Download The peripheral nervous system links the brain to the “real” world

Peripheral Nervous System
Peripheral Nervous System
Organization of Nervous System:
The peripheral nervous system links
the brain to the “real” world
Nervous system
Integration
Central nervous system
Peripheral nervous system
(CNS)
(PNS)
Motor
output
Brain
Spinal cord
Ganglia
(neuron cell bodies)
Sensory
input
Motor division
Sensory division
(efferent)
(afferent)
Nerve Types:
Nerves
(bundles of axons)
1) Sensory nerves (contains only afferent fibers)
Autonomic nervous system
Somatic nervous system
(involuntary; smooth & cardiac muscle)
(voluntary; skeletal muscle)
2) Motor nerves (contains only efferent fibers)
3) Mixed nerves (contains afferent / efferent fibers)
Sensory
receptors
Most nerves in the human body
are mixed nerves
Sympathetic division
Parasympathetic division
Motor
output
Peripheral Nervous System
Peripheral Nervous System
Nerve Structure:
Endoneurium
A. Epineurium:
Classification of Nerve Fibers:
• Classified according to conduction velocity
• Outside nerve covering
Perineurium
• Dense network of collagen fibers
Classification
Type
Relative
diameter
Relative conduction
velocity
A alpha (A)
Largest
Fastest (120 m / s)
Yes
A beta (A)
Medium
Medium
Yes
A gamma (A)
Medium
Medium
Yes
A delta (A)
Small
Medium
Yes
B
Small
Medium
Yes
C
Smallest
Slowest (0.2 m / s)
No
Ia
Largest
Fastest
Yes
Myelination
B. Perineurium:
• Divides nerve into fascicles
• Contains blood vessels
Epineurium
C. Endoneurium:
Sensory and
Motor
• Surrounds individual axons and ties
them together
(Erlanger & Gasser)
Fascicle
Ib
Largest
Fastest
Yes
Sensory only
II
Medium
Medium
Yes
(Lloyd & Hunt)
III
Small
Medium
Yes
IV
Smallest
Slowest
No
Marieb & Hoehn – Figure 13.26
Sensory Systems
Peripheral Nervous System
Organization of Nervous System:
“Nothing is in the mind that does not pass through the senses”
Nervous system
Aristotle
(~ 350 B.C.)
Integration
Central nervous system
Peripheral nervous system
(CNS)
(PNS)
Motor
output
Brain
Spinal cord
Sensory
input
Motor division
Sensory division
(efferent)
(afferent)
Autonomic nervous system
Somatic nervous system
(involuntary; smooth & cardiac muscle)
(voluntary; skeletal muscle)
Sympathetic division
Parasympathetic division
• Sensory receptors provide the only channels of communication from the external
world to the nervous system:
• Detections: Electrical impulses - triggered by receptor cells
• Sensations: Electrical impulses - reach brain via neurons
• Perceptions: Interpretation of electrical impulses by brain
Subjective
Sensations / perceptions are not inherent in stimuli themselves;
instead they depend on the neural processing of the stimuli
1
Sensory Systems
Sensory Systems
Sensory Pathways in NS:
Fourth-order
neuron
midline
1) Receptor:
Type of Receptor:
• Converts stimuli to electrochemical energy
Modality:
Category of sensation
Sensory Receptors:
Modality:
Location:
Cerebral
cortex
• Structure variable
• Specialized epithelial cells (e.g., vision)
• Modified neurons (e.g., olfaction)
Third-order
neuron
• Skin (touch)
• Inner ear (audition)
• Inner ear (vestibular)
Mechanoreceptors
2) First-order afferent neuron:
Audition
Touch
• Cell body located in ganglion (PNS)
Vestibular
Thalamus
Photoreceptors
Second-order
neuron
3) Second-order afferent neuron:
• Eye (vision)
Vision
• Synapse with first-order neuron at relay nuclei
• Many first-order synapse with one second-order
Brain
stem
• Interneurons present (signal processing)
Chemoreceptors
• Cross at midline (decussate)
Taste
Olfaction
Osmolarity
• Tongue (taste)
• Nose (olfaction)
• Vessels (osmolarity)
4) Third-order afferent neuron:
• Reside in thalamus (relay nuclei)
5) Fourth-order afferent neuron:
Thermoreceptors
Relay nucleus
• Interneurons present (signal processing)
Temperature
Receptor
Spinal
cord
First-order
neuron
• Reside in appropriate sensory area
• Skin (pain)
Nociceptors
Pain
Guyton & Hall – Figure 46.2 / 46.3
Sensory Systems
• Skin (temperature)
Costanzo – Figure 3.5 / 3.6
Sensory Systems
Sensory Receptors:
Sensory Receptors:
Sensory transduction: The process by which environmental stimuli activates a
receptor and is converted into electrical energy
Receptor Field: An area of the body that when stimulated results in a change in
firing rate of a sensory neuron
• Receptor excitation = Change in membrane potential (Receptor potential)
Receptor fields may be excitatory or inhibitory
Receptor fields exist for first-,
second-, third-, and fourth-order neurons
(usually via opening of ion channels)
Excitatory receptor fields increase firing rate
Inhibitory receptor fields decrease firing rate
Example: Pacinian corpuscle
1) Pressure causes a change in the membrane
properties (e.g., mechanical deformation)
Receptor Potential > Threshold  Action Potential
( Intensity =  Receptor potential =  AP frequency)
2) Ion channels open; receptor potential develops
• Amplitude correlates with stimulus size
3) Receptor potential triggers action potential
The smaller a receptor field, the more
precisely the sensation can be localized
Sensory Systems
Lateral inhibition defines boundaries
and provides a contrasting border
Sensory Systems
Adaptation depends on how quickly
receptor potential drops below threshold
Sensory Receptors:
Sensory Receptors:
Sensory coding: Various aspects of perceived stimuli are encoded and sent to the
proper location in the CNS
Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency
of action potentials generated declines over time
Features encoded:
1) Modality
2) Location
Threshold of Detection:
Weakest signal that will
produce a receptor response
50% of the time
Labeled Line Principle:
Receptors respond to
single modality
Touch
receptor
Tonic receptors
(slowly adapting receptors)
3) Threshold
Somatosensory
cortex
Receptor fields
Vision:
1 photon of light
Hearing:
Bending of hair equal to
diameter of H atom
4) Intensity
5) Duration
• Number of receptors activated
Length of time a sensory
neuron fires
• Firing rate of sensory neurons
• Types of receptors activated
Can encode
stimulus intensity
Will adapt to extinction;
may take hours / days
Pattern of adaptation differs among
different types of receptors
During prolonged stimulus,
receptors “adapt” to the stimulus
• Transmit impulses as long as
stimulus present
• Keep brain appraised of body
status
Guyton & Hall – Figure 46.5
Costanzo – Figure 3.7
2
Costanzo – Figure 3.8
Sensory Systems
Sensory Receptors:
Adaptation depends on how quickly
receptor potential drops below threshold
Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency
of action potentials generated declines over time
Sensory Systems
Rapid adapters = Change in velocity
Somatosensory System:
Slow adapters = Intensity; duration
• Processes information about touch, position, pain, and temperature
Mechanoreceptors:
Phasic receptors
Two-point
discrimination
(rapidly adapting receptors)
Can not encode
stimulus intensity
Pattern of adaptation differs among
different types of receptors
Pacinian corpuscle
Meissner’s corpuscle
Location = Subcutaneous; intramuscular
Detection = Vibration; tapping
Adaptation = Very rapidly
Location = Non-hairy skin
Detection = Tapping; fluttering
Adaptation = Rapidly
• Transmit impulses only when
change is taking place
Hair plexus
Ruffini’s corpuscle
• Important for predicting future
position / condition of body
Location = Hairy skin
Detection = Velocity; movement direction
Adaptation = Rapidly
Location = Skin; joint capsules
Detection = Stretch; joint rotation
Adaptation = Slowly
Guyton & Hall – Figure 46.5
Costanzo – Figure 3.7
Costanzo – Figure 3.8 / 3.9
Sensory Systems
Sensory Systems
Rapid adapters = Change in velocity
Slow adapters = Intensity; duration
Somatosensory System:
• Processes information about touch, position, pain, and temperature
Somatosensory System:
• Processes information about touch, position, pain, and temperature
Mechanoreceptors:
Nociceptors:
• Respond to noxious stimuli that
can produce tissue damage
Mechanical nociceptors:
• Respond to mechanical stimuli
Merkel’s receptor / Tactile disc
Location = Non-hairy skin / Hairy skin
Detection = Vertical indentation
Adaptation = Slowly
• A delta (A) afferent neurons (myelinated)
Polymodal nociceptors:
• Respond to multiple stimuli:
Thermoreceptors:
• High-intensity chemical stimuli
• Hot / Cold stimuli
• C afferent neurons (unmyelinated)
• Slowly adapting receptors; located in skin
Process where axons of nociceptors release
substances that sensitize the nociceptors to
stimuli that were not previously painful
• Overlap in response ranges
• Do not respond at extreme ranges (nociceptors)
Sensory Systems
• H+
• K+
• ATP
• Prostaglandins
• Bradykinins
• Substance P
Sensory Systems
Somatosensory Pathways:
Somatosensory Pathways:
midline
1) Dorsal column system
midline
2) Anterolateral (spinothalamic) system
• Information transmitted:
• Information transmitted:
Cerebral
cortex
a) Discriminative touch
b) Pressure / Vibration
c) Two-point discrimination
Cerebral
cortex
a) Pain
b) Temperature / Light touch
• Consists mainly of groups III and IV fibers
• Fast pain (e.g., pin prick)
d) Proprioception
Thalamus
• Consists mainly of groups I and II fibers
Thalamus
• A delta fibers; group II & group III fibers
• Rapid onset / offset; precisely localized
• Slow pain (e.g., burn)
• Fibers decussate in brain stem (second-order neurons)
• Nucleus gracilis = Info from lower body
• Nucleus cuneatus = Info from upper body
Fasciculus
gracilis
Chemical cues released at damaged /
Intense activity areas include:
Hyperalgesia:
• Cold receptors vs. warm receptors
Brain
stem
Fasciculus
cuneatus
• C fibers; group IV fibers
Brain
stem
• Aching / throbbing; poorly localized
Lateral
spinothalamic
(pain / temperature)
Receptor
Marieb & Hoehn – Figure 12.33
Receptor
Spinal
cord
Ventral
spinothalamic
Marieb & Hoehn – Figure 12.33
Spinal
cord
(light touch)
3
Sensory Systems
Sensory Systems
Somatosensory Pathways:
Vision:
midline
2) Anterolateral (spinothalamic) system
70% of sensory receptors
(~ 1 ml / day)
Referred Pain:
50% of cerebral cortex
Cerebral
cortex
Thalamus
Brain
stem
Contained in orbit of skull:
1) Orbital fat (cushions / insulates eye)
2) Extrinsic eye muscles (6)
3) Lacrimal gland: Produces tears
Dematomal rule:
Sites on the skin are innervated by nerves arising
from the same spinal cord segments as those
innervating the visceral organs
• Lubricates eye
Strabismus:
• Supplies nutrients / oxygen
Receptor
Spinal
cord
A condition in which an eye rotates
medially / laterally
• Provides antibacterial enzymes
Marieb & Hoehn – Figure 14.8
Marieb & Hoehn – Figure 15.3
Sensory Systems
Cornea is the only tissue that can be
transplanted with limited rejection issues
Vision:
Layers of the Eye:
1) Fibrous Layer: (outermost)
Cornea:
Sclera:
Transparent window
“White of the eye”
• Offers structurally support / protection
Sclera
• Acts as anchoring point for muscles
Cornea
• Assists in light focusing
2) Vascular Layer:
Optic Disk (blind spot):
Origin of optic nerve; no
photoreceptors present
Layers of the Eye:
3) Retina: (innermost)
• Pigmented layer (light absorption)
Sclera
• Neural layer (light detection)
Cornea
Choroid
Choroid
(middle)
• Contains blood vessels / lymph vessels
Iris
• Controls shape of lens
Iris:
Regulates size of pupil
radial
muscles
constrict
Fovea
Pigmented Layer
Incoming
light
• Regulates light entering eye
circular
muscles
constrict
Marieb & Hoehn – Figure 15.4 / 15.6
Sensory Systems
Vision:
Ciliary
body
Iris
Optic Disk
Ciliary
body
Retina
Choroid:
Pigmented layer; delivers nutrients to retina
Ciliary Body:
Parasympathetic
Sympathetic
Smooth, muscular ring; controls lens shape
• Only contain brown pigment
Marieb & Hoehn – Figure 15.4
Fovea centralis
• Photoreceptors (light detectors)
• Rods (light sensitive - ~ 125 million / eye)
• Cones (color sensitive - ~ 6 million / eye)
Cones clustered in fovea of macula lutea (sharp focus)
Sensory Systems
Sensory Systems
Vision:
Posterior
segment
Lasik eye
surgery
Vision:
Segments of the Eye:
For clear vision, light must be focused on the retina
Aqueous Humor:
Circulating fluid in anterior segment
• Nutrient / waste transport
• Cushioning of eye
• Retention of eye shape
Vitreous Humor:
Gelatinous mass in posterior segment
• Transmits light
• Stabilize eye shape
• Holds retina firmly in place
Anterior
segment
Close objects =  focal distance
Canal of Schlemm:
Drains
aqueous humor
Accommodation:
1) Cornea: 85% of refraction (fixed)
Intraocular Pressure = 12 – 21 mm Hg
Glaucoma:
Disease where intraocular pressure
becomes pathologically high (~ 70 mm Hg)
Rounder lens =  focal distance
Refraction:
The bending of light when it passes
between mediums of different density
Changing the shape of a lens
to keep an image in focus
2) Lens: 15% of refraction (variable)
• Composed of lens fibers (cells)
Ciliary body produces
aqueous humor
• Crystallins
(1 – 2 l / minute)
(transparent proteins)
Cataract:
(cloudy lens)
(constant focal length)
Marieb & Hoehn – Figure 15.8
4
Sensory Systems
Sensory Systems
20 / 20 = Standard visual acuity
Near point of vision:
Vision:
Inner limit of clear vision
Accommodation:
Loss of lens
elasticity
20 / 200 = Legally blind
Vision:
When Things Go Wrong:
Children = 7 – 9 cm
Young Adults = 15 – 20 cm
Elderly Adults = 70 – 85 cm
tightens
suspensory
ligaments
Near-sighted
Far-sighted
relaxes
suspensory
ligaments
Emmetropia
Lens flattens for distant object focus;
Ciliary body RELAXES
Myopia
Hyperopia
Lens bulges for near object focus;
Ciliary body CONTRACTS
Astigmatism:
In Addition:
• Pupils constrict
• Eyeballs converge
Warping of the lens or cornea
leading to image distortion
Randall et al. – Figure 7.38
Sensory Systems
Correct with diverging lens
Correct with converging lens
Sensory Systems
Horizontal cells / Amacrine cells
Vision:
(facilitate / inhibit serial links – retinal processing)
Vision:
Retinal Layers:
Pigment cell layer
• Absorbs stray light
• Storage site for retinal
(R) = Receptor cells
Photoreceptor layer
• Rods / cones
(H) = Horizontal cells
Outer nuclear layer
• Nuclei of receptor cells
Outer plexiform layer
• Synapses between receptors
and interneurons
Inner nuclear layer
• Nuclei of interneurons
Inner plexiform layer
• Synapses between interneurons
and ganglion cells
Ganglion cell layer
• Nuclei of ganglion cells
(B) = Bipolar cells
(A) = Amacrine cells
Photon:
Basic unit of visible light
High Energy
Low Energy
(short wavelength)
(long wavelength)
Optic nerve layer
• Leave eye via optic disc
Serial link
Costanzo – Figure 3.13
Sensory Systems
Sensory Systems
Vision:
Vision:
Retinal Layers:
Photoreception:
A single photon of light can activate a rod;
several hundred photons of light are
required to activate a cone
(Day vision)
(R) = Receptor cells
(H) = Horizontal cells
(B) = Bipolar cells
(A) = Amacrine cells
Cones demonstrate high acuity
and low sensitivity
Rod Structure:
Cone Structure:
(more abundant
In rods)
• Only a few cones synapse on a single
bipolar cell
Light-sensitive
photochemicals
• In fovea, a single cone synapses on
a single bipolar cell
Outer
segment
• A single bipolar cell synapses on a single
ganglion cell
(Night vision)
Rods demonstrate low acuity
and high sensitivity
Cytoplasm /
cytoplasmic organelles
• Many rods synapse on a single bipolar cell
Inner
segment
• Light striking any one rod will activate
the bipolar cell
• Multiple bipolar cells may synapse on a single
ganglion cell
Costanzo – Figure 3.13
Connection to
neural elements
Connected via
individual cilium
Synaptic
terminal
Costanzo – Figure 3.14
5
Sensory Systems
Sensory Systems
Vision:
Vision:
Photoreception:
Photoreception:
Photochemistry of Vision:
Rod Receptor Potential:
• Rods and cones contain chemicals that decompose on exposure to light
• Rods = Rhodopsin (visual purple)
• Cones = Cone pigments (color pigments)
Similar compositions
• At rest (in dark…), rod receptor potential ~ - 40 mV (receptor average
= - 80 / - 90 mV)
• Cause: Outer segment highly permeable to Na +
Dark Current
Rhodopsin decomposition triggers
hyperpolarization of receptor cell
Example: Rhodopsin
Structure:
• Opsin = membrane protein
• Retinal = light-absorbing pigment
Hyperpolarize
(aldehyde of vitamin A)
The greater the amount
of light, the greater the
hyperpolarization
Two sterically distinct retinal states in the retina
Light
(photoisomerization)
cis-form
trans-form
constant NT release
NT release
NT = Glutamate
Sensory Systems
Sensory Systems
Night Blindness:
Reduced photosensitivity of eyes
Vision:
Vision:
Photoreception:
Photoreception:
Rhodopsin Activation:
(nutritional deficiency of vitamin A)
Regeneration of Rhodopsin:
Storage location
cGMP no longer
present to bind
to ion channels
1) Conformational change in opsin
(picked up from blood)
4) Closure ion channels
(Free of opsin)
Trans-retinal
detaches
Opsin converted to
metarhodopsin II
3) Activation of phosphodiesterase (enzyme)
Bleaching: The fading of photopigment upon absorption of light
2) Activation of transducin (G protein)
Randall et al. – Figure 7.44
Randall et al. – Figure 7.45
Sensory Systems
Sensory Systems
Vision:
Vision:
Photoreception:
Photoreception:
Sex-linked
(1% of males affected)
8% of women carriers
Very rare
(autosomal recessive)
Color Vision:
• Three classes of cones recognized
• Trichromacy Theory
• Humans: Blue, Green, Orange
• Sensation of color = CNS processing
• Color sensitivity depends on opsin structure,
not light-absorbing molecule
• Humans (~ 50% analogy with a.a. of rods)
Normal
Protanopia
Deuteranopia
Tritanopia
(lacking red cones)
(lacking green cones)
(lacking blue cones)
Test Yourself…
• Blue = autosomal chromosome
• Red / Green = X chromosome
•  similarity (gene duplication)
Colorblindness:
Defect in cone opsin gene
Guyton & Hall – Figure 50.10
6
Sensory Systems
Vision:
Temporal
field
Optic Pathways:
Martini & Nath – Figure 13.18
Sensory Systems
Nasal
field
L R
L R
Temporal
field
Auditory:
Auricle
Tympanic
membrane
Hemianopia:
Loss of vision in half the visual field
of one or both eyes
Oval
window
Inner ear
Sensory receptors
Semicircular canals /
Vestibule (equilibrum)
Cochlea (hearing)
Optic nerve
1)
Visual Field
Left
Optic chiasma
Right
2)
External
acoustic meatus
Optic tract
3)
1)
Malleus
Incus
Lateral
geniculate nuclei
2)
(thalamus)
Middle ear
Geniculocalcarine
tract
Converts sound
waves Into
mechanical movements
(amplification)
3)
External ear
Auditory
tube
Collect / direct sound waves
= deficit
Stapes
Primary visual cortex
Costanzo – Figure 3.17
Marieb & Hoehn – Figure 15.28
Marieb & Hoehn – Figure 15.30
Sensory Systems
Cochlea (“snail”):
Spiral-shaped chamber that
houses the receptors for hearing
Auditory:
Sensory Systems
Sound:
A pressure disturbance produced
by a vibrating object
Auditory:
The Nature of Sound:
Cochlear
nerve
Scala
media
Scala
Vestibuli
(contains endolymph)
(contains perilymph)
High pitch
Low pitch
Cochlea
Frequency:
Number of waves passing a given
point in a given time
Pitch = Sensory perception of frequency
Organ of
Corti
Soft
Tectorial
membrane
Loud
Amplitude:
Organ of Corti:
Intensity of sound
Loudness = Sensory perception of amplitude
As basilar membrane
vibrates, hair cells
bump up against
tectorial membrane
Average human speech = 65 dB
Potential damage = > 100 dB
Pain = > 120 dB
Scala tempani
(contains perilymph)
Basilar membrane
Marieb & Hoehn – Figure 15.31
Marieb & Hoehn – Figure 15.31
Sensory Systems
Oscillating NT release produces
intermittent firing of afferent nerves
Auditory:
Amplification of signal:
1) Lever action of ossicles
2) Large window  small window
Sensory Systems
Auditory:
Sound waves enter auditory meatus
Tympanic membrane vibrates
High frequency sounds displace
basilar membrane near base
Basilar membrane differs
in stiffness along length
Ossicles vibrate
Hearing Range:
20 Hz to 20,000 Hz
Oval window vibrates
Cochlear microphonic
potential
Pressure waves exit
via round window
(most sensitive = 2000 – 5000 Hz)
Medium frequency sounds displace
basilar membrane near middle
Pressure waves develop
in scali vestibuli
Tonotopic map
Hyperpolarization
Depolarization
• Shear force bends hairs
• K+ conductance changes
• Depolarization leads to NT release
NT
Organ of Corti vibrates
Low frequency sounds displace
basilar membrane near end
Conduction Deafness:
Sound is not able to be
transferred to internal ear
Sensorineural Deafness:
Damage occurs in neural
pathway (e.g., hair cells)
7
Marieb & Hoehn – Figure 15.34
Sensory Systems
Sensory Systems
Auditory cortex
(tonotopic map preserved)
Auditory:
Vestibule:
Region of inner ear housing
receptors that respond to gravity
sensation / linear acceleration
(static equilibrium)
Vestibular System:
midline
Auditory Pathways:
Cerebral
cortex
Medial
geniculate
nuclei
Central lesions do not cause
deafness because fibers from
each ear are intermixed in CNS
• Provides stable visual image for retina
• Adjust posture to maintain balance
(thalamus)
Maculae anatomy:
Vestibule
Otoliths
Thalamus
Inferior
colliculus
nuclei
Otolithic
membrane
(midbrain)
Brain
stem
Cochlear
nerve
Hair
cell
Cochlear
nuclei
Saccule
(vertical)
Maculae:
Sensory receptors for static equilibrium
(medulla)
Spinal
cord
Marieb & Hoehn – Figure 15.34 / 15.36
Utricle
(horizontal)
Sensory Systems
Vestibular System:
Sensory Systems
Semicircular canals:
Region of inner ear housing
receptors that respond to rotational
movements of head
(dynamic equilibrium)
Analyze three (3) rotational planes
(“yes” / “no” / head tilt)
Crista ampullaris anatomy:
Semicircular canal
Head acceleration (e.g, forward) causes
otolithic membrane to slide; subsequent bending
of hair cells modifies AP firing rate
For every position of the head, there is a unique
pattern of AP activity for the utricle & saccule
Cupula
Vestibular System:
Modified hair cell
Example:
Counterclockwise rotation of head
1) Cupula begins to move; endolymph
is stationary
2) Eventually, endolymph catches up
with cupula (hair cells quiescent)
Ampulla
3) When stopped, endolymph briefly
moves hair cells in opposite direction
Endolymph
(fluid)
(reverses firing rates)
Rotation of head stimulates
semicircular cells on same
side rotation is occurring
Hair cell
Crista ampullaris:
Sensory receptor for dynamic
equilibrium
Head rotation (e.g., spinning) causes endolymph
to push against cupula; subsequent bending
of hair cells modifies AP firing rate
(stereocilia bent
toward kinocilium)
Costanzo – Figure 3.23
Sensory Systems
Vestibular Pathways:
Nucleus
Medial nucleus
Superior nucleus
Sensory Systems
Visual reflexes
(e.g., nystagmus)
Vestibular System:
(stereocilia bent
away from kinocilium)
Olfaction:
midline
Olfactory Organ: (nasal cavity)
Postural reflexes
Input
Output
Semicircular
canals
Extraocular
muscles
Lateral nucleus
Vestibule
Spinal cord
(utricles)
(vestibulospinal tract)
Inferior nucleus
Vestibule /
Semicircular
canals
Cerebellum
Basal
cell
Cerebral
cortex
Olfactory
receptor
cell
Supporting
cell
Thalamus
Nasal
epithelium
Brain
stem
• Olfactory receptor cells (~ 1 million / cm2)
Vestibular
nerve
• Primary afferent neurons (bipolar)
• Ciliated (~ 20 cilia / cell)
• Odorant-binding proteins
Vestibular
nuclei
(medulla)
Vestibulospinal tract
• Odorants must by water / lipid soluble
Spinal
cord
•  4 molecules = receptor activation
• ~ 1000 receptor types (discriminate ~ 10,000)
• Supporting cells
• Basal cells (stem cells  new receptors)
• Continuous neurogenesis
8
Sensory Systems
Sensory Systems
Across-fiber pattern code:
Each odorant produces unique
pattern of activity across a
population of receptors
Olfaction:
Olfactory Transduction:
Olfaction:
• cAMP triggers ligand-gated
Na+
midline
Olfactory Pathways:
• G protein (Golf) coupled to adenylate cyclase
• ATP  cAMP
~ 1000 receptor cells synapse
with a single mitral cell
Olfactory
cortex
Cerebral
cortex
channels
Mitral cells
Glomeruli
(olfactory tract)
(Olfactory bulb)
Thalamus
Limbic system
Cribiform plate
Olfactory Encoding:
Brain
stem
We don’t exactly know how…
• What is known:
Nasal
epithelium
1) Olfactory receptors are not dedicated to a single odorant
2) Olfactory receptors are selective (variable response…)
Spinal
cord
3) Different receptor proteins have different responses to the same odors
Marieb & Hoehn – Figure 15.23
Sensory Systems
Gustation:
Sensory Systems
Gustation:
Taste bud: (~ 10,000 / tongue)
Taste Transduction:
Basal
cell
Taste
pore
Umami
Bitter
Sour
Salty
Sweet
transient receptor
potential
Gustatory
cell
• Gustatory cells (taste receptors)
• Slender microvilli (taste hairs)
• 10 day lifespan
Taste buds spread
across tongue;
different thresholds
in different regions
• Basal cells (stem cells  new receptors)
• Taste buds on lingual papillae:
Circumvallate
Fungiform
Foliate
(majority of taste buds)
(anteriorly located)
(laterally located)
Costanzo – Figure 3.37 / 3.28
Sensory Systems
Gustation:
midline
Taste Pathways:
Second-order neurons
project ipsilaterally
Ventral
posteromedial
nucleus
Taste
cortex
Cerebral
cortex
(thalamus)
Solitary
nucleus
Thalamus
(medulla)
Taste bud
Brain
stem
Posterior 1/3 of tongue (bitter / sour):
Glossopharyngeal nerve (IX)
Anterior 2/3 of tongue (sweet / salty / umami):
Facial nerve (VII)
Spinal
cord
9