Ascending Sensory Pathways

Transcript

0:00 – 0:30  [Basic Definition]  For each sense, the ascending sensory pathway is a neural circuit that begins with receptors in the periphery and ends with the first cortical area that receives signals about that sense.  For example, the ascending gustatory pathway begins with taste cells in the mouth and ends with neurons in the insular cortex.  Except for the olfactory system, all ascending sensory pathways include a synapse in the thalamus.  Some sensory systems like vision and somatosensation are characterized by a strong lateralization where stimuli originating from the left are processed initially by the right side of the cerebrum.

0:30-2:30  

So now the taste system obviously conveys information about tastes, which convey meaningful information about the nutritiousness of food, or the safety of things that we put in our mouths.In 202, you guys learned about the receptors on the tongue that convey information about different kinds of tastes, and now we’ll follow that information into the brain here.

You guys learned about taste cells in the tongue that express taste receptors. Those are not neurons in the case of the gustatory system, but they’re innervated by neurons, peripheral sensory neurons, just like the peripheral sensory neurons in the rest of the body. We have neurons with peripheral cell bodies that extend axons into the periphery, innervating the taste receptor cells. They also extend axons into the central nervous system that synapse on cells in a region of the brainstem called the nucleus of the solitary tract.And here we actually see three more cranial nerves: seven, nine and ten, that each convey information from taste cells in the different tissues here in the mouth and throat.

And so we have a relay here in the brainstem, but the nucleus of the solitary tract is itself important for driving certain behavioral responses. But very basic ones like swallowing, regurgitating, choking responses, vomiting, those sorts of things. These behaviors that are very deep below our consciousness, are driven by neurons here in the brainstem.

But taste information is also conveyed up to the thalamus. Axons from the nucleus of the solitary tract reach the thalamus and in particular a certain nucleus in the thalamus: the ventral posterior medial nucleus, sometimes called VPM. We’ll hear about this nucleus a number of times during this class.

And then from the thalamus, we have projections from the thalamic neurons to the cortex. There’s actually a couple of different regions of cortex here, part of the insular cortex and more frontal areas as well. These regions seem to be important for our conscious experience of taste, and the important decisions we have to make, like what kind of fast food to buy and which flavor of Ben and Jerry’s. Those decisions are under conscious control thanks to these pathways.

These taste pathways follow the  canonical sensory pathway, where you have a peripheral nerve with peripheral cell bodies that carries information into a brainstem nucleus. From there, there’s a projection to a thalamic nucleus, and from there there’s a projection to a primary sensory cortex.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about the ascending sensory pathways.  But this may be shortened to simply “ascending pathways”.  Rather than ascending sensory “pathways”, you may hear about ascending sensory “tracts”, which would include only those parts of the pathway that are located in the central nervous system.  You may also hear more generally about “sensory pathways”, and this term would include not only the ascending projections to the brain, but also the parallel projections to reflex circuits and the descending pathways that modulate activity in the ascending pathways.

3:00-4:00  [Here’s a real world example]  Detailed knowledge of the ascending sensory pathways plays an important role in clinical neuroscience. During the pandemic, you probably learned that a loss of smell, which is technically called anosmia, is an early indicator of COVID-19 infection.  However, did you know that anosmia is also commonly caused by head trauma, like the whiplash injury that can occur during a car crash. In the ascending olfactory pathways, olfactory receptor neurons project to the olfactory bulb by passing through the cribriform plate, which is a structure on the base of the skull that has many small openings providing a passage from the nasal cavity to the cranial cavity. From the olfactory bulb, olfactory signals are conveyed to the olfactory cortex which contributes to the conscious awareness of smell.  Head trauma can produce anosmia by tearing the axons of olfactory receptors neurons as they pass through the small openings of the cribriform plate. When these axons are damaged, olfactory signals do not reach the olfactory bulb, so there is no conscious awareness of smell.  Importantly, the olfactory receptor neurons have potential to regenerate and re-establish connections in the ascending olfactory pathway.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapters 8-12 cover the Sensory Systems

2) Principles of Human Physiology (Stanfield)

• Chapter 10: “The Nervous System: Sensory Systems”

Media attributions

Image from “Foundations of Neuroscience” by Casey Henley is licensed under a Creative Commons Attribution NonCommercial Sharealike license (CC-by-NC-SA 4.0).