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Sensory Mechanisms: Chapter 12,
Lecture 32
Receptors Receive and Convert Stimuli
•
▪ Stimulus: sensory input that causes some change within or
outside the body
– Heat, pressure, sound waves, light, chemicals
•
▪ Receptor: structure that detects stimulus and
converts its energy into another form
•
▪ Different kinds of receptors
– Mechanoreceptor – Thermoreceptor – Pain receptor
– Chemoreceptor
– Photoreceptor
1. Receptors Are Classified according to Stimulus
▪
Mechanoreceptors
– Respond to mechanical energy (sound waves change
in fluid pressure, stretching, gravity)
▪
Thermoreceptors
– Respond to heat or cold
▪
Pain receptors
– Respond to tissue damage or excessive heat or
pressure (tissue damage, excessive pressure and temperature)
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Receptors Are Classified according to Stimulus
▪
Chemoreceptors
– Respond to presence of chemicals
▪
Photoreceptors – Respond to light
CNS Interprets Nerve Impulses Based on Origin and
Frequency
•
▪ Nerve impulses are transmitted from receptors to specific
portions of brain
•
▪ Stronger stimuli
1. Activate a greater number of receptors
2. Trigger a greater frequency of action potentials in sensory
neurons
1. Some Receptors Adapt to Continuing Stimuli
▪
Some Inputs are Ignored ▪ Sensory adaptation
o – Sensor neuron stops sending impulses even though the
original stimulus is still present
o – Allows the CNS to concentrate on important stimuli and
ignore noncritical ones to maintain homeostasis
▪
Receptors that adapt
– Light touch, pressure, and smell receptors
▪
Receptors that do not adapt
– Pain, joint, and muscle monitoring receptors
Somatic Sensations and Special Senses Provide Sensory
Information
▪
Somatic sensations
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•
– Arise from receptors located throughout the body
•
– Temperature, pressure, touch, vibration, pain, awareness of body
positions
•
Special senses
•
– Arise from receptors restricted to specific areas of the body
•
– Five special senses: Taste, smell, hearing, balance, vision
Somatic Sensations Arise from Receptors throughout the
Body
▪
Located throughout the body in
o ▪ Skin, joints, skeletal muscle, tendons, internal organs
o ▪ Detect touch, pressure, vibration, pain, and body position
and movements
o ▪ Send impulses to the primary somatosensory area of
parietal lobe of cerebral cortex (see fig. 11.7)
o ▪ Somatosensory area processes the information and sends it
to the primary motor area in the frontal lobe
Skin Contains a Variety of Sensory Receptors
1. Unencapsulated dendrites: detect pain, light pressure, changes
in temperature (around hairs and near skin surface)
o Merkel disks: detect light touch and pressure
o Meissner’s corpuscles: detect beginning and end of light
touch and pressure
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o Ruffini endings: respond to ongoing pressure
o Pacinian corpuscles: detect deep pressure and high-
frequency vibration (encapsulated in dermis)
Mechanoreceptors Detect Limb Position, Muscle Length,
and Tension
▪
Mechanoreceptors
1. In joints: detect joint position
2. In skeletal muscles: muscle spindles, specialized mechanoreceptors
for monitoring muscle length, which relay information about limb
position 3. In tendons: detect tension
Thermoreceptors Detect Temperature
•
▪ Thermoreceptors near skin surface provide information about
external environment
– Surface thermoreceptors adapt quickly
•
▪ Thermoreceptors in thoracic and abdominal organs monitor core
temperature
– Core temperature receptors do not adapt quickly
Pain Receptors Signal Discomfort
o ▪ Unencapsulated nerve endings respond to injury from
excessive pressure, heat, light, or chemicals
o ▪ Fast pain (acute, sharp)
– Occurs very quickly
– Informs us of stimuli to be avoided
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o ▪ Slow pain
▪
– Occurs more slowly
▪
– Originates in muscles or internal organs
▪
– Referred pain may be perceived as originating in a
different area of the body
o ▪ Pain receptors do not adapt
Taste: Chemoreceptors Bind with Dissolved Substances
▪
Taste buds
– Chemoreceptors bind with dissolved substances
– Most are distributed around edge, front, and back of
tongue
▪
Taste categories
– Sweet
-Salty
- Sour
– Bitter
– Umami
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Smell: Chemoreceptors Bind with Odorants
▪
Olfactory receptor cells
– Chemoreceptors that bind with odorants – Detect >1,000 different
odorants
▪
Correlation between taste and smell
– Chewed food releases chemicals that come in contact
with olfactory receptors
Hearing: Mechanoreceptors Detect Sound Waves
▪
Properties of sound and sound waves – Loudness
– Related to amplitude of sound waves
– Measured in decibels – Pitch (tone)
– Related to frequency (number of wave cycles/sec) – higher
frequency: higher pitch
– lower frequency: lower pitch
The Outer Ear Channels Sound Waves
:
•
Pinna: outer visible portion of ear, directs sound waves to auditory
canal
•
Auditory canal: directs sound waves to the tympanic membrane
•
Tympanic membrane (ear drum): separates outer-ear from middle
ear, vibrates in response to sound waves and passes along
vibrations to the malleus bone (“hammer”) in inner ear
The Middle Ear Amplifies Sound
•
▪ Air-filled space with three small bones
– Malleus (hammer), Incus (anvil), Stapes (stirrup)
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•
▪ Sound is converted to vibrations of tympanic membrane, and
then passes through these three bones in sequence
•
▪ There is a several-fold amplification of sound
•
▪ Stapes passes vibrations along to oval window from which they
will enter the inner ear (cochlea)
•
▪ Auditory tube (eustachian tube)
– Connects to throat – Equalizes pressure
The Inner Ear Sorts and Converts Sounds
▪
Cochlea converts sound to action potentials
•
Structure of cochlea
•
– Looks like a coiled snail
•
– Uncoiled, it is a tapered tube with two outer canals
(vestibular and tympanic canals) and inner fluid-filled
duct (cochlear duct)
•
– Contains hair cells (mechanoreceptors), with hair-like
extensions embedded in tectorial membrane – Organ of Corti
– Hair cells (mechanoreceptors) and tectorial membrane
The Inner Ear Sorts and Converts Sounds
•
▪ Vibrations of oval window pass through cochlear fluid as
pressure waves
•
▪ Pressure waves result in physical bending of hair cells in organ
of Corti
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•
▪ Differently pitched sounds result in stimulation of hair cells
(mechanoreceptors) in different regions of the cochlea
– High pitches: near base of cochlea, close to oval and round
windows
– Low pitches: closer to cochlear tip
•
▪ Action potentials are carried by auditory nerve to
vestibulocochlear nerve to brain
The Inner Ear Plays an Essential Role in Balance
▪
Vestibular apparatus
– Three semicircular canals and vestibule
– Contain hair cells (mechanoreceptors) embedded in gel-like material
– Hair cells bend in response to movement ▪ Sensing rotational
movement
– Movement of fluid in semicircular canals bends hair cells
(mechanoreceptors)
▪
Head position and linear acceleration
– Movement of otoliths (crystals) bends hair cells in vestibule (utricle
and saccule)
