neurobiology exam 4

Optogenetics

Uses light-responsive proteins to control cellular activity with high temporal and spatial precision

Optogenetics mechanism

Light specific wavelengths triggers conformational changes in opsins, opening or closing ion channels or activating signaling pathways on targeted cells

How does channelrhodopsin-2 (ChR2) induce neuronal firing?

It is a light-gated cation channel that opens up blue light (~470nm) illumination. The influx of sodium and calcium depolarizes the membrane, generating action potentials when it reaches the threshold.

Halorhodopsin’s role in neural inhibition

It is a light driven chloride pump activated by yellow light (~590nm). It hyperpolarizes neurons by importing chloride, decreasing the probability of reaching the action potential threshold.

Step-function opsins

Exhibit prolonged open states after a brief light pulse, enabling bistable control of membrane potential

Standard ChR variants

Close rapidly, requiring continuous or pulsed illumination to maintain activation

Light-delivery methods for deep brain targets

Implanted optical fibers, gradient-index lenses for imaging-coupled access, and micro-LED probes

Opsin spectral tuning

Modifies opsins to shift activation wavelengths, enabling multi-color control. Ex) blue-shifted ChR2 variants and red-shifted opsins like Chrimson

Temporal precision

Reflects how accurately light pulses map to cellular responses. Depends on opsin kinetics, light pulses width and membrane time constants.

Strategies for Cell-type-specific opsin expression

Specificity is achieved via promoter selection, cre-lox systems with floxed opsin constructs, or viral vectors with retrograde or anterograde tropism.

All-optical physiology

Combines optogenetic stimulation with simultaneous optical readouts. Enables close-loop interrogation or circuits by reading and writing activity without electrodes

Safety and artifact considerations

Excessive irradiance can cause heating, phototoxicity, or opsin desensitization; power density and duty cycle must be managed.

Channelrhodopsin-2 (ChR2)

A light-sensitive ion channels used to activate neurons in optogenetic experiments

Archaerhodopsin (Arch)

A light-driven proton pump used to inhibit neural activity in optogenetics

Cre recombinase

An enzyme facilitating Cre-LoxP recombination, enabling genetic targeting in specific tissues or cells

Promoter

DNA sequence controlling gene expression, important for targeting optogenetic tools to select cell types

Opsins

Family of light-sensitive proteins used as tools in optogenetics for controlling cell activity with light

Viral vectors

Vehicles used to deliver optogenetic gens into cells

Onset or beginning of a seizure

Where seizures start in the brain

Level of awareness during a seizure

Tells us about the type of seizure

Focal onset

Seizures that start in one specific area of one hemisphere of the brain

Generalized onset

Seizures that begin in networks that involve both sides of the brain from the start, usually impair awareness or consciousness

Tonic-clonic

Stiffening and rhythmic jerking of the muscles during a seizure

Clonic

Jerking

Atonic

Muscles become limp or weak

Tonic

Tense or rigid muscles

Myoclonus

Brief muscle twitching