S E N S O R Y P R O C E S S E S P R O V I D E I N F O R M A T I O N O N

A N I M A L S E X T E R N A L E N V I R O N M E N T A N D I N T E R N A L

S T A T U S

3 4 . 4

INTRODUCTION• Animals need information about their

external environments to move, locate food, find mates, and avoid danger.

– Echolocation

– Pheromones

– Key star

• They also need information regarding internal conditions, such as partial pressure of O2 and CO2 in the blood, and tension in contracting muscles.

SENSORY RECEPTOR CELLS TRANSFORM STIMULI INTO ELECTRIC SIGNALS

• Sensory receptor cells: neurons specialized for sensory transduction—transforming the energy of a stimulus into an electric signal.

• The electric signal generates action potentials. Action potentials convey the sensory information to the brain or other areas of the nervous system.

• Sensory receptor cells are highly specific in the stimuli to which they respond.

SENSORY RECEPTOR CELLS DEPEND ON SPECIFIC RECEPTOR PROTEINS AND ARE IONOTROPIC OR METABOTROPIC • Sensory receptor proteins: membrane proteins in sensory receptor cells that initially

detect a stimulus

• They produce graded membrane potentials (receptor potentials).

• Receptor cell membranes are often modified to have a large surface area, such as microvilli, cilia, or folding. This allows more receptor molecules and greater sensitivity.

SENSORY RECEPTOR CELLS DEPEND ON SPECIFIC RECEPTOR PROTEINS AND ARE IONOTROPIC OR METABOTROPIC

• Cone cells in vertebrate eyes have highly folded membranes with great numbers of photoreceptor molecules.

Cone cells in vertebrate eyes have tightly folded cell membranes in which great numbers of photoreceptor molecules are found

SENSORY RECEPTOR CELLS DEPEND ON SPECIFIC RECEPTOR PROTEINS AND ARE IONOTROPIC OR METABOTROPIC

• Ionotropic receptor cells: receptor proteins are typically stimulus-gated Na+

channels.

– Stimulus opens the channel, Na+ moves in, and receptor potential is generated.

• Metabotropic receptor cells are typically G protein-linked receptors; activation leads to change in a second messenger. This can directly or indirectly produce a receptor potential.

SENSATION DEPENDS ON WHICH NEURONS IN THE BRAIN RECEIVE ACTION POTENTIALS FROM SENSORY CELLS

• In the brain, different regions receive and process information from different sensory systems.

• Examples: – Axons from the eyes travel to the

visual cortex

– Axons from the inner ear travel to the auditory cortex

• Intensity of sensations is coded by the frequency of the action potentials.

SENSATION OF STRETCH AND SMELL EXEMPLIFY IONOTROPIC AND METABOTROPIC RECEPTION

• Mechanoreceptors respond to mechanical distortion of the cell membrane; most are ionotropic.

• Stretch receptor cells in muscles respond when the muscle contracts.

• Stretch receptors are specialized neurons. When cell membranes of the dendrites are stretched, Na+

channels open and produce a graded receptor potential that spreads to the axon hillock.

• Action potentials are generated if depolarization is above threshold.

1. Stretching a muscle cell is the stimulus…

2. …that activates the opening of ion channels in stretch receptor dendrites, giving rise to graded receptor potential…

3. …which spreads to the axon hillock, which generates action potentials when the receptor potential is great enough to trigger them.

4. The action potentials travel to the brain along the axon.

SENSATION OF STRETCH AND SMELL EXEMPLIFY IONOTROPIC AND METABOTROPIC RECEPTION• Stretch receptors in the biceps help adjust the muscle’s strength of contraction to match the

load the muscle must sustain.

• Receptors in the muscle detect how much the muscle is being lengthened by a load; this information is used to keep the arm stable as the load increases.

• Mechanoreceptor Simulation

1. Muscle Spindles detect lengthening caused by stretch. When muscles spindles are stretched…

2. …Sensory neurons associated with them transmit action potentials to the nervous system. These signals stimulate motor neurons to increase muscle contraction.

3. Each line represents one action potential

CHEMORECEPTORS• The sense of smell (olfaction) involves metabotropic receptors.

• Chemicals detected by smell are odorants. The sensory cells are chemoreceptors.

• In mammals, the receptors are in the lining of the nasal cavity. Axons extend to the olfactory bulb in the brain.

• Odorants must be dissolved in the liquid mucus to be detected.

CHEMORECEPTORS

• When an odorant binds to a receptor protein, it activates a G protein, which activates a second messenger.

• The second messenger opens ion channels and an influx of Na+

depolarizes the olfactory neuron.

CHEMORECEPTORS• Different receptor cells have different G

proteins that bind different odorants.

• With tens of thousands of olfactory receptor cells—each with one type of receptor protein—a wide range of odorants canbe detected.

• The combination of olfactory cells stimulatedby a particular odorant is unique to that odorant. The brain can interpret the patternof signals as pointing to a particular smell.

CHEMORECEPTORS• Some animals have extremely specific olfactory receptor cells that are extremely sensitive.

• Some male moths have thousands of receptors on their antennae that respond to pheromones emitted by the females.

• They can detect a female when the pheromone level is as low as 0.000000000000001% of the air molecules.

AUDITORY SYSTEMS USE MECHANORECEPTORS TO SENSE SOUND PRESSURE WAVES

• Auditory systems use mechanoreceptors to sense sound pressure waves.

• Many hearing organs have a membrane that moves in and out when sound pressure waves hit it.

• In mammals, this is the tympanic membrane (ear drum).

1. Sound pressure waves travel through the auditory canal and vibrate the tympanic membrane

AUDITORY SYSTEMS USE MECHANORECEPTORS TO SENSE SOUND PRESSURE WAVES

• The middle ear is an air-filled cavity with three bones (ossicles)—the malleus, incus, stapes.

• They transmit vibrations of the tympanic membrane to another membrane, the oval window.

• The oval window connects to the cochlea, a coiled, fluid-filled tube where sound energy is transduced into electric signals.

2. The ossicles transmit vibrations of the tympanic membrane to the oval window of the cochlea.

3. Vibrations at oval window create pressure waves in fluid-filled cochlear canals.

AUDITORY SYSTEMS USE MECHANORECEPTORS TO SENSE SOUND PRESSURE WAVES

• The cochlea has three parallel canals separated by membranes.

• The basilar membrane changes in width and stiffness over its length.

• Different pitches, or frequency of vibration, cause different regions of the basilar membrane to oscillate.

4. Pressure waves in cochlear canals flex adjacent membranes

Pressure waves travel down the vestibular canal and flex the basilar membrane at a frequency-specific region. Hair cells in that region are stimulated to send signals to the brain

Membrane width is diagrammed. It increases gradually from one end to the other

Red numbers represent the sound frequencies, in Hertz, to which various parts of the membrane maximally respond

Sound transduction in the human ear animation

AUDITORY SYSTEMS USE MECHANORECEPTORS TO SENSE SOUND PRESSURE WAVES

• The organ of Corti, on the basilar membrane, has mechanoreceptor cells called hair cells.

• Hair cells have projections called stereocilia that project into the semi-rigid tectorial membrane. When the basilar membrane moves, the stereocilia are bent.

• Hair cells transduce the bending motions of the stereocilia into electric signals and synapse with neurons that produce action potentials.

5. When basilar membrane is flexed, it bends stereocilia on hair cells in the organ of Corti 6. The movements of stereocilia are transduced

into action potentials in the auditory nerve.

THE PHOTORECEPTORS INVOLVED IN VISION DETECT LIGHT USING RHODOPSINS

• Photoreceptors—receptor cells sensitive to light

• Photoreceptors are metabotropic; the receptor proteins are all in the family of pigments called rhodopsins, which act as G protein–linked receptors.

• Rhodopsin consists of the protein opsin and a light-absorbing group, 11-cis-retinal.

THE PHOTORECEPTORS INVOLVED IN VISION DETECT LIGHT USING RHODOPSINS• An advantage of metabotropic control is that signals can be amplified.

• With large numbers of rhodopsin molecules and strong amplification, some photoreceptor cells undergo a measurable change in membrane potential in response to just a single photon of light.

Photosensitivity animation

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• Connective tissue forms the transparent cornea on front of eye.

• Iris (pigmented)—controls amount of light reaching photoreceptors; opening is the pupil

• Lens—crystalline protein, focuses image, can change shape for focusing

• Retina—photosensitive layer at the back of the eye

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• The retina has several types of neurons including the photoreceptors (rods and cones).

• Rods and cones have large membrane surface area and many rhodopsin molecules. In humans, rods outnumber cones.

• Rods are more sensitive to light; important in dim light situations. Cones provide color vision.

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS

• Humans have three types of cone cells with slightly different molecular 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.

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• Rod and cone cells only produce graded membrane potentials (not action potentials).

• When stimulated, they hyperpolarize—the opposite of other sensory cells responding to stimuli.

• In darkness, Na+ channels are open and Na+ continually enters the cells. When stimulated, the 2nd messenger closes the channels, and inside of membrane becomes more negative.

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• The effect of the hyperpolarization is graded, depending on light intensity.

• Rod and cone cells synapse with other neurons in the retina and relay signals to them by graded neurotransmitter release.

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• The retina has four types of integrating

neurons arranged in layers.

• Rods and cones synapse with bipolar cells, which synapse with ganglion cells.

• Horizontal cells and amacrine cells communicate laterally within the retina.

• Ganglion cells are the only ones that produce action potentials. Their axons converge to form the optic nerves.

Integrating neurons –consisting of ganglion, amacrine, bipolar, and horizontal cells – are found in layered arrangement in the retina

1. Light travels through integrating neurons, which are transparent…

2…and is absorbed by the rods and cones (the visual photoreceptor cells) at the back of the retina.

4… and finally converges on ganglion cells, which send action potentials via their axons to the brain

3. Visual information from the rods and cones is processed by the integrating neurons…

THE VERTEBRATE RETINA IS A DEVELOPMENTAL OUTGROWTH OF THE BRAIN AND CONSISTS OF SPECIALIZED NEURONS• Each ganglion cell has a receptive field—a defined, circular field formed by the cells from which

it receives signals.

• Different ganglion cells are excited by light falling on the center versus the periphery of the receptive field.

• This enables ganglion cells to communicate information to the brain about visual patterns such as spots, edges, and areas of contrast.

Structure of the Eye video

SOME RETINAL GANGLION CELLS ARE PHOTORECEPTIVE AND INTERACT WITH THE CIRCADIAN CLOCK

• About 1–2% of ganglion cells are photosensitive. The receptor protein is believed to be melanopsin.

• They provide information on the presence of light and its intensity.

• This information is relayed to the suprachiasmatic nuclei, where it is used to entrain the master circadian biological clock.

• It also regulates pupil size so they are large in dim light and small in bright light.

ARTHROPODS HAVE COMPOUND EYES

• Arthropods have compound eyes consisting of units called ommatidia.

• Each ommatidium has a lens to focus light onto photoreceptor cells containing rhodopsin.

• Each ommatidium points in a slightly different direction. The more ommatidia, the higher the image resolution. Fast-flying predators such as dragonflies have up to 30,000.

ARTHROPODS HAVE COMPOUND EYESThe compound eye of a fruit fly contains hundreds of ommatidia, each of which looks somewhat like a dot on the eye surface

ANIMALS EVOLVED DIVERSITY• Some animals have evolved unusual sensory abilities to sense information such as electric fields

or Earth’s magnetic field.

• Some animals perceive their environment very differently than humans do, for example, hearing sound frequencies that we cannot hear, or seeing electromagnetic wavelengths that we cannot see.

ANIMALS EVOLVED DIVERSITY• Our eyes can see wavelengths between 0.4 and 0.7 μm.

• Infrared wavelengths—longer than 0.7 μm

• Ultraviolet wavelengths—shorter than 0.4 μm

• Some snakes can see infrared wavelengths, detecting warmth in darkness, which helps them find prey.

Pit vipers have infrared sensing organs in pits near the eyes.

ANIMALS EVOLVED DIVERSITY

• Many animals can see ultraviolet wavelengths—bees, some birds.

• Male and female cedar waxwings look alike to us, but researchers have discovered that the birds themselves can tell the difference, with UV wavelengths.

ANIMALS EVOLVED DIVERSITY• Some animals can sense electric fields.

• Some fish produce weak electric fields and detect distortions in these fields caused by objects in their environment.

• The fish can perceive objects even in complete darkness.

This African fish is nocturnal. They find food using self-produced electric fields