Sensory Systems

gustatory receptors

- detect molecules dissolved or suspended in a liquid medium to produce the sense of taste. The ligands are called tastants.

ciliated mechanoreceptors

- these mechanoreceptors open/close in response to pressure on cilia that extend from the cell membrane and are embedded in an extracellular gel (often called a cupula). Sensory epithelial cells that contain ciliated mechanoreceptors are often called hair cells.

fovea

center of field of view for each eye, which contains a high concentration of cones (color sensitive photoreceptors).

ganglion cells

closest to incoming light, outermost layer of retina. The only retinal neurons capable of generating action potentials and transmitting visual information to the brain. Ganglion cells receive NT from bipolar and amacrine cells, generating a graded membrane potential that will trigger an action potential, if strong enough. Visual information from several rods and cones is integrated through the interactions between horizontal, amacrine, and bipolar cells before a ganglion sends an action potential to the brain. Ganglion cells receive information on color, brightness, and motion. You can think of the information a ganglion cell transmits as making a single pixel in your visual image. Intrinsically photosensitive ganglion cells contain melanopsin and are sensitive to blue light. These directly photosensitive ganglion cells send signals to the CNS to help regulate circadian rhythm.

cones

conical photoreceptors that contain photopsins. Each of the three cone types is most sensitive to specific wavelengths of visible lightbecause they possess only one kind of photopsin (S - blue, M - green, L - red). Graded potentials generated just like rods.

rods

cylindrical photoreceptors that contain rhodopsin. Maintained in a depolarized state in the dark via open Na+ channels (see dark current), repolarize in the light when Na+ channels are closed. Release glutamate (NT) in the dark, which hyperpolarizes (inhibits) bipolar cells. Light excites retinal changing its conformation (cis- to trans-), which activates rhodopsin to split transducin (a G-protein). Transducin activates an enzyme (a phosphodiesterase) that closes Na+ channels (by converting cGMP to GMP).

olfactory receptors

detect airborne molecules to produce the sense of smell. The ligands are called odorants. Each olfactory receptor detects one specific chemical ligand and many recognizable smells are composed of several different odorants, so hundreds of different olfactory receptors are required for the human sense of smell. Compare this to insects that rely on olfactory sense having 10,000 or more different olfactory receptors.

sensory cell

either a specialized neuron or epithelial cell with sensory receptors in the cell membrane. Sensory cells depolarize/hyperpolarize in response to sensory stimuli. Strong depolarization of most sensory neurons can produce an action potential. Depolarization of sensory epithelial cells (and some sensory neurons) results in release of neurotransmitters that can excite or inhibit synapsing neurons capable of producing action potentials.

ocelli

eyespots, cup-shaped group of pigment containing photoreceptors with a cluster of sensory neurons. Capable of detecting light/dark, but not of forming images (image formation requires a lens to focus light).

photoreceptors

sensory receptors composed of light-sensitive pigments that detect photons of light. Humans have four different photoreceptors to detect visible light. Other animals are capable of detecting UV or IR light. IR light is also experienced as heat and IR receptors are often categorized as thermoreceptors.

blind spot

Exit site for optic nerve (collection of ganglion cell axons), no photoreceptors present. You don't experience it most of the time because your brain is able to integrate information from the other eye and from slight movements of the field of view to cover the gaps in your visual field.

mecanoreceptors

Sensory receptors that are sensitive to pressure. Mechanoreceptors are responsible for our sense of touch (tactile sense), proprioception, hearing, and equilibrium. Pressure physically changes the conformation of the mechanoreceptor, allowing ion influx/efflux.

thermoreceptors

Sensory receptors that are temperature sensitive ion channels. Many thermoreceptors are also sensitive to chemical ligands such as capsaicin (spicy tastant produced by peppers) and menthol (refreshing tastant/odorant produced by mints). These tastants literally make you feel hot and cold.

nociceptors

Sensory receptors that detect pain. Typically, mechanoreceptors, thermoreceptors, etc. that transmit a signal interpreted by the CNS as pain. Involves the NT substance P.

opsins

a group of structurally similar photosensitive protein pigments that bind to the organic molecule retinal (derived from vitamin A). Rhodopsin, Melanopsin, and Photopsins (S, M, and L) are the opsins found in the retina of the vertebrate eye.

sensory processing disorder

a potentially disabling condition in which sensory integration does not occur in the typical fashion. Sensory signals may be experienced separately rather than being summed, or "background" sensory signals are consciously perceived rather than being ignored. People with SPD often feel overwhelmed by too much sensory information under "normal" sensory conditions such as walking along a crowed street, sitting in a movie theater, or shopping in a large, brightly-lit store.

ciliary body

a ring of muscle that changes the shape of the lens to adjust focal distance. Rounder lens focuses light from near objects, flatter lens focuses light from far objects.

cornea

a thin transparent section of sclera at the anterior end of the eye. Serves as protection and a fixed lens (refracts light).

sensory receptor

a transmembrane protein receptor that changes conformation when it detects a sensory stimulus. A conformation change in a sensory receptor results in the opening/closing of ion channels. Similar to ionotropic and metabotropic receptors, some sensory receptors are ion channels and others activate second messengers that open/close ion channels. Clarification: your book sometimes uses sensory receptor to refer to the entire sensory cell or the sensory structure/organ that contains sensory cells. To prevent confusion, I will only use "sensory receptor" to refer to the transmembrane protein.

sensory amplification

cellular signal amplification in response to sensory stimulus. Sensory amplification can occur if the sensory receptor is produces a second messenger that opens/closes ion channels. One stimulus results in many ion channels opening/closing.

lateral line system

fish and some aquatic amphibians have a lateral line system, paired channels that run down each the side of the body, with many hair cells contained within the channel. The cilia are embedded in a paddle-like gel structure called a cupula. Pressure waves in the water produced by objects and animals near the fish cause water to move in the lateral line system, this puts pressure on the cupula and opens/closes the ciliated mechanoreceptors

sensory integration

integration involves the summation of many small stimuli (potentially both positive and negative) to produce a meaningful signal. The CNS is responsible for a great deal of sensory integration (part of sensory perception), but integration of sensory information occurs during sensory transduction, transmission, and perception.

horizontal cell

interneuron that integrates information from multiple rods and cones and inhibits or excites bipolar cells in response. Helps to integrate information on color and brightness.

amacrine cell

interneuron that integrates signals between bipolar and ganglion cells. Important for adaptation to different light levels.

camera eye (single-lens eye)

light is focused by a lens onto a sheet of photoreceptors. Information from many photoreceptors is integrated (typically by a brain) to form a single image. A group of Cnidarians called Cubozoa (box jellyfish) possess four true eyes and twenty ocelli can are capable of object detection even without a CNS to integrate visual information. Cephalopods (octopi, squid, cuttlefish) within the Mollusca, some Annelids, and Arachnids within the Arthropoda, and Vertebrates have camera eyes.

choroid

middle tissue layer of the eye. Pigmented to absorb extra light. Highly vascularized to provide blood supply to retina.

conjunctiva

mucous membrane that surrounds the sclera to lubricate the sclera and provides protection against bacterial infection.

iris

pigmented layer of smooth muscle tissue that rings the opening to the eye (pupil). Can constrict to allow less light into the eye or dilate to allow more light into the eye.

proprioceptors

pressure sensing structures found in human muscle. Proprioceptors detect tension and contraction of muscle fibers. The Parietal Lobe integrates this information so that you can perceive the position of your body parts in space (e.g., touch your nose with your eyes closed).

lamellae

pressure sensing structures found in human skin. Lamellae are modified dendrites of sensory neurons and are important for the tactile sense. Pressure directly causes a conformation change, making the mechanoreceptors in the lamella leaky when pressed.

electroreceptors

sensory receptors that detect electrical current in water. Electroreceptors can be used for electrolocation or electrocommunication. Sharks and rays use electroreceptors for passive electrolocation - detecting animals by the electrical current generated by muscle and neuron action potentials. Weakly electric fish are capable of producing their own electrical fields and use them to actively electrolocate by detecting distortions in their own electric field from nearby animals and objects (like a bat or dolphin echolocating with sound). Weakly electric fish can also produce electrical current as a means of communication to other weakly electric fish with electroreceptors to detect these communications (i.e., "mating calls").

magnetoreceptors

sensory receptors that detect the Earth's magnetic field and allow magnetic orientation and navigation. Found in many different animals, but exact mechanisms not fully understood (magnetite is not always necessary).

chemoreceptors

sensory receptors that open/close ion channels in response to a chemical stimulus. Chemoreceptors can be ligand-gated ion channels or might activate second messengers in response to a ligand. Internal chemoreceptors are important for maintaining homeostasis by detecting ion concentrations and carbon dioxide concentrations. You could think of hormone receptors and even NT receptors as being internal chemoreceptors.

dark current

sounds super cool; the flow of Na+ into a photosensitive sensory neuron (rod or cone) in the dark, maintaining a depolarized membrane potential.

biopolar cells

synapse with all types of retinal neurons, but are primarily responsible for transmitting graded potentials from photoreceptors (rods and cones) to ganglion cells. Bipolar cells are kept in a hyperpolarized state (inhibited) by glutamate from rods and cones in the dark and only become depolarized in the light. Depolarization releases a NT to depolarize ganglion cells, possibly generating action potentials and transmitting the signal to the brain. Bipolar cells are excited or inhibited by horizontal cells and amacrine cells. This helps to integrate visual information before sending a signal to the brain via a ganglion cell.

sensory transmission

the afferent flow of action potentials to the appropriate association center of the CNS. Only involves neurons.

sensory transduction

the cellular response to the detection of the stimulus. This involves the processes from the initial depolarization/hyperpolarization of the sensory cell to the generation of an action potential. Transduction usually occurs within the sensory cell, but might involve the transduction of sensory information through graded potentials across several cells before an action potential is generated.

compound eye

the eye of arthropods consist of multiple ommatidia, each of which uses a lens to focus light onto one or two photoreceptors. This detects light, but doesn't detect enough variation to form an image. Sensory information from all ommatidia is integrated in the brain to form a composite mosaic image.

mammal inner ear

the inner ear of mammals is used for hearing (auditory sense), balance (equilibrium) and sensing acceleration. Acceleration in three dimensions is detected by hair cells with cupulae in the semicircular canals. For auditory sense, sound waves (pressure waves conducted through a medium) are conducted through several ear structures to the basilar membrane in the cochlea. The vibrations push sensory hair cells against the tectorial membrane, thus opening/closing the ciliated mechanoreceptors. Equilibrium (balance) is essentially about knowing the direction of gravity (which way down is). Gravity is detected as the result of many hair cells embedded in an otolithic membrane (gel, like a big cupula) covered in mineral otoliths. The otoliths are pulled down by gravity, depressing the otolithic membrane, and stimulating hair cells.

retina

the layer of tissue that includes photoreceptors (within rod and cone sensory neurons) and sensory neurons and is photosensitive.

lens

the lens of the vertebrate eye focuses light onto the retina. The lens is somewhat elastic and can adjust focus (see: ciliary body) to near objects by becoming rounder and focus on far objects by becoming thinner.

sensory reception

the process by which a sensory cell detects a sensory stimulus. The sensory stimulus activates a sensory receptor, which results in the opening/closing of ion channels in the cell membrane of the sensory cell.

sensory adaptation

the process by which sensory cells reduce or increase sensitivity to sensory stimuli in order to cope with a changing environment or to sort out important "signals" from unimportant background "noise". Rods and cones (the visual sensory cells) adjust sensitivity to light as light intensity changes becoming more sensitive in low light and less sensitive in bright light (this is due to changes in the concentration of the enzymes that reset photoreceptors). Olfactory cells become less sensitive to persistent odors. Presumably, olfactory adaption operates on the basis that new smells or changing smells (signal) are more important than old smells (noise).

sensory perception

the process of integration and association of sensory information by the appropriate center in the brain that creates awareness of the sensory stimulus in a meaningful way.

sensory pathway

the set of sensory cells and afferent neurons that allow sensory information to be received, transduced, transmitted to, and perceived by the CNS.

aqueous humor

the transparent fluid produced by the ciliary body that fills space between the cornea and the lens. Glaucoma can be caused by a build-up of aqueous humor and can lead to blindness.

vitreous body (vitreous humor)

the transparent gel that fills the space between the lens and the retina. Maintains eye shape.

uvea

thin pigmented layer that provides nourishment to retina and absorbs light so that light is not reflected around inside the eye.

sclera

tough outer layer of the vertebrate eye; primarily connective tissue; provides protection and support. Includes the cornea.

Electromagnetic receptors

• sensory receptors that detect electromagnetic energy (light), electrical fields, or magnetic fields.