Electrophysiology 1

GABA receptor activation usually has a ________ effect on the post synaptic cell

hyperpolarizing

Benzodiazepines bind to a ___________ site than GABA, causing a _______ in the receptor’s response to GABA binding

different, increase

The part of a neuron that receives input signals from other neurons is called the ________ and the part that transmits outputs is the ________

dendrite, axon

(True/False) Potassium ions travel out of the cell through leak channels at the typical resting membrane potential of -70mV.

true

(True/False) The net drive on potassium ions at a resting membrane potential of -70mV is 0.

False

(True/False) The diffusion drive on potassium at the resting membrane potential of - 70mV is 0.

false

(True/False) At 0 mV, there is no ion flow across a typical neuron membrane because the electrochemical drive is 0.

False

The equilibrium potential of sodium (Na+) is around +55mV. Given this, which of the following is true?

check all that apply

-Opening sodium channels from rest would lead to sodium ion flow into the neuron

-Sodium ions would flow out of the neuron at +65mV

-Opening sodium channels at rest would lead to membrane depolarization

-All of the above

all of the above

What is the function of the sodium-potassium pump?

-Maintain concentration gradients across the membrane

-Keep the resting membrane potential positive

-Maintain negatively charged ions on the inside of the membrane

-All of the above

Maintain concentration gradients across the membrane

Which of the following is universally true of ion channels?

-They enable non-polar molecules to pass through the membrane

-Each is permeable to a wide variety of ions

-They are composed of unique polypeptide chains

-They do not pass ions at the resting membrane potential

they are composed of unique polypeptide chains

Nissl Stain

showed that a class of basic dyes could stain the nuclei of all the cells and surrounding material, called Nissl bodies

-the dye was purple and could not express the details of a neuron

dendrites

receive inputs from other neurons

axons

‘wires’ that transmit output to other neurons

soma

cell body; integrates and processes signals, then sends the signals down the axon

True or False: input equals output

False; input rarely equals output

• a dendrite can receive 1 signal from another neuron and the information can reach the soma and trigger 800 signals to be passed down the axon and be delivered to another neuron

myelin sheath

fatty substance that protects the neuron and improves the speed of electrical information through the axon

neural networks

cells are connected within a particular brain region across the brain

interneurons

send projections to neurons within the same brain regions

-stellate cells

projection neurons

project to other regions of the brain, different from the one that they’re in

-pyramidal neurons

glial cells

• make up 50-80% of cells in the brain

• support and modulate communication across neurons

• provide structural support

• regulate blood flow

• capable of signaling although not in the same way as a neuron

voltage

an electrical charge across the membrane

permeability

state of a membrane that allows substances to pass through it

• Neuron membranes are semi-permeable

what do ion channels do to the membrane

they change the permeability of the membrane

• how open the ion channels are determines the permeability of the membrane

a typical neuron has a voltage of about

-70 mV; This is because the reversal potentials for Na+ and K+ represent two extremes, with the cell’s resting membrane falling somewhere in between. Since the membrane is more permeable to K+ and there is a higher concentration of K+ in the cell when the cell is at rest, the resting membrane potential is much closer to the reversal potential of K+ (-80mV ) than for Na+ (+55mV).

what causes the channel gates to open and close?

• voltage

• neurotransmitter binding

leak channels

channels that are always open to a particular type of ion

-potassium leak channels allow for the flow of potassium out of the cell in the resting potential phase

concentration gradient

when the concentration of particles is higher in one area than another

how does the diffusion drive regular K+ within the membrane

it pushes potassium out of the membrane in order to adhere to the concentration gradient

electrochemical drive

pushes positively charged potassium into the membrane to move toward the negatively charged cell

If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be

depolarized

If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be

hyperpolarized

membrane potential

the difference in the electrical charge between the inside and the outside of a neuronal membrane

How does the diffusion drive push K+ ions outside the membrane? How does the electrochemical drive work against that?

-when the membrane physiology changes, K+ leak channels on the membrane open, pushing K+ ions outside the membrane

-the force that pushes these ions outside the membrane is the diffusion drive

-electrochemistry explains that ions of the same charge repel each other, and opposite charges are attracted to one another. When the potassium leak channels open, the positively charged K+ ions flow outside, increasing the charge of the extracellular fluid

-The intracellular fluid of the membrane is now more negatively charged, attracting the positively charged K+ ions and pushing them back inside the membrane

-When the diffusion drive and the electrochemical drive completely counteract, an equilibrium is reached

net current flow

a type of flow associated with an ion

-If there is a positive current, the cell is hyperpolarizing

-If there is a negative current, the cell is depolarizing

sodium-potassium pump

a transport protein that uses ATP to constantly pump 3 Na+ ions out of the cell and pumps 2 K+ ions into the cell against their concentration gradient to maintain the resting membrane potential

Nernst Equation

gives the equilibrium potential for a single ion

True or False: The equilibrium potential is proportional to the ratio of concentrations inside and outside, and the polarity (+ or -) is given by the charge on the ion

true

Goldman-Hodgkin-Katz equation

-is the weighted function of the concentration of the ions weighted by their permeability

-determines the equilibrium potential for the whole membrane when the membrane is permeable to multiple ions

intracellular recordings through patchclamp electrophysiology

measuring inputs coming into the dendrites, measure the computations within the soma, and sometimes, even measure the outputs from the axon through identifying voltage changes

excitatory post synaptic potentials (EPSP)

a temporary depolarization of the post synaptic membrane caused by the flow of positively charged ions into the post synaptic cell

inhibitory post synaptic potentials

a temporary hyperpolarization of post synaptic membrane caused by the flow of negatively charged ions into the post synaptic cell

current generated post synaptic potenitals propagate through _______, to the ________

dendrites, soma

graded potentials

a small change in the membrane potential of a cell. This change in potential occurs in response to a stimulus and can be either depolarizing or hyperpolarizing

voltage gated ion channels

-they change the membrane potential, thus allowing for an action potential

-low permeability

-no flow of of preferential ion at rest

voltage gated sodium channels

-what allows Na+ to enter the cell during an action potential

-the Na+ enters the cell before K+ leaves, causing the depolarization of the cell

-when the membrane gets depolarized, voltage gated sodium channels start to open and sodium starts flowing in

-open and close in response to changes in membrane potential

action potential

sequence of voltage changes across the membrane when a signal is firing through a membrane

-all or nothing response

-the change in voltage needs to be equal to -55mV or no action potential will occur

where are action potenitals initiated?

in the axon hillock

-they are then propagated along the axon through voltage changes at the nodes of ranvier

-they pass through the axon smoothly by insulation by the myelin sheath

what do refractory periods do?

they restrict the activation of another action potential in the same area where one just occurred

saltatory conduction

currents from the action potential leap from one hap to another in the myelin sheath

-these gaps are the nodes of ranvier

the larger the axon, the _________ they propagate

faster

How is tetrodotoxin poisoning lethal?

it blocks voltage gated Na+ channels which blocks sodium from entering the membrane

channelopathies

diseases which exist due to mutations to various types of ion channels in the brain

-can contribute to conditions such as epilepsy, which involves changes in neuronal excitability

when the action potential invades the pre-synaptic terminal,

-there is a change in the conductance of Ca2+ as a result of the activation of the voltage-gated calcium channel

-the action potential comes in, and it depolarizes the pre-synaptic terminals

-that depolarization activates the calcium voltage channels

*Ca2+ has an equilibrium potential of +137mV

-Ca2+ does not contribute much to the resting potential but it plays a huge role at axon terminals in synaptic transmission

when voltage gated Ca2+ channels open,

Ca2+ will want to rush into the presynaptic terminal

-it will depolarize the membrane potential of the presynaptic cell

summary of sequence of events in synaptic transmission

-action potential depolarizes the axon terminal

-voltage gated Ca2+ channels open

-calcium triggers neurotransmitter release

-neurotransmitter binds to receptors on the post synaptic terminal

(dendrites)

-receptors gate ion channels, causing changes in the post synaptic membrane voltage (PSPs)

How does calcium trigger neurotransmitter release?

neurotransmitter is packed into vesicles in the pre-synaptic membrane

-when the calcium enters, the vesicles will bind with the presynaptic membrane and release their contents into the synaptic cleft

quanta

-the minimum amount of neurotransmitter that can be released from the presynaptic neuron (in units of quanta)

-the amount of neurotransmitter emptied into the cleft for each of the vesicles

how do we study presynaptic cells?

through using electron microscopes

-these microscopes use a beam of accelerated electrons, rather than photons, allowing them to achieve a much larger magnification (up to 10,000,000x) than the traditional microscope

what do miniature end plate potentials indicate?

the smallest possible post-synaptic response elicited by the release of a single ‘quanta’

how are quanta released at the synapse?

through exocytosis

-vesicles filled with neurotransmitters express proteins called SNARE proteins, important for docking the vesicles to the surface of the membrane

-a molecule called ‘complexin’ interacts with SNARE proteins to dock the vesicles to the membrane, getting it ready to release

-the way complexing reacts with the membrane is the vesicle load does not release until the complexin detaches from the SNARE complex

-when Ca2+ enters the cell, it binds to a protein called synaptotagmin, which serves as a calcium sensor

-when it binds to calcium, it kicks away the complexin, allowing the vesicle to fuse with the membrane and dump the neurotransmitter content into the synaptic cleft (fusion)

-this entire process happens in less than one millisecond

*mutations in SNARE proteins are responsible for a lot of neural disorders, including epilepsy, autism, and movement disorders

endocytosis

when vesicles are fused to the membrane during exocytosis, they have to be reformed at the end of the process and this reformation is endocytosi

-the vesicles are reformed and the neurotransmitter is repackaged

why does synaptic fatigue occur?

Too much stimulation depletes the pool of available vesicles

-it results in a temporary suspension of impulse transmission in the region of synapses

-the endocytosis and exocytosis cannot keep up, tiring the synapse

-depolarization is less effective

Flash and Freeze Electron Microscope

-stimulate tissue during exp and endocytosis, then high pressure freezes the tissue

-they then look at the tissue through an electron microscope to view the processes of endo and exocytosis

Basic ideas of neurotransmitter release

-Neurotransmitter is packaged into vesicles in the pre-synaptic terminal

-exocytosis of vesicles is triggered by Ca2+ which releases neurotransmitter into the synaptic cleft. This release is triggered by calcium entering the pre-synaptic terminal and binding to synaptotagamin which then changes the interaction of the vesicles with the membrane via the SNARE proteins

-Each of these vesicles released a fixed amount of neurotransmitter, producing a fixed change in voltage in the post synaptic cell via receptors on the post synaptic membrane

-Quantal size (amount of neurotransmitter release per vesicle) contributes to the amplitude of the post-synaptic response

-after the release of the neurotransmitter is released, the vesicles are recycled by endocytosis

neural types differ because of the

type of neurotransmitter they release

-they typically release one or sometimes two (rarely three)

-the type of neurotransmitter released depends on the expression of enzymes in the synthesis pathway of that neurotransmitter along with the ‘machinery’ for that release

what determines which enzymes are expressed in a given cell type?

transcription factors and gene regulation

Glutamate

(usually excitatory) produces a depolarization of the post synaptic membrane

-binds with AMPA-R, NMDa-R, and mGluR1 receptors

GABA

(usually inhibitory) produces a hyperpolarization of the post synaptic membrane

-binds with GABAa and GABAb receptors

metabotrophic receptors

bind to receptors not directly linked with ion channels but linked with G proteins

-activation of receptor changes the activation of the G protein

-binding of neurotransmitter to the metabotropic receptor can determine whether the g protein is activated or not

-when g protein is activated, it can change the expression of intracellular messengers, which can directly modulate ion channels and control whether they open or close

in some cases, it affects gene transcription and can have an impact on the membrane potential

-slow-acting (takes 100s of milliseconds to days)

-mGluR1 and GABAb are metabotropic receptors

ionotropic receptors

-directly linked to gates and ion channels

-increases membrane permeability to Na+ and K+

-the net effect of the AMPA receptor on a membrane is a combination of the increased permeability to Na+ and K+

-when glutamate binds to AMPA receptors at resting membrane potential, the channel opens and the membrane depolarizes

-its reversal potential is in between sodium and potassium’s because it is permeable to both

What happens when you have multiple excitatory and inhibitory inputs entering the neuron at once, and the IPSP signal is closer to the soma than the EPSP?

the EPSP will be blocked by the IPSP and may not have an effect on the soma

What is the end plate potential? What did the quantal release of neurotransmitter experiment show?

it is essentially a depolarization at the muscle end plate at the neuromuscular junction

-in the neuromuscular junction, an impulse at the nerve junction causes the release of acetylcholine

-The acetylcholine opens Na+ channels on the muscle end plates

-The entry of sodium into the membrane raises the membrane potential, and this change is known as the end plate potential

When testing the quanta, they put an electrode in the motor end plate and tracked how there were very small changes in the membrane of the post-synaptic cell

-these changes, called miniature end plate potentials, correspond to the release of neurotransmitters from a single quanta

-the experiments noted that changes in voltage came in increments when plotting the number of times they observed a change in voltage of a particular amplitude

-when the magnitude of the end plate potential was plotted, the histogram showed there was a large peak at .4mV and another peak at .8mV and more in a 2x pattern, exemplifying that there are multiples of a single quanta

-if there was no quantal release, there would be a uniform histogram

-the multiple peaks show that vesicles contain a fixed amount of neurotransmitter

What is the peak voltage of an action potential?

+30 mV

ionotropic receptors are _____ gated ion channels

ligand

if the membrane is suddenly hyperpolarized, the net current flow is

negative

Passive current spread

-the current from a post-synaptic potential spreads across the membrane from its initial site in the center of the membrane (at the synapse)

-the current decays as it spreads bilaterally down the membrane because of membrane permeability

• the K+ ions will flow out of leak channels

some neurons are directly linked through

gap junctions

-they link pre and post synaptic cells through ion channels

-the transmission occurs through passive current spread

the area where the vesicles fuse to the membrane to perform exocytosis is called

the active zone

Types of Neurotransmitters

(fast acting)

Act through ionotropic receptors, which activate ligand-gated ion channels, but sometimes they act through slow metabotropic receptors

-GABA (inhibitory usually)

-Glutamate (excitatory usually)

-Glycine (inhibitory usually, in the spinal cord)

Types of Neuromodulators

Act through metabotropic receptors, but sometimes they act through ionotropic receptors

-Dopamine

-Norepinephrine

-Serotonin

-Acetylcholine

-Neuropeptides

divergence

one pre synaptic neuron can influence thousands of post synaptic neurons

convergence

many different neurons can project to a single neuron

Volume transmission

occurs when chemical messengers are released from the pre-synaptic terminal into the synapse

-other molecules can spill over into the extracellular space and act on compatible receptors beyond the synaptic cleft

Reuptake

The process in which neurotransmitters are transported from the synaptic cleft back into the pre synaptic neuron

-there are specific proteins called transporter proteins which perform reuptake

pharmacology

branch of medicine concerned with the use, effects, and modes of action of various drugs

agonists

-mimic the effects of naturally occurring endorphins in the brain

-bind to receptors to activate them

-ex. heroin and other types of oxycodones/amphetamines

Antagonists

-binds to receptor sites to block the neurotransmitter

-ex. Haloperidol

allosteric modulators

drugs that bind to receptors at different sites than the primary ligand to increase or decrease the response to ligand binding

-it will modulate the response on the receptor to the ligand

-ex. benzodiazepines

• they treat anxiety and sleep disorders

• bind to a location on the GABA receptors to increase the responsiveness of those receptors to GABA

How to drugs prolong reuptake?

Drugs like cocaine act of the DAT

-they block DAT’s ability to reuptake the dopamine and repackage it into the vesicles (endocytosis), resulting in the prolonged exposure of the postsynaptic cell to receive a longer hit of dopamine

-it can cause spillover where the dopamine spills out of the synaptic cleft and diffuse to nearby neurons

How do drugs block reuptake? (THIS ONE IS NOT ABOUT TRANSPORT PROTEINS)

SSRIs block reuptake to prolong the post synaptic neuron’s exposure to serotonin as a way to treat depression

neuromodulators act within an ______ range

optimal; too much and too little greatly impact function and behavior

things that happen in presynaptic cells

-whether or not vesicles are released

-Quantal size

-# of quanta

-Reuptake (duration and speed)

-Type of neurotransmitter

Computations in post synaptic cells

-ionotropic/metabotropic receptors

• inhibitory/excitatory

• number of receptors

the same input/change in voltage to the dendrites of two different types of neurons can result in different patterns of action potential firing because of?

-morphology

-difference in expression of ion channels

simple neurocircuits: convergence

several inputs from different neurons can converge on the same post synaptic neuron

-only if A,B,and C are excited, then D is excited

-or if a, b, or c, is excited, then D is excited

simple neurocircuits: divergence

one neuron projects to many different neurons

-if D is excited, then A, B, and C are excited

-important for broadcasting a common signal