S1
Sensing the world
• Sensory coding: sensory systems code for
modality, intensity, location, and duration
of external stimuli.
• Transduction: the conversion of a physical
stimulus into a change in membrane
potential (electrochemical signal)
– Signals are transmitted in the form of
graded potentials, action potentials,
and synaptic interaction
• Receptors: cells or regions of cells that
respond to specific stimuli and perform
transduction
– The process of sensory coding starts
here
– Specificity: receptors are often
sensitive to specific stimuli; varies with
receptor type
S2
Changes of membrane potential
depend on ion channels
Reminder: leak channels are not gated.
S3
Receptors for
gustation
5 different types of receptor proteins
(but not all in the same cell)
Receptor proteins
Receptor cell
1st order sensory neuron
S4
Receptors for audition (hearing)
and
equilibrium
(movement and orientation to gravity)
Hair cells =
specialized receptor cell with
mechanically-gated
K+ channels.
Receptor cell
1st order sensory neuron
Scanning electron micrograph of hair cells
from the bullfrog inner ear, which contain the
mechanically-gated ion channel TRPA1.
(Image courtesy of Howard Hughes Medical
Institute)
S6
Receptor cells with
receptor proteins.
Encapsulated
TZ
TZ
TZ
S7
Somatosensory and the sensation of touch
Skin is largest sense organ:
up to 2 million receptors
Sensory system must code for
where (location),
how hard (intensity),
how long (duration), and
modality (sense of touch, temperature, vibration, wet/dry, damage (pain; noxious)).
S8
Somatosensory: sensation of touch,
vibration, pain, and temperature
“Adequate” stimulus: the
stimulus type to which a
receptor responds best
(lowest threshold.)
Deep
Encapsulated nerve endings
Superficial
Sustained stimulus
Fluctuating stimulus
S9
This diagram is
misleading:
Different types
of receptors
are NOT
part of the
same sensory
neuron!
S 10
Labeled Lines:
Different sensory
modalities are
transmitted separately
along distinct
pathways.
S 11
Figure 7.15
Must Code for
1) Stimulus Localization
& 2) Modality
& 3) Intensity
& 4) Duration
Three neurons to
the cortex!
Labeled Line
S 12
Right side
Right side
3
2
Anterolateral (spinothalamic) tract
1st order synapses onto 2nd order in spinal cord, 2nd
order axon decussates in spinal cord, travels to
thalamus in contralateral anterolateral tract, synpases
onto 3rd order neuron in thalamus, which sends its axon
to SSC.
Right side
Dorsal column-medial lemniscal tract
1st order axons ascend in ipsilateral dorsal columns to
synapses onto 2nd order in dorsal column nuclei of
brainstem, 2nd order axon decussates in brainstem,
travel to thalamus to synpases onto 3rd order neuron,
which sends it axons to SSC.
Right side
1
Proprioception
S 13
Somatotopy in the Somatosensory Cortext (post-central gyrus.)
Regions more densely innervated by sensory receptors occupy more cortical tissue.
S 14
Referred pain
What about referred pain from the heart?
Who
cares?
S 16
Referred Pain
Fig 7-18 p. 200
of text
S 17
Receptive field = the area of the
body that, when stimulated,
leads to activity in a neuron.
= 1st order sensory neuron
Could be encapsulated
S 18
Right side
Right side
Receptive field = the area of the body
that, when stimulated, leads to activity
in a neuron.
1st, 2nd, and 3rd order sensory neurons
each have receptive fields.
Right side
Dorsal column-medial lemniscal tract
1st order axons ascend in ipsilateral dorsal
columns to synapses onto 2nd order in dorsal
column nuclei of brainstem, 2nd order axon
decussates in brainstem, travel to thalamus to
synpases onto 3rd order neuron, which sends it
axons to SSC.
Right side
Proprioception
S 19
Receptor potentials
e.g. somatosensory
e.g. gustatory
• Graded potentials are the result of transduction within a receptor.
Transduction produces a receptor potential
• Amplitude is usually in proportion to the stimulus intensity
• Specialty receptor cells with no axon (visual, gustatory, auditory, and
vestibular systems). The graded receptor potentials will directly change
amount of NT secretion onto 1st order sensory neuron.
• Receptors with axons (somatosensory and olfactory systems) have a
trigger zone and receptor potentials generate action potentials
S 20
Receptors and transduction
Could be
encapsulated
Activation of mechanically gated channels
S 21
Sensations to touch
Mechanoreceptors contain receptor proteins that respond to stretching,
distortion, or pressure on the peripheral plasma membrane
S 23
Summary to now:
Modality encoded by _______.
Intensity encoded by _______.
What’s left?
Location encoded by _______.
Duration encoded by _______.
Stimulus Localization
S 24
Figure 7.06
The size of receptive fields varies
dramatically in different regions of
skin (i.e. lips, palm, fingertip, calf).
For touch discrimination, small receptive fields allow greater accuracy in “two
point discrimination” test (in lab!)
S 25
Stimulus Localization
& Intensity
Receptive fields of different
neurons often overlap such
that any patch of skin may
have several receptors of
the same type (modality)
and receptors of different
types (different modalities
I.e. touch, pain temperature,
etc.)
Overlapping receptive fields
of touch receptors
(Meissners, Merkels) allow
for more precise localization
of a stimulus via the
mechanism of lateral
inhibition (next slide.)
S 26
Stimulus Localization & Intensity
Presynpatic
Inhibition
Lateral inhibition exaggerates the
difference in stimulus intensity detected by
adjacent neurons, aids with LOCALIZATION.
Mild
inhibition
Massive
inhibition
S 27
Stimulus Localization
& Intensity
Lateral inhibition improves
stimulus localization.
Which is easier to localize and why:
a light touch or a moderate touch?
Right side
3
2
Right side
Right side
1
Proprioception
1QQ # 12 Answer one
1. Which of the following are correct statements:
a) The intensity of a stimulus is proportional to the size of the
graded potential in the receptive membrane.
b) The modality of a stimulus is encoded by which type or types
of sensory receptors are activated.
c) The intensity of a stimulus is encoded by the frequency of
action potentials.
d) Some ascending spinal neurons receive synaptic inputs from
more than one part of the body.
e) Each taste receptor cell has at least two different categories of
tastant receptor proteins.
2. What is the meaning of labeled line in the context of
sensory physiology?
S 28
Coding for stimulus duration.
Adaptation = decrease in frequency of APs
with sustained stimulus
S 29
Figure 7.11
“Flutter”
receptors
are rapidly
adapting.
Examples: waistband of underwear, top of socks, earrings,
mechanoreceptors in carotid arteries for blood pressure
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