Biomedical Science - Sensory tracts

sensory pathways

-ascending

-routes by which information received by sensory receptors in different parts of the body are conveyed to the sensory cortex in the brain

Somatosensory cortex

-The primary somatosensory cortex is located in the post central gyrus in the parietal lobe

-receives all the sensations of touch, pain, pressure

Somatosensory homunculus map

-is a visual representation of brain's somatosensory cortex, it shows:

-where sensory info from different regions is processed

-disproportionate amount of cortex area dedicated to different parts of the body based on sensory input

sensory mapping - dermatones

each segment of the spinal cord receives sensory info from 1 dermatome, which is a particular area of the body

exteroception

-Sensation may arise from outside the body (exteroception)

-Somatosensory receptors on the body surface/linings convey touch, temperature, pain and pressure

-Special sensory receptors conveying taste, smell, vision, balance or hearing from the special sense organs through pathways in cranial nerves and brain unique to each sense

proprioception

Receptors in muscles, joints and tendons continuously convey info to the brain as to the position, location, orientation and movement of the body

viscerosensation (interoception)

-Mechanoreceptors eg. stretch receptors when lungs inflate or sense high blood pressure

-Chemoreceptors (PH/CO2/O2)

-Nociceptors (sense internal pain)

somatosensory pathway

-have developed 2 separate routes throughout evolution but both pathways have the same general framework

-direction of sensory pathways are ALWAYS AFFERENT and info moves from the receptor to the sensory cortex

sensory pathways mostly have X neuron pathways

3

First order neurons

sensory component of a spinal nerve, conveys info from the receptor to the CNS

second order neurons

convey info from lower areas of the CNS to thalamus (in the diencephalon) of the opposite side

third order neurons

convey info from the thalamus to the sensory cortex

2 main routes for sensory info from body

Dorsal pathway & spinothalamic/spinoreticular tracts

spinothalamic tracts

non discriminative touch

temperature

sharp pain

spinoreticular tracts

-pain perception

-also synapse in the Reticular Formation in the medulla and pons

dorsal column pathways process

-The primary afferent neurons (1) do NOT synapse in the spinal cord but pass into the dorsal white matter (dorsal columns) on the same side (ipsilateral)(2)

-They ascend to the dorsal column nuclei on the posterior aspect of the medulla (3) before synapsing with a neuron (4) that will ascend to the thalamus

-This neuron (4) leaves the dorsal column nuclei and crosses (decussates) to the opposite side before ascending in the medial lemniscus tract (5) to the thalamus (the ventral posterior thalamic nucleus) (6) and then to the sensory cortex (7)

spinothalamic and spinoreticular tracts process

-The first order neurons synapse in the dorsal horn of the same side of spinal cord (1)

-The second order neurons decussate (cross over) the spinal cord to the white matter of the spinal cord on the opposite side (2)

-The spinothalamic tracts (3) [please note that only one tract is shown but this represents the anterior and the lateral spinothalamic tract] ascend to the thalamus (4) and on to the somatosensory cortex (5)

-The spinoreticular tracts (6) ascend to the reticular formation in the medulla (7) and then ascend to the thalamus (8) and then to the somatosensory cortex (9)

-the spinoreticular tract has four order neurons in its pathway rather than 3 - but this is an exception

sensory info from head and face

-No DCML or spinothalamic/ spinoreticular nerves for face

-Sensation is sensed by a nerve with 3 branches called Trigeminal nerve, Cranial nerve V

-Info on sensation passes to pons, then up to opposite hemisphere of the brain