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Sensory Systems (Chapter 41)

Terms in this set (40)

Olfaction—sense of smell; depends on chemoreceptors.
Olfactory sensors are embedded in epithelial tissue at top of nasal cavity (in vertebrates).
Axons extend to the olfactory bulb in the brain, dendrites end in olfactory hairs on the nasal epithelium.
Olfactory receptors communicate directly with the brain; they are very close to the brain spatially.
There are two olfactory bulbs in the brain (for symmetry).

Odorant molecules (things that smell) enter the nasal cavity when we breath. they brush against and get trapped in the mucous film on the olfactory epithelium at the top of nasal cavity. g-protein coupled chemoreceptors on the dendrites of olfactory receptor cells reaching into the mucous film bind with the odorant molecules. the olfactory receptor cells have axons reaching to the glomeruli of the olfactory bulb in the brain right above the nose.
In mammals, the main olfactory system detects odorants that are inhaled through the nose, where they contact the main olfactory epithelium, which contains various olfactory receptors. Olfactory neurons transduce receptor activation into electrical signals in neurons. The signals travel along the olfactory nerve. This nerve terminates in the olfactory bulb (which is part of the CNS). The olfactory tract takes info from the olfactory bulb to the cerebral cortex.

Rather than binding only to specific ligands like most receptors, olfactory receptors display affinity for a range of odor molecules. The complex set of olfactory receptors on different olfactory neurons can distinguish a new odor from the background environmental odors and determine the concentration of the odor.

In the olfactory system, odorant molecules in the mucus bind to G-protein coupled receptors on olfactory cells. The G-protein activates a downstream signalling cascade that causes increased level of cyclic-AMP (cAMP), which trigger neurotransmitter release. How it works:
1) odorant ligands bind to g-protein coupled chemoreceptor embedded in membrane of dendrite.
2) this activated the attached olfactory g-protein, which goes over and activates the AC
4) the now activated adenylate cyclase (AC) converts cytosolic ATP into cAMP.
5)the higher cytosolic concentration of cAMP activates nearby gated cation channels to let in Na+ and Ca2+, depolarizing cell and causing the major changes that Ca2+ causes when it enters a cell.
6)the presence of Ca2+ inside the cell opens a gated Cl- channel and Cl- flows out of the cell, further depolarizing it.
7)the olfactory receptor cells bring the generated electrical signal to a glomerulus in the olfactory bulb.
The cochlea has three fluid-filled sections or canals (the vestibular canal, the middle canal, and the tympanic canal), and contains a fluid wave. the fluid wave is driven by pressure (from the oval window) across the basilar membrane separating the middle and tympanic canals.

The vestibular and middle canals are separated by the Reissner's membrane. The middle and tympanic canals are separated by the basilar membrane. The organ of corti sits inside the middle canal, on the basilar membrane.The tectorial membrane is inside the middle canal and connected to the organ of Corti. The auditory nerve fibers come in to the cochlea between the tectorial and basilar membranes and connect to the organ of Corti to receive the electrical info that it generates.

The cochlea is filled with a watery liquid, the perilymph, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, the cochlear partition (basilar membrane and organ of Corti) moves; thousands of hair cells sense the motion via their stereocilia, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. These primary auditory neurons transform the signals into electrochemical impulses known as action potentials, which travel along the auditory nerve to structures in the brainstem for further processing.

The round window is a flexible membrane at the end of the canal, where pressure waves coming along the tympanic canal can leave the inner ear.
Pressure waves can travel all the way around the entire spiral to reach the round window, or take a "shortcut" across the basilar membrane—flexes the membrane. Low frequency sounds (low Hertz) will go slowly through the whole inner ear, while high frequency sounds (high Hz) will go across the basilar membrane in a "short-cut" to the round window. the higher the frequency of the wave hitting the ear drum, the higher-pitched it sounds to us.