Test 2 Notes
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Date Created: 04/06/16
VISION The process by which it translates one form of energy (the environmental stimulus) into another (the neural impulse) is known as transduction A receptor cell is a biological transducer Receptors convert physical stimuli into neural impulses indirectly Receptors convert physical stimuli into electrical signals, and it is these signals – known as generator or receptor potential – that produce the neural impulse The intensity of a stimulus has to exceed the absolute threshold – the minimal intensity of a stimulus that can be detected about 50% of the time – in order to be detected by receptor cells A stimulus that is below the absolute threshold is said to be subliminal and will not be able to influence behaviors The absolute thresholds for the five major senses are: o Vision – a candle light that is 30 miles away in a totally dark night o Sound – a ticking watch that is 20 feet away in a quiet room o Taste – a teaspoon of sugar in 2 gallons of water o Smell – a drop of perfume in a 3-room apartment o Touch – the falling of a bee’s wing that is 1 cm away from the cheek Psychophysicists measured not only absolute thresholds, but also what they called the differential threshold – often referred to as the just noticeable difference (JND) Max Weber and Gustav Fechner were interested in finding out the least amount of change in stimulation that would be noticeable o i.e. the differential threshold or JND JNDs, said Weber, are a constant proportion of a stimulus Fechner labeled this Weber’s Law Brightness -- .08; Taste -- .08; Loudness -- .05; Heaviness -- .02; and Electric Shock -- .01 We possess a variety of receptors; every receptor has a membrane that is surrounded by ions The presence of such a membrane and ions creates a resting potential in the receptor in much the same way that it does in the neuron convert a stimulus into an electric signal (generator potential) - the environmental stimuli change the permeability of the cell membrane and Na+ ions flow into the cell, causing it to depolarize The resulting generator potential travels to the sensory neuron, where it produces the neural impulse (action potential) Every receptor has the capacity to convert a stimulus to a generator potential The membrane of each type of receptor is sensitive to a specific type of environmental stimulus The generator potential must do 2 things to produce the action potential on the sensory neuron o 1. Generator potential spreads to the site of impulse initiation, usually the axon of the sensory neuron o 2. Generator potential produces depolarization sufficient to reach threshold – it must depolarize the axon of the sensory neuron from -70 mv to -55 mv in order to fire the all-or-none action potential that travels to the brain The generator potential is a graded potential (its magnitude depends directly on the intensity of the stimulus) The generator potential is decremental (it decreases as it spreads to the axon of the sensory neuron) A single generator potential is usually insufficient to produce the action potential Multiple generator potentials (all below threshold) must work collectively in order to exceed the threshold Once the generator potential passes the threshold, it elicits not only the action potential but also the train of action potentials The greater the magnitude of the generator potential, the greater the frequency of the action potential Stimuli of long duration produce decreasing generator potential and progressively fewer action potentials This is called adaptation – a decrease in the resultant sensory experience Sensory coding is the one-to-one correspondence between some aspect of the physical stimulus and some aspect of the nervous system’s activity Where the neural impulse travels and how it travels correlate with certain sensory experiences – sights, sounds, smells, touches, and so forth We refer to these correlates as codes, and to the brain’s capacity to produce them as coding Because the human nervous system is capable of coding an enormous variety of stimuli, we can differentiate among a multitude of colors, sounds, tastes, smells, and tactile feelings We also make discriminations among stimuli on the basis of their intensity There are two types of sensory coding: the anatomical coding and the functional coding The anatomical coding is used to describe the correlation between sensation and brain area It is a theory proposed in 1826 by Johannes Muller, who called it the law of specific nerve energies (1838) The doctrine of specific nerve energies states that sensation depend less on the environmental stimuli that activate them than on the nerves that are stimulated and ultimately on the part of the brain that nerves stimulate Each sensory nerve is ordinarily excited by only one kind of energy, and the brain interprets any stimulation of that nerve as being that kind of energy According to Muller’s theory, sound and light produce different sensations because auditory nerves and optic nerves travel to different parts of the brain The functional coding involves the differences in neural activity (the frequency of neural impulses) triggered by varying amounts of environmental stimuli It states that various sensations do not necessarily correspond only to specific anatomical areas; they are also differentiated in accordance with the degree of neural activity within an area The sensation of brightness or loudness, for instance, varies in accordance with the number of neural impulses arriving per unit of time (per second) at the visual or auditory cortex An intense stimulus, for instance, will increase the level of firing and produce more impulses than a less intense stimulus A stimulus does not initiate the firing of the neural impulse Sensory areas in the brain and the neurons leading to these areas are spontaneously firing even if no stimulus is present What the stimulus does is to modulate this spontaneous activity All stimuli must produce 2 effects in the nervous system in order to be recognized: o 1. They must be received by an aroused brain i.e. the brain must be prime to process the stimuli; o 2. They must be attended to i.e. the brain must receive specific sensory information Humans have a highly sophisticated visual system that enables us to detect shapes, follow movement, differentiate colors, and use vision to judge distance But the physiological basis for this versatility is not yet fully understood To understand vision, you must first understand light Light is a form of electromagnetic energy generated by the movement of elementary particles known as photons This movement takes the form of light waves that vary in 3 respects: o Wavelength o Amplitude o Purity All visual sensations are produced by the relative differences in the wavelength, the amplitude, and the purity of wavelengths, and these sensations fall into 3 general categories: o Hue (wavelength) o Brightness (amplitude) o Saturation (purity) Hue is the sensation of color – is produced by differences in the length of electromagnetic waves Only a small fraction of waves can trigger a visual experience in humans We measure wavelength of light in nanometers (nm), one nanometer is equal to 1/1,000,000 millimeters The light waves visible to the human eye fall within a range from 380 nm to around 760 nm – which are capable of being transduced i.e. of producing the neural impulse The shortest wavelength visible to humans, 380 nm produces violet color The longest wavelength, 760 nm produces red color Wavelengths shorter than 380 nm (including ultraviolet rays, X-rays, and gamma rays) are not visible to the naked eye Wavelengths longer than 760 nm (including infrared waves, radar, FM, AM) are also invisible The optical process involves preliminary gathering and bending of light by non-neural cornea and lens in the eye The neural process involves the working of the retina and its relationship to the brain The eyeball is basically round and is sheathed by a fibrous layer that contains 2 parts o About 5/6 of the surface is covered by an opaque white coating called the sclera, or white of the eye o The only area the sclera does not cover 1/6 is the little bulge that is covered by a transparent shield known as the cornea Light enters the eye through the cornea, whose principal function is to initiate the focusing process The cornea has no blood vessels It draws its fuel from a fluid-like substance known as the aqueous humor, which occupies the chamber between the cornea and the lens The next layer within the eye, adjacent to the sclera, is the choroid layer or coat This darkly pigmented layer of tissue has 2 basic functions: o 1. To support the blood vessels that supply fuel to the retina; and o 2. To absorb light waves that have scattered after corneal refraction (cat has tapetum, act as a mirror, reflecting light back to the eye for the nocturnal sight) As light passes through the cornea and the aqueous humor, it encounters the iris, a colored contractile membrane (the color of the iris is the color of the eyes) A small opening in the center of the iris, the pupil, controls the amount of light that reaches the back of the eye th The pupil can dilate to about 5/16 of an incthin diameter at its widest; at its narrowest, it measures about 1/16 of an inch The widening and narrowing of the pupil are governed by two sets of smooth muscles under the control of the autonomic nervous system In periods of stress of intense concentration, the sympathetic nervous system stimulates the pupil to dilate (increases of light) In periods of relaxation or in bright sunshine, the parasympathetic nervous system stimulates the pupil to contract (decreases of light) Myopia results from the overreaction of rays of light The focal point of the lens is located in front of the retina because of the lengthened eyeball, resulting in a blurred image The myopic need to wear a concave lens to adjust the vision Hyperopia results from under-refraction of rays of light; the focal point of the lens is located behind the retina because of the shortened eyeball, resulting in a blurred image The hyperopic need to wear a convex lens to adjust the vision, the retina is the place where the neural processing of visual information begins The retina is a membrane consisting of 3 layers of cells One layer is make up of 2 types of receptor cells: rods and cones The other 2 layers are made up of neurons that take 2 forms: bipolar and ganglion cells There are other neurons in the retina: o 1. Horizontal cells interconnect the receptors; and o 2. Amacrine cells interconnect the ganglion cells The arrangement of 3 layers is unusual The photoreceptor cells (rods and cones) are located behind the 2 neural layers This means that when light hits the retina, it must filter through the neural layers (first the ganglion cell layer, then the bipolar cell layer) before reaching and activating the photoreceptor cells It also means that the neural impulse, once it is triggered by the receptors in the rear portion of the retina, travels toward the front of the retina through the bipolar and ganglion cells Ultimately, the neural impulse is routed to the back of the retina through axons of the ganglion cells that make up the optic nerve Rods and cones are named for the shapes Rods are slender and cylindrical Cones are broad and bulbous Rats only have rods; turtles have only cones, humans have both rods and cones Humans have about 125 million rods and 7 million cones Rods are located primarily in peripheral areas, and cones are more numerous in the interior The center of the retina (smaller than a square mm) only has cones, more than 50,000 of them, packed closely together This area is called fovea centralis Both rods and cones are sensitive to light Rods have a low threshold of excitation That rods operate primarily in conditions of low illumination, such as exist at night Rods do not abstract color from the light Rods function in ways that are achromatic (colorless) and scotopic (related to darkness) The more rods that are stimulated by a particular wavelength, the brighter an object will appear Rods are less adept at visual acuity (the capacity to discern detail) Cones function in ways that are chromatic (colorful) and photopic (related to light) With a much higher threshold of excitation to light than rods, they function mainly under high illumination conditions, such as exist during the day The more cones that are stimulated by a particular wavelength, the brighter and more colorful an object will appear Both cones and rods are not equally sensitive to all wavelengths of light The Purkinje effect is an excellent illustration of the varying sensitivity of rods and comes and their relationship to color sensation At dusk some colors seem vibrant than others Green grass, for instance, seems brighter than red roses The reason is that cones are not equally sensitive to all wavelengths When illumination begins to dim, fewer cones are activated by long wavelengths than by short ones. In the low illumination of evening, the rods come into play, because of their super-sensitivity to light Remember, rods are completely insensitive to long wavelengths, therefore in the evening a rod rose may appear black and green grass may appear a somewhat brighter shade of gray After transduction takes place in the retina, the converted signals are converted from the retina to the brain via axons that issue from ganglion cells and are collected into a bundle called optic nerve At the point where the optic nerve leaves the retina of each eye, there are neither rods nor cones This area is called the blind spot or optic disc Each eye has its own optic nerve, but the two nerves converge at the base of the brain at a place called the optic chiasm There they go through a kind of restoring process, but they do not form synapses there In frogs (a lower animal), the optic nerves from two eyes simply cross to the opposite sides of the brain In humans, only half of the neurons contained in the two optic nerves cross The neurons that cross are those that originate in the nasal half of each retina The neurons that do not cross are those that originate in the temporal half of each retina The convergence of rods on ganglion cells increases their sensitivity; spatial and temporal summation can fire a neural impulse The ability to distinguish a form as distinct from other adjacent forms cells for the ability to see its contour Ratliff and Hartline discovered that the eye of the horseshoe crab is ideally suited for visual analysis What makes the eye of the crab so unique is that it contains only 800 or so photosensitive cells and those cells that happen to be rather large – large enough to allow for detailed examination of an individual cell Each of these cells is called an ommatidium Each has its own lens and its own neuron In other words, each ommatidium is a miniature eye In one of Ratiff and Hartline’s experiments, they placed microelectrodes on a single neuron (ganglion cell) leading form a single ommatidium The first thing they noted was that the single neuron has a spontaneous firing rate Then they projected a fine beam of light on the ommatidium connected with that ganglion cell, and the firing increased, but the neighboring ganglion cell’s spontaneous firing decreased They concluded that the stimulation of an ommatidium not only has an excitatory effect on the ganglion cell with which it is connected, but also has an inhibitory effect, through cross connections on neighboring ganglion cells They called this phenomenon the lateral inhibition The lateral inhibition is they key to the contour coding process and may account for a property of contour called Mach bands Black background with a white square patch, the contrast is sharpest right at the border where the white edge meets the black edge The ultra bright edges and the ultra dark edges are called Mach bands It has been theorized that the neural code for the ultra dark and ultra bright edges depends on the lateral inhibition The white edge of the band appears ultra bright because the neurons are stimulated by the band have a higher firing rate than those neurons stimulated by neighboring white areas, because only half of their neighbors are stimulated, the inhibitory effect is diminished by half The black edge of the band appears ultra dark because the neurons stimulated by it have a lower firing rate than the neurons stimulated by the neighboring black areas, because the neighboring black areas are not stimulated and the inhibitory effect is not existed There are 3 types of ganglion cells: X cells, Y cells, and W cells The ganglion cells from the retina travel in two directions Y cells (5%) branch off to the superior colliculum in the midbrain, but X cells (55%) go to the lateral geniculate nucleus in the thalamus, another 40% of ganglion cells (W cells) go to both sites and elsewhere in the brain HEARING Sound wave moves relatively slowly, much more slowly than light: 330 meters per second (738 miles/hour) as compared to 300,000,000 meters per second (187,500 miles/sec) for light Sound wave has been measured in 3 respects: frequency, amplitude, and timbre Frequency is the rate of vibration. Sound wave frequencies are commonly measured in cycles per second referred to as Hertz (Hz) The lowest frequency detectable by the human ear is about 16 Hz (15 to 20 Hz), which is a long wavelength; the highest frequency that most human beings can respond to is 20,000 Hz, which is a very short wave length Frequencies within this range produce the sound sensation known as pitch The higher the frequency, the higher the pitch; the lower the frequency, the lower the pitch Infants can pick up frequencies as high as 40,000 Hz; dogs can generally detect frequency up to 30,00 Hz (dog whistles) Bats, moths, porpoises can pick up frequencies as high as 100,000 Hz For middle aged adults, the upper limit for hearing decreases by about 80 Hz for every 6 months The upper limit drops even faster for those exposed to loud noises Amplitude is used to denote the degree of pressure in a sound wave – the number of molecules compressed into a given area Amplitude can also be thought of as the force that the vibrating object exerts on the molecules It is the variation in amplitude that is perceived as loud and soft The more molecules moved or compressed into a given area, the greater the pressure, and the louder the sensation The loudness is measured in decibels (db), which is the basic unit of intensity of sound What saturation is to light, timbre is to sound. Timbre refers to the purity of quality of a tone A simple vibrating object such as a tuning fork produces a simple sound wave – or sine wave, whose pressure vibrations produce a simple sound Most of the sounds we hear, come from complex vibrating objects that consist of a number of vibrating parts, each with its own amplitude and frequency Harmonics is usually defined as the science of musical sounds, and harmonic analysis is defined as the method by which sounds are measure and analyzed Analysis of complex sounds indicated that fundamental tones (low frequency – high amplitude) and overtimes (high frequency – low amplitude) interact with one another When sound waves coincide, they are known as periodic, and the effect is usually pleasant; when the waves are produced at random, they are said to be aperiodic, and the effect is not always pleasant, is called noise Sound is produced initially by an object that disturbs molecules when it vibrates Once disturbed, these vibrating molecules spread in all directions The degree to which sound waves are sustained depends basically on 2 factors: o 1. The distance they travel, and o 2. The environment in which they make their journey Distance reduces amplitude (loudness) but does not affect frequency (pitch) Environment can affect both The ear can be considered to have 3 parts: o The outer ear – collects sound o The middle ear – amplifies sound o The inner ear – transduces sound Pinna or auricle, is what you normally think of as your ear The pinna plays only a minor role in hearing The pinna helps us to distinguish the direction of sound, especially sounds originating in front of in back of us A small hole in the pinna opens in an inch-long tunnel commonly known as the auditory canal, which serves to conduct air-borne sound inward At the end of the passage is the membrane (the tympanic membrane) that is commonly known as the eardrum The cone-shaped eardrum is pulled over the opening of the auditory canal and acts rather like the diaphragm of a microphone Its chief quality it its elasticity – its ability to move back and forth – which allows it to match the incoming sound in frequencies and amplitudes Most of the space within the middle ear cavity is taken up by 3 bones, collectively called ossicles: o The malleus (hammer) o The incus (anvil) and o The stapes (stirrup) They are usually referred to by the shapes they roughly resemble They begin at the back of the eardrum and end at the oval window which opens into the inner ear They act like a piston Two muscles control the movements of the bones in the middle ear: o 1. The tensor muscle is attached to the malleus o 2. The stapedius muscle is attached to the stapes Both the stapedius muscle and the tensor muscle protect the inner ear fron intense sound Also attached to the bottom of the middle ear is a thin tube known as the Eustachian tube It connects with the back of the mouth, and it maintains a necessary equilibrium of the pressure inside and outside the middle ear The ossicles are among the smallest bones in the body (206 bones in an adult body: 29, 26, 25, 64 and 62) but their function in sound conduction is crucial They serve to amplify sound pressure exerted against the oval window of the inner ear The surface of the eardrum measure approximately 100 square mm This area is much larger than that of the stirrup, which measures approximately 3 square mm The power of the sound wave is amplified some 22 times before it reaches the oval window Inflammation of the middle ear is called otitis media which is characterized by pain, dizziness, and impaired hearing The inner ear begins on the other side of the oval window and constitutes the most intricately structured of all the ear components It consists of 3 main structures: o 1. The 3 semicircular canals o 2. The vestibular sacs (2 otoliths), the above two parts are found in the upper portion, not involved in hearing, are to help maintain body balance and posture o 3. The cochlea – in the lower portion – is the organ of hearing (the retina of the ear) The cochlea consists of 3 fluid-filled canals within a single coiled tube: o 1. The vestibular canal (scala vestibule) o 2. The chochlear canal (scale media) and o 3. The tympanic canal (scale tympani) The vestibular canal begins at the oval window and tapers to a small opening called the helicotrema The tympanic canal, which interconnects with the vestibular canal at the helicotrema, ends at the round window The cochlear canal lies between the other two canals The organ of Corti is named after its discoverer, Alfonso Corti It rests on the floor of the cochlear canal, supported by the basilar membrane Inside are auditory receptor cells that connect with neurons making up the auditory nerve The receptor cells are called hair cells because that are what they look like They are located on the basilar membrane, beneath the surface of an overhanging gelatinous structure known as the tectorial membrane In the human ear there are about 28,500 hair cells and they are arranged in 2 layers (25,000 in the outer layer and 3,500 in the inner layer) Hair calls are activated as soon as sound has been transmitted through the ossicles of the middle ear and the fluid of the inner ear has been pushed from the vestibular canal to the tympanic canal The compressed fluid causes an upward push of the basilar membrane, which heaves the hair cells up and down against the relatively immobile tectorial membrane Hair cells, like neurons, are polarized with ions that causes depolarization Once the hair cells have been bent, the resultant generator potential sums with other generator potential to produce a neural impulse that then travels to the brain via the auditory nerve The auditory nerve is a bundle of bipolar neurons whose dendrites form synapses with the hair cells and whose cell bodies collect in the spinal ganglion located just outside the walls of the cochlea The axons of these bipolar neurons, one bundle from each cochlea, course inward, entering the brainstem at the level of the medulla There they form synapses in the cochlear nucleus, one on each side of the brainstem From the cochlear nucleus, the neurons travel in 2 directions They either ascent the brainstem directly or cross to the other side of the brainstem and then ascend In either case, the ascending tract is known as the lateral lemniscus (a bundle or band of sensory nerve fibers) The crossing pathways are referred to as trapezoid bodies
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