Describe the 2 different pathways for afferent somatosensory information to asce

Question

Describe the 2 different pathways for afferent somatosensory information to ascend to the brain. How can this knowledge allow you to clinically assess where damage may have occurred in the somatosensory pathway?

a. Why does the “homunculus” mapped out in the somatosensory cortex not look like a normally proportioned human being?

b. Describe the pathway for olfactory information from the olfactory neurons to the cortex.

c. How is information about odor transduced in the olfactory nerve? How is it processed on its way to the cortex (i.e., is there convergence and of what?)?

d. What are the 6 subtypes of gustation?

Explanation / Answer

The two different pathways are first order and second order afferent.

The first order afferent is pseudounipolar neuron, the cell body of which is located in peripheral ganglion. The peripheral axon of this forms somatosensory receptors and central process which synapses with second order afferent neurons in spinal cord or brain stem nucleus. This second order then ascend the neuraxis and synapse with third order afferent neurons in thalamus.

There is a Decussation(axon cross the midline to the opposite side of spinal cord or brain stem) in each somatosensory pathway below the thalamus. If afferent pathway is damaged somewhere below the Decussation, the sensory loss will be on the side of ipsilateral(same side) to the lesion. If the afferent pathway is damaged somewhere above the Decussation, the sensory loss will be on the side contralateral(opposite side) to the lesion.

Homunculus does not look like normally proportioned human being because it is based on neurological map of the areas and proportion of the human brain dedicated to motor functions and sensory functions for different parts of the body.

The scent(odorant) enters our body by primary (nose) and secondary(pharynx). From there it enters nasal cavity where it gets attached to the olfactory receptors neurons present on olfactory mucosa(olfactory epithelium). From there it is transducted to the left and right olfactory lobe. From there information is conducted to Lateral olfactory Tract to the primary olfactory cortex. From there it goes to thalamus and then to orbito frontal cortex where the smell is perceived. Orbito cortex has connections with limbic system structures like amygdala, hippocampus and hypothalamus. The limbic system is responsible for affective component of smell.

Yes there is convergence, convergence of sensory neurons expressing the same odorant.

The six types of gustations are sweet, sour, bitter, salty, pungent and astringent.

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