The Senses

Chapter 14

Points to Ponder

• What are sensory receptors?

• How do we detect the sense of taste and smell?

• What is the anatomy of the eye?

• How do we focus images?

• What are some eye abnormalities?

• What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing?

14.1 Sensory receptors and sensations

Sensory receptors

• Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors : detect stimuli from outside the body (e.g. taste, hearing, vision).

• Continually send messages to the CNS regarding environmental conditions – Interoceptors : receive stimuli from inside the body (e.g. change in blood pressure) • Receptors are activated only when stimuli is present 1. Pressoreceptores: change in blood pressure 2. Osmoreceptors: change in water-salt balance 3. Chemoreceptors: monitor pH of the blood

• • •

Types of sensory receptors

Chemoreceptors – respond to nearby chemicals – – Taste, smell, monitor pH in the blood • pH low  chemoreceptors in carotid arteries and aorta activated breathing rate increases  more CO2 is expired   blood ph raises Pain receptors – a type of chemoreceptors • respond to chemicals released by damaged tissue Photoreceptors – respond to light energy – Sensitive to light rays and provide us with vision – Rod cells: black and white vision – Cone cells: color vision Thermoreceptors changes – stimulated by temperature – Located in the hypothalamus and the skin

Types of sensory receptors

• Mechanoreceptors such as pressure – Hearing : • sound waves are converted to fluid-borne pressure waves that are detected y mechanoreceptors in the inner ear – – respond to mechanical forces Balance and equilibrium : • mechanoreceptors response to fluid-borne pressure waves in the vestibule and semicircular canals of the inner ear – – – Touch receptors : skin Pressoreceptors : in the certain arteries Proprioceptors : respond to stretching of muscle fibers, tendons, joints, and ligaments • Allow body to determine position of our limbs

14.1 Sensory receptors and sensations

Senses and the receptors involved

How does sensation occur?

1. Sensory receptors respond to environmental stimuli 2. Nerve impulses travel to cerebral cortex 3. Sensation (conscious perception) of stimuli occurs 4. Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e. odor) – Two Possible Pathways to adaptation 1. Sensory receptors stop sending impulses to brain 2. RAS filters out ongoing stimuli - Sensory info  thalamus  cortex via RAS brain stem to cerebral 

Sensation

- Sensory receptors Free nerve endings or specialized cells associated with neurons Plasma membrane of the receptors contains receptor proteins that react to the stimulus Ex. Chemoreceptors bind to particular molecule Once receptor is stimulated Ion channels open, and ions flow across the plasma membrane Impulse carried to the PNS and CNS Travel ascending tract to the brain  cerebral cortex  to particular area depending on impulse (i.e. visual, auditory) Results in sensation and perception

Proprioceptors and Cutaneous Receptors

• Sensory receptors in the muscles, joints, tendons, internal organs and skin • Sensory receptor stimulation  travels to spinal cord  impulse travels ascending tracts to the brain  somatosensory areas of the cerebral cortex

Proprioceptors

• Mechanoreceptors involved in reflex actions that maintain muscle tone • Determines position of limbs by detecting degree of muscle relaxation, stretch of tendons, and movement of ligaments • Muscle spindles – Sensory nerve endings wrapped around muscle cells – Nerve impulses generated when muscle relaxes and stretching is low

• • • •

Cutaneous receptors

Receptors in the dermis make the skin sensitive to touch, pressure, pain and temperature – – Temperature receptors in epidermis Free nerve endings Hot and cold receptors Receptors sensitive 1.

2.

3.

to touch in dermis Meissner corpuscles Merkel disks Root hair plexus Receptors sensitive 1.

2.

to pressure in dermis Pacinian corpuscles Ruffini endings

Pain Receptors

• Sensitive to chemicals released by damaged issues • Drugs inhibit the synthesis of certain chemicals • Referred Pain – Pain from an internal organ is referred to the skin – Nerve impulse from pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin • i.e. pain from the heart is felt in the left shoulder and arm

Senses of Taste and Smell

• Chemical senses – Taste cells and olfactory cells have chemoreceptors sensitive to particular molecules in food and air

Taste receptors

• Taste receptors = taste buds ~ 3,000 taste buds mostly on the tongue – Also present on hard palate, pharynx, and epiglottis • Sensitive to sweet, sour, salty and bitter tastes in food • 80-90% of what we perceive as taste is actually due to the sense of smell • From Taste buds  Brain – Taste bud opens at taste pore – Taste bud has supporting cells and taste cells with microvilli – Tastant binds to chemoreceptors of taste cells – Nerve impulse is generated in sensory nerve fibers – Impulse travels to thalamus – Relayed to the gustatory (taste) cortex – Cortex integrates incoming info and gives us our sensation

Taste Receptors

14.3 Senses of taste and smell

Smell receptors

• 10-20 million olfactory cells (modified neurons) in the roof of the nasal cavity • Each cell end has olfactory cilia with receptors for odorants (odor molecules) • From Olfactory receptors  Brains – Each olfactory cells has receptors for one odorant, but nerve fibers from different cells lead to the same neuron in the olfactory bulb – Odorant bind to chemoreceptors on olfactory cilia – Stimulation of olfactory cell – Impulse travels via that sensory nerve fiber – Synapse with neuron in olfactory bulb – Impulse travels via the olfactory tract to the • Olfactory areas of the cerebral cortex • Limbic system: center for emotion and memory • Number of olfactory cells decline with age, remaining ones become less sensitive

Olfactory Receptors

14.4 Sense of vision

Anatomy of the eye

• 2 compartments: – Anterior chamber : • between cornea and lens filled with a clear fluid called aqueous humor • Typically drains via ducts • Glaucoma occurs when drainage ducts are blocked – Resulting pressure compresses arteries that serve nerve fibers of retina resulting is partial or complete blindness – Posterior chamber : • most of the eye, behind the lens contains gelatinous material called vitreous humor • Made of 3 layers/coats – A.

Sclera : mostly white and fibrous except the cornea B.

Choroid : darkly, pigmented vascular layer C.

Retina : inner layer containing photoreceptors

14.4 Sense of vision

Anatomy of the eye

14.4 Sense of vision

A. The eye: Sclera

• Sclera – the white of the eye that maintains eye shape – Cornea : transparent portion of the sclera that is important in refracting light – Pupil : a hole that allows light into the eyeball

14.4 Sense of vision

B. The eye: Choroid

• • Choroid – middle layer that absorbs light rays that are not absorbed by the retina – Iris : donut-shaped, colored structure that regulates the size of the pupil • Color of irus correlates with its pigmentation – Ciliary body : a structure behind the iris that contains a muscle that control the shape of the lens Lens – attached to the ciliary body and functions to refract and focus light rays

14.4 Sense of vision

Anatomy of the eye

• The lens is flexible, transparent and concave • The lens accommodates, changes shape, to focus light on retina to form an image – Shape controlled by ciliary muscles – Distant objects • Ciliary muscles relax • Suspensory ligament stretch • Lens remains flat – Near objects • Ciliary muscle contract • Releasing tension on ligaments • Lens rounds up • As we age the lens loses elasticity and we use glasses to correct for this

The eye: The lens

14.4 Sense of vision

C. The eye: Retina

• Contains photoreceptors – rods and cones • Rods are sensitive to light • Cones require bright light and see wavelengths of light (color) • The fovea centralis is an area of the retina – densely packed with cones where images are focused • Sensory receptors from the retina form the optic nerve – Optic nerve takes impulses to the brain • The blindspot is where the optic nerve attaches and lacks vision

14.4 Sense of vision

Anatomy of the retina

C. The eye: Photoreceptors of the retina

• • Embedded in the membrane of the retina Rods : – Contain a visual pigment called rhodopsin • Made of opsin and retinal (light absorbing molecule)

Stimulation of Photoreceptors

1. Rod absorbs light 2. Rhodopsin splits into opsin and retinals 3. This split leads to reactions that cause the closure of ion channels in the rod cell’s plasma membrane 4. Release of inhibitory transmitter molecules from rod’s synaptic vesicles ceases 5. Signal goes to other neurons in the retina – Important for peripheral and night vision – Vitamin A is important for proper functioning

C. The eye: Photoreceptors of the retina

•

Cones

: – Located mostly in the fovea – Allow us to detect fine detail and color – 3 different kinds of cones containing red, green and blue pigments • Each pigment is made up of retinal and opsin but with differences in the opsin structure which accounts for individual absorption patterns

14.4 Sense of vision

Rods and cones in the retina

Visual Pathway to the Brain

1. Stimulation of Photoreceptors (previously explained) 2. Photoreceptors synapse with bipolar cells and stimulate cells 3. Bipolar cells synapse ganglion cells and stimulate these cells – Integration of signals coming from photoreceptors are processed by the bipolar and ganglion cells before ganglion cells generate nerve impulses 4. Nerve impulse is generated and travels via the sensory fibers (axons) of ganglion cells which become optic nerve 5. Optic nerve carry impulses from the eyes to the optic chiasma (X shape, crossing of optic nerve fibers) and optic tract 6. Optic tract synapse with nuclei within thalamus 7. Thalamic nuclei axons form optic radiations that take nerve impulses to the visual cortex in the occipital lobe

14.4 Sense of vision

Summary of eye structures

14.4 Sense of vision • • • • • •

Abnormalities of the eye

Color blindness – genetic disease most common in males – usually cannot see red or green Cataracts Glaucoma – lens of the eye is cloudy – fluid pressure builds up in the eye Astigmatism image – cornea or lens is uneven leading to a fuzzy Nearsightedness far away objects – eyeball is too long making it hard to see – Rays focus in front of the retina – Concave corrective lens improve vision Farsightedness near objects – eyeball is too short making it hard to see – Rays focus behind the retina – Convex corrective lens improve vision

14.4 Sense of vision Abnormalities of the eye that are corrected with lenses

14.5 Sense of hearing

Anatomy of the ear

• The ear functions in hearing and balance • 3 divisions: A.

Outer ear : functions in hearing; filled with air B.

Middle ear : functions in hearing; filled with air C.

Inner ear : functions in hearing and balance; filled with fluid

14.5 Sense of hearing

A. The ear: Outer ear

• Includes: – Pinna : the external ear flap that catches sound waves – Auditory canal : directs sound waves to the tympanic membrane • lined with fine hairs and modified sweat glands that secrete earwax – Was helps guard against entrance of foreign materials

14.5 Sense of hearing

B. The ear: Middle ear

• Includes: – Tympanic membrane (eardrum): membrane that vibrates to carry the wave to the bones – 3 small bones called ossicles : amplify sound waves • Malleus (hammer), incus (anvil), stapes (stirrup) • Malleus adheres to tympanic membrane • Stapes touches the oval window – Auditory tube : a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst

14.5 Sense of hearing

Following the sound wave

14.5 Sense of hearing

C. The ear: Inner ear

• Important for both hearing and balance • • 3 areas: cochlea, semicircular canals, vestibule • • • Stapes (middle ear bone) vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea Cochlea – hearing Vestibule – gravitational equilibrium Semicircular canals – rotational equilibrium

• • • • 14.5 Sense of hearing

Auditory Pathway

Begin at the auditory canal Travel by vibrations of molecules Waves strike the tympanic membrane – Malleus takes the pressure from the inner surface of the tympanic membrane and passes it to incus, and to the stapes This method multiplies the pressure about 20 times • Stapes strikes the membrane of the oval window causing it to vibrate • • Pressure is passed to the fluid within the cochlea Cochlae contains the organ of Corti organ containing hairs for hearing (spiral organ) sense 1. Pressure waves cause the basilar membrane to move up and down 2. Bending of embedded hairs (stereocilia) occurs when they are pushed against the tectorial membrane 3. Bending of stereocilia causes the generation of a nerve impulses that travels to cochlear nerve and then to brain

Pitch and Volume

• Pitch – Determined by varying wave frequencies that are detected by different parts of the organ of Corti – Sensation depends on the region of the basilar membrane that vibrates and the area of the auditory cortex that is stimulated • Volume – Determined by the amplitude of sound waves – More pressure in canal causes basilar membrane to vibrate to a greater extent • Result is increased stimulation interpreted as increased volume

14.5 Sense of hearing

14.Sense of equilibrium

The inner ear: Semicircular canals and vestibule

• Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) – Depends on hair cells in the utricle and saccule • Detects angular movement (rotational equilibrium) – Depends on hair cells at the base of each semicircular canal (ampulla)

14.5 Sense of hearing

The inner ear: Balance

Rotational Equilibrium Pathway

• Mechanoreceptors in semicircular canals – Detect rotation/angular movement of head • Three semicircular canals – One is each dimension of space • Base of each canal = ampulla – Each ampulla responds to head rotation in a different plane of space • Little hair cells found in ampulla are associated with a cupula • As fluid within a semicircular canal flows over at displaces the cupula • Stereocilia bend, and pattern of impulses carried by the vestibular nerve to the brain changes • Brain uses info from hair cells to maintain equilibrium

Gravitational Equilibrium Pathway

• Mechanoreceptors in the utricle and saccule – Detect movement of head in the vertical and horizontal place • Utricle and saccule contain little hair cells • Stereocilia are embedded within otolithic membrane – Otolithic consists of otoliths (calcium carbonate granules) • When Body is Still – otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells • When Head Bends – Otoliths are displace and the otolithic membrane sags – Stereocilia of the hair cells bend – If stereocilia move toward the kinocilium • nerve impulses increase in the vestibular nerve – If stereocilia move away from the kinocilium • nerve impulses decrease in the vestibular nerve – Frequency of nerve impulses indicates if movement is up or down