21-Spinal Cord Tracts I

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21-Spinal Cord Tracts I Powered By Docstoc
					Sensory & Motor

  Dr. Zeenat Zaidi
n   There is a continuous flow of information between the
    brain, spinal cord, and peripheral nerves. This
    information is relayed by sensory (ascending) and
    motor (descending) ‘pathways’.
n   Generally the pathways:
    § Consists of a chain of tracts, associated nuclei and

      varying number of relays (synapses)
    § Consist of two or three neurons

    § Exhibit somatotopy (precise spatial relationships)

    § Decussate

    § Involve both the brain and spinal cord

    § Are paired (bilaterally and symmetrically)
Somatic Sensory Pathways
              Sensory Pathways
n   Monitor conditions both inside the body and in the
    external environment
n   Sensation-stimulated receptor passes information
    to the CNS via afferent (sensory) fibers
n   Most sensory information is processed in the spinal
    cord , thalamus, or brain stem. Only 1% reaches
    the cerebral cortex and our conscious awareness
n   Processing in the spinal cord can produce a rapid
    motor response (stretch reflex)
n   Processing within the brain stem may result in
    complex motor activities (positional changes in the
    eye, head, trunk)
                Sensory Pathways
n   Contain a sequence of THREE
    neurons from the receptor to the
    cerebral cortex
n   First order neuron: Sensory            3
    neuron that delivers information
    from the receptor to the CNS.
n   Cell body located in the dorsal
    root ganglion. The Axon (central
    process) passes to the spinal
    cord through the dorsal root of    2
    spinal nerve gives many
    collaterals which take part in
    spinal cord reflexes runs          1
    ipsilaterally and synapses with
    second-order neurons in the
    cord and medulla oblongata
n   Second order neuron:
    u Has cell body in the
      spinal cord or medulla
    u Axon decussate &
    u Terminate on 3rd order

n   Third order neuron:
     u Has cell body in
     u Axon terminates on
       cerebral cortex
    White Matter: Pathway Generalizations

n   Ascending and descending fibers are organized in
    distinct bundles which occupy particular areas and
    regions in the white matter
n   Generally long tracts are located peripherally in the white
    matter, while shorter tracts are found near the gray

     • The TRACT is a bundle of nerve fibers (within CNS)
       having the same origin, course, destination &
     • The name of the tract indicates the origin and
       destination of its fibers
     • The axons within each tract are grouped according
       to the body region innervated
      Tracts of the Spinal Cord
n   Tracts that serve to join brain to the spinal
     u Ascending

     u Descending

n   Fibers that interconnect adjacent or distant
    segments of the spinal cord
     u Intersegmental (propriospinal)
              Intersegmental Tracts
n   Extensive fiber connections
    between spinal segments
n   Fasciculus proprius
     u Short ascending &
       descending fibers
     u Both crossed &
     u Begin and end within the
       spinal cord
     u Participate in
       intersegmental spinal
     u Present in all funiculi
       adjacent to gray matter
         Intersegmental Tracts
n   Dorsolateral tract of
    Lissauer: Primary sensory
    fibers carrying pain,
    temperature and touch
    information bifurcate upon
    entering the spinal cord.
    Their branches ascend and
    descend for several spinal
    segments in the dorsolateral
    tract, before synapsing in
    the dorsal horn
      Intersegmental fibers, establishing connections with
      neurons in the opposite half of the spinal cord, cross
      the midline in the anterior white commissure
      Ascending Spinal Tracts
Transmit impulses:
n Concerned with specific sensory modalities: pain,
  temperature, touch, proprioception, that reach a
  conscious level (cerebral cortex)
   u Dorsal column funiculi

   u Spinothalmic tracts

n From tactile and stretch receptors to
  subconscious centers (cerebellum)
   u Spinocerebellar tracts
n   Three major pathways carry sensory information
     Ø Posterior column pathway (gracile & cuneate
     Ø Anterolateral pathway (spinothalamic)

     Ø Spinocerebellar pathway
          Ascending Spinal Tracts
n   Dorsal white column
n   Lateral spinothalamic
n   Anterior spinothalamic
n   Anterior spinocerebellar
n   Posterior spinocerebellar
n   Cuneocerebellar
n   Spinotectal
n   Spinoreticulr
n   Spino-olivary
n   Visceral sensory tracts
                  Dorsal Column
n   Contains two tracts, Fasciculus
    gracilis (FG) & fasciculus
    cuneatus (FC)
n   Carry impulses concerned with
    proprioception and
    discriminative touch from
    ipsilateral side of body
n   Contain the axons of primary
    afferent neurons that have
    entered cord through dorsal
    roots of spinal nerves
FG contains fibers received at sacral, lumbar and lower
thoracic levels, FC contains fibers received at upper
thoracic and cervical levels
n   Fibers ascend without
    interruption where they
    terminate upon 2nd order
    neurons in nucleus gracilis and
    nucleus cuneatus
n   The axons of the 2nd order
    neurons decussate in the
    medulla as internal arcuate
    fibers and ascend through the
    brain stem as medial
n   The medial lemniscus
    terminates in the ventral
    posterior nucleus of the
    thalamus upon 3rd order
    neurons, which project to the
    somatosensory cortex
    (thalamocortical fibers)
                Spinothalamic Tracts
n   Located lateral and ventral to
    the ventral horn
n   Carry impulses concerned
    with pain and thermal
    sensations (lateral tract) and
    also non- discriminative
    touch and pressure (medial
n   Fibers of the two tracts are
    intermingled to some extent
n   In brain stem, constitute the
    spinal lemniscus
n   Fibers are highly somato-
    topically arranged, with those Information is sent to the
    for the lower limb lying most primary sensory cortex on
    superficially and those for the the opposite side of the body
    upper limb lying deeply
          Lateral Spinothalamic Tract
n   Carries impulses concerned
    with pain and thermal
n   Axons of 1st order neurons
    terminate in the dorsal horn
n   Axons of 2nd order neuron
    (mostly in the nucleus
    proprius), decussate within
    one segment of their origin, by
    passing through the ventral
    white commissure & terminate
    on 3rd order neurons in ventral
    posterior nucleus of the
n   Thalamic neurons project to
    the somatosensory cortex
        Anterior Spinothalamic Tract
n   Carries impulses concerned
    with non- discriminative touch
    and pressure
n   Axons of 1st order neurons
    enter cord terminate in the
    dorsal horn
n   Axons of 2nd order neuron
    (mostly in the nucleus
    proprius) may ascend several
    segments before crossing to
    opposite side by passing
    through the ventral white
    commissure & terminate on 3rd
    order neurons in ventral
    posterior nucleus of the
n   Thalamic neurons project to
    the somatosensory cortex
    Spino-reticulo-thalamic System
n The system represents an additional route
  by which dull, aching pain is transmitted to
  a conscious level
n Some 2nd order neurons terminate in the
  reticular formation of the brain stem,
  mainly within the medulla
n Reticulothalamic fibers ascend to
  intralaminar nuclei of thalamus, which in
  turn activate the cerebral cortex
              Spinocerebellar Tracts
n   The spinocerebellar system
    consists of a sequence of
    only two neurons
n   Two tracts: Posterior &
n   Located near the
    dorsolateral and
    ventrolateral surfaces of the
n   Contain axons of the second
    order neurons
n   Carry information derived
    from muscle spindles, Golgi
    tendon organs and tectile
    receptors to the cerebellum
    for the control of posture and
    coordination of movements
     Posterior Spinocerebellar Tracts
n   Present only above level
n   The cell bodies of 2nd
    order neuron lie in
    Clark’s column
n   Axons of 2nd order
    neuron terminate
    ipsilaterally (uncrossed)
    in the cerebellar cortex
    by entering through the
    inferior cerebellar
        Ventral Spinocerebellar Tracts
n   The cell bodies of 2nd order
    neuron lie in base of the dorsal
    horn of the lumbosacral segments
n   Axons of 2nd order neuron cross to
    opposite side, ascend as far as
    the midbrain, and then make a
    sharp turn caudally and enter the
    superior cerebellar peduncle
n   The fibers cross the midline for a
    second time within the cerebellum
    before terminating in the
    cerebellar cortex
v   Both spinocerebellar tracts convey
    sensory information to the same
    side of the cerebellum
                 Spinotectal Tract
n   Ascends in the anterolateral
    part in close association with
    spinothalamic system
n   Primary afferents reach dorsal
    horn through dorsal roots and
    terminate on 2nd order neurons
n   The cell bodies of 2nd order
    neuron lie in base of the dorsal
n   Axons of 2nd order neuron
    cross to opposite side, and
    project to the periaquiductal
    gray matter and superior
    colliculus in the midbrain
       Spino - olivary Tract

n Indirect spinocerebellar pathway (spino-
n Impulses from the spinal cord are
  relayed to the cerebellum via inferior
  olivary nucleus
n Conveys sensory information to the
n Fibers arise at all level of the spinal
             Spinoreticular Tract
n   Originates in laminae IV-
n   Contains uncrossed
    fibers that end in
    medullary reticular
    formation & crossed &
    uncrossed fibers that
    terminate in pontine
    reticular formation
n   Form part of the
    ascending reticular
    activating system
Somatic Motor Pathways
               Motor Pathways
n   CNS issues motor commands in response to
    information provided by sensory systems, sent by
    the somatic nervous system (SNS) and the
    autonomic nervous system (ANS)
n   Conscious and subconscious motor commands
    control skeletal muscles by traveling over 3
    integrated motor pathways
n   The corticospinal pathway – voluntary control of
    motor activity
     u Corticobulbar tracts
     u Corticospinal tracts
n   The medial and lateral pathways – modify or direct
    skeletal muscle contractions by stimulating,
    facilitating, or inhibiting lower motor neurons
                     Motor Pathways
•   Contain a sequence of TWO
    neurons from the cerebral
    cortex or brain stem to the
•   Upper motor neuron : has cell
    body in the cerebral cortex or
    brain stem, axon decussates
    before terminating on the
    lower motor neuron
•   Lower motor neuron: has cell
    body in the ventral horn of the
    spinal cord, axon runs in the
    ipsilateral ventral root of the
    spinal nerve and supply the         LMN
    Descending Spinal Tracts

n   Originate from the cerebral cortex & brain
n   Concerned with:
    v Control of movements

    v Muscle tone

    v Spinal reflexes & equilibrium

    v Modulation of sensory transmission to
       higher centers
    v Spinal autonomic functions
n   The motor pathways are
    divided into two groups
     u Direct pathways
       (voluntary motion
       pathways) - the
       pyramidal tracts
     u Indirect pathways
       (postural pathways),
       essentially all others -
       the extrapyramidal
       Direct (Pyramidal) System
n   Regulates fast and fine (skilled) movements
n   Originate in the pyramidal neurons in the
    precentral gyri,
n   Impulses are sent through the corticospinal
    tracts and synapse in the anterior horn
n   Stimulation of anterior horn neurons activates
    skeletal muscles
n   Part of the direct pathway, called corticobulbar
    tracts, innervates cranial nerve nuclei
    Indirect (Extrapyramidal) System

n   Complex and multisynaptic pathways
n   The system includes:
     • Rubrospinal tracts: control flexor muscles

     • Vestibulospinal tracts: maintain balance and
     • Tectospinal tracts: mediate head neck, and eye
     • Reticulospinal tracts
           Descending Spinal Tracts
n   Pyramidal
    u   Corticospinal
n   Extrapyramidal
    u   Rubrospinal
    u   Tectospinal
    u   Vestibulospinal
    u   Olivospinal
    u   Reticulospinal
n   Descending
    Autonomic Fibers
             Corticospinal Tracts
n   Concerned with
    voluntary, discrete,
    skilled movements,
    especially those of distal
    parts of the limbs
    (fractionated movements)
n   Innervate the
    contralateral side of the
    spinal cord
n   Provide rapid direct
    method for controlling
    skeletal muscle
n   Origin: motor and sensory
n   Axons pass through corona
    radiata, internal capsule, crus
    cerebri and pyramid of
    medulla oblongata
n   In the caudal medulla about
    75-90% of the fibers
    decussate and form the
    lateral corticospinal tract
n   Rest of the fibers remain
    ipsilateral and form anterior
    corticospinal tract. They
    also decussate before
n   Distribution:
     u 55% terminate at
        cervical region
     u 20% at thoracic

     u 25% at lumbosacral
n   Termination: Ventral horn
    neurons (mostly through
    interneurons, a few fibers
    terminate directly)
n   Corticobulbar tracts end
    at the motor nuclei of CNs
    of the contralateral side
               Rubrospinal Tract
n   Controls the tone of limb
    flexor muscles, being
    excitatory to motor neurons
    of these muscles
n   Origin: Red nucleus
n   Axons course ventro-
    medially, cross in ventral
    tegmental decussation,
    descend in spinal cord
    ventral to the lateral
    corticospinal tract
n   Cortico-rubro-spinal pathway
                  Tectospinal Tract
n   Mediates reflex movements of
    the head and neck in response
    to visual stimuli
n   Origin: Superior colliculus
n   Axons course ventro-medially
    around the periaqueductal gray
    matter, cross in dorsal
    tegmental decussation,
    descend in spinal cord near the
    ventral median fissure,
    terminate mainly in cervical
n   Cortico-tecto-spinal pathway
            Vestibulospinal Tracts
n   Lateral Vestibulospinal
n   Origin: lateral vestibular
    (Deiter’s) nucleus
n   Axons descend ipsilaterally in
    the ventral funiculus
n   Terminate on ventral horn
    cells throughout the length of
    spinal cord
n   Has excitatory influences
    upon extensor motor neurons,
    control extensor muscle tone
    in the antigravity maintenance
    of posture
              Vestibulospinal Tracts
n   Medial vestibulospinal
n   Origin: medial vestibular
n   Axons descend bilaterally in
    the ventral funiculus, with the
    medial longitudinal fasciculus
n   Most of the fibers end in the
    cervical region, some reaching
    upper thoracic segments
n   Involved in movements of the
    head required for maintaining
                Reticulospinal Tracts
n   Influence voluntary movement,
    reflex activity and muscle tone by
    controlling the activity of both
    alpha and gamma motor neurons
n   Mediate pressor and depressor
    effect on the circulatory system
n   Are involved in control of breathing
n   Origin: pontine & medullary
    reticular formation
n   Medial (pontine) reticulospinal
    tract descends ipsilaterally
n   Lateral (medullary) reticulospinal
    tract descends bilaterally
n   Both tracts located in the ventral
         Descending Autonomic Fibers
n   The higher centers
    associated with the control of
    autonomic activity are
    situated mainly in the
n   The fibers run in the
    reticulospinal tracts
n   Terminate on the autonomic
    neurons in the lateral horn of
    thoracic & upper lumbar
    (sympathetic) and sacral
    segments (parasympathetic)
    levels of the spinal cord

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