Final answer:
Severance of sensory fibers on cranial nerve IX does not cause loss of taste because the sense of taste involves multiple cranial nerves, including the glossopharyngeal, facial, and a branch of the vagus nerve. Taste pathways are redundant, with each nerve serving different parts of the tongue and synapsing at different points in the brain. Therefore, damage to one does not result in a total loss of the ability to taste.
Step-by-step explanation:
The reason severance of sensory fibers on cranial nerve IX will not cause loss of taste is that taste sensations from the tongue are carried not only by the glossopharyngeal nerve (cranial nerve IX) but also by the facial nerve (cranial nerve VII) for the anterior two-thirds of the tongue and a branch of the vagus nerve (cranial nerve X) for the posterior part. The glossopharyngeal nerve mainly carries taste sensations from the posterior third of the tongue.
The sensory pathway for gustation, or taste, involves multiple cranial nerves including the facial and glossopharyngeal nerves, which synapse with neurons of the solitary nucleus in the brain stem. From there, axons project to the ventral posterior nucleus of the thalamus before reaching the gustatory cortex where taste is consciously perceived. This redundant pathway means that severing the sensory fibers of one nerve may reduce but not entirely eliminate the sense of taste.
Additionally, the gustatory cells once activated, release neurotransmitters onto the dendrites of sensory neurons. These are part of multiple cranial nerves, ensuring that taste perception is a complex process involving several neural pathways, which is why severance of the sensory fibers of the glossopharyngeal nerve alone does not lead to complete taste loss.