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Sensory Physiology

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Title: Sensory Physiology


1
Sensory Physiology Our sensory receptors
transduce the energy from the outside world in
the forms of light, sound, heat, and pressure
into the energy of nerve impulse that travels in
our nervous system.
2
Several categories of sensory receptor 1.   
Chemoreceptors receptors in taste buds of
the tongue, 2.    Photoreceptor in the
retina of the eyes, 3.    Thermoreceptors -
those that sense temperature change, 4.   
Mechanoreceptors - sense touch and
pressure, 5.    Nociceptors - pain receptors  
3
  • When classified according to the types of
  • sensation
  • Proprioreceptor located in muscle, joints
  • and tendons providing sense of position and
  • allow fine tune of skeletal movement.
  • 2. Cutaneous receptor sense touch, temperature
  • and pain.
  • 3. Special senses involves receptors that mediate
  • taste, smell, sight and hearing.

4
Sensory Transduction Generator potential and
threshold In response to stimulus, sensory
receptors produce changes in membrane potential
called receptor or generator potential (similar
to EPSP).  
5
Increased stimulation on receptor can cause an
increase in the magnitude of generator potential.
An action potential can be induced when the
generator potential reaches a threshold. (Fig.
10.2). The action potential is then conducted
to the CNS.
6
  Further increase in the amplitude of generator
potential will cause an increase in the
frequency of action potentials. By changing
the frequency of action potentials, a sensory
neuron can provide information about the
relative intensity of a stimulus to CNS.
7
  • Sensory Pathways
  • Most sensory pathways involve three neurons
  • (or more).
  • Cutaneous receptors and proprioceptor ?
  • first order neuron ? medulla oblongata.
  • second order neuron ? thalamus on the
  • other side ? third order neuron ? Postcentral
  • gyrus (primary sensory cortex)

8
Cutaneous Sensations - General Sense Different
sensory receptors Naked nerve endings,
Pacinian corpuscle nerve ending surrounded
by connective tissue Meissner's corpuscle.
Overhead
9
Sensation of Pain Pain receptors are called
nociceptors, which are free nerve endings widely
spread in the superficial layer of the skin,
joint capsules and the periosteium of bones.
There are very few pain receptors in visceral
organs. Pain receptors have higher threshold
than other sensory receptors.
10
When the stimulation is strong and prolonged,
receptors of other sensations (thermoreceptors
and mechanoreceptors) may also be involved in
pain transmission   The neurotransmitters that
are related with pain signal transduction
include glutamate and Substance P, an eleven
amino acid polypeptide.
11
Hot temperature produce pain sensation through a
specific receptor capsaicin receptor which is
associated with ion channels. Opening of Ca and
Na channels can produce depolarization. The
signal is transmitted to CNS, and perceived as
heat and pain.
12
Two types of neural fibers that conduct pain
signals Myelinated - fast conduction for
sharp pain, usually induce somatic reflex.
Unmyelinated fibers - slow conduction for dull
and persistent ache such as aching in
joints.
13
There are pain centers in CNS (cortex, brain stem
and spinal cord). Therefore, pain sensation
is closely related with mental perception.
After long term pain stimulation, central
adaptation may change pain perception, and
decrease pain sensitivity.  
14
The painkillers, such as Aspirin, ibuprofen, and
Tylenol can only relief somatic pain sensation.

Somatesthetic Sensations Sensations from
cutaneous and proprioceptors.
15
How do narcotics relief pain? Morphine can relief
pain at 3 levels. 1). Decreases the threshold
for pain sensation,
2). Blocks pain nerve impulse transmission, by
decreasing the production of substance P,
3). Changes the pain perception in CNS.
4). Morphine also relieves visceral pain.
16
Receptive Fields and Sensory Acuity A receptive
field is the skin area served by a single
sensory neuron. Sizes of receptive fields are
negatively correlated with the density of
sensory receptors.
If a large skin area contains few sensory
receptors, the receptive field of each neuron is
relatively large.  
17
Two-point touch threshold The minimum distance
at which the touch of two points can be
distinguished.   The value of the two-point
touch threshold is proportional to the size of
receptive fields and inversely correlated to the
sensitivity if the skin area.   A matching
question two-point touch value 42mm back
2 mm finger .
18
General Processing and Adaptation Adaptation is
a reduction in sensitivity to a constant
stimulus. Peripheral adaptation receptors or
sensory neurons alter their levels of activity.
The receptor responds strongly at first, but
the activity gradually declines thereafter.
19
Examples Fast-adapting receptors temperature
receptors. Slow-adapting receptor pain
receptors, which show little peripheral
adaptation.
20
  Central adaptation For exp., Sensation of
smell decreases by time. Central adaptation,
generally involves the inhibition of nuclei
along the pathway.
21
Special Senses Taste and Smell Sense of taste
and smell is mediated by chemoreceptors. The
receptors respond to the molecules that are
dissolved in the surrounding fluid.
22
Sense of Taste Gustation An adult has about
3000 taste buds. Each taste bud contains 50-100
gustatory cells that act as taste receptors.
Each gustatory cell has many microvilli (taste
hairs ) on the surface to increase the
sensitivity. These gustatory cells can
depolarize when stimulated, producing action
potentials and release neurotransmitters.
23
Taste sensation is conducted by cranial nerves
VII (facial), IX (glossopharyngeal). Facial
nerve monitors the taste buds in the anterior
two-thirds of the tongue, glosso- pharyngeal
posterior third of the tongue.
24
Four major modalities of taste on the tongue,
depending on the concentration of different
taste buds in each region.
Sweet taste in the front, bitter in the back,
sour on the sides and salty over most of the
tongue. Fig. 10.8
25
For humans, there are another type of receptors,
umami, which are sensitive to amino acids,
(glutamate), small peptides and nucleotides.
These receptors produce pleasant sensation in
response to glutamate compounds such as MSG.
26
Cellular and molecular mechanism of gustation
The salty taste of food is due to the presence
of Na, which can pass through Na channels of
the sensitive cells to cause depolarization and
action potential.  
27
Similar to salty taste, sour taste is produced by
movement of H ions into the cells. The taste
of most organic molecules is mediated by
interaction of the molecules and receptors.
Sugar through G protein - cAMP. Amino acids
G protein - IP3
28
Smell Olfaction An olfactory receptor cell is
a bipolar neuron that has one dendrite
projecting into the nasal cavity and ends at a
knob containing cilia (olfactory organ on either
side of the nasal septum). Overhead
29
The bipolar neuron also sends out axon that run
through the holes in the cribriform plate
() and reach the olfactory bulb of the cerebrum
where it synapse with the second order neuron,
which project to the olfactory cortex in the
temporal lobes.
30
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31
Unlike other sensory pathways, the sense of
smell is transmitted directly to the brain from
the first order neuron. The information
processing begins in the olfactory bulb.  
32
The olfactory center in the temporal lobe is
associated with hypothalamus, which is part of
the limbic system. Limbic system is known to
be involved in emotion and rewarding. This
explains why a specific odor can evoke emotional
memory.
33
 Equilibrium and Hearing   Anatomy of the ear
The ear is divided into three regions Outer
(external), middle and internal ear.   The outer
ear includes Auricle, external auditory meatus
and tympanum or eardrum.
34
The middle ear is the cavity between the tympanic
membrane and the cochlea of the inner ear, and is
normally filled with air.
Auditory ossicles malleus, incus and
stapes.   Malleus is attached to tympanic
membrane. The vibration of the eardrum is
transmitted by malleus to incus and to stapes.

35
The stapes is attached to oval window (a
membrane in the cochlea), which vibrates in
response to vibration in tympanic membrane.
36
The auditory tube (pharyngo-tympanic tube) is a
passageway between the middle ear and
nasopharynx, which allows the balance of air
pressure between the two sides of the eardrum.
Microorganisms can travel from nasopharynx
to the middle ear and cause ear infection
37
Inner ear cochlea and vestibular
apparatus.   Cochlea Is a bony structure and
shapes like a snail shell.
Each turn of the spiral can be divided into three
chambers scala vestibuli, cochlear duct and
scala tympani. (overhead) Cochlea duct forms
part of the membranous labyrinth and contains
hearing receptors.
38
Two membranes Vestibular memebrane separates
scala vestibuli and cochlea duct Basilar
membrane separates cochlea duct and scala
tympani. (Overhead)  
Scala vestibuli and scala tympani contain
perilymph fluid that is similar to cerebral
spinal fluid Cochlea duct contains endolymph
fluid that is similar to intracellular fluid.
39
  The walls of the bony cochlea is formed by
dense bone except for two small areas near the
base of the spiral. One is round window the
other is oval window, which is connected to
stapes.
40
Vibration of the tympanic membrane causes the
vibration of stapes and the oval window, which
disturbs the perilymph fluid in scala vestibuli.

The waves of perilymph fluid then travel to
scala tympani and reach the base of the cochlea.
Sound waves are thus transmitted from scala
vestibuli to the scala tympani. Overhead
41
The waves of pressure produce displacement of
the vestibular membrane and basilar membrane.
The latter is directly related with hearing.
42
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43
Spiral organ A functional unit Basilar
membrane hair cells tectorial membrane
(gelatinous material).
44
When there is a sound stimulus, pressure waves
of perilymph fluid will create a shearing force
between basilar membrane and tectorial
membrane. This causes the stereocilia to move
and bend.
45
Such movement causes ion channel to open and
depolarization to occur. The depolarized hair
cells can release transmitter chemical,
glutamate, to stimulate an associated sensory
neuron. The activated sensory neuron will in
turn, produce action potential that is conducted
to the brain.
46
A stronger sound stimulation ? strong bending of
stereocilia ? large amount of glutamate released
by the hair cells ? High frequency of action
potentials ? interpreted as a louder sound.  
47
Displacement of basilar membrane is essential
for pitch discrimination Sounds of low pitch
cause the vibration of the basilar membrane far
away from the stapes. High pitch sound causes
vibration of the membrane Near the stapes.
48
Equilibrium and the Inner Ear   Inner ear
contains cochlear and vestibular apparatus.
Vestibular apparatus provides the sensation of
equilibrium.
The sensory receptors in vestibular apparatus are
also hair cells.
49
One of the stereocilia is larger than the
others and is called kinocilium.   The movement
of the head causes stereocilia to bend.
  • When the stereocilia are bended towards the
  • kinocilium, ? depolarization? release of
  • transmitter chemical ? action potential fired
  • by the associated sensory neurons.

50
When the stereocilia are bended in opposite
direction, ? hyperpolarization, no action
potential is fired.   The frequency of the
action potentials fired by sensory neurons
carries the information about how hair cells are
bent
51
Vestibular apparatus consists of two parts the
otolith organs (utricle and saccule) and
semicircular canals.
52
In utricle and saccule,  the hair cells project
into the endolymph, and the hairs are embedded
in gelatinous otolithic membrane. Overhead.
53
  A horizontal forward movement will cause
stereo- cilia to bend backward, because the
otolithic membrane stays behind. The bending
will produce action potential ? CNS. Because
the different orientation, saccule is more
sensitive to vertical acceleration.
54
  Utricle and saccule also provide the
information in respect to gravity. If you
stand with your head tilted to one side, these
receptors will report the angle involved and
whether your head tilts forward or backward.
55
Semicircular Canals Provide information of
rotational or angular movement of the
head. Semicircular canals are oriented in three
planes with different angles.
56
At the base of each semicircular duct there is
ampulla, which contains sensory hair cells.
The hairs are embedded in a gelatinous cupula
membrane. The working mechanism of position
detection is similar to that of otolithic
organs.
57
  • Neural pathways
  • Auditory pathways involves more than three
  • Neurons.
  • Hair cells ? hearing sensory neurons (1st order)
  • form cochlear branch of the vestibulocochlear
  • nerve (VIII) ? medulla oblongata (cochlear
  • Nucleus) ? 2nd neuron ? cross over to midbrain,
  • (3rd neuron) ? thalamus (4th neuron) ? auditory
  • cortex in the temporal lobe.
  • Fig. 10.23

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59
Equilibrium Pathways (Fig. 10.16) Neural fibers
from vestibular apparatus form vestibular
branch (vestibulocochlear nerve)
vestibular nuclei in Medulla Cerebellum
Oculomotor center in brain stem
Spinal cord
Body balance
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62
Motion sickness The mechanism is not clear may
be the central processing is confused by the
conflicting signals from the sensory organs,
which cause nausea, vomiting. For example, when
sitting in a moving car, your eyes are telling
you that you are in the space that is not
changing, but the information from the labyrinth
reposts that your body is lurching or turning
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