Sense Organs Sense Organs Sense organs contain a
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Sense Organs
Sense Organs • Sense organs contain a high concentration of receptor cells which are able to detect stimuli to bring about a response
THE EYE
The human eye
The human eye
The human eye • The receptors that detect light are called rods and cones. • These are situated in the retina at the back of the human eye. • The eyes are found as a pair positioned at the front of the skull. • The field of vision from each eye overlaps which allows a slightly different image of the same object from each eye. Two types of imaging occur as a result: • Binocular vision: vision using two eyes with overlapping fields of view so that the separate images are combined and interpreted as one image by the brain. • Stereoscopic vision: the ability to form three dimensional images which provides the ability to judge distance, depth and the size of an object.
Structure of the eye
The human eye • The eyeballs lie inside a bony cavity. • Connective tissue and fat surround the eyeball to protect it from mechanical injury. • The front of the eye is protected by eyelids, which have eyelashes to stop foreign particles from entering the eye. • The coloured part of the eye is the known as the iris. • The opening in the centre of the iris is the pupil. The sclera is a white outer covering layer. • The internal eye can be divided into three layers: the sclera, choroid and the retina
The human eye
The human eye
The human eye
Accommodation • Accommodation is the ability of the eye to alter the shape of the lens to ensure that a clear image always falls on the retina whether the object is near or distant
Accommodation • Refers to the ability of the eye to change the shape (convexity) of the lens to ensure a clear image is formed on the retina whether the image is near or distant. • to bend the light more to focus it • Fat lens needed
Near vision • When an object is less than 6 m away. • the ciliary muscles contract • the sclera is pulled forward • the suspensory ligaments slacken • the tension on the lens decreases • the lens becomes more convex • the refractive power of the lens increases • a clear image is formed on the retina
Distant vision • When an object is 6 m away • Ciliary muscles relax • Sclera goes back to normal position • Suspensory ligaments become taut • Tension on the lens increases • The lens becomes less convex • The refractive power of the lens decreases • The clear image is formed on the retina
Changing lens thickness • The lens is slightly elastic, its relaxed state is short and fat. l. Cilary muscles are attached to the lens, when contracted they pull the lens thin
Pupillary mechanism • Pupillary mechanism refers to the process by which the diameter of the pupil is altered to control the amount of light entering the eye. • The intensity of the light is the stimulus that changes the size of the pupil. • The iris controls the amount of light entering the pupil. It has circular and radial muscles which act antagonistically to change the size of the pupil.
In bright light • The circular muscles of the iris contract radial muscles relax pupil constricts amount of light entering the eye is reduced
In dim light • The radial muscles of the iris contracts • The circular muscles relax • The pupil dilates • The amount of light entering the eye is increased
Refractive Errors - Hyperopia (Farsightedness Hyperopia is more commonly referred to as farsightedness and is the opposite of myopia. Those who are farsighted have difficulty focusing on distant objects and are even blurrier up close.
• Myopia (short sightedness) • Causes: • Eyeball being too long • Inability of the lens of the eye to become less convex. • Treatment • Wear glasses with a concave lens.
• Astigmatisation • The front surface of the cornea is curved more in one direction that in the other.
Symptoms: Distortion or blurring of images at all distances Headache and fatigue squinting and eye discomfort and irritation Treatment Prescription glasses are required if the degree of astigmatisation is great enough to cause eye strain and head ache, or distortion of vision.
• Cataracts • Refers to: The cloudy, opaque part of the lens. • not clear understanding of its causes. • Treatment: • Surgical removal of the lens. • replacing the lens with a synthetic lens.
Ear
The human ear
The human ear
The human ear
The human ear • As shown above, the ear may be divided into three sections: • the outer ear • the middle ear • and the inner ear.
The outer ear
The outer ear
The middle ear • The middle ear is an air-filled cavity within the skull. • It is separated from the outer ear by the tympanic membrane and • is separated from the inner ear by the round window and the oval window
The middle ear
The middle ear
The inner ear • The inner ear lies within the bones of the skull and is made up of the semi-circular canals, vestibule (sacculus and utriculus) and the cochlear which are continuous with each other. • These structures are made up of bony cavities and are called the bony labyrinth. This labyrinth is filled with a fluid called perilymph. • Suspended in the perilymph is a system of membranes called the membranous labyrinth. • This labyrinth contains a fluid called endolymph.
The inner ear
The inner ear
Functioning of the human ear • The human ear has two functions: • hearing • maintaining balance
Hearing • Hearing is the process in which sound waves are transmitted through the ear and impulses are generated which are sent to the cerebrum for interpretation. • This process occurs as follows:
Hearing • The pinna traps and directs sound waves into the auditory canal towards the tympanic membrane. • The tympanic membrane vibrates as the sound waves strike against it. • The vibrating tympanic membrane causes the ossicles to vibrate. • The hammer, anvil and stirrup amplify and transmit the vibrations to the oval window. • the oval window is smaller than the tympanic membrane. As a result, the pressure increases, causing the sound to be amplified. • The vibrating oval window causes pressure waves to travel through the endolymph in the cochlea.
Hearing • the organ of Corti in the middle chamber of the cochlea is stimulated. • The stimulus is converted into a nerve impulse which is transmitted to the auditory nerve. • The auditory nerve transmits the impulse to the cerebrum for interpretation. • The pressure waves in the cochlea are absorbed into the middle ear through the round window and exit the body via the Eustachian tube.
Maintaining balance • Balance is the process in which receptors in the inner ear detect changes in the position of the head and respond to gravity as well as changes in speed and direction of the body. • Balance is brought about by the cristae ampullae of the semicircular canals and the maculae in the sacculus and utriculus. • This process occurs as follows
Maintaining balance
Maintaining balance
Maintaining balance • The sacculae and utriculae • contain special receptors called maculae that are stimulated. • When the position of the head changes, • the pull of gravity stimulates • sensory hair cells in the maculae to generate impulses. • The impulses generated from the maculae are sent to the cerebellum. • The cerebellum sends impulses to the skeletal muscles to restore the balance.
Maintaining balance • The ampullae • are situated at the end of semi-circular canals contain cristae. • The three semi-circular canals are positioned in three different planes • and therefore, any sudden changes in the speed and direction of body movement, • cause the endolymph to move in at least one of the semicircular canals. • The movement of endolymph stimulates • the cristae to generate impulses which are sent to the cerebellum. • The cerebellum sends impulses to the skeletal muscles to restore the balance.
Hearing defects • Middle ear infection occurs when excess fluid builds up in the middle ear. It is caused by pathogens entering through the Eustachian tube. The fluid cannot drain through the Eustachian tube due to the infection from the pathogen which causes it to become inflamed. • Treatment: • medication • grommets are used for young children. A grommet is a draining tube which is put into the tympanic membrane through surgery which allows moisture from behind the tympanic membrane to drain out.
Hearing defects • Deafness refers to a total or partial hearing loss. It may be caused by: • injury to parts of the ear, nerves or parts of the brain responsible for hearing • hardening of ear tissues such as the ossicles • Treatment: • hearing aids • cochlear implants
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