Sensors
Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
Sensory receptor cells, or sensors or receptors, transduce physical and chemical stimuli into a change in membrane potential.
The change in membrane potential may generate an action potential that conveys the sensory information to the CNS for processing.
Sensory transduction—begins with a receptor protein that can detect a specific stimulus.
The receptor protein opens or closes ion channels in the membrane, changing the resting potential.
Receptor potentials—graded membrane potentials that travel a short distance.
Receptor potentials can generate action potentials in two ways:
•Can generate action potentials in the receptor cell
•Can trigger release of neurotransmitter so that a postsynaptic neuron generates an action potential
Stretch receptors in crayfish cause receptor potentials when the attached muscle is stretched.
Receptor potentials spread to the base of the axon and generate action potentials.
The rate of firing depends on the magnitude of the receptor potential, which depends on the amount of stretching.
Different sensory receptors respond to particular stimuli:
•Mechanoreceptors detect physicalforces such as pressure (touch) and variations in pressure (sound waves).
•Thermoreceptors respond to temperature.
•Electrosensors are sensitive to changes in membrane potential.
•Chemoreceptorsrespond to the presence or absence of certain chemicals.
•Photoreceptors detect light.
Some sensory receptor cells are organized with other cells in sensory organs, such as eyes and ears.
Sensory systems include sensory cells, associated structures, and neural networks that process the information.
Sensation depends on which part of the CNS receives the sensory messages.
Intensity of sensation is coded as the frequency of action potentials.
Some sensory cells transmit information to the brain about internal conditions, without conscious sensation.
Adaptation—diminishing response to repeated stimulation.
Enables animals to ignore background conditions but remain sensitive to changing or new stimuli.
Some sensory cells don’t adapt (e.g., mechanoreceptors for balance).
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
Chemoreceptors—receptor proteins that bind to various molecules, responsible for taste and smell.
Also monitor internal environment, such as CO2 levels in blood.
Olfaction—sense of smell; depends on chemoreceptive neurons embedded in epithelial tissue at top of nasal cavity (in vertebrates).
Axons from olfactory sensors extend to the olfactory bulb in the brain—dendrites end in olfactory hairs on the nasal epithelium.
Odorant—a molecule that activates an olfactory receptor protein
Odorants bind to receptor proteins on the olfactory cilia.
Olfactory receptor proteins are specific for particular odorants.
When an odorant binds to a receptor protein, it activates a G protein, which activates a second messenger (cAMP).
The second messenger causes an influx of Na+ and depolarizes the olfactory neuron.
Many more odorants can be discriminated than there are olfactory receptors.
In the olfactory bulb, axons from neurons with the same receptors converge on glomeruli.
Pheromones—chemical signals used by insects to attract mates.
Example: Female silkworm moth releases bombykol. Male has receptors for bombykol on the antennae.
One molecule of bombykol is enough to generate action potentials.
Vomeronasal organ (VNO) is found in many vertebrates—specialized for pheromones
It is a paired tubular structure embedded in the nasal epithelium.
When animal sniffs, the VNO draws a sample of fluid over chemoreceptors in walls.
Information goes to an accessory olfactory bulb and on to other brain regions.
Gustation is the sense of taste.
Taste buds—clusters of chemoreceptors.
Some fish have taste buds on the skin; the duck-billed platypus has taste buds on its bill.
Human taste buds are embedded in the tongue epithelium, on papillae. The sensory cells generate action potentials when they detect certain chemicals.
Humans taste salty, sour, sweet, bitter, and umami—a savory, meaty taste.
“Salty” receptors respond to Na+ depolarizing the cell.
“Sour” receptors detect acidity as H+, and “sweet” receptors bind different sugars.
Umami receptors detect the presence of amino acids, as in MSG.
Bitterness is more complicated and involves at least 30 different receptors.
Concept 35.3 Mechanoreceptors Detect Physical Forces
Mechanoreceptors are cells that detect physical forces.
Distortion of their membrane causes ion channels to open and a receptor potential to occur.
This may lead to the release of a neurotransmitter.
The skin has diverse mechanoreceptors:
•Free nerve endings detect heat, cold, and pain.
•Merkel’s discs: Adapt slowly, give continuous information.
•Meissner’s corpuscles: Adapt quickly, give information about change.
•Ruffini endings: Deep, adapt slowly, react to vibrating stimuli of low frequencies.
•Pacinian corpuscles: Deep, adapt rapidly, react to vibrating stimuli at high frequencies.
Muscle spindles: Mechanoreceptors in muscle cells, called stretch receptors.
When muscle is stretched, action potentials are generated in neurons.
CNS adjusts strength of contraction to match load on muscle.
Golgi tendon organ: Another mechanoreceptor, in tendons and ligaments.
Provides information about the force generated by muscle; prevents muscle tearing.
Hair cells—mechanoreceptors in organs of hearing and equilibrium.
Hair cells have projections called stereocilia that bend in response to pressure.
Bending of stereocilia can depolarize or hyperpolarize the membrane.
Auditory systems use hair cells to convert pressure waves to receptor potentials.
Outer ear:
Pinnae collect sound waves and direct them to the auditory canal.
The tympanic membrane covers the end of the auditory canal and vibrates in response to pressure waves.
Middle ear—air filled cavity:
Open to the throat via the eustachian tube. Eustachian tubes equilibrate air pressure between the middle ear and the outside.
Ossicles—malleus, incus, stapes— transmit vibrations of tympanic membrane to the oval window.
Innerear has two sets of canals—the vestibular system for balance and the cochlea for hearing.
The cochlea is a tapered and coiled chamber composed of three parallel canals separated by Reissner’s membrane and the basilar membrane.
The organ of Corti sits on the basilar membrane—transduces pressure waves into action potentials.
Contains hair cells with stereocilia—tips are embedded in the tectorial membrane.
Hair cells bend and create a graded potential that can alter neurotransmitter release.
Upper and lower canals of the cochlea are joined at distal end.
The round window is a flexible membrane at the end of the canal.
Traveling pressure waves of different frequencies will produce flexion of the basilar membrane.
Different pitches, or frequency of vibration, flex the basilar membrane at different locations.
Action potentials stimulated by mechanoreceptors at different positions along the organ of Corti are transmitted to regions of the auditory cortex via the auditory nerve.
Conduction deafness: Loss of function of tympanic membrane or ossicles.
Nerve deafness: Damage to inner ear or auditory nerve pathways.
Hair cells in the organ of Corti can be damaged by loud sounds. This damage is cumulative and irreversible.
The vestibular system in the mammalian inner ear has three semicircular canals at angles to each other, and two chambers—the saccule and the utricle.
Hair cells sense position and orientation of head by shifting of endolymph.
Cupulaein canalscontain hair cell stereocilia—otoliths in membrane exert pressure and bend stereocilia.
Concept 35.4 Photoreceptors Detect Light
Photosensitivity—sensitivity to light
A range of animal species from simple to complex can sense and respond to light.
All use same pigments—rhodopsins.
Rhodopsin molecule consists of opsin (a protein) and a light-absorbing group, 11-cis-retinal.
Rhodopsin molecule sits in plasma membrane of a photoreceptor cell.
11-cis-retinal absorbs photons of light and changes to the isomer all-trans-retinal—changes the conformation of opsin.
In vertebrate eyes, the retinal and opsin eventually separate, called bleaching.
A series of enzymatic reactions is required to return all-trans-retinal back to 11-cis-retinal, which recombines with opsin to become photosensitive rhodopsin again.
Rod cells are modified neurons with:
•An outer segment with discs of plasma membrane containing rhodopsin to capture photons
•An inner segment that contains the nucleus and organelles
•A synaptic terminal where the rod cell communicates with other neurons
Stimulation of rod cells by light makes the membrane potential more negative (hyperpolarized)—the opposite of other sensory cells responding to their stimuli.
The dark current is a flow of Na+ ions that continually enters the rod cell in the dark.
Rod cell is depolarized and releases neurotransmitter continually.
Hyperpolarizing effect of light decreases neurotransmitter release.
When rhodopsin absorbs a photon of light, a cascade of events begins, starting with the activation of a G protein, transducin.
Transducin activates PDE which converts cGMP to GMP—the Na+ channels close, and the membrane is hyperpolarized.
Rhodopsin in a variety of visual systems:
Flatworms—photoreceptor cells in paired eye cups.
Arthropods—compound eyes. Each eye consists of units called ommatidia.
Each ommatidium has a lens to focus light onto photoreceptor cells.
Vertebrates have image-forming eyes—bounded by sclera, connective tissue that becomes transparent cornea on front of eye.
Iris (pigmented)—controls amount of light reaching photoreceptors; opening—pupil.
Lens—crystalline protein, focuses image, allows accommodation,can change shape.
Retina—photosensitive layer, back of eye.
The retina has five layers of neurons including photoreceptors (rods and cones) at the back.
Photoreceptors send information to bipolar cells, which send information to the ganglion cell layer.
Axons fromganglion cells conduct information to the brain.
A receptive field—a groupof photoreceptorsthat receive information from a small area of the visual field and activate one ganglion cell.
The receptive field of a ganglion cell results from a pattern of synapses between photoreceptors, bipolar cells and lateral connections.
Receptive fields have two concentric regions, a center and a surround.
A field can be either on- or off-center.
Light falling on an on-centerreceptive field excites the ganglion cell, while light falling on an off-centerreceptive field inhibits the ganglion cell.
The surround area has the opposite effect so ganglion cell activity depends on which part of the field is stimulated.
Neurons of the visual cortex, like retinal ganglion cells, have receptive fields.
Cortical neurons are stimulated by bars of light in a particular orientation, corresponding to rows of circular receptive fields of ganglion cells.
The brain assembles a mental image of the world by analyzing the edges in patterns of light and dark.
Vertebrate photoreceptors consist of rod cells and cone cells.
Rod cells are responsible for night vision; cone cells are responsible for color vision.
Fovea—area where cone cell density is highest.
Humans have three types of cone cells with slightly different opsin molecules—they absorb different wavelengths of light.
This allows the brain to interpret input from the different cones as a full range of color.
Color blindness is the loss of function of a type of cone cell—the result of a nonfunctional gene.
Answer to Opening Question
All of these animals make use of other senses besides vision to perceive their surroundings in the dark.
Information is also conveyed through tactile stimuli, olfaction, heat-detection, and auditory input.