Ascending Spinal Tracts

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Sensory Motor Pathways

• Dr. Zeenat Zaidi

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• 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.
• 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)

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Somatic Sensory Pathways
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Sensory Pathways

• Monitor conditions both inside the body and in
  the external environment
• Sensation-stimulated receptor passes information
  to the CNS via afferent (sensory) fibers
• Most sensory information is processed in the
  spinal cord , thalamus, or brain stem. Only 1
  reaches the cerebral cortex and our conscious
  awareness
• Processing in the spinal cord can produce a rapid
  motor response (stretch reflex)
• Processing within the brain stem may result in
  complex motor activities (positional changes in
  the eye, head, trunk)

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Sensory Pathways

• Contain a sequence of THREE neurons from the
  receptor to the cerebral cortex
• First order neuron Sensory neuron that delivers
  information from the receptor to the CNS.
• Cell body located in the dorsal root ganglion.
  The Axon (central process) passes to the spinal
  cord through the dorsal root of spinal nerve
  gives many collaterals which take part in spinal
  cord reflexes runs ipsilaterally and synapses
  with second-order neurons in the cord and medulla
  oblongata

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• Second order neuron
• Has cell body in the spinal cord or medulla
  oblongata
• Axon decussate
• Terminate on 3rd order neuron
• Third order neuron
• Has cell body in thalamus
• Axon terminates on cerebral cortex ipsilaterally

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White Matter Pathway Generalizations

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

• The TRACT is a bundle of nerve fibers (within
  CNS) having the same origin, course, destination
  function
• 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

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Tracts of the Spinal Cord

• Tracts that serve to join brain to the spinal
  cord
• Ascending
• Descending
• Fibers that interconnect adjacent or distant
  segments of the spinal cord
• Intersegmental (propriospinal)

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Intersegmental Tracts

• Extensive fiber connections between spinal
  segments
• Fasciculus proprius
• Short ascending descending fibers
• Both crossed uncrossed
• Begin and end within the spinal cord
• Participate in intersegmental spinal reflexes
• Present in all funiculi adjacent to gray matter

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Intersegmental Tracts

• 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
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Ascending Spinal Tracts

• Transmit impulses
• Concerned with specific sensory modalities pain,
  temperature, touch, proprioception, that reach a
  conscious level (cerebral cortex)
• Dorsal column funiculi
• Spinothalmic tracts
• From tactile and stretch receptors to
  subconscious centers (cerebellum)
• Spinocerebellar tracts

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• Three major pathways carry sensory information
• Posterior column pathway (gracile cuneate
  fasciculi)
• Anterolateral pathway (spinothalamic)
• Spinocerebellar pathway

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Ascending Spinal Tracts

• Dorsal white column
• Lateral spinothalamic
• Anterior spinothalamic
• Anterior spinocerebellar
• Posterior spinocerebellar
• Cuneocerebellar
• Spinotectal
• Spinoreticulr
• Spino-olivary
• Visceral sensory tracts

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Dorsal Column

• Contains two tracts, Fasciculus gracilis (FG)
  fasciculus cuneatus (FC)
• Carry impulses concerned with proprioception and
  discriminative touch from ipsilateral side of
  body
• 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
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• Fibers ascend without interruption where they
  terminate upon 2nd order neurons in nucleus
  gracilis and nucleus cuneatus
• The axons of the 2nd order neurons decussate in
  the medulla as internal arcuate fibers and ascend
  through the brain stem as medial lemniscus.
• The medial lemniscus terminates in the ventral
  posterior nucleus of the thalamus upon 3rd order
  neurons, which project to the somatosensory
  cortex (thalamocortical fibers)

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Spinothalamic Tracts

• Located lateral and ventral to the ventral horn
• Carry impulses concerned with pain and thermal
  sensations (lateral tract) and also non-
  discriminative touch and pressure (medial tract)
• Fibers of the two tracts are intermingled to some
  extent
• In brain stem, constitute the spinal lemniscus
• Fibers are highly somato-topically arranged, with
  those for the lower limb lying most superficially
  and those for the upper limb lying deeply

Information is sent to the primary sensory cortex
on the opposite side of the body
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Lateral Spinothalamic Tract

• Carries impulses concerned with pain and thermal
  sensations.
• Axons of 1st order neurons terminate in the
  dorsal horn
• 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 thalamus
• Thalamic neurons project to the somatosensory
  cortex

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Anterior Spinothalamic Tract

• Carries impulses concerned with non-
  discriminative touch and pressure
• Axons of 1st order neurons enter cord terminate
  in the dorsal horn
• 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
  thalamus
• Thalamic neurons project to the somatosensory
  cortex

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Spino-reticulo-thalamic System

• The system represents an additional route by
  which dull, aching pain is transmitted to a
  conscious level
• Some 2nd order neurons terminate in the reticular
  formation of the brain stem, mainly within the
  medulla
• Reticulothalamic fibers ascend to intralaminar
  nuclei of thalamus, which in turn activate the
  cerebral cortex

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Spinocerebellar Tracts

• The spinocerebellar system consists of a
  sequence of only two neurons
• Two tracts Posterior Anterior
• Located near the dorsolateral and ventrolateral
  surfaces of the cord
• Contain axons of the second order neurons
• Carry information derived from muscle spindles,
  Golgi tendon organs and tectile receptors to the
  cerebellum for the control of posture and
  coordination of movements

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Posterior Spinocerebellar Tracts

• Present only above level L3
• The cell bodies of 2nd order neuron lie in
  Clarks column
• Axons of 2nd order neuron terminate ipsilaterally
  (uncrossed) in the cerebellar cortex by entering
  through the inferior cerebellar peduncle

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Ventral Spinocerebellar Tracts

• The cell bodies of 2nd order neuron lie in base
  of the dorsal horn of the lumbosacral segments
• 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
• The fibers cross the midline for a second time
  within the cerebellum before terminating in the
  cerebellar cortex
• Both spinocerebellar tracts convey sensory
  information to the same side of the cerebellum

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Spinotectal Tract

• Ascends in the anterolateral part in close
  association with spinothalamic system
• Primary afferents reach dorsal horn through
  dorsal roots and terminate on 2nd order neurons
• The cell bodies of 2nd order neuron lie in base
  of the dorsal horn
• Axons of 2nd order neuron cross to opposite
  side, and project to the periaquiductal gray
  matter and superior colliculus in the midbrain

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Spino - olivary Tract

• Indirect spinocerebellar pathway
  (spino-olivo-cerebellar)
• Impulses from the spinal cord are relayed to the
  cerebellum via inferior olivary nucleus
• Conveys sensory information to the cerebellum
• Fibers arise at all level of the spinal cord

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Spinoreticular Tract

• Originates in laminae IV-VIII
• Contains uncrossed fibers that end in medullary
  reticular formation crossed uncrossed fibers
  that terminate in pontine reticular formation
• Form part of the ascending reticular activating
  system

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Somatic Motor Pathways
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Motor Pathways

• 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)
• Conscious and subconscious motor commands control
  skeletal muscles by traveling over 3 integrated
  motor pathways
• The corticospinal pathway voluntary control of
  motor activity
• Corticobulbar tracts
• Corticospinal tracts
• The medial and lateral pathways modify or
  direct skeletal muscle contractions by
  stimulating, facilitating, or inhibiting lower
  motor neurons

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Motor Pathways

• Contain a sequence of TWO neurons from the
  cerebral cortex or brain stem to the muscles
• 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 muscle.

UMN
LMN
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Descending Spinal Tracts

• Originate from the cerebral cortex brain stem
• Concerned with
• Control of movements
• Muscle tone
• Spinal reflexes equilibrium
• Modulation of sensory transmission to higher
  centers
• Spinal autonomic functions

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• The motor pathways are divided into two groups
• Direct pathways (voluntary motion pathways) - the
  pyramidal tracts
• Indirect pathways (postural pathways),
  essentially all others - the extrapyramidal
  pathways

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Direct (Pyramidal) System

• Regulates fast and fine (skilled) movements
• Originate in the pyramidal neurons in the
  precentral gyri,
• Impulses are sent through the corticospinal
  tracts and synapse in the anterior horn
• Stimulation of anterior horn neurons activates
  skeletal muscles
• Part of the direct pathway, called corticobulbar
  tracts, innervates cranial nerve nuclei

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Indirect (Extrapyramidal) System

• Complex and multisynaptic pathways
• The system includes
• Rubrospinal tracts control flexor muscles
• Vestibulospinal tracts maintain balance and
  posture
• Tectospinal tracts mediate head neck, and eye
  movement
• Reticulospinal tracts

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Descending Spinal Tracts

• Pyramidal
• Corticospinal
• Extrapyramidal
• Rubrospinal
• Tectospinal
• Vestibulospinal
• Olivospinal
• Reticulospinal
• Descending Autonomic Fibers

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Corticospinal Tracts

• Concerned with voluntary, discrete, skilled
  movements, especially those of distal parts of
  the limbs (fractionated movements)
• Innervate the contralateral side of the spinal
  cord
• Provide rapid direct method for controlling
  skeletal muscle

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• Origin motor and sensory cortices
• Axons pass through corona radiata, internal
  capsule, crus cerebri and pyramid of medulla
  oblongata
• In the caudal medulla about 75-90 of the fibers
  decussate and form the lateral corticospinal
  tract
• Rest of the fibers remain ipsilateral and form
  anterior corticospinal tract. They also decussate
  before termination

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• Distribution
• 55 terminate at cervical region
• 20 at thoracic
• 25 at lumbosacral level
• Termination Ventral horn neurons (mostly through
  interneurons, a few fibers terminate directly)
• Corticobulbar tracts end at the motor nuclei of
  CNs of the contralateral side

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Rubrospinal Tract

• Controls the tone of limb flexor muscles, being
  excitatory to motor neurons of these muscles
• Origin Red nucleus
• Axons course ventro-medially, cross in ventral
  tegmental decussation, descend in spinal cord
  ventral to the lateral corticospinal tract
• Cortico-rubro-spinal pathway (Extrapyramidal)

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Tectospinal Tract

• Mediates reflex movements of the head and neck in
  response to visual stimuli
• Origin Superior colliculus
• 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 segments
• Cortico-tecto-spinal pathway (Extrapyramidal)

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Vestibulospinal Tracts

• Lateral Vestibulospinal Tracts
• Origin lateral vestibular (Deiters) nucleus
• Axons descend ipsilaterally in the ventral
  funiculus
• Terminate on ventral horn cells throughout the
  length of spinal cord
• Has excitatory influences upon extensor motor
  neurons, control extensor muscle tone in the
  antigravity maintenance of posture

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Vestibulospinal Tracts

• Medial vestibulospinal tract
• Origin medial vestibular nucleus
• Axons descend bilaterally in the ventral
  funiculus, with the medial longitudinal
  fasciculus
• Most of the fibers end in the cervical region,
  some reaching upper thoracic segments
• Involved in movements of the head required for
  maintaining equilibrium

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Reticulospinal Tracts

• Influence voluntary movement, reflex activity and
  muscle tone by controlling the activity of both
  alpha and gamma motor neurons
• Mediate pressor and depressor effect on the
  circulatory system
• Are involved in control of breathing
• Origin pontine medullary reticular formation
• Medial (pontine) reticulospinal tract descends
  ipsilaterally
• Lateral (medullary) reticulospinal tract descends
  bilaterally
• Both tracts located in the ventral funiculus

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Descending Autonomic Fibers

• The higher centers associated with the control of
  autonomic activity are situated mainly in the
  hypothalmaus
• The fibers run in the reticulospinal tracts
• 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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