ASCENDING PATHWAYS

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ASCENDING PATHWAYS
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Ascending Pathways

• Three-neuron pathways
• Primary sensory neurons
• From external receptors
• Travel through dorsal roots of spinal cord
• Secondary neurons
• Make up tracts in spinal cord and brainstem
• Tertiary neurons
• From thalamus to primary sensory cortex
• Travel through internal capsule

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Ascending Pathways

• For conscious perception
• Spinothalamic system
• Medial Lemniscal system
• For unconscious perception
• Spinocerebellar
• Spino-olivary
• Spinotectal
• Spinoreticular

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Spinothalamic System

• Lateral spinothalamic tract
• Anterior spinothalamic tract

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Lateral Spinothalamic Tract

• Carries pain and temperature
• Primary fibers ascend or descend 1-2 spinal cord
  segments before synapsing with secondary fibers.

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Lateral Spinothalamic Tract

• Secondary axons decussate through anterior gray
  and white commissures.
• Secondary axons make up the lateral spinothalamic
  tract traveling in the lateral column of the
  spinal cord.

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Lateral Spinothalamic
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Lateral Spinothalamic Tract

• Secondary fibers are joined in brainstem by
  fibers of the trigeminothalamic tract
• (Pain and temperature from face and teeth.)

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Lateral Spinothalamic Tract

• Secondary fiber collaterals project to reticular
  formation
• Stimulate wakefulness and consciousness.
• Secondary fibers project to ventral
  posterolateral (VPL) nucleus of thalamus.

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Lateral Spinothalamic Tract

• Secondary fibers synapse with tertiary fibers in
  VPL.
• Tertiary fibers (corticopetal fibers) synapse in
  postcentral gyrus
• Somatic sensory areas 3, 1, 2
• Tertiary fibers form part of internal capsule.

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Anterior Spinothalamic Tract

• Carries light touch (crude touch), pressure,
  tickle, itch
• Primary neurons may ascend 8-10 spinal cord
  segments before synapsing with secondary neurons.
• Secondary fibers decussate in anterior gray or
  white commissures.

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Anterior Spinothalamic Tract

• Secondary fibers ascend to synapse with tertiary
  fibers in VPL nucleus of thalamus.
• Tertiary fibers ascend through internal capsule
  to primary sensory cortex.

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Lateral Spinothalamic Tract

• Red
• 1
• Blue
• 2
• Green
• 3

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Lateral Spinothalamic Tract

• Red
• 1
• Blue
• 2
• Green
• 3

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Anterior Spinothalamic
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Anterior Spinothalamic Tract

• Red
• 1
• Blue
• 2

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Anterior Spinothalamic Tract
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Medial Lemniscus System

• Also called posterior column system.
• Carries sensations for two-point sensation (fine
  touch), pressure, and vibration.

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Medial Lemniscus System

• Primary fibers ascend entire length of spinal
  cord and synapse with secondary neurons in
  medulla
• Fasciculus gracilis
• Fasciculus cuneatus

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Medial Lemniscus

• Red
• Gracilis
• Blue
• cuneatus

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Medial Lemniscus System

• Fibers of fasciculus gracilis synapse in nucleus
  gracilis
• Convey sensations from below midthoracic level.
• Fibers of fasciculus cuneatus synapse in nucleus
  cuneatus
• Convey sensations from above midthoracic level.
• Also conveys proprioceptive sensation from arms
  to cerebellum.

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Medial Lemniscus System

• Secondary fibers decussate.
• Secondary fibers ascend to synapse in VPL of
  thalamus.
• Tertiary fibers ascend through internal capsule
  to primary sensory cortex.

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Medial Lemniscus
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Medial Lemniscus
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Posterior Spinocerebellar Tract

• Originates in thoracic and upper lumbar regions.
• Consists of uncrossed fibers that enter
  cerebellum through inferior cerebellar peduncles.
• Transmits ipsilateral proprioceptive information
  to cerebellum.

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Anterior Spinocerebellar Tract

• Originates in lower trunk and lower limbs.
• Consists of crossed fibers that recross in pons
  and enter cerebellum through superior cerebellar
  peduncles.
• Transmits ipsilateral proprioceptive information
  to cerebellum.

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Spinocerebellar Tracts
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Spino-Olivary Tracts

• Project to accessory olivary nuclei and
  cerebellum.
• Contribute to movement coordination associated
  primarily with balance.

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Spinotectal Tracts

• Project to superior colliculi of midbrain.
• Involved in reflexive turning of the head and
  eyes toward a point of cutaneous stimulation.

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Spinoreticular Tracts

• Involved in arousing consciousness in the
  reticular activating system through cutaneous
  stimulation.

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Stretch (myotactic) Reflex

• Muscle spindle receptor
• 3-10 small, specialized intrafusal muscle
  fibers
• Contractile only at ends.
• Non-contractile center.
• Afferent neurons from center of intrafusal fibers
  travel through dorsal root of spinal nerve to
  synapse directly with alpha motor neurons of
  extrafusal fibers in which muscle spindle is
  embedded.

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Stretch (myotactic) Reflex

• Afferent neurons from muscle spindle also synapse
  with ascending fibers within spinal cord.
• Gamma motor neurons supply intrafusal fibers
  of muscle spindle
• Regulate sensitivity of intrafusal fibers.
• Gamma neurons are modulated by descending
  fibers within spinal cord.
• Refer to syllabus for specific stretch reflexes.

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Golgi-Tendon Reflex

• Golgi tendon organs
• Encapsulated nerve endings
• End with numerous terminal branches with
  small swellings associated with individual
  tendon fascicles.
• Lie within tendons near the muscle-tendon
  junction.
• Stimulated when tendon is stretched.

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Golgi-Tendon Organs/Reflex

• Afferent neurons from Golgi organs pass through
  dorsal root of spinal nerve and synapse with
  inhibitory association neurons in posterior gray
  matter of spinal cord.

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Golgi-Tendon Organs/Reflex

• Association neurons synapse with alpha motor
  neurons that innervate muscle fibers associated
  with tendon.
• Causes relaxation of associated muscles and
  prevents damage to the tendon due to excessive
  tension.

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