Ch 15 Special Senses Part 1 - Smell, Taste, Vision PDF

CH 15 SPECIAL SENSES OLFACTION (SMELL) GUSTATIONS (TASTE) VISION EQUILIBRIUM (BALANCE) HEARING All originate at sensory receptor cells, they could be neurons or specialized receptor cells tha...

CH 15 SPECIAL SENSES OLFACTION (SMELL) GUSTATIONS (TASTE) VISION EQUILIBRIUM (BALANCE) HEARING All originate at sensory receptor cells, they could be neurons or specialized receptor cells that communicate Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Olfaction Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved CRANIAL NERVE I (olfactory nerve) Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Physiology of Olfaction Odorants in inhaled air are detected by olfactory neurons; chemical stimuli are transduced to electrical signals; transmitted to various regions of brain for identification Activation of olfactory receptors: odorants are dissolved in mucus surrounding olfactory neurons cilia; odorant-binding proteins transport odorants through mucus to receptors on cilia of olfactory neuron Once action potential is generated, odorant has been transduced from chemical stimulus to electrical (neural) signal TRANSDUCTION – the conversion of stimulus into an electrical signal the brain can interpret (odorants) Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Physiology of Olfaction Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Physiology of Olfaction – Olfactory tract is only sensory pathway that bypasses thalamus – Primary olfactory cortex – – responsible for awareness and identification of odor – Amygdala, hippocampus, hypothalamus, and components of limbic system receive information from primary olfactory cortex; evokes emotional and visceral responses to odors Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  TASTE (GUSTATION) Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Structures of Gustation Taste reception = gustatory receptor cells Distributed over superior surface of tongue and adjacent portions of the pharynx and larynx Taste buds – tongue is covered with papillae (rounded projections) that can be further classified based on shape: CHEMORECEPTORS – Vallate (circumvallate) papillae – largest of four classes; dome-shaped – Fungiform papillae – mushroom-shaped; contain few taste buds – Foliate papillae – ridges on sides of tongue; only contain taste buds in childhood MECHANORECEPTOR – Filiform papillae – long, thin cylinders scattered across tongue; devoid of taste buds; contain sensory nerve endings; detect food texture and temperature. ▪ provide friction; helps tongue move objects around in mouth Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Structures of Gustation Taste buds – usually located on lateral surfaces of papillae; Gustatory (taste) cells –display receptors that detect different tastes; associated sensory neurons carry taste stimuli to CNS via CN VII, CN IX, or CN X Each gustatory receptor cell extends slender microvilli (taste hairs) into the surround fluids through the taste pore, a narrow opening. A typical gustatory receptor cells services about 10 days before it is replaced. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Physiology of Gustation Sense of taste involves signal transduction of chemicals in food into electrical signals that can be sent to CNS 4 primary taste Taste sensations: taste relies on detection of five classes of chemicals: sensations – Sweet tastes – elicited by simple sugars (glucose, fructose) – Sour tastes – produced by hydrogen ions (citric acid in lemon juice) – Salty tastes – elicited by presence of metal ions (sodium and potassium ions) – Bitter flavors – produced by nitrogen-containing compounds – Umami – taste associated with meat or broth; produced by glutamate or other amino acids – Water – demonstrated in humans and concentrated in pharynx, sensory output for these receptors processed in hypothalamus and affects water balance and regulation of blood volume. There is some evidence that sensitivity to the four primary taste sensation varies along the long axis of the tongue. However, there are no differences in the structure of the taste buds and taste buds in all portions of the tongue provide all four primary taste sensations. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Detection of taste begins, like olfaction, by dissolved chemicals that binds to receptors on microvilli (taste hairs) and either; ▪ diffuse through membrane leak channels or ▪ bind to receptor proteins of gustatory receptor cells – Taste receptors – classified by substance they detect, with only one type of receptor associated with individual gustatory cell – Activation of taste receptors – substance must first dissolve in saliva before it can reach taste bud where it may be detected as gustatory stimulus ~90% of gustatory receptor cells respond to two or more different taste stimuli, the different tastes involve different receptor mechanisms Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  The released neurotransmitters enter synapses with the dendrites of adjacent sensory neurons. Depolarization of sensory neurons leads to a generator potential and the propagation of action potentials along the gustatory pathway to CNS Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved VISION Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Accessory structures of eye; Provides protection while allowing light to enter the eye Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Lacrimal Apparatus - produces, distributes and removes tears reduce friction remove debris prevent bacterial infection provide nutrients and oxygen Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved The Eyeball Figure 15.9 Midsagittal section of internal structures of the eye. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  The Eyeball Two muscles control the amount of light entering the eye and passing through the lens; 1. Dilator pupillae 2. Sphincter pupillae Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved When an ophthalmologist uses an ophthalmoscope to look into your eye they see the following view of the retina. In the center of the retina is the optic nerve, a circular to oval white area measuring about 2 x 1.5 mm across. From the center of the optic nerve radiates the major blood vessels of the retina. Approximately 17 degrees (4.5-5 mm), or two and half disc diameters to the left of the disc, can be seen the slightly oval-shaped, blood vessel-free reddish spot, the fovea, which is at the center of the area known as the macula. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Vision Vision – perception of light reflected by various objects Eyes and visual pathways in CNS can determine object’s size, shape, and color Object distance, rate, and direction of movement can also be interpreted Some energy exists as electromagnetic radiation; range of wavelengths measured in nanometers (nm) Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Focusing Light on the Retina Clear vision requires that light rays are focused on the retina, not in front or behind it. Two-thirds of eye’s refractive (light bending) power occurs as light passes through cornea; has refractive index close to water Lens provides for fine tuning and refractive adjustment Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Focal point – specific point on the retina Focal distance – distance between center of the lens and it’s focal point Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  An object in view is not a single point, the image consists of a large number of individual points like pixels on a screen. Light from each point is focused on the retina creating a miniature image of the original, however the image is inverted and reversed. The brain compensates for this and we are not aware of any difference. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Focusing Errors Near point of vision – The greatest amount of refraction is required to view objects that are very close to the lens, is determined by the degree of elasticity in the lens. ▪ Children typically can focus 7-9cm (3-4in) from the eye ▪ Young adult; 15-20cm (6-8in) ▪ Age 60 – 83cm (33in) Errors of refraction – limited accommodation due to aging lens or shape of eyeball Near point of accommodation – closest point at which eye can focus on an object; increases with age as lens becomes less flexible Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Errors of refraction – Myopia (nearsightedness)– distance between cornea and lens is too great or cornea is too curved ▪ Lens is unable to flatten enough and incoming light is focused in front of retina, blurring objects viewed at distance ▪ Concave lenses correct myopia by diverging incoming light before it contacts lens; redirects focus onto retina Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved – Presbyopia (age-related farsightedness) – individual’s near point of accommodation is 10–20 inches or greater ▪ Individual is usually in their 5th decade ▪ Difficulty reading; can be corrected with reading glasses or bifocals (lenses for both distance and near vision) – Hyperopia (farsightedness) – eyeball is too short or cornea is too flat ▪ Lens is unable to accommodate (become thick enough) to focus light on retina; instead focuses behind retina, causing blurry vision when looking at close objects ▪ Convex lenses correct hyperopia by causing more light to converge on retina Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Neural layer of the retina contains multiple layers of specialized photoreceptors, neurons and supporting cells Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Photoreceptors and the Retina Cell types layered in inner (neural) layer of retina Two types of photoreceptors, rods and cones; adjacent to outer pigmented epithelial layer of retina ▪ Cones – function best in bright light for processing high-resolution color vision ▪ Rods – do not detect colors; most sensitive in low light and as component of peripheral vision – Photoreceptors synapse with bipolar cells; neurons that communicate with retinal ganglion cells – Retinal ganglion cells – in anteriormost region of retina; axons form optic nerve (CN II) – Horizontal cells and amacrine cells – involved in image processing Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Photoreceptors and the Retina 2 color bands in this illustration of the retinal surface indicate the relative densities of cones and rods on either side of a horizontal line passing through the fovea centralis and optic disc of the right eye. When looking directly at an object the image falls on the fovea centralis – the center of color vision and image sharpness. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Photoreceptors and the Retina Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Photoreceptors and the Retina Structural features of rods – outer segments contain 1000s of flattened membranous plates or discs containing visual pigment molecules. – inner segments contains the photoreceptors major organelles and is responsible for all cell functions other than photoreception. Rhodopsin – all cones have, it does not distinguish color – Photopsin (outer segment)– similar to opsin but has slightly altered structure; allows it to absorb different wavelengths of light – Three forms of photopsin allow response to wavelengths perceived as blue, green, or red Specific to each cell Visual pigments are derivatives of the compound rhodopsin, the visual pigment found in rods. Consists of protein opsin, bound to pigment retinal, which is synthesized from Vitamin A. The type of opsin present determines the wavelength of light that can be absorbed by retinal. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved Rods and Cones All rods contain the same type of opsin and are most sensitive to blue-green wavelengths of light Three types of cones: – blue, green and red cones – each have different form of opsin sensitive to different range of wavelengths – their stimulation is various combinations is basis for color vision. – If all 3 stimulated we perceive the color as white – also perceive white if rods (but not cones) are stimulated, which is why everything appears black and white when light is dim. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  In darkness, gated sodium ion channels are kept open in the presence of cGMP (cyclic guanosine monophosphate). Because channels are open the membrane potential is -40mV rather than typical -70mV of resting neurons. At -40mV the photoreceptor is continuous liy releasing neurotransmitters across the synapses to bipolar cells. The inner segment also continuously pumps sodium ions out of the cytosol. The bound retinal molecule of rhodopsin has two possible configurations: bent/curved 11-cis form and the linear 11-trans form. On absorbing light, it changes, activating the opsin molecule continued… Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved  Opsin then activates transducin, a G protein bound to the disc membrane, then in turn activates the enzyme phosphodiesterase (PDE). PDE breaks down cGMP, the removal of cGMP from the gated sodium channels results in their inactivation. the rate of sodium into the cytosol then decreased Rhodopsin cannot respond to additional photons until its retinal components regain it original shape. The entire rhodopsin molecule must be broken down into retinal and opsin in a process called bleaching. This process requires energy in the form of ATP Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved The Visual Pathway Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved The Visual Field Visual images from left and right eyes overlap, the visual cortex of each cerebral hemisphere receives information from both eyes. The information is sorted, however, so that – the left visual cortex gets information on the right half of the visual field, and – the right visual cortex receives information on the left half of the visual field. Depth perception is the ability to judge depth or distance by interpreting the three- dimensional relationships among objects in view. Your brain perceives depth by comparing the relative positions of objects within the images received by both eyes. The map in the visual cortex is upside down and backward, duplicating the orientation of the visual image at the retina. Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved