Ch. 41 Sensory Systems

The basilar membrane is narrow and stiff at the oval window end, and wide and floppy at its other end.

(base) higher frequency: narrow end causes it to vibrate in lower frequency: wide end (apex)

The point where the optic nerve exits the eye lacks photoreceptors

, resulting in a blind spot several millimeters in diameter in the human eye.

Overview of Sensory Receptors and Pathways

- A stimulus (- feedback) is a change in the internal or external environment that can be detected by the organism's body - Examples: heat, light, sound, mechanical forces, air pressure, specific chemicals in the air or water. - Sensory stimuli are detected by receptor proteins in sensory receptors - Eyes, ears, skin, and other surface organs detect varying

Single-Lens Eyes

- An eye type that works by changing the amount of light allowed to enter into the eye and by focusing this incoming light with a lens. - The single-lens eye of cephalopods, like a vertebrate eye, operates like a camera - Light enters through the cornea, a lens concentrates the light, and a layer of photoreceptors (retina) records the image - Muscles in the iris adjust the size of the pupil to vary the amount of light entering the eye - Muscles move the lens to focus on objects at different distances (accommodation)

What are the functions of proprioceptors?

Proprioceptors detect stimuli that are processed to provide the animal with information about movements and position of the body.

Receptor proteins

Protein that recognizes and binds molecules from other cells that act as chemical signals.

Visual Processing in the Retina

The retina of mammals contains four types of neurons: - bipolar cells where processing of visual information begins and Rods and cones form synaptic connections - Ganglion cells receive signals from bipolar cells - axons of ganglion cells collect at the back of the eyeball to form the optic nerve - Horizontal cells integrate inputs from multiple photoreceptors - lateral to the region of light reception - Amacrine cells receive inputs from bipolar cells and excite ganglion cells

Incus

The second of the three sound-conducting middle ear bones in vertebrates, located between the malleus and the stapes.

Nociception

The sensation of pain.

somatosensation

The sense that informs the brain about the body's contact with objects in the environment, and about the movement, position, and balance of body parts.

All sensory neurons send signals to the CNS in the same way. How are stimuli of different kinds perceived as being different?

The signals from particular sensory receptors are routed by afferent neurons to specific regions of the CNS. Processing of the incoming signals by those regions gives the "sense" of the stimulus—a smell, pain, and so forth.

eyespot (ocellus)

The simplest eye, which detects light but does not form an image. eyespots are found in a variety of animals, including some insects, arthropods, and mollusks.

fovea

The small region of the retina around which cones are concentrated in mammals and birds with eyes specialized for daytime vision.

stapes

The smallest of three sound-conducting bones in the middle ear of tetrapod vertebrates.

Two main types of image-forming eyes have evolved in invertebrates:

compound eyes and single-lens eyes.

Horizontal cells and amacrine cells spread their dendrites laterally across part of the surface of the retina, to

integrate inputs from multiple other retinal neurons. They then inhibit or excite neurons at one point on the retina, based on lateral input from other points on the retina. This type of neural circuit is called lateral inhibition or lateral excitation: A neural circuit in which neurons responding to input from one part of a sensory organ inhibit signaling in adjacent parts of the sensory organ, to enhance contrast between adjacent sensory inputs.

The human perception of pain:

is activated by mechanoreceptors, thermoreceptors, and chemoreceptors.

In humans, the fovea receives

light from the fixation point, which is the point in the visual field at which you are looking at any given moment (in other words, the point on which you concentrate your attention).

How do thermoreceptor proteins work?

Two types of thermoreceptors have been identified, both of which are ion channel proteins that change shape in response to changes in temperature. One opens at temperatures above and the other opens at temperatures below 25C.

What is visible light and how does it differ from other types of radiant energy?

Visible light is electromagnetic radiation that has wavelengths between approximately 400 and 700 nm. Other types of radiant energy have longer or shorter wavelengths.

The chemical ligands for olfactory and gustatory receptor proteins are called

odorants and tastants.

Which sense involves the greatest number of different receptor proteins in terrestrial animals?

olfaction

In mammals and other terrestrial vertebrates,

olfaction is the perception of airborne molecules and gustation is the perception of molecules in the saliva on the tongue.

Structures in the vertebrate ear are activated by sound waves in the following order:

pinna, tympanic membrane, malleus, incus, stapes, oval window, cochlear duct.

Pacinian corpuscles

respond to deep pressure & vibration. have structures surrounding the nerve endings that contribute to reception of stimuli.

Protection from Damage

- Another system protects the tympanic membrane from damage by changes in environmental atmospheric pressure - The system depends on the Eustachian tube (also called the auditory tube), a duct that leads from the air-filled middle ear to the throat - When swelling or congestion due to infections prevents the tube from admitting air, we complain of having stopped-up ears— we can sense that a pressure difference between the outer and middle ear is bulging the tympanic membrane, interfering with the transmission of sounds and causing pain

Chemoreceptors

- Chemoreceptors form the basis of taste (gustation) and smell (olfaction), and measure the internal levels of molecules such as oxygen, carbon dioxide, and hydrogen ions - Many invertebrates use the same receptors for smell and taste - these receptors may be confined to certain locations or distributed over the body surface - Some terrestrial invertebrates, particularly insects, have clearly differentiated taste and smell receptors

Nociceptors

- Def: A sensory neuron that detects tissue damage or noxious chemicals; their activity registers as pain. - Signals from receptors that detect damaging stimuli (nociceptors) are interpreted by the brain as pain - Often pain elicits a protective reflex response (such as withdrawing the hand from a hot surface) that proceeds before we are consciously aware of the sensation - Various stimuli cause pain, including mechanical damage and temperature extremes - some nociceptors are specific for a particular type of damaging stimulus

Inner ear, organ of corti

- Def: That part of the ear, particularly the cochlea, that converts mechanical vibrations (sound) into neural messages that are sent to the brain. - The inner ear contains the semicircular canals, utricle, saccule, and the cochlea (a spiraled tube, snail-shaped structure in the inner ear containing the organ of hearing.) - The cochlea is divided into three longitudinal chambers: the vestibular canal (top), the cochlear duct (mid.), and the tympanic canal (bottom) - Within the cochlear duct, the organ of Corti (spiral organ, an organ within the cochlear duct that contains the sensory hair cells detecting sound vibrations transmitted to the inner ear) - The vibrations of the oval window pass through the fluid in the vestibular canal, make the turn at the end, and travel back through the fluid in the tympanic canal. At the end of the tympanic canal, they are transmitted to the round window, a thin membrane that faces the middle ear.

Compound Eyes

- Def: The eye of most insects and some crustaceans, composed of many faceted, light sensitive units called ommatidia fitted closely together, each having its own refractive system and each forming a portion of an image. - The compound eye of insects and crustaceans contains many faceted visual units (ommatidia) fitted closely together - Light entering an ommatidium is focused by a transparent cornea and crystalline cone onto photoreceptor cells - Microvilli of these cells form a central axis rich in rhodopsin, a photopigment (retinal bound to opsin) which absorbs light and causes action potentials to be generated - The brain receives a motion-sensitive mosaic image

Invertebrate Eyes

- Earthworms have photoreceptors in their skin that allow them to sense and respond to light - Planarians have a cuplike ocellus which detects light but does not form an image - photoreceptor cells are connected to dendrites of afferent neurons, which are bundled into nerves that travel from ocelli to the cerebral ganglion - Planarians orient themselves so that the amount of light falling on their two ocelli is equal and diminishes as they swim

The Lateral Line System

- Fishes and some aquatic amphibians detect vibrations and currents in water through a series of mechanoreceptors in the lateral line system - Def: The complex of mechanosensory neurons and related structures along the sides of some fishes and aquatic amphibians that detect vibrations in the water. - In fish, the mechanoreceptors (neuromasts, each of which contains a dome-shaped cupula filled with fluid) also provide information about orientation with respect to gravity and swimming velocity - The lateral line allows fishes to avoid obstacles, orient in a current, monitor the presence of other moving objects in the water, and move in unison in a school

Receptors for Touch and Pressure

- Human skin contains several types of touch and pressure receptors, including free nerve endings, Pacinian corpuscles, Ruffini endings, and Meissner's corpuscles - In vertebrates, mechanoreceptors for touch and pressure are embedded in the skin, skeletal muscles, walls of blood vessels, and internal organs

Color Vision

- Humans and other primates have three types of cones, each containing one of three different photopigments in which retinal is combined with different opsins - The three photopsins have overlapping absorption ranges with different peak absorptions: 445 nm (blue light), 535 nm (green light), and 570 nm (red light) - Light that stimulates all three receptor types equally is seen as white

Taste Receptors in humans

- Humans have about 10,000 taste buds scattered over the tongue, roof of the mouth, and throat. Those on the tongue are embedded in outgrowths called papillae (singular, papilla) (papula = pimple), which give the surface of the tongue its rough texture. - Taste receptors on the human tongue respond to five basic tastes: sweet, sour, salty, bitter, and umami (savory, the amino acid glutamate - as in MSG). All sensed by G protein coupled receptors. - The classes of receptors may all have many subtypes, each binding a specific molecule within that class

Taste Receptors in Insects

- In insects, taste receptors occur inside hollow sensory bristles (sensilla) on the antennae, mouthparts, or feet - Pores in sensilla admit molecules to the chemoreceptors, which are specialized to detect sugars, salts, amino acids, or other chemicals - Many female insects have chemoreceptors on their ovipositors, which allow them to lay eggs on food appropriate for the hatching larvae

Receptors Reduce Their Response

- In some systems, the frequency of action potentials generated in afferent neurons is REDUCED when the intensity of a stimulus remains constant (sensory adaptation) - Sensory adaptation INC the sensitivity of receptor systems to changes in environmental stimuli, which may be more important to survival than constant factors - Nonadapting receptors, such as those detecting pain, are also essential for survival

Visual Processing in the Retina Cont.

- In the light, decrease in neurotransmitter release from rods and cones results in hyperpolarization of depolarized bipolar cells and depolarization of hyperpolarized bipolar cells - Signals from rods and cones move vertically from bipolar cells to ganglion cells - each ganglion cell receives signals from a defined set of photoreceptors (its receptive field) - Visual processing through horizontal cells (lateral inhibition) sharpens the edges and enhances contrast in an image

Olfactory Receptors in Insects

- Insect olfactory receptors detect airborne molecules such as pheromones released as sexual attractants by female moths - Olfactory receptors in the bristles of male silkworm moth antennae can detect pheromones in concentrations as low as one attractant molecule per 1017 air molecules - Ants, bees, and wasps identify members of the same hive or nest and communicate by means of odor molecules

Statocysts

- Many aquatic invertebrates, including jellyfish, some gastropods, and some arthropods, have organs of equilibrium called statocysts (A mechanosensory neuron in invertebrates that senses gravity and motion using statoliths) - Most statocysts are fluid-filled chambers with walls that contain hair cells enclosing one or more movable stonelike bodies called statoliths (A movable starch-or carbonate-containing stonelike body involved in sensing gravitational pull) - When the animal moves, statoliths lag behind the movement, bending the sensory hairs and triggering action potentials - Statocysts signal the brain about the body's position and orientation with respect to gravity

Electroreceptors

- Many sharks and bony fishes, and some amphibians and mammals, have specialized electroreceptors that detect electrical fields - The plasma membrane of an electroreceptor is depolarized by an electrical field, leading to the generation of action potentials - Electrical stimuli detected by the receptors are used to locate prey or navigate around obstacles in muddy water - or, by some fishes, to communicate - Some electroreception systems are passive - they detect electric fields in the environment, not the animal's own electric currents - passive systems are used mainly to find prey - Other electroreception systems are active - the animal emits and receives low voltage electrical signals that are generated by special electric organs - A few species, such as the electric eel and electric catfish, produce discharges on the order of several hundred volts that are used to stun or kill prey

Proprioceptors

- Mechanoreceptors (Mechanosensory neurons) called proprioceptors detect stimuli used in the CNS to maintain body balance and equilibrium and to monitor the position of the head and limbs - Sensory receptors that detect the length of a muscle and its contraction are muscle spindles - bundles of small specialized muscle cells wrapped with the dendrites of afferent neurons

Hair cells

- Mechanosensory neurons that have membrane-bound stereocilia at one end, each of which contains a mechanoreceptor ion channel that opens when the stereocilium is bent in one direction, depolarizing the hair cell. - In vertebrates, hair cells in the semicircular canal communicate signals in the vestibular system, and hair cells in the cochlea communicate auditory signals.

Sound Detection in Invertebrates

- Most invertebrates detect sound and other vibrations through mechanoreceptors in their skin or other surface structures - Cephalopods (squid & octopuses) have a system of mechanoreceptors on their head and tentacles, similar to the lateral line of fishes, which detects vibrations in the surrounding water - Many insects have sensory hairs or bristles that vibrate in response to sound waves, often at particular frequencies - Some insects (e.g., moths, grasshoppers, crickets) have complex auditory organs on either side of the abdomen or on the first pair of walking legs - These organs consist of a thinned region of the insect's exoskeleton that forms a tympanum over a hollow chamber - Sounds reaching the tympanum cause it to vibrate - mechanoreceptors connected to the tympanum translate the vibrations into nerve impulses

Olfactory neurons in nasal cavity in vertebrates

- Olfactory neurons are located in a specialized olfactory epithelium in the nasal cavity which in fish is filled with water, while it is filled with air in terrestrial vertebrates. - In fish, odorant receptor proteins (ORPs) bind molecules in the water, while the ORPs of terrestrial vertebrates have evolved to bind volatile molecules in the air. - Each olfactory neuron has many sensory cilia that project into a layer of mucus that covers the olfactory epithelium. - Airborne molecules dissolve in the watery mucus, and then bind to ORPs in the membranes of sensory cilia - Axons extend from the other end of olfactory neurons and synapse on neurons in the olfactory bulbs. - The olfactory bulbs project directly to olfactory cortex and to a part of the amygdala that processes olfactory information. - Olfactory neurons are the only sensory neurons that project directly into the CNS, rather than synapsing on afferent neurons in the PNS, and olfaction is the only sense whose signals are not relayed through the thalamus. - Brain regions that integrate olfactory signals are closely associated with brain regions involved in identifying the emotional significance of experiences and in forming emotional memories

Nociceptors Neurotransmitters

- PNS nociception releases glutamate (sharp) or substance P (dull) the create pain sensations. - The CNS has a pain-suppressing system - in response to exercise, sex, and stress, the brain releases natural painkillers (endorphins) that bind to receptors on substance P neurons, reducing the amount of neurotransmitter released

Photoreceptors and Vision

- Photoreceptors detect light at particular wavelengths - centers in a brain or central ganglion integrate signals arriving from the receptors into a perception of light - All animals use forms of a single pigment, retinal (synthesized from vitamin A), in photoreceptors to absorb light energy - Retinal is bound to proteins in the G-protein-coupled receptor family, called opsins, forming a retinal-opsin complex called a photopigment. When retinal absorbs a photon it changes shape, causing the opsin to change shape so that it can then activate a G protein. - The simplest eyes only distinguish light from dark - the most complex eyes distinguish shapes and colors, and focus an accurate image of objects being viewed

olfactory neurons

A neuron that is activated in the presence of specific odorant molecules.

Rhodopsin

A photopigment consisting of retinal bound covalently to an opsin protein.

The Semicircular Canals

- Positioned at angles corresponding to the 3 planes of space, detect rotational ("spinning") motions. - Each canal has an ampulla at its base, topped with sensory hair cells that synapse with afferent neurons - surface of a hair cell facing the endolymph is covered with stereocilia - Microvilli (stereocilia) extend into a gelatinous structure, the cupula (cupula = little cup), which protrudes into the endolymph of the canals. - When the head rotates, endolymph in the semicircular canal corresponding to that direction lags behind, pulling the cupula with it. The displacement of the cupula bends the stereocilia, opening ion channels at their tips, so K+ enters the hair cell and depolarizes it. The depolarization opens voltage-gated Ca2+ channels in the main part of the hair cell, and entry of Ca2+ causes the release of NT from the hair cell into the synapse. The neurotransmitter triggers action potentials in the afferent neuron.

Receptive fields in humans

- Receptive fields for touch in the skin are smallest in the fingertips, lips, and tip of the tongue, giving these regions the greatest sensitivity to mechanical stimuli. - In other areas, such as the skin of the back, arms, and legs, the receptive fields are much larger.

Olfactory Receptors

- Receptors that detect odors are located in the nasal cavities - One end of each olfactory receptor cell has sensory hairs that project into a mucus layer that dissolves airborne molecules - The other end synapses with interneurons in the olfactory bulbs - olfactory receptors are the only receptor cells that make direct connections with brain interneurons - From the olfactory bulbs, signals are conducted to olfactory centers of the cerebral cortex, where they are integrated

The Retina

- Retinal neurons are layered over photoreceptor cells - light passes through neurons before reaching photoreceptors - The human retina contains about 120 million rods and 6 million cones organized into a densely packed, single layer - The image focused by the lens is centered on the fovea, a small region of the retina where cones are concentrated - Mammals and birds with eyes specialized for night vision lack a defined fovea. contain mostly rods - Rods are spread over the remainder of the retina, which provides peripheral vision - The image focused by the lens is centered on the fovea, which is circular and less than a millimeter in diameter in humans. The rods are spread over the remainder of the retina - The rods of mammals are much more sensitive than cones to low-intensity light - In dim light, we can detect objects better by looking slightly to the side of the object, to direct the image toward light-sensitive rods in surrounding regions of the retina

Rhodopsin and Signal Transduction

- Rhodopsin is a membrane-embedded G Protein-coupled receptor - activated rhodopsin triggers a pathway that leads to the closure of Na+ channels in the plasma membrane - Closure of the channels hyperpolarizes the photoreceptor membrane, decreasing neurotransmitter release - The response is graded - as light absorption increases, the amount of neurotransmitter released is reduced proportionately. if light absorption decreases, neurotransmitter release increases proportionately.

frequency in ear

- Sensory hair cells in a particular location in the basilar membrane are tuned to a particular sound frequency (pitch) - Hair cells are distributed in small groups along the basilar membrane - each group is connected by synapses to afferent neurons which are bundled together in the cochlear nerve - The cochlear nerve carries information through intermediate regions to the thalamus, which routes signals to specific regions in the temporal lobe, which then integrates the information

Sensory Transduction

- Sensory receptors respond to stimuli by undergoing a change in receptor potential (mem. pot. caused by opening or closing of ion channels, thus changing the flow of positive & negative ions across the plasma membrane) - a graded membrane potential - Change is caused by changes in the rate of conduction of positive ions (Na+, K+, Ca2+) across the plasma membrane - The conversion of a stimulus into a receptor potential is called sensory transduction. - If the receptor potential is large enough, it triggers an action potential in the afferent neuron that travels along the axon into the interneuron networks of the CNS - The interneurons integrate the sensory stimuli, and the brain formulates a compensating response

Magnetoreceptors

- Some animals gain information about their environment by sensing magnetic or electrical fields - stimuli that humans can detect only with scientific instruments - Migrating butterflies, beluga whales, sea turtles, homing pigeons, and foraging honeybees have magnetoreceptors that allow them to detect and use Earth's magnetic field as a source of directional information - The pattern of Earth's magnetic field differs from region to region - animals with magnetic receptors can use this information to monitor their location

Taste Receptors (gustatory neurons)

- Taste receptors of most vertebrates are located in taste buds - the sensory hairs of taste receptors pass through the pore of a taste bud and project to the exterior - In humans, taste buds are embedded in outgrowths of the tongue called papillae - The opposite ends of the receptor cells form synapses with dendrites of an afferent neuron which relays signals from the taste receptors to the thalamus - From the thalamus, some signals lead to gustatory centers in the cerebral cortex, which integrate them into the perception of taste - others lead to the brain stem and limbic system

Perception

- The brain uses action potentials from sensory receptors to generate an interpretation (perception) of the external and internal environments - Def: The conscious awareness of our external and internal environments derived from the processing of sensory input. Perception differs from reality: - Lack receptors for some types of energy (e.g., X-rays) - Sensory input is processed in various ways - Human perception of the world is significantly different from the perceptions of other organisms

Processing Visual Information

- The optic nerves converge before entering the base of the brain - a portion of each optic nerve crosses over to the opposite side, forming the optic chiasm (Location just behind the eyes where the optic nerves converge before entering the base of the brain, a portion of each optic nerve crossing over to the opposite side.) - Most axons enter the lateral geniculate nuclei in the thalamus, where they synapse with interneurons leading to the visual cortex - The left half of the image seen by each eye is transmitted to the right hemisphere, and the right half is transmitted to the left hemisphere - Communication between right and left hemispheres integrates this information into a perception of the entire visual field seen by two eyes - Because the view seen by each eye is slightly different, the brain creates a picture with a sense of distance and depth - The two optic nerves together contain more afferent neurons than the rest of the body put together - almost one-third of the cerebral cortex in humans is devoted to visual informatio

Human Ears Are Representative of Mammals

- The outer ear has an external structure (pinna) which concentrates and focuses sound waves & meatus - Sound waves enter the auditory canal and strike the tympanic membrane (Also called the eardrum, a sheet of tissue forming the boundary between the outer ear and the middle ear that vibrates in response to sound waves that move through the auditory canal), which vibrates in response - The middle ear an air-filled cavity containing 3 small, interconnected bones: the malleus (hammer), incus (anvil), and stapes (stirrup) - The stapes fits within an elastic membrane (oval window) and is held in place by a ligament

The Vestibular Apparatus

- The specialized sensory structure of the inner ear of most terrestrial vertebrates that is responsible for detecting rotational movement of the head and provides information about the up-down positioning of the head as well as changes in the rate of linear movement of the head. The vestibular apparatus consists of 3 semicircular canals, the utricle, and the saccule. all filled with a fluid called endolymph. - perceive the position and motion of the head

Basilar and tectorial membrane

- The vibrations traveling through the inner ear cause the basilar membrane to vibrate in response. - The basilar membrane, which forms part of the floor of the cochlear duct, anchors sensory hair cells in the organ of Corti and which vibrates in response to vibrations moving through the inner ear. - Stereocilia of these cells are embedded in the tectorial membrane (A membrane that extends the length of the cochlear canal of the inner ear in which the stereocilia of the sensory hair cells of the organ of Corti are embedded.) - When the basilar membrane vibrates, the stereocilia are bent in relation to the stationary tectorial membrane - Neurotransmitter release from hair cells triggers action potentials in afferent neurons at the same frequency

Thermoreceptors

- Thermoreceptors are sensory receptor that detects the flow of heat energy/changes in temperature - Both types of receptors consist of free nerve endings formed by dendrites of afferent neurons, with no specialized receptor structures surrounding them - Some invertebrates, such as mosquitoes and ticks, use thermoreceptors to locate warm-blooded prey - Some snakes, including rattlesnakes and pythons, use pit organs to detect warm-blooded prey - Some neurons in the hypothalamus of mammals also function as thermoreceptors that sense changes in brain temperature and receive afferent thermal information - These neurons are highly sensitive to shifts from normal body temperature, and trigger involuntary responses such as sweating, panting, or shivering, which restore normal body temperature - Two thermoreceptor proteins have been discovered, both of which are cation channels.

The gustatory neurons of most vertebrates are called taste cells.

- form part of a structure called a taste bud, a small, pear-shaped capsule with a pore at the top that opens to the exterior - The sensory cilia of taste cells pass through the pore of a taste bud and project to the exterior. - At their opposite end, taste cells form synapses with dendrites of afferent neurons. - Taste cells in vertebrates are concentrated in the mouth.

The Vertebrate Eye

- light entering the eye through the cornea passes through the pupil and then the lens, which focuses an image on photoreceptor neurons in the retina - axons of afferent neurons converge to form the optic nerve leading from the (eye) retina to the brain - A clear aqueous humor fills the space between the cornea and lens - the main chamber of the eye is filled with jellylike vitreous humor - The outer wall contains a tough layer of connective tissue (sclera) - inside it is a darkly pigmented layer (choroid:prevents light from entering except through the pupil, blood vessels, nurish retina) - Two types of photoreceptors are in the retina: rods and cones

Mechanoreceptors and the Tactile and Spatial Senses

- mechanosensory neurons detecting touch & pressure - A mechanical stimulus creates tension in the plasma membrane of a receptor which causes ion channels to open, producing a receptor potential - If the receptor potential is large enough, an action potential is triggered in the afferent neuron leading to the CNS - Sensory information from the receptors provides the brain with information on the movement, position, and balance of body parts, and underlies the sense of hearing

The Utricle and Saccule

- provide information about the position of the head with respect to gravity (up versus down), as well as changes in the rate of linear movement of the head. - The utricle and saccule contain sensory hair cells covered with a gelatinous otolithic membrane in which small crystals of calcium carbonate (otoliths) are embedded - When an animal is upright, sensory hairs in the utricle are oriented horizontal - those in the saccule are vertical - When the head is tilted, or there is change in linear motion of the body, sensory hairs bend and action potentials are generated

taste cells

A gustatory neuron, located within a taste bud, activated in response to specific tastant molecules in the saliva.

retina

A light-sensitive membrane lining the posterior part of the inside of the eye.

taste bud

A small, pear-shaped capsule with a pore at the top that opens to the exterior that contains the taste cells. Found in most vertebrates.

bipolar cells

A type of neuron in the retina of the eye that connects the rods and cones with the ganglion cells.

ganglion cells

A type of neuron in the retina of the eye that receives visual information from photoreceptors via various intermediate cells such as bipolar cells, amacrine cells, and horizontal cells.

Horizontal cells

A type of neuron that forms lateral connections among photoreceptor cells in the retina of the eye.

amacrine cells

A type of neuron that forms lateral connections in the retina of the eye, connecting bipolar cells and ganglion cells.

What vibration-detecting systems are found in octopuses and insects?

An octopus has mechanoreceptors on its head and tentacles, similar to the lateral line of fishes. These mechanoreceptors detect vibrations in the water. Many insects have sensory receptors in the form of hairs or bristles that vibrate in response to sound waves, whereas some insects (for example, moths, grasshoppers, and crickets) have auditory organs on either side of the abdomen or on the first pair of walking legs.

oval window

An opening in the bony wall that separates the middle ear from the inner ear.

papillae

An outgrowth on the tongue. Taste buds are embedded in the papillae.

Which of the following events does not occur during light absorption in the vertebrate eye?

As light absorption increases, the rhodopsin response causes an increase in the release of neurotransmitters.

What do the eyes of cephalopods and vertebrates have in common, and what does this tell us about their visual perception?

Both cephalopod and vertebrate eyes contain lenses that focus light from one point in space onto a single point on the retina. These structures enable cephalopods and vertebrates to form visual images and thus recognize objects.

lateral geniculate nuclei

Clusters of neurons located in the thalamus that receive visual information from the optic nerves and send it on to the visual cortex.

Why do vertebrates express so many different odorant receptor proteins?

Each odorant receptor protein binds a specific range of odorant molecules. A large number of different odorant receptor proteins are needed because there are so many different biologically relevant molecules in an animal's environment.

Sensory Receptor Fields

Each sensory receptor has a defined receptive field: region surrounding a sensory neuron within which the neuron responds to a stimulus.

What are three ways electroreceptors are used in aquatic vertebrates?

Electroreceptors are used in aquatic vertebrates for electrolocation (locating other animals such as prey), electrocommunication (communicating with other members of the same species), and killing prey (involving high-voltage discharge).

Accommodation: Focusing on a Distant Object

Fine ligaments anchor the lens to the muscular ciliary body, which changes the shape of the lens to focus light from distant or nearby objects on the retina (accommodation)

What are two uses of hair cells in vertebrates?

Hearing and maintaining balance

Which two senses make use of mechanoreceptor proteins?

Hearing and somatosensation

What are the different types of neurons in the retina, and how does each one contribute to transmitting and integrating the visual input?

In addition to rods and cones, retinas also contain horizontal cells, bipolar cells, amacrine cells, and ganglion cells. Rods and cones synapse on bipolar cells, which synapse on ganglion cells, whose axons form the optic nerve that transmits visual information to the brain. This feedforward excitation circuit transmits visual information to the brain. Horizontal cells enhance photoreceptor signaling at the boundaries between light and dark areas, such as the edges of objects. The diverse functions of amacrine cells are poorly understood, but one type is known to contribute to making one type of ganglion cells sensitive to motion in specific directions.

Cupula

In certain mechanoceptors, a gelatinous structure with stereocilia extending into it that moves with pressure changes in the surrounding water; movement of the cupula bends the stereocilia, which triggers release of neurotransmitters.

How are sounds of particular frequencies distinguished by humans?

In humans, vibrations representing sound frequencies are transmitted into the fluid-filled inner ear. They travel through the inner ear and cause the basilar membrane to vibrate in response, bending the sensory hair cells and stimulating them to release a neurotransmitter that triggers action potentials in afferent neurons leading from the inner ear. The vibrations from a particular sound frequency cause the basilar membrane to vibrate maximally at one particular location, stimulating the hair cells in that region to initiate action potentials. That information is sent to the brain, which integrates it into a perception of the sound stimulus.

cones

In the vertebrate eye, a photoreceptor in the retina that is specialized for detection of different wavelengths (colors). In cone-bearing plants, a cluster of sporophylls.

rods

In the vertebrate eye, a type of photoreceptor in the retina that is specialized for detection of light at low intensities.

Taste and Smell Receptors in Terrestrial Vertebrates

Information from taste receptors is typically processed in the parietal lobes - information from smell receptors is processed in the olfactory bulbs and temporal lobes

Afferent Neurons

Intensity and extent of a stimulus are registered in 2 ways: - By the frequency of action potentials traveling along each axon of an afferent pathway - By the number and types of afferent neurons that the stimulus activates to generate action potentials in the pathway

odorant receptor proteins

One of a family of G-protein-coupled receptors that binds a specific range of odorants, activating an olfactory neuron.

opsins

One of several different proteins that bond covalently with the light-absorbing pigment of rods and cones (retinal).

Why is magnetoreception useful in navigation?

Magnetoreception is useful in navigation because the Earth has a magnetic field. Animals that sense magnetic fields can know in which compass direction they are traveling.

How do vertebrates distinguish different colors of light?

Mammalian retinas contain two types of photoreceptor neurons: rods and cones. Rods are more sensitive to very low light levels and thus contribute to night vision. Cones enable mammals to distinguish different colors of light. Each cone cell expresses one of a small number of different types of cone opsin proteins (three types in humans). Each type of cone opsin responds to a different range of wavelengths of light. Specific wavelengths are detected based on the relative responses of different types of cones.

How do mechanoreceptor proteins depolarize cells in response to mechanical forces?

Mechanoreceptors are ion channel proteins that open or close as they change shape in response to physical forces. If a mechanoreceptor ion channel is permeable to Na+ ions, then those ions rush in when the channel opens, depolarizing the mechanosensory neuron.

What are the main types of sensory receptor proteins, and which sense or senses does each type make possible?

Mechanoreceptors detect physical forces that act on the membranes of sensory neurons. Photoreceptors absorb photons of visible light. Chemoreceptors bind specific biologically relevant molecules as ligands. Thermoreceptors detect changes in temperature.

Five Basic Types of Sensory Receptors

Mechanoreceptors: - Detect mechanical energy when it deforms membranes - Physical forces that push or pull on the membrane of a sensory neuron, thereby changing the shape of the receptor protein. - Most are ion channel proteins that open or close when they are deformed by forces acting on them - Basis for the vertebrate senses of touch and hearing. - Ex: Auditory receptors in the ears Photoreceptors: - Detect the energy of light - transduction mechanism in the sense of vision - Ex: Photoreceptors in the retina of the eye Chemoreceptors: - Detect specific molecules or chemical conditions (acidity) - detect specific molecules that act as ligands, activating the receptors. - detect food, smell - Ex: taste buds on the tongue Thermoreceptors: - Detect the flow of heat energy - transduction mechanism in the sense of touch. - Ex: temp. receptors in the skin Nociceptors: - Detect tissue damage or noxious chemicals - Register the activity as pain - Pain receptors are located in the skin and also in some internal organs

stereocilia

Microvilli covering the surface of hair cells clustered in the base of neuromasts.

What types of receptor proteins do nociceptor neurons express?

Nociceptor neurons express mechanoreceptors, thermoreceptors, and/or chemoreceptors.

iris

Of the eye, the colored muscular membrane that lies behind the cornea and in front of the lens, which by opening or closing determines the size of the pupil and hence the amount of light entering the eye.

Describe the signal transduction pathway that causes vertebrate photoreceptors to release less neurotransmitter in the light than they do in the dark.

Photons of light are absorbed by retinal molecules bound within opsin proteins, causing retinal and thus the opsin to change shape, activating the opsin. The G protein transducin binds to and is activated by the activated opsin. It then binds to the enzyme phosphodiesterase, which breaks down cGMP to GMP. The concentration of cGMP in photoreceptor neurons is high in the dark, so the cGMP binds to and activates Na+ channels, increasing Na+ influx and depolarizing the neuron. When the cGMP concentration is decreased by activated phosphodiesterase in the light, the Na+ channels close, so less Na+ enters the neuron and the neuron is hyperpolarized. Depolarized photoreceptor neurons release more neurotransmitters because depolarization activates voltage-gated Ca2+ channels, increasing intracellular Ca2+, which triggers fusion of neurotransmitter vesicles with the plasma membrane.

Photoreceptors

Photoreceptors (rods & cones) have three parts: - An outer segment consisting of stacked, flattened, membranous discs - An inner segment where metabolic activities occur - The synaptic terminal, where neurotransmitter molecules are stored and released - The light-absorbing pigment retinal is bonded to opsins to produce photopigments, which are embedded in the membranous discs of the photoreceptors' outer segments - Rhodopsin is the opsin expressed by rods, and three other opsins are expressed by the three types of cones in the human eye - The retinal-opsin photopigment in rods is rhodopsin - In the dark, the retinal of rhodopsin is inactive (cisretinal) and the rods steadily release the neurotransmitter glutamate - When rhodopsin absorbs a photon of light, retinal converts to its active form, trans-retinal, and the rods decrease the amount of glutamate they release

Define sensory transduction.

Sensory transduction is the conversion of a stimulus into a change in membrane potential in a neuron.

Mechanoreceptors and Hearing

Sounds are vibrations that travel as waves produced by the alternating compression and rarefaction of the air ↓ The loudness, or intensity of a sound depends on the amplitude (height) of the wave ↓ The pitch of a sound (such as a high note or a low note) depends on the frequency of the waves, measured in hertz (cycles per second) - The more cycles per second, the higher the pitch

List the different tastes, and describe the receptors for each taste.

Sweet taste senses sugars in food, and is transduced by a G-protein-coupled receptor that binds sucrose and other sugars. Umami taste senses proteins in food, and is transduced by a G-protein-coupled receptor that binds the amino acid glutamate. Bitter taste senses toxins in food, and is transduced by a family of G-proteincoupled receptors, each of which binds a different range of potentially toxic molecules in food. Sour taste senses acidic solutions, and is transduced by cation channel proteins that are inhibited by H+. Salty taste senses Na+ in food, and is transduced by at least two different receptor proteins, one of which is a Na+ channel.

sensory transduction

The conversion of a stimulus into a change in membrane potential (thru release of NT).

pupil

The dark center in the middle of the iris through which light passes to the back of the eye.

How is the human vestibular system structured, and what does it detect?

The human vestibular apparatus consists of 3 semicircular canals that are positioned so the fluid within them moves in response to rotational movements in each of the 3 spatial dimensions. The motion of the fluid bends stereocilia in hair cells within the semicircular canals. Ion channels in the tips of those stereocilia open when the stereocilia are bent in one direction, causing K+ to enter the hair cells and depolarize them. The vestibular apparatus detects the movement of the head in space.

Compare the lateral line system with the mammalian ear.

The lateral line system and the mammalian ear both contain mechanosensory neurons that are depolarized when movement of their stereocilia opens mechanoreceptor ion channels. Both sets of mechanosensory neurons detect vibrations in a fluid. In the lateral line system the vibrating fluid is water surrounding the fish; in the mammalian ear, fluid in the cochlea vibrates in response to air pressure waves.

Why is the vertebrate olfactory system less well-understood than the visual system?

The olfactory system is poorly understood in part because there are so many different odorant receptor proteins, and identifying the range of ligands for each receptor is a huge task. Another challenge is that integration of olfactory information in the brain does not appear to be as consistently organized as integration of visual information.

Malleus

The outermost of the sound-conducting bones of the middle ear in vertebrates.

cornea

The transparent layer that forms the front wall of the eye, covering the iris.

lens

The transparent, biconvex intraocular tissue that helps bring rays of light to a focus on the retina.

What do all sensory receptor proteins have in common?

They produce a change in membrane potential when activated.

The eyes of vertebrates and cephalopods are similar in structure and function. A difference between the vertebrate eye and the cephalopod eye is that the vertebrate eye has:

a lens that changes shape when focusing, whereas in cephalopods the lens moves back and forth to focus.

The variety of color seen by humans is directly dependent on the:

activation of three different opsins in cones.

Accommodation in the vertebrate eye

does not involve forward and backward movement of the lens, as in cephalopods.

The sense organ of the lateral line, the neuromast, has a

gelatinous cupula that is pushed and pulled by vibrations and currents transmitted through the lateral line canal. As the cupula moves, the stereocilia of the hair cells are bent, generating action potentials in afferent neurons that lead to the brain.

free nerve endings

respond to pain & temperature. the dendrites of afferent neurons with no specialized structures surrounding them. Wrapped around hair follicles respond when the hair is bent, making you instantly aware, for example, of a breath of air, or a fly on your arm.

You notice an odor when you walk into a room but soon stop smelling it, even though someone else smells it when they walk into the room. This response is due to:

sensory adaptation.

mechanosensation

the transduction of mechanical forces caused by moving objects and by pressure are detected by mechanoreceptor proteins, which are ion channels in the membranes of mechanosensory neurons. A mechanical stimulus changes the conformation of a mechanoreceptor, causing the ion channel to open and let Na+ in ions that depolarize the neuron.

inner ear has two specialized sensory structures,

the vestibular apparatus and the cochlea.