Lecture 8 - Sensory System Flashcards
Sensory Systems
- Olfaction
- Taste
- Touch
- Hearing
- Vision
Chemical Stimuli
(chemoreceptors) Olfaction Taste
Mechanical Stimuli
(mechanoreceptors)
touch hearing
Electromagnetic Stimuli/light waves
(photoreceptors)
vision
Sensory Receptor Cells - Initial stimuli Conversion
- convert physical and chemical stimuli into neural signals
- stimulus causes change in neurotransmitter release
1. sensory receptor detects stimuli and directly or indirectly opens/closes ion channels
2. change in membrane potential –> neurotransmitter release - some sensory cells do not use action potentials
- graded depolarization causes neurotransmitter release
- released neurotransmitter can cause AP in subsequent neurons downstrea,
What is a sensation and how is it interpreted? How are different ones interpreted?
- the sensations we perceive
- specific sensation = which combinations of neurons are firing action potentials (smell vs round, rose vs vanilla)
- intensity of stimulus = frequency of action potentials (how many APs fire, not which ones)
Olfactory Receptor Neurons
Location: epithelium on top pf nasal cavity
- receptors on cilia of cells extending into nasal cavity
- chemoreceptors
- odorant
- axons extend up to olfactory bulb in brain
*travel farther, DO need action potential
* humans have 20 million olfactory receptors in our nose * dogs have 1 billion
Olfaction
How? Process?
- odorant - molecules that activate an olfactory receptor
- olfactory receptors - sensitive for particular types of odorant molecules
- one cell receives the stimulus and carries signal all the way to the brain
Process:
- Odorant binds to receptor
- activation of receptor causes increased levels of cAMP - second messenger (odorant stays outside)
- cAMP opens Na+ channels
- Na+ influx –> depolarization –> action potential
Distinctness of olfactory receptors
~ 350 different types in humans (mice have 1,000)
- each receptor recognizes a unique aspect of smell
- each type of receptor is found in a limited number of receptor neurons
- each receptor neuron expresses just one type of receptor
*not a 1:1 relationship of smell:receptor
How a “smell” originates
- the combined activity of multiple distinct receptors
- more than one “odor” can activate a receptor
- one odor can activate more than one receptor
- almost infinite combinations
- population coding
- a particular smell = unique combo of receptors activated
Taste Receptor Cells
PAPILLAE - large bumps
–> each papillae has several hundred TASTE BUDS
–> each taste bud has 50-150 TASTE RECEPTOR CELLS
5 Basic Tastes
Bitter
Sour
Salty
Sweet
Umami
5 Types of taste receptor cells
Each cell recognizes one of the 5 tastes: salty, sweet, sour, bitter, umami
Process of Taste Reception
- cause depolarization of cell (NO action potential) (different mechanism of depolarization for each cell)
- depolarization leads to the Ca++ channels to open
- triggers neurotransmitter release onto sensory neurons
- sensory neurons convey the information to the brain via action potential
Transduction for Salty Taste
NaCl/Na+
type of Na+ pump that is always open
- increase in sodium concentration from food
- ions flow across this channel into receptor cell
- depolarization
- ca++ influx
- neurotransmitter release
- other cell carries signal to brain
Transmission for sour taste
H+ - sourness = acidity
- H+ flows in through Na+ channel (and H+ blocks K+ channel)
- depolarization
- Ca++ influx
- neurotransmitter release
- Other cell carries signal to brain
Transduction for bitter, sweet and umami
- molecule binds to outside
- activates second messengers on inside - not ion channels themselves
- second messengers open unique type of Na+ channel
- Na+ flows in = depolarization
- ca++ influx
- neurotransmitter release
- other cell carries signal to brain
Similarities and differences between bitter, sweet and umami receptors
- different types of receptors, same cellular effect
- same intracellular pathway
- expressed on different taste cells, each connected to different target
- for all sensory systems, end product of difference types of stimuli is always the same (neurotransmitter release)
- matter of WHICH cells are activated
Population Coding (for taste)
- gives us infinite flavors from 5 types of taste receptors
- particular combination of different receptors activated (and level of activation) once again gives us large array of possible flavors
- olfaction also influences our taste
Miraculin
- binds to sweet receptors (but does not activate them)
- in the presence of acid changes conformation
- activates the receptors
- sour food tastes sweet
Taste Transduction Summary
Salty: Na+ flows directly into Na+ channels
Sour: protons flow in through the Na+ channel and close K+ channel
Bitter, sweet and umami: receptors that are not ion channels increase levels of second messengers. these open Na+ channels
* for all: depolarization opens voltage gated Ca++ channels and causes neurotransmitter release
Touch
- mechanoreceptors
- physical distortion of plasma membrane causes ion channels to open
- generates depolarization and action potential
- diverse - generates varied sensations
- receptors distributed throughout the body -
respond to different kinds of stimuli (touch, temperature, pain, body position)
Types of touch receptors
- human skin packed with many different types of mechanoreceptors that generate varied sensations
- provide different aspects of sensory information
- sensitivity
- rapid vs slowly adapting (change)
- pressure and vibration
Spatial resolution of touch
Why more sensitive in some places?
- ability to discriminate detailed features of a stimulus varies greatly across different points in the body
- two point discrimination test
- reasons for better resolution
1. higher density of mechanoreceptors
2. enriched mechanoreceptors w/small receptive field
3. more brain tissue devoted to the sensory info of each mm of skin
How APs travel from skin to brain
- Travel along one long axon
- mechanoreceptors are actually located on axon
1. stimulation induces action potential
2. travel up spinal cord to where cell body lies
3. axon continues on, ascending through spinal cord
4. synapse where spinal cord meets medulla
5. information then continues on to higher brain areas, including sensory cortex
Somatosensory Complex
- perietal lobe, behind central sulcus Somatotopic map -homounculus (little man)
- receptory fields of sensory neurons produce an orderly map of the body on the cortex
- not scaled like the human body
- plasticity –> disuse causes atrophy of a region, taken over by others –> increased use causes expansion of a region
What is hearing?
our perception of sound waves sound
- pressure waves auditory receptors are mechanoreceptors that convert those pressure waves into changes in membrane potential
Parts of the Ear
outer ear - collects sound
middle ear - amplifies sound
- ossicles transmit vibrations from eardrum (tympanic membrane) to oval window
inner ear - turns sound into a neural signal
in cochlea, hair cells release neurotransmitters
Cochlea
- organ of hearing
- composed of theee parallel canals separated by two membranes
Basilar membrane
Basilar membrane
- transduces pressure waves into action potentials
- contains hair cells
- middle canal has very high concentration of k+
Hair cells
- on basilar membrane
- sterocilia extend out of the top
- tips of sterocilia are embedded in over-hanging membrane
- when basilar membrane flexes, it pushes the sterocilia up against the overhanging membrane and bends them
What causes basilar membrane to flex?
- basilar membrane extends along cochlea
- pressure wave from ossicles pushing on oval window causes movement o the fluid inside the cochlea
- creates traveling waves in basilar membrane Base: narrow and stiff Apex: think and floppy
Process
Bending of sterocilia opens mechanically gated ion channels
- ion channels are at the tips of sterocilia and covered by a lid linked to other sterocilia
- bending of sterocilia pulls lids open or pushes them closed
- K+ influx –> depolarization
- AT REST (straight cilial) - channel partly open (small leak of K+ in)
1. movement of cilia in one direction
2. tension on tip link opens channels, K+ influx –> DEPOLARIZATION
3. cilia go other direction
4. close tip links - HYPERPOLARIZATION
How does depolarization induce neurotransmitter release
- without action potential - hair cells do not fire action potential
- graded depolarization or hyperpolarization
1. depolarization causes opening of Ca++ channels
2. cause neurotransmitter release
3. neurotransmitter binds to sensory neuron that sends action potentials to brain
Wave frequency corresponds with…
cycles of polarization and depolarization
Frequency map in basilar membrane
- different frequencies of pressure waves cause basilar membrane to bend at different locations - high frequency sounds - bend base of membrane - low frequency sounds - bend apex of membrane - which hair cells are activated informs brain about what frequency the sound is - combined with pattern of neurotransmitter release
Cornea and lens
- focus the light rays of the retina
- muscles contract around the lens, changing its shape to focus images at different distances into our retinas
The retina
- contains the cells that detect light
- photoreceptors
- plus other layers of cells that help process information from the photoreceptors
- inside-out organization
- light must pass through all the layers of cells to reach retina
Two types of photoreceptors
rods and cones stack of membranous disks containing light sensitive photopigment
Rods
- more of them
- more disks 1000X more sensitive to light
- low light conditions
- one type of photopigment
- lower acuity
Cones
- less sensitive to light
- daylight conditions
- three types of photopigment
- color
- higher acuity
photo transduction
dark = depolarized light = closes Na+ channels, hyperpolarizes cell
Rhodopsin
Basis for vision
- photosensitivity is due to the fact that photoreceptor cells contain pigment call rhodopsin
- rhodopsin absorbs photon of light and undergoes a change in shape
Before rhodopsin activation
(dark) - in the dark Na+ channels are OPEN in the photoreceptor cell
1. Na+ flows in freely
2. depolarized
3. neurotransmitters released (NO AP)
Activation of rhodopsin by light
Process
- induces Na+ channels to close
1. rhodopsin activates a cascade of second messengers
2. second messengers cause Na+ channels to close (degrade molecules that held them open)
3. causes hyperpolarization
4. NO neurotransmitter release
Transduction summary
dark = depolarized (neurotransmitter released)
light = hyperpolarized (NO neurotransmitter released)
Phototransduction in cones
- similar to rod phototransduction
- 3 different rhodopsins (differ in wavelengths they absorb best, red, green, blue)
- color detection
- relative contributions of blue, green and red cones mix to give hundreds of visible colors
Ex: at 470, relatively equal activation of blue and green = teal
Red/green color blindness
~10% of men of euro descent -
sex linked (gene for both red and green cones is on x chromosome)
- cant distinguish green from red
- caused by lacking either green cones or red cones
- cannot tease apart wavelengths in the 500-600nm range
Color vision
- some animals have fewer than 3 cones (dogs have 2 - blue and yellow)
- some animals have more than 3
- many birds have 4
- shrimp have 16
- can see shades of color we cant see also infrared and UV spectra
Flow of visual info
- photoreceptors
- bipolar cells
- ganglion cells (first action potential)
exit retina to brain
Cerebrum
- surrounds hypothalamus and thalamus
- CEREBRAL CORTEX = outer layer
- LIMBIC SYSTEM = in between thalamus/hypothalamus Two hemispheres: left and right
- control opposite sides of the body
- connected by corpus callosum
Two hemispheres of the brain
- most information is being perceived and processed simultaneously by both hemispheres
- integrated together lateralization of some cognitive processes
Right hemisphere
- controls left side of body
- receives inputs from the left side of the body
- facial recognition
- artistic perception
Left Hemispheres
- controls the right side of the body
- receives inputs from the right side of the body
- language
- exact calculation and fact retrieval
- interpret and explain
Corpus Callosum
- “white matter”
- huge bundle of axons that connects the two cerebral hemispheres
- allows communication between the two halves of the brain
Results of split brain experiments
Right hemisphere
- left visual field
- cannot name object but can recognize it, point to it
- facial recognition Left hemispheres
- right visual field
- can name object
- interpret and explain
Limbic System
- emotion
- sensation of pleasure, pain, rage, fear, memories
- motivation and reward AMYGDALA
- fear
- kluver bucy syndrome: lack of fear, inability to recognize it in others HIPPOCAMPUS
- declarative memory formation
- transfer of information from short term to long term memory
- patient H,M.
- spatial learning (london taxi drivers, rats navigating mazes)
4 Lobes of cerebrum + cerebellum
Frontal
- front of head
- motor functions, executive functions
Parietal
- top
- sensory info
Temporal
- middle
- hearing, speech, memory
Occipital
- visual processing
Cerebellum
- at brainstem
- motor coordination
Frontal Lobe
- motor function: primary motor cortex
- executive functions:
- decision making and planning
- attention
- inhibition
- personality
- desires, intentions, beleifs
*last to mature –> why teenagers lack inhibition etc
Parietal Lobe
- sensory info: primary somatosensory cortex (especially spatial sense)
Temporal lobe
- auditory perception
- semantics and understanding of language
- recognizing, identifying and naming objects including faces
Occippital lobe
- visual processing
Homounculus
- Primary sensor cortex
- primary motor cortex
Primary motor cortex
- controls muscles in specific parts of the body
- parts of the body with fine motor control have disproportionate representation
- if stimulated, muscle contraction in that area
Primary somatosensory cortex
- receives sensory information relayed from body through thalamus
- areas of the body that have a high density of tactile mechanoreceptors capable of making fine discriminations of touch have large representation
- if stimulated, person will report feeling touch/sensation in that part of the body
*plasticicity - if lose a finger, others will take up the space designated for it
Cerebellum
“little brain” (smaller lump at the bottom of the brain)
- motor coordination - controls coordination of movement (fine tuning, auto pilot)
- receives input from the cortex about the intended movements and calculates the sequence of muscle contractions needed to achieve those intended movements
- damage results in uncoordinated and inaccurate movements (NOT paralysis)
Brainstem
vital functions, motor and sensory info 3 components: midbrain
pons
medulla
Function:
- breathing, heart rate, blood pressure, temperature
- reflexes such as coughing, sneezing, swallowing, vomiting
- relays info between cortex and cerebellum
- pathway of sensory and motor info damage –> “locked in syndrome” (paralyzed except for eyes) or death
Thalamus
relay station of sensory information integrates incoming info
- relay between subcortical areas and cortex
- relays and integrates sensory information
*damage can lead to many different presentations
- usually a disruption in sensory integration
hypothalamus
under the thalamus, closely related to pituitary below it physiological and endocrine processes
- links the nervous system to the endocrine system
- regulates physiologial functions (thirst, hunger, temperature, fatiges, sleep, circadian shythms, sex hormones)
cerebrum
- corpus callosum - connect two hemispheres
- amygdala - emotions, especially fear
- hippocampus - formation of new memories
- cortex - voluntary sensory and motor, higher cognition
Brain structure
Bumps and grooves Bumps = gyri
Grooves = sulci
increases surface area of the cortex pack more nerves in
Gyri
bumps
sulci
grooves
Major Structures of the Brain
“Oldest” evolutionary speaking
- Cerbellum
- Brainstem
Second “oldest”
- thalamus
- Hypothalamus
- cerebrum (corpus callosum, amygdala, Hippocampus, cortex)
The brain across species
Compare brain size versus body weight
Our brains are relatively large compared to our body weight
Tastes that Use primary Messengers
Salty Sour
Tastes that use Secondary Messengers
Bitter Sweet Umami
Common to All taste receptors
depolarization opens voltage-gated Ca++ channels and causes neurotransmitter release
