Processing Auditory, Somatosensory, Olfactory, and Gustatory Information

Summary

Professor Dave revisits the senses, explaining how the brain processes sensory information from sight, sound, touch, smell, and taste. Each sensory system follows a hierarchical pathway, from receptors to the cortex, demonstrating functional segregation and parallel processing. Auditory processing involves vibrations traveling from the eardrum to the cochlea, while the somatosensory system covers exteroceptive, proprioceptive, and interoceptive signals. Smell and taste utilize olfactory receptors and taste buds to perceive chemical stimuli. The lecture highlights the pathways and processing mechanisms involved in sensory perception, providing a comprehensive overview of how we experience our environment.

Takeaways

• 👁️ Sensory systems share a hierarchical structure where information travels from receptors to the brain, progressively increasing in complexity.
• 🔊 The auditory system involves the reception of sound waves, which are processed through the cochlea and travel through the brain to produce the perception of sound.
• 👂 Sound travels through the cochlear nuclei, superior olives, inferior colliculi, and medial geniculate nuclei before reaching the auditory cortex.
• 👋 The somatosensory system processes touch and consists of the exteroceptive, proprioceptive, and interoceptive systems, each detecting different stimuli such as touch, body position, and internal sensations.
• 🖐️ Different types of cutaneous receptors like Merkel’s disks, Ruffini endings, and Pacinian corpuscles are responsible for detecting different types of tactile stimuli such as pressure, stretch, and vibration.
• 💥 The dorsal-column medial-lemniscus system and the anterolateral system are the main pathways for touch and temperature/pain information to reach the brain.
• 🧠 The primary somatosensory cortex is organized as a somatosensory homunculus, mapping regions of the body based on the amount of cortical space dedicated to each part.
• 👃 Smell (olfaction) involves molecules interacting with receptors in the nose, which send signals directly to the brain without a thalamic relay, connecting to the amygdala and piriform cortex.
• 👅 Taste (gustation) involves taste buds, each detecting one of the five basic tastes (salty, sweet, bitter, sour, and savory), with signals sent to the gustatory cortex via the thalamus.
• 🧠 Sensory systems operate in parallel processing, where signals are split into multiple pathways, and the integration of this information in the brain generates our perception of reality.

Q & A

What is the general pathway for sensory information to travel from receptors to the brain?

Sensory information typically travels from receptors to thalamic relay nuclei, then to a primary sensory cortex, followed by a secondary sensory cortex, and finally to an association cortex. Each tier in this hierarchy performs increasingly specific and complex analyses.

What is the difference between sensation and perception?

Sensation refers to the detection of a stimulus at sensory organs, while perception is the integration and interpretation of that information, which occurs exclusively in the brain.

How does auditory processing resemble visual processing?

Like visual processing, auditory processing is hierarchical, functionally segregated, and involves parallel processing. Sound waves activate hair cells in the cochlea, and the resulting signals travel through specific brain pathways, including the cochlear nuclei, superior olives, and auditory cortex.

What are the three divisions of the exteroceptive system in somatosensation?

The exteroceptive system has three divisions: one for mechanical stimuli (tactile), one for thermal stimuli (temperature changes), and one for nociceptive stimuli (pain).

How is sensory information from the skin transmitted to the brain?

Sensory information from the skin is transmitted to the brain via nerves that relay information to the spinal cord through dorsal roots. It then travels up one of two main pathways: the dorsal-column medial-lemniscus system (for touch and proprioception) or the anterolateral system (for pain and temperature).

What is the somatosensory homunculus?

The somatosensory homunculus is a map of the surface of the body in the primary somatosensory cortex, where different regions correspond to specific body parts. The size of each body part on the homunculus is proportional to the amount of cortex dedicated to it.

How do the olfactory and gustatory systems detect chemical stimuli?

The olfactory system detects volatile molecules in the air through receptors in the nose, while the gustatory system detects chemical molecules in food using taste buds on the tongue. Both systems send signals to the brain, where perception occurs.

What is the role of the olfactory bulb in smell perception?

The olfactory bulb receives input from olfactory receptor cells in the nose and relays this information to the brain. Neurons in the olfactory bulb project to the medial temporal lobes and the limbic system, influencing both odor perception and emotional responses to odors.

What are the five types of taste receptors found on the tongue?

The five types of taste receptors on the tongue correspond to the sensations of salty, sweet, bitter, sour, and savory (umami). These receptors are clustered in taste buds that send signals to the brain for perception of flavor.

How do sensory systems demonstrate parallel processing?

Sensory systems demonstrate parallel processing by splitting signals into multiple pathways, allowing different aspects of the sensory information to be processed simultaneously, leading to multiple effects and a richer perception of stimuli.

Outlines

00:00

👁️ Understanding the Hierarchy of Sensory Systems

In this section, the speaker revisits the sensory systems, comparing the processing of visual information to other senses. He introduces the hierarchical organization of sensory systems, where information travels from receptors through thalamic relay nuclei, to primary and secondary sensory cortices, and finally to association cortices. This hierarchy reflects increasing specificity and complexity of stimuli processing. The difference between sensation (detection at sensory organs) and perception (interpretation in the brain) is also emphasized. Additionally, sensory systems feature parallel processing, where signals split into multiple pathways, ensuring complex outputs. The principles of hierarchy, functional segregation, and parallel processing are foundational to how sensory systems shape our perception of reality.

05:04

✋ The Complex Nature of the Somatosensory System

Touch is explored in detail, focusing on its diverse origins in the somatosensory system, which encompasses three subsystems: the exteroceptive (external stimuli on the skin), proprioceptive (body position and balance), and interoceptive (internal body information). The exteroceptive system, responsible for tactile sensations, is divided into three divisions: mechanical (touch), thermal (temperature), and nociceptive (pain) stimuli, all detected by various types of cutaneous receptors. These receptors include free nerve endings, Merkel’s disks, Ruffini endings, and Pacinian corpuscles, each specialized for different types of touch. Signals from the skin are relayed to the brain via dermatomes in the spinal cord and follow pathways like the dorsal-column medial-lemniscus and anterolateral systems to the thalamus and somatosensory cortex, which maps sensations across the body via the homunculus.

10:07

👃 Olfactory and Gustatory Systems: Processing Smell and Taste

This section explores the olfactory and gustatory systems, responsible for processing smell and taste, respectively. Olfaction is driven by volatile molecules that interact with receptors in the olfactory mucosa, sending signals to the olfactory bulb and subsequently to areas in the medial temporal lobes like the amygdala and piriform cortex. This system bypasses the thalamus initially, with signals later reaching the orbitofrontal cortex for odor perception. The gustatory system, on the other hand, involves taste buds on the tongue, which detect five primary tastes: salty, sweet, bitter, sour, and savory. These signals are transmitted via cranial nerves to the medulla, then the thalamus, and finally to the primary and secondary gustatory cortices, where the perception of taste occurs. Both systems are crucial for interpreting the chemical environment around us.

Keywords

💡Hierarchical Organization

Hierarchical organization refers to the structured process in which sensory information is processed in increasingly complex and specific stages. The video explains that this applies to all senses, as signals move from receptors to higher brain regions, such as the primary sensory cortex and association cortex, with each stage adding more complexity to the information.

💡Sensation

Sensation is the detection of a stimulus that occurs at sensory organs, like the eyes, ears, or skin. In the video, sensation is contrasted with perception, which occurs in the brain. For example, the auditory system detects sound waves as vibrations in the ear, which then become neural signals.

💡Perception

Perception refers to the brain's interpretation of sensory information, leading to the subjective experience of reality. The video emphasizes that while sensation happens at sensory organs, perception occurs in the brain, allowing us to understand and interact with the world around us. This process happens in all sensory systems.

💡Parallel Processing

Parallel processing is the phenomenon where a single sensory signal is split into different pathways, allowing multiple aspects of the stimulus to be processed simultaneously. The video mentions this in the context of sensory systems, where different pathways analyze stimuli, such as the dorsal and ventral streams in vision or auditory pathways.

💡Auditory System

The auditory system is responsible for hearing, which involves the detection and processing of sound waves. The video outlines how sound vibrations are transmitted through the ear, from the tympanic membrane to the cochlea, and how different frequencies stimulate various parts of the basilar membrane, sending signals to the brain for auditory perception.

💡Somatosensory System

The somatosensory system is responsible for sensing touch, pressure, temperature, and pain. The video breaks this down into three subsystems: exteroceptive (touch), proprioceptive (body position), and interoceptive (internal body states). The system relays information from receptors in the skin to the brain via the spinal cord.

💡Cutaneous Receptors

Cutaneous receptors are sensory receptors found in the skin that respond to various stimuli such as pressure, temperature, and pain. The video describes several types, including free nerve endings (for pain and temperature), Merkel's disks (for skin indentation), and Pacinian corpuscles (for sudden skin displacement).

💡Olfaction

Olfaction, or the sense of smell, is the sensory system that detects airborne chemicals. In the video, olfaction involves receptors in the olfactory mucosa that send signals directly to the olfactory bulb and subsequently to areas like the limbic system, linking smell to emotions and perception.

💡Gustation

Gustation is the sense of taste, which detects chemical compounds in food through taste buds on the tongue. The video explains how different taste receptors are sensitive to basic tastes such as salty, sweet, sour, bitter, and savory, and how these signals are transmitted to the brain for flavor perception.

💡Thalamus

The thalamus is a key relay center in the brain that processes sensory information before it is sent to the cortex. The video explains that most sensory signals pass through the thalamus (e.g., in the visual, auditory, and somatosensory systems), where they are relayed to primary sensory cortices for further processing.

Highlights

The brain receives information from sensory organs through mechanisms similar to visual processing.

Sensory systems feature hierarchical organization, going from receptors to thalamic relay nuclei to primary, secondary, and association cortices.

Sensation and perception are distinguished as detection at sensory organs versus integration and interpretation in the brain.

Parallel processing is a key feature of sensory systems, allowing signals to produce multiple effects simultaneously.

Auditory processing involves sound waves stimulating the tympanic membrane and traveling through the ear's structures to reach the brain.

The cochlea's internal membrane, the organ of Corti, is the primary auditory receptor organ, with hair cells encoding different frequencies.

Sound information follows a pathway through the cochlear nuclei, superior olives, inferior colliculi, and medial geniculate nuclei to the auditory cortex.

The auditory system features anterior and posterior pathways, similar to dorsal and ventral streams in visual processing.

Somatosensation originates from exteroceptive, proprioceptive, and interoceptive systems, with a focus on tactile stimuli from the exteroceptive system.

Cutaneous receptors, including free nerve endings, Merkel's disks, Ruffini endings, and Pacinian corpuscles, contribute to touch perception.

The somatosensory system's pathway includes nerves relaying information to the spinal cord and then ascending through various tracts to the brain.

The primary somatosensory cortex is organized as a homunculus, mapping body parts based on the amount of cortex dedicated to them.

Olfactory processing involves volatile molecules interacting with receptors, sending signals through the olfactory bulb to brain regions such as the amygdala and piriform cortex.

Gustatory processing occurs via taste buds detecting salty, sweet, bitter, sour, and savory flavors, with signals traveling to gustatory cortices.

Overall, sensory systems are hierarchical, functionally segregated, and parallel, contributing to our perception and experience of reality.

Transcripts

Professor Dave again, let’s revisit the senses.

We just went through an overview of how visual information makes it to the brain, so what

about the other senses?

Once again, we looked at the ears and nose and tongue from an anatomical standpoint in

the anatomy and physiology playlist, so make sure to view this tutorial if you’re rusty

on some of the terminology.

Otherwise, let’s go ahead and take a look at the mechanisms by which the brain receives

information from these sensory organs, just like we did for the eyes.

When we looked at visual processing, we learned about hierarchical organization.

We saw how light is received by photoreceptors and transduction occurs.

This results in neural signals which travel to the lateral geniculate nuclei, then to

the primary visual cortex, then the secondary visual cortex, and then the visual association cortex.

A similar kind of hierarchy applies to all the senses.

We can show this in a more general way, going from receptors to some thalamic relay nuclei,

to a primary sensory cortex, then secondary, finishing up at an association cortex.

As information travels through this pathway, the neurons exhibit greater specificity and

complexity in their response to stimuli, demonstrating functional segregation, where each tier performs

different kinds of analysis.

We can also make the distinction between sensation and perception.

Sensation involves the detection of a stimulus, which occurs at the sensory organs, while

perception involves the integration of information and its interpretation, which happens exclusively

in the brain.

Sensory systems heavily feature parallel processing, such that a signal can be split up into parallel

pathways, and thus produce multiple effects.

Putting this together, we can say that any sensory system is hierarchical, functionally

segregated, and parallel.

The combined activity in all of these regions is what generates perception, and therefore

our subjective experience of reality.

Let’s apply this understanding to each sensory system, starting with the auditory system.

As we know, sound involves the vibrations of molecules, typically the molecules in the

air, and these vibrations produce longitudinal waves that interact with the ears in such

a way so as to produce perception that we call hearing.

Just the way that vision is limited to a specific region of the electromagnetic spectrum, hearing

is also limited to certain frequencies, and for most people that would be from around

20 hertz to 20,000 hertz, where hertz are inverse seconds, meaning a particular number

of cycles or wavelengths per second.

Most of the sounds we hear around us produce very complex waveforms, as they are combinations

of a number of different sine waves.

We’ve already learned about the tympanic membrane, or eardrum, found in the inner ear,

and we know that this is the structure that sound waves stimulate to allow for hearing.

The vibrations travel through these three small bones, into the oval window, and then

to the cochlea.

The internal membrane of the cochlea is the organ of Corti, which is the auditory receptor organ.

Vibrations cause hair cells in this region to stimulate firing in the axons of the auditory nerve.

Different frequencies of sound stimulate hair cells at different points along the basilar

membrane, and this information is encoded in the resulting signal.

This signal travels to the cochlear nuclei, then to the superior olives, then the inferior

colliculi, and then the medial geniculate nuclei of the thalamus.

From here, as expected, information heads to the primary auditory cortex, secondary

auditory cortex, and auditory association cortex.

Just like the dorsal and ventral streams for visual processing, there seems to be an anterior

auditory pathway leading to the prefrontal cortex, and a posterior auditory pathway leading

to the posterior parietal cortex.

Much about the auditory system is still being actively researched, so let’s move on to

the somatosensory system.

The sense of touch is not as simple to define as the others, which each have a distinct

organ responsible for producing sensations.

Somatosensations actually originate in three different ways.

The exteroceptive system senses things that touch your skin, the proprioceptive system

monitors the position and balance of your body, and the interoceptive system provides

general information about things inside the body.

Because we typically associate the sense of touch as things that we touch with our hands

or that brush up against our bodies, let’s focus on the exteroceptive system.

This has three divisions.

One perceives mechanical stimuli, which is the more rigidly tactile division, one perceives

thermal stimuli, or changes in temperature, and one perceives nociceptive stimuli, which

means pain.

All of this is due to cutaneous receptors that sit in the skin all over the body.

These can be of a few different varieties.

The simplest are just free nerve endings that detect temperature changes and pain.

From there we can see Merkel’s disks, which respond to skin indentation, and slightly

larger Ruffini endings, which respond to gradual skin stretching.

Finally the largest are the Pacinian corpuscles.

These respond to sudden displacements of the skin rather than constant pressure.

The difference between these functionalities helps distinguish the difference in sensation

between being tapped on the shoulder versus faintly feeling your clothes on your body

throughout the day.

So how does information get from these receptors to the brain?

Nerves relay this information to the spinal cord via dorsal roots, and each section of

the body that is innervated by the dorsal roots of a particular segment of the spinal

cord is called a dermatome.

We can see all the dermatomes of the human body here, and the specific vertebrae they

correspond to, although there is considerable overlap.

From here, information will head up one of several pathways to the brain, and the two

major ones are the dorsal-column medial-lemniscus system, which typically carries information

about touch and proprioception, and the anterolateral system, which typically deals with pain and

temperature.

For the first of these, neurons ascend in dorsal columns and enter the brain at the

dorsal column nuclei.

They then cross over to the other side of the brain, ascend in the medial lemniscus

towards the ventral posterior nucleus of the thalamus, and then typically project to the

primary somatosensory cortex.

The anterolateral system is comprised of the spinothalamic tract, the spinoreticular tract,

and the spinotectal tract, which go up the spinal cord all the way to the thalamus, where

information is then distributed to the somatosensory cortex.

The primary somatosensory cortex is organized according to a map of the surface of the body,

so specific regions correspond to specific body parts.

This is illustrated on the somatosensory homunculus, which designates the corresponding regions,

with body parts shown in sizes that are proportional to the amount of the cortex that is dedicated

to them.

It is clear that certain areas get the bulk of the focus, like the hands, as well as the

lips and tongue, for obvious reasons.

Signals that travel from these body parts to the somatosensory cortex are then shuttled

to areas of the association cortex in the prefrontal and posterior parietal cortex.

Finally, let’s take a look at smell and taste, also referred to as olfaction and gustation.

These are similar senses in that they monitor the chemical environment.

Whereas vision utilizes light as stimulus, these senses use molecules.

When we inhale through the nose, molecules in the air interact with receptors in the nose.

Any substance that is volatile, meaning it will readily enter the gas phase, can potentially

interact with these receptors and send a signal to the brain such that it can be perceived.

Likewise, when food is placed on the tongue, different types of molecules can interact

with receptors called taste buds and send signals to the brain such that we can perceive

a particular flavor.

We introduced these structures in the anatomy and physiology course, so check back there

for the basics.

To connect things to the brain, for olfaction we will notice the olfactory receptor cells

sitting in the olfactory mucosa, with axons that lead to the olfactory bulb.

The clusters of neurons near the surface are called olfactory glomeruli.

Each of these receives input from a few thousand receptor cells containing the same type of

receptor protein, of which there are about three hundred fifty.

From here, neurons project to the brain via olfactory tracts, without any thalamic relay

cells in between.

They land in regions of the medial temporal lobes, like the amygdala and the piriform cortex.

From there, some signals spread around the limbic system, and some go through the thalamus

to the orbitofrontal cortex, right behind the eyes.

We believe the first of these pathways regulates emotional response to odors, whereas the latter

is associated with general odor perception.

With the tongue, we are probably familiar with taste buds, which are receptors found

in clusters of fifty to a hundred.

There are only a few different taste receptors, which correspond to salty, sweet, bitter,

sour, and savory flavors.

This is in contrast with odor receptors which can be involved in combinations that yield

one trillion unique odors.

Signals originating here in the tongue will leave the mouth along several cranial nerves

and head to the solitary nucleus of the medulla, then on to the ventral posterior nucleus of

the thalamus, and then to the primary gustatory cortex, and the nearby secondary gustatory cortex.

We will expand on all of these pathways in the future, for now, let’s move on to some

other topics.