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Chapter 12. Senses. Chapter Outcomes. Explain the difference between sensory reception, sensation, and perception Describe the process of sensory adaptation Distinguish between the major sensory receptors in the human body
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Chapter 12 Senses
Chapter Outcomes • Explain the difference between sensory reception, sensation, and perception • Describe the process of sensory adaptation • Distinguish between the major sensory receptors in the human body • Describe the principal structures of the human eye and their functions
Chapter Outcomes • Observe the principal features of the mammalian eye and perform experiments that demonstrate the functions of the human eye • Describe several eye disorders and treatments • Describe how the structures of the human ear support the functions of hearing and balance
Chapter Outcomes • Explain how humans sense their environment through taste, smell, and touch • Explain how small doses of neurotoxins can be used as painkillers
12.1- Sensory Reception, Sensation & Perception • What is the difference between these three? • Reception • Sensation • Perception
Sensory Receptors • Our sensory neurons are attached to receptors that are activated by specific stimuli • These sensory receptors are highly modified ends (dendrites) of sensory neurons • We have a number of different types of receptors in our body
Groups of Receptors • Often receptors are grouped in specific organs which are specialized to respond to a single stimuli (such as organs for taste, smell, hearing and vision) • The sensations that we receive from these receptors are actually produced in the brain – if transmission from the sensory neuron is blocked, the sensation stops
In general, the stimuli that we respond to are those most relevant to our survival • For example, our range of hearing and vision is limited compared to other animals, even though the other stimuli are present • Our senses can also undergo sensory adaptation • This occurs when a receptor becomes accustomed to a particular stimulus being present
Sensory Adaptation Examples • Ever notice that some strong smells, over time, seem to disappear? • However, if you leave that environment and return, the smell has seemingly reappeared • This phenomenon is due to your sense of smell becoming accustomed to that strong smell
We can also become accustomed to temperature changes • For instance, before you step into a warm shower, the bathroom might seem relatively warm • However, after the shower, you step out and feel very cold • This is because your body becomes accustomed to the warmer temperatures of the shower
12.2 - The Eye • The eye consists of three layers: • The Sclera • Outermost portion of the eye • Includes the cornea & aqueous humor
Choroid Layer • Contains pigments that prevent light from scattering • Includes the: • Iris • Pupil • Ciliary Muscles
Retina • Composed of three layers of cells: • Rods & Cones • Bipolar Cells • Ganglion Cell Layer
The Fovea Centralis • This is a region in the center of the retina that contains a dense bundle of cones • The lens of the eye focuses the majority of the light on this area • The fovea produces sharp colour images • Surrounding this area are rods which pick up low-intensity black & white light
Vision – The Lens • Images form on the retina because of the focal length of the lens • Unlike plastic or glass lenses, the lens of the eye can change its shape, which makes it able to focus on near and far objects • Objects 6 m (20 ft) from the eye should be focused without any change to the lens’ normal shape
The Chemistry of Vision • Rods and cones contain a light-sensitive pigment known as rhodopsin • In the absence of light, rods release inhibitory neurotransmitters that inhibits nearby nerve cells • When light hits this pigment it is split into two components: Opsin (a protein) and retinene (a form of vitamin A) • This division stops the release of the inhibitory transmitter, allowing transmission of an impulse to the optic nerve
Regeneration • As indicated by the previous diagram, the breakdown of rhodopsin is much faster than its regeneration • This is responsible for the afterimages that are often seen after looking at a single object for a long time or at a bright light • Bright light can cause temporary blindness because the rhodopsin is not regenerated in sufficient amounts to maintain vision
Colour Vision • The cones used for colour vision come in three varieties – red, green and blue • Slight changes in the opsin component of rhodopsin are responsible for the various cones’ sensitivities to different colours of light • The following diagram shows the subtle differences in the opsin molecules
As you can see, there are subtle changes to the amino acids that make up these proteins
Colour Blindness • Colour blindness is caused when one or more of the colour cones are defective • This is caused by a mutation in the genes that create the opsin molecules • These mutations alter the sequence of amino acids that make up the opsin, and therefore change its shape and function
Types of Colour Blindness • There are a number of different types of colour blindness • A rare case, known as monochromacy, occurs when a person lacks all three colour pigments and can distinguish no colour at all • More common is dichromacy, where one of the pigments is absent – this is often inherited and affects males more often than females
Types of Colour Blindness • A third type of colour blindness is anomalous trichromacy, where all three pigments are present, but have altered spectral sensitivity • It often results in a difficulty in distinguishing between red and green hues (most common) or yellow and blue hues (very rare)
Types of Dichromacy • Protanopia – an absence of red colour receptors; red will appear dark • Deuteranopia – green photoreceptors are absent, and it affects red-green colour distinction • Tritanopia – total absence of blue receptors What does dichromacy look like?
Other Common Vision Defects • There are a number of other common vision defects • Glaucoma • Caused by increased pressure in the aqueous humor • This pressure causes the blood vessels in the retina to collapse • The rods and cones die because of a lack of oxygen and other nutrients
Glaucoma can be treated with medication or surgery • Medications aim at reducing the pressure within the aqueous humor by either helping it drain or reducing the production of the aqueous humor • Laser or microsurgery can be used to cut a small hole to relieve the fluid pressure, but this is not a permanent solution
2. Cataracts • Cataracts are caused by the lens becoming more opaque • This prevents light from coming through and reaching the retina • Cataracts can be treated by replacing the damaged lens with an artificial one using surgery http://upload.wikimedia.org
Astigmatism • Astigmatism occurs when the lens is irregularly-shaped and only correctly focuses in one plane • This can be countered by using an external lens to compensate for the irregular shape of the lens in the eye
Myopia (Nearsightedness) • Myopia occurs when the eyeball is “too long” and the image from the lens focuses in front of the retina • This is treated by using a biconcave lens to diverge the light rays before they reach the lens
Hyperopia (farsightedness) • The main contributing factor to hyperopia is an eye that is “too short”, resulting in the image being focused behind the retina • A convex lens can be used to converge the light rays before they reach the lens, which refocuses the light on the retina • As well, as we age, our lens becomes less elastic and we lose the ability to focus on near objects
The Blind Spot • Where the ganglion cells merge, they form the optic nerve • At the point where the optic nerve enters the retina, it creates a region that has no rods or cones • This is known as the blind spot
Visual Interpretation • Messages from the eyes travel through the optic nerves to the brain • Once in the brain, the pieces of visual information are sorted, processed, and integrated to produce a 3-D image • Aspects of sight such as movement, colour, depth, and shape are handled by different parts of the occipital lobe • This speeds up the processing of the visual image
Note that images from the right eye are interpreted on the left side of the occipital lobe
12.3 - The Ear • The ear carries out two functions – it is used for balance and for hearing • Both of these senses use specialized hair cells that are very tiny and respond to the movement of fluids in the ear
The Outer Ear • The outer ear consists of: • The pinna • The auditory canal
The Middle Ear • The middle ear produces the sound nerve impulses that are sent to the brain • It consists of several parts: • The tympanic membrane (tympanum) • The ossicles
The oval window • The Eustachian tube
The Inner Ear • The inner ear contains: • The cochlea • The semicircular canals • The vestibule
Hearing and Sound • Our hearing can detect sound energy as low as 1.0×10-12 Watts • Sound travels as pressure waves through a material, and therefore will not pass through a vacuum • Sounds travel most rapidly through solids, and most slowly through gases
The Organ of Corti • The Organ of Corti consists of three structures: • The basilar membrane, which contains many hair cells • The hair cells, which have many tiny projections known as stereocilia • The tectorial membrane, into which are embedded the ends of the stereocilia
Production of a Sound Impulse • The tympanic membrane vibrates as pressure waves hit it • These vibrations are passed on to the ossicles, which amplify the sound • The vibrations of the ossicles move the oval window; the round window moves as well, producing waves of fluid in the inner ear
These waves of fluid travel through the cochlea • The movement of fluid causes a thin membrane known as the basilar membrane to move. This membrane is attached to hair cells located in the organ of Corti • The movement of the cilia of the hair cells against the tectorial membrane produces a nerve impulse which is sent to the brain Production of a Sound Impulse - Animation
Hearing and Pitch • Different pitches of sound can be heard by the human ear (a range of about 20 – 20,000 cycles per second) • Low-pitched sounds stimulate the hair cells near the far end of the cochlea, while high-pitched sounds stimulate hair cells near to the oval window
Hearing Loss • Hearing loss generally results from nerve damage (generally damage to the hair cells) or damage to the sound-conduction system of the outer and middle ear • Repeated loud noise destroys stereocilia • Any noise over 80 dB can damage hair cells
Hearing Loss Treatment • For people who have conduction deafness, hearing aids are often used • However, patients with nerve deafness can have a device implanted in the ear that picks up sounds and transmits them directly to the auditory nerve • Scientists have also been able to use viruses to insert genes that allow the growth of new stereocilia in guinea pigs
Perception of Sound • Nerve transmissions from the ears eventually reach the temporal lobes • Depending on the neurons stimulated, the brain interprets the sounds as specific pitches and intensities • As well, neurons in our temporal lobes can also generalize the area from which the sound originated