Chapter 45
Sensory Systems
Sensory Systems
Sensory Cells and Transduction of Stimuli
Chemoreceptors: Responding to Specific Molecules
Mechanoreceptors: Detecting Stimuli that Distort Membranes
Photoreceptors and Visual Systems: Responding to Light
Sensory Worlds Beyond Human Experience
Sensory Cells and Transduction of Stimuli
Sensory cells transduce physical or chemical stimuli into signals that are transmittable and interpretable.
Most sensory cells (or receptor cells) are modified neurons, specialized for detecting different kinds of stimuli, such as pressure, heat, or light.
Sensory Cells and Transduction of Stimuli
Most sensory cells have membrane receptor proteins that detect a stimulus and respond by altering the flow of ions across the plasma membrane.
The resulting change in membrane potential causes the sensory cell to fire action potentials or to change its secretion of a neurotransmitter onto an associated neuron that fires action potentials.
The intensity of the __________ is encoded in the frequency of the action potentials produced.
Sensory Cells and Transduction of Stimuli
Although they are simply __________ events, sensory data are interpreted in different ways according to the different places in the CNS where messages from different kinds of sensory cells arrive.
A small patch of skin, for example, contains various sensory cells capable of detecting heat, pressure, movement, and tissue damage (pain).
Whether a stimulus is interpreted as one or another sensation depends on which cells of the central nervous system receive the signal.
Sensory Cells and Transduction of Stimuli
Some information is __________ without our being conscious of it.
For example, the brain receives continuous information about levels of CO2, blood sugar, and O2. Such information is important for the maintenance of homeostasis.
Sensory cells and other types of cells form sensory organs, such as eyes, ears, and noses.
Sensory systems include the sensory cells, the associated structures, and the neuronal networks that process the information.
Sensory Cells and Transduction of Stimuli
Sensory cells __________ the __________ from a __________ into action potentials.
The first step is activation of a receptor protein in the plasma membrane of a sensory cell by a stimulus.
The activated protein opens or closes ion channels.
Sensory Cells and Transduction of Stimuli
In ionotropic sensory detection, the receptor protein itself is part of the ion channel and, by changing its conformation, opens or closes the channel pore.
In metabotropic sensory detection, the receptor protein is linked to a G protein that activates a cascade of intracellular events that eventually open or close ion channels.
Sensory Cells and Transduction of Stimuli
The affected receptor must trigger an action potential if the signal is to be transmitted by the nervous system.
A change in the resting membrane potential of a sensory cell in response to a stimulus is called a receptor potential.
Sensory Cells and Transduction of Stimuli
Primary sensory cells generate action potentials directly. An example is the crayfish stretch receptor.
Secondary sensory cells generate action potentials indirectly by inducing the release of neurotransmitter.
Sensory Cells and Transduction of Stimuli
Some sensory cells respond less when stimulation is repeated, a phenomenon called adaptation.
The ability of animals to ignore continuous stimuli while remaining sensitive to changing stimuli is sometimes due to sensory cell __________.
Chemoreceptors:
Responding to Specific Molecules
Chemoreceptors detect chemical stimuli.
Chemoreceptors are responsible for smell and taste, and for monitoring internal environmental factors such as CO2 and O2 in the blood.
Corals, for example, can detect protein or even a single type of amino acid, causing them to extend tentacles in search of food.
Chemoreceptors:
Responding to Specific Molecules
Arthropods use chemical signals called __________ to attract mates.
Female silkworm moths release a pheromone called bombykol from glands at the tip of the abdomen, and males have receptors for bombykol on their antennae.
A single molecule of the pheromone can stimulate a perceivable action potential.
When 200 hairs or more per second are activated, the male flies upwind in search of the female, following a __________ __________ to the female.
Chemoreceptors:
Responding to Specific Molecules
Olfaction, the sense of smell, also depends on chemoreceptors.
In vertebrates, olfactory sensors are neurons embedded in a layer of epithelial cells at the top of the nasal cavity.
The axons of these sensors project to the olfactory bulb of the brain.
The dendrites end in olfactory hairs at the surface of the nasal epithelium.
Molecules from the environment diffuse through nasal mucus to reach the surface of the olfactory hairs.
Chemoreceptors:
Responding to Specific Molecules
Odorants are chemicals that bind to olfactory receptor proteins.
Each olfactory receptor protein binds particular odorant molecules, which activates a G protein.
The G protein then activates an enzyme that increases levels of a second messenger, such as cAMP.
The second messenger binds to sodium channels in the plasma membrane and opens them. The influx of Na+ depolarizes the membrane and an action potential is fired.
Chemoreceptors:
Responding to Specific Molecules
The number of odorant molecules greatly exceeds the number of different receptor proteins.
Each odorant may bind to one or more specific receptor proteins.
A specific odorant is distinguished according to the different and unique __________ of cells it activates.
The strength of the odor depends on the number of odorant molecules detected.
More odorant molecules produce more action potentials per unit of time and are perceived as stronger odors.
Chemoreceptors:
Responding to Specific Molecules
The vomeronasal organ (VNO) is a small, paired tubular structure embedded in the nasal epithelium.
The VNO has a pore opening into the nasal cavity; when an animal sniffs it draws a sample of nasal fluid over the chemoreceptors of the VNO.
The information from the VNO chemoreceptors goes to an accessory olfactory bulb in the brain.
From the olfactory bulb, information is routed to regions of the brain involved in sexual and other instinctive behaviors.
Chemoreceptors:
Responding to Specific Molecules
Experiments with mice have confirmed that the VNO detects pheromones.
In snakes, the VNO opens into the mouth cavity. The snakes forked tongue fits into the VNO and molecules collected from the air contact the chemoreceptors in the VNO.
The snake uses its tongue to smell its environment.
Chemoreceptors:
Responding to Specific Molecules
Gustation, the sense of taste, depends on clusters of sensory cells called taste buds.
Humans have 10,000 taste buds embedded in the epithelium of the tongue.
Many are in raised papillae, the small bumps on human tongues.
The outer surface of each bud has a pore that exposes the tips of sensory cells. Microvilli increase the surface area of the cells.
The sensory cells form synapses with dendrites of sensory neurons.
Chemoreceptors:
Responding to Specific Molecules
Receptor proteins in the microvilli bind specific molecules. This causes the release of neurotransmitters to the dendrites of associated sensory neurons.
Taste buds are replaced every few days, but the associated neurons live on.
Taste buds can distinguish sweet, salty, sour, and bitter tastes.
Recently the savory meaty taste umami has been added to the list of distinguishable tastes.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Mechanoreceptors are cells that are sensitive to mechanical forces.
They are involved in many sensory systems, including skin sensations and sensing blood pressure.
Physical distortion of a mechanoreceptors plasma membrane causes ion channels to open, which leads to the generation of action potentials.
The rate of the action potentials is related to the strength of the stimulus.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Skin is packed with diverse mechanoreceptors that cause various sensations.
Merkels discs provide continuous information about things touching the skin.
Meissners corpuscles are very sensitive mechanoreceptors found mostly in non-hairy skin. They provide information about changes in things touching the skin.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Two other mechanoreceptors are found deeper in the skin:
Ruffini endings provide information about vibrating stimuli of low frequencies.
Pacinian corpuscles provide information about vibrating stimuli of higher frequencies.
Dendrites of sensory neurons are also wrapped around hair follicles, which detect displacement of the hairs.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Density of tactile mechanoreceptors influences how finely stimulation can be resolved.
On the back, two stimuli must be fairly far apart before they can be __________.
On fingertips, finer spatial discrimination is possible because mechanoreceptors are much more dense.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Stretch receptors provide an animal with information about the position of its limbs and the stresses on its muscles and joints. They feed information continuously to the CNS.
Stretch receptors embedded in connective tissues in skeletal muscle are called muscle spindles.
They are modified muscle fibers that are innervated in the center with extensions of sensory neurons.
The CNS uses information from muscle spindles to maintain muscle tone.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
The Golgi tendon organ is a stretch receptor found in tendons and ligaments.
When a muscle contraction becomes too forceful, the Golgi tendon organ sends signals to the CNS that inhibits motor neurons and the muscle relaxes.
This prevents muscle damage by limiting the force of contracting muscles when excessive force could injure connective tissue.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Hair cells are also mechanoreceptors.
Each hair cell has a set of stereocilia (microvilli).
When the stereocilia are bent in one direction, receptor potential becomes more negative; when they are bent in the other direction, it becomes more positive.
When the membrane potential becomes more positive, the hair cell releases a neurotransmitter to the sensory neuron associated with it, and the sensory neuron sends action potentials to the CNS.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Hair cells are found in the lateral line system of fishes, providing information about movement through the water and moving objects that cause pressure waves in water.
Vertebrate organs of equilibrium use hair cells to detect the position of the body with respect to gravity.
Semicircular canals and the vestibular apparatus in the mammalian inner ear use hair cells to detect position and orientation of the head, as well as acceleration produced by movement.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Auditory systems use mechanoreceptors to convert pressure waves into action potentials.
Pinnae collect sound waves and direct them into the auditory canal, which leads to the middle inner ear.
The eardrum (tympanic membrane) covers the end of the auditory canal and vibrates in response to pressure waves. On the other side is the fluid-filled middle ear.
Pressure on both sides of the eardrum equilibrates because the Eustachian tube allows airflow.
Mechanoreceptors:
Detecting Stimuli that __________ Membranes
Three delicate bones in the middle ear called the ear ossicles (the malleus, incus, and stapes) transfer the vibrations of the eardrum to the oval window.
Behind the oval window is the fluid-filled inner ear. Movements of the oval window result in pressure changes in the inner ear.
The inner ear is a long, tapered, coiled chamber called the cochlea, composed of three parallel canals separated by two membranes, Reissners membrane and the basilar membrane.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
The organ of Corti rests on the basilar membrane.
The organ of Corti actually transduces pressure waves into action potentials in the auditory nerve.
The organ of Corti contains hair cells whose stereocilia are in contact with the tectorial membrane.
When the basilar membrane flexes, the tectorial membrane bends the hair cell stereocilia.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
What causes the basilar membrane to flex?
The cochlea is filled with fluid and the upper and lower canals are connected at the distal end. Pressure waves displace the fluid in the upper canal of the cochlea.
Instead of traveling all the way around the canals, the waves of fluid cross the basilar membrane, causing it to flex.
High frequency causes the basilar membrane nearest the oval window to flex.
Low frequency causes flexing farther down the membrane.
Mechanoreceptors:
Detecting Stimuli that Distort Membranes
Deafness has two general causes:
__________ deafness is loss of function of the tympanic membrane or ossicles of the middle ear. The ossicles stiffen with age causing loss of ability to hear high frequency sound.
__________ deafness is caused by inner ear or auditory pathway damage, including damage to hair cells.
Rock music and other loud noises can cause damage to hair cells. This damage is cumulative and permanent.
Photoreceptors and Visual Systems:
Responding to Light
Photosensitivity is the sensitivity to light.
It ranges from the ability to orient to the sun to the ability to see.
Evolution has conserved molecules used for __________ across the entire range of animal species. These are a family of pigments called rhodopsins.
Photoreceptors and Visual Systems:
Responding to Light
Rhodopsin molecules can absorb photons of light and undergo shape changes.
Rhodopsin molecules consist of a protein called opsin and a light-absorbing group, 11-cis-retinal.
The retinal group is in the center of the opsin, and the entire complex is within the plasma membrane of a photoreceptor cell.
When 11-cis-retinal absorbs a photon, it changes to all-trans-retinal, which changes the conformation of the opsin. This change signals detection of light.
Photoreceptors and Visual Systems:
Responding to Light
The all-trans form of retinal and opsin complex passes through several intermediate stages.
One stage, known as photoexcited rhodopsin, triggers a cascade that results in alteration of membrane potential of a neuron.
Photoreceptors and Visual Systems:
Responding to Light
A rod cell is a modified neuron. It releases neurotransmitters that influences other neurons.
Rod cells have an outer segment, an inner segment, and a synaptic terminal.
The inner segment has the nucleus and many mitochondria.
The outer segment has a stack of discs of plasma membrane densely packed with rhodopsin. The discs function to capture photons.
Photoreceptors and Visual Systems:
Responding to Light
When a rod cell is in the dark, it has a depolarized resting potential. Na+ ions can continually enter the outer segment.
When light flashes on the rod cell, the outer segment becomes more negative, or hyperpolarized.
When light is absorbed by rhodopsin, it becomes photoexcited and activates a G protein called transducin.
The activated transducin activates a phosphodiesterase, which converts cGMP to GMP.
cGMP keeps sodium channels open; in light, GMP levels rise and channels close.
Photoreceptors and Visual Systems:
Responding to Light
The advantage of this system is that it amplifies the signal.
Each single photon can cause activation of several hundred transducin molecules, which in turn, activate many phosphodiesterase molecules.
A single photon can close a huge number of sodium channels.
Photoreceptors and Visual Systems:
Responding to Light
Invertebrates have a variety of visual systems.
Flatworms obtain directional information from photoreceptors that are organized into paired eye cups, shielded by layers of pigmented cells.
Because of the shielding, photoreceptors on the two sides of the animal are unequally stimulated unless the animal is facing directly toward or away from the light.
Photoreceptors and Visual Systems:
Responding to Light
Arthropods have compound eyes consisting of many optical units called ommatidia.
Each ommatidium has a lens that directs light onto photoreceptor cells (retinula cells). These cells have microvilli with rhodopsin, and their axons communicate with the nervous system.
Each ommatidium gives a slightly different view, resulting in broken-up images.
The number of ommatidia in an eye varies, from a few in certain ants to 10,000 in dragonflies.
Photoreceptors and Visual Systems:
Responding to Light
Both vertebrates and cephalopod mollusks have highly evolved eyes.
Vertebrate eyes are fluid-filled spheres bound by tough connective tissue called sclera.
A transparent cornea in the front allows light passage.
Inside the cornea is the pigmented iris, which controls the amount of light that can enter.
The pupil is the region where light enters.
The lens makes fine adjustments in the focus of images on the photosensitive retina at the back of the eye.
Photoreceptors and Visual Systems:
Responding to Light
The most sensitive area of the retina is the __________.
The lenses allow the eyes to focus light.
Fishes, amphibians, and reptiles focus by moving the lenses of their eyes closer to or farther from their retinas.
Mammals and birds alter the shape of the lens to focus.
Photoreceptors and Visual Systems:
Responding to Light
The shape of the lens changes due to the action of two structures.
Connective tissue surrounding the lens keeps it spherical, but suspensory ligaments pull it into a flatter shape.
Ciliary muscles counteract the pull of the ligaments and allow the lens to become round.
The flatter lens is able to focus distant images but not nearer ones, which need the light-bending properties of the round lens to bring close images into focus.
Lenses become less elastic with age and we lose the ability to focus on objects close at hand.
Photoreceptors and Visual Systems:
Responding to Light
The retina includes layers of cells that process visual information from the photoreceptors and produce an output signal that is transmitted via the optic nerve.
Light must pass through all the layers of cells before photons are captured by rhodopsin.
There are two types of vertebrate photoreceptors: cones and rods.
Rod cells are more sensitive to light. Cone cells respond to different wavelengths of light for color vision.
Cones also provide the __________ vision. The fovea has only cone cells.
Photoreceptors and Visual Systems:
Responding to Light
Humans have three kinds of cone cells: One type absorbs violet and blue wavelengths, one absorbs green, and one absorbs yellow and red.
The human fovea has about __________ cone cells per square millimeter; a hawk has __________.
Hawks also have two foveas per eye and can see both their flight path and the ground below.
There are no photoreceptors where blood vessels and bundles of axons going to the brain pass through the back of the eye. This creates a __________ spot on the retina.
Photoreceptors and Visual Systems:
Responding to Light
The human retina is organized into five layers of cells.
Cells at the front of the retina are ganglion cells. They fire action potentials and their axons form the optic nerves.
The photoreceptor cells are at the back of the retina. Ganglion cells and photoreceptors are connected by bipolar cells.
Release of neurotransmitter from the photoreceptor cells in turn causes the rate of neurotransmitter release from the bipolar cells to change.
Release of neurotransmitter from bipolar cells causes ganglion cells to fire action potentials.
Photoreceptors and Visual Systems:
Responding to Light
Horizontal cells connect neighboring pairs of photoreceptors and bipolar cells.
This provides a means for the lateral flow of information.
Amacrine cells connect neighboring pairs of bipolar cells and ganglion cells.
These help make eyes more sensitive to small but rapid changes.
Photoreceptors and Visual Systems:
Responding to Light
Each ganglion cell has a well-defined receptive field, which consists of a specific group of photoreceptor cells.
This integrates the light signal into one output.
The receptive field of a ganglion cell can be divided into two concentric areas, called the center and the surround.
Photoreceptors and Visual Systems:
Responding to Light
There are two kinds of receptive fields: on-center and off-center.
Ganglia with on-center receptive fields are maximally excited by light falling on the center.
Ganglia with off-center receptive fields are maximally stimulated by light falling on the surround.
Center effects are always stronger than surround effects.
The photoreceptors in the center of the receptive field of a ganglion cell are connected to that ganglion via bipolar cells.
Sensory Worlds Beyond Human Experience
Some species can see infrared and ultraviolet light.
One of the seven photoreceptors in each ommatidium of a fruit fly is sensitive to ultraviolet light.
Some flowers have patterns that are invisible to humans but can be seen by flies.
Pit vipers have pit organs, one in front of each eye, which can sense and locate infrared radiation in total darkness.
Sensory Worlds Beyond Human Experience
Elephants can communicate with infrasound, sounds below the range of human hearing.
The advantage of using low frequency sound to communicate is that it carries over very long distances.
Sensory Worlds Beyond Human Experience
Echolocation is sensing the world through reflected sound.
Dolphins, bats, and whales can use noises to echolocate.
They generate sounds at frequencies above human hearing.
These animals use muscles in the middle ear to dampen their sensitivity to sound while they are emitting sounds in order to protect their hearing.
To hear the returning echoes, they relax the muscles.
Sensory Worlds Beyond Human Experience
Some fish can sense __________ fields.
Lateral lines of some species, such as catfish, contain electroreceptors.
These enable the fish to detect weak electric fields, which helps them locate prey.
Some fishes, such as electric fish, can use electric fields to navigate. Rocks, plants, and other structures disrupt their field and are interpreted.
Animation 45.1 Sound Transduction in the Human Ear
Animation 45.2 Information Processing in the Retina
Video 45.1 Human ear drums and bones
Video 45.2 Hair cells of the cochlea responding to music
Video 45.3 Eyespots of Volvox
Video 45.4 Human iris responding to changes in light
Video 45.5 Human retina