Little change in concentration

외부의 자극에 의해 흥분되지 않은 resting 상태에서 그대로 일정한membrane potential(MP) 을 유지 ---> resting potential(RP)

• nerve, muscle 에서 항상 negative, constant, cell type 에 특징적e.g.: warm-blooded animal: -55 to -100 mV; smooth muscle: -30 mV

•이러한 RP 은 막의 채널을 통해 ion 들이 passive하게 이동함으로서 발생한다 .

•Resting 시 대부분의 열린 채널은 K+ ion, 따라서 RP 은 주로 K+ ion 에 대한 transmembrane concentration gradient 에 의해 주로 결정된다 .

A-:proteins, polypeptides, organophosphates (such as DNA, RNA, nucleotides such as ATP, and an array of nutrients.

They all behave as organic acids, giving off a hydrogen ion (that is incorporated with oxygen to form metabolic water) and leaving the negative ion inside the cell. This establishes a negative charge dispersed throughout the cytoplasm, and positive ions are attracted.

Selective permeability

Potassium equilibrium-90 mV

Balance of diffusion & attraction

Sodium equilibrium

Nernst Equation By the end of the 19th century, it was known that the cytoplasm was high in K+ and that [Na+] was very low--and that this relationship was reversed outside the cell.

The assumption was made that the cell membrane was permeable to K+ but not to Na+.

Direct measurement of the transmembrane potential was not yet possible, but an effort was made to calculate this voltage using the Nernst equation, shown at left.

E?+ (mV) = –58 mV = 58 / +1 • log (10 mM / 100 mM)

EK+ (mV) = –81 mV = 58 / +1 • log (5 mM / 125 mM)

ENa+ (mV) = +58 mV = 58 / +1 • log (150 mM / 15 mM)

ECa+ (mV) = +145 mV = 58/ +2 • log(100mM / 0.001mM)

ECl– (mV) = –58 mV = 58 / –1 • log ( 100 mM / 10 mM)

Nernst equationEion is the “equilibrium potential” for a

single permeant ion

Eion (mV) = 58 / charge • log ([ion]out / [ion]in)

Goldman equation was derived to solve for transmembrane potential using all ions involved simultaneously.

For most animal cells, the only important ions are K+, Na+, and Cl-.

Goldman includes the permiability factor, p, of each ion. Solution of the Goldman equation for a cell membrane yields an accurate model of the transmembrane voltage at any particular set of concentrations and temperatures.

the inclusion of chloride ion concentrations in Goldman equations is redundant. Omit the choride ion, and you will get the same answer.

Later, when you think about action potentials, you will realize that only the sodium and potassium ions migrate across the membrane in significant numbers, so only they are practically important in understanding membrane--ion interrelationships.

Resting Potential

lessons:

• The resting potential is based on the fact that K+ is actively pumped into all cells.

• Given the concentration gradients of K+, & the selective permeability of the resting to K+, the resting potential results by diffusion with no energy required.

• Ions pass membrane through protein pores called channels.

• Equilibrium is reached when there is a balance between tendency to diffuse & electrical attraction.

• The membrane of cells is permeable to both K+ and Na+, so both ions contribute to the resting potential.

• Na+ “leakage” into cell must be counter-acted with a Na/K exchange pump.• Electrical potential changes as current spreads along membranes.• Electrical properties of membrane determine how the potential changes over distance and over time.

Electroneutrality ChannelsMembrane potentialGeneration of a Membrane Potential ORIGIN OF THE NERNST POTENTIAL Membrane Potential Affects Ion MovementK+ channel subunit structureK+ channel, extracellular view, S5 and S6 segmentsVoltage Sensor: rest vs. activeK+ channel structure & K+ current measurementNa+ channel alpha subunit

Depolarization

The animation below illustrates how the flow of positively charged ions into the axon leads the axon to become positively charged relative to the outside.

This initial phase of the action potential is called the depolarization phase. At the end of the depolarization phase, the voltage of the inside of the axon relative to the outside is positive and the relative concentration of sodium ions inside the axon is greater than at the beginning of the action potential.

Movie: action potential

Trigeminal Ganglion Cell: this is about 2 seconds of activity that was recorded from a rat ganglion cell after a single whisker (vibrissa) was moved and held in position.

This neuron was firing about 100 action potentials every second.

Each action potential in this record is separated by about 10 milliseconds. There are 21 action potentials displayed in this picture of the recording - count them!

Action potential recording

The action potential indicates what happens when the neuron transmits information from one cell to another. Neuroscientists use other words, such as a "spike" or an "impulse" to describe the action potential.

The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire.

Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a complete action potential is fired - this is the "ALL OR NONE" principle.

The "cause" of the action potential is an exchange of ions across the neuron membrane. A stimulus first results in the opening of sodium channels. Since there are a lot more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized.

It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization).

The action potential actually goes past -70 mV (a hyperpolarization) since the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.

A.P. due to V-gated channels

TTX?

Distribution of channels

Leak channels everywhere

Voltage-gated Na+ channel

But “ready” Not “ready”Pass current

@ “threshold voltage”

Positive feedback loop

Reach “threshold”?

If YES, then...

Action potential initiation

S.I.Z.

Action potential terminationThink

“votes”

Voltage Clamp

If resistance decreases and current stays the same, then voltage also decreases so that the result of V/R stays the same - I. So if R decreases and I is constant, V decreases.

V is held constant while R is decreasing. If R decreases, then I must increase to keep V constant. So if V is constant while R is decreasing, then I must increase.

If you can measure I and V, then R can be easily calculated by re-arranging the equation to:

This is the same as question 2. If voltage is constant and resistance decreases, what happens to current? Current must increase if voltage remains constant when resistance deceases. You can see this in the figure on the right - ion channels open and positively charged ions can flow into the cell.

Sodium Conductance Activation and Inactivation

Changes in sodium and potassium conductance with different depolarizations.

activation & inactivation. When inactivation is present, sodium channels cannot open again. The peak conductance increases with greater depolarization.

inactivation slowly goes away, eventually allowing sodium channels to open again if depolarization occurs.

During the period of inactivation, it is more difficult to increase gNa, and therefore the presence of inactivation underlies refractoriness, i.e. the inability to trigger an action potential for some time after another action potential has occurred.

At membrane potentials above +50 mV sodium actually exits the nerve cell. At +50 mV there is no sodium flow

timing of important events in the cell

sodium influx is at a maximum at about 1 millisecond. Potassium efflux on the other hand is at a maximum at about 3 ms.

TTX (tetrodotoxin) effectively blocks the sodium channels in nerve cells. A drug called TEA (tetraethylammonium) will effectively block potassium flow and allow for the testing of membrane conductance and timing for sodium influx.

Voltage clamp analysis

Refractory periods

Hodgkin-Huxley equations

Hodgkin-Huxley equations describe the generation and propagation of the action potentials in neurons in terms of the dynamics of the potassium and sodium channels in the axon membrane. The conductance of the channels is described in terms of internal state variables of the channel (h, m and n below). The probability that a channel will open is a function of the state variables, the total conductance of a population channels is the product of the channel opening probability, the single channel conductance and the total number of channels. The Hodgkin-Huxley equations are:

Where each of n, m and h satisfy equations of the type:

Here w, z, , , Ileak, VNa and VK are experimentally determined constants, e and k are constants of nature, T is the temperature and and experimentally determined, voltage dependent, rate constants. An input current injection, Iinj may cause the membrane potential to exceed a threshold so that it starts firing action potentials. Action potentials last around 10ms, and the important time scale is about 10-100 s.

Propagation speed

Action Potential Propagation

A.P. propagation

A.P. propagation

The animation to the left shows an idealized neuron. The red ovals represent Na+ ion channels and the green ovals represent K+ ion channels.

You can start the movie and it will show an action potential (shown as a colored curve) move down the axon. When the sodium channels open during the depolarization (the red section of the action potential curve), the Na+ rushes in because both of the greater concentration of Na+ on the outside and the more positive voltage on the outside of the axon.

When the Na+ channels close and the K+ channels open (the green section of the action potential curve), the K+ now leaves the axon due both to the greater concentration of K+ on the inside and the reversed voltage levels.

Thus, in many ways the action potential is not the movement of voltage or ions but the flow of these ion channels opening and closing moving down the axon. This movement of the ion channels explains why the action potential is slow relative to the normal flow of electricity. The normal flow electricity is the flow of electrons and travels at the speed of light while these ion channels movement is considerably more slowly. These are mechanical movements and cannot move nearly at the speed of light.

Myelin wraps around a nerve cell axon in the spinal cord. In the CNS, glial cells called oligodendrocytes produce myelin, which is composed of multiple layers of oligodendrocyte membranes that wrap concentrically around one or more axons. (In the PNS, Schwann cells form myelin.)

The myelin sheath around an axon acts like an electrical insulator, allowing nerve impulses to be conducted very quickly.

Myelin appears white and shiny because the layers of membrane that form it contain large amounts of fatty substances called lipids.

The central nervous system consists of neurons and glial cells. Neurons constitue about half the volume of the CNS and glial cells make up the rest.

Glial cells provide support and protection for neurons. They are thus known as the "supporting cells" of the nervous system. The four main functions of glial cells are:

1. to surround neurons and hold them in place,

2. to supply nutrients and oxygen to neurons,

3. to insulate one neuron from another, and

4. to destroy and remove the carcasses of dead neurons (clean up).

The three types of CNS supporting cells are Astrocytes, Oligodendrocytes, and Microglia. The supporting cells of the PNS are known as Schwann Cells.

AstrocytesAstrocytes are star-shaped glial cells of the CNS. The end-feet processes line blood vessels and make up part of the blood-brain barrier. Bar = 30 Microns

Microglia are the smallest of the glial cells.

Some act as phagocytes cleaning up CNS debris.

Most serve as representatives of the immune system in the brain.

Microglia protect the brain from invading microorganisms and are thought to be similar in nature to microphages in the blood system.

Electrical synapses are so-called because they permit the rapid flow of charge directly from the cytoplasm of one cell to another. The name given to such synapses are GAP JUNCTIONS.

Electrical synapse (gap junction)

Ion channels

Electrical synaptic current

- gap junction channels have a large conductance.- NO synaptic delay (current spread from cell to cell is instantaneous). However, chemical synpases (below) do have a significant delay.- commonly found in other cell types as well i.e. glia.- can be modulated by intracellular Ca+2, pH, membrane voltage, calmodulin. - clusters of proteins that span the gap such that ions and small molecules can pass directly from one cell to another-6 protein subunits arranged around a central pore, made up of the connexin protein.- two sides come together to make a complete unit of 12 proteins around the central pore.- cloned from many tissues and organisms and are extremely well conserved.

Discovery of Neurotransmitters Back in 1920, an Austrian scientist named Otto Loewi discovered the first neurotransmitter. In his experiment (which came to him in a dream), he used two frog hearts. One heart (heart #1) was still connected to the vagus nerve. Heart #1 was placed in a chamber that was filled with saline. This chamber was connected to a second chamber that contained heart #2. So, fluid from chamber #1 was allowed to flow into chamber #2. Electrical stimulation of the vagus nerve (which was attached to heart #1) caused heart #1 to slow down. Loewi also observed that after a delay, heart #2 also slowed down. From this experiment, Loewi hypothesized that electrical stimulation of the vagus nerve released a chemical into the fluid of chamber #1 that flowed into chamber #2. He called this chemical "Vagusstoff". We now know this chemical as the neurotransmitter called acetylcholine

Chemical synapses

Synaptic knobs

-a chemical transmitter is released and diffuses to bind to receptors on postsynaptic side- bind leads (directly or indirectly) to changes in the postsynaptic membrane potential (usually by opening or closing transmitter sensitive ion channels)

-- the response of the neurotransmitter receptor can depolarizes (excitatory postsynaptic potential; epsp) or hyperpolarizes (inhibitory postsynaptic potential; ipsp) the post-synaptic cell and changes its activity- significant delay in signal (1 msec) but far more flexible than electrical synapse

Chemical Synapse-most common type of synapse

-- electrical signal in the presynaptic cell is communicated to the postsynaptic cell by a chemical (the neurotransmitter),

-- separation between presynaptic and postsynaptic membranes is about 20 to 30 nm

Basic Function of Chemical Synapse

1. Nerve impulse arrives at pre-synaptic terminal 2. Depolarization causes voltage-gated Ca+2 channels to open

- increases Ca+2 influx, a transient elevation of internal Ca+2 ~100 mM 3. Vesicle exocytosis

- increase in Ca+2 induces fusion of synaptic vesicles to membrane- vesicles contain neurotransmitters

4. Vesicle fusion to membrane releases stored neurotransmitter 5. Transmitter diffuses across cleft to postsynaptic side 6. Neurotransmitters bind to receptor either:

i) ligand-gated ion channel or ii) receptors linked to 2nd messenger systems

7. Binding results in a conductance change - channels open or close or - binding results in modulation of postsynaptic side

8. Postsynaptic response - change in membrane potential (e.g. muscle contraction in the case of a motor neuron at a neuromuscular junction)

9. Neurotransmitter is removed from the cleft by two mechanismsi) transmitter is destroyed by an enzyme such as acetylcholine esteraseii) transmitter is taken back up into the pre-synaptic cell and recyclede.g. - acetylcholine esterase, breaks down acetylcholine in cleft, choline is recycled back into the pre-synaptic terminal

Neurotransmitter Criteria must be produced within a neuron. must be found within a neuron. When a neuron is stimulated (depolarized), a neuron must release the chemical. When a chemical is released, it must act on a post-synaptic receptor and cause a biological effect. After a chemical is released, it must be inactivated. Inactivation can be through a reuptake mechanism or by an enzyme that stops the action of the chemical. If the chemical is applied on the post-synaptic membrane, it should have the same effect as when it is released by a neuron.

Neurotransmitter Types

Vesicle Exocytosis

- a group of 6 to 7 proteins work together to respond to Ca+2 influx and regulate vesicle fusion- many of these proteins are common to other secretory pathways- after exocytosis the synaptic vesicle membranes are reinternalized by endocytosis and reused (reloaded with neurotransmitter by a transmitter transporter system)

-vesicles are also transported from the cell body to the nerve terminal transmitter is synthesized in the terminal and loaded into the vesiclesenzymes and substrates necessary are present in the terminal

-- i.e. acetylcholine, acetyl-CoA + choline used by choline acetyltransferase

i) non-peptide transmitters - exocytosis only occurs after an increase of internal Ca+2 (due to depolarization) - at active zones (regions in the presynaptic membrane adjacent to the cleft)- a group of 6 to 7 proteins work together to respond to Ca+2 influx and regulate vesicle fusion

ii) peptide-transmitters (same as for non-peptide transmitters except:)- exocytosis is NOT restricted to active zones- exocytosis is triggered by trains of action potentials

Neurotransmitter Synthesis

all transmitters except peptides are made in the nerve terminal

i) non-peptide transmitters - fast synaptic signalling

- synthetic enzymes + precursors transported into nerve terminal

- subject to feedback inhibition (from recycled neurotransmitters)

- can be stimulated to increase activity (via Ca+2 stimulated phosphorylation)

ii) peptide transmitters

- peptide neurotransmitters are made from large precursor proteins in the cell body

- specific proteases cleave the precursor into the appropriate peptides (occur in the cell body, in the vesicle during transport or at the nerve terminal)

- responsible for modulatory signalling and distant signalling

Neurotransmitter Packaging

into vesicles

- neurotransmitters packaged into vesicles

i) for small non-peptide neurotransmitters

- packaged in small "classical" vesicles

- involves a pump powered by a pH gradient between outside and inside of vesicle

- pump blocked by drugs and these block neurotransmitter release

ii) for peptide neurotransmitters - packaged into vesicles in the cell body and transported to terminals (anterograde transport) - found the large dense core vesicles

Some types of chemical synapse include:i) Excitatory - excite (depolarize the postsynaptic cell)ii) Inhibitory - inhibit (hyperpolarize the postsynaptic cell)iv) Modulatory - modulates the postsynaptic cells response to other synapses

Directly acting neurotransmitters

Many receptors are physically part of an ion channel.

Binding neurotransmitter to a receptor on the postsynaptic cell causes a change in the shape of the receptor.

This can open, or in some cases close, the ion channel.

Neurotransmitters that bind to ion channels are said to act directly.

They cause a brief, rapid change in the membrane potential of the postsynaptic cell.

Directly-acting neurotransmitters include acetylcholine, glutamate, GABA, and glycine.

Ion channels typically have multiple binding sites for neurotransmitters and require the binding of more than one neurotransmitter molecule to open or close the channel.

Ligand-gated ion channels (ionotropic receptors)- binds neurotransmitter and opens ion channel- allows ions to flow in or out of cell,i) Na+ channels - excitatory (generates an excitatory postsynaptic potential)ii) Cl- channels - inhibitory (generates an inhibitory postsynaptic potential)- brief time period- classic synaptic transmissione.g. acetylcholine, GABA, glycine, glutamate- the specificity of a transmitter response is a function of the receptor type NOT the transmitter itself. (i.e. Ach can be excitatory when binding to one type of AchR (NMJ)) and inhibitory when binding to another type of receptorThe following (Table 21-1) list some ligand gated ion channels and the ions they are permeable to.

Metabotropic receptor

(2nd messenger system)

Metabotropic transmission is

amplified

Some neurotransmitters bind to receptors that are separate from ion channels. This process most often leads to production of intracellular second messengers, which ultimately alter ion channels. Such neurotransmitters are said to act indirectly.

The receptor is coupled to the ion channel by a G protein. At rest, guanosine diphosphate, or GDP, is bound to the G protein. When norepinephrine binds to the receptor, the G protein is activated, releases GDP, and binds guanosine triphosphate, or GTP. GTP is a high-energy molecule. Part of the activated G protein travels in the membrane and activates an enzyme, which induces production of a second messenger. The neurotransmitter is the first messenger. The second messenger activates an intracellular enzyme, which phosphorylates a potassium ion channel and closes it. The resulting synaptic potential is slow in onset, and long in duration. Besides excitation, indirectly-acting neurotransmitters can also produce slow inhibition.

The neurotransmitters acetylcholine, glutamate, GABA, and serotonin can act indirectly as well as directly, depending on the receptor to which they bind. The catecholamines (norepinephrine, epinephrine, and dopamine) and peptide neurotransmitters ONLY act indirectly.

Neurotransmitter Receptors There are two main types of receptors ionotropic receptors and metabotropic receptors.

When an ionotropic receptor binds with its neurotransmitter it opens a gate or channel of some sort in the membrane.

Glutamate, GABA receptor

When a neurotransmitter binds to a metabotropic receptor a chain of reactions are set into motion. The end result of these reactions may be to alter the shape of the cell slightly, change the way in which the neuron makes certain proteins or even to open ion channels. Ionotropic receptor effects act immediately and last 10-20msec, faster then metabotropic effects which can take up to 30msec to start and can last for as long as a second.

Many neurons also contain receptors for neuromodulators these substances only work in conjunction with ‘classic’ neurotransmitters and make the neuron slightly more or slightly less excitable.

Finally there are often several receptors for each neurotransmitter - some neurotransmitters even have both ionotropic and metabotropic receptors. Each receptor type has slightly different effects on postsynaptic neurons - for example they may only be effective when another chemical is present. This adds another layer of complexity to brain function (and understanding it!).

Chemically gated Ion Channels in Neuron

located on the dendrites, and the cell body. Chemically-gated channels are responsible for producing synaptic potentials.

Voltage gated Ion Channels in Neuron

Most voltage-gated channels are found on the axon hillock, all along unmyelinated axons, and at the nodes of Ranvier in myelinated axons.

responsible for generation and propagation of the action potential.

Synapses in the brain or central nervous system (CNS)

-a single synapse on a target is seldom found in brain

-large neurons in the brain typically receive many inputs (1000 to 80,000 per cell)

-the inputs are integrated in the receiving neuron such that a "decision" is made to pass on the information onto other cells

-this "decision" is often whether or not to generate an action potential

-each synaptic input usually only gives only a small depolarization so many inputs must cooperate (or summate) to reach threshold to fire an action potential

-for example in the case of the motor neuron to get an epsp of +20 mV would need in the order of 20 terminals to simultaneously discharge (process called summation)

-Movie: synapse

EPSP - an excitatory impulse, an excitatory post-synaptic potential raises the membrane potential above rest

i) an excitatory impulse at a synapse on the soma causes a depolarization of the whole soma including the beginning of the axon. This is because the diameter of the soma or cell body is so large that the internal resistance is very low so current flow extremely well through the cell body.

- the beginning of the axon is also known as the spike initialization zone or axon hillock and is so packed with Na+ channels, an epsp of +15 to +20 mV triggers an action potential in the zone

ii) an epsp in a dendrite has less of a chance of triggering an action potential than an epsp generated at the soma

in theory an action potential can also be triggered in the soma and dendrites but because there are so few Na+ channels this is very unlikely.

IPSP - an inhibitory impulse is called an i.p.s.p (inhibitory post-synaptic potential) and lowers the membrane potential below rest (hyperpolarizes)

-synaptic transmission triggers the opening ligand gated Cl- channels or indirectly through other mechanisms the opening of K+ channels

-Cl- flows into the cell- K+ flows out of the cell - hyperpolarizes the soma- harder to trigger an action potential therefore inhibitory

- an ipsp on the dendrite will have less effect due to current loss than in ipsp in the soma.

binding to GABAA receptors opens chloride channels in the neuron. binding to GABAB receptors opens potassium channels.

Synaptic current (in or out)

Chemical synaptic transmissionA.P. invasion

E.g. Acetyl choline synapse

Presynaptic cell

Postsynaptic cell

Ca++ entry / N.T. release

ACh receptor/channel

ACh current both Na+ and K+

Movie

N.T. binding / channel opening

Spread of ions (“potential”)

Termination

diffusion

breakdown

reuptake

Inactivation of NeurotransmittersThe action of neurotransmitters can be stopped by four different mechanisms.

1. Diffusion: the neurotransmitter drifts away, out of the synaptic cleft where it can no longer act on a receptor.

2. Enzymatic degradation (deactivation): a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor. For example, acetylcholinesterase is the enzyme that breaks acetylcholine into choline and acetate.

3. Glial cells: astrocytes remove neurotransmitters from the synaptic cleft.

4. Reuptake: the whole neurotransmitter molecule is taken back into the axon terminal that released it. This is a common way the action of norepinephrine, dopamine and serotonin is stopped...these neurotransmitters are removed from the synaptic cleft so they cannot bind to receptors.

ACh recycling

Synaptic current

Passive current flow in dendrites and cell bodies- dendrites extend 0.5 to 1 mm in all directions from soma and receive signals from a large area- 80-90% of all presynaptic terminals terminate on dendrites- most can't produce action potentials (too few or no Na+ channels) - transmit current by passive spread down dendrites to the soma- therefore the membrane potential decreases as move along dendrite due to current loss thanks to our friends ri, rm and cm- because dendrites have no voltage gated Na+ channels (few exceptions to this rule) and cell bodies have little or no voltage-gated Na+ channels current flow is solely dependent on the Cable Properties of the dendrites and soma - called cable because analogous to properties of long copper telecommunication cables

Inhibitory current

Inhibition Movie

Excitation v. inhibition

Temporal summation at 1

synapse

Postsynaptic potential

Spatial summation of 2 synapses

Distant synapses less effective

Integration Movie

Shunting inhibition

Facilitation to multiple spikes

http://faculty.uca.edu/~jmurray/BIOL4425/lec/lectures.asp