Download Chapter 8 The Special Senses Sensory receptors – dendrites

Chapter 8 The Special Senses
Sensory receptors – dendrites specialized to detect certain types of stimuli
 Exteroceptors detect stimuli from outside the body (e.g. taste, hearing, vision)
 Interoceptors receive stimuli from inside the body (e.g. change in blood
pressure)
Types of sensory receptors
 Chemoreceptors – respond to the binding of certain chemicals (smell and
taste)
 Photoreceptors – respond to light energy (vision)
 Mechanoreceptors – respond to mechanical forces such as pressure (hearing
and balance)
The special senses
 Sensory adaptation can occur when perception of a particular sense decreases
to the point where we are no longer aware of it
o
This is usually due to repeated or constant stimulation
 The special senses likely evolved to protect organisms as they moved through
their environment
Olfaction
 The chemoreceptors responsible for sensing smell are located in the upper
chambers of the nasal passage
 The chemicals will dissolve in the mucus coating the receptors
 There are specific receptors for specific molecules
 When a particular receptor binds its specific odor molecule, a sensory impulse
is sent to the olfactory bulb and then to the brain
 The olfactory bulb also connects with the limbic system of the brain
o
This explains why smells can trigger such deep memories and
emotions
Taste
 The chemoreceptors responsible for sensing taste are located in the
approximately 10,000 buds located in small bumps (papillae) on the tongue
 They can distinguish 5 categories of flavor - sweet, sour, salty, bitter, and
umami (savory)
 Binding of the flavor molecules initiates signal transmission to the brain
Anatomy of the ear
The ear functions in hearing and balance
 3 divisions
o
Outer ear functions in hearing; filled with air
o
Middle ear functions in hearing; filled with air
o
Inner ear functions in hearing and balance; filled with fluid
A. The ear: Outer ear
 Includes
o
Pinna - the external ear flap that catches sound waves
o
Auditory canal - directs sound waves to the tympanic membrane
 Lined with fine hairs and modified sweat glands that secrete
earwax
B. The ear: Middle ear
 Includes
o
Tympanic membrane (eardrum) - membrane that vibrates to carry
the wave to the bones
o
3 small bones (malleus, incus, stapes) - amplify the sound waves
o
Eustachian tube - a tube that connects from the throat to the middle
ear used to equalize pressure so the eardrum does not burst
C. The ear: Inner ear
 Important for both hearing and balance
 3 areas - cochlea, semicircular canals, vestibule
o
Stapes (middle ear bone) vibrates and strikes the membrane of the
oval window causing fluid waves in the cochlea
o
Vestibule – gravitational equilibrium
o
Semicircular canals – rotational equilibrium
Cochlea
 Converts vibrations into nerve impulses
 Contains the organ of Corti (spiral organ), a sense organ containing hairs for
hearing
 Bending of embedded hairs cause vibrations that send nerve impulses to the
cochlear nerve and then to the brain
 Pitch is determined by varying wave frequencies that are detected by different
parts of the organ of Corti
 Volume is determined by the amplitude of sound waves
Semicircular canals and vestibule
 Detects movement of the head in the vertical and horizontal planes
(gravitational equilibrium)
o
Depends on hair cells in the utricle and saccule
 Detects angular movement (rotational equilibrium)
o
Depends on hair cells at the base of each semicircular canal
(ampulla)
Anatomy of the eye
 Made of 3 layers/coats –
o
Sclera - mostly white and fibrous except the cornea
o
Choroid - darkly, pigmented vascular layer
o
Retina - inner layer containing photoreceptors
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
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
 Ciliary process - a structure behind the iris that contains a muscle that controls
the shape of the lens
 Lens – attached to the ciliary body and functions to refract and focus light rays
o
The lens is a flexible, transparent, biconvex structure
o
The lens changes shape (accommodates) to focus light on the retina
in order to form an image
o
As we age the lens loses elasticity and we use glasses to correct for
this
C. The eye: Retina
 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 that takes impulses to
the brain
o
The blindspot is where the optic nerve attaches
C. The eye: Photoreceptors of the retina
 Rods
o
Contain a visual pigment called rhodopsin
o
Important for peripheral and night vision
o
Vitamin A is important for proper functioning
 Cones
o
Located mostly in the fovea
o
Allow us to detect fine detail and color
o
3 different kinds of cones containing red, green, and blue pigments
Abnormalities of the eye
o Cataracts – lens of the eye is cloudy
o Glaucoma – fluid pressure builds up in the eye
o Astigmatism – condition in which the cornea or lens is uneven leading to a
fuzzy image
o Nearsightedness – eyeball is too long making it hard to see distant objects
o Farsightedness – eyeball is too short making it hard to see close objects
o Color blindness – genetic disease more common in males in which they cannot
distinguish red and green
o
Scientists have used gene therapy to cure red-green colorblindness
in squirrel monkeys