Ch 6 - Sensors I: Remote Sensing

sensory transduction

the process by which an external stimulus (e.g., sound) is converted into a change in the electrical activity of sensory neurons or other sensory cells

photoreceptor

biological sensors of light, including both rod and cone photoreceptors

blind spot

the location in the visual field where you cannot detect a visual stimulus (as long as you are looking through just one eye) b/c the corresponding location on the retina (the optic nerve head) lacks photoreceptors

optic nerve head

the location where the axons of retinal ganglion cells cross through the retina to exit the eye and enter the optic nerve

• is responsible for creating the blind spot

macula

a small, highly pigmented region of the retina that contains the fovea

macular degeneration

degeneration of retinal cells in the region of the macula

• often develops in old age and can cause major vision problems

fovea

a small region of the retina that (in humans) contains a very high density of cone photoreceptors and very few rod receptors

• many cones, few rods

rod photoreceptor

a type of photoreceptor cell that is more light sensitive than cone photoreceptors but is not used in color vision

• sensitive to light; not used in color vision

outer segment

the part of a photoreceptor cell that faces toward the back of the eye and contains most of the photosensitive pigments (rhodopsin or one of the cone opsins)

rhodopsin

the photosensitive pigment in rod photoreceptors

opsin

a type of G protein-coupled receptor that is found in photoreceptors and helps make them light-sensitive

retinal

a small molecule that dissociates from opsin molecules when it is “activated” by light

• ex: converted from cis isomer to the all-trans isomer, causing change in molecule’s shape

phototransduction cascade

the series of event that starts w/ photons activating the molecule retinal and ends w/ a change in the membrane potential of a photoreceptor cell

• photons activate retinal → change in membrane potential of photoreceptor

transducin

a G protein that can be activated by light-activated opsins

phosphodiesterase

an enzyme that can be activated by activated transducin and degrades cyclic GMP

hyperpolarization

a decrease in membrane potential to below (more negative than) the resting potential

• more negative

rod bipolar cell

a type of retinal neuron that…

• receives input from rod photoreceptors

• expresses metabotropic glutamate receptors (which elicit hyperpolarization in response to glutamate)

• synapses onto amacrine cells

dark adaptation

a set of processes that makes photoreceptors more sensitive to light after they have been in darkness (or low-light conditions) for a while

S-cones

cone photoreceptors that are maximally sensitive to short (S) wavelengths of light

• blue

M-cones

cone photoreceptors that are maximally sensitive to medium (M) wavelengths of light

• green

L-cones

cone photoreceptors that are maximally sensitive to long (L) wavelengths of light

• yellow/red

color-blindness

an inability to discriminate some colors that the majority of people can discriminate

• comes in a variety of forms

OFF bipolar cell

a type of retinal neuron that receives input from cone photoreceptors and is depolarized when a light stimulus turns OFF

• expresses ionotropic glutamate receptors (which elicit depolarization in response to glutamate)

ON bipolar cell

a type of retinal neuron that receives input from cone photoreceptors and is depolarized when a light stimulus comes ON

• expresses metabotropic glutamate receptors (which elicit hyperpolarization in response to glutamate)

retinal ganglion cell

a type of retinal neuron that receives input from bipolar cells and sends its axon thru the optic nerve into the brain

amacrine cell

a diverse class of retinal neurons, some of which receive input from rod bipolar cells and synapse onto OFF and ON cone bipolar cells w/ inhibitory and electrical synapses, respectively

• info from rods → pass thru amacrine and cones to get out of retina

spatial receptive field

the set of locations where stimuli must be presented in order to increase (or suppress) a neuron’s firing rate significantly

melanopsin

a visual pigment expressed by a subset of light-sensitive retinal ganglion cells

pigment epithelium

a highly pigmented layer of cells covering the outer segments of retinal photoreceptors

• involved in retinoid recycling

retinoid recycling

the conversion of all-trans retinal back to cis-retinal, which can reassociate w/ opsins and make them light sensitive

• consumes significant amount of metabolic energy and explains why pigment epithelium is covered w/ dense capillary bed (choroid)

choroid

a capillary bed that covers the retina’s pigment epithelium, supplying it w/ metabolic energy

Müller cell

a type of glial cell in the retina

• acts as a light guide, channeling light from the front of the retina to the photoreceptors at the back of the retina

tapetum lucidum

a reflective layer in the retina of many nocturnal species

• accounts for the “eye shine” that arises when you shine a light straight into those animals’ eyes

olfactory epithelium

the layer of cells in the roof of the nasal cavity that houses the olfactory sensory neurons

olfactory sensory neuron

neurons in the olfactory epithelium that express olfactory receptor molecules and project to the olfactory bulb

odorant binding protein

proteins that bind odorant molecules and shuttle them through the mucus covering the olfactory epithelium to the olfactory receptors

olfactory receptor molecule

a large family of G protein-coupled receptors that are expressed in the cilia of olfactory sensory neurons and bind to odorants

pseudogene

a gene that has become nonfunctional during the course of evolution

epitope

a molecular feature

• in context of olfaction, a feature of an odorant to which an olfactory receptor may bind selectively

  • the specific molecular feature (____) that each olfactory receptor type binds to

olfactory bulb

the most rostral part of the telencephalon in most vertebrates, located on the inferior surface of the frontal lobe in humans

• receives inputs from the olfactory epithelium and projects to olfactory cortex

glomeruli

one of many spherical structures in the olfactory bulb where the axons of olfactory sensory neurons synapse onto the dendrites of mitral cells

2-deoxyglucose (2-DG)

a radioactive substance that neurons partially metabolize as if it were glucose

• used in rats to research chemotopic organization of the olfactory bulb

chemotopy

the idea that similar odorants are represented in adjacent locations (notably within the olfactory bulb)

• orderly mapping between odorant structure and spatial location of activated glomeruli

  • involved in odor discrimination

retinotopic

the idea that adjacent neurons in a visually responsive brain region respond to stimulation of adjacent locations in the retina

• visual environment systematically “mapped” onto retina

sensory map

the idea that sensory representations in the brain resemble geographic maps insofar as adjacent locations in the world correspond to adjacent locations in the neural representation

• most brain areas involved in sensory processing contain sensory maps

mitral cells

the neurons in the olfactory bulb that receive input from olfactory sensory neurons and project to olfactory cortex

granule cell

a neuron w/ a very small cell body that is tightly packed together w/ other similar cells

• in olfactory bulb, they are inhibitory and interconnect mitral cells

  • most of these (___) cells reside in the cerebellum

reciprocal dendrodendritic synapse

synapses between dendrites that are arranged so that a synapse that transmits signals in one direction (from one dendrite to the other) lies adjacent to a synapse going in the opposite direction

• granule cells form these types of synapses w/ mitral cells

  • create negative feedback loop (excitation of mitral cell excites granule cells, then inhibits same mitral cell, & shuts down activity)

lateral inhibition

the process by which nearby neurons inhibit each other

• second function of reciprocal synapses between granule and mitral cells

  • narrows range of odorants that mitral cell responds to

pinna

the outer ear (flap)

ear canal

the tubular passage from the outer ear to the tympanic membrane

tympanic membrane

the thin membrane separating the ear canal from the middle ear; vibrates in response to airborne sounds

• AKA eardrum

middle ear

the portion of the ear that lies between the tympanic membrane (eardrum) and the inner ear (cochlea)

• contains middle ear bones

  • malleus (hammer), incus (anvil), stapes (stirrup)

malleus (hammer)

one of the middle ear bones

• attaches to tympanic membrane

incus (anvil)

one of the middle ear bones

• connects the malleus to the stapes

stapes (stirrup)

one of the middle ear bones

• attaches to the oval window

oval window

a thin membrane covering the entrance to the cochlea

inner ear

the part of the ear that lies deepest within the skull

• includes both cochlea and the vestibular apparatus (semicircular canals, saccule, and utricle)

middle ear muscles

small muscles that affect movements of the tympanic membrane and middle ear bones, thereby affecting acoustic sensitivity

cochlea

the spiraling, tubular part of the inner ear that is responsible for sensing sounds

scala vestibuli

the half of the cochlear tube that extends from oval window to the top (apex) of the cochlea

scala tympani

the half of the cochlear tube that extends from the top (apex) of the cochlea to the round window

round window

a thin membrane covering the basal end of the scala tympani

• always vibrates together w/ oval window but in opposite direction

  • enables sound waves to propagate within cochlea

scala media

a fluid-filled compartment in the cochlea, sandwiched between scala vestibuli and scala tympani

basilar membrane

a thin membrane inside the cochlea

• inner and outer hair cells sit on top

hair cell

sensory cells that sit on top of basilar membrane of cochlea

• extend stereocilia into scale media (inner hair cells) or tectorial membrane (outer hair cells)

  • also found in vestibule and semicircular canals

stereocilia

the “hairs” of the hair cells

• are actually microvilli, rather than cilia or hairs

inner hair cell

the hair cells closest to the center of the spiraling cochlea

• sense vibrations of the basilar membrane

  • sensory function

outer hair cell

the 3 rows of hair cells furthest away from the center of the spiraling cochlea

• main function is to amplify vibrations of the basilar membrane (and increase acoustic sensitivity)

  • sensory and motor function

tectorial membrane

a relatively thick membrane that lies on top of hair cells of the basilar membrane

• slides sideways against basilar membrane when vibrating up/down (fluid inside moves sideways)

  • vibrations of stereocilia cause oscillations in hair cell’s membrane potential

endolymph

the fluid inside the scala media

• fluid in which stereocilia are bathed

  • contains unusually high concentration of potassium ions

tip link

a thin filament that connects the tip of one stereocilium to adjacent, taller stereocilium

• linked to an ion channel that opens when the stereocilia are bent towards tallest stereocilium

ribbon synapse

a type of chemical synapse in which numerous synaptic vesicles are arranged around a central “ribbon”

• found in hair cells, allowing them to release large amounts of glutamate over long periods of time

spiral ganglion

a spiraling string of cell bodies inside cochlea

• its neurons innervate cochlear hair cells and project to cochlear nuclei in the brain

  • postsynaptic terminals located inside this

cochlear nucleus

a set of neurons in the upper medulla that receives input from cochlea and projects to inferior colliculus

• receives the neurotransmitter release from hair cells’ ribbon synapses

perilymph

the fluid surrounding the cell bodies of cochlear hair cells

• has much lower concentration of potassium ions than the endolymph

  • involved in hair cell repolarization

stria vascularis

specialized epithelial cells in the outer wall of the cochlea that pump potassium ions into the scala media

• provides high metabolic energy for high K+ concentration in scala media’s endolymph

  • hair cell repolarization

tonotopy

the idea that neurons tuned to similar sound frequencies are located adjacent to one another

• collectively form a neural “map” of sound frequency

cochlear tonotopy

the higher the frequency of sound, the closer to cochlea’s base will be hair cells activated by that tone

• high-frequency → base of cochlea

• low-frequency → apex of cochlea

sensory modality

the classification of sensations and perceptions into distinct categories

• ex: vision is sensory modality, and hearing and touch

background adaptation

the idea that sensors gradually stop responding to steady, persistent stimuli

labeled line

idea that activity in a neural pathway is interpreted by the nervous system according to a fixed “label”

• if axons normally carry specific type of visual info, that info becomes their label

  • brain interprets nerve activity according to labeled line code

topographic arrangement

labeled lines exhibit orderly, map-like organization

• topographic arrangement minimizes axonal connection lengths