L10 Sensory Systems

What are chemoreceptors responsible for mediating?

What are the two main categories of chemoreceptors, and what sensory functions do they include?

Chemoreceptors mediate the detection of chemicals

The two main categories are exteroceptors, which include olfaction (smell), gustatory (taste), and nociception (pain), and interoceptors, which include blood pH, chemosensors in the stomach, and internal nociception (pain)

Where is the olfactory epithelium located in the mammalian olfactory system?

What is the function of the mucus layer in the olfactory epithelium?

What is the role of odorant binding proteins in olfaction?

What kind of neurons are olfactory receptor cells?

What type of receptors do odorant receptor neurons (ORNs) express?

Where are odorant receptor proteins located within ORNs?

The olfactory epithelium is located in the roof of the nasal cavity

The mucus layer moistens the olfactory epithelium and dissolves odorants

Odorant binding proteins allow lipophilic odorants to dissolve in mucus

Olfactory receptor cells are ciliated bipolar neurons

Odorant receptor neurons (ORNs) express G-protein coupled receptors

Odorant receptor proteins are located in the ciliated dendrites of bipolar odorant receptor neurons (ORNs)

What happens when an odorant binds to an odorant receptor?

Describe the signal transduction pathway initiated by odorant binding in ORNs

How many odorant receptor proteins does each olfactory neuron express?

Can a single odorant receptor recognize more than one odorant?

Can a single odorant stimulate more than one receptor?

How are odors perceived by the brain?

Binding of an odorant to an odorant receptor causes a conformational change, leading to ORN depolarization and action potential firing

The activated G protein, Golf, moves through the membrane and activates adenylate cyclase. Adenylate cyclase converts ATP into cAMP. cAMP opens cAMP-gated ion channels, allowing Ca^{2+} and Na^{+} to enter the cell, causing a generator potential. The Ca^{2+} also opens Ca^{2+}-activated CI channels, causing CI to leave the cell and increasing depolarization. The generator potential then opens voltage-gated Na^{+} channels, triggering action potentials

Each olfactory neuron expresses only one odorant receptor protein

Yes, each receptor can recognize more than one odorant

Yes, each odorant can stimulate more than one receptor

Odors are perceived by the brain using a combinatorial code, where each perceivable odor activates a unique set of odorant receptors and olfactory neurons

What are vertebrate gustatory receptors composed of?

How are taste receptor cells distributed across the tongue?

What are the steps of signal transduction for salty taste?

What are the steps of signal transduction for sour taste?

What are the steps of signal transduction for sweet, bitter, or umami taste?

Vertebrate gustatory receptors are taste buds, which are composed of neuroepithelial taste receptor cells (TRCs) and accessory cells

Taste receptor cells are distributed across the tongue with regional differences in sensitivity to certain modalities such as salty, sour, sweet, bitter, or umami

Na^{+} from salty food enters through sodium channels (ENaCs). The resulting depolarization triggers action potentials through voltage-gated sodium channels. ATP is then released from the taste receptor cell, which activates P2X receptors on the GRN

H^{+} from sour foods activates pH-sensitive proton channels (Otop1) and enters the cell. The lowered pH inhibits K^{+} channels and depolarizes the membrane, activating voltage-gated Na^{+} (VGNC) and Ca^{2+} (VGCC) channels. Increased intracellular Ca^{2+} leads to neurotransmitter release and activation of the GRN

The sweet, bitter, or umami substance binds to its receptor, causing a conformational change. The activated G protein, via its ßy subunits, activates phospholipase C (PLC). PLC catalyzes the conversion of PIP_{2} into the second messenger IP_{3}. IP_{3} causes the release of Ca^{2+} from intracellular stores. Ca^{2+} activates TRPM channels, which are non-selective, and Na^{+} enters the cell. This membrane depolarization opens a voltage-gated channel (CALHM1/3). ATP is then released from the cell and serves as a neurotransmitter by binding to P2X receptors on the afferent neuron

How do mechanoreceptors convert mechanical stimuli into cellular signals?

What are some variations of mechanoreception found in vertebrates?

Where are touch receptors most densely innervated in the skin?

What are the different types of touch receptors based on their receptive fields and adaptation properties?

Mechanoreceptors couple mechanical stimuli to ion channels

Variations include touch/pressure, proprioception, equilibrium/balance, hearing, and baroreception (blood pressure sensing)

The hands and face are the most densely innervated regions

Touch receptors can have large vs small receptive fields and exhibit fast vs slow sensory adaptation

What is the role of proprioceptors?

What do muscle spindles and Golgi tendon organs detect?

Proprioceptors monitor the position of the body in space

Spindle fibers detect the stretch of muscles, and Golgi tendon organs detect the stretch of tendons

What type of cells are mechanosensors for hearing and balance in vertebrates?

What is the ionic composition of the endolymph that bathes hair cells?

What happens when stereocilia move in hair cells?

Do hair cells release neurotransmitters in the absence of deflection?

How do stereocilia deflections affect hair cells?

Where are the mechanoreceptors for equilibrium located?

What structures within the vestibular apparatus detect movements?

What do the maculae of the utricle and saccule detect?

What do the cristae of the semicircular canals detect?

Hair cells are mechanosensors for hearing and balance in vertebrates

Hair cells are bathed in "endolymph," which contains high K^{+} relative to the cell

Movement of the stereocilia opens the mechanically-gated cation channels (TMC1/2)

Yes, hair cells maintain transmitter release onto afferent neurons, even in the absence of deflection

Stereocilia deflections modulate K^{+} conductance and transmitter release in hair cells

The mechanoreceptors for equilibrium (balance) are located in the inner ear

Hair cells of the vestibular apparatus detect movements

Maculae of the utricle and saccule detect linear acceleration and tilting

Cristae of the semicircular canals detect angular acceleration

What three parts of the ear are involved in hearing?

Describe the process of sound transduction in the ear

What do hair cells in the Organ of Corti detect?

What is the function of outer hair cells in hearing?

Sound waves enter the ear canal and vibrate the tympanic membrane. Middle ear bones transmit the vibration to the cochlea, causing the oval window to vibrate. Pressure waves in the perilymph of the cochlea cause the basilar membrane and tectorial membrane to vibrate. Stereocilia on the hair cells bend, causing the hair cells to depolarize. Inner hair cells release the neurotransmitter glutamate, which excites the afferent neuron. The round window serves as a pressure valve

Hair cells in the Organ of Corti detect basilar membrane movements

Outer hair cells amplify sounds by "somatic electromotility," where they change length in response to stimulation, increasing the deflection of the basilar membrane and amplifying signals to inner hair cells

hat is the primary function of photoreceptors?

What type of receptors are photoreceptive opsins?

Where do opsins localize in vertebrate photoreceptors?

How does phototransduction occur?

Summarize the light-induced events that occur during phototransduction

What are the two classes of photoreceptors found throughout the animal kingdom?

What is the role of rhabdomeric photoreceptors discovered in vertebrates?

How do rhabdomeric photoreceptors signal?

How do ciliary photoreceptors signal?

Photoreceptors convert light energy into changes in membrane potential

Photoreceptive opsins are seven-membrane-spanning GPCRs

Opsins localize to membranes in the outer segments of vertebrate photoreceptors

Phototransduction occurs through chromophore isomerization

Opsins covalently bind vitamin-A derived chromophores. Photons cause isomerization of the chromophore, which leads to conformational changes in the opsin and dissociation of the chromophore from opsin ("bleaching"). These G-protein signaling events cause changes in membrane potential

The two classes are rhabdomeric photoreceptors and ciliary photoreceptors

These photoreceptors help entrain our internal clocks to environmental light/dark cycles

Rhabdomeric photoreceptors signal through Gq

Ciliary photoreceptors signal through Gi/transducin via cyclic nucleotide-gated (CNG) channels

What is the primary function of the mammalian eye?

How do the muscle fibers of the iris control light entry into the eye?

What is the role of the cornea in vision?

What is the role of the lens in vision?

What happens to light after it passes through the retina?

What is the function of the choroid?

What is the reflective layer found in the choroids of nocturnal animals?

Where does most light refraction occur in the eye?

What is accommodation in the context of the eye?

How does the eye accommodate to focus on nearby objects?

The mammalian eye allows for the formation of a bright, focused image

Muscle fibers of the iris change the pupil diameter to control the amount of light entering the eye

The cornea refracts light and focuses it onto the lens

The lens is responsible for fine-tuning the focus onto the retina and changes shape depending on the distance of the light source in a process called accommodation

Light passes through the retina and excites the photoreceptors

The choroid is a pigmented layer that absorbs light

The tapetum lucidum is a reflective layer found in the choroids of nocturnal animals that reflects and amplifies dim light, making their eyes "glow"

Most refraction occurs at the cornea

Accommodation refers to the ability of the eye to focus light from different distances

During accommodation, the ciliary muscle contracts, the ligaments slacken, and the lens bulges, restoring the focal length for nearby objects

Rods and cones are located at the back of the retina

Where are rods and cones located in the retina?

What are the two types of photoreceptor cells found in mammals?

Describe the characteristics of rods

Describe the characteristics of cones

How do rod pathways exhibit convergence?

How do ganglion cells in the fovea centralis differ from rod pathways in terms of input?

What is the fovea centralis specialized for?

What are the important characteristics of the fovea centralis?

What is the basis for color discrimination in humans?

Mammals have two types of photoreceptor cells: rods and cones

Rods are ciliary photoreceptors with a rod-shaped outer segment and are sensitive to very dim light

Cones are ciliary photoreceptors with a cone-shaped outer segment, are sensitive to brighter light, and are responsible for color detection

Many rods synapse on a single bipolar cell, and multiple bipolar cells synapse onto a single ganglion cell, leading to convergence and a large receptive field for each ganglion cell

Ganglion cells in the fovea centralis receive input from a single bipolar cell, which in turn receives input from a single cone, resulting in smaller receptive fields and greater acuity

The fovea centralis is specialized to provide sharp, central vision

The fovea centralis is exclusively cones, devoid of capillaries, and has displaced obstructing cell layers

The absorbance spectra of human rods and cones are the basis for color discrimination

What is the function of retinal ganglion cells' receptive fields?

How do horizontal and amacrine cells affect neighboring cells?

Describe the neural circuitry of retinal processing involving light hyperpolarization

Retinal ganglion cells have complex receptive fields that enhance borders and contrast

Activated horizontal and/or amacrine cells inhibit neighboring cells. Horizontal cells function at the photoreceptor and bipolar cells, while amacrine cells function at the bipolar and ganglion cells

Light hyperpolarizes the photoreceptor, reducing transmitter release. A decrease in inhibitory signaling depolarizes the bipolar cell, and this depolarization increases the excitation of the ganglion cell