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
Sensory Receptors
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
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.
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
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
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
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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
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- again
G-Protein-Coupled Receptors
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G-protein and adenylate cyclase
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The inositol-phospholipid signaling pathway
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.
Sensory Receptors – Part II
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
Mechanoreceptors
• Transform mechanical stimuli into electrical signals
• All organisms and cells can sense and respond to mechanical
stimuli
• Two main types
• ENaC – epithelial sodium channels
• TRP – transient receptor potential
Touch and Pressure
Three classes
• Baroreceptors – interoceptors that detect pressure
changes
• Tactile receptors – exteroceptors that detect touch,
pressure, and vibration on the body surface
• Proprioceptors – monitor the position of the body
Insects
Two types of mechanoreceptors
Type I – External Surface
Two common types of
sensilla
• Trichoid – hairlike
• Campaniform – bellshaped
Type II – Internal Surface
• Scolopidia – bipolar
neuron and complex
accessory cell (scolopale)
• Can be isolated or
grouped to form
chordotonal organs
• Most function in
proprioception
• Can be modified into
tympanal organs for sound
detection
Vertebrate Tactile Receptors
• Widely dispersed
• Function as isolated sensory cells
• Free nerves endings or enclosed in accessory structures
(e.g., Pacinian corpuscle)
Proprioceptors
Monitor the position of the body
• Three major groups
• Muscle spindles – located on the surface of
the muscle and monitor muscle length
• Golgi tendon organs – located at the junction
between skeletal muscles and tendons and
monitor tendon tension
• Joint capsule receptors – located in the
capsules that enclose joints and detect
pressure, tension, and movement in the joint
Equilibrium and Hearing
• Utilize mechanoreceptors
• Equilibrium or balance – detecting position of the body
relative to gravity
• Hearing – detecting and interpreting sound waves
• Vertebrates: ear is responsible for both equilibrium and
hearing
• Invertebrates: organs for equilibrium are different from
organs of hearing (e.g., tympanal organs)
Statocysts
• Organ of equilibrium in invertebrates
• Hollow, fluid filled cavities lined with mechanosensory
neurons
• Contain statoliths – dense particles of calcium carbonate
Hair cells
• Mechanoreceptor cells
used for hearing and
balance in vertebrates
• Modified epithelial cells
• Have extensive
extracellular structures and
cilia that extend from the
apical end
Signal Transduction in Hair Cells
Can detect movement and direction
Fish
• Use hair cells in ears for hearing
and for detecting body position
and orientation
• Have neuromasts that detect
water movement
• Neuromast – hair cell and
accessory cupula
• Lateral line system – array of
neuromasts within pits or tubes
running along the side of the
body
Vertebrate Ears
Function in both equilibrium and hearing
Equilibrium
• Vestibular apparatus detects movements
• Vestibular apparatus – three semi-circular canals with
enlarged region at one end (ampulla) and two sacklike
swellings (utricle and saccule)
• All regions contain hair cells
Vestibular Apparatus
• Utricle and saccule contain mineralized otoliths suspended in
a macula covering >100,000 hair cells
• Ampullae lack otoliths and contain cristae (hair cells located
in a cupula)
Maculae Detect Linear Acceleration and Tilting
Cristae Detect Angular Acceleration
Sound Detection
• Inner ear detects sound
• In fish, incoming sound waves cause otoliths to move
which bend cilia of hair cells
• Some fish use the swim bladder to amplify sounds
Terrestrial Vertebrates
• Hearing involves the inner,
middle, and outer ears
• Problem: sound transfers
poorly between air and the
fluid-filled inner ear
• Solution: amply sound
• Pinna acts as a funnel to
collect more sound
• Middle ear bones increase
the amplitude of vibrations
from the tympanic
membrane to the oval
window
Mammalian Inner Ear
• Specialized for sound detection
• Cochlea is coiled in mammals
• Perilymph – fills vestibular and tympanic ducts and is similar to
extracellular fluids
• Endolymph – fills cochlea duct and is high in K+ and low in Na+
• Organ of Corti contains hair cells and sits on basilar membrane
• Two types of hair cells
• Inner hair cells detect sound
• Outer hair cells amplify sounds
Sound Transduction
Steps
• Incoming sound
• Oval window vibrates
• Waves in perilymph of vestibular duct
• Basilar membrane vibrates
• Stereocilia on the inner hair cells bend
• Depolarization
• Release of neurotransmitter (glutamate)
• Excite sensory neuron
Round window serves as a pressure valve
Sound Encoding
Basilar membrane is stiff and narrow at the proximal end
and flexible and wide at distal end
Frequency
• High  stiff end vibrates
• Low  flexible end vibrates
Amplification
Loudness
• Loud sounds   movement of basilar membrane 
 depolarization of inner hair cells   AP frequency
Outer hair cells
• Change shape in response to sound instead of
releasing neurotransmitter
• Change in shape causes basilar membrane to move
more and causes a larger stimulus to the inner hair
cells
• Amplifies sound
Sound Location
• Brain uses information on time lags and differences in
sound intensity
• Sound to right ear first  sound located to the right
• Sound louder in right ear  sound located to the right
Photoreception
• Ability to detect a
small proportion of the
electromagnetic
spectrum from
ultraviolet to near
infrared
• Concentration on this
range or wavelengths
supports idea that
animals evolved in
water
Photoreceptors
Organs range from single light-sensitive cells to complex,
image forming eyes
Two major types
• Ciliary photoreceptors – have single, highly folded
cilium; folds form disks that contain photopigments
• Rhabdomeric photoreceptors – apical surface is
covered with multiple outfoldings called microvillar
projections
Photopigments - molecules that absorb energy from
photons
Vertebrate Photoreceptors
All are ciliary photoreceptors
Two types
• Rods
• Cones
Characteristics of Rods and Cones
Nocturnal animals have relatively more rods
Photopigments
Photopigments have two covalently bonded parts
• Chromophore – pigment that is a derivative of vitamin
A, e.g., retinal
• Opsin – G-protein-coupled receptors
Steps in photoreception
• Chromophore absorbs energy from photon
• Chromophore changes shape
• Photoreceptor protein changes shape
• Signal transduction cascade
• Change in membrane potential
Bleaching – process where activated retinal no longer
bonds to opsin, thereby activating opsin
Phototransduction
Transduction cascades differ in rhabdomeric and
ciliary photoreceptors
The Eye
• Eyespots are single cells or regions of a cell that contain
photosensitive pigment, e.g., protist Euglena
• Eyes are complex organs
Flat-sheet Eyes
• Provide some sense of light direction and intensity
• Most often seen in larval forms or as accessory eyes in
adults
Cup-shaped Eyes
• Retinal sheet is folded to form a narrow aperture
• Better discrimination of light direction and intensity
• Seen in the Nautilus
Vesicular Eyes
• Use a lens in the aperture to improve clarity and intensity
• Lens refracts light and focuses it onto a single point on
the retina
• Present in most vertebrates
Convex Eye
•Photoreceptors radiate outward forming a convex retina
•Present in annelids, molluscs, and arthropods
(eeeeeeeeeek)
Compound Eyes
Most complex convex eyes found in arthropods
Composed of ommatidia
Form images in two ways
• Apposition compound eyes – ommatidium operate
independently; afferent neurons make interconnection
to generate an image
• Superposition compound eyes – ommatidium work
together to form an image on the retina
The Vertebrate Eye
Forms bright, focused
images
Parts
• Sclera – white of the
eye
• Cornea – transparent
layer
• Choroid – pigmented
layer
• Tapetum – layer in
the choroid of
nocturnal animals
that reflects light
The Vertebrate Eye, Cont.
Parts
• Iris – two layers of pigmented
smooth muscle
• Pupil – opening in iris
• Lens – focuses image
• Ciliary body – muscles for
changing lens shape
• Aqueous humor – fluid in the
anterior chamber
• Vitreous humor – gelatinous
mass in the posterior
chamber
Image Formation
• Refraction – bending light
rays
• Both the cornea and the lens
act as converting lens to
focus light on the retina
• In terrestrial vertebrates, most
of the refraction occurs
between the air and the
cornea
Image Accommodation
• Accommodation - incoming light rays must converge on the retina
to produce a clear image
• Focal point – point at which light waves converge
• Focal distance – distance from a lens to its focal point
• Distant object: light rays are parallel when entering the lens
• Close object: light rays are not parallel when entering the lens and
must be refracted more
• Light rays are focused on the retina by changing the shape of the
lens
The Retina
• Arranged into several layers
• Rods and cones are are at
the back and their tips face
backwards
• Axons of ganglion cells join
together to form the optic
nerve
• Optic nerve exits the retina at
the optic disk (“blind spot”)
The Fovea
• Small depression in
the center of the
retina where overlying
bipolar and ganglion
cells are pushed to
the side
• Contains only cones
• Provides the sharpest
images
Signal Processing in the Retina
Rods and cones form different images
Rods
• Principle of convergence – as many as 100 rods
synapse with a single bipolar cell  many bipolar
cells synapse with a ganglion cell
• Large visual field
• Fuzzy image
Cones
• One cone synapses with one bipolar cell which
connects to one ganglion cell
• Small visual field
• High resolution image
Signal Processing in the Retina, Cont.
Complex “on” and “off”
regions of the receptive
fields of ganglion cells
improve their ability to
detect contrasts
between light and dark
The Brain Processes the Visual Signal
• Optic nerves  optic
chiasm  optic tract 
lateral geniculate
nucleus  visual cortex
Color Vision
Detecting different wavelengths of light
Requires multiple types of photoreceptors with different
maximal sensitivities
• Humans: three (trichromatic)
• Most mammals: two (dichromatic)
• Some bird, reptiles and fish: three, four, or five (pentachromatic)
Thermoreception
Central thermoreceptors – located in the hypothalamus and
monitor internal temperature
Peripheral thermoreceptors – monitor environmental
temperature
• Warm-sensitive
• Cold-sensitive
• Thermal nociceptors – detect painfully hot stimuli
ThermoTRPs – TRP ion channel thermoreceptor proteins
Specialized Thermoreception
• Specialized organs for detecting heat radiating objects at
a distance
• Pit organs – pit found between the eye and the nostril of
pit vipers
• Can detect 0.003°C changes (0.5°C for humans)
Magnetoreception
• Ability to detect magnetic fields
e.g., migratory birds, homing salmon
• Neurons in the olfactory epithelium of rainbow trout contain
particles that resemble magnetite