Nervous system II 3/3 - Biology As Poetry

Special senses

Balance, hearing, sight, smell, and taste.

The special senses are all associated with a small number of somewhat complex sensory organs. This contrasts with various other senses – collectively, general senses – such as the sense of touch or our ability to experience pain, which are associated with large numbers of comparatively simple receptors that are found more or less in isolation from each other.

Note that both balance and hearing are associated with the same organ, the ear. Sight of course is associated with eyes while smell with the nose and taste, at least in humans, with the tongue.

Equilibrioception

Sense of balance.

The sense of balance requires a coordination of perception of where the parts of the body are located in space (proprioception), where the body is located with regard to visually observable things (perception of visual cues), and perception of where the head is located relative to the center of the Earth as well as its acceleration (as mediated by the vestibular system).

The body is able to remain upright while standing or walking (or running) due to the coordination of these various systems, and the body mostly unconsciously makes adjustments to the action of various muscles to resist the otherwise inevitability of falling over.

Vestibular system

Inner ear associated means by which mammals sustain their balance.

The vestibular system consists of a combination of semicircular canals and otolith organs, both associated with the inner ear. The vestibulo-ocular reflex maintains a clarity of vision by helping to coordinate head movement with eye movement, resulting in a stabilization of images on the retina.

One need only compare the jostling of a head-mounted camera while walking to the stability of one's vision under identical conditions to appreciate the degree to which we are capable of stabilizing our own visual perception while our bodies otherwise are moving. An unaided video camera is completely lacking in this regard.

In the above video (top) I take a walk in Columbus (Ohio), South from the Ohio State University campus, to see how well I could reduce, with some effort, the moving about of the camera's field of view while I walked, i.e., as one might compare to the stability of one's own visual perception as due to the action of the vestibulo-ocular reflex. For the sake of comparison, the video found on the bottom I've put through YouTube's image stabilization software (level 10). I also "Auto-fixed" the lower video solely for the sake of aesthetics. The effect of stabilization of videos using software is that the edges of the video are cropped. Nonetheless, as we don't perceive the world as bouncing about as we walk, but instead closer to the view seen in the lower video, it should be obvious that the impact of the vestibular system on our vision, while moving, must be fairly dramatic.

Semicircular canals

Three tubes found in each ear that detect the acceleration of the head.

The semicircular canals are literally three canals or tubes that form semicircles. These semicircles are oriented at angles to one another so that they can detect acceleration (that is, changes in speed or direction) along all three geometric axes, x, y, and z. These are located within the inner ear, three per ear, and they function by detecting the movement of liquid found within the canal relative to the canal.

Acceleration of the head in a given plane generates movement in those canals that are oriented such that liquid will move, but not in all of the canals at once. In this way it is possible for the brain to identify in what geometric plane or planes acceleration has occurred, which can be very helpful for keeping balance particularly given rapid movement that involves also rapid changes in direction.

The above video describes the means by which movement of fluid within the semicircular canals is detected.

The above video considers more than just the semicircular canals, and is a bit disjointed particularly towards the end, but nonetheless provides a good introduction to how the semicircular canals function.

Otolith organs

Means by which changes in head position are detected such as relative to the center of the Earth.

The otolith organs possess otoliths which are tiny stones of calcium carbonate. These stones are able to move in response to gravity. As the head changes in orientation, the otoliths are consistently pulled downward, but the direction that is downward changes.

Technically, these are responses to linear acceleration, such as the acceleration provided by gravity. The result is the bending of receptor hairs, which provide information to the brain that allows perception of head orientation in relation to linear acceleration (i.e., such as the pull of gravity).

The names of our otolith organs are the saccule and utricle and these are found in the vicinity of the semicircular canals, that is within what is known as the bony labyrinth (osseous labyrinth), though they provide separate information from that delivered by the semicircular canals. In particular, rather than having hair cells that respond to the movement of fluid they instead have hair cells that respond to the direction of gravity.

Another perspective is that otolith organs detect a more or less constant force of acceleration, such as that stemming from the pull of gravity, while the semicircular canals detect changes in acceleration, such as occurs when changing direction.

The above video ends abruptly, but does provide an indication of how the otolith organs function.

Tympanic membrane	The site of the first step in the conversion of vibrations of sound into vibration of tissue.
	That site, in tetrapods (land-dwelling vertebrates), is the eardrum, or tympanic membrane. The eardrum vibrates in response to the vibrations of air, i.e., sound, and converts those vibrations into movement of ossicles, the bones of the inner ear.

Eustachian tube

Means of air pressure equalization as well as drainage of the middle ear.

The Eustachian tube connects the middle ear to the cavity connecting the nasal cavity and throat, collectively the nasopharynx. It passes as an elongated opening – that is, as a tube – through both bone and cartilage in making this connection.

The tube is only episodically open, such as in the course of swallowing or yawning, which allows periodic equalization of the pressure between the middle ear and the atmosphere. Inability to open the Eustachian tube – such as due to tissue swelling within the tube as can occur particularly with small children – can lead to otitis media, that is, a middle ear infection.

The above video is a fairly graphic depiction of how pressure equalization is accomplished via the opening of the Eustachian tube.

Cochlea

Inner ear organ which converts fluid movement that originated as sound vibrations into action potentials.

The cochlea is a spiral-shaped organ that is a portion of the bony labyrinth of the inner ear. Within the cochlea are three fluid filled chambers, two of which are continuous. Vibrations transmitted from the middle ear begin in the cochlea, and thereby the inner ear, at what is known as the oval window.

These vibrations then continue along the length of the cochlea within the scala vestibuli, and then double back towards what is known as the round window within the scala tympani. Between these two chambers lies the cochlear duct.

Vibrations within the scala vestibuli and scala tympani are imparted on the cochlear duct, vibrating hair cells and thereby generating the action potentials which then travel along the auditory nerves to the brain. A cochlear implant involves the placement of electrode arrays into cochlea to partially restore hearing.

The above video provides an alternative perspective on the same process, the detection of sound vibrations within the cochlea.

Yet another perspective on the detection of vibrations within the cochlea; if you are going to watch only one of these videos on the mechanics of hearing, perhaps watch this one.

The above video is a better introduction to the ear than previous, but also is about 3× longer.

The above video, titled "What Does A Cochlear Implant Sound Like?", is a bit frustraiting since it doesn't actually attempt to replicate what having a cochlear implant sounds like, though it still does a good job of describing what a cochlear implant actually is.

Eye

Animal light gathering and processing organ.

There exists substantial variation among the eyes of different species. Ours is a complex eye, as too is the eye of an octopus.

In addition to the quality with which our eyes are able to gather as well as distinguish among photons of light, so too our brains devote substantial amounts of processing power towards interpreting as well as understanding the visual world that our eyes make available to us.

The above video takes a look at the more superficial aspects of the eye, that is, the more outside or focusing portions of the eye; it is an outstanding video!

The above video presents more than just a bit of comparative anatomy, here in terms of variation in the functionality and anatomy of animal eyes.

Cornea

Transparent front of the eye.

The cornea seals the eye as equivalent to a transparent covering of skin (which in fact essentially is what the cornea is, i.e., stratified epithelium). What the cornea seals in is the aqueous humor, which is the fluid that fills the eyeball.

The cornea also contributes to the focusing of incoming light. Indeed, the cornea contributes more to the focusing of that light than does the eye's lens.

Due to a high density of nociceptors within the cornea, the cornea is extremely sensitive to pain, which is of obvious benefit as one consequence is an extreme reluctance to do anything that has the potential to damage the cornea.

A scratched cornea is the partial loss of the surface epithelial layer. This loss tends to not only be temporary but also relatively brief since the cornea heals relatively quickly.

Vision

Consequence of the sense of sight.

Vision is a consequence of the action of eyes in combination with interpretation provided by both the eye and the brain. The latter, interpretation, can be viewed as a kind of data processing

We have two eyes and these are forward facing, resulting in us in a stereoscopic perspective of the world, that is, we perceive the world as three dimensional. Our vision also is in color, owing to the presence of three types of cone cells in our retina, each of which can detect a different color (which are red, blue, and green).

The above video provides a wonderful overview of how the eye works, concentrating on the retina.

Visual receptors	Light sensitive cells that are capable of converting light detection into some form of information.
	In humans, the visual receptors are predominately rod cells and cone cells that are found in association with the retina of the eye. Photons striking these cells can give rise to action potentials that are relayed – through the optic nerve – to the brain where interpretation of this light-generated signal occurs.

Retina

Location of the visual receptors of the eye.

The retina is the location of the rods and cones of the eye. Between the visual receptors and the source of light, i.e., the cornea and lens, are the associated neurons, making it relatively easy to mistake the orientation of the various cells involved in visual reception, which basically is backwards from what otherwise would "make sense".

The axons of the neurons that are known as ganglion cells then pass along the surface of the retina until those axons combine to form the optic nerve at what is known as the optic disc. The latter is also known as the blind spot, or physiological blind spot, due to its local absence of visual receptors.

The above video discusses the anatomy along with cells associated with the retina; warning, this is all pretty complicated stuff.

Rods

Low light visual receptors of the eye.

The rods do not distinguish light in terms of color but instead are optimized for the reception of large numbers of photons. The rods are much more sensitive to individual photons and also pool their detection across multiple cells in terms of the brain's perception of the acquired light, resulting in much greater light sensitivity than one sees with cones but also much lower image resolution.

Rods also respond more slowly to reception of photons than do cones. The rods, further, are concentrated on the outer portion of the retina, i.e., that portion responsible for peripheral vision.

Cones

Visual receptors of the eye specialized for distinguishing among colors.

In comparison with the rods, the cones have much less sensitivity to photons. Cones in addition – and particularly within the fovea where focused light is concentrated on the retina – can be singly synapsed such that the afferent signal generated by such a cell ultimately is recognized by the brain as more or less a single, high-resolution pixel of visual perception.

That is, for reasons that are independent of the ability of cones to distinguish among colors, cones also are associated with a visual acuity that is not similarly observed with rods, and this is so since unlike cones, often multiple rods synapse with individual ganglion cells, resulting in much lower image resolution. Together there are three types of cone cells, with sensitivities concentrated in light wavelengths corresponding to blue, green, and red.

Optic nerve

Neural connection between the eyes and the brain.

The optic nerves house the axons of the ganglion cells of the retina. These axons pass from the retina through what is known as the optic disc, a.k.a., the physiological blind spot of the eye. The nerve then splits with one half of the optic nerve associated with each eye going to one hemisphere and the other to the other hemisphere.

The result is that approximately one half of the visual field of each eye is interpreted by one hemisphere, and the other half the other. Indeed, the left side of the field of vision of each eye is routed to the left hemisphere of the cerebral cortex while the right side of each field of vision is routed to the right hemisphere.

Big error early on, but the narrator recovers! Nevertheless this video does a very good job showing what goes where (retina to brain). The last example, though, is a lot more obvious than the narrator suggests…

Focusing particularly on vision, the above video considers what the brain does with the information that it receives from your senses, constructing a sense of what is going on in the world around it, that is, perception, from what otherwise would be numerous disparate pieces of incoming information, such as received via the optic nerve.

The above video provides an introduction to all of the other special senses, that is, that are in addition to vision, with brief reference to our sense of touch as well.

Olfaction

Perception of odors.

Olfaction is achieved in our bodies as the sense of smell. This process is one of detecting the chemical makeup of compounds that are able to dissolve into specialized mucous membranes found in the upper regions of our nasal cavities, that is, within our noses.

The chemicals are received by olfactory receptors which then convey action potentials through a thin, porous bone into our brains to what is known as the olfactory bulb. The olfactory nerve then conveys the information gleaned by the olfactory bulb to the olfactory centers of the cerebrum for higher-level processing.

Of interest, the olfactory receptors in invertebrates such as insects can be found in their antennae.

Taste receptor

Proteins associated with specialized cells found on the tongue that are responsible for detecting the flavors bitter, salty, sour, sweet, and umami.

These taste receptor cells synapse with afferent neurons that carry the reception of these various "tastes" to the brain. The taste of foods, however, involves more than the interaction of taste receptors with dissolved food at the tongue but is also strongly influenced by the smell of food as detected in the nose (and hence why a stuffed nose can interfere with the taste of foods).

The taste umami is obtained via the detection of the amino acid glutamic acid which in salt form is known instead as glutamate (as in monosodium glutamate, i.e., MSG).

Cataract

Loss of transparency of the lens of the eye.

As the lens of the eye consists of protein, a cataract represents an opacity of the otherwise transparent complex of protein making up the lens. This opacity can be partial or instead total, that is, affecting only a portion or instead the entirety of the lens.

Cataracts commonly lead to blindness – and in fact are a leading cause of blindness – but otherwise are routinely treatable, on an outpatient basis, via surgery. The surgery involves a combination of removal of the discolored lens and its replacement with an artificial lens (plastic lens). The key risk factors leading to cataracts are UV exposure such as from sunlight in combination with smoking.

The above video depicts the surgical removal of a cataract and replacement with an artificial lens, here with greater detail supplied during the narration (and a little coughing at the end to really freak you out ;–>).

Hank Green explains why we shouldn't stare at the sun, or even much look at the sun if we can help it.

Glaucoma

Eye disease characterized by excess pressure of the aqueous humor and blindness if left untreated.

The aqueous humor is the fluid filling the front of the eye and the pressure of this fluid can increase if rates of fluid production come to exceed rates of fluid drainage. The consequence of the resulting excessive pressure of the aqueous humor is excessive pressure on the vitreous humor, the fluid that fills the larger chamber of the eyeball as found behind the lens.

Excessive vitreous humor pressure, in turn, can result in death of neurons of the retina and consequent permanent loss of sight. So too glaucoma can result in damage to the optic nerve.

The above video gives an overview of what glaucoma is all about, with also fair bit of eye anatomy.

The above video gives an another overview of what glaucoma is all about, though from a more patient-associated perspective.

Otitis media	Infection of the middle ear.
	The result is an earache, potentially severe, and as particularly affects small children. These infections often are precipitated by Eustachian tube blockage, that is, particularly temporary loss of the middle ear's ability to drain, which also is more likely in small children. Confirmation of diagnosis is usually accomplished via visualization of the tympanic membrane, i.e., the eardrum.

The above video is an introduction to anesthesia, which is very much a manifestion of the nervous system and in a sense an induced pathology.

Special Senses