Neuro exam 2

Taste (gustation)

Detection of hydrophilic chemicals whether for distinguishing poison and food, types of food, and control of feeding`

Tastant

A chemical that stimulates the sense of taste

Saltiness

NaCl, receptor is the Na+ channel and passes through ion channels

Sourness

H+, receptor is the OTOP1 (H+ channel) and PKD2L1 (K+ channel), passes through ion channels

Sweetness

Sucrose, receptor is T1R2 + T1R3 and activates through GPCRs

Bitterness

Caffiene, T2R (~25 types), activates through GCPRs

Umami

Glutamate, T1R1 + T1R3, activates through GPCRs

How many taste buds are there on your tongue?

About 2000-5000

TRCs Taste Receptor Cells

50-150 within a taste bud, respond to stimuli (tastants), depolarize and release transmitters

Receptor potential

Stimulus induced change in membrane potential of a sensory receptor cell

TRCs preferences

Can respond to more than one basic taste but tend to have preferences for how much the cell will polarize/depolarize

TRC transmitters

Have excitatory effects on downstream neurons

Detection of saltiness

Will activate Na+ selective channels and cause depolarization (more positive membrane potential), also releases serotonin to activate gustatory afferent axons)

Detection of sourness

Through proton (H+) sensitive channels, also blocks K+ selective channels, causes depolarization and also releases serotonin to activate gustatory axons (there are multiple ways to detect sourness)

Detection of bitterness, sweetness and umami

Through GPCRs, transduction process is activating GPCRs, triggering PLC → IP3 → Ca2+ signaling cascade, causes depolarization and also releases ATP to activate gustatory afferent axons

Central taste pathway 

Taste receptor cells → Gustatory nucleus → VPM of thalamus → Gustatory cortex

Smell (olfaction)

Detection of airborne chemicals, warns of incoming harm, combines with taste for identification of food, serves as mode for communication

Organ of smell 

Olfactory epithelium

ORNs olfactory receptor neurons

Activated by odorants, chemical stimulants of smell

Mechanism of olfactory transduction

Uses Cl- in an intracellular manner, as Cl- allows Cl- to leave cell via Ca2+ activated Cl- channels, leading to depolarization

Pathway of olfactory transduction

Odorants bind to GPCRs → activates G-protein and adenylyl cyclase → Increase cAMP level → Open cAMP-gated cation channel (Na+ and Ca+ influx) → Open Ca2+-activates channel (Cl- flow out of cell) → become depolarized and fire action potential 

Simplified central olfactory pathway

Olfactory receptor cells → Glomeruli → 2nd order olfactory neurons → Olfactory cortex

Population coding

A combination of responses from a large number of broadly tuned neurons specify the identity of a particular stimulus

Glomerulus

Bulb in olfactory bulb, each one is connected to many ORNs and are the first to receive olfactory information

Sensory map

An orderly arrangement of neurons that correlates with features of the environment, created by the mapping of glomeruli and ORNS

Olfactory map

When specific spatial representation of olfactory information at the olfactory bulb occur, creating a “map”

VGCC

Voltage-gated calcium channel

Pathway for calcium and neuron activity

Neuron activation → VGCCs open → Ca2+ rise → release neurotransmitter

GCaMPs

A class of genetically coded Ca2+ indicators

Temporal coding

The representation of information encoded through the timing of action potentials rather than by their rate

Audible Range for humans

20 Hz to 20,000 Hz

Outer Ear parts

Pinna, auditory canal

Middle Ear

Ossicles

Inner Ear

Cochlea

Five stages of auditory pathway

Sound wave → Tympanic membrane → Ossicles → Oval window → Fluid in cochlea → Auditory sensory neurons (hair cells)

Ossicles

Handle sound force amplification that is then transmitted to cochlea

Organ of Corti

Contains auditory receptor cells (hair cells)

Number of chambers in cochlea

Three

Endolymphj

A liquid with 150 mM K+ and 1mM Na+

Perilymph

A liquid with 7mM K+ and 140mM Na+

Vibration pathway in ear

Vibrations travel in cochlear fluid → then lead to the vibration of the basilar membrane

Stereocilia

Part of hair cells, have a hair like structure in the apical surface

Have both inner and outer hair cells, IHC and OHC

By bending stereocilia, sounds can cause receptor potential in hair cells

Sensory transduction to depolarize hair cells

As stereocilia bends and tip link is stretched, the mechanically gated K+ channels open and K+ ions enter the cell. Then there is a depolarization and an ensuing calcium influx through VGCC, which then causes the release of the transmitter glutamate

Ratio of IHC to OHC and synaptic output to SGCs

3:1 of OHC to IHC, 1 IHC feeds about 10 spiral ganglion cells (95% of output)

Prestin

Motor protein that can be compressed to result in depolarization of OHC

OHC function for basilar membrane

OHCs amplify basilar membrane deflections, causing further stretching

Cochlear amplifier 

Loop mechanism  in which the hearing sensitivity is boosted by causing IHC to bend more through the OHC amplifying basilar membrane deflections

Characteristic frequency

The given intensity frequency at which a neuron is most responsive, auditory nerves will still experience a number of spikes per second but have a peak for which they are specially tuned for

Properties of basilar membrane BASE

Stiffer and more narrow, tends to have higher frequency for maximum amplitude

Properties of basilar membrane at apex

More flexible, wider, tends to have have lower frequency for maximum amplitude

Tonotopy

Displays high frequency at the base, low frequency at the apex

The systemic organization within an auditory structure based on the sound frequency, a sensory map

Phase locking

The consistent firing of a neuron at the same phase of a sound wave, which helps encoding for low frequency sound

When does phase locking occur

With sound waves up to 5000 Hz, high frequency sound cannot elicit phase-locked response in neurons

Mechanism at 20-200Hz frequency

Phase locking

Mechanism for 200-5000Hz

Tonotopy and phase locking

Mechanism for 5000 to 20000

Tonotopy alone

Localization of sound in horizontal plane

Requires Interaural Time Delay and Interaural Intensity Difference, also depends on sound frequency

ITD Interaural Time Delay

Difference in time for the same sound to reach each ear, works for frequency range of 20-2000Hz

Interaural Intensity Difference

Difference in intensity of the same sound at each ear, works for frequency range of 2,000 to 20,000 Hz

Superior olive sensitivity 

Has neurons sensitive to interaural time delay, as such biaural neurons (from both left and right cochlear nuclei)

Localization of sound in vertical plane

Is based on reflections from the pinna (outer ear), delays between direct path and reflected path changes as the sound source moves vertically

What makes somatic sensory system unique

Broadly distributed, responds to many kinds of stimuli, has many different kinds of sensory neurons

Somatic sensory system enables:

Ability to sense pressure, pain, and temperature, as well as proprioception and interoception

Proprioception

Ability to sense position/movement of body parts

Interoception

Sense of internal organ function

Skin

Beginning of somatosensory system, the largest sensory organ

Touch stimuli

Pressure on the skin

Mechanoreceptors

Used to detect touch

Within Epidermis (upper layer)

Merkel’s disks

Within Dermis (inner)

Pacinian corpuscles, Ruffini’s endings, Meissner’s corpuscles

Receptive field

The region of a sensory surface which, when stimulated, changes the membrane potential of a neuron

Small receptive field size

Meissner’s corpuscle and Merkel’s disk

Large receptive size

Pacinian corpuscle and Ruffini’s ending

Slow adaption

Merkel’s disk and Ruffini’s ending

Rapid adaption

Meissner’s corpuscle and Pacinian corpuscle

Small field size and rapid adaption

Meissner’s corpuscle

Large field size and rapid adaption

Pacinian corpuscle

Small receptive field size and slow adaption 

Merkel’s disk

Large receptive field size and slow adaption

Ruffini’s ending

Reasoning behind Pacinian corpuscle’s unique response profile

Due to special ending (corpuscle), as it only fires when the probe makes and breaks contact

How do mechanosensitive ion channels open

Due to the force from the lipid membrane as depolarization occurs, or due to force on extracellular structures like proteins, or from internal forces like cytoskeletal protein

Piezo1 and Piezo2

Mechanosensitive gates, non-selective cation channels that are important for touch sensation, unfurl with force

Gene knockout: Cre

A site specific recombinase that knocks out a gene, cuts out the DNA section between two LoxP sites in the same orientation

Gene knockout: LoxP

A short sequence from bacteriophage P1, which is recognized by Cre

Types of primary afferent axons for somatic sensory system

A alpha, A beta, A S and C, in decreasing order of thickness and insulation

Divisions of spinal gray matter

Dorsal horn, intermediate zone, ventral horn

Abeta axon goes to brain in spinal chord, contains the mechanoreceptors of skin

Segmental organization of spinal cord

Divided in 30 segments between four divisions

Dermatomes

The area of the skin innervated by the right and left dorsal roots of a single spinal segment

Somatotopy

The topographic organization of somatic sensory pathway in which neighboring receptors in the skin feed information to neighboring cells in the target brain structure

A sensory map for touch sensation

The mapping of the body surface on the primary somatosensory cortex

Aα fibers

For proprioceptors of a skeletal muscle

Aβ fibers

Mechanoreceptors of skin

Aδ fibers

Pain and temperature (first pain)

C fibers

Pain temperature and itch (second pain)

Nociceptors

Pain receptor neurons

Types of nociceptors

Mechanical thermal and chemical

Methods of opening ion channels in nociceptors

Strong mechanical stimulation, temperature extremes, oxygen deprivation and chemicals, as well as substances released by damaged cells (proteases, ATP, K+ ions, histamine)

Capsaicin

Spicy, activates TRPV1

TRPV1

Ion channel that responds to both heat and capsaicin

TRP family

Are cation channels activated by various external stimuli