Chapter 38 The Senses

Sensory Reception A. Sense organs contains specialized sensory receptor cells that detect stimuli & send the information to CNS

B. Signal Transduction – mechanism of B. Signal Transduction – mechanism of changing environmental stimuli into neural impulses (action potentials) 1. Occurs in plasma membrane of receptor cell. Stimulus opens & closes membrane channels (Na+) 2. Produces a receptor potential, or generator potential, which is a change in membrane polarization

3. Most receptor cells are producing 3. Most receptor cells are producing receptor potentials all the time, but at a slow steady rate, even when there is no stimulus a) When a stimulus IS present the receptor cell releases more transmitter than usual & increases the rate of action potentials generated in sensory neuron. b) This signals the brain that a stimulus was detected.

C. How do identical action potentials C. How do identical action potentials communicate different information? 1. By WHICH interneurons the sensory neurons are synapsing with in the brain. D. How do action potentials communicate information about intensity of stimuli? 1. By RATE of transmission of action potentials to the brain  Stronger stimuli generate more frequent action potentials being sent to the brain

E. Sensory Adaptation 1. The tendency of sensory receptor cells to become less sensitive when they are stimulated repeatedly. a) This will trigger fewer action potentials b) Brain may lose awareness of the stimuli

II. Specialized Sensory Receptors A. Skin contains: 1. Pain receptors 2. Thermoreceptors (heat & cold) 3. Mechanoreceptors (touch) a) In each of these senses one neuron serves as the receptor and the sensory neuron. b) Dendrites transduce stimuli & axons send info. to CNS

B. Pain receptors 1. Naked receptors 2. Serve to warn us of danger or injury 3. Located everywhere except brain C. Thermoreceptors 1. Skin receptors detect heat & cold 2. Deeper sensors monitor temperature of blood a) Hypothalamus is body’s thermostat

D. Mechanoreceptors 1. Very diverse 2. Stimulated by touch, pressure, stretching, motion and sound 3. All work by some force bending or stretching membrane of receptor cell. 4. As membrane changes shape, it becomes more permeable to + ions a) Touch receptors around bases of hairs detect hair movements b) Stretch receptors monitor positions of body parts

E. Hair Cells 1. Detect sound waves & other movements in air or water 2. “Hairs” are specialized cilia or microvilli. a) When hair is bent in one direction, membrane is stretched, increasing permeability to + ions. Increases action potentials. b) When bent in opposite direction, less transmitter is released & action potentials decrease.

F. Chemoreceptors 1. Located in nose and taste buds 2. React to chemicals in the external or internal environment a) CO2 detectors in blood b) Silkworm moth antennae detect sex attractant produced by female.

G. Electromagnetic receptors 1. Respond to electricity, magnetism and light a) Some fish detect electric fields given off by their prey. b) Electric fish discharge electrical bursts into the water to stun prey c) Many species have magnetite, an iron-containing mineral • May help them navigate in world

2. Photoreceptors a) Most common type of electro- magnetic receptor b) Detect light (visible, infrared, or ultraviolet) c) Snakes have two kinds: • Normal eyes for visible light • Infrared detectors - located in pits below normal eyes. Detect body heat from up to 1 m away. d) Lizards may have UV detectors in parietal eye

Vomeronasal Organ Parietal Eye

III. Taste & Smell A. Receptors for both senses are chemoreceptors. 1. They interact with each other B. Smell 1. Receptors located in noses of vertebrates. Contain cilia. 2. Located on antennae of many insects

Olfactory Cell Location & Anatomy

C. Taste 1. In humans, receptors are located in taste buds on tongue. a. Four types of taste buds:  Sweet  Sour  Salty  Bitter 2. Insects have chemoreceptors for taste in sensory hairs on their feet

Taste Buds in Humans

IV. Vision A. Three types of photoreceptors: 1. Eye cup (eye spot) - Planaria 2. Compound eye - arthropods 3. Single-lens eye - vertebrates

B. Eye Cup 1. Photoreceptor cells contain molecules of a visual pigment a) These molecules absorb light b) Cause light energy to be transduced c) Action potentials sent to brain d) Do NOT form images. Show intensity and direction of light

C. Compound Eye 1. Have lenses that focus light and form images a) Consist of tiny light-detecting units called ommatidia b) Each has its own transparent covering called a cornea & a lens c) Lens focuses light on several photoreceptor cells d) Function as acute motion detectors e) Some insects see UV radiation

Compound Eye

Nectar Guides

D. Single-lens eyes 1. Anatomy - know functions of each a) Sclera b) Choroid c) Cornea d) Conjunctiva e) Iris f) Pupil g) Lens h) Retina i) Fovea j) Blind spot k) Vitreous humor l) Aqueous humor

2. How to Focus (Accommodation) a) Lens focuses light onto retina. • Bends rays b) Two ways to focus: • Rigid lens moves back & forth • Muscles change shape of an elastic lens  Thicker lens - bends light more sharply

c) Focusing in Humans • Shape of lens controlled by ciliary muscles attached to choroid  When eye focuses on nearby object, these muscles contract (get a smaller diameter)  This makes ligaments that hold lens become slack.  Elastic lens then becomes thicker and rounder  Reverse happens when focusing on distant object. Ciliary muscle relaxes. Lens becomes flatter.

d) Know how corrective lens work to correct each of the following conditions:  Near-sightedness (Myopia) Eyeball longer than normal Can’t focus well on distant objects  Far-sightedness (Hyperopia) Eyeball shorter than normal Can’t focus well on nearby objects  Astigmatism (misshapen lens or cornea)

Focusing of the Human Eye

3. Photoreceptors at Work a) About 130 million photoreceptors of two types: • Cones - Color receptors. Provide very sharp images but need plenty of light. Don’t function at night • Rods - very sensitive to light. Allow us to see in dark situations but only in shades of gray.

b) Arrangement of Photoreceptors:  Cones - Densest in the fovea. Best to look directly at object. Allows for sharp daytime vision  Rods - Absent from fovea. Greatest density at outer edges of retina. Best to look at an angle at night to see dim objects more clearly

See spectrum picture on next slide c) Visual Pigments of Photoreceptors:  Each rod & cone contains a stack of disks containing light-absorbing pigments.  Rod pigment = rhodopsin  Cone pigment = 3 different photopsins Have 3 types of cones: blue green red See spectrum picture on next slide

Photoreceptors of the Human Eye

Absorption Spectrum of Photoreceptors

d) Visual Sensory Transduction:  When pigments absorb light, they change chemically  Changes permeability of membrane  Creates receptor potential which will ultimately be sent to the brain  Complex integration occurs due to arrangement of cells in the retina

e) Visual Signal Pathway:  Receptor tips embedded in BACK of retina  Light must pass through the several other layers of cells which are between receptors and the incoming light  Information transduced by rods & cones must then pass in the opposite direction through the network of neurons  Axons make up optic nerve to brain

Structure & Function of the Retina

V. Hearing A. Involves hair cells in fluid-filled canals. B. Ear is composed of 3 regions: 1. Outer ear consists of pinna and auditory canal Eardrum a) Collects sound waves b) Directs sound to the tympanic membrane (eardrum)

2. Middle Ear a) Eardrum vibrates at the same frequency as the original sound. • Frequency is measured in hertz (Hz) = # vibrations/sec. b) Eardrum causes middle ear bones (malleus, incus, stapes) to vibrate in sequence. c) Bones transmit vibration to the oval window, which is much smaller in diameter than eardrum • This amplifies the vibrations

Anatomy of the Human Ear

d) Middle ear is connected to pharynx via the Eustachian tube  Allows air pressure to be equalized on each side of the eardrum

3. Inner Ear a) Oval window transmits sound waves to the snail-shaped cochlea b) Cochlea is a long-coiled tube that contains 3 separate fluid-filled canals:  Vestibular - upper canal. Begins at oval window.  Tympanic - lower canal. Connects to vestibular canal & ends at round window  Cochlear - between the other two

Cochlea

Cochlea

 Actual organ of hearing  Long, thin spiral within cochlear canal c) Organ of Corti  Actual organ of hearing  Long, thin spiral within cochlear canal  Consists of hair cells embedded in the basilar membrane  Tips of hairs touch overlying shelf Basilar membrane

Mechanoreceptors for Hearing

C. Physiology of Hearing 1. Oval window causes waves in the fluid of the vestibular canal 2. At top of coil, pressure waves enter the tympanic canal 3. Waves start the cochlear canal and basilar membrane to vibrating 4. Causes some hair cells to be bent.  Ion channels open and K+ enters 5. Receptor potential is generated. 6. Action potential sent down sensory neuron axon (auditory nerve) to brain

D. How are Volume & Pitch Determined? 1. Volume (measured in decibels)  Louder sounds produce bigger waves which create more bending of hair cells  This sends more action potentials down auditory nerve towards brain  Human hearing ranges from 0 dB to 120 dB (point of intolerable pain)  > 90 dB sounds can destroy hairs

2. How is Pitch (Frequency) Detected? a) Basilar membrane is not uniform in width along its spiraling length  End near oval window is narrower  End near tip of coil is wider b) High-pitched sounds generate high frequency pressure waves • Causes narrower parts of basilar membrane to vibrate c) Low-pitched sounds cause the wider parts of basilar membrane to vibrate * Humans hear from 20-20,000 Hz

Frequency Detection

VI. Balance A. Involves two types of receptors: 1. Semicircular canals 2. Two chambers (saccule & utricle) B. Canals - detect rotational motion of head in any of 3 planes C. Chambers - detect position of head in relation to gravity

Mechanoreceptors for Equilibrium

Sensory Receptors in Other Animals