The Neuroscience of the Five Senses

Photo by Gregory Bourolias | Unsplash.com
Photo by Gregory Bourolias | Unsplash.com

This is part two of our four-part introduction to neuroscience series. For a review of the foundational concepts in neuroscience, please see our previous article. We have included a review of the anatomical terms of location below to assist readers unfamiliar with anatomical terminology.

Anatomical Terms of Location

A quick note regarding neuroanatomical nomenclature: the adjective anterior indicates that a structure is towards the front (face side) of the brain, while posterior indicates that the structure is towards the back. Superior indicates that a structure is towards the top (crown) of the head, while inferior means that the structure is towards the bottom of the brain. In the brain, dorsal can be considered synonymous with superior (like a dorsal fin), and ventral is synonymous with inferior. Medial suggests that a structure is towards the middle of the brain, while lateral indicates that a structure is towards the outside of the brain. A final word of clarification: these descriptors are often used to describe a structure’s relationship to another structure. For example, the brain is superior to the spinal cord.

Five Senses

The brain is obviously more than just a wakefulness-producing, appetite-stimulating, reproduction-oriented machine. The human brain provides the conscious connection to the environment that allows us to enjoy a beautiful sunset, fall in love, and appreciate music. But how does it do this?

Our connection to the outside world is established through our five senses: sight, sound, taste, smell, and touch. Our senses gather a massive amount of data points from the external and internal environment and then transmit them to our brain for processing and refinement. Our conscious experience is made possible by the cortical and subcortical elaboration and integration of the various senses across time. Let’s take a look at each sense individually before examining them together as a whole.

Chemical Senses: Smell and Taste

First, let’s examine the evolutionarily ancient chemical senses: smell and taste. Our sense of smell is unique in that some of its connections bypass the trip through the thalamus/relay that every other sense undertakes before arriving at the cortex.1 Olfactory signals plug directly into the cortex (forebrain). This is why odor can be such a visceral trigger for the resurgence of emotional content and memories.1

Brain_MedialThe olfactory receptors in our nose are each tuned to a subset of chemical cues. The receptors transmit their information to the olfactory bulb, which acts as a hub of informational distribution. One of these olfactory bulb projections leads to the amygdala and the hippocampus; these two structures are central to the formation of memories and our experience of emotions. If memory were a painting, then the hippocampus would provide the black and white outline of the event while the amygdala would add the emotional color. It is this descriptive role of the hippocampus in memory formation that led us to choose the mnemonic hippocampus/memorizer to aid recall; and the emotional paintbrush with which the amygdala colors our memories that led us to choose the mnemonic amygdala/emoter. In addition to its projections to the hippocampus/memorizer and the amygdala/emoter, the olfactory bulb also projects to the nearby olfactory cortex in the medial temporal lobe.1

The olfactory bulb sends a final projection to the thalamus/relay, which in turn relays this information on to the orbitofrontal cortex (OFC), a part of the prefrontal cortex (frontal lobe). This final projection to the OFC generates our conscious experience of smell.1

The human tongue can taste five distinct tastes: sweet, bitter, sour, salty, and umami.2 The sense of taste is extremely primal, having evolved to discriminate the nutritious from the toxic sources of calories in our environment.

At the cellular level, 50-100 taste cells make up one taste bud.1 Each taste cell is tuned to a different taste. The taste information is transmitted to the brain along three cranial nerves (of which there are twelve) that emanate from the brainstem in pairs: the seventh (facial nerve), ninth (glossopharyngeal), and tenth (vagus). The taste signals coalesce in a brainstem/CPU nucleus known as the nucleus tractus solitarii (NTS). The NTS passes the taste information to the thalamus/relay, which relays the information along to the anterior insula, the frontal operculum (frontal lobe), the amygdala/“Emoter,” and the hypothalamus/“Cruise Control.”1 We will return to the insula when we discuss the sense of touch.

Mechanical Senses: Hearing, Touch, and Vision

Sound travels down our ear canal, arriving at the cochlea, a structure that functions as the primary receptive source for auditory information. The cochlea contains the organ of Corti, which houses the actual sound-detecting hair cells.2 The hair cells translate sound waves into neural impulses that travel to the brainstem/CPU along the eighth (vestibulocochlear) cranial nerve. Impulses from the eighth cranial nerve conglomerate in the cochlear nuclei in the brainstem/CPU before continuing their journey upwards along the brainstem/CPU to the superior olivary nucleus and the inferior colliculus in the midbrain.1 From here, auditory signals make a stop at the medial geniculate nucleus in the thalamus/relay before being relayed on to the primary auditory cortex (A1) in the superior temporal gyrus.2

Signals in A1 are arranged tonotopically, meaning that different parts of A1 are responsible for processing specific frequencies (pitches) of sound. This raw auditory information is transferred from A1 to the auditory association area (Wernicke’s area) in the superior temporal gyrus. Wernicke’s area is involved in interpreting meaning from the sound of words – more on this later.1

By OpenStax College | CC License
By OpenStax College | CC License

Our sense of touch begins in specialized nerve endings distributed throughout our skin and muscles (among other locations). Most nerve fibers transmit their somatosensory touch signals to the spinal cord. From the spinal cord, most sensory information synapses in various nuclei in the thalamus/relay before moving on to the primary somatosensory cortex (S1) in the postcentral gyrus of the parietal lobe; because of its role in sensing the external environment we will use the mnemonic S1/external sensor to recall S1’s function. In the S1/external sensor, sensation is arranged somatotopically, resembling a human body draped head down along the side of the brain.3

The insula completes our somatosensory picture of the environment by providing information on our internal states. The insula generates our perceptual experience of our heartbeat, our breathing, the gurgling of our stomach, or the experience of a full bladder.1 For the aforementioned reasons, the insula will be recalled using the mnemonic insula/internal sensor.

Let’s return to the S1/external sensor. The S1/external sensor, having interpreted the raw somatosensory information from the external environment, then passes this somatosensory data along a ventral and a dorsal pathway. We should recall that in the brain, ventral refers to structures towards the bottom of the brain, while dorsal refers to the structures towards the top of the brain (like a shark’s dorsal fin).

The ventral pathway, originating from the S1/external sensor, leads to the secondary somatosensory cortex (S2) located in the parietal operculum. Operculum is Latin for “little lid,” and refers to various structures that sit on top of the insula/internal sensor. S2 is a somatosensory association area where the perception of an object’s texture and shape are formed.2 Additionally, S2 acts as a gateway for somatosensory information to travel to the medial temporal lobe, the amygdala/emoter, and the hippocampus/memorizer.3 It is here in the medial temporal lobe that somatosensory information can be recorded into long-term memory.5 S2 also projects to the premotor cortex in the frontal lobe and the prefrontal cortex, both of which will become important in future discussions.

The dorsal somatosensory pathway from the S1/external sensor is also known as the How Pathway and it projects to the posterior parietal cortex. This route to the posterior parietal cortex is referred to as the How Pathway because it is involved in planning how to execute movements in a given environment. The posterior parietal lobe receives a great deal of input from the visual system, which we will turn to next. Together these two sensory systems help establish our cognitive spatial representation of a given environment.6

The final sense that we will discuss is vision. Vision turns out to be one of the most complex senses. This complexity is unsurprising in a species that relies so heavily on vision. In fact, greater than 50% of the human brain is involved in visual processing.1

Blausen.com - Wikiversity Journal of Medicine | CC License
Blausen.com – Wikiversity Journal of Medicine | CC License

Visual perception begins in the cells of the retina. The visual signals project along the second cranial nerve (optic) to the superior colliculus and the lateral geniculate nucleus (LGN) of the thalamus/relay. Visual signals travel from the LGN to the primary visual cortex (V1) located in the occipital lobe. From V1 visual signals, like the somatosensory sensory signals discussed earlier, split into ventral and dorsal streams.

The ventral pathway from V1 is also known as the What Pathway and projects to the inferior temporal lobe.1 The What Pathway is, unsurprisingly, involved in identifying what an object is. When we recognize the shiny, red, and round object as an apple, we can thank our inferior temporal lobe and the What Pathway for this perceptual induction.

The dorsal pathway from V1 is known as the Where Pathway, projecting to the medial temporal area (MT). Visual information is sent from the MT to the medial superior temporal area (MST). Both the MT and the MST are motion sensitive association cortices and are involved in locating where an object is in space and assessing its movement.7 Where Pathway information sends spatial information to the posterior parietal cortex to integrate with information from the How Pathway. Visual and somatosensory information generate multimodal (i.e. more than one sense modality) representations of our environment. The multimodal visuospatial information from the posterior parietal cortex is sent forward to the prefrontal cortex and comprises the substrate for spatial working memory.7

Photo by Edan Cohen | Unsplash.com
Photo by Edan Cohen | Unsplash.com

Now that we have reviewed the five senses, let’s combine a few to create more elaborate sensory representations of our environment. We already discussed the parietal lobe’s role in generating visuospatial information. This visuospatial information allows us to plan how far we need to extend our hand to reach a glass of water, or to estimate how long we have to turn out of our driveway before an oncoming car poses a danger.

Another multimodal sensory association area lies in the superior temporal polysensory area (STP). The STP integrates visual, auditory, and somatosensory information. The STP seems to also be involved in visuospatial perception, but responds especially strongly to emotions in facial expression.5

The aforementioned pattern of elaboration and integration of multiple sensory modalities generally increases as one moves anteriorly, from the back to the front of the brain. The integration and elaboration is made possible in large part by a process known as corticothalamic feedback. As we have seen, all sensory information, with the exception of olfactory, passes through the thalamus/relay before arriving in the cortex. What we have yet to consider is that the thalamus/relay is not simply a passive output machine that transmits information to the cortex, it also receives a vast amount of input back from the cortex.2

This input from the cortex can inhibit or excite thalamic/relay cells, dynamically fine-tuning their responsiveness to various sensory signals.3 In this way, the cortex can dial up or down specific senses of interest, almost like a selective volume knob. Additionally, the interconnectivity of the cortex and thalamus/relay produces a resonance that unifies many cortical regions. As we will see in our later discussions of the prefrontal cortex and cognition, this corticothalamic integration process generates the foundation of our conscious experience.8

This completes the second part of our four-part introduction to neuroscience series. In our next article, we will explore how our five senses inform the motor systems within the brain.

References

  1. Squire LR, ed. Fundamental Neuroscience. 4th ed. Amsterdam; Boston: Elsevier/Academic Press; 2013.
  2. Haines DE, Ard MD, eds. Fundamental Neuroscience for Basic and Clinical Applications [study Smart with Student Consult]. 4th ed. Philadelphia: Elsevier, Saunders; 2013.
  3. Vanderah TW, Gould DJ, Nolte J. Nolte’s The Human Brain: An Introduction to Its Functional Anatomy. Seventh edition. Philadelphia, PA: Elsevier; 2016.
  4. Baynes J, Dominiczak MH. Medical Biochemistry.; 2014. http://www.clinicalkey.com/dura/browse/bookChapter/3-s2.0-C20110076986. Accessed July 9, 2016.
  5. Bruce C, Desimone R, Gross CG. Visual properties of neurons in a polysensory area in superior temporal sulcus of the macaque. J Neurophysiol. 1981;46(2):369-384.
  6. Gottfried JA, ed. Neurobiology of Sensation and Reward. Boca Raton, FL: CRC Press; 2011.
  7. Barbey AK, Krueger F, Grafman J. Structured event complexes in the medial prefrontal cortex support counterfactual representations for future planning. Philos Trans R Soc Lond B Biol Sci. 2009;364(1521):1291-1300. doi:10.1098/rstb.2008.0315.
  8. Alitto HJ, Usrey WM. Corticothalamic feedback and sensory processing. Curr Opin Neurobiol. 2003;13(4):440-445.

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