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ASCENDING PATHWAYS. 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

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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 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

  • Blue

  • Green


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Lateral Spinothalamic Tract

  • Red

  • Blue

  • Green



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Anterior Spinothalamic Tract

  • Red:

  • Blue



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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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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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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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