Quiz 3 Notes
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Date Created: 03/21/16
WEEK 8 NOTES 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 the 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 rain 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 WEEK 9 NOTES 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 The pupil can dilate to about 5/16 of an inch in diameter at its widest; th 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 and 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
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