Chapter 14
Senses
Sensory Receptors and Sensations
Sensory receptors initiate the nerve impulses that travel to the CNS. Sensation only occurs once the impulse reaches the cerebral cortex. There are four types of receptors:
- chemoreceptors are receptors that respond to chemicals and are found in the
taste buds and olfactory epithelium.
- pain receptors are specialized receptors that respond to chemicals released by
cells surrounding the receptor.
- photoreceptors are receptors that respond to light energy and are found in the
rods and cones in the retina.
- mechanoreceptors are receptors that respond to pressure and are found in the
hair cells of the inner ear and respond to sound pressure for hearing, motion for rotational equilibrium, and gravity for gravitational equilibrium.
- thermoreceptors are receptors that respond to temperature changes. There are
two types, warmth and cold receptors. They are located in the skin and the
hypothalamus.
Proprioceptors and Cutaneous Receptors
- proprioceptors are special mechanoreceptors that involve the reflex action that
maintain muscle tone and as a result help coordinate posture and
equilibrium.
- cutaneous receptors are the receptors in the dermis layer of the skin
responsible for touch, pressure, pain, and temperature.
Senses of Taste and Smell
We can taste and smell due to chemoreceptors that respond to protein molecules
and are found in the taste buds and olfactory epithelium. The taste buds allow the brain to distinguish five tastes; sweet, sour, salty, bitter, and umami. Our sense of smell is much more acute with the ability to distinguish tens of thousands of smells if not more.
Sense of Vision
Has basically three parts; the eye, the optic nerve, and the cerebral cortex.
Anatomy and Physiology of the Eye
The eye itself is comprised of three layers:
-the sclera, which is the outer layer, supporting and protecting the eye, it
also contains the cornea.
-the choroid, which is the middle layer, absorbs stray light, and also
contains the iris and ciliary body.
-the retina, which is the inner layer, which contains the sensory receptors
(rods, cons, and fovea centralis).
http://www.hollows.org/upload/7727.pdf
Function of the Lens
Light travels through the cornea, lens, and the vitreous humor onto the retina. The ciliary muscle and the suspensory muscle work together to flatten the lens for distant objects and rounded for near objects.
Visual Pathway to the Brain
The pathway to the brain begins as light strikes the retina, stimulating the
photoreceptors (rods distinguish black/white, shapes and motion; and
cones distinguish color), continuing to the optic nerve to the optic
chiasma then on to the occipital lobe of the brain.
Abnormalities of the Eye
- Distance vision is attributed to short eyeballs, where the image is in
focus past the retina.
- Near vision is attributed to long eyeballs, where the image is in
focus before the retina.
- Astigmatism is where the lens or cornea is uneven and cannot evenly
focus an image on the retina.
Sense of Hearing
A sense of hearing depends on the ear, the cochlear nerve and the auditory centers of the cerebral cortex. Sensory inputs for equilibrium (balance) also happen in the ear. This done through mechanoreceptors called stereocilia attached to hair cells in the inner ear.
Anatomy and Physiology of the Ear
The ear has three specific parts:
- the outer ear, containing the pinna, the auditory canal and the tympanic
membrane used in directing pressure wave to the middle ear.
- the middle ear, containing the tympanic membrane, the malleus, the
incus, the stapes, used in amplifying the pressure waves and the oval and
round windows, which are covered by a membrane, separating the
middle ear from the inner ear .
- the inner ear, which has two functions; one containing the semicircular
canals, the vestibule, and the vestibule nerve which are used in
equilibrium (balance); and two containing the cochlea and the cochlear
nerve used in converting sound into impulses to send to the auditory
centers of the brain.
Auditory Pathway to the Brain
The auditory pathway begins as a pressure wave travels down the
auditory canal to the tympanic membrane to the middle ear. Then is
amplified by the middle ear through the membrane at the oval window
and into the inner ear. Nerve impulses are generated by
mechanoreceptors called stereocilia attached to hair cells in the inner
ear. These impulses travel down the cochlear nerve to the auditory center
of the brain.
Sense of Equilibrium
There are three semicircular canals. Sensory inputs for equilibrium (balance) also happen in the ear. This done through mechanoreceptors called stereocilia attached to hair cells in the inner ear (ampulla, utricle and saccule). But the brain also integrates vision and positional information from receptors in the muscles and tendons for a total balance equation.
Rotational Equilibrium Pathway
Rotational and/or angular movements of the head are determined when the
mechanoreceptors called stereocilia attached to hair cells in the ampulla
send impulses into the vestibular nerve and to the brain
GravitationalEquilibrium Pathway
Vertical or horizontal movements of the head are determined when the
mechanoreceptors called stereocilia attached to hair cells in the saccule
and the utricle send impulses into the vestibular nerve and to the brain.
Sources:
Human Biology 10th Edition by Sylvia S. Mader
Larry M. Frolich Power Point Unit III
http://www.hollows.org/upload/7727.pdf
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