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E.2.1 Outline the diversity of stimuli that can
be detected by human sensory receptors,
including mechanoreceptors, chemoreceptors,
thermoreceptors, and photoreceptors
Section 46.1 pg. 970
•
Sensory receptors are structures that are specialized
to respond to changes in their environment (stimuli)
• Activation of sensory receptors triggers nerve
impulses along the afferent fibers coursing to the CNS
• There are 3 ways to classify sensory receptors:
1. by the type of stimulus they detect
2. by their body location
3. by the relative complexity of their structures
• Sensory receptors named according to the stimuli that
activate them are:
1. Mechanoreceptors
• Generate nerve impulses when they, or adjacent
tissues, are deformed by mechanical forces such as touch,
pressure (including blood pressure), vibration, stretch, and
itch
• Examples include: hair follicle receptors, Meissner’s
corpuscles (found on the surface of hairless skin), Pacinian
corpuscles (deep pressure-sensitive receptors), and Ruffini
and Merkel cells which are touch-sensitive; also, hair cell
receptors of the inner ear and stretch receptors in tendons
Mechanoreceptors
2. Thermoreceptors
• Sensitive to temperature changes
• Examples include: cold receptors located directly
below the epidermis, and warm receptors which are located
slightly deeper in the dermis
• They are also found in the hypothalamus where they
regulate the temperature of circulating blood thus providing
the CNS with information about the body’s “core”
temperature
Thermoreceptors
3. Photoreceptors
• Respond to light energy
• Examples include: rods and cones located in the
retina; the photopigments contained within them break down
when exposed to light, thus generating an action potential
• An action potential is an event that results in polarity
reversal at the cell membrane of a nerve cell or muscle cell
Photoreceptors
4. Chemoreceptors
• Respond to chemicals in solution (molecules smelled
or tasted, or changes in blood chemistry)
• Examples include: taste buds, peripheral
chemoreceptors of the aorta and carotid bodies which
monitor plasma pH, central chemoreceptors of the medulla
which monitor pH of CSF, and olfactory receptors located in
the upper nasal passages
E2.2 Label a diagram of the structure of the human
eye
• The eye is protected inside a bony socket of the skull
and is moved by a set of 6 muscles.
• Around the eye, we find eyelids and tear glands to
keep the eye moist and clean, eyelashes to keep dust out,
and eyebrows as a protector against sweat running down
from the brow
• The eye is supported by the hydrostatic pressure of
the aqueous and vitreous humours
Conjunctiva – thin transparent layer continuous with the
epithelium of the eyelids
Cornea – transparent front of the sclera; the curved surface
is very important in refracting the light towards the retina
Aqueous humour – clear solution of salts in the anterior
chambers of the eye
Pupil – opening in the center of the iris through which light
enters the eye
Lens – transparent, elastic bi-convex structure which
focuses light onto the retina
Iris – the visible, colored part of the eye made up of 2
smooth muscles layers which will vary the pupil size in
response to light intensity
Ciliary body – a thickened ring of tissue made up of smooth
muscles bundles called ciliary muscles that surrounds the
lens and controls its shape
Suspensary ligaments – attach the lens to the ciliary
muscles
Vitreous humour – a clear gel the fills the posterior
chamber
Sclera – white, protective covering of the eye (fibrous)
Choroid – the dark brown, vascular, middle layer of the eye
that prevents light from scattering and reflecting within the
eye
Retina – the innermost layer of the eye which contains the
photoreceptor cells (rods and cones) which play a direct role
in vision
Fovea – a small pit located in the macula lutea (yellow spot)
where light is allowed to pass directly to the photoreceptor
cells without passing through several layers of retina;
enhances visual acuity
Blind spot (optic disc) – region of the eye that lacks
photoreceptor cells; light focused on it cannot be seen; point
where the optic nerve exits the eye
Optic nerve – collection of axons from the retinal ganglion
cells that exit from the back of the eyeball; carries impulses
to the brain
E2.3 Annotate a diagram of the human retina to
show the cell types and the direction in which light
moves
***the direction of receptor potentials in the retina is opposite to
the direction of light
E2.4 Compare rod and cone cells
• The photoreceptor cells become active when light
is focused onto the retina
• Photoreceptor cells are modified neurons that
resemble tall epithelial cells with their tips immersed in
the pigmented layer of the retina
• Most of the cones are located in the central region
of the retina called the fovea or fovea centralis, where
the eye forms the sharpest image
• Rods are almost completely absent from the fovea
• The rods and each of the 3 cone types (blue, red, and
green) absorb different wavelengths of light and have
different thresholds for activation
• Rods are very sensitive, responding to very dim light,
making them best suited for night vision and peripheral
vision
• Rods absorb all wavelengths of visual light but are
perceived only in grey tones; they do not distinguish color
•
Cones need bright light to be activated (low
sensitivity), but have pigments that furnish a vividly colored
view of the world
•
Cones absorb light in only 3 wavelengths; blue at
420 nm, green at or close to 530 nm, and red at or close to
560 nm
•
In addition, rods and cones are “wired” differently to
other retinal neurons
• Rods participate in converging pathways, with as
many as 100 rods feeding into each ganglion cell resulting
in the rods’ effects being considered collectively with vision
appearing fuzzy and indistinct
• The visual cortex has no way of knowing which rods
of the large number influencing a particular ganglion cell are
actually activated
• By contrast, each cone in the fovea (or at most a few)
has a straight-through pathway via its “own personal bipolar
cell” to a ganglion cell with information from each cone
going directly to the higher visual centers
• This accounts for the detailed, high-resolution view
of very small areas of the visual field provided by the cones
E2.5 Explain the processing of visual stimuli,
including edge enhancement and contralateral
processing
• The retina is made up of 3 layers of cells (See
Fig. 46.21 Raven)
• Rods and cones are found in the layer closest to
the external surface of the eyeball
• The next layer contains the bipolar cells and the
layer closest to the eye cavity is made up of ganglion
cells
• Light must first pass through the ganglion cells
followed by the bipolar cells before it reaches the rods and
cones
• The rods and cones synapse with the bipolar cells and
the bipolar cells synapse with the ganglion cells which will
then transmit impulses to the brain by way of the optic nerve
• At the x-shaped optic chiasma, fibers from the medial
aspect of each eye cross over to the opposite side and then
continue on via the optic tracts
• This is known as contralateral processing whereby
the right brain processes information from the left visual
field and vice versa
• Contralateral processing is often illustrated in the
abnormal perceptions of patients with brain lesions
• Action potentials in the optic nerves are relayed from
the retina via the optic tracts to the lateral geniculate nuclei
located in the thalamus, and from there, the visual
information is relayed to the visual cortex of the occipital
lobes ( See fig. 46.22 Raven)
• In the processing of visual information edge
enhancement occurs
• Edge enhancement is the result of lateral inhibition
whereby the capacity of an excited neuron reduces the
activity of its neighbors leading to increased contrast and
sharpness in the visual response particularly at the edges of
images when there is contrasting background
• Edge enhancement can be demonstrated using the
Hermann grid illusion which is a type of illusion that
deceives a person’s vision
Hermann grid illusion
E.2.6 Label a diagram of the ear
E.2.7 Explain how sound is perceived by the ear,
including the roles of the eardrum, bones of the
middle ear, oval and round windows, and the hair
cells of the cochlea
The process of hearing can be divided into 6 basic steps:
1.
Sound waves enter the external auditory canal and
travel toward the tympanic membrane (eardrum)
2.
Movement of the tympanic membrane causes
displacement of the auditory ossicles (bones of the middle ear);
when the tympanic membrane vibrates, so do the malleus and
through their articulations, the incus and stapes, and the sound is
amplified
3. Movement of the stapes at the oval window
establishes pressure waves in the fluid (perilymph) of the
vestibular duct
4. Pressure waves distort the basilar membrane on their
way to the round window of the tympanic duct; the lower
the frequency of the sound, the longer the wavelength, and
the farther from the oval window the area of maximum
distortion will be. Frequency information is translated into
position information
5. Vibration of the basilar membrane causes vibration of
hair cells against the tectorial membrane; this movement
leads to displacement of the stereocilia (bend), which in turn
leads to depolarization of the hair cells and stimulation of
the sensory neurons
6. Information about the region and intensity of
stimulation is relayed to the CNS over the cochlear branch
of the vestibulocochlear nerve (auditory nerve) NVIII (Text
46.12, pg.981)
 TOK: Other organisms can detect stimuli that humans
cannot. For example, some pollinators can detect
electromagnetic radiation in the non-visible range. As a
consequence, they might perceive a flower as patterned
when we perceive it as plain. To what extent, therefore, is
what we perceive merely a construction of reality? To
what extent are we dependent upon technology to “know”
the biological world?