10-11-12

Lecture 10 - smell, taste, pain

Sensation and perception - collecting data from the world and interpreting

Sensation - the transduction of information by specialized neurons (receptors(, usually in the peripheral nervous system getting the data

Energy -> Action Potentials

Perception - the interpretation of sensory signals in the brain interpreting the data

Boundary between sensation and perception is ambiguous

Transduction - converting one form on energy and convert them to another form of energy (AP)

Vision and smell - get sent to thalamus and then to the cortex

Olfaction - one of the oldest senses, avoids the thalamus does not use it until later on

Connects directly with the limbic system which important for memory and emotion.

That’s why smells can produce strong emotion/memory

Other senses project to brainstem and then thalamus

SMELL (OLFACTORY)

Receptors are on the olfactory epithelium which is near the nasal pathway - neurons responsible for smell

Smells binding with neurons on your nose

Smell is more subject to adaptation - means that smells something bad, but if your in the room long enough you don’t smell it ‘nose blindness’

Olfactory bulb project to the back of the nasal passage and have neurons that have dendrites that connect directly to the air

Mice and humans possess 1000 genes for odor receptors

Humans only 350 appear to be fully functional

Many mammals have 10X more than humans

Sensitivity reflects evolutionary demands

Olfactory sensory transduction

All receptors are GPCRs - all use metatropic receptors (kinda slow)

Idea is that when something binds with smell, produced increases cyclin AMP production which lead to Na channel opening which polarizes the cell leading to a AP

‘Never grow neurons’ - not true! Olfactory receptors can be replaced throughout adult life - # stars decraseing when you are 2

Likely due to chemicals/viruses that the OE is commonly exposed to

Humans have 350 olfact. Receptor genes although we can only discriminate -5000 odors

Odors must be perceived by the activation of a particular combination of receptors

Distributed coding and coarse coding - no receptors code a specific smell, by looking at a range of receptors you can distinguish a cell

Olfaction pathway tied to limbic system - most likely due to motivational aspects - predator odor, mate selection, localizing food source

Olfactory seems to have a shortcut to your emotions

Women are better at smelling - have more olfactory receptors

How do you study smell? Sniffing when an odorant is present versus absent

Low vs high absorption

Use fmri - sagittal scan

When he told people to sniff, that activated the primary olfactory cortex was sensitive to how much you were sniffing, but didn’t care about what odor there was

Secondary olfactory cortex - seemed to care about whether there was an odor in it for not

Different ordrants absorbed at different times

Low absorption odorants - want it to be slow

One nostril has better air flow than the other, shifts over every six hours - because one is high absorption and the other one is low.

Taste receptors

Sweet, sour, salty, bitter, umami

Spicy is pain!

Tastebud have all 5 receptors in them, you can taste a flavor from any part of your tongue

Gustatroy system

Each type of papillae can detect all five tastes

Salty - Na+ is transported across cell membrane via channels leading to depolarization

Sour- acid H+ ions,block K+ channels, and induce depolarization by preventing K+ outflow

Sweet - detected by combination of two members of taste receptor family (T1R and T2R) all TRs are GPCRs

Bitter - activated by toxic substances, highly sensitive

Compromised of about 30 T2R receptors, they aren’t discriminated well in mouth because each neurons has all 30 types of receptors, the neurons respond to all of them

Umami - ‘meaty or brothy’ sensation followed by a long-lasting mouthwatering and coating sensation over the tongue

Two components that synergize to activate umami taste receptors

L-glutamate

Ribonucleotides thru variants of T1R and T3R mediate the umami taste

Taste

Genetic factors and hormones can account for some differences in taste sensitivity

Variations in taste sensitivity are related to the number of fungiform papillae near the tip of the tongue

Supertasters have higher sensitivity to all tastes and mouth sensations in general

Taste uses labeled-line coding, axons encode info relevant for each sense

If you block taste receptors for one sense, does not impair sensation of others

Other factors contribute to taste perception (smell, thermosensitive receptors)

Taste pathways to the brain

Cranial nerves Vll, lX, and X relay taste information into the CNS

Taste information first enters into the brainstem, vis the NST

Then to thalamus and then to insular cortex

PET used to see how long someone enjoyed a food

A limitation of PET is that you can only take a picture of the brain every 10 minutes

5th or 6th trial they get tired, primary insula, orbital frontal cortex = disgust

Gustation and satiation

Somatosensation

Meissner corpuscle

Free nerve endings

Merkel cells

Kenyon cells

Ruffini corpuscles

Somatosensory pathways receptor to cortex

Info from touch receptors in the head enters the CNS through the cranial nerves

Info from below the head enters the spinal cord from dorsal spinal nerves and up the cord to the CNS

Info ascends ipsilaterally (same side) in the dorsal column system -> crosses the midline to contralateral side in the brain stem

In the door, out the vent

Somatosensory cortex - caudal to the central sulcus, being this is the secondary somatosensory cortex

Primary SC - receives touch information from the opposite side of the body

Secondary SC - maps both sides of the body

Somatosensory system remaps itself as your body changes and interact with the world

Vionlist - when did they start? Use FMRI use voxels did movement of fingers activate

Controls - normal people that don’t play

Nociception - pain

Pain - discomfort normally associated with tissue damage

Conveyed by activation of receptors on free nerve endings

Nociceptors - name of snesory receptors that convey pain-realted info

Other types of pain receptors

TRPV1- pain relevant to hear

TRP2 - detects hotter temps that TRPV1

2 components of pain, e.g., from touching a hot stove – a rapid, sharp

short-lasting pain, and a slower-acting, throbbing pain:

1. A-delta fibers – large myelinated fibers; convey info rapidly to

spinal cord and brain

-transduced by TRP2 receptors

2. C fibers – small, unmyelinated fibers; convey info more slowly

-transduced by TRPV1 receptors

• Other receptors have been isolated that convey cold, and itch

sensations, and these are also conveyed via C fibers

• SCN9 gene – encodes a Na+ channel in nociceptors; people with

defects in this gene can not feel any pain

Cold sensitivity – is mediated via a novel class of TRPs,

called CMRs (cool menthol receptors)

• Diversity of TRPs (e.g., TRPV1, CMR1) play roles in

temperature, as well as taste, sensations

Examples of foods that activate these receptors:

-menthol, eucalyptus, peppermint (CMR1)

-capsaicin, garlic (other TRPs)

Pain pathways to brain

• Pain pathways ascend in the spinothalamic

tract (already contralateral once entering

the spinal cord)

• Pain-sensitive cells in the spinal cord relay

information to several areas of the brain:

– S1

– amygdala, hippocampus, prefrontal cortex -emotional

components of pain processing

– Periaqueductal gray area can block pain processing

via descending pathways back to spinal cord

Audition

• Audition refers to our sense of hearing

• Audition depends upon our ability to detect sound waves

• Sound waves are periodic compressions of air, water or other media

Properties of sound waves

• Sound is a pressure wave generated by certain types of mechanical force

• Involves longitudinal oscillations (increases and decreases) in air pressure

• Sound can be described in terms of its wave-like properties, although it is a longitudinal wave

• Similar to light in the visible spectrum, sound can be characterized by amplitude and frequency:

• frequency is measured in cycles / second (i.e., the no. of wavelengths passing over a point in space per unit time)

• pitch – term used to define the perception of a sound (usually in terms of frequencies)

• Higher frequency (i.e., more waves per second) – usually perceived as higher pitch

• loudness - is defined by the amplitude, or height, of the sound wave

• Intensity of sound usually stated in decibels (dB) – logarithmic scale (i.e., 40 dB is 100X louder than 20dB)

• most sounds are not pure tones -e.g., instruments produce harmonics

• Fourier analysis – a set of mathematical formulas for deconstructing complex wave forms (i.e.,

sound) into separable sine waves

• therefore, sound - and auditory perception - can be understood in terms of their component frequencies and amplitudes

Humans can process sound waves from about 15 Hz - 20,000 Hz

– outer ear - pinnae

– middle ear - tympanic membrane, ossicles

– inner ear - cochlea

Outer Ear

• includes the pinna, the structure of flesh and cartilage attached to each side of the head.

• Responsible for:

– Altering the reflection of sound waves into the middle ear from the outer ear.

– Helping to locate the source of a sound.

Middle Ear

• The middle ear contains the tympanic membrane (eardrum), which vibrates at the

same rate when struck by sound waves

• Connects to three tiny bones, called ossicles (malleus, incus, & stapes) that transmit

larger waves from the tympanic membrane to the smaller oval window

• oval window - membrane on the cochlea (inner ear)

– Transmits waves through the viscous fluid of the inner ear

Inner Ear

• The inner ear contains a snail shaped structure called the cochlea

• Hair cells (HCs) - auditory receptors residing in the cochlea, between the basilar and

tectorial membranes

• When displaced by vibrations in the fluid of the cochlea, hair cells excite the cells of

the auditory nerve

• Organ of Corti – collective name of all structures in scala media (hair cells, et al.) that

convert sounds into neural activity

Sound transduction

• Stereocilia – hair-like objects on hair cells, that are deflected by pressure (i.e., sound) and depolarize membrane

• Non-selective K+ and Ca2+ channels are opened in hair cells when they move

• Hair cells depolarize (i.e., via entry of K+ and Ca2+), membrane depolarization spreads to other side of cell, releases NT and activates afferent neurons that project to brain

How is sound coded into electrical impulses?

2 Basic Mechanisms:

•Temporal coding – encoding of sound as a function of number of action potentials per

second at same frequency of the actual sound

•AP generated will have the exact same pattern frequency as the soundwave

•BUT! Max AP frequency is only 1 ms

•Thus, temporal coding only works for low frequencies (i.e., <1,000 Hz), but functions up

to 4,000 Hz

•Place coding - each area along the basilar membrane has hair cells sensitive to only one

specific frequency of sound wave

•BUT, the basilar membrane is only long enough to include ~4,000 – 20,000 Hz

•This is the classic “labeled line coding” strategy

Amusia and perfect pitch

• People vary in sensitivity to pitch

• Amusia (tone deafness)- the impaired detection of frequency changes

• Maybe genetic

• speculated that amusia may be analogous to color blindness in vision

• Perfect pitch is the ability to hear a note and identify it.

• Either high accuracy or none

• More common among people who speak tonal languages (i.e., Vietnamese, Chinese)

Auditory pathways

• Afferents from the cochlea travel via the vestibulocochlear nerve (cranial nerve VIII), ipsilaterally, to the cochlear nucleus in the brain stem

• Axons then project both ipsi- and contralaterally to superior olivary nucleus

• From here: inferior colliculus medial geniculate nucleus (MGN) cortical regions important for auditory processing

• The superior olivary nucleus and inferior colliculus play important roles in sound localization (why? Think temporal segregation of bilateral inputs)

• primary auditory cortex (A1) – located in the superior temporal cortex, is the destination for most information from the auditory system

• Each hemisphere receives most of its information from the contralateral side (again, similar to visual information)

Auditory cortex

• Not involved in basic auditory discrimination; this can be accomplished at lower levels of system

• A1 is important for processing complex sounds, i.e., most in our daily experience

• Two main streams of auditory processing in cortex (similar to how visual processing works... for later):

-dorsal stream – focuses on localization

-ventral stream - analyzes components of sound

• A1 is necessary for the advanced processing of hearing.

– Damage to A1 does not necessarily cause deafness unless damage extends to the

subcortical areas.

• A1 provides a tonotopic map in which cells in the primary auditory cortex are more

responsive to preferred tones.

– Some cells respond better to complex sounds than pure tones.

• A2 responds more to changes in sound than to prolonged sounds, and also more complex sounds, e.g., animal cries, machinery noise, music, etc.

Perception as Construction

• The percept created by an object depends on the

interaction of several factors:

– Energy sources (environmental & object)

– Object properties (permanent & temporary)

– Environment properties (other objects, walls, etc.)

– Laws of physics (how does energy change over time and

space?)

– Receptor properties (sensitivity to different types of

energy)

– Previous experience and knowledge

– Algorithms and heuristics

Visual Stimuli

• Perceptual Dimensions of Color

• Hue: the dominant wavelength

• Brightness: intensity

• Saturation: purity

Receptors

• Retina

• Photoreceptors

• Rods

• Cones

• Short (~blue)

• Medium (~green)

• Long (~red)

Receptor Properties

• Some people (usually men) have only 2 types, and their perception of color is different

red/green colorblind

We have color constancy mechanisms that partially cancel the effects of different light sources

Laws of Physics

• As sound travels from a source, the intensity decreases as a function of the inverse of the square of the distance from the source

• Many laws of physics govern the reflection of light; these are used in computer animation

Deficits in Visual Perception: Scotomas

• A Discrete Region of Blindness

Deficits in Visual Perception: Akinetopsia

• Selective Loss of Motion Perception