Neuro 14 Ascending Tracts Student

8/2/2019 Neuro 14 Ascending Tracts Student

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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

SpinotectalSpinoreticular


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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 1-2spinal cord segments before synapsingwith secondary fibers.


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Secondary axons decussate throughanterior gray and white commissures.

Secondary axons make up the lateralspinothalamic tract traveling in the lateralcolumn of the spinal cord.


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Lateral Spinothalamic


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Secondary fibers are joined in brainstemby fibers of the trigeminothalamic tract:

(Pain and temperature from face and teeth.)


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Secondary fiber collaterals project toreticular formation:

Stimulate wakefulness and consciousness.

Secondary fibers project to ventralposterolateral (VPL) nucleus of thalamus.


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Secondary fibers synapse with tertiaryfibers in VPL.

Tertiary fibers (corticopetal fibers) synapsein postcentral gyrus:

Somatic sensory areas 3, 1, 2

Tertiary fibers form part of internalcapsule.


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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 withsecondary neurons.

Secondary fibers decussate in anterior

gray or white commissures.


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Secondary fibers ascend to synapse withtertiary fibers in VPL nucleus of thalamus.

Tertiary fibers ascend through internal

capsule to primary sensory cortex.


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Red

1

Blue

2

Green

3


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Red

1

Blue

2

Green

3


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Anterior Spinothalamic


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Red:

1

Blue

2


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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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Primary fibers ascend entire length ofspinal cord and synapse with secondaryneurons in medulla:

Fasciculus gracilis

Fasciculus cuneatus


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Medial Lemniscus

Red:

Gracilis

Blue

cuneatus


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Fibers of fasciculus gracilis synapse innucleus gracilis:

Convey sensations from below midthoracic level.

Fibers of fasciculus cuneatus synapse innucleus cuneatus:

Convey sensations from above midthoracic level.

Also conveys proprioceptive sensation from arms tocerebellum.


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Secondary fibers decussate.

Secondary fibers ascend to synapse inVPL of thalamus.

Tertiary fibers ascend through internalcapsule 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 lumbarregions.

Consists of uncrossed fibers that entercerebellum through inferior cerebellarpeduncles.

Transmits ipsilateral proprioceptiveinformation to cerebellum.


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Anterior Spinocerebellar Tract

Originates in lower trunk and lower limbs.

Consists of crossed fibers that recross inpons and enter cerebellum throughsuperior cerebellar peduncles.

Transmits ipsilateral proprioceptiveinformation to cerebellum.


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Spinocerebellar Tracts


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Spino-Olivary Tracts

Project to accessory olivary nuclei andcerebellum.

Contribute to movement coordinationassociated primarily with balance.


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Spinotectal Tracts

Project to superior colliculi of midbrain.

Involved in reflexive turning of the headand eyes toward a point of cutaneousstimulation.


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Spinoreticular Tracts

Involved in arousing consciousness in thereticular activating system throughcutaneous 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 fiberstravel through dorsal root of spinal nerve tosynapse directly with alpha motor neurons of

extrafusal fibers in which muscle spindle isembedded.


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Stretch (myotactic) Reflex

Afferent neurons from muscle spindle alsosynapse with ascending fibers withinspinal cord.

Gamma motor neurons supply intrafusalfibers 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 individualtendon 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 passthrough dorsal root of spinal nerve andsynapse with inhibitory association

neurons in posterior gray matter of spinalcord.


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Golgi-Tendon Organs/Reflex

Association neurons synapse with alphamotor neurons that innervate muscle fibersassociated with tendon.

Causes relaxation of associated musclesand prevents damage to the tendon due toexcessive tension.

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Neuro 14 Ascending Tracts Student