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Lecture

Lecture 3


Department
Biology
Course Code
BIO271H1
Professor
Ohana

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Lecture 3
Sensory Physiology: Chemoreception
Neurotransmitter Receptor Function
oIonotrophic receptors
Ligand-gated ion channels
Fast
Ex. Nicotinic Ach receptor
oMetabotrophic receptors
Receptor changes shape
Formation of second messenger
Alters opening of ion channel
Slow
May lead to long-term changes via other cellular function
oSensation begins in sensory receptor cells
oThe important feature of any stimulus is its ability to modify the
conformation of a receptor molecule
Sensory Receptors
oRange from single cells to complex sense organs
oOur ability to distinguish between different types of sensory
stimuli is based on our specialized sensory receptors and their
respective integrating centers
o**Chemoreceptors detect chemical stimuli; chemicals bind to
receptor, causing conformational change and activating a signal
transduction pathway that opens or closes ion channels, while
alters the membrane potential of the sensory cell
o**Mechanoreceptors detect stretch or tension on the cell
membrane; when a pressure stimulus distorts the cell
membrane, it changes the conformation of the mechanoreceptor
protein, opening ion channels and changing the membrane
potential of the sensory cell
o**Photoreceptors detect light by absorbing the energy carried
by the incoming light stimulus, and changing shape, activating
a signal transduction pathway that opens or closes ion channels,
resulting in a change in the membrane potential of the sensory
cell
Classification of Sensory Receptors
oBased on type of stimulus (stimulus modality) the receptors
detect
Chemoreceptors
Chemicals
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For example, smell and taste
Mechanoreceptors
Pressure and movement
For example, touch, hearing, balance, blood
pressure
Photoreceptors
Light
For example, vision
Electroreceptors
Electrical fields
Magnetoreceptors
Magnetic fields
Thermoreceptors
Temperature
Sensory Systems
oTranduction: the process by which receptor cells change
stimulus energy into the energy of a nerve impulse
oAll receptors transduce incoming stimuli into changes in
membrane potential
Receptor protein detects stimulus
Opening or closing of ion channel
Change in membrane potential
Signal sent to integrating center (central nervous system
Graded Potentials
oGenerator potential
Sensory receptor is also the primary afferent neuron
Change in membrane potential spreads along membrane
oReceptor potential
Sensory receptor is separate from the afferent neuron
Change in membrane potential triggers release of
neurotransmitter
Sensitivity to Multiple Modalities
oAdequate stimulus
Preferred (most sensitive) stimulus modality
oMany receptors can be excited by other stimuli, if sufficiently
strong
Ex. Pressure on eyelid perceive light
oPolymodal receptors
Sensitive to more than one stimulus modality
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Ex. Nociceptors; polymodal receptors for multiple
stimulations that are perceived as pain
Stimulus Encoding
oAll stimuli are ultimately converted into action potentials in a
primary afferent neuron
oHow can organisms differentiate among stimuli or detect the
strength of the signal?
oSensory receptors and sensory neurons must encode four types
of information
Stimulus modality
Stimulus location
Stimulus intensity
Stimulus duration
Stimulus Modality and Location
oReceptor location encodes stimulus modality and location
oIntegrating center interprets modality and location
oModality
Polymodal receptors are exceptions encode modality via
temporal patterns of APs
oLocation
Theory of labeled lines
Receptive Field and Location of Stimulus
oReceptive field
Region of the sensory surface that causes a response
when stimulated
Smaller receptive field allows more precise location of the
stimulus (i.e. greater acuity, or spatial resolution)
oImproved ability to localize stimuli by
Using more than one sensory receptor cell (overlapping
area between several receptors; population coding)
Later inhibition
Signals from neurons at the center of the receptive
field inhibit neurons on the periphery
Dynamic Range
oSensory neurons code stimulus intensity by changes in action
potential frequency
Ex. Strong stimuli high frequency
oDynamic range
Range of stimulus intensities over which a receptor
exhibits an increased response
Threshold of detection
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