Ascending sensory pathways and physiological significance
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Definition
Ascending pathways relay sensory information from receptors to the sensory cortex through first, second and third order neurons arranged in series.
Second order neurons cross the midline, so thalamus and cortex represent sensation from the opposite side of the body.
Neurons in Ascending Pathways
First order neurons are primary afferents with cell bodies in the dorsal root ganglion (DRG), entering the spinal cord via the dorsal root.
Second order neurons: located in the spinal cord or brainstem, relay impulses to the thalamus.
Third order neurons: run from specific thalamic nuclei to the sensory cortex, making the thalamus the main sensory relay centre.
Fourth order neurons, arising from third order terminals in the sensory cortex, project to cortical association areas for further processing and are not usually included in the sensory pathway proper.