Transcript Neuron Structure and Function

Sensory Receptors
Sensory Receptors
Range from simple neurons to complex sense
organs
Types: chemoreceptors, mechanoreceptors,
photoreceptors, electroreceptors,
magnetoreceptors, thermoreceptors
All transduce incoming stimuli into changes in
membrane potential
Figure 7.1
Sensory Receptors
Figure 7.1
Classification of Sensory Receptors
Based on stimulus location
Telereceptors – detect distant stimuli, e.g.,
vision and hearing
Exteroceptors – detect stimuli on the outside
of the body, e.g., pressure and temperature
Interoceptors – detect stimuli inside the body,
e.g., blood pressure and blood oxygen
Classification of Sensory Receptors
Based on type of stimuli the receptors can detect
(stimulus modality)
Chemoreceptors – chemicals, e.g., smell and taste
Mechanoreceptors – pressure and movement, e.g.,
touch, hearing, balance, blood pressure
Photoreceptors – light, e.g., vision; detect photons
Electroreceptors – electrical fields
Magnetoreceptors – magnetic fields
Thermoreceptors - temperature
Receptors and stimulus
Location: Can distinguish the location of
the stimulus (touch, light or odour)
Duration: Determine length of stimulus by
responding to the stimulus for the duration
of the stimulus.
Intensity: Increase in action potential
frequency or increase in neurotransmitter
release.
Sensitivity to Multiple Modalities
• Adequate stimulus – preferred or most sensitive stimulus
modality
• Many receptors can also be excited by other stimuli, if
sufficiently large, e.g., pressure on eyelid  perceive
bright light
• Polymodal receptors – naturally sensitive to more than
one stimulus modality, e.g., ampullae of Lorenzini in
sharks
• Nociceptors – sensitive to strong stimuli, e.g., pain; many
are polymodal receptors
Stimulus Encoding
 All stimuli are ultimately converted into action potentials
in the primary afferent neurons
 How can organisms differentiate among stimuli or detect
the strength of the signal?
 Sensory receptors must encode four types of information
Stimulus modality
Stimulus location
Stimulus intensity
Stimulus duration
Dynamic Range
• Action potentials code
stimulus intensity through
changes in frequency, e.g.,
strong stimuli  high
frequency
• Dynamic range – range of
intensities for which
receptors can encode stimuli
• Threshold detection –
weakest stimulus that
produces a response in a
receptor 50% of the time
• Saturation – top of the
dynamic range; all available
proteins have been stimulated
Figure 7.4a
Range Fractionation
Relationships between stimulus
intensity and AP frequency
• Linear across large range of
intensities: large change in
stimulus causes a small
change in AP frequency 
large dynamic range, poor
sensory discrimination
• Linear across small range of
intensities: small change in
stimulus causes a large
change in AP frequency 
small dynamic range, high
sensory discrimination
Range fractionation – groups of
receptors work together to
increase dynamic range without
decreasing sensory
discrimination
Figure 7.4b-c
Tonic and Phasic Receptors
Two classes of receptors that encode stimulus duration
• Phasic – produce APs only at the beginning or end of the
stimulus  encode changes in stimulus, but not stimulus
duration
• Tonic – produce APs as long as the stimulus continues
• Receptor adaptation – AP frequency decreases if stimulus
intensity is maintained at the same level
Tonic and Phasic Receptors, Cont.
Figure 7.5
Pain 
• Pain and itching are mediated by Nocireceptors
• Itch comes form Nocireceptors in the skin. Higher
pathways for itch are not well understood
• Pain is s subjective perception
Chemoreception
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Most cells can sense incoming chemical signals
Animals have many types of chemoreceptors
Multicellular organisms typically use taste and smell
Olfaction – sense of smell
• Detection of chemicals carried in air
Gustation – sense of taste
• Detection of chemicals emitted from ingested food
Distinct due to structural criteria
Performed by different sense organs
Use different signal transduction mechanisms
Are processed in different integrating centers
The Olfactory System
Evolved independently in vertebrates and insects
Vertebrate olfactory system
• Can distinguish thousands of odorants
• Located in the roof of the nasal cavity
• Mucus layer to moisten olfactory epithelium
• Odorant binding proteins – allow lipophilic odorants to dissolve in mucus
• Receptor cells are bipolar neurons and are covered in cilia
• Odorant receptor proteins are located in the cilia
Odorant Receptors are G Proteins
• Each olfactory neuron expresses only one
odorant receptor protein
• Each odorant receptor can recognize more than
one odorant
Figure 7.7
Pheromones
Vomeronasal organ – detects
pheromones
Structurally and molecularly distinct
from the primary olfactory
epithelium
• Location
• Base of nasal cavity near the
septum in mammals
• Palate in reptiles
• Transduction
• Activates a phospholipase Cbased signal transduction
system; adenylate cyclasecAMP in other olfactory
receptors
Figure 7.8
Taste Buds in Vertebrates
Group of taste receptor cells
Located on tongue, soft palate, larynx, and esophagus;
external surface of the body in some fish
Taste Buds in Vertebrates
 50 to 150 taste cells
 Epithelial cells that have apical and basal sides and joined by tight
junctions
 Life span of 10-14 days
 Basal stem cells divide to regenerate taste cells
 Microvilli on its apical surface that project into the mucus of the tongue
 Taste receptor proteins are found in the microvilli
 Chemicals are soluble and diffuse to the bind to their receptors
 Different cells in the same bud can detect NaCl, sucrose, H+ and quinine
(bitter)
 Taste cell forms a chemical synapse with a sensory neuron that projects
to the brain from the tongue
Taste buds and peripheral innervation
Figure 7.11c-d
A generic taste cell.
Apical surface: both channels and G-proteincoupled receptors that are activated by
chemical stimuli
Basolateral surface: voltage-gated Na+, K+,
and Ca2+ channels, as well as all the
machinery for synaptic transmission
mediated by serotonin
The increase in intracellular Ca2+ is either by
the activation of voltage-gated Ca2+ channels
or via the release from intracellular stores
causes synaptic vesicles to fuse and release
their transmitter onto receptors on primary
sensory neurons
Each cell contains the standard complement
of neuronal proteins including Na+/K+
ATPase at the basal level, voltage-gated Na+
and Ca2+ channels, leak K+ channel
A generic taste cell…cont.
The response to the chemical is mediated by
the expression of receptors for that chemical
in the microvilli
The response is a depolarization of the cell
sometimes enough to generate an action
potential
The signaling of the cell to the sensory
neuron depends on a sufficient
depolarization to open the voltage-gated Ca2+
channels necessary for vesicle fusion and
neurotransmitter release.
Transduction mechanisms
G-Protein-Coupled Receptors
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G-protein and adenylate cyclase
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The inositol-phospholipid signaling pathway
You don’t have to memorize this  weeeeeee
but be aware of it and know which taste is transmitted
using this pathway ie bitter
Salt taste
 The Na+ enters into the cell through the passive
amiloride-sensitive Na+ channel
 These proteins are found in frog skin and kidney
 Amiloride will block Na+ salt taste reception
 Entry of Na+ into the cell of course causes the
cell to depolarize
 Need a large concentration of Na+ to trigger a
sufficient depolarization to signal to the postsynaptic sensory neuron
Salt taste
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Sour taste
Taste response produced by acids, excess protons (H+). These
positive ions enter the cell through a H+, cation specific ion channel
and in turn depolarize the cell to threshold for an action potential.
Sour taste
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Sweet taste
 There are specific membrane receptors for different sweeteners and sugars
 These receptors are not ligand gated ion channels but rather are
metabotropic receptors
 These receptors belong to the family of seven transmembrane domain
proteins that are linked to signaling cascades through G proteins.
In mammals a combination of the T1R2/T1R3 receptors have a response to
sugars and sweeteners
 These receptors stimulate a G protein (Gp) which in this case activates
phosopholipase C (PLC)
 PLC breaks down PIP2 (phosphatidylinositol 4,5-bisphosphate) into IP3
(inositol triphosphosphate) and DAG
 IP3 will bind to and activate a ion channel (TRP channel called TRPM5)
which allows Ca2+ to influx into the cell
 This pathway leads to a depolarization and threshold is reached to trigger
an action potential
Sweet taste
• In other animals sugars also appear to bind to receptors that stimulate G
proteins (Gs) that activate adenylate cyclase
• This results in an increase in cAMP in the cell that activates a protein
kinase (PKA) which in turn phosphorylates a K+ channel to close the
channel
• Once the K+ channel is close the cell will depolarize
• Both these signaling cascades are used in multiple biological systems
• In the nervous system neurotransmitter binding to specific metabotropic
receptors can trigger these cascades
• Photoreceptor and olfactory neurons also use parts of these cascades for
their sensory transduction
Sweet taste
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Bitter taste
Different cells have different mechanisms of bitter taste transduction
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In mammals the bitter receptor is a metabotropic receptor called T2R.
There are about 30 different subtypes in mammals
These signal through a G protein called gustducin to PLC and thus
generate IP3
Like sweet receptors the IP3 activates a TRPM5 channel to open and
allow Ca2+ to influx into the cell.
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Some bitter chemicals such as quinine bind to and block specific K+
channels and thus result in depolarization of the cell
Bitter taste
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Amino acid taste cells
In some animals (catfish) there are a high number of amino acid taste cells
There appears to be multiple ways that animals respond to amino aicds
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In fish and other amphibians, amino acids such as L-arginine and Lproline bind to specific receptors which are ligand gated ion channels
2. In mammals there are taste cells that respond to L-glutamate. In these
cells L-glutamate activates a metabotropic receptor glutamate receptor
linked to a G protein. Glutamate binds to many different metabotropic
receptors and in taste cells it is the mGluR4 that is responsible for the
taste transduction
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In mammals there are also two metabotropic receptors T1R1/T1R3 that
combine to respond to the standard 20 amino acids. This combination
signals through G protein activation of PLC and the generation of IP3
and the activation of the TRPM5 channel.