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Sensory Systems
Inputs to the Nervous System
Oh, I forgot to mention…
• a postsynaptic membrane integrates synaptic inputs
– a nerve impulse (action potential) is all-or-none
• membrane depolarization must reach a
threshold
– firing of an action potential depends on the sum of
all incoming information
• hyperpolarizing neurotransmitters cause an
inhibitory post-synaptic potential (IPSP)
• an axon hillock receives EPSP/IPSP from all
dendrites and the cell body
>post-synaptic signal integration
>many post-synaptic potentials influence
the firing of an action potential
Figure 44.15
spatial summation
temporal summation
Sensory Systems
• sensory cells respond to stimuli
– perceive stimuli through membrane proteins
• detect stimuli
• alter membrane ion permeability
– transduce stimuli into action potentials
• directly (modified neurons)
• indirectly (cells associated with neurons)
– encode the intensity of the stimulus by the
action potential frequency
membrane proteins in sensors
Figure 45.1
Sensory cell stimulation
neuronal
sensor
of
muscle
stretching
Figure 45.2
Sensory Systems
• sensory organs
– sensor cells combine with other cells to
enhance
• collection
• filtering
• amplification
Sensory Systems
• sensory transduction
– a sensory cell receptor protein is activated
– the receptor opens or closes ion channels
• direct or indirect
– changed potential = receptor potential
• a generator potential fires an action
potential in a sensory neuron
• or, the receptor potential causes release of
neurotransmitter in a non-neuronal cell
alternate
signal
generation
paths
in
different
types
of
sensory
cells
Sensory Systems
• sensation depends on the CNS
– different sensors/different parts of the CNS
• visual, auditory, olfactory
– specific pathways transmit sensory signals
• homeostatic sensors produce internal signals
– signals are received and processed by CNS
– signals don’t produce conscious sensation
Sensory Systems
• receptor adaptation to repeated stimulation
– allows animal to ignore background data
– remains responsive to changes or new data
– adaptation rates/capacities vary in different
receptors
female silkworm moth, Bombyx sp.
male silkworm moth, Bombyx sp.
Figure 45.3
Sensory Systems
• Chemoreceptors - specific molecular stimuli
– pheromonal signals in arthropod sexual
attraction
• female moths release a species-specific
pheromone
• male moths perceive the signal through
chemosensory antennae hairs
• male moths fly up the concentration
gradient of pheromone
Sensory Systems
• Chemoreceptors - specific molecular stimuli
– olfaction - the sense of smell
• neuronal sensors in the nasal cavity
–dendrites extend receptors into the
mucus layer of nasal epithelium
–axons extend to the olfactory bulb
above the nasal cavity
Location
Figure 45.4
Organization
Figure 45.4
Sensory Systems
• Chemoreceptors - specific molecular stimuli
– olfaction - sense of smell
• odorant binds the matching receptor
• a the receptor activates a G protein
• the G protein activates adenylyl cyclase
• cAMP opens gated Na+ channels
• depolarization causes an action potential
• intensity number of action potentials
Taste Pore of taste bud
Figure 45.5
taste
sensors
and
associated
sensory
neurons
Figure 45.5
Sensory Systems
• Chemoreceptors - specific molecular stimuli
– gustation - sense of taste
• sensors are clustered in “taste buds”
• taste buds are in the tongue epithelium
• taste pores expose sensory microvilli to
the mouth’s contents
• sensory cells form synapses with
associated sensory neurons
• sensory neuron fires an action potential
Sensory Systems
• Mechanoreceptors - membrane distortion
– conscious sensations
• touch, tickle, pressure
• hearing
– homeostatic monitoring
• stretch in muscle, tendon, ligament
• stretch in blood vessel
• hair movement
Sensory Systems
• Mechanoreceptors - membrane distortion
– membrane distortion opens channels
– the membrane potential changes
– the membrane fires an action potential
– stimulus strength determines the rate of
action potentials
some skin mechanoreceptors
Figure 45.6
fast
slow
slow
fast
Sensory Systems
• Mechanoreceptors
– muscle spindles
• detect muscle stretching
• associated neurons fire action potentials
• motor neurons stimulate contraction
– Golgi tendon organs
• monitor force of muscle contraction
• associated neurons fire action potentials
• inhibit motor neuron; relax muscle
stretch
sensors
participate
in muscle
contraction,
relaxation
Figure 45.7
Sensory Systems
• Mechanoreceptors
– hair cells
• microvilli project from the cell body
• displacement of hair produces receptor
potential
• depolarization to threshold releases
neurotransmitter
• sensory neuron fires action potentials
lateral line
system of
fish
Figure 45.8
semicircular
canal
system
Figure 45.9
Sensory Systems
• vertebrate equilibrium organs
– semicircular canals
• right angles to each other
• contain cupules
• register direction of head movement
– vestibule
• otoliths positioned on gelatinous matrix
• body movement displaces microvilli
otoliths in
the
vestibular
apparatus
Figure 45.9
pressure waves to
vibrations to pressure waves
Figure 45.10
cochlear canals
Figure 45.10
Sensory Systems
• auditory system - hearing
– transduces pressure waves into action
potentials
• tympanic membrane vibrates
• ossicles amplify vibration to oval window
• oval window makes waves in cochlear
canal
• waves displace basilar membrane
• hairs are displaced in organ of Corti
• auditory nerve fires action potentials
source
of
pitch
in the
auditory
system
Figure 45.11
retinal
isomerization
Figure 45.12
location of rhodopsin
a rod cell:
a modified neuron,
sensitive to light
Figure 45.13
light
absorption
causes
membrane
hyperpolarization
Figure 45.13
Sensory Systems
• photosensitivity and sight
– rhodopsin receptors
• opsin + 11-cis-retinal
• in membrane of photoreceptor cell
• isomerized by light absorption to alltrans-retinal
–causes opsin conformational change
–excited rhodopsin hyperpolarizes cell
–neurotransmitter release decreases
pathway
of
hyperpolarization
in a
rod cell
Figure 45.14
Sensory Systems
• photosensitivity and sight
– flatworms detect differences in light
intensity using rhodopsin
– arthropods use clusters of ommatidia in their
compound eyes
• retinula cells in the ommatidia contain
rhodopsin
components
of a
compound
eye
Figure 45.15
Figure 45.18
Rods & Cones Absorb Different
Wavelengths of Light
Sensory Systems
• vertebrate eye
– the retina is the site of
• light absorption
• signal processing
– the retina contains
• rods - light perception
• cones - color perception
– the fovea contains highest density of
receptors
– the blind spot lacks receptors
Human Eye
Figure 45.16
focusing
the
mammalian
eye
Figure 45.17
retinal structure in the human eye
Figure 45.20
Sensory Systems
• vertebrate eye
– receptors are deep in the retina
• receptors synapse with
–bipolar cells
–horizontal cells
• bipolar cells synapse with
–ganglion cells
–amacrine cells
• ganglion cell axons form the optic nerve
measure ganglion signals
Figure 45.21
Sensory Systems
• vertebrate eye
– 100,000,000 receptors
– 1,000,000 ganglion cells
• a ganglion cell receives and processes
information from its receptive field of
receptors
–receptive fields include center &
surround
–receptive fields are on-center or offcenter
ganglion signals in response to light
Figure 45.21
Sensory Systems
• Specialized senses
– EM or IR radiation detection
• some animals sense EM radiation outside
the human visible spectrum
–insects “see” UV radiation
–pit vipers sense IR radiation in the
“visible dark”
Sensory Systems
• Elephants communicate with ultra-low sound
frequencies
Sensory Systems
• Specialized senses
– echolocation
• some animals “map” their environment by
emitting sounds and detecting their echoes
–bats emit high-pitched sounds
–marine mammals emit lower-pitched
sounds
Sensory Systems
• Specialized senses
– electroreception
• some fish are equipped with electrical
detectors in their lateral lines
• electric fish generate electric fields and
detect disruptions in them