Animal Sensory Systems

Learning Objectives

  • Distinguish between the functions of classes of sensory receptors (mechanoreceptors, chemoreceptors, photoreceptors, nociceptors)
  • Explain the three general mechanisms by which the magnitude or degree of the stimulus (analog data) is accurately communicated via action potentials (digital signals) to the CNS and identify these mechanisms in example sensory systems
  • Explain, compare, and contrast how stimuli related to touch, hearing, sight, taste, smell, and pain are detected, transmitted, and interpreted by the brain
  • Compare and contrast mechanisms of visual and vestibular perception among different animals lineages

Sensory Processing in Animals

The information below was adapted from OpenStax Biology 36.0 and OpenStax Biology 36.1

The sensory system detects signals from the outside environment and communicates it to the body via the nervous system. The sensory system relies on specialized sensory receptor cells that transduce external stimuli into changes in membrane potentials. If the changes in membrane potential are sufficient to induce an action potential, then these action potentials are then communicated along neurons within the afferent division of the PNS to the CNS for information processing. The CNS integrates and interprets the incoming signals to effect a response to the appropriate body systems via the efferent division of the PNS.

Sensory receptor cells can be:

  • specialized neurons (the receptor cell is also a neuron)
  • specialized sensory cells which synapse with a neuron (the receptor cell secretes neurotransmitters to stimulate changes in membrane potential in the synapsed neuron)

Sensory receptor cells transduce (convert into changes in membrane potential) incoming signals and may either depolarize or hyperpolarize in response to the stimulus, depending on the sensory system. In vertebrates, each sensory system transmits signals to a different specialized portion of the brain such as the olfactory bulb (smell) or occipital lobe (sight), where the signal is integrated and interpreted to effect some sort of response (often motor output) via the PNS.

Different sensory receptor cells are specialized for different types of stimuli, and are categorized by the type of stimulus they detect. Sensory receptor cells include (but are not limited to!)

  • Mechanoreceptors: respond to physical deformation of the cell membrane from mechanical energy or pressure, including touch, stretch, motion, or sound
  • Chemoreceptors: respond to specific molecules, often dissolved in a specific medium (such as saliva or mucus), or airborne molecules
  • Photorecetpors: respond to radiant energy (visible light in most vertebrates; visible as well as UV light in many insects)
  • Nociceptors: respond to “noxious” stimuli, or essentially anything that causes tissue damage
  • Thermoreceptors: respond to heat or cold

All bilaterally symmetric animals have a sensory system, and the development of any species’ sensory system has been driven by natural selection; thus, sensory systems differ among species according to their history of natural selection. For example, the shark, unlike most fish predators, is electrosensitive- that is, sensitive to electrical fields produced by other animals in its environment.

Humans and many other vertebrates have at least five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration.

Analog to Digital: Encoding and Transmission of Sensory Information

Sensory stimuli vary in intensity. For example, a sound can be a whisper or a shout. Yet the stimulus is transmitted via action potentials in the sensory neurons, which are “all or nothing” events that do not vary in intensity. How do we detect and perceive the difference between a shout and a whisper? The intensity or degree of a stimulus is often encoded in three different ways:

  • The rate or frequency of action potentials produced by the sensory receptor. For example, an intense stimulus will produce a more rapid series of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. (Note the intensity affects the rate of production of action potentials, not the speed with which the action potential travels along the axon.)
  • The number of receptors activated. For example, an intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors.
  • Which specific receptors are activated. For example, a low pitch or will initiate action potentials in one set of sensory neurons, while a high pitch will initiate action potentials in a different set of sensory neurons.

Look for these patterns described above as you complete the remaining material on this page.

Survey of Sensory Systems

Below are discussions of several different sensory systems in different types of animals. For each sensory system, be prepared to identify, compare, and contrast each of the following:

  • What is the stimulus and what type of receptor detects it?
  • Is the receptor a receptor cell, or a specialized neuron?
  • How is the stimulus transduced into a change in membrane potential?
  • What happens in the sensory receptor when the stimulus is recognized?
  • How is the intensity/degree/magnitude of the stimulus communicated to the CNS?

Mechanoreceptors: Touch, Sound, Balance

Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.

Touch: the Somatosensory System

The information below was adapted from OpenStax Biology 36.1

Somatosensation is the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations as well. The sense of touch is detected by a variety of different types of mechanorecetpors that are embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system. In fact, what is commonly referred to as “touch” involves more than one kind of stimulus and more than one kind of receptor. Touch in humans includes four primary tactile mechanoreceptors in the skin. You don’t need to know the specific name for each type of touch receptor (shown below), but you should be able to recognize that

  • some types of mechanoreceptors are located near the upper layers of the skin, and are thus more sensitive to lighter touch and are able to precisely localize gentle touch
  • other types of mechanoreceptors are located deeper in the skin, and are thus only activated by stronger pressure and are not as highly sensitive to identify the precise location of the touch.

A light touch activates only the mechanoreceptors near the upper layer of the skin, while a firmer touch activates mechanoreceptors deeper in the skin, in addition to the mechanoreceptors near the surface of the skin. A firmer touch will also activate a greater number of receptors, and may induce more frequent action potentials in the receptors than a lighter touch.,

Four of the primary mechanoreceptors in human skin are shown. Merkel’s disks, which are unencapsulated, respond to light touch. Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, and Krause end bulbs are all encapsulated. Meissner’s corpuscles respond to touch and low-frequency vibration. Ruffini endings detect stretch, deformation within joints, and warmth. Pacinian corpuscles detect transient pressure and high-frequency vibration. Krause end bulbs detect cold. Image credit: OpenStax Biology.

 

Sound: the Auditory System

The information below was adapted from OpenStax Biology 36.4

Auditory stimuli are sound waves, which are mechanical pressure waves that move through a medium, such as air or water. (There are no sound waves in a vacuum since there are no air molecules to move in waves.) Because sound waves exert pressure, sound is detected by mechanoreceptors.

As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude. Three of these are important for understanding how hearing works:

  • Frequency is the number of waves per unit of time, which is heard as pitch. Frequency is also related to wavelength, where high-frequency (15,000 Hz) sounds are higher-pitched and shorter wavelength than low-frequency, long wavelength (100 Hz) sounds. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz; bats, 100,000 Hz; and dolphins 150,000 Hz, and some fish can hear 180,000 Hz.
  • Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume. The sound waves of louder sounds have greater amplitude than those of softer sounds. For sound, volume is measured in decibels (dB). In the figure below, the softest sound that a human can hear is the zero point. Humans speak normally at 60 decibels.

For sound waves, wavelength corresponds to pitch. Amplitude of the wave corresponds to volume. The sound wave shown with a dashed line is softer in volume than the sound wave shown with a solid line. (credit: NIH via OpenStax Biology)

  • Outer ear:
    • Sound waves are collected by the external, cartilaginous part of the ear
    • Sound waves then travel through the auditory canal and cause vibration of the ear drum (tympanic membrane)
  • Middle ear:
    • The eardrum transmits sound to the middle ear by vibrating the ossicles, the three small bones of the middle ear which collect force and amplify sounds.  The three ossicles are unique to mammals.
  • Inner ear:
    • The ossicles transmit the vibrations to a thin membrane called the oval window, which is the outermost structure of the inner ear.
    • The vibrations of the oval window create pressure waves in the fluid inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal
    • Inside the cochlea, the basilar membrane is a flexible membrane that runs the length of the cochlea, and contains the mechanoreceptors called hair cells which transduce sound waves into action potentials. The basilar membrane vibrates in response to sound pressure waves, pressing the hair cells against the tectorial membrane, which physically bends the hair cell cilia, initiating action potentials in the afferent neurons that communicate sounds stimuli to the brain.

Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea. (credit: OpenStax Biology, modification of work by Lars Chittka, Axel Brockmann)

When the sound waves in the cochlear fluid contact the basilar membrane, the basilar flexes back and forth. The basilar membrane’s flexibility changes along its length, such that it is thicker, stiffer, and narrower at one end of the chochlea, and thinner, floppier, and broader at the other end. As a result, different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea, with the stiffer region vibrating in response to high frequency (higher-pitched) sounds, and the more flexible region vibrating in response to low frequency (lower-pitched) sounds. In other worlds, pitch is detected based on which region of the basilar membrane vibrates in response to a sound (and thus which hair cells are activated).

In the mammalian ear, sound waves cause the stapes to press against the oval window. Vibrations travel up the fluid-filled interior of the cochlea. The basilar membrane that lines the cochlea gets continuously thinner toward the apex of the cochlea. Different thicknesses of membrane vibrate in response to different frequencies of sound. Sound waves then exit through the round window. In the cross section of the cochlea (top right figure), note that in addition to the upper canal and lower canal, the cochlea also has a middle canal. The organ of Corti (bottom image) is the site of sound transduction. Movement of stereocilia on hair cells results in an action potential that travels along the auditory nerve. Image credit: OpenStax Biology.

The site of transduction from sound waves to action potentials is in the organ of Corti (spiral organ). How is sound transduced from a wave to an action potential? Within the organ of Corti, hair cells are held in place above the basilar membrane with their hair-like stereocilia embedded in the tectorial membrane above them. When a sound wave flexes the basilar membrane:

  1. The hair cells on the basilar membrane are flexed against the tectorial membrane, which bends the stereocilia
  2. The bending of the stereocilia causes potassium ion channels to open in the cell membrane; the hair cell is bathed in a solution high in potassium, so potassium rushes into the cell through the open channels, depolarizing the hair cell
  3. Just like in a synaptic terminal, membrane depolarization ultimately causes synaptic vesicles in the hair cell to fuse with the plasma membrane, releasing neurotransmitters into the synaptic cleft between the hair cell and its synapsed afferent neuron
  4. If the resulting depolarization is sufficient, an action potential is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated, while pitch is determined by which particular hair cells along the basilar membrane are stimulated

 

Bending of cilia (yellow) in hair cells in the inner ear in response to sound pressure waves results in opening of potassium channels that depolarize the hair cells, cause release of neurotransmitters on the synapsed sensory neurons (shown in blue), and can trigger action potentials (APs) in the axons of those neurons. Image credit: Modification of work by Thomas.haslwanter – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14585601

This video provides a quick overview of mammalian cochlear function in hearing:

Balance and Movement: the Vestibular System

The stimuli associated with the vestibular system are linear acceleration (gravity) and angular acceleration and deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. In vertebrates, gravity is detected through head position. Angular acceleration and deceleration are expressed through turning or tilting of the head.

The vestibular system in vertebrates has some similarities with the auditory system. It utilizes hair cells located within the ear in a structure called the vestibular labyrinth (located adjacent to the cochlea), but it activates them in a different way compared to the auditory system. Hair cells in the vesibular labyrinth detect signals in two ways:

  • Some hair cells lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystalslike tiny rocks, that move in response to gravity. Any time the head is tilted at an angle or is subject to acceleration or deceleration, these crystals cause the gelatin to shift, bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance.
  • Some hair cells project into a gelatinous cap called the cupula. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. When movement stops, the movement of the fluid within the canals slows or stops.

Hair cells in the semicircular canals have projections that extend into cupulas, which move in response to changes in acceleration or deceleration. Image credit: By NASA – Source: The Effects of Space Flight on the Human Vestibular System, an online educational article by the U.S. government’s National Aeronautics and Space Administration (NASA), Public Domain, https://commons.wikimedia.org/w/index.php?curid=6765835

This video provides a quick overview of the mammalian vestibular system:

A similar system involving hair cells and a cupula is present at the lateral line of bony fish, which are used to detect changes in water pressure.

The lateral line extends down the body of bony fishes. The canal of the lateral line is lined with sensory hair cells with projections that extend into cupulas. The canal allows water to enter from the surrounding environment, which can band the cupulas as water pressure changes, triggering action potentials in the sensory neurons synapsed with the hair cells. By Thomas.haslwanter – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=21451808

Many invertebrates detect balance through a structure called a statocyst, a ball-shaped organ lined with inward-facing hair cells and containing statoliths, dense particles similar to calcium carbonate crystals in the vertebrate utricle and saccule. Any movement causes the statoliths to change location inside the statocyst, activating different hair cells as they move.

Statocysts contain inward-facing hair cells (brown) that detect movements of statoliths (purple), dense particles that move in response to gravity. The hair cells are synapsed with sensory neurons (black lines) that communicate sensory information to the brain. Image credit: Davis, W. J. (1968) – http://caspar.bgsu.edu/~courses/Neuroethology/Labs/Images/Statocyst.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=22495818

Photoreceptors: Vision

The information below was adapted from OpenStax Biology 36.5

As with auditory stimuli, light travels in waves. The pressure waves that create sound must travel in a medium: a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no medium; light can travel in a vacuum (this is why we can see stars from space.) The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light: a packet of electromagnetic radiation called a photon. Humans can perceive just a small slice of the entire electromagnetic spectrum, which includes radiation that we cannot see as light because it is below the frequency of visible red light and above the frequency of visible violet light, which are the limits of vertebrate light detection.

Certain variables are important when discussing perception of light:

  • Wavelength (which varies inversely with frequency) is detected as hue, or color. Light at the red end of the visible spectrum has longer wavelengths (and is lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).
  • Wave amplitude is perceived as luminous intensity, or brightness.

Detection of light occurs through photoreceptors, cells that contain pigment-absorbing molecules that absorb light. Photoreceptor cells are typically located in light-collecting organs called eyes. Eyes vary in structure in different types of animals and include:

  • eye cups in flatworms, which are dimple-shaped structures that detect the direction of a light source
  • compound eyes of arthropods, which contain multiple lenses and detect shapes, patterns, and movements
  • pinhole eyes in the nautilus, which contain no lens and forms simple, low-resolution images
  • simple eyes of cephalopods and vertebrates, which contain a single lens and form high-resolution images

Many of these types of eyes, which are present in living organisms today, may have also represented a pathway of evolution from a simple patch of photosensitive cells to simple lens eyes of vertebrates and cephalopods:

By Matticus78 at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2748615

This video describes the evolutionary origins of the human eye:

The vertebrate eye contains the following structures:

  • Sclera: tough outer tissue (white of the eye)
  • Cornea: transparent sheet of connective tissue, functions with the lens to focus light on the retina
  • Iris: pigmented ring of muscle that controls amount of light entering eye
  • Pupil: hole in center of iris
  • Lens: crystalline, curved structure that focuses light on the cornea (by bending, not by moving) in conjunction with the cornea
  • Retina: thin layer of photoreceptor cells and neurons
    • Photoreceptor cells: light-detecting sensory cells
    • Bipolar cells: intermediate connecting neurons
    • Ganglion cells: neurons whose axons project to the brain via the optic nerve
    • Fovea: site of retina with only cones, area of highest visual resolution
  • Optic nerve: axons of the ganglion cells

(a) The human eye is shown in cross section. (b) A blowup shows the layers of the retina. Image credit: OpenStax Biology

The vertebrate eye is pretty good at forming high-resolution images, though perhaps we are a little biased in making that assessment. But it turns out that that the vertebrate eye is actually far from ideal. In fact, the cephalopod eye, which looks the same on the surface but evolved independently, is far better. Here is a comparison between the vertebrate and cephalopod eyes:

  • Cephalopod eyes move the lens to focus (like in a camera), rather than changing lens shape as in the vertebrate eye; the result in vertebrates is age-related loss of resolution as the lens loses flexibility over time
  • Vertebrate eyes have an “inverted” retina, where blood vessels and nerves are in front of the photreceptor cells, unlike in the cephalopod eye where the nerves are behind the photoreceptor cells; the result in vertebrates is the blind spot as well age-related macular degeneration and increased likelihood of retina detachment (a severe medical emergency) due to the loose association between the retina and the back of the eye

The vertebrate and cephalopod eyes are very similar but with key differences, including the relative locations of the photoreceptors and sensory neurons (the retina is “inverted” in the vertebrate eye, resulting in a blind spot where the optic nerve exits the retina), and the movable lens in cephalopods compared with a flexible lens in vertebrates. Image credit: modification of work by By Philcha, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4612105

 

Regardless of the structure of the eye, all photoreception relies on light-absorbing pigment molecules embedded in the photoreceptor cells. This pigment is called retinal, and it is contained in a protein called opsin. Together, they form a complex called rhodopsin, that allow us to detect light and color. Both the protein and the pigment are essential for this process:

  • The pigment: retinal, reversibly changes shape when it is hit by a photon of light (this process is extremely similar to detection of red vs far-red light by phytochrome in plants.)
  • The protein: opsin, holds the retinal pigment and changes shape/activity when the retinal changes shape in response to absorption of light. Opsins are responsible for our ability to perceive differences in color or hue. Humans have three color-sensitive opsins: S opsin (short-wavelength opsin), M opsin (medium-wavelength opsin), and L opsin (long-wavelength opsin).
  • The photoreceptor cells: rods and cones each contain a unique opsion that causes the cell the cell to be most sensitive to a specific wavelength of light. Keep in mind that the retinal is always the same; it is the opsin that varies in different photoreceptor cells
    • Cones (which are cone-shaped) each contain a single type of color-sensitive opsin, making each cone most sensitive to a particular hue or color of light.  Though we only have three types of cones (due to the three types of color-sensitive opsins), we are able to detect so much variation in hue to do activation of different combinations of cones at the same time. Cones require high-levels of light to work, which is why we can’t perceive color well in the dark. Cones are heavily concentrated at the fovea and are useful for focusing on important visual details.
    • Rods (which are rod-shaped) each contain a fourth type of opsin called rod opsin, which is activated by an intermediate wavelength of light and is capable of working in low-levels of light. Rods are not color-sensitive, but allow us to see in low levels of light. Rods are heavily concentrated at the periphery (outer edges) of the retina, and are useful for detecting movement in our field of vision.

Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected. Image credit: OpenStax Biology

This video provides a brief explanation of how we can perceive so many different colors with only three types of cones:

 

What happens when a photon activates rhodosin?  Unlike all the other sensory systems we have discussed, a rod or cone cell  hyperpolarizes when its rhodopsins are activated by light, and it depolarizes when its rhodopsins are in the dark.  This means that, when in the dark, our rods and cones are depolarized and thus releasing neurotrasnsmitters to their synapsed bipolar cells.  When a rod or cone cell is activated by light, it hyperpolarizes and stops releasing neurotransmitter.

 

Chemoreceptors: Taste (Gustation) and Smell (Olfaction)

The information below was adapted from OpenStax Biology 36.3

Taste, also called gustation, and smell, also called olfaction, are interconnected senses. Both involve molecules of the stimulus entering the body and bonding to receptors, relying on chemoreceptors, receptors that are sensitive to specific chemicals. Just as each photoreceptor cell is sensitive to a specific wavelength of light based on the opsin present in the cell, each chemoreceptor cell is sensitive to a particular molecule based on the protein receptor present in the cell.

Taste: the Gustatory System

The primary tastes detected by humans are sweet, sour, bitter, salty and umami (savoriness, which tends to indicate that a food is high in protein).

Detecting a taste relies on activation of specific chemical receptors in taste receptor cells (gustatory receptors). When the specific chemical (tastant) binds the receptor, the receptor cell becomes depolarized and releases neurotransmitter on its synapsed afferent neuron. As in other sensory systems, specificity in taste occurs because each taste receptor cell has only one type of protein receptor which is sensitive to either sweet, sour, bitter, salty, or umami. The process of depolarization differs based on the specific type of gustatory receptor:

  • A salty tastant (containing NaCl) provides the sodium ions (Na+) that enter the taste receptor cells and excite them directly.
  • Sour tastants are acids cause an increase hydrogen ion (H+) concentrations in the taste receptor cells, thus depolarizing them.
  • Sweet, bitter, and umami tastants cause activation of an enzyme that causes opening of an ion channel, thus depolarizing the taste receptor cells.
  • Spiciness isn’t detected by taste buds at all, but is actually due to activation of pain receptors (nociceptors)! (More on pain perception at the bottom of this page)

The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors (taste receptor cells) that are located within the bumps on the tongue called papillae (singular: papilla). Each taste bud contains all five types of gustatory receptors (the taste map is totally false), which are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore. Tastants must be dissolved in saliva to bind with and stimulate the receptors on the microvilli, which is why the sense of taste isn’t as strong when your mouth is dry.

Pores in the tongue allow tastants to interact with and activate gustatory receptors in the taste bud. Each receptor cell contains the receptors for only one type of tastant, but each taste bud contains all types of taste receptor cells. (credit: OpenStax Biology, modification of work by Vincenzo Rizzo)

 

Smell: the Olfactory System

Flavor includes a lot more than just the five primary tastes; most people can detect a difference between the sweet flavors of different types of fruit rather than detecting all of them as just “sweet.” But the nuance of flavor doesn’t actually come from gustation at all; it comes from our sense of smell. This is why many people temporarily lose their sense of taste when they have a cold or other severe nasal congestion.

All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow us to sense about 10,000 different odors. (As you may be expecting by now, each olfactory receptor cell has only one type of olfactory receptor protein, meaning each receptor cell is specific to only one type of odorant.) For comparison, mice have about 1,300 olfactory receptor types and therefore probably sense many more odors than do humans.

How does the sense of smell work?

  • Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, located at the back of the nasal cavity. The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm2 in humans. Similar to tastants, which must be dissolved in saliva, an odorant molecule must be dissolved in mucus in order to be detected by its receptor.
  • An olfactory receptor, which is a dendrite of a specialized sensory neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds, dogs selectively bred for their sense of smell, have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans.
  • Each olfactory neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and slight variations in the protein sequences make them sensitive to different odorants.
  • Each olfactory sensory neuron has only one type of receptor protein on its cilia, and the receptors are specialized to detect specific odorants, so each olfactory neuron is capable of detecting only a single type of odorant molecule.
  • When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is is depolarized and relays action potentials to the brain.
  • Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.

In the human olfactory system, (a) bipolar olfactory neurons extend from (b) the olfactory epithelium, where olfactory receptors are located, to the olfactory bulb. (Image credit: OpenStax Biology, modification of work by Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist)

Our ability to detect and interpret flavor is due to the combination of gustatory and olfactory senses:

  • Taste receptors are responsible for the sense of how salty, sweet, bitter, sour, or savory a food is, via tastants dissolved in saliva
  • Olfactory receptors are responsible for the flavor of a food, via odorants detected in the olfactory epithelium during chewing, through a process called retronasal olfaction (the flow of air from the back of the throat up to the olfactory epithelium via the back of the nose)

This video review how olfaction works:

Nociceptors: Tissue Damage and Pain

Pain is the name given to nociception, which is the neural processing in response to tissue damage. Pain is caused by both true sources of injury, such as contact with a corrosive chemical, and also by harmless stimuli that mimic the action of damaging stimuli, such as contact with capsaicins, the compounds that cause peppers to taste hot (and which are used in self-defense pepper sprays and certain topical medications). Peppers taste “hot” because the protein receptors that bind capsaicin open the same calcium channels that are activated by heat-sensitive thermoreceptors.

There are many different types of nociceptors and we will not describe them in this course, but the important thing to understand is that different types of nociceptors are activated by different types of tissue damage, including extremes of hot or cold, toxic chemicals, and extreme mechanical deformation such as stretching, cuts, and tears.

Nociception starts at the sensory receptors, but pain does not actually start until it is communicated to the brain; pain is the interpretation of the tissue damage, not the actual stimulus itself. There are several nociceptive pathways to and through the brain, including through the thalamus (like most other sensory systems) and directly to the hypothalamus , which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system, where it can directly activate the fight-or-flight response.

This video discusses nociceptor function while explaining how pain relievers work (the details of how pain relievers work are not relevant to this course, though you may find the discussion interesting):

 

Here are some additional videos to help you put this all together (in a more entertaining way than many of the videos above). Note that these videos do not provide any new information, but they may help you better integrate all the information previously discussed:

This video provides an engaging review of hearing and balance (bonus points if you get the movie reference!)

 

This video provides an engaging review of vertebrate eye function:

 

This video provides an engaging review of olfaction and gustation: