MIDTERM 2 (L09-L17) Flashcards
What is sensation?
It refers to how cells of the nervous system detect stimuli in the environment (such as light, sound, heat, etc.), and how they transduce these signals into a change in membrane potential and neurotransmitter release
What is perception?
It refers to the conscious experience and interpretation of sensory information
what are sensory neurons?
also known as Sensory Receptors
they are specialized neurons that detect a specific category of physical events
they express receptor proteins that are sensitive to a specific feature of the external environment, such as
- the presence of specific molecules (via chemical interactions)
- smell, taste, nausea, pain
- physical pressure
- touch, stretch, vibration, acceleration, gravity, balance, hearing, thirst, pain
- temperature
- heat, cold, pain
- pH (acidity, basicity)
- sour taste, suffocation, pain
- electromagnetic radiation (light)
- vision
what are some other senses that non-human animals have?
examples:
- the ability to detect electrical and magnetic fields, humidity, and water pressure
what is sensory transduction?
process by which sensory stimuli are transduced (converted) into receptor potentials
What is a receptor potential?
Graded change in the membrane potential of a sensory neuron caused by sensory stimuli
What is a sensory neuron?
Specialized neuron that detects a particular category of physical events (sensory stimuli)
- e.g., photoreceptor (cells) transduce light into receptor potentials
what don’t all sensory neurons have?
not all sensory neurons have axons or action potentials, but they all release neurotransmitter
the sensory neurons that do not have action potentials releases neurotransmitter in a graded fashion, dependent on their membrane potential
The more depolarized they are, the more neurostransmitter they release
What are opsins?
Receptor proteins that are sensitive to light
the opsins in our eyes gain their sensitivity to light by binding a molecule of retinal
What are the four different types of opsin proteins we use to detect light?
rhodopsin
red cone opsins
green cone opsins
blue cone opsins
What are the four different photoreceptor cells in our eye?
Each photoreceptor cell in our eye contains only one kind of opsin protein, so we have four different types of photoreceptor cells:
- rod cells express rhodopsin protein
- red cone cells express the red cone opsin
- green cone cells express the green cone opsin
- blue cone cells express the blue cone opsin
what is a photoreceptor cell?
the sensory neuron responsible for vision
- they transduce the electromagnetic energy of visible light into receptor potentials
the human retina contains 4 types of photoreceptor cells; each expresses a different type of opsin protein
What is retinal?
small molecule (synthesized from vitamin A) that attaches to the opsin proteins in our eye
the retinal molecule is what actually absorbs the electromagnetic energy of visible light in our eyes
The wavelength of light it can detect depends on the opsin protein it is attached to
What type of receptors are the opsins in our photoreceptor cells? (ionotropic or metabotropic)
inhibitory metabotropic receptors
What does visible light refer to?
It refers to electromagnetic energy that has a wavelength between 380 and 760 nm
we detect this light using four kinds of photoreceptor cells (1 rod cell and 3 cone cells)
What is trichromatic coding?
The three cone opsins are sensitive to different wavelengths of light:
- blue cone opsins are most sensitive to short wavelengths
- green cone opsins are most sensitive to medium wavelengths
- red cone opsins are most sensitive to long wavelengths
What is color perception a function of?
Colour perception is a function of the relative rates of activity in the three types of cone cells (i.e., colours are discriminated by the ratio of activity across the three types of cones)
What does the amount that any cone will be activated depend on?
The amount that any one cone will be activated depends both on the wavelength of the light and the amount of it (its intensity)
If shown three colours separately (blue, green, and red) at the same intensity, what colour do people often say is the brightest?
green, because green cone opsins are the most sensitive to light
Why is paint different than light?
Paint doesn’t create light; its absorbs some and reflects some
When it comes to painting, yellow, magenta, and cyan are the three primary colours because each of these dyes only absorb (subtracts) one colour from white light
What happens when you mix chemicals that absorb all wavelengths of visible light?
you end up with a black piece of paper
What are the three dimensions of our perception of light?
brightness - intensity (luminance, amount)
saturation - purity (in terms of composite wavelengths)
hue - dominant wavelength (colour)
What happens when brightness is 0%?
Your image will be black
hue and saturation have no impact if there is no brightness
What happens when saturation is 0%?
You are in the middle of the colour cone where there is no colour (equal contribution from all wavelengths) which means you have a black and white image
what is protanopia?
absence of the red cone opsin (1% of males)
visual acuity is normal because red cone cells get filled with green cone opsin
people with this inherited condition have trouble distinguishing colours in the green-yellow-red section of the spectrum
simple mutations in the red cone opsin (1% of males) produce less pronounced deficits in colour vision
mutation in the red cone opsin hinder colour vision if they make the red cone opsin act more like the green cone opsin (in terms of what light it can detect)
What is tritanopia?
absence of the blue cone opsin (1% of the population)
blue cone cells do not compensate for this in any way, but the blue cone opsin is not that sensitive to light anyway, so visual acuity is not noticeably affected
What is deuteranopia?
absence of the green cone opsin (1% of males)
visual acuity is normal because green cones get filled with red cone opsin
- people with this inherited condition have trouble distinguishing colours in the green-yellow-red section of the spectrum
simple mutation of the green cone opsin (6% of males) produce less pronounced deficits in colour vision
What is true colour blindness called?
achromatopsia
it is typically caused by mutations in the g protein signalling cascade that is similar in all cone cells
Why do cone cells encode colour vision?
cone cells are not particularly sensitive to light, but because there are three different kinds of them, each sensitive to different wavelengths, they encode colour vision
What is the cornea?
the outer, front layer of the eye
it focuses incoming light a fixed amount
What is the iris?
it’s a ring of muscle
the contraction and relaxation of this muscle determines the size of the pupil, which determines how much light enters the eye
What is the conjuctiva?
it is a mucous membrane that lines the eyelid
what is the sclera?
it is opaque and does not permit entry of light
the white part of the eyeball
What is the lens of the eye?
the lens consists of several transparent layers
the shape of the lens can change to allow the eye to focus, a process known as accomodation
What is the retina?
the interior lining (furthest back part) of the eye is the retina
the retina contains photoreceptor cells
in humans, photoreceptor cells are classified as rod or cone cells
what is the vitreous humour?
light passes through the lens and crosses the vitreous humour, a clear, gelatinous fluid
What is the fovea?
the central region of the retina is the fovea
there is very little compression of visual information in the fovea, so this is where we have the highest visual acuity
primarily contains cone cells, each of which connects to a single bipolar cell, which in turn connect to a single ganglion cell
- thus, photoreceptors in the fovea can register the exact location of the light, enabling high resolution, colour vision
What is the optic disk?
it is where the optic nerve exits through the back of the eye
there are no photoreceptors here, so it is a blind spot in our eye
What are the three different neurons in the retina?
photoreceptors
bipolar cells
ganglion cells
What kind of cells are found in the periphery of the retina?
In the periphery, many rod cells (containing rhodopsin) converge onto fewer and fewer downstream collections of neurons (bipolar and ganglion cells)
With this compression of information, there is little information about the exact location and shape of the light in peripheral vision, but we can easily detect dim light, moving lights, and general shapes
What is fovea vision sensitive to?
detail and colour (high resolution colour vision)
What is peripheral vision sensitive to?
dim light (low resolution grayscale vision)
What do images deficient in high frequency information look like?
unfocused, but we can still make out the form
How is the high visual acuity needed for reading made possible?
The high visual acuity needed for reading is only possible in the fovea, which is primarily where the cone cells are located
What muscles move the eye around and hold it in place in the orbit?
Six extraocular muscles are attached to the sclera: the tough, outer white of the eye
What are the two different types of movement of the eye?
saccadic movements
- rapid, jerky shifts in your gace from one point to another
- our eyes scan a scene by making saccadic movements
pursuit movements allows us to maintain an image of a moving object
what are bipolar cells?
neurons that relay information from photoreceptor cells to ganglion cells
what are ganglion cells
the only neurons in the retina that send axons out of the eye
they receive information from bipolar cells and project to the rest of the brain; their axons give rise to the optic nerve, which leaves the retina through the optic disk (i.e., the blind spot of the retina)
What are horizontal cells?
neurons that interconnect and regulate the excitability of adjacent photoreceptor and bipolar cells
they adjust the sensitivity of these neurons to light in general
what are amacrine cells?
neurons that interconnect and regulat the excitability of adjacent bipolar and ganglion cells
there are many different types of amacrine cells, and each have different functions
in addition to the regular leak potassium ion channels that all neurons have, what do photoreceptor cells express?
they express “leaky” sodium ion channles which are open in the dark (when the cells are at rest)
What happens in the dark to photoreceptor cells?
in the dark, sodium continually enters through these ion channels, which depolarizes the photoreceptor cell membrane to -40 mV
at this depolarized membrane potential, photoreceptor cells continuously release glutamate
What happens when the retinal portion the retinal-opsin complex absorbs light?
it causes a conformational change (change in shape) in the opsin receptor protein
this launches a g-protein signalling cascade that closes the open sodium ion channels
the closing of these channels hyperpolarizes the membrane to -70 mV, at which point the photoreceptor cell largely stops releasing glutamate
so, photoreceptor cells are more depolarized and release more glutamate in the dark than in the light
What are the two main kinds of bipolar cells?
OFF bipolar cell
- express ionotropic glutamate receptors, so they are depolarized by glutamate
- because photoreceptor cells release glutamate in the dark, OFF bipolar cells are more active (more depolarized) in the dark than in the light
ON bipolar cells
- only have inhibitory metabotropic glutamate receptors, so they are uncommonly inhibited by glutamate
- thus, ON bipolar cells are more active (more depolarized) in the light vs. the dark
What type of neurons are ganglion cells in the retina?
retinal ganglion cells (RGCs) are typical neurons
they have action potentials and are generally excited by glutamate
What are receptive fields?
the receptive field of a neuron involved in visual processing is defined as the area of visual space where the presence of light influences the firing rate of that neuron (i.e., the part of space in which light must fall to get a response from the neuron)
How do you identify the receptive field of a particular neuron?
you record from the neuron as the animal maintains focus on a central fixation point
you then shine light in different areas of visual space (e.g., near or far from the fixation point in any direction, usually on a computer screen) and see where in visual space a change in light alters the spiking activity of the neuron
What is the first cell in the pathway of a receptive field?
when the correct wavelength of light is presented in a photoreceptor cell’s receptive field, the photoreceptor cell hyperpolarizes and becomes less active (releases less glutamate)
What is the second cell in the pathway of a receptive field?
there are two main types of bipolar cells (ON & OFF)
when light is presented in the receptive field of ON bipolar cells, they depolarize and release more glutamate
when light is presented in the receptive field of OFF bipolar cells, they hyperpolarize and release less glutamate
They respond differently to the changes in photoreceptor cell activity because they have different kinds of glutamate receptors
- ON bipolar cells only have inhibitory glutamate receptors; OFF bipolar cells have excitatory glutamate receptors
What is the third cell in the pathway of a receptive field?
retinal ganglion cells generally integrate information from many ON and OFF bipolar cells
their receptive fields often have a “center-surround” organization, and they are called ON or OFF cells, depending on whether they show increased or decreased spiking activity when light is presented in the center of their receptive field
ON cells are excited by light in the center and are inhibited by light in the surorund
OFF cells are excited by light in the surround and are inhibited by light in the center
Visual information is relayed from where to where?
visual information is relayed from retinal ganglion cells (RGCs) to the thalamus (the lateral geniculate nucleus) to area V1 in the ceberal cortex (primary visual cortex)
the receptive fields of V1 neurons are the sum of many RGCs
What are are simple cells in the primary visual cortex?
Simple cells in primary visual cortex are sensitive to lines of light, and their receptive fields are typically organized in a center-surround fashion
What is the primary visual cortex?
Also known as area V1 or striate cortex
neurons in V1 have larger receptive fields than the retinal ganglion cells
- they are most activated when a line of light in a particular orientation is detected in the receptive field
cells respond best when a line is in a particular position - a particular orientation
some neurons respond best to vertical lines, some to horizontal lines, and some to lines oriented somewhere in between
What are neurons in the eye trying to identify?
every little spot in your visual field is rigorously analyzed
is there light in that spot and is it oriented this way or that way?
the neurons are trying to identify areas where there are sharp transitions between light and dark (or between two colours)
the neurons are trying to identify borders, edges, corners
Neurons downstream of V1 are going to put all this information together to identify objects and their relative position in space
what is considered the visual association cortex?
All of the occipital lobe surrounding primary visual cortex is considered visual association cortex
visual association cortex extends into the parietal and temporal lobes, forming respectively the dorsal and ventral streams of visual information processing
What is the “where” visual stream of the brain?
The dorsal stream of visual information starts in primary visual cortex and ends in posterior parietal cortex
it is involved in identifying spatial location
it encodes where objects are, if they are moving, and how you should move to interact with them or avoid them
What is the “what” visual stream in the brain?
the ventral stream starts in primary visual cortex and ends in inferior temporal cortex
it is involved in identifying form (shape)
It encodes what the object is and its colour
what is monocular vision?
some V1 neurons respond to visual input from just one eye
What is binocular vision?
Most V1 neurons respond to visual input from both eyes
What is depth perception?
there are many monocular cues that can be used to estimate depth, such as relative size, amount of detail, relative movement as we move our eyes, etc.
these are the cues we use to appreciate depth when looking at a 2-dimensional image (e.g., on a photograph or TV screen)
What is stereopsis?
The perception of depth that emerges from the fusion of two slightly different projections of an image on two retinas
the difference between the images from the two eyes is called retinal disparity
it results from the horizontal separation of the two eyes
it improves the precision of depth perception, which is particularly helpful when trying to quickly plan movements to interact with objects moving in space
What is an agnosia?
An agnosia is a deficit (problem) in the ability to recognize or comprehend certain sensory information, like specific features of objects, people, sounds, shapes, or smells, although the specific sense is not defective nor is there any significant memory loss
What does agnosia relate to?
An agnosia relates to a problem in some sensory association cortex (typically in a single sensory modality) - not to problems that relate to the sensory neurons themselves or to the primary sensory areas
example: visual agnosia relates to damage located downstream of primary visual cortex (in visual association cortex, or the dorsal visual stream in the parietal cortex, or the ventral visual stream in the temporal cortex)
What is akinetopsia?
a type of visual agnosia caused by damage in an area of the dorsal visual stream (in the parietal lobe of the cerebral cortex)
it is a deficit in the ability to perceive movement
What is cerebral achromatopisa?
in contrast to regular achromatopsia, which is complete colour blindness due to defective cone opsin signalling, cerebral achromatopsia is a visual agnosia caused by damage to the ventral visual stream
people with cerebral achromatopsia deny having any perception of colour
they say everything looks dull or drab, and that it is all just “shades of grey”
- people with regular achromatopsia don’t say those things, because they have no conception of colour
What is prosopagnosia?
failure to recognize particular people by sight of their faces; caused by damage to the fusiform gyrus (fusiform face area)
What does descending neural activity from the top areas reflect?
to some extent, descending neural activity from the top areas reflect predictions about what the input is most likely be in the next moment (based on previous experience)
this descending information cancels out the correctly predicted ascending information, so the only information that actually ascends are errors in visual predictions
how do feedback connections function in the visual cortex wiring diagram?
each level of the network (except the lowest level, which represents the image) attempts to predict the responses at the next lower level via feedback connections
what propagates up is the prediction error signal, which is used to improve future predictions
this is the predictive coding theory of perception
what is the role of the thalamus in visual processing?
the axons leaving the retinal ganglion cells go to the:
- thalamus (specifically the lateral geniculate nucleus, which in turn projects to the primary visual cortex in the occipital lobe): visual information is processed in this pathway to determine what you are looking at
- this pathway creates an internal (mental) representation of your entire visual space –> the objects in it, their position, and their attentional value
- Midbrain (specifically the superior colliculi): visual information is used here to control fast visually-guided movements
- the midbrain doesn’t really know what you are looking at, but it knows where light is moving in visual space
- Hypothalamus: visual information is used here to control circadian rhythms (such as sleep-wake cycles)
- the hypothalamus doesn’t know what you are looking at, but it knows how much light is present in your environment
What is the somatosensory system?
the somatosensory system provides information about touch, pressure, temperature, and pain on the surface of the skin and inside the body
what are the three interacting somatosensory systems?
the exteroceptive system (cutaneous/skin senses) responds to external stimuli applied to the skin (e.g., touch and temperature)
the interceptive system (organic senses) provides information about conditions within the body and is responsible for efficient regulation of its internal milieu (e.g., heart rate, breathing, hunger, bladder)
the proprioceptive (kinesthesia) system monitors information about the position of the body, posture, and movement (e.g., the tension of the muscles inside the body)
What are the different types of external stimuli that cutaneous senses encode?
the cutaneous senses (skin) encode different types of external stimuli:
- pressure (touch) is caused by mechanical deformation of the skin
- vibrations occur when we move our fingers across a rough surface
- temperature is produced by objects that heat or cool the skin
- pain can be caused by many different types of stimuli, but primarily tissue damage
What is the outermost layer of the skin called?
the outermost layer of skin is called epidermis (“above dermis”)
cells here get oxygen from the air (not the blood)
What is the middle layer of skin called?
the middle layer is called dermis
What is the deepest layer of skin called?
the deepest layer is called the hypodermis (or subcutaneous, “below the skin”)
sensory neurons are scattered throughout these layers (epidermis, dermis, hypodermis)
What is glabrous skin?
Glabrous skin is “hairless” skin (e.g., palms of hands and feet)
What do Merkel’s disks respond to? (EPIDERMIS)
Local skin indentations (simple touch)
What are Ruffini corpuscles sensitive to? (DERMIS)
stretch and the kinesthetic sense of finger position and movement
What do Pacinian corpuscles respond to? (DERMIS)
skin vibrations
What do free nerve endings primarily respond to?
temperature and pain
Where are Meissner’s corpuscles only found?
in glabrous skin
they detect very light touch and localized edge contours (brail-like stimuli)
How does perception of temperature work?
There are two categories of thermal receptors: those that respond to warmth and those that respond to coolness
- pain information is also conveyed by some of these cells
this information is poorly localized, and the axons that carry it to the CNS are unmyelinated or thinly myelinated
some of the receptor proteins that are sensitive to temperature can also be activated by certain ligands (e.g., capsaicin molecules activate heat receptors and menthol molecules activate cold receptors)
how does perception of pain work?
Sensations of pain and temperature are transduced by free nerve endings in the skin
there are several types of pain receptor cells (usually referred to as nociceptors - “detectors of noxious stimuli”)
one type is the high-threshold mechanoreceptors (pressure receptor cells), which are free nerve endings that respond to intense pressure, like striking, stretching, or pinching
other types of free nerve endings respond to extreme heat (or the presence of chemicals such as capsaicin, the active ingredient in chili peppers)
Axons from skin, muscles, and internal organs enter the CNS via spinal nerves. What are the 2 main pathways that they can take?
- poorly localized information (e.g., crude touch, temperature, and pain) crosses over the midline in the spinal cord, just after the first synaptic connection
- this information ascends to the thalamus through the spinothalamic tract - highly localized information (e..g, fine touch) ascends ipsilaterally through the dorsal column of the spinal cord
- the first synapse in this pathway is in the medulla
- from there, the information crosses over to the contralateral side as it ascends to the thalamus
both pathways get bundled together in the midbrain before synapsing in the thalamus
- from there, information goes to primary somatosensory cortex in the parietal lobe
What is the somatosensory homunculus?
When different sites of primary somatosensory cortex are electrically stimulated, patients report somatosensory sensations in specific parts of their bodies
the relationship between cortical stimulations and body sensations is reflected in a somatotopic map of the body surface
the somatotopic map is often referred to as the somatosensory homunculus (“little man”)
What is tactile agnosia?
Patients with tactile agnosia have trouble identifying objects by touch alone
when touching an object, people think this is that:
- pine cone –> brush
- ribbon –> rubber band
- snail shell –> bottle cap
however, these patients can often draw objects that they are touching, without looking, and they can sometimes identify objects from their drawings
What is phantom limb?
phantom limb is a form of pain sensation that occurs after a limb has been amputated
amputees report that the missing limb still exists and that it often hurts
one idea is that phantom limb sensation is due to confusion in the somatosensory cortices (primary and association)
- the brain gets nonsense signals (in part from the cut axons) and it has difficulty interpreting them
What are the 6 different categories of taste receptors?
- sweetness (molecules of sugar)
- detected with a single metabotorpic receptor - umami (molecules of glutamate/glutamine)
- detected with a single metabotropic receptor
- the most common amino acid in animal products (meat and cheese)
- MSG (monosodium glutamate) activates both salt and umami receptors - bitterness (a variety of molecules)
- detected with 50 different metabotropic receptors that bind different bitter molecules - saltiness (positive ions such as sodium)
- detected with an ion channel that is highly permeable to sodium - sourness (pH level; the concentration of free hydrogen ions)
- detected with an ion channel that is highly permeable to free protons
- cells that detect sourness are also responsible for the detection of astringency (e.g., the flavour of “salty licorice”) and carbonation (bubbles), but the details are murky - fat (fatty acids)
- detected with metabotropic receptors and fatty acid transporters
How does perception of gustatory information work?
transduction of taste is similar to chemical transmission that takes place at synapses
when a tasted molecule binds to a taste receptor protein, it produces a change in membrane potential (either directly through an ion channel or through g protein signalling cascades)
different tastes relate to the activation of different types of taste receptor proteins
taste buds contain 20 to 50 taste receptor cells
- each taste bud is dedicated to processing one type of taste (sugar, umami, bitter, salt, sour, or fat), which means that every cell within a taste bud expresses the same taste receptor protein
taste receptor cells do not have traditional action potentials
- they release neurotransmitter in a graded fashion
taste receptor cells are replaced about every ten days, because they are directly exposed to a rather hostile environment
to study the taste system, researchers often manipulate the DNA of mice
- for example, to identify the sugar taste receptor, researchers remove specific genes from their genome and then test if mice can discriminate between regular water and sugar water
researchers have even created mice where the sugar receptor gene was replaced with a bitter receptor gene
- these genetically engineered mice cannot taste sugar, but they love the bitter molecule that now activates the cells in their sweet taste buds
these studies demonstrate that much taste processing is innate (hard-wried from birth to be either pleasurable or aversive)
sugar and umami taste receptor cells are instinctively rewarding/reinforcing
- direct stimulation of them (or their downstream structures in the cerebral cortex) is inherently reinforcing
- bitter taste receptor cells are instinctively aversive
- animals can grow to appreciate some bitter taste cell activity, but it is an acquired taste
interestingly, the entire cat family (including leopards, lions, tigers, cheetahs, jaguars, etc.) cannot taste sugar
- they have evolved to only enjoy the savory, salty taste of meat
Where is the primary gustatory cortex located?
primary gustatory cortex is in the insular lobe of the cerebral cortex
How does the olfactory system work?
the olfactory system is specialized for identifying specific molecules called odorants
the receptor proteins that transduce odorants into a change in membrane potential are metabotropic g protein-coupled receptors
- each one is sensitive to a specific molecule
odorant molecules are volatile substances that have a molecular weight in range of approximately 15 to 300
- most of them are lipid soluble and of organic origin, however many substances that meet these criteria have no odour
what is the olfactory epithelium?
the tissue of the nasal sinus that sits underneath the skull (the cribriform plate) and contains olfactory receptor cells
- each olfactory cell expresses only one type of olfactory receptor protein
Where do olfactory receptor cells synapse?
in glomeruli in the olfactory bulb, which in turn sends axons into the brain
How many types of olfactory receptor cells does each glomerulus process information from?
just one (expressing a particular type of olfactory receptor protein)
thus, each glomerulus processes a distinct odour
How can humans recognize up to ten thousand different odorants even though they only have ~400 different olfactory receptor protein/cell types?
through combinatorial processing
Are odours largely hard wired to be innately good or bad?
unlike taste, odours are largely not hard wired to be innately good or bad
whether we like or dislike an odour is related to learned associations
Does olfactory information relay in the thalamus?
No, olfactory information does not relay in the thalamus
it goes directly to primary olfactory cortex in the temporal lobe and the amygdala
What are pheromones?
Although most odours are not innately perceived as good or bad in young animals, pheromones are different
pheromones are molecules released by one animal to signal something to another member of the same species
- behavioural responses to pheromones are largely innate (hard-wired from birth)
pheromones strongly influence the behaviour of many organisms, from prokaryotic cells to complex multicellular animals, but their existence in humans is controversial
in many animals, especially insects, pheromones are used to:
- attract or repel other members of the same species
- signal attractiveness and sexual receptivity
- mark a path to follow (as seen in ants)
- signal danger
how does pheromone signalling work?
in mammals, the initial transduction and processing of pheromones occurs in the vomeronasal organ and “accessory olfactory bulb”, which are next to but distinct from the regular olfactory epithelium and “main olfactory bulb” which process regular odours
pheromones are detected by metabotropic vomeronasal receptors
- these receptors are only distantly related to the olfactory receptors that detect normal odours, highlighting their different role
humans, apes, and birds do not have funcitonal vomeronasal organs
- they only have regular olfactory epithelium that detects normal odours (but it is certainly possible that some pheromone-like signalling occurs in this structure in humans)
How do pheromones affect rodent behaviour?
many mammals release pheromones in their urine
- these molecules are usually not airborne
- they must be actively sniffed or tasted to be detected
rodents often sniff each other’s genitals and each other’s urine, and pheromone detection strongly influences their sexual behaviour
female to male pheromone signalling is especially powerful
- if the vomeronasal system is functional in a male mouse, they will only attempt to mate with female mice that are in heat
- if the male vomeronasal system is damaged, they try to mate indiscriminately with any mouse, male or female
male to female pheromone effects are more subtle
- females prefer males that have healthy testosterone levels (vs. castrated males), presumably because of testosterone-induced male sex pheromone signalling
what are some rodent male to female pheromone effects?
Lee-Boot effect
- when female mice are housed together (without any male urine present), their estrous cycles slow down and eventually stop
Whitten Effect
- pheromones in the urine of male mice can trigger synchronous estrous cycles in groups of female mice
Vandenbergh Effect
- Earlier onset of puberty seen in female animals that are house with males
Bruce Effect
- the tendency for female rodents to terminate their pregnancies following exposure to the scent of an unfamiliar male
THE URINE OF CASTRATED MALES DOES NOT PRODUCE THESE EFFECTS
in general, rodent male to female pheromone effects are subtle and hard to reproduce
- and human pheromone effects are notoriously hard to replicate
- although several studies have found that females living in close proximity tend to have synchronized menstrual cycles, more recent work suggests that human menstrual synchrony does not exist
What are soundwaves?
when an object vibrates, its movement causes the molecules of air surrounding it to alternately condense and rarefy (pull apart)
these fluctuations in air pressure travel away from the source as a sound wave at approximately 700 miles per hour
Under what conditions can the human ear transduce fluctuations in air pressure?
if the length of the sound wave is between 1.7cm and 17m
this corresponds to sound waves that oscillate at frequencies between 20 and 20,000 times per second
What are the 3 physical dimensions of sound?
LOUDNESS corresponds to the amplitude or intensity of the molecular vibrations
- corresponds to the total number of hair cells that are active and their overall activity levels
PITCH (tone) corresponds to the frequency of the molecular vibrations
- it is measured in hertz (Hz) or cycles per second
- moderate to high frequencies are encoded by place coding
- low frequencies are partly encoded by rate coding
TIMBRE corresponds to the complexity of the sound
- we use timbre to help identify the source of the sound wave (through learning processes)
- perceived by assessing the precise mixture of hair cells that are active throughout the entire cochlea
Describe the anatomy of the ear
- sound is funnelled through the PINNA (the outer ear)
- Sounds coming down the ear canal cause the TYMPANIC MEMBRANE (the eardrum) to vibrate
- these vibrations are transferred to the middle ear - the middle ear is comprised of three OSSICLES (small bones): the malleus, incus, and stapes
- vibrations of the ossicles are transferred to the membrane behind the OVAL WINDOW
- these vibrations are transmitted to the fluid-filled COCHLEA (the inner ear), which is a long coiled tube-like structure that contains sensory neurons
What does the basilar membrane do?
the basilar membrane encodes high notes on the end closest to the oval window
like a xylophone, the low notes correspond to the longest (widest) section
Describe the cross section through the cochlea
the cochlea is divided into three longitudinal divisions: scala vestibuli, scala media, and scala tympani
the receptive organ is the organ of Corti
- it consists of the basilar membrane on the bottom, the tectorial membrane on the top, and auditory hair cells in the middle
the cells that transduce sound are called hair cells because of their physical appearance
- their hair-like extensions are called cilia
outer hair cells have cilia that are physically attached to the rigid tectorial membrane
- the cilia of inner hair cells are not attached to anything
- they sway back and forth with the movement of the solution
sound waves cause the basilar membrane to move relative to the tectorial membrane, which causes hair cell cilia to stretch and bend
- the movement of the cilia pulls open ion channels, which changes the membrane potential of hair cells
What are inner and outer hair cells?
although there are 3 times more outer hair cells than inner hair cells, only inner hair cells transmit auditory information to the brain
outer hair cells contract like muscles to adjust the sensitivity of the tectorial membrane to vibrations
- by regulating the flexibility of the tectorial membrane, outer hair cells influence the sensitivity of inner hair cells to specific frequencies of sound (i.e., different notes)
people that do not have working inner hair cells are completely deaf
- whereas people that do not have functional outer hair cells can hear, but not very well
Explain the working of hair cell cilia
the cilia of hair cells are connected to each other by TIP LINKS - elastic filaments that attach the tip of one cilium to the side of adjacent cilium
the point of attachment of a tip link to a cilium is called an insertional plaque
each insertional plaque has a single ion channel in it that opens and closes according to the amount of stretch exerted by the tip link
What does loud noise do to hair cell cilia?
loud noises can easily break the tip links that interconnect each cilia
- and hair cells cannot transmit auditory information without tip links
fortunately, tip links usually grow back within a few hours
- tip link breakage generally corresponds to temporary hearing loss (such as after a loud bang, or loud concert)
tip link breakage is probably a protective measure, because too much glutamate release onto the cochlear nerve causes permanent cell death (excitotoxicity)
20% of 20-year-olds seem to have noise-induced hearing loss, presumably due to cochlear nerve damage
How does perception of pitch work?
the major principle of auditory coding is that different frequencies of sound produce maximal stimulation of hair cells at different points on the basilar membrane
this approach to encoding sensory information is known as PLACE CODING
- the position of the most active hair cell in the cochlea indicates the fundamental frequency (the pitch) of the sound wave
moderate to high frequencies are entirely encoded by place encoding
- human speech is in this frequency range
very low frequencies are largely encoded by RATE CODING
What is Place Coding?
because of how the cochlea and basilar membrane are constructed, acoustic stimuli of different frequencies cause different amounts of movement along the basilar membrane
- higher frequency sounds cause bending of the basilar membrane closest to the stapes, resulting in more hair cell activity in that area
what is Rate Coding?
very low frequency sounds are processed using a rate coding system: the rate of neurotransmitter release from the hair cells deepest in the cochlea (furthest from the stapes) determines the perception of low frequency sounds
What do V-shaped tuning curves tell us about sound?
Low points of three solid curves indicate that these inner hair cells will respond to faint sound only if it is of a specific frequency
if the sound is louder, cells will respond to frequencies above and below their preferred frequencies
lesions targeted to outer hair cells disrupt the responsiveness of inner hair cells to specific sounds
What are overtones?
the fundamental frequency of a sound is the lowest frequency in the wave
natural sounds are comprised of a fundamental frequency and a collection of overtones, which are generally integer multiples of the fundamental frequency
- because strings (and membranes) are clamped on each end, oscillations tend to only occur at integer multiples of the fundamental frequency
What is timber?
fundamental frequency - the lowest and most intense frequency of a complex sound
- this frequency is what is most often perceived as sound’s basic pitch
overtone - sound wave frequencies that occurs at integer multiples of the fundamental frequency
the timbre of sound refers to the specific mixture of frequencies (fundamental frequency plus overtones) that different instruments emit when the same note is played
- it is the complexity of the sound wave
- we analyze the timbre of a sound and how the timbre changes over time to identify which instrument made the sound
How do cochlear implants work?
Typically, 20-24 electrodes are positioned along the length of the cochlea
understanding human speech is often best when frequency positions corresponding to 250 Hz to 6500 Hz are stimulated
the fundamental frequency of human speech is 85-180 Hz for men and 165-255 Hz for women, but the abundance of overtones can give the impression of the fundamental tone
How does perception of spatial location work?
Phase differences and low-frequency sounds
- we detect the source of continuous low-pitched sounds by means of phase differences
- are the ears detecting compressed air at the same time? If so, the source must be equidistant from the two ears
- the auditory system can identify phase differences of sound waves under 800 Hz, because these sound waves have a half wavelength that is larger than the dimension of the head
Loudness and high-frequency sounds
- we detect the source of high-pitched sounds by analyzing differences in loudness between ears
- high frequency sounds are significantly dampened as they pass through our heads
How does timbre relate to the perception of spatial location?
Sound waves bounce off the folds and ridges of the pinna (outer most part of ear) before entering the ear canal
depending on the angle at which sound waves strike these folds and ridges, different frequencies of sound can be enhanced or attenuated (changing the timbre of the sound)
animals can analyze how the timbre changes as they move their ears or head around
- this helps them identify where the source of the sound is located
for this to work, animals must first learn to recognize subtle changes in timbre when sound enters the ears from different angles
