fasciculi gracilis
  • Transmit information from the fine touch and
    pressure receptors and joint proprioceptors
  • These tracts comprize what is referred to as
    discriminative touch and conscious proprioception

39
Ascending (Sensory) Tracts
  • Lateral and anterior funiculi
  • Lateral spinothalamic tract
  • Anterior spinothalamic tract
  • Convey information on pain, temperature, deep
    pressure and course touch (undiscriminated)

40
Ascending (Sensory) Tracts
  • Anterior and posterior funiculi
  • Anterior spinocerebellar tract
  • Posterior spinocerebellar tract
  • Convey information from proprioceptors (muscle
    and tendon stretch) to the cerebellum which uses
    this information to coordinate skeletal muscle
    activity

41
Ascending (Sensory) Tracts
  • Since the spinocerebellar tracts do not terminate
    in the cortex, these pathways do not contribute
    to conscious sensation
  • The spinocerebellar tracts do not decussate and
    thus contribute to ipsilateral innervation

42
Descending (Motor) Tracts
  • The descending motor tracts that deliver impulses
    from the brain to the spinal cord are divided
    into two groups
  • Pyramidal tracts
  • All others

43
Descending (Motor) Tracts
  • Motor pathways involve two neurons, referred to
    as upper and lower motor neurons
  • The pyramidal cells of the motor cortex, as well
    as the neurons in subcortical motor nuclei that
    give rise to other descending motor pathways, are
    called upper motor neurons
  • The anterior horn motor neurons, which actually
    innervate the skeletal muscles are called lower
    motor neurons

44
Descending (Motor) Tracts
  • The lateral (pyramdial) and anterior
    corticospinal tracts are the major motor pathways
    concerned with voluntary movement, particularly
    precise or skilled movement

45
Descending (Motor) Tracts
  • The pyramdial tracts are also called the direct
    pathways because their axons descend without
    synapsing from the pyramidal cells of the primary
    motor cortex all the way to the spinal cord

46
Descending (Motor) Tracts
  • Pyramidal tracts synapse primarily with
    interneurons, but also directly with anterior
    horn motor neurons, principally those controlling
    limb muscles
  • The anterior horn motor neurons activate the
    skeletal muscles with which they are associated

47
Descending (Motor) Tracts
  • The remaining descending tracts include
  • Rubrospinal
  • Anterior reticulospinal
  • Lateral reticulospinal
  • Vestibulospinal
  • Tectospinal

48
Descending (Motor) Tracts
  • The remaining tracts originate in different
    subcortical motor nuclei of the brain stem
  • These tracts were formerly lumped together as the
    extrapyramidal tracts
  • The current term is to label them indirect
    pathways or just the names of the individual
    pathway

49
Descending (Motor) Tracts
  • Although the cerebellum coordinates voluntary
    muscle activity, no motor efferents descend
    directly from the cerebellum to the spinal cord
  • The cerebellum influences motor activity by
    acting through relays on the motor cortex

50
Spinal Cord Trauma
  • Damage to the spinal cord is associated with some
    form of loss of function
  • Paralysis / loss of function
  • Paresthesis / sensory loss
  • Flaccid paralysis / motor loss
  • Spastic paralysis / upper motor neuron loss
  • Body regions below lesion
  • Quadriplegia / spinal cord injury - 4 limbs
  • Paraplegia / spinal cord injury - 2 limbs
  • Hemiplegia / brain injury - one side of body

51
Developmental Aspects of CNS
  • Fetal alcohol syndrome
  • Cerebral palsy
  • Anencephaly (without brain)
  • Spina bifida (forked spine)
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