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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Primary sensory neurons:
From external receptors
Travel through dorsal roots of spinal cord
Make up tracts in spinal cord and brainstem
From thalamus to primary sensory cortex
Travel through internal capsule
Medial Lemniscal system
(Pain and temperature from face and teeth.)
Stimulate wakefulness and consciousness.
Somatic sensory areas 3, 1, 2
Convey sensations from below midthoracic level.
Convey sensations from above midthoracic level.
Also conveys proprioceptive sensation from arms to cerebellum.
3-10 small, specialized intrafusal muscle fibers:
Contractile only at ends.
Gamma motor neurons supply intrafusal fibers of muscle spindle:
Regulate sensitivity of intrafusal fibers.
Gamma neurons are modulated by descending fibers within spinal cord.
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.