Term
| Explain how stimulus modality is determined: |
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Definition
| The different modalities result from differences in neural pathways and synaptic connections. ex. The brain interprets impulses arriving at the auditory nerve as sound, and from the optic nerve as sight, even though the impulses are identical in the two nerves. The senses act as energy filters that allow one to perceive only a narrow range of frequency. |
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Term
| How do phasic receptors relate to sensory adaptation? |
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Definition
| They are receptors that respond with a burst of activity when the stimulus is first applied, but then quickly decrease their firing rate. Some respond with a quick burst when the stimulus is first applied, then with another quick burst when the simulus is removed. This could be understood as the "turning on" and "turning off" of a stimulus. |
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Term
| Describe the nature of the receptor generator potential: |
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Definition
| It is usually analagous to the depolarizations that produce EPSPs. In response to and environmental stimulus, the sensory endings produce local, graded changes in the membrane potential. Once the receptor potemntial reaches the threshold value of depolarization, and action potential is generated. |
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Term
| Describe the significance of the receptor generator potential: |
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Definition
ex. Pacinian corpuscle: When a light touch is applied to the receptor, a small depolarization (the geerator potential) is produced. Increasing the pressure on the pacinian corpuscle increases the magnitude of the generator potential unil it reaches the threshold depolarization required to produce an action potential. |
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Term
| Our perceptions are products of our brains; they relate to physical reality only indirectly and incompletely. Explain this statement using vison and cold as an example: |
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Definition
Vision: Vision is limited to light in a small range of electromagnetic wavelengths known as the visible spectrum. UV, X-rays, infared and radiowaves are the same type of energy as the visible light, but it cannot excite photoreceptors. Cold: There is no such thing as cold, only varying degrees of heat. The perception of cold is entirely a product of the nervous system, and is critical for survival. |
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Term
| What is meant by the law of specific nerve energies and the adequate stimulus? |
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Definition
| Stimulation of a sensory nerve fiber only produces one sensation-- touch, cold, pain, ect. According to the law of specific nerve energies, the sensation characteristic of each sensory neuron is that produced by is normal stimulus, or adequate stimulus (the least amount of energy required to activate its receptor). |
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Term
| Use examples of vision and cold to explain the law of specific nerve energies: |
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Definition
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Term
| Describe sensory adaptation in olfactory and pain receptors: |
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Definition
| Olfactory receptors would be considered phasic (fast adapting), because the initial stimulus would cause neurons to fire at a high frequency, and then quickly slow the firing rate, even though the stimulus is still there. This explains why someone may smell something, but then quickly not smell it anymore, even though the odorant molecules are still present. Pain receptors would be considered tonic (slow adapting), because the initial stimulus would cause neurons to fire rapidly nd at a consistent rate, as long as the stimulus is there. This is critical for survival, because pain is an important perception. |
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Term
| Explain how the magnitude of a sensory stimulus is transduced into a receptor potential, and how the magnitude of the receptor potential is coded in the sensory nerve fiber: |
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Definition
| When a tonic receptor is stimulated, the generator potential it produces is proportional to the intensity of the stimulus. After the threshold depolarization is produced, increases in the amplitude of the generator potential results in increases of the frequency with which action potentials are produced. In this way, the frequency of action potentials that are conducted into the CNS serves as a code for the strength of the stimulus. Frequency code is needed because the amplitude of action potentials is constant (all or none). Acting through changes in action potential frequency , tonic receptors provide information about the relative intensity of a stimulus. |
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Term
| Describe the sensory pathway in regards to proprioceptors and pressure receptors from the skin to the postcentral gyrus: |
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Definition
| This is a somatethetic sense; its pathway involves 3 orders of neurons in series. Sensory information from proprioceptors and pressure receptors is first carried by large, myelinated nerve fibers that ascend in the dorsal columns of the spinal cord on the same side. After the fibers synapse in the medulla with other 2nd order sensory neurons, information in the latter neurons crosses over to the contralateral side as it ascends via a fiber tract (medial leminiscus) to the thalamus. These fibers ascend in the anterior spinothalamic tract. 3rd order sensory neurons in the thalamus receive this input and in turn project to the postcentral gyrus. |
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Term
| Describe the sensory pathway from the skin to the postcentral gyrus, in regards to thermoreceptors and nocioceptors: |
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Definition
| Sensations of heat, cold and pain are carried into the spinal cord by thin myelinated axons (A-delta fibers) and thin unmyelinated fibers (C fibers). Within the spinal cord, these neurons synapse with 2nd order association neurons that cross over to the contralateral side and ascend to the brain in the lateral spinothalamic tract. Then, the 2nd order neurons of the spinothalamic tract synapse with 3rd order neurons in the thalamus, which in turn project to the postcentral gyrus. |
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Term
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Definition
| Sensory acuity, or tactile acuity, is the sharpness of touch sensation. An example of this is the two-point threshold, which is the minimum distance at which two points of touch couold be perceievd as seperate. It is a measure of the distance between receptive fields. The fingertips have the sharpest sensory acuity, the calf has the dullest sensory acuity. |
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Term
| How is sensory acuity affected by receptor density: |
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Definition
| The acuity of perception of sensation is directly related to the density of receptors in the region. The area of the receptive field of the back is served by relatively few neurons, serving a large area of skin, whereas the receptive field of the fingertips are innervated by a large number of neurons, serving a small area of skin. So the sensory acuity of the back is much duller than the fingertips. |
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Term
| Explain how sensory acuity is affected by lateral inhibition: |
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Definition
| When a blunt object touches the skin, a number of receptive fields are stimulated-- some more than others. The receptive fields in the center areas where touch is the strongest will be stimulated more than those in neighboring fields where the touch is lighter. Stimulation will gradually diminish from the point of greatest contact, without a clear, sharp, border. A fuzzy sensation is not what is felt, as may be predicted, but a single touch with well-defined borders. This is a sharpening of sensation, producing a higher degree of sensory acuity. |
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Term
| Explain how the modalities of salty and sour taste are produced: |
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Definition
| They are produced by the different chemicals that come into contact with the microvilli of the taste cells. The salty taste is due to the prescence of Na+, which activate specific receptor cells for the salty taste. Na+ passes into the sensitive receptor cells through channels in the apical membranes. This causes a depolarization in the cells, which opens Ca2+ channels, causing them to release their neurotransmitter. The mechanism for the sour taste is the same as in the salty taste, except the ion responsible for the sour taste is H+, and the degree of sourness corresponds to the fall in pH within the cells. |
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Term
| Explain how the modalities of sweet and umami taste are produced: |
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Definition
| They are produced when sugars bind to the microvilli of the taste cells, coupled to G-proteins called gustducins. The gustducin G-protein subunit dissociates and activates 2nd messenger systems, leading to the closing of K+ and the depolarization of the receptor cell, causing it to release its neurotransmitter. Sugars taste sweet to varying degrees when bound to the G-protein-coupled receptor "tuned" for detection of a sweet taste. Umami evokes a "meaty" sensation in response to proteins, through the binding of amino acids L-glutamate and L-aspartate along with the sweet taste, but both umami and sweet modalities use the same mechanism. |
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Term
| Explain how the modality of bitter taste is produced: |
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Definition
| It is produced when quinine binds to the microvilli of the taste cell, coupled to G-proteins called gustducins. The gustducin G-protein subunit dissociates and activates 2nd messenger systems. Instead of causing a depolarization, it causes Ca2+ channels to open (like with salty and sour) but the Ca2+ is released from the smooth ER, causing it to release its neurotransmitter. |
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Term
| Explain how odorant molecules stimulate their receptors: |
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Definition
| The axon from each olfactory sensory neuron conveys information relating only to the odorant molecule that binds to and activates its specific receptor protein (a G-protein-coupled receptor). Before the odorant molecule binds to its receptor, the receptor is associated with 3 G-protein subunits (a, B and y). When the odorant molecule binds to its receptor the subunit dissociates, and move into the plasma membrane and a subunit binds to adenylate cyclase and activates this enzyme. Adenylate cyclase catalyzes the conversion of ATP into cAMP and PPi. cAMP acts as a 2nd messenger, opening ion channels tha allow inward diffusion of Na+ and Ca2+, producing a receptor potential, which stimulates production of action potentials. |
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Term
| How is the information after odorant molecules stimulate their receptors sent to the brain? |
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Definition
| Once the action potential is produced, the bipolar sensory neurons (which project through holes in the cribriform plate of the ethmoid bone) synapse with neurons located in glomeruli, which are located in the olfactory bulb. After the bipolar neurons synapse with the 2nd order neurons the information is transmitted directly to the cerebral cortex. |
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Term
| Why is our sense of smell so keen? |
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Definition
| Because the brain integrates the information from many different receptor inputs (each axon relates to 1 of 350 olfactory receptor proteins), and interprets the pattern as a characteristic "fingerprint" for a particular odor. |
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Term
| Describe the structures of the utricle and saccule: |
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Definition
| Part of the vestibular apparatus, they are the "otolith organs." Each have a patch of specialized epithelium called a macula, consisting of hair cells and supporting cells. The hair cells contain 20-50 extensions called stereocilia and one larger extension called a kinocilium. these hair cells project into the endolymph-filled membranous labyrinth, and the hairs are imbedded in a gelatinous otolithic membrane containing microscopic crystals called otoliths. |
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Term
| Describe the structure of the semicircular canals: |
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Definition
| Part of the vestibular apparatus, the 3 semi-circular canals project in 3 different planes at nearly right angles to one another. Each canal contains an inner extension of the membranous labyrinth called the semicircular duct, and at the base of each duct is an enlarged swelling- the ampulla. The crista ampularis (an elevated area of the ampulla) is where the sensory hair cells are located. The processes of the hair cells are imbedded in a gelatinous membrane called the cupula, which has a higher density than the surrounding endolymph. |
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Term
| Explain how the utricle and saccule function to produce a sense of equilibrium: |
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Definition
| Static equilibrium- The utricle is more sensitive to horizontal acceleration and the saccule to vertical acceleration. In the utricle during forward or backward acceleration, the otolithic membrane pushes the hair cells backward, or forward, respecively; and when a person accelerates downward or upward (as in an elevator) the hair cells are pushed upward or downward, respectively. the inertia produces a changed pattern of action potentials in sensory nerve fibers, maintaining equilibrium. |
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Term
| Explain how the semicircular canals function to provide a sense of equilibrium: |
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Definition
| Kinetic equilibrium--The semicircular canals are sensitive to rotational acceleration. The endolymph provides inertia in that rotation acceleration bends the cupula (in which the sensory processes of the hair cells are imbedded). The inertia produces a changed pattern of action potentials in the sensory nerve fibers, maintaining equilibrium. Hair cells in the anterior semicircular canal are stimulated during a somersault, in the posterior semicircular canal are stimulated during a cartwheel and in the lateral semicircular canal are stimulated during the spinning of the long axis of the body. |
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Term
| Explain how sound waves result in movements of the oval window and then the basilar membrane: |
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Definition
| Vibrations of the stapes and oval window displace perilymph fluid within the scala vestibuli (the upper of the 3 chambers in the cochlea). If frequency (pitch) is low, pressure waves that move the peilymph in the scala vestibuli passes to the scala tympani (the lower of the 3 chambers), because the perilymph is continuous at the apex, where it ends blindly in the helicotrema. Movements of perilymph within the scala tympani then displaces the round window, because perilymph cannot be compressed. If frequency is high, the pressure waves don't have time to travel all the way to the apex and must be transmitted through the vestibular membrane, which seperates the scala vestibuli from the cochlear duct and through the basilar mambrane, which seperates the cochlear duct to the scala tympani. This way, sound waves transmitted through perilymph from the scala vestibuli to the scala tympani produces displacement of the vestibular membrane and the basilar membrane. |
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Term
| Explain how movements of the basilar membrane affect hair cells: |
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Definition
| The hair cells are located on the basilar membrane, and their stereocilia project into the endolymph of the cochlear duct. There are two categories of hair cells, inner, which extends the length of the basilar membrane, which relay information regarding sound to the brain, and outer, which aid the sensory function of the inner hair cells. The stereocilia of the hair cells are imbedded in the gelatinous tectorial membrane, which overhangs the hair cells in the cochlear duct. The basilar membrane, hair cells and tectorial membrane forms the organ of Corti. When the cochlear duct is displaced by pressure waves of perilymph, the shearing force created by the basilar membrane and tectorial membrane causes stereocilia to bend, mechanically opening ion channels in the tops of the stereocilia. This causes a depolarization releasing glutamate as a neurotransmitter. The greater the displacement of the basilar membrane and bending of the stereocilia, the greater the amount of neurotransmitter released by the inner hair cell, and therfore a greater receptor potential. By this means, a greater bending of stereocilia will result in a higher frequency of action potentials, which will be percieved as a louder sound. |
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Term
Explain how movements of the basilar membrane at different sound frequencies affect hair cells: |
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Definition
| The basilar membrane vibrates maximally at the locations determined by the sound frequency (pitch). ex. The smaller apex of the cochlea vibrates most in response to low frequencies, the base responds most in response to high frequencies. This vibration stimulates hair cells at those locations, which activate sensory neurons of CN VIII that convey action potentials to the brain. The brain then interprets those action potentials from different regions of the cochlea as sounds of different pitches. |
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Term
| How are different pitches of sounds distinguished by the cochlea, and how does this affect the function of the cochlea? |
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Definition
| Different frequencies of sound stimulate different sensory neurons that project to different places in the auditory cortex. This means that the cochlea is like a "frequency analyzer" and the analysis is based on which hair cells activate the sensory neurons, and this, in turn, is related to the position of the hair cells on the basilar membrane. this is known as the place theory of pitch, and since the different sensory neurons project to different places in the auditory cortex, the organization of this cortex is said to be tonotopic. |
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Term
| How can hair cells stimulate associated sensory neurons? |
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Definition
| When the stereocilia bend, ion channels in the tops of the stereocilia are opened mechanically. This causes a rapid diffusion of cations into the hair cells, and thus a depolarization. The depolarized inner hair cells then release glutamate as a neurotransmitter, which stimulates associated sensory neurons. |
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Term
| Describe the neural pathway for hearing once an action potential has been produced: |
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Definition
| Sensory neurons in the vestibulocochlear nerve (CN VIII) synapse with neurons in the medulla oblongata that project to the inferior colliculus of the midbrain. Neurons in this area project to the thalamus, which sends axons to the auditory cortex of the temporal lobe. By means of this pathway, neurons in different regions of the basilar membrane stimulate neurons in corresponding areas of the auditory cortex. Each area of this cortex thus represents a different part of the basilar membrane and a different pitch. |
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Term
| Describe the structures of the eye, and how these focus light on the retina: |
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Definition
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Term
| Explain how accomodation at different distances is accomplished: |
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Definition
| Accomodation is the ability of the eyes to keep an image focused on the retina, as the difference between the eyes and the object varies. This results from contraction of the ciliary muscle, which is like a sphincter. When the ciliary muscle is relaxed, its apeture is wide, and this places tension on the zonular fibers of the suspensory ligament and pulls the lens taut and flat. This is for vision 20 feet or more from a normal eye. As an object moves closer to the eyes, the ciliary muscle contracts and its apeture is narrow. This releases tension on the zonular fibers, making the shape of the lens more rounded and convex. This is for vision of an object 20 feet and closer. |
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Term
| Explain common disorders of refraction: |
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Definition
Presbyopia- Loss of accomodating ability with age; due to reduced flexibility of the lens, and forward movement of the zonular fibers to the lens. Hyperopia- Rays focus behind the retina; when the eyeball is too short; also known as farsightedness, and is corrected with convex lenses. Myopia- Rays focus in front of the retina; when the eyeball is too long; also known as nearsightedness, corrected by concave lenses. Astigmatism- Rays do not focus on the retina; due to significant asymmetry of the cornea and lens, corrected by uneven lens to compensate for the asymmetry. |
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Term
| Why is an inverse image produced on the retina? |
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Definition
| Because of the way light is refacted on the lens. The image is formed on the retina upside down and backwards, and the cornea and lens focus the right part of the visual field in the left half of the retina of each eye, and the left part of the image on the right half of the retina of each eye. |
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Term
| Describe the structure of the retina: |
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Definition
| It consists of a single cell-thick pigmented epithelium, photoreceptor neurons (rods and cones) and layers of other neurons. The neural layers face outward towards the incoming light, so it must pass through several neural layers before striking the photoreceptors. These layers are the scelera, choroid layer and pigment epithelium. Then, the light strikes the photoreceptors and these synapse with bipolar cells and then ganglion cells, which contribute axons to the optic nerve. In addition to the flow of photoreceptors to bipolar neurons to the ganglion cells, horizontal cells synapse with several photorecetors (and possibly bipolar cells) and neurons called amacrine cells synapse with several ganglion cells. |
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Term
| How does light affect rhodopsin? |
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Definition
| Each rod contains thousands of molecules of a purple pigmant called rhodopsin in discs of the outer segments of the receptor cells. When rhodopsin responds to absorbed light, it dissociates into its two components- the pigment retinaldehyde (or retinene or retinal) and a protein called opsin. This reaction is called the bleaching reaction. Retinal exist in two possible configurations- all-trans and 11-cis. Only 11-cis is found attached to opsin. Upon exposure to light, 11-cis retinal and opsin is converted into all-trans retinal and dissociated from opsin. This photochemical reaction induces changes in ionic permeablity, which results in stimulation of ganglion cells in the retina. Once the absorbtion of light causes the formation of all-trans retinal, the all-trans retinal is transported from the photoreceptors to the pigment epithelial cells. There, it is reisomerized into the 11-cis retinal form and transported back to the photoreceptors, where it can bind to opsin and form rhodopsin again. |
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Term
| Explain how light effects synaptic activity in the retina: |
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Definition
A. In the dark, the continuous dark current 1) depolarizes the photoreceptors because the Na+ channels are kept open by cGMP and causes them to 2) release their neurotransmitter at their synapses with bipolar cells, and the bipolar cell doesn't stimulate the ganglion cell. B. In the light, 1) cGMP declines due to its conversion to GMP, which closes the Na+ channel, hyperpolarizing the photoreceptors. As a result, 2) the release of inhibitory neurotransmitter is stopped because they are not stimulated in the light. 3) Bipolar cells release excitatory neurotransmitter at their synapses with ganglion cells, so that the ganglion cells are stimulated to produce action potentials. |
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Term
| Describe the neural pathways of vision: |
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Definition
| The right part of the visual field is focused on the left half of the retina, and the left part of the visual field is focused on the right half of the retina. Axons from ganglion cells in the left half of the left retina pass to the left lateral geniculate nucleus of the thalamus, and axons from the nasal half of the right retina decussate in the optic chiasma and also synapse in the left lateral geniculate nucleus. Axons from ganglion cells in the right half of the right retina pass to the right lateral geniculate nucleus of the thalamus, and axons from the nasal half of the left retina decussate in the optic chiasma and also synapse in the right lateral geniculate nucleus. Both lateral geniculate bodies of the thalamus project to the striate cortex of the occipiotal lobe, which is called the geniculostriate system, and is involved in perception of the visual field. |
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Term
| Compare the function of rods and cones: |
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Definition
| Rods are for less accurate black and white vision in dim lighting. Rods are more sensitive to light than cones, and they absorb more green light than red and blue light (because of the high amounts of rhodopsin). Cone are for acute color vision in bright light. Cones are less sensitive to light than rods, and in bright light the rods are bleached out. |
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Term
| Describe the significance of the fovea centralis: |
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Definition
| It is the pinhead-sized area of the retina with the highest visual acuity, because there are about 4,000 cones in the fovea which synapse with 4,000 ganglion cells- a ratio of 1:1 compared to other areas of the retina with about 120 million rods and about 6 million cones (mixed) with an overall convergence ratio of about 150:1. Since each cone in the fovea has a private line to a ganglion cell, and each ganglion cell receives input from a tiny region of the retina, visual acuity is greatest and sensitivity to low light is poorest when light falls on the fovea. |
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Term
| Explain how dark adaptation occurs: |
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Definition
| The bleaching reaction that occurs in the light results in a lowered amount of rhodopsin in the rods and lowered amount of pigments in the cones. When a light-adapted person first walks into a dark room, sensitivity to light is high, and vision is poor. Photoreceptor sensitivity gradually increases, (dark adaptation) reaching its maximum in about 20 minutes. This increased sensitivity to low light is partly due to increased visual pigments produced in the dark, but the main way dark adaptation is produced is by increased rhodopsin uin the rods. |
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Term
| Explain what is meant by the tri-chromatic theory of color vision: |
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Definition
| There are 3 different cones responsible for human color vision- blue, green and red. Blue cones are S cones (short wavelengths) green cones are M cones (medium wavelengths) and red cones are L cones (long wavelengths). One layer of the lateral geniculate nucleus of the thalamus adds the input from the L and M cones to obtain information about light intensity. Another layer subtracts light from the L and M cones to obtain red-green color information. A third layer subtracts input from the S cones from the combined L and M input to obtain blue-yellow color information. These 3 "channels" stay anatomically seperate as they project from the primary visual cortex. |
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Term
| Compare the architecture of the fovea centralis with more peripheral regions of the retina; and how does this architecture relate to visual acuity and sensitivity: |
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Definition
| Since the fovea centralis is the point of the retina that provides the highest visual acuity due to the high concentration of cones that are each innervated by a single ganglion cell, the peripheral regions do not percieve images as acutely. This is because in the periphery, there are 120 million rods and 6 million cones, mixed. The ratio of rods and cones to ganglion cells is about 150:1. Visual acuity is greatest and sensitivity to low light is poorest when light falls on the fovea. In dim light, only the rods are activated, and vision is best out of the corners of the eye when an image falls away from the fovea. The convergence of large numbers of rods on 1 bipolar cell and the convergence of large numbers of biplar cells on a single ganglion cell increase sensitivity to low light at the expense of visual acuity, so night vision is therefore less distinct than day vision. |
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Term
| Describe the neural pathway for the tectal system. What are the functions of these pathways? |
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Definition
| 20-30% of the fibers from the retina follow a path to the superior colliculus of the midbrain, (optic tectum) which lead to activation of motor pathways leading to the eye and body movements. Its function is tracking of movements of objects in the visual field. |
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Term
| Describe some of the higher processing of visual information: |
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Definition
| In the dark, ganglion cells discharge at a slow rate. When the lights turn on, the firing rate of many (but not all) ganglion cells increases slightly. With some ganglion cells however, a small spot of light causes a large increase in firing rate. A small spot of light can be more effective than a large area of light. When a spot of light is moved only a short distance away from the receptive field, the cell that was originally stimulated is inhibited by light in its periphery. The responses produced by light in the center and produced by light in the "surround" of the visual field are antagonistic. The ganglion cells stimulated by light at the center of the visual fields are said to have on-center fields; those inhibited by light in the center and stimulated by light in the surround are said to have off-center fields. The reason for this is that diffuse lighting gives the ganglion cell conflicting orders- on and off. Because of the antagonism between the center and surround, the activity of each ganglion cell is due to the result of the difference in light intensity between the center and surround of the visual field. This is a form of lateral inhibition, which helps accentuate contours of images and improve visual acuity. |
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Term
| Describe the stimulus requirements of simple cortical neurons: |
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Definition
| They are best stimulated by a bar or a slit of light located in a precise part of the visual field (of either eye) at a precise orientation. |
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