The Senses
rev 6-11
• Neurons communicate with our muscles or glands to
produce a reaction within our body. They communicate
through chemicals called neurotransmitters.
• However, in order to receive a stimulus from the
environment, there is a specialized type of neuron called a
receptor.
• Sensory input (called a stimulus) causes a change within
or outside the body. The receptor accepts the stimulus (a
form of energy such as heat, pressure, or sound). and
converts the energy into a form that can be understood by
the body.
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The Senses
• Receptors are usually categorized by the kind of
stimulus to which they respond.
– Chemoreceptors are stimulated by chemicals
that attach to receptors at the end of the sensory
neuron.
• sense of taste and smell.
– Pain Receptors
– Mechanoreceptors are the largest class of
receptors and include:
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The Senses-mechanoreceptors
• stretch receptors
• proprioceptors
• tactile(touch and vibration) • hair cells (ear)
• baroreceptors (sensitive to changes in blood
pressure)
– Thermoreceptors (temperature)
– Photoreceptors (light)
Receptor Adaptation when exposed to the same
degree of stimulus, receptors stop sending
impulses so you no longer respond to the stimulus.
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The Senses
The EYE
• composed of 3 tissue layers
– outer layer is the sclera
– middle layer is the choroid
– inner layer is the retina
• The Sclera (or white of the eye)
– is composed of tough, white connective tissue
that is modified in the front
• eye muscles attach to the sclera
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The Senses
– this modified “spot” is called the cornea and is
clear so that light can pass through
• cornea is the most exposed part of the eye and is
vulnerable to damage from dust, dirt, etc.
• has high ability to regenerate and repair itself
– can also be replaced surgically
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The Senses
• Light passes through the cornea and a small fluid
filled space called the aqueous humor
– the aqueous humor is located between the
cornea and the lens (the anterior chamber of the eye)
– it nourishes and cushions the cornea and lens
• The vitreous body or vitreous humor is located
behind the lens (in the posterior chamber of the
eye)
– this is a gel-like fluid
– the fluid transports nutrients and wastes and
maintains the shape of the eye
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The Senses
• Choroid layer
– middle layer which contains a large portion of
the blood supply for the eye
– is darkly pigmented to absorb light and prevent
it from scattering and reflecting within the eye
– Some red light is reflected back from the
choroid however and causes the “red-eye
effect” in photographs
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The Senses
The Choroid Layer
– Contains the iris
• a ring of smooth muscles that regulates the
opening of the pupil (the central opening of
the iris which allows light to enter the eye)
• the iris gives the eye its color
– and the ciliary body
• a ring of muscles that encircles the lens and
can change the shape of the lens
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The Senses
• The lens
• composed of transparent crystalline proteins
• focuses the visual image on the retina
• is held in the center of the ciliary body by
suspensory ligaments
• precise focusing requires thickening or
thinning of the lens
• the changing shape allows for focusing on
near or distant objects
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The Senses
• The Retina
– innermost layer of tissue which contains 2
kinds of photoreceptors (light sensitive
receptors)
• RODS sensitive to low levels of light (night
time vision)
– Allow us to see in black and white
• CONES require high levels of light
– come in 3 varieties (red, green and blue)
so we can see in color
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The Senses
– a dense collection of cones, the macula
lutea, is found at the back of the eye near
the fovea centralis, the center of focus
– anything we wish to see in great detail is
focused on the fovea centralis
– the optic disc
• blind spot where nerve impulses that have
been focused on the retina exit the eye via
the optic nerve
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The Senses
• Eye Protection
– bony socket of the skull
– layer of adipose tissue that cushions the socket
– blinking
– tears from the lacrimal gland cleanse, lubricate
and protect the eye
• tears contain mucus, antibodies and
lysozyme, a bacteria destroying enzyme
• the lacrimal ducts take excess tears from the
eyes and drain them into the nose
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EYE DISSECTION-SAFETY
• may get splashed as you open the eye
• Dissection
– follow instructions on page 91 of text
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The Senses
The Ear
• Is an organ of both hearing and balance
• Outer Ear
– consists of the pinna (visible portion of the ear)
and the auditory or acoustic canal
– sound waves arrive at the pinna and are directed
into the auditory canal
– the canal channels the sound to the eardrum or
tympanic membrane which is a partition
between the outer and middle ears
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The Senses
• The Middle Ear is an air filled chamber within the
temporal bone and contains 3 bones: the malleus
(hammer), incus (anvil) and stapes (stirrup)
– when soundwaves strike the eardrum, it causes the
membrane to vibrate. The vibrations cause the malleus,
incus and stapes to vibrate also.
– The stapes touches another membrane, called the oval
window (the beginning of the inner ear), and causes
this membrane to vibrate.
– The Eustachian tube is a connection between the
middle ear and throat.
• It acts to equalize air pressure in the middle ear with
that of the surrounding atmosphere
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The Senses
• So, we have air conduction of sound via the outer
ear and this is changed to bone conduction of
sound in the middle ear
• Inner Ear
– consists of the cochlea (where sound is
converted into fluid movement), the vestibule
and 3 semicircular canals
– within the cochlea is the Organ of Corti which
contains tactile receptors or hair cells
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The Senses
– the hair cells vibrate and bend which releases a
neurotransmitter that generates impulses to
nearby sensory neurons
– different parts of the organ of Corti respond to
different pitched tones so we can hear
variations in sounds
• These impulses travel to the auditory nerve and we
“hear” the sound
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The Senses
• Balance and Equilibrium
– the Vestibular Apparatus consists of
• the 3 semicircular canals for sensing
rotational movement of the head and body
(dynamic equilibrium). This tells us where
we are in space.
• the vestibule which senses static (stationary)
balance and linear acceleration of the head
(static equilibrium)
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The Senses
• The vestibule
– contains hair cells, gel and otoliths (hard
crystals of bone like material)
– when the head tips forward, the otoliths
slide toward the gravitational pull causing
the hair cells to bend
– this produces impulses in nearby sensory
neurons which are transmitted to the
brain for interpretation
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REMINDER, Page 1 of 2:
1. Long lab; you can work in groups of 3 if
needed.
2. Perform the receptor tests on page 87 and
88.
3. Learn the parts of the eye on the eye
models and the sheep eye. Use the bulleted
list on page 91 for parts to identify on both.
4. Remember to follow safety procedures.
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REMINDER, Page 2 of 2:
5. When finished with the sheep eyes, discard them
in the "discard" bucket.
6. Perform the eye tests on pages 90-92. You do not
need to do the color blindness tests.
7. Learn the parts of the ear on the ear models. Use
the bulleted list on page 94 to know what
structures to locate.
8. Perform the ear tests on page 94. Do not perform
the frequency test.
9. Strike the tuning fork on your palm, not on the table.
Hold the fork by the bottom peg.
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