Color Vision (PSL 300) - University of Toronto PDF

Color Vision PSL 300 University of Toronto  Question of the day Computer screens contain only red, green, and blue lights, so how can they show yellow...

Color Vision PSL 300 University of Toronto  Question of the day Computer screens contain only red, green, and blue lights, so how can they show yellow objects? Photopigments undergo chemical changes when they absorb light. They are the non-protein, chromophore parts of chromoproteins (e.g. photoreceptor proteins), but medical writers often use the word to refer to the whole combined molecule (”rhodopsin is a photopigment”). “Opsin” sometimes means the non-chromophore part of a chromoprotein (”rhodopsin is opsin bound to retinal”), but sometimes it means the whole chromoprotein, so that rhodopsin is a kind of opsin. Some say “opsin proteins” bind chromophores. Different opsins (e.g. rhodopsin and cyanolabe) differ in a few amino acids. All human opsins bind (or include) the same chromophore: retinal, whose absorption depends on the protein it binds. In 2014 I said opsins consist of opsin proteins bound to a chromophore (retinal in humans), and that rhodopsin is an opsin, consisting of rhodopsin protein bound to retinal. In 2015 rhodopsin etc. were photopigments.  Outline n Color Wavelength n Color and Photoreceptors Cone sensitivities Trichromatic vision n n Processing Color n n Opponent channels n Color Blindness n Color Constancy Color  Color depends on wavelengths of light Ultraviolet Infrared 400 700 Wavelength (nm) n Light has a wavelength — the distance from one wave peak to the next — and different wavelengths correspond to different colors. The wavelengths we normally see range from 400 nm for violet to 700 nm for red. n We can also see extremely powerful infrared lights, and people who have had their lenses removed can see some ultraviolet. But even so, the visible range is only a tiny part of the electromagnetic spectrum.  Why did evolution give us eyes that see 400–700 nm? Power 250 1000 2000 2500 Wavelength (nm) n The power in sunlight peaks there. Also, Earth’s atmosphere is most transparent to these wavelengths. n n And sea water, where eyes first evolved, is most transparent < 500 nm. continuingeducation.construction.com Color and Photoreceptors  Most humans sense color with 3 kinds of cone n The 3 types are called red, green, and blue. In most people, ~63% of the cones are red, 31% are green, and just 6% are blue. n Each type of cone has its own type of visual pigment. All these pigments are similar to rhodopsin but not identical to it, and so all prefer different wavelengths of light. n Because we sense color with 3 types of cone, we are called trichromats. n Blue cones evolved first, maybe because blue light penetrates sea water better than longer wavelengths. Red and green cones evolved from a common ancestor 35 million years ago.  Different visual pigments prefer different wavelengths Absorbance (%) 100 50 0 400 420 498 533 564 700 Wavelength (nm) melanopsin 479 n The plot shows how much light the visual pigments absorb (as a percentage of the maximum for that pigment) at each wavelength. n Red and green cone pigments prefer yellow and yellow-green light. Blue cone pigment prefers blue. Rhodopsin (black curve) prefers blue-green. Melanopsin (not shown) prefers blue. Bowmaker & Dartnall (1980)  The brain infers color by comparing data from the 3 types of cone Absorbance (%) 100 98% 83% 50 0% 0 400 500 630 700 Wavelength (nm) n E.g. yellow light affects red and green cones but not blue ones, so if your red and cones are hyperpolarized and your blue ones aren’t then your brain perceives yellow. n You can be fooled: a red and a green light, with no yellow, can produce the same cone activities as a yellow light would, and so the brain sees yellow either way.  We can produce any color perception by mixing 3 wavelengths n Any color we can experience corresponds to a pattern of activity in our 3 types of cone. It is possible to produce any such pattern by mixing just 3 colors of light, the primary colors red, green, and blue. n So TVs, computer screens, and projectors can show full color with just red, green, and blue lights — there is no yellow light coming from this slide. 24.media.tumblr.com  Not all creatures are trichromats Mantis shrimp n Most mammals are dichromats, with 2 types of cone, but Old World primates are trichromats, maybe because we eat fruit — “Monkeys are to fruit as bees are to flowers”. n Many birds are tetra- or pentachromats, with 4 or 5 types of cone. For them, color TV would need 4 or 5 kinds of light. n Mantis shrimps have 16 kinds of photoreceptor. ecouterre.com  Not all the colors are in the rainbow n Spectral colors are those that can be evoked by light of a single wavelength. They are the rainbow colors, from violet through blue, green, yellow and orange to red. n Extraspectral colors such as purple or white are evoked only by a mix of wavelengths, e.g. we see purple when 2 or more wavelengths affect red and blue cones more than green cones. en.wikipedia.org Processing Color  Ganglion cell color signals are combinations of cone signals n Some ganglion cells are excited by red light and by green light. They are called R + G cells, or the yellow channel (because red and green make yellow). n Some ganglion cells are excited by red light and inhibited by green (R – G). Others are inhibited by red and excited by green (G – R). These 2 types form the red-green opponent channel. n n Some are excited by blue light and inhibited by red and green, (B – R – G, which is the same as B – (R + G), i.e. blue minus yellow). Others are yellow minus blue. These 2 types form the blue-yellow opponent channel.  Opponent channels are thought to explain afterimages n As you stare at something green, your G – R cells gradually fatigue. When you look away, those fatigued cells are less active than your R – G’s, and so you see red. And similarly for blue versus yellow. n We aren’t sure the responsible cells are in the retina, as there are color-opponent cells in LGN and visual cortex as well. Color Blindness  The most common variant is red-green color blindness Dalton’s theory was that his ocular humors were blue. He left his eyes to science, and the day after his death, his theory was disproved. In 1990 it was found that he had no green cones. n In 1794 John Dalton wrote “I discovered last summer... that colours appear different to me to what they do to others... my brother excepted, who seems to see as I do”. n Today, Daltonism is a name for red-green color blindness, where people have trouble distinguishing those colors. colorvisiontesting.com  Color blindness was the first human trait to be linked to a chromosome n The inheritance pattern of Daltonism is that color blind fathers have color-normal daughters who have color blind sons. n The genes for red and green cone visual pigments lie on the X- chromosome. Problems at these loci underlie 95% of all variations in color vision. (The “blue” gene, on chromosome 7, is more stable). n Women are seldom color blind because if one X-chromosome codes a faulty pigment then the other X-chromosome compensates. If her 2 X-chromosomes code, say, 2 different functional “red” cone pigments then a woman may be a tetrachromat. Color Constancy  We call the intrinsic color of a surface its reflectance n The reflectance of a surface is its tendency to reflect certain wave- lengths of light and absorb others, e.g. a yellow banana reflects yellow light more than other wavelengths; a green banana reflects green. n The reflectance of an object carries information about it, e.g. about ripeness. boswerlk.wordpress.com  The light an object sends to our eyes depends on reflectance and illumination n E.g. if you put a ripe banana in greenish light then the banana’s reflectance doesn’t change (it is still a yellow-colored object) but now it sends mainly green light to our eyes. n Nonetheless our brains can usually infer the reflectance, so we see the ripe banana as yellow even in green light. This crucial ability is called color constancy. www.lottolab.org  A demonstration of color constancy n The 2 pictures show the same cube in yellow and in blue light. You correctly see the blue facets in the left picture as blue, and the yellow facets in the right picture as yellow. n But the light these facets send to your eyes is the same, as you can see in the next slide, where I hide everything except one blue facet on the left and one yellow facet on the right... www.lottolab.org  These 2 facets, one intrinsically blue and one yellow, both send gray light to your eyes n Here I’ve placed a white rectangle with 2 holes in front of the pictures, so everything is hidden except 2 facets: a blue one in the left picture and a yellow one in the right. n Now you can see that both facets send the same, gray light to your eyes. www.lottolab.org  Your visual system infers reflectances by comparing parts of the image n In yellowish light, blue facets reflect a lot of yellow and only a little blue (because there isn’t much blue light), and so they send a grayish mix of wavelengths to the eye. n Your brain sees a lot of yellow throughout the image and correctly infers that the lighting is yellow and the facets are blue. Without the context it can’t deduce lighting or reflectance. hubel.ed.harvard.edu  These comparisons which underlie constancy give rise to illusions n In each vertical pair, the center squares are identical in color, but their surroundings affect our perception of brightness, hue, and saturation.  There are Chevreul illusions for color as for brightness n These uniformly colored bands look redder near their left edges. Reading in Silverthorn’s Human Physiology n 7th edition: Pages 348–349 including Figure 10.31, from the start of the section “Photoreceptors Transduce Light into Electrical Signals” to the start of the subsection “Phototransduction”. n 6th edition: Pages 365–367, from the start of the section “Photoreceptors Transduce Light into Electrical Signals”, and Figure 10.31. Smell and Taste PSL 300 University of Toronto  Question of the day Why do we say chilies are hot and mints are cool?  Outline n Smell n n Olfactory neurons n Projections n Limbic system n Pheromones n Taste n Taste buds n Receptor cells Gut receptors n Projections  Smell and taste are forms of chemoreception n Chemoreception is evolutionarily old: bacteria use it to guide their movements; animals without brains use it to find food and mates. n n Chemoreception may have evolved into chemical synaptic com- munication. Smell  The olfactory receptors are in the olfactory epithelium Olfactory epithelium 2 n This epithelium lies at the top of the nasal cavity, covering ~3 cm in each of the 2 sides. It contains ~10 million receptor cells in total. n n The epithelium is pigmented. No one knows why, but the richness of its color correlates with olfactory sensitivity: in us it is pale yellow, in 1 cats a “dark mustard brown”. 1 Ackermann, A Natural History of the Senses  The receptor neurons are ciliated neurons n Each cell has a single dendrite that extends into the olfactory epi- thelium. There it branches to form nonmotile cilia that increase the surface area of the cell, so it has a greater chance of catching odorant molecules. n n Each receptor cell has (many copies of) one type of odorant receptor molecule on its membrane. We have ~400 kinds of receptor cell, i.e. ~ 400 “primary odors”. www.physiology.vcu.edu  Olfactory receptor cells have G protein-coupled receptor molecules in their membranes n The genes for these receptor molecules form the largest known gene family in vertebrates — 1000 genes, or ~3–5% of the genome — though only ~400 are expressed in humans. n n When an odorant molecule binds its receptor, it activates a G protein, Golf, which increases the local concentration of cAMP. n n cAMP-gated cation channels open, depolarizing the receptor neurons and triggering an action potential that travels along the cell’s axon to the olfactory bulb. Dr Hans Hatt et al. of Ruhr Uni Bochum found in 2014 that >15 of the 400 olfactory receptor molecules are found in the skin. They and others have also found olfactory receptors in heart, lungs, liver, kidney, colon, brain, prostate, testes, and sperm.  The receptor cells are sensitive Don't sweat, and wear thick socks. Olympic pool 375m ml = 75m teaspoons = 7.5b drops n Some of them can detect a single molecule of their preferred chemical, though ~40 cells must react before we experience a smell. n n Our receptors are as sensitive as a dog’s, but dogs have 20 times more. Bloodhounds can detect butyric acid (an ingredient in sweat) 8 at one part in 10. They can track us by molecules that seep through the soles of our shoes. www.pixsoriginadventures.co.uk  Olfactory receptor cells have unusual properties n They are pinocytotic, continually sipping in fluid and sending it along the nerves into the brain. We don’t know why. n n They are short-lived, degenerating after a month or 2, to be re- placed by new ones from below. n n They send their axons into the brain through tiny holes in the cribriform (“sievelike”) plate — the bone at the base of the cranial cavity. In 2012 a British-Polish team used olfactory cells to treat a spinal-cord injury in a man who had been paralysed from the chest down since a stabbing 2 years earlier. Surgeons cultured 500,000 olfactory ensheathing cells (glia which wrap around axons and assist axonal regenration) from the patient’s olfactory bulb and injected them into his cord above and below his injury. Two years later, in 2014, he was able to walk with a frame.  The receptor cells project to the olfactory bulb Olfactory bulb n The bulb is an extension of the cerebrum, and lies on the under- side of the frontal lobes. n n The projection from the receptors to the bulb is called the olfactory nerve, or cranial nerve I.  Many receptor cells converge on each bulb neuron Olfactory bulb with secondary neurons Cribiform plate Primary sensory neurons Blindfolded humans can crawl along a path defined by mint-soaked rope. Head injuries can damage the cribriform plate, causing anosmia. n As with rods converging on ganglion cells, this arrangement en- hances sensitivity but discards spatial information.  The bulb projects directly to olfactory cortex, bypassing thalamus Olfactory Bulb cortex Olfactory Olfactory nerve tract Receptors Olfactory cortex is in the frontal and temporal lobes Olfactory  The bulb also projects to the limbic system Cingulate gyrus Hippocampus Amygdala n This is an old group of brain regions concerned with motivation and emotion. For early animals, motivation was tightly linked to smell: they used their noses to identify food and poisons, mates and predators. n n Our emotions are no longer so smell-related (e.g. we like money) but they are still handled by these old olfactory areas. Maybe that is why odors call up emotional memories, e.g. Proust’s madeleines, Dickens’s label paste.  Olfaction adapts slowly but completely n Sewer workers don’t notice anything objectionable, and people are often unaware of their own body odors. n n Food evaluators take steps to avoid adapting, e.g. wine tasters eat biscuits or cheese between sips, and Scottish cheese tasters sip whisky.  Rodents and maybe humans have pheromones n Pheromones are chemicals released by an animal into the environment which affect the physiology or behavior of other members of its species. n Rodents have an olfactory structure in the nasal cavity called the vomeronasal organ (VNO), which is involved in their behavioral responses to sex pheromones. n A newborn will drag itself 50 cm to a cloth wiped on its mother’s nipple. n In humans,Men thewearVNO disappears undershirts during for 2 days without bathing. fetal Women smell development, the undershirts and but we say how much they’d like to sleep with that man. They favor men whose major do respond tohistocompatibility airborne complex chemical signals. (MHC) molecules (a.k.a. human leukocyte antigen HLA) are different from their own. If a group of women live together then their menstrual cycles synchronize, e.g. in summer camp, dorms, women’s prisons. Animal pheromones such as civet and musk are used in perfume and incense. In animals, pheromones trigger arousal, sperm production, abortion. There is a sperm-release pheromone that makes male fish ejaculate. Taste  Our main taste receptor cells are clustered in taste buds n We have ~5000 taste buds, mainly on the top of the tongue but also on the soft palate, epiglottis and upper esophagus. Babies have 10,000, parrots 400, cows 25,000. A taste bud lives only ~10 days. n n Each taste bud contains ~100 receptor cells, which are epithelial cells (not neurons) arranged like petals. They contact the oral cavity through a small opening, the taste pore.  A typical taste bud contains at least 5 kinds of receptor cell n Each kind of receptor cell detects one flavor, and all 5 have clear biological roles: n Sweet and umami receptor cells detect sugar (energy) and the amino acid glutamate (indicating protein), respectively. n Bitter receptor cells detect poison. + + n Salty and sour receptor cells detect Na and H — 2 important ions. n n The tongue may also have receptors for fatty acids. Not just mouth feel.  There are receptor cells of all 5 kinds all over the top of the tongue n For instance, it is not true that sweetness is sensed only by the tip of the tongue. n n But different areas of the tongue do vary slightly in their thresholds for different flavors.  Taste receptor cells are grouped into 3 types Tight junctions Type I cells may sense salt Type II cells Type III sense sweet, bitter cells sense sour and umami ATP Serotonin Gustatory neurons n Only type III cells form synapses with sensory neurons, activating them with serotonin. n n Type II cells release ATP, which acts on neurons and type IIIs.  Different kinds of cell employ different membrane proteins n Cells for sweet, umami, and bitter have receptor molecules coupled to a G protein called gustducin, which activates signal path- 2+ ways, increasing intracellular [Ca ] and triggering release of ATP. n n Detection of salt and sour involves ion channels which are not linked with G proteins.  Our experience of food depends on other sensors besides the taste buds n It depends on smell, temperature, pain, texture, crunch, appear- ance, and cognition — if I tell you some lousy food is a delicacy then you like it better. n Canned meatballs. n Nerve endings in the walls of the mouth have TRP channels sen- sitive to temperature and chemicals, e.g. vanilloid receptors respond to heat and to capsaicin in chilies; TRPM8 channels respond to cold and to menthol. n Chemoreceptors in our stomach and intestines monitor their contents; some of these receptors resemble ones on the tongue, e.g. for sweet and umami. Receptors similar to the ones that sense heat and capsacin in the human mouth are used by snakes for thermal imaging and by vampire bats for homing in on warm blood at night. Hydroxy-alpha-sanshool in Szechuan peppers activates Meissner receptors in the lips, and feels like 50-Hz vibration.  Taste signals take several paths to the brain Gustatory Thalamus Medulla n Receptor cells in the taste buds excite fibers of cranial nerves VII, IX, and X, the facial, glossopharyngeal, and vagus nerves. These pathways synapse in medulla and thalamus en route to the cortex. n n TRP receptors in the walls of the mouth excite cranial nerve V, the trigeminal. Reading in Silverthorn’s Human Physiology n 8th edition: Section 10.3 “Chemoreception: Smell and Taste” (pages 322–327). n 7th edition: “Chemoreception: Smell and Taste”, pages 324–329. n 6th edition: “Chemoreception: Smell and Taste”, pages 341–346. Hearing and Equilibrium PSL 300 University of Toronto  Question of the day Why did Beethoven have a bite bar on his piano?  Outline n Hearing n n Sound n Anatomy of the ear n n Cochlea n n Auditory Processing n n Hearing Loss n Equilibrium n n Semicircular canals n Utricle and saccule  The ear is the organ of hearing and equilibrium Pinna Vestibular apparatus Canal Eardrum Cochlea Middle ear Eustachian tube n The external ear consists of the pinna and the ear canal, sealed at its end by the tympanic membrane, or eardrum. n n Beyond the eardrum is the middle ear, an air-filled space connec- ted to the pharynx by the Eustachian tube. n n The inner ear contains the sensors: cochlea for hearing and the vestibular apparatus for equilibrium.  Hearing Ultrasound is used to heal sports injuries. Cranked up, it can heat small spaces almost as hot as the sun. Acoustical levitation uses ultrasound as intense as a jet engine. Ultrasonic horns repel teenagers and scare dogs and deer off the roads. Pet collars, so high-pitched even cats and dogs can’t hear them, repel fleas. Crickets communicate ultrasonically; the chirping we hear from them is just a by- product. Different species have their own frequency bands, like radio stations. Frogs, and some snakes and lizards, hear through their lungs. Porpoises and dolphins may hear through an oil-filled lower jaw. Sperm whales and bottlenose dolphins may use sound as a weapon, emitting bangs that stun large prey and cause small fish to hemorrhage internally. The deaf can enjoy music: Helen Keller wrote about holding a radio to feel a concert.  Sound is pressure waves n At the peaks of the waves, the molecules are crowded together and the pressure is high; at the troughs the molecules are far apart and the pressure is low.  Frequency is the number of wave peaks per second Pressure Time Middle-aged hear up to ~14 kHz n We perceive frequency as pitch: low frequencies as low-pitched sounds, high frequencies as high-pitched. n n Frequency is measured in waves per second, i.e. in hertz (Hz). n n Humans hear sounds in the range 16–20,000 Hz — ~10 octaves. Acuity is highest 1000–3000 Hz. Some bats and dolphins can hear 200 kHz. Elephants and crocodiles hear infrasonics. Middle C is ~261.6 Hz, adult male voices ~100 Hz, female ~150 Hz.  Amplitude is the pressure difference between peak and trough Wavelength } Pressure } Amplitude Time n Amplitude is the main factor that determines our perception of loudness: the larger the amplitude, the louder the sound (for any one sound frequency). n n Loudness depends on frequency as well, e.g. a sound of 30,000 Hz is beyond the range of human hearing, so it won’t be loud no matter how large its amplitude. Powerful ultrasonics don’t damage hearing unless they make objects resonate in the audible range.  Sound waves vibrate the eardrum Pinna Canal Eardrum Middle ear n The eardrum separates the outer ear from the middle ear.  A chain of small bones conveys vibrations through the middle ear Incus Malleus Stapes Oval window Eardrum n The eardrum vibrates the malleus (hammer) bone, which moves the incus (anvil), which moves the stapes (stirrup), which pushes like a piston against the oval window, a membrane between middle and inner ear. n n These 3 bones, called the ossicles, are the smallest in the body. They act as a lever system carrying vibrations from the eardrum to the much smaller oval window. The oval window leads into the cochlea, which contains the receptor cells Oval window Cochlea Cochlea  Uncoiling the cochlea makes its anatomy clearer Saccule Vestibular duct Oval Cochlear duct window Round window Helicotrema Tympanic duct n The vestibular duct (or scala vestibuli) and tympanic duct (scala tympani) contain perilymph (a fluid similar to plasma). These 2 ducts communicate at the helicotrema. n n The cochlear duct (scala media) contains endolymph (similar to intracellular fluid).  The oval window vibrates, setting up waves in the perilymph Auditory nerve Round Perilymph window Cochlear duct n Wave energy enters the cochlea at the oval window and exits, eventually, back into the middle ear through another membrane called the round window. n n En route, the waves shake the cochlear duct, which contains the auditory receptor cells (hair cells), though to see those cells we have to zoom in... A cross section shows that the cochlear duct contains the organ of Corti Organ of Corti Vestibular duct Tympanic duct  The organ of Corti sits on the basilar membrane and under the tectorial membrane Tectorial membrane Hair cell Fibers of Basilar auditory membrane nerve n The organ of Corti contains the auditory receptor cells — mech- anoreceptors called hair cells. They are epithelial cells, not neurons, and number ~20,000 per cochlea. n n Each hair cell has 50–100 stiff “hairs” called stereocilia, which extend into the tectorial membrane. They bend when waves in the perilymph deform the basilar and tectorial membranes.  When its cilia bend toward the longest cilium, the hair cell excites its neuron Hair cell Neuron n The hair cell depolarizes and releases transmitter, activating a primary sensory neuron. n n Axons of these neurons form the auditory nerve (also called the cochlear nerve), a branch of cranial nerve VIII.  When its cilia bend away, the hair cell releases less transmitter Hair cell Neuron n The hair cell hyperpolarizes, so it releases less transmitter and doesn’t excite its neuron as much. The basilar membrane responds to different frequencies at different points High Low frequencies frequencies Stiff near Basilar Flexible near oval window membrane helicotrema n The membrane is narrow and stiff near the round and oval win- dows, wider and more flexible at its other end. n n High-frequency waves maximally displace the membrane at the oval-window end; low-frequency waves maximally displace the other end. So the brain can deduce the frequency by noting which hair cells are most active. The pattern of membrane motion reveals pitch to the brain 3 100 Hz Membrane motion (:m) 0 3 400 Hz 0 3 1600 Hz 0 0 10 20 30 Distance from oval window (mm) Auditory Processing  Auditory signals pass from each ear to both sides of the brain Auditory cortex Thalamus Medial geniculate nucleus Midbrain To cerebellum Cochlear nuclei Auditory nerve in medulla These nuclei are tonotopic, i.e. neighboring cells in them prefer similar frequencies. they project to the inferior colliculus (IC), directly and via the superior olive (SO). Primary auditory cortex (A1) is in the temporal lobe A1 mcb.berkeley.edu  The brain localizes sounds based on loudness and timing n If a sound is louder in the right ear than in the left then it is coming from the right side of the head. Loudness is conveyed by firing frequency, i.e. louder sounds make auditory sensory neurons fire at a faster rate. n n If the sound reaches the right ear before the left then it is coming from the right side of the head. Hearing Loss  There are 3 kinds of hearing loss n In conductive hearing loss, sound can't be transmitted through the external or middle ear. n n In sensorineural hearing loss, there is damage to the hair cells or elsewhere in the inner ear. Mammals can't replace dead hair cells, though birds can. 90% of hearing loss in the elderly (presbycusis) is sensorineural. n n In central hearing loss, there is damage to the cortex or the path- ways from cochlea to cortex. Typically the patient’s trouble is in recognizing and interpreting sounds, rather than in detecting them.  Clinical tests distinguish conductive from sensorineural loss n In the Rinne test you hold a tuning fork against the mastoid bone and then beside the ear, and ask when the sound is louder. Normally it is louder through the ear canal. If it is louder through the bone, there is conductive loss. n n In the Weber test you hold the tuning fork to the patient’s forehead, in the midline, and ask in which ear the sound is louder. With sensorineural loss, it is louder in the good ear. With conductive loss, it is louder in the bad ear, because it doesn't have to compete with sounds heard through that ear canal. Remember finger-in-ear test. Heinrich Adolf Rinne (1819--1868) was not a Frenchman; he was German. So was Ernst Heinrich Weber (1795--1878). Equilibrium  Different parts of the vestibular apparatus sense head position and motion Semicircular canals: Utricle Superior Saccule Posterior Horizontal n The utricle and saccule contain hair cells that are activated when the head tilts relative to gravity. n n The semicircular canals are fluid-filled hoops that detect head rotation, e.g. when your head turns rightward, the fluid in the tubes sloshes leftward, activating hair cells.  Equilibrium pathways project mainly to the cerebellum n Vestibular hair cells activate primary sensory neurons of the vesti- bular nerve, which is a branch of cranial nerve VIII. n n These neurons may either pass directly to cerebellum or synapse in the medulla, whence they proceed to the cerebellum or up through thalamus to cortex. n n Your brain uses vestibular information to infer your own position and motion, and keep you upright. Reading in Silverthorn’s Human Physiology n 8th edition: Sections 10.4 “The Ear: Hearing” and 10.5 “The Ear: Equilibrium” (pages 328–337), and “Location of the Stimulus” (page 313). n 7th edition: “The Ear: Hearing” and “The Ear: Equilibrium” (pages 329–340), and “Location of the Stimulus” (page 315). n 6th edition: “The Ear: Hearing” and “The Ear: Equilibrium” (pages 346–356) and Figure 10.23, and “Location of the Stimulus” (pages 331–332). Somatic Senses PSL 300 University of Toronto  Question of the day How can a heart attack make your arm feel sore?  Outline n Somatosensory receptors Free endings Merkel, Meissner, Pacinian n Pathways and processing n n C fibers, Aδ, Aβ n Projections to cortex n Somatotopic map n Pain n Fast, slow, referred n Gating n Analgesics  There are 4 somatic senses: touch, temperature, proprioception, and nociception n Proprioception is awareness of the position of body parts relative to each other; e.g. even with your eyes closed you can sense how much your elbow is flexed. n n Nociception detects tissue damage or the threat of it, and is per- ceived as pain or itch. Somatosensory Receptors  Somatosensory receptor cells are all neurons Cell bodies Spinal cord Afferent fibers n Receptors for somatic sensation below the chin have their cell bodies in the dorsal root ganglia. Receptors for the head have their cell bodies in the brain. n The parts of these neurons that transduce touch, pressure etc., into electrical signals are in their nerve endings, i.e. in the tips of their fibers, in the skin and viscera.  There are a variety of receptors in the skin Pacinian Merkel Meissner Free Ruffini n Free nerve endings detect mechanical stimuli, temperature, and chemicals. n Merkel receptors (or Merkel disks) are mechanoreceptor nerve endings in contact with specialized epithelial cells called Merkel cells. n n Encapsulated receptors, e.g. Meissner and Pacinian corpuscles, are mechanoreceptors sheathed in connective tissue.  At the bottom of the epidermis are saucer-shaped Merkel disks 20 μm n They are very sensitive to deformation of the skin, and are more tonic than phasic, i.e. they send a sustained message as long as the deformation persists. n They signal contact. www.physiology.columbia.edu  Most mechanoreceptors are phasic n Given a sustained, constant stimulus, the nerve ending’s mem- brane depolarizes but then returns to baseline in ~3 ms — i.e. it registers changes, not steady levels. n So you don’t perceive much unless the stimulation is changing: if you run your hand along a surface, you get a vivid impression of its texture; after your hand stops, you sense far less.  At the top of the dermis are egg-shaped Meissner corpuscles n They are mainly in the tongue and hairless skin — erogenous zones, palms and fingertips (which have 5000/cm2 when we are 10 2 years old, but just 1000/cm when we are 50). n Inside each corpuscle are many looping endings, like the coils of a spring mattress. They detect sideways shearing, as when you stroke a surface or lift something with your fingertips. n n They are phasic, so they sense changes in shear. missinglink.ucsf.edu  Deep in the dermis are onion-shaped Pacinian corpuscles n The nerve endings are sheathed in many layers. They can sense tiny displacements (10 μm) if the motion is quick. n They are phasic, and so they respond strongly to vibration and other fast-changing stimuli. bioweb.wku.edu  Receptors are not uniformly distributed over the body surface n Palms, fingertips, and lips are the foveas of the somatosensory system: they have more densely packed receptors, and therefore higher acuity, than other areas. n n The test of acuity is 2-point discrimination: if your skin is touched at 2 places simultaneously, can you tell whether there are one or 2 contact points? n n On your lips and fingertips you can distinguish points 2–4 mm apart, but on your calves you need 40 mm.  Thermoreceptors are free nerve endings n Cold receptors respond maximally at ~30°C (which is well below body temperature); warm receptors at ~45°C. Both are phasic-tonic, which is why we get used to a hot bath or a cold lake. n n Above 45°C, pain receptors are activated. Cold fibers also respond briefly to temperatures > 45°C, causing paradoxical cold: a hot object, touched briefly, may feel cold. n n We have more cold receptors than warm, and few thermo- receptors in total — as few as 1000 fibers may carry temperature information up the spinal cord to the brain (precise localization isn’t crucial for temperature).  Nociceptors are free nerve endings that respond to noxious stimuli n Some respond to damaging mechanical stimuli, others to damag- ing heat or chemicals. n + n Some respond to chemicals released from damaged cells (K , histamine, prostaglandins) or to serotonin released by platelets in response to injury. Pathways and Processing  Somatosensory afferents fall into 2 groups: small and large n The small fibers, called C and Aδ (A-delta), come mainly from free nerve endings. C fibers are unmyelinated, and conduct spikes at speeds up to 2 m/s. Aδ’s are thicker than C’s, myelinated, and conduct at up to 30 m/s. n n Different small fibers respond to different adequate stimuli, such as mechanical stimuli, chemicals, or temperature. n n Large fibers, called Aβ (A-beta), come from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles. They are myelinated, and conduct at 70 m/s.  Large and small fibers project differently Medulla Spinothalamic Dorsal column tract Large Small Spinothalamic tract n Large fibers turn upward on reaching the spinal cord, and run ipsilaterally up to the medulla in tracts called the dorsal columns. In the medulla they synapse on cells whose axons cross the midline. n n Small fibers synapse directly or via interneurons on motoneurons (for reflex responses) or on dorsal-horn neurons whose axons cross the midline and run in the spinothalamic tracts, in the lateral part of the cord, between the dorsal and ventral horns.  This anatomy reflects the fibers’ different functions n Large fibers provide feedback to the brain, especially to motor cortex, as it manipulates objects. Their information has to travel a long way (up to the brain) quickly. n n Small fibers evoke simple responses to specific stimuli: with- drawing from pain, brushing away a bug, thermoregulatory and sex- ual responses. Many of these tasks can be handled in the spinal cord, without immediate input from the brain.  Signals travel via thalamus to cortex Cortex Thalamus Medulla Spinal cord n Signals from the spinal cord travel via the ventroposterolateral (VPL) nucleus of the thalamus. Signals from the head (not shown) travel via the ventroposteromedial nucleus (VPM). n n Both pass to the primary somatosensory cortex, S1.  Primary somatosensory cortex is somatotopic n Neighboring areas of skin project to neighboring cells in cortex, so S1 is a map of the contralateral body surface. n n The map is distorted, as areas of high sensitivity and acuity (such as hands and lips) get a lot of cortical space, just as the foveas do in the visual system. Primary somatosensory cortex (S1) is in the parietal lobe S1 mcb.berkeley.edu  There is lateral inhibition among somatosensory fibers n As in the visual system, lateral inhibition enhances spatial differ- ences, i.e. edges. n n If you step into a very hot bath, you feel the most discomfort not in your foot but at the line formed by the water surface around your leg, because that is the temperature edge. n n This is a somatosensory version of the Chevreul illusion. Pain  Many nociceptors have ion channels of the TRP type n Many nociceptors (and also thermoreceptors) have ion channels belonging to a family called transient receptor potential (TRP) channels. n e.g. TRPV1 channels, called vanilloid receptors, respond to dam- aging heat and to chemicals, including the capsaicin in chili peppers; TRPM8 channels respond to cold and to menthol.  Nociceptive signals report damage or danger, and evoke pain or itch n People with congenital analgesia usually die before they are 20, because of injury and infection. n n We have 2 types of pain: fast and slow, e.g. when you stub your toe, you feel an immediate sharp pain, followed ~1 s later by a duller sensation. Fast pain is carried by Aδ fibers, slow by C fibers. n n The reason for the 2 types is likely that pain evokes 2 distinct responses: quick withdrawal (to get away from the painful thing) and prolonged immobilization (to promote healing).  Nociceptive signals evoke responses from the CNS n Nociceptive signals trigger withdrawal, e.g. pulling your hand back from a hot stove. This is a spinal reflex, and so it doesn’t need immediate input from the brain. n n Nociceptive signals also reach the limbic system and hypothal- amus, causing emotional distress, nausea, vomiting, and sweating. n n Descending pathways through the thalamus can block noci- ceptive cells in the spinal cord, e.g. in emergencies where survival depends on ignoring pain.  Pain in an internal organ is often felt on the body surface — referred pain Skin To brain Ureter n Nociceptors from different locations converge on a single as- cending tract. So when that tract sends signals to the brain, the brain doesn’t know where the stimulus came from. n n As pain is more common in skin than in internal organs, the brain assumes the problem is on the body surface.  Pains from different organs are referred to different regions on the body surface Stomach Small intestine Appendix Ureters Colon  Pains from different organs are referred to different regions on the body surface Liver and Heart gallbladder  Pain can be gated by Aβ activity Aβ fiber To brain Touch Injury C fiber To brain Interneuron Secondary - neuron n In the dorsal horn, C fibers contact secondary neurons. Those secondaries are inhibited by Aβ fibers via interneurons. n n So Aβ’s can block or dampen pain signals, e.g. if you rub a sore shoulder, it feels better.  Analgesics work by various mechanisms n Acetylsalicylic acid (aspirin) inhibits prostaglandins and inflam- mation, and slows transmission of pain signals. n n Opioids (such as morphine and codeine) decrease transmitter release from primary sensory neurons and postsynaptically inhibit secondary sensory neurons. n n The body makes natural painkillers such as endorphins, enkeph- alins, and dynorphins. Reading in Silverthorn’s Human Physiology n 8th edition: Section 10.2 “Somatic Senses”, (pages 315–322). n 7th edition: “Somatic Senses”, pages 317–324. n 6th edition: “Somatic Senses”, pages 335–341.