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Flashcards in Sensation Deck (43):

How is the phototransduction cascade set off in rods?

Upon activation of rhodopsin bumping into transducin, a phosphodiesterase that degrades cGMP becomes active. This causes cGMP-gated cation channels to close, hyperpolarizing the cell. This causes some voltage gated calcium channels to close, causing less neurotransmitter (glutamate) to be released by the rod's axon.


What happens after the rods slow their rate of neurotransmitter release?

The bipolar cells they synapse onto are receiving less glutumate from the rod cell, which causes fewer of the metabotropic glutamate receptors on the bipolar cells to be active, leading to overall depolarization of the cell. More light = more active bipolar cell by less hyperpolarization.


What happens to rods when the retina is in complete darkness?

The rod cells become so saturated releasing cGMP that the phosphodiesterases begin deactivating, eventually lowering their baseline activation. This is referred to as dark adaptation, and it allows increased sensitivity at low light levels.


What are the three different types of cones?

S-cones, which respond to 425nm wavelengths (dark blue light), M-cones, which respond to 530nm light (green light), and L-cones, which respond to 560nm light (yellow/red). However, the full response curves for each of these receptors overlaps considerably.


How does the transduction structure differ for cones when compared to rods? How are they similar?

In addition to the ON metabotropic bipolar cells rods have, cones also have an OFF ionotropic bipolar cell they can synapse on. Cones' bipolar cells also synapse directly onto retinal ganglion cells (ON or OFF), while rod bipolar cells synapse onto amacrine cells, which then synapse onto ON cone bipolar cells via electrical synapses or OFF cone bidpolar cells via inhibitory synapses.


What does the increased sensitivity of rods come at the price of?

At the price of resolution, which the cones have in abundance.


How does retinoid recycling work?

After the retinal is converted to the all-trans isomer and float away from opsin, it is transferred to the pigment epithelium, a layer of cells that cover the rod/cone outer segments covered with a dense capillary bed called the choroid, which re-isomerizes the retinal and returns it to the cells to be reunited with opsin.


Why is the inverted design of the retina not such a terrible idea?

Because the transparent parts are between the outside and the photoreceptors, while the opaque parts are all behind the photoreceptors.


What are Muller cells?

These cells take light and focus it on the outer segments of the photoreceptors, allowing for bypassing the translucent layers in between.


Why do animals eyes shine when photographed sometimes?

Because some animals have a highly reflective layer called the tepetum lucidum in their pigment epithelium or the choroid, which allows for a "second chance" for the photons that missed the photoreceptors to interact with it as they bounce back from being reflected. Cool.


Where are odor-sensing cells located?

In the olfactory epithelium, which is in the roof of the nasal cavity, the external side of which is coated with mucus and contains the cilia of the olfactory sensory neurons.


How does mucus conduct smell?

By containing many odorant binding proteins, or by simply dissolving the odorants.


Are olfactory neurons replaced?

Yes, about once every month, but some can live for much longer as evidenced by mouse tracer experiments.


How many different olfactory receptors are there? How do they bind to their ligand?

Around 350 known receptors in humans. They bind to an odorant molecule through a specific epitope in that molecule. The same molecule may have multiple epitopes, and may be bound by multiple receptor types.


Describe the process of olfactory transduction.

First an odorant binds to an olfactory receptor. After this happens the G-protein Golf becomes active, which in turn activates adenylate cyclase, which converts AMP to cAMP, which binds to and opens a cAMP-gated cation channel, depolarizing the cell through the influx of Na+/Ca2+. This calcium also opens calcium-gated chloride channels, which causes Cl- to rush out of the cell, further depolarizing it. These olfactory sensory neurons can fire action potentials.


How many olfactory receptors does each olfactory sensory neuron contain?

One unique olfactory receptor,


How does the olfactory system encode information?

Combinatorially, since each odorant has multiple epitopes, the combination of which are unique to that odorant, the olfactory sensory neurons use this to encode smell combinatorially.


Where does the information from olfactory sensory neurons go?

From the olfactory sensory neuron's axons to glomeruli (globular structures around the axons) in the olfactory bulb through tiny holes in a bony plate that separates the two.


How many glomeruli does the olfactory bulb contain? How many inputs does this come from? Of what kind?

About 5,500 glomeruli, which comes from a staggering 7 million olfactory sensory neuron axons. Each glomerulus is highly specific to a certain kind of odorant receptor.


What is chemotopy?

The tendency of similar odorant molecules to be encoded in similar locations, and for molecules that are similar to each other in carbon structure to be encoded closer to each other.


What are mitral cells? How do they work?

Mitral cells, which project from the olfactory bulb to the olfactory cortex allowing for further smell processing. They are connected to glomeruli in an extremely specific way by granule cells, which are the most common cells in the olfactory bulb. They also contain long-range inhibitory projections to other mitral cells.


What are olfactory bulb granule cells?

These interesting cells have no axons, but rather reciprocal drodendritic synapses, which may allow for shutdown of specific olfactory sensations by feedback. The granule cell recieves excitatory input from the mitral cell and sends inhibitory input back to the mitral cell. These cells also synapse onto adjacent mitral cells, allowing for lateral inhibition.


What is the range of human hearing?

Around 20Hz to 20kHz in frequency and just above 0dB.


How much louder is a 110dB sound than a 90dB sound?

10^2 times larger. The decibel system is base-10 logarithmic, with 10 units representing one base of 10.


Describe the anatomy of the ear.

The outside of the ear is called the pinna, after which you have the ear canal. The ear canal connects to the timpannic membrane, which vibrates in response to sound (collectively the outer ear). This in turn is connected to the malleus/incus/stapes system, which pushes fluid in the cochlea through the oval window, which flexes out the round window (the middle ear). The cochlea then projects auditory axons away to the auditory cortex for processing (the inner ear).


What are the components of the cochlea?

The scala vestibuli is the up-winding section of the cochlea, from the oval window to the apex, and the scala tympani is the down-winding section of the cochlea, from the apex to the round window. They sit on top of each other and sandwiched between we have them (except at the apex) is the scala media, separated from the scala tympani by the basilar membrane.


What is the function of the malleus/incus/stapes system with the tymapanic membrane?

This system allows for amplification of sound waves and focus onto a single point (the oval window).


How fast does sound travel in air? Water?

Sound waves travel 343m/s in air, and 1,560m/s in saltwater.


What is the scala media?

The scala media is the part of the cochlea that sits between the scala vestibuli and the scala tympani, and allows for sound transduction. It sits on the basilar membrane, which can vibrate, allowing the scala media to move. The tectorial membrane vibrates similarly but in the opposite direction. This movement causes hair cells (outer and inner) to brush up against the tectorial membrane, causing sensory transduction.


Where do low-frequency sounds mostly go to? High-frequency sounds? Why?

Low-frequency sounds tend to vibrate the basilar membrane mostly at the apex, while high-frequency sounds tend to vibrate it mostly closer to the base. This is because the basilar membrane's width, tension, and stiffness decreases along the cochlea toward the apex.


How is sound transduced?

Vibration of the basilar membrane causes rows of stereocillia to brush up against the tectorial membrane, whose tip links stretch and cause depolarization of the hair cells by K+ and Ca2+ influx. When the hair cells are brushed towards the longest hair cell, the cells depolarize. When they are brushed towards the shortest hair cells, the stretch ion channels close. Hair cells, connected via ribbon synapses, then release glutamate onto postsynaptic terminals. The spiral ganglion cell terminals then cause an action potential to fire, which traverses the cell body and terminates in the cochlear nucleus.


How is the endolymph of the cochlea unique?

It is extremely high in K+, which is the depolarizing agent along with Ca2+ of the hair cells present.


How do hair cells repolarize?

They cell bodies are pathed in perilymph, which has a very low K+ ion concentration, allowing K+ diffusion out of the cell and into the perilymph, repolarizing the cell. The perilymph and endolymph are separated by tight junctions.


What maintains the concentration gradients in the endolymph and the perilymph?

Specialized epithelia/l cells in as tructure called the stria vascularis, which sits on a rich capillary bed.


What are the sensors in tendons and joints?

There exist joint sensors, but they provide relatively minor information. Golgi tendon organs, whose axon terminals innervate various points along the tendon and allow sensing of compression and convey information about muscle tension.


How is muscle movement sensed?

This is sensed by muscle spindles, which are sandwiched between the principal muscle fibers of skeletal muscle, and innervated by a few axons. Some of these form annulospiral endings, which wrap around fibers near their center, while tohrers terminate off-center and form flower-spray endings. Both of these are activated when the muscle is stretched. Annulospiral endings respond most to the angular velocity of a stretch, and can sense the direction of stretch, while flower-spray endings encode stretch magnitude.


Where do muscle tension sensors project to?

Golgi tendon organs and annulospiral endings project via A-alpha fibers to Rexed's laminae VI, VII, and IX, as well as to the dorsal column tracts, terminating before the dorsal column nuclei in the accessory cuneate nucleus and Clarke's coclumn, which project to the ipsalateral cerebellum. These are called spinocerebellar projections.


Where do muscle velocity sensor project to?

Flower-spray endings project via A-beta fibers to Rexed's laminae III through V.


How is vestibular information sensed?

Through the vestibule and semicircular canals.


How do we sense head tilt?

Clusters of hair cells called maculae sit in the utricle of the vestibulae and the sacculus of the vestibulae, as well as a kinocillium adjacent to the tallest stereocillia.


What are some common themes of contact sensor organization?

1. Laballed lines.
2. Variablility in sensor range and sensitivity
3. Sensory maps


Where does the vestibular apparatus project to?

This projects to the brain via the eighth cranial nerve, also known as the vestibulocochlear nerve, because it also contains auditory fibers from the cochlea. These axons terminate primarily in the vestibular complex of the dorsal medulla. Axons from the saccule, utricle, and semicircular canals terminate in separate but overlapping portions of the vestibular complex, and these project in turn to a wide variety of targets including in the spinal cord and brainstem, to control eye and head movements.


How does vestibular transduction in the utricle work?

When hair cells are deflected toward the tallest stereocillia, they are depolarized by K+/Ca2+ entry, and release glutamate onto the innervating nerve cells. They bend in response to movement of the otoconial membrane as it moves sideways.