Other Sensory Receptors
Modules 7.1-7.3

 

Figure portraying the sound waves of compressed and decompressed air molecules radiating outward from the vibrating source

 

Figure showing the major features of sound waves – the 2 we will talk about are

1) the amplitude of the waves (how big are the waves of air molecules, which determine the loudness that we perceive

2) the frequency of the waves per second (how fast is the original vibration and the waves it produces) which determines the pitch that we perceive. Frequency may be expressed as cps (cycles per second) or “Hertz”

 

Diagram of the Structure of the Ear (Outer, Middle & Inner Ear)

Sound waves are focused into the external auditory canal by the somewhat funnel shaped pinna (outer ear) and cause vibration of the tympanic membrane (or eardrum) at the end of the canal. The backside of the eardrum is attached to a series of 3 tiny bones (the “ossicles” or, individually, the hammer, anvil and stirrup shaped bones) which reach across (kind of like the bones of your arm) the air-filled middle ear, so that the last bone presses against the fluid filled inner ear (or cochlea) at a point called the oval window.

With each movement of the eardrum the stirrup presses against the cochlea causing the fluid inside to move (kind of like pressing on a water balloon), and when the eardrum vibrates back in the opposite direction, so does the stirrup, and the fluid inside the cochles sloshes back to its original location. The cochlea is shaped like a snail shell – a long spiral – and the fluid movements are from the wide beginning part of the spiral towards the inner most part of the spiral (and then sloshing back again). Each different frequency of sound produces  a slightly different pattern of fluid movement inside the cochlea, with the “peak” of the fluid wave occurring at a different point within the length of the cochlea spiral.

 

Figure of interior of cochlea : If we could “unroll” the spiral shape of the cochlea and then cut a cross section through it to look at the interior,  we’d see that inside there are membranes that divide the interior into 3 lengthwise “canals”, with the auditory receptors (hair cells) lined up on the floor (basilar membrane)  of the middlemost of these canals. The fluid movements down the length of the cochlea cause this basilar membrane to move up and down on the surface of those “waves”.

 

Detail of the arrangement of the Hair Cells on the Basilar Membrane: The hair cells are lined up in a structure called the Organ of Corti, which sits on the basilar membrane and runs the entire length of the cochlea spiral. Hair cells are called that because they have hair-like structures coming out of the top of each cell. The tips of those hairs just barely touch an overhanging membrane (tectorial membrane) when no sound is entering the ear.

(Imagine that someone standing still on a trampoline (basilar membrane) in a roofed carport (roof= tectorial membrane) can just barely touch the roof with their arms (the hairs) completely extended. But if they bounce on the trampoline (i.e. when the basilar membrane moves up and down because of fluid waves) they would be able to touch the roof more firmly and in fact would have to bend their arms to prevent them from smashing against the roof . Similarly the hair of the hair cells are pressed  and bent against the tectorial membrane by the fluid movements triggered by sounds. That pressure is what actually generates electrical changes in these receptors cells which can then be transmitted to the brain, alerting it  about the presence of a sound.

 

Auditory Hair Cell Closeup Photograph


Photo of many hair cells lined up on Basilar Membrane

 

Figure portraying

Fluid Waves Traveling Down Length of Cochlea Causing Basilar Membrane Movement Up and Down

 

Different Pitches MaximallyActivate Different hair cells down length of cochlea (because of fluid movement differences). High pitches activate hair cells at the beginning of cochlea (close to oval window), middle pitches activate hair cells about midway down the length and with low pitches the peak of the “wave” and hair cell activation reaches the inner most tip of the cochlea spiral.

 

Photo: Normal & “Trampled” Hair Cells Exposed to Loud Sounds (hair cells get squashed down by excessive exposure to loud sound (like a rock concert) like grass gets trampled at a crowded picnic. (If this is an occasional event they perk back up, but repeated abuse can permanently damage hair cells, just like the grass in a heavily used path eventually dies.) If the noise is always a specific frequency (e.g. the frequencies of a jet engine), only the hair cells activated by that particular frequency will be damaged.

 

Sound Localization

      Relies on brain detecting differences in what 2 ears hear to locate sounds on left or right

   Intensity differences (high frequencies especially)

   Time of onset differences (any)

   Phase differences (low frequencies especially)

If you only had one good ear you could move your head to see which direction is louder.

 

      Tell front sounds from back sounds because of pinna-related differences (sounds from behind us are more muffled)

 

Air Pressure Must be between middle ear and outside world balanced for Normal Hearing

(e.g. it may get unbalanced when you go up or down in an airplane, and your hearing will get more and more muffled until you “pop” your ears to equalize the pressure on both sides of the eardrum.

 

Types of Deafness

      Conductive Deafness – auditory stimulus does not pass normally through middle ear to cochlea

      Nerve Deafness – deafness due to damage to inner ear hair cells or auditory nerve

 

Knowing the Arrangement and Functioning of the Hair Cells Allowed the development of Cochlear Implants (electrodes fed into cochlea to directly stimulate auditory nerves in those without functioning hair cells.

 

Auditory nerves enter brainstem (auditory input processed in several brainstem areas (superior olive of medulla, inferior colliculus of midbrain, medial geniculate nucleus of thalamus), before reaching the auditory cortex in the top gyrus of the temporal lobe (where our conscious awareness and analysis of the sound takes place). The strip of primary auditory cortex is organized in a “tonotopic” fashion (from cells that process deep bass sounds to cells that process high pitches)

 

Vestibular System – Another Set of Fluid Filled Structures of the Inner Ear, Just Above Cochlea

      Sense movement of head (and body)

      Use that input to :

   Help maintain balance

   Control eye movements when head moves

n      Contributes to our motor control

 

Vestibular System Figure showing 3 semi-circular fluid filled canals, each oriented in a different plane, but all joined,at their base to 3 sphere- shaped structures. (Imagine you had a mini-boom box with sphere shaped speakers on either side. There are 3 large semi-circular handles on the boom box – one vertically oriented handle on the top which attaches at the front and the back of the top panel, one vertical handle sticks out from the side of the box, attaching to the top and bottom of the back panel, and a third handle is a horizontal handle on the back of the box, attaching parallel to the bottom)

 

There are hair cells located within the end of  each of the semicircular canal, close to the junction with the spherical structures. With each movement of your head fluid sloshes within your semicircular canals, more so in particular canals depending on the movement. When you spin on your chair one canal is activated, when you bend forward another is activated, when you tilt sideways another is maximally activated. (so canals are sensitive to spinning, tilting, bending). Fluid movement causes friction against hairs cells, initiating electrical messages (about position and movement), similar to transduction in the cochlea.

 

Rocks in Your Head

 

The sphere shaped structures also contain a row of hair cells inside. In one sphere (the utricle) that row of hair cells is horizontally positioned on the floor of the sphere and in the other (the saccule) the row of hair cells is vertically oriented, with the hair cells lined up on the side of the sphere. In both spheres the tips of the hair cells are embedded in a jelly like layer with rock-like crystals stuck to the surface of the jelly. Each time you move forwards or backwards (in your car for example) those rock crystals drag in the opposite direction, causing pull or friction against the hair cells beneath them. (Analagous to how, whatever I have setting on the seat of my car tends to move back on the seat when I accelerate and forward on the seat when I brake – the rock crystals in the utricle are doing the same. The crystals in the saccule drag across their hair cells when you go up or down in an elevator, allowing you to “feel”  which way you are going.

 

Vestibular overstimulation triggers motion sickness and/or dizziness.

      Vestibular malfunction can cause Meniere’s Disease- extreme dizziness, nausea, vomiting, dysequilibrium, difficulty moving because of faulty balance and postion perception.

 

We have a variety of cutaneous receptors that respond to touch, temperature pain, etc.

Sensations from the body feed into the dorsal roots of our 31 pairs of spinal nerves (and 1 pair of cranial nerves) as we described earlier this semester. Each pair of spinal nerves collects sensations from its specific “slice” of the body surface. This “territory” is known as its dermatome (“skin region”). We are going to concentrate on pain.

 

The Experience of Pain

 

      Tissue injury leads to release of irritating chemicals (histamine, prostaglandins & others) which activate pain receptors and also make receptors more sensitive; in addition some receptors specifically activated by high heat or acid

      Receptors release glutamate (when pain is mild) & Substance P (when pain is more intense)

      Experience influenced by other sensory inputs:   Melzack & Wall’s Gate theory or gate-control theory of pain: the message sending by the neurons receiving those glutamate and Substance P transmissions mentioned above can be modified by nearby tiny inter-neurons. If those inter-neurons are activated by either 1) other incoming non-pain sensory messages OR 2) pain suppression commands coming down from the brain OR 3) the presence of opiate drugs or natural endorphins, the inter-neurons can inhibit or decrease the number of pain messages relayed on to the brain (“closing the gate” to some of the pain messages). The neurotransmitter released by those inter-neurons is enkephalin.

 

 

 

Figure showing destination of incoming body sensations: Somatosensory Cortex

 

Taste

 

Taste buds containing the taste receptors are located along the sides of the individual “bumps” or Fungiform Papillae that we see on the surface of our tongue.

 

Figure of Taste Buds Along Sides of Papilla

Individual taste receptors have hair like cilia which stick out of the taste bud into the crevice between adjacent papillae, where those cilia are bathed by saliva or liquids in the mouth.

 

      Many receptors in a bud

      Replaced every 10-14 days like skin cells

      Have excitable membranes and release transmitter like neurons

      Have receptors sensitive to salty, sour, sweet, bitter and “umami” (first 2 open ion channels, last 3 activate G protein associated metabotropic receptors)

 

Taste

      Humans vary in # of  taste buds and taste sensitivity

    E.g. genetically based difference in ability to taste the bitter compound PTC (see also p.10)

    Low sensitivity (tt), medium (Tt), and high sensitivity (TT)

    We also vary in the number of fungiform papillae & taste buds.

    About 25% have less than 1000 (“nontasters”), 25% have 10,000+ (“supertasters”), and about 50% of us are in the middle (“tasters”)

    Supertasters are oversensitive to sweet, bitter, and capsaicin (hot pepper) and fatty feel of food.

 

Taste Cortex?

      Some taste messages go to the tongue region of somatosensory cortex, but fMRI studies show that the greatest activation from taste occurs in a cortical region hidden deep in the lateral fissure: the insula

 

      Experiencing flavor requires both taste and smell.

 

Figure showing location of Olfactory Receptors in patch of nasal membrane at the very top of nasal passages.

 

Olfactory Receptors also have cilia – these stick out from the nasal membrane so that molecules of chemical in the air entering our nasal passages can bind to specifically shaped receptors on the cilia. The chemicals that produce different smells have different shapes.

 

Cluster of axons from olfactory receptors pass through tiny pinpoint holes in the bone beneath the frontal lobes, and synapse in the long skinny “olfactory bulbs” on the bottom of the frontal lobe (not part of frontal cortex – separate ancient structures you can lift up from the surface of the frontal lobe – about ¼ inch wide and 3 inches long, attaching to limbic system structures.

 

Olfactory Nerves in a precarious position in head injuries because the brain moves within skull and can shear off those axon connections in car accident, for example.

 

The VNO

      A 2nd (unconscious) olfactory system for sensing pheromones (pheromones are low-level chemical signals released by 1 member of a species that influence the physiology and/or behavior of another member of species)

      Prominent in other species; almost gone in humans

Human Pheromones

      Pheromone inducing menstrual synchrony in women living together

      Male pheromone stimulating menstrual regularity in females