Notes Chapter 17 Special Senses PDF

Preview

Summary

notes...

Description

Biology 235 – Athabasca University

June 8th 2021

Chapter 17: The Special Senses (pg 572)

General Senses include: somatic senses (tactile, thermal, pain, and proprioceptive) and visceral senses. Special Senses include: smell, taste, vision, hearing, and equilibrium

Olfaction: Sense of Smell (pg. 573) -

-

Est. that humans can recognize 10,000 different smells using 10 million to 100 million receptors contained with region called olfactory epithelium (in superior part of nasal cavity) Olfactory epithelium consists of three types of cells: o olfactory receptor cells: 1st order neurons of the olfactory pathway. Contains olfactory cilia where olfactory transduction occurs (transduction is conversion from stimulus energy into a graded potential in a sensory receptor) o supporting cells: columnar epithelial cells of the mucous membrane lining of the nose. Provide physical support, nourishment, and electrical insulation for receptor cells and detoxify chemicals that come into contact with olfactory epithelium. o basal cells: stem cells located at bases of supporting cells. Continually undergo cell division and produce new olfactory receptor cells that have ~one month life span (remarkable considering most neurons are not replaced). odorants are chemicals that bind to and stimulate olfactory receptors in olfactory cilia olfactory glands (bowman's glands): produce mucous that is carried to surface of epithelium by ducts. Secretion moistens surface and dissolves odorants so that transduction can occur. Nasal epithelium and glands innervated by facial nerve (also innervates lacrimal glands). This is why some odorants can cause tears and runny nose.

Physiology of Olfaction -

ability to recognize 10,000 different odours because of activation of many combinations of olfactory receptor cells. olfactory receptor cells generate potential (depolarization) after reacting with specific stimuli odorant. Triggers nerve impulse (called olfactory transduction). Olfaction has low threshold with adaptation (decreasing sensitivity) happening rapidly (up to 50% in the first minute).

Olfactory Pathway -

On each side of nose, about 40 unmyelinated axons of olfactory receptors extend through foramina in ethmoid bone and make up left and right olfactory nerves Olfactory nerves terminate in brain in masses of grey matter called olfactory bulbs Axons of olfactory bulbs extend into primary olfactory area of the cerebral cortex (where consciousness of smell begins). Olfactory tract: where olfactory bulb neurons extend to (see diagram) Other axons extend into limbic system and hypothalamus which accounts for emotional and memory responses to smells. Women have better sense of smell than men, especially during ovulation

Gestation: Sense of Taste -

5 primary tastes can be distinguished: sour, sweet, bitter, salty, and umami (savoury) taste receptors located on taste buds. Each taste bud has three kinds of cells: supporting cells, gustatory receptor cells, and gustatory microvilli. Taste pore is a opening in the taste bud from which gustatory microvilli (hairs) project. Gustatory receptor cells have life span of about 10 days and are produced anew from basal cells (stem cells)

Papilla (plural : papillae) -

Elevations on tongue where taste buds are found. Increases surface area and provide rough textured surface to the upper surface of the tongue. Three types: o Vallate papillae: circular v-shaped at back of tongue. Have 100-300 taste buds. o Fungiform papillae: mushroom shaped elevations all over tongue have about 5 taste buds each. o Foliate papillae: located on margins of tongue. Taste beds usually degenerate in childhood. o Also, filiform papillae (threadlike structures) no taste buds but help increase friction between tongue and food to move around in oral cavity.

Physiology of Gestation (taste) -

Chemicals that stimulate gustatory receptors cells are called tastants Once tastant dissolved in saliva, it makes contact with plasma membrane of gustatory microvilli (site of taste transduction; i.e. stimuli turned into impulse) Tastants bind to receptor and then G proteins activate chemical secondary messangers that then cause depolaraization

Taste Thresholds and Adaptation -

Threshold varies for each taste. Bitter is lowest (protection mechanism from poisons which are often bitter). Adaptation of a specific taste can occur in 1 to 5 minutes of continuous stimulation

Gustatory Pathway -

3 cranial nerves contain axons of 1st order gustatory neurons that innervate the taste buds:

-

-

o Facial nerve for taste buds in anterior 2/3 of tongue o Glossopharyngeal nerve serves taste buds in posterior 1/3 of tongue o Vagus nerve serve taste buds in throat and epiglottis From these nerves, impulses propagate to gustatory nucleus in the medulla oblongata. From there, some axons carry it to limbic system and hypothalamus. Taste signals that arrive in the primary gustatory area in the parietal lobe of the cerebral cortex give rise to conscious perception of taste

Vision -

Half of all sensory receptors of body are in eye. Large part of cerebral cortex is devoted to processing information.

Accessory Structures of the Eyes Eyelids (palpebrae): shade eyes during sleep, protect eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs. Palpebral fissure: opening between lower and upper eyelids Lacrimal caruncle: whitish, reddish elevation mostly medially. Contains sebaceous (oil) and sudoriferous (sweat) glands. Where "sleep" sometimes accumulates. Tarsal Plate: connective tissue giving eyelid it's form and support Conjunctiva: thin, protective, mucous membrane over sclera (white of eye) Eyelashes and eyelashes: help protect from foreign objects, perspiration, and direct rays of sun. Lacrimal Apparatus: structures that produce and drain lacrimal fluid (tears) in process called lacrimation

-

-

Glands (size of almond) secrete lacrimal fluid Lacrimal ducts: 6 to 12 which empty tears on surface of conjunctiva of upper lid. Lacrimal puncta: small openings before ducts where tears enter Lacrimal canalculi: ducts which carry tears to nasolacrimal sac and then nasolacrimal duct which carries tears to nasal cavity where it mixes with mucous (hence why crying makes us need to blow our noses). lysozyme: protective bactericidal enzyme contained in lacrimal fluid. Protects, cleans, lubricates, and moistens eyeball

-

only humans express happiness or sadness by crying (in response to parasympathetic stimulation)

Extrinsic eye muscles: eyes sit in bony depressions in skull called orbits. Orbits protect eyes and stabilize them and anchor them to the muscles that make them move. Extrinsic eye muscles extend from walls of bony orbit to the sclera (white) of the eye and are surrounded by periorbital fat 1. 2. 3. 4. 5. 6.

superior rectus inferior rectus lateral rectus Medial rectus Superior oblique Inferior oblique

Anatomy of the Eyeball -

About 2.5 cm across. Only anterior 1/6 exposed. Wall of eyeball has three layers: o Fibrous tunic: superficial layer of eye. Consists of anterior cornea (transparent coat that covers the coloured iris) , is curved to direct light. And the sclera: covers the whole eye, connective tissue, gives shape and rigidity. At junction of sclera and cornea is opening known as scleral venous sinus where aqueous humor drains into. o Vascular tunic: middle layer of eyeball. Has three parts:  



choroid (full of blood vessels and gives nutrients to eye), ciliary body: from ora serrate (margin of retina) to junction of sclera and cornea. Ciliary processes: protrusions that secrete aqueous humor. Zonular fibres extend and attach to lens. Ciliary muscles contract or relax to alter shape of lens and adapt for near or far vision. Iris: coloured part of eye. Shape like flattened donut. Function is to regulate amount of light entering eyeball through pupil. Pupil has radial muscles (dilates) and circular muscles (constricts)

o Retina (inner tunic): third and inner layer of the eyeball. Beginning of visual pathway. Has optic disc, where optic nerve exits the eyeball.  Retina consists of pigmented layer and neural layer. Pigmented layer is melanin containing epithelial cells and helps absorb stray light rays.  The neural (sensory) layer of the retina is a multi-layered outgrowth of the brain that processes visual data extensively before sending nerve impulses into axons that form the optic nerve. Three layers of neural layer are:  Photoreceptor layer  Bipolar cell layer  Ganglion cell layer

Also in bipolar layer are horizontal cells and amacrine cells that form laterally directed neural circuits that modify signals being sent along pathway from photo receptors to bipolar cells and ganglion cells. There are two types of specialized cells in photoreceptor layer, rods and cones  Rods allow us to see in dim light but do not provide colour vision (we see in gray scale in dim light)  Brighter light stimulates cones which produce colour vision. There blue cones, red cones, and green cones, each sensitive to that wavelength of light. Information from photoreceptor cells flows to bipolar cells then to ganglion cells then through axons to optic disc and exit eyeball in optic nerve. Optic disc also called 'blind spot' because there are no rods and cones there. Macula lutea is the exact centre of the posterior part of the retina. The fovea centralis is a small depression in the macula lutea and is the area of highest visual acuity 







-

Lens: behind pupil and iris is the lens which helps focus images on the retina to facilitate clear vision

Interior of the Eyeball -

-

the lens divides the interior of the eyeball into anterior cavity and virtreous chamber. The anterior cavity (anterior of lens), is further divided into the anterior chamber (between cornea and iris) and posterior chamber which is behind iris and in front of lens. Both chambers are filled with aqueous humour, a transparent watery fluid that nourishes lens and cornea (filtered out from blood and replaced about every 90 or so minutes) the larger posterior cavity of the eyeball is the vitreous chamber which is between the lens and retina. Within vitreous chamber is vitreous body, transparent jelly like substance that holds retina flush against choroid

-

the pressure in the eyeball, called intraocular pressure produced mainly by aqueous humour and partly by vitreous body maintains shape of eyeball and prevents it from collapsing.

Image Formation -

-

eye like camera, optic elements focus image of object on light sensitive 'film' (retina) while allowing correct amount of light (iris) to give proper 'exposure' three processes that happen: o the refraction or bending of light by the lens and cornea o the change of the shape of the lens (accommodation) o constriction or narrowing of pupil

Refraction of Light Rays -

-

-

light rays bend (refract) at junctions of mediums that they are passing through (i.e. air and water) as light ways enter eye, they are refracted at the anterior and posterior surfaces of the cornea. Both surfaces of lens further refract light rays so they come into exact focus on retina images projected onto the retina are inverted and left to right reversed. Brain learns early in life to correct this. Additional refraction by lens called accommodation (lens becomes more spherical) (because incoming light rays are divergent when from objects closer than 6m - see figure)

-

Near point of vision is the minimum distance from eye that object can be clearly focused (about 10cm)

Refraction Abnormalities: Presbyopia: lens loses elasticity and cannot curve to focus on objects as close. Begins ~ mid forties. Age 40: 20 cm; age 60, 80 cm). Some need reading glasses because of this. Emmetropic eye: normal eye that can focus rays from 6m for clear image on retina Myopia (nearsightedness): can see close objects clearly, but not far (eyeball too long or lens thicker than normal) Hyperopia (hypermetropia) (farsightedness): can see far objects clearly, but not close Astigmatism: cornea or lens has irregular curvature (images out of focus and blurred)

Constriction of the Pupil: narrowing of diameter of the hole due to circular muscles of the iris. Happens automatically during accommodation and prevents light from entering from periphery of lens (which would result in blurred vision) Convergence: humans have binocular vision - both eyes focus on one object (vs. other animals, i.e horses). Allows for perception of depth and ability to perceive three dimensions of objects

Physiology of Vision Photoreceptors and Photopigments

-

1st step in visual transduction is absorption of light by a photopigment. The type of photopigment in rods is called rhodopsin. Photopigments also contain glycoprotein called opsin and vitamin A derivative called retinal. Bleaching: part of cycle of how photopigment responds to light. The final product of this step of the process is colourless, hence the name Regeneration: when cis-retinal binds to opsin and reforms a photopigment.

Light Adaptation: emerge from dark to light and visual system adjusts Dark adaptation: when going from light to dark, sensitivity to light increases slowly over few minutes. Release of neurotransmitter by photoreceptor: see figure 17.16 Colour blindness: usually inherited inability to distinguish between certain colours resulting from absence of one of the three types of cones (red-green blindness most common). Night blindness (nyctalopia) inability to see well at low light levels.

The Visual Pathway -

Visual signals in the retina undergo processing at synapses at various neurons in the retina. Then axons from retinal ganglion cells provide output from the retina to the brain, exciting the eyeball at the optic nerve.

Processing of Visual Input in the Retina (see image above)

Brain Pathway and Visual Fields -

The axons within the optic nerve pass through the optic chiasm, a crossing point of the optic nerves After that, they are part of the optic tract Everything that can be seen by one eye is that eye's visual field We have binocular visual field where the two eyes overlap The visual field of each eye is divided into two regions: the nasal (central half) and the temporal (peripheral half)

Hearing and Equilibrium Hearing is the ability to perceive sounds. Ear is amazing b/c its sensory receptors can transduce sound vibrations with amplitudes as small as atom of gold into electrical signals 1000 faster than photoreceptors can respond to light. Ear also contains receptors for equilibrium, the sense that helps you maintain balance.

Anatomy of the Ear: divided into three main regions 1. External ear: collects sound waves and channels them inward 2. Middle ear: conveys sound vibrations to the oval window 3. Internal ear: houses receptors for hearing and equilibrium

External Ear - consists of: -

Auricle (or pinna): flap of elastic cartlidge shaped like the flared end of a trumpet and covered by skin External auditory canal: curved tube (2.5cm long) that lies in the temporal bone and leads to eardrum Eardrum (or tympanic membrane): thin, semi-transparent partition between external auditory canal and middle ear. Ceruminous glands: near the exterior opening, the external auditory canal contains a few haris and specialized sweat glands called ceruminous glands that produce earwax (or cerumen). The hair and ear wax help prevent dust and foreign objects from entering ear and protects delicate skin from water and insects.

Middle Ear: -

-

-

small, air filled cavity in the petrous portion of the temporal bone that is lined by epithelium. Separated from external ear by tympanic membrane (eardrum) and from internal ear by thin bony partition that contains two small openings: the oval window and the round window. Extending across middle ear and attached by ligaments are 3 smallest bones of bodyL o Auditory ossicles: malleus (hammer), incus (anvil), and stapes, (stirrup) The stapedius muscle (supplied by facial nerve) smallest muscle in body and dampens loud noises to protect oval window and decreases sensitivity of hearing. They take a second to contract so protect the ear from prolonged loud noises but not from exceptionally short ones (like gunshot). Hyperacusia is paralysis of the stapedius muscle which makes for abnormally sensitive hearing. Auditory tube (Eustachian tube) opens to anterior wall of middle ear and nasopharynx (superior portion of throat). Normally closed except when swallowing or yawning to equalize pressure.

Internal (inner) Ear: also called labyrinth because of its complicated series of canals. -

-

-

Two main division: outer bony labyrinth that encloses inner membranous labyrinth the bony labyrinth is a series of cavities in the petrous portion of the temporal bone divided into: o semicircular canals o vestibule o cochlea bony labyrinth lined with periosteum and contains perilymph (fluid similar to cerebrospinal fluid) the epithelial membranous labyrinth contains endolymph (extracellular fluid with lots of potassium) vestibule is the oval central portion of bony labyrinth inside that are two sacs called utricle and saccule Projecting from the vestibule are 3 bony semicircular canals. At one end of each canal is a swollen enlargement called the ampulla The cochlea is a bony spiral canal that resembles a snail's shell and is divided into three ducts, the cochlear duct, scala vestibule, and the scala tympani. the vestibular membrane separates the cochlear duct from the scala vestibuli and the basilar membrane resting on basilar membrane is the spiral organ (organ of corti) which is a coiled sheet of epithelial cells and about 16,000 hair cells which are the receptors for hearing. At the tip of each hair cell are 40 80 sterocilia and kinocilium (cilium) that extend into endolymph of the cochlear duct. The tectorial membrane, a flexible gelatinous membrane covers the hair cells of the spiral organ.

The Nature of Sound Waves -

Alternating high and low pressure regions travelling in the same direction through some medium Originate from a vibrating object (much like ripples from a stone being tossed into water). Frequency (pitch) is the distance between the waves. The intensity (amplitude) is the loudness. We hear from 0 decibels (dB). 120 decibels is uncomfortable, 140 painful. These amplitudes damage the hair cells in the cochlea of our ears

The hair cells transduce mechanical vibrations into electrical signals. The Auditory Pathway -

-

Bending of the stereocilia of the hair cells of the spiral organ causes the release of a neurotransmitter (probably glutamate) which generates nerve impulses in the sensory neurons that innervate hair cells. Impulses eventually propagate to primary auditory area of the cerebral cortex in the temporal lobe of the cerebrum. Because impulses travel at slightly different speeds from each ear we can tell where noises are coming from.

Physiology of Equilibrium -

Two types of equilibrium: o Static equilibrium: maintenance of body position (mainly head) relative to the force of gravity. This includes tilting the head and linear acceleration (elevator or car speeding up or slowing down) o Dynamic equilibrium: maintenance of body position (mainly head) in response to sudden movements such as rotational acceleration or deceleration. o The receptor organs resp...