lec 2.5 (neuron)

Question 1

Action Potiental

Answer 1

The neuronal membrane structure and fucntion
The resting potential of nuersons
Electrochemical communication in the nervous system

• 9 glia : 1 nueron
• CNS: ogliodentrocytes
• PNS: schwann cells
• soma: cell body of nueron, NT made here (protein synthesis)
• dendrites: branches; receives synaptic input
• axon: long and contain info that flows down (conducts action potiental to the axon terminal)
• myelin: fatty insulation; (contains ogliodendrogyctes (CNS) and schwann cells (PNS); allows info to move fast (salatroy conduction)
• axon terminal/ buton: end of axon branch and sypases to another cell ; stores and releases NT; site of Nt reuptake and recycling (this is where action poteinal is concered intp a chemical signal)
• nodes: gaps in the myealin sheath on axon ; rich in voltage gated sodiumm channels; exposed to outer cellular space; allows influx of Na+ during action poteitnal; regenrates action potiental and boosts signal ; enables saltory conduction and increases speed (nueron get an influx of sodium and helps boost condution)

myelin responsinle for conductance
cystosol inside nueron
brain is made

Question 2

types of channels

Answer 2

voltage gated channels (allows dosium in )

ligrand- gated channels (are found in dendrited, where a message is delivered)

Question 3

lidocaine

Answer 3

blocks sodium channels and blocks the message (pain)

Question 4

Information Transfer in the Neuron

Answer 4

Very rapid process over long distances

Neurons use electrochemical communication
“Electro” – ions
“Chemical” – neurotransmitters
- lipids: make up wall of the nueron

The neuronal membrane is essential to this process
It is an “excitable” membrane
“Excited” or “at rest”

Question 5

action potiental notes

Answer 5

** at rest: polorized **
- more neg than postive
- depolarization (becomes more +

• ions only move to avoid other like them (electrostatic pressure)
• nuerson is a cell that has a wall and has ions flow inside and out
• at rest:
• 70 mv
• more sodium outisde

Question 6

Electrochemical Communication - Fluids

Answer 6

Ions – molecules with an electrical charge (moves in and out of nueron)
Cations – positive charge > K+, Na+, Ca++
Anions – negative charge > Cl-

Opposites attract
Cl- + Na+ = Salt

Question 7

Electrochemical Communication

Answer 7

• The neuronal membrane is specialized
• Action potential – movement of info within the axon (consists of changes in the electrical environment inside and outside the neuron.
• Coded patterns of electrical impulses

“Potential” > difference in electrical environment between inside/outside
“Action” > alterations in these environments

Resting membrane potential
Inside > K+
Outside > Na+, Cl-
more potassium inside the neuron and more sodium and chloride outside

Action Potiental begins with Ligand, but moves forward to voltage -gated

Question 8

channels

Answer 8

1. volatage-gated
   * changes when inside is more + than -
   * axon hilock and nodes > drive saltatory conduction
   * node are driven by this (action potiental)
   * moves forward here
2. ligand gated
   * NT, chnages formations to allow sodium in
   * more on dendrites
   * recieves NT
   * begins here

Question 9

Electrochemical Communication – The Membrane

Answer 9

The phospholipid bilayer

Phosphate group – polar ‘head’
* Hydrophilic – loves water

Lipid group – nonpolar ‘tail’
* Hydrophobic – hates water

“Bi”layer
* Separates the extracellular fluid from the cytosol

nonpolar and covalent

Question 10

Electrochemical Communication – Proteins

Answer 10

Ion exchange requires specialized proteins:

ion channel (passive - allow passive movement of ions across the membrane)
- open and closes
- doesnt pull in ions

ion pump (active- actively move ions, requiring energy to do so
- example: sodium potassium pump
- these do pull ions

Question 11

Ion channel

Answer 11

• very selective (only allows specfic ions based on the size of that ion channel)
• This selectivity is key to the neuron’s ability to control its internal environment.

Question 12

ion pump

Answer 12

• Ion pumps – membrane-spanning molecules that rotate
• Require energy (ATP > ADP 3 sodium for 2 potassium)
• crucial for maintaining ion gradients across the membrane

Transport ions
Na+, K+

Notes:
- binds and give energy to pull in potassium
- still leaves a postive charge
- causes the nuerson to hyerpolarize
keeps nueron at resting moves ions

Question 13

Maintenance of Ion Distribution

Answer 13

• Na+/K+ pump exchanges internal Na+ for external K+
• This requires energy > ATP
• Maintenance of proper ionic concentrations
• pump is important for maintaiing nueron’s resting state
• pump never turns off, but they are slow
• during action potiental, they turn on at high volume

Question 14

Electrochemical Communication (potiental and equilibrium)

Answer 14

• Potential - capable of being or becoming, possibility
• Equilibrium - a state of rest or balance due to the equal action of opposing forces
• These concepts are crucial for understanding how neurons maintain their resting state and generate action potentials.
• when nueron is depolarized it is closer to 0 (there is same amount of ions inside and outside)
• (the difference in charge between the inside and outside of the cell is decreasing, as positive ions flow into the cell, making the internal charge less negative and closer to the outside charge)

Question 15

Electrochemical Communication – Ion movement (diffusion)

Answer 15

• Diffusion – movement of ions from an area of high concentration to low concentration
• Ion channels allow for this

During AP: Occurs constantly, but is especially important when ion channels open, allowing ions to move down their concentration gradients.

Question 16

Electrochemical Communication – Ion Movement (electricity)

Answer 16

• electrical charges affect ion movement
• Electricity – movement of ions towards an opposite charge
• Opposites attract
• Likes repel
• Oppositely charged particles attract each other, while like charges repel.
• Electrostatic gradient
• This creates an electrostatic gradient, which, along with concentration gradients, influences ion movement across the neuronal membrane.

During AP: The movement of ions creates electrical currents across the membrane throughout the AP.

Question 17

Electrochemical gradient vs Concentration gradient vs electro

Answer 17

1. Electrochemical gradient: The combined effect of both the electrical gradient and the concentration gradient.

During AP: Always present, drives ion movement when channels open.

2 . Concentration gradient: The difference in concentration of a substance (ion) across the membrane.

During AP: Established by ion pumps during rest, drives initial ion movements when channels open.

3 . Electrical gradient: The difference in electrical charge across the membrane.

During AP: Changes rapidly as ions move, influencing further ion movement.

Question 18

During an Action Potential

Answer 18

During an Action Potential:

1. At rest: Concentration gradients are maintained by ion pumps. There’s an electrical gradient (inside negative).
2. Initiation: Na+ channels open. Na+ diffuses in, following both concentration and electrical gradients.
3. Rising phase: More Na+ enters, reducing both gradients for Na+. This creates a stronger electrical gradient for K+ to leave.
4. Falling phase: K+ channels open. K+ diffuses out, following its concentration gradient and the new electrical gradient.
5. Hyperpolarization: K+ continues to leave briefly, driven mainly by the electrical gradient.
6. Recovery: Na+/K+ pumps restore original ion concentrations against the concentration gradients, using energy (ATP).

Question 19

Electrochemical Communication – Ion Movement (Impermeable membrane, semi-permeable, no net movement)

Answer 19

Impermeable membrane
* High salt concentration inside, low outside
* Because no ions can move, there’s no electrical gradient despite the concentration difference.
* doesnt allow any ions to pass through
* mimics resting

Insert a K+ channel > semi-permeable
* Now we add a channel that allows only potassium (K+) to pass through.
* The concentration gradient causes K+ to move out of the cell (from high to low concentration).
* As K+ (positive ions) moves out, it creates an electrical gradient - the inside becomes more negative relative to the outside.

Equilibrium is established – no net movement
* K+ continues to move out until the electrical gradient (which wants to pull K+ in) balances the concentration gradient (which pushes K+ out).
* At this point, there’s no net movement of K+ - for every K+ that moves out, one moves in.
* However, there’s still a difference in charge across the membrane (inside negative, outside positive).
* Forces interact > leaves a difference in charge

concentration gradients and electrical gradients creates a stable electrical difference across the membrane, which is crucial for the neuron’s ability to generate and propagate action potentials.

This movement creates both concentration and electrical gradients

Question 20

The (real) neuron exists in two states

Answer 20

neurons have two primary states:
1. The resting state
- No net changes in onic concentration
- NO action potential

2. The active state
- HUGE changes in ionic concentration
- Action potential

Question 21

The (real) neuron exists in two states (class notes)

Answer 21

Neuron Activation and Action Potential Process

1 . Resting State:
- Resting membrane potential: -70 mV (inside negative relative to outside)

2 . Initial Excitation:
- Dendrites: Receive information via neurotransmitters (NT)
- Ligand-gated channels on dendrites open in response to NTs
- Some inputs are excitatory (EPSPs - Excitatory Postsynaptic Potentials)
- Some inputs are inhibitory (IPSPs - Inhibitory Postsynaptic Potentials)
- Net excitation must exceed inhibition to reach threshold

3 . Reaching Threshold:
- Threshold potential: approximately -55 mV
- If EPSPs sufficiently outweigh IPSPs, membrane potential reaches threshold

4 . Action Potential Initiation:
- Once threshold is reached, voltage-gated Na+ channels in the axon hillock open
- This marks the switch from ligand-gated (on dendrites) to voltage-gated (on axon) channel activity

5 . Action Potential Phases:

a. Depolarization:
- Rapid influx of Na+ causes membrane potential to become more positive
- Membrane potential rises quickly, overshooting to about +30 mV

b. Repolarization:
- Voltage-gated K+ channels open, K+ flows out of the cell
- Membrane potential begins to return towards resting level

c. Hyperpolarization:
- K+ continues to flow out briefly, causing membrane potential to become more negative than resting state

d. Return to Resting Potential:
- Na+/K+ pumps activate to restore ion concentrations
- Membrane potential returns to -70 mV resting state

6 . Propagation:
- This process occurs along the axon, allowing the signal to travel

Note: The exact voltage values can vary slightly between different types of neurons and conditions.

Question 22

The Resting Membrane Potential

Answer 22

• The voltage across the neuronal membrane at rest
• Vm (Measured by a voltmeter)
• At rest, the potential is uneven!!
• Vm = ~ -65 mV
• Inside is more negative
• Absolutely necessary for transfer of information

Question 23

Distribution of Ions at Rest

Answer 23

Vm depends on the ionic environment both inside/outside the cell

Intracellular (inside the neuron):
* High concentration of K+ (potassium) and A- (large organic anions)
* Low concentration of Na+ (sodium)

Extracellular (outside the neuron):
* High concentration of Na+ (sodium), Cl- (chloride), and some Ca++ (calcium)
* Low concentration of K+ (potassium)

K+ is more concentrated inside, Na+ is more concentrated outside

• This uneven distribution is crucial for maintaining the resting potential and for the neuron’s ability to generate action potentials. It’s maintained by the selective permeability of the membrane and the action of ion pumps.

Question 24

The Trigger – Starting an AP

Answer 24

• how an action potential is initiated
• An AP is essentially a reversal of the Resting Membrane Potential (RMP)
• It begins with tiny excitatory inputs, often at synapses on dendrites
• The AP typically originates in the axon hillock (where the axon meets the cell body)
• It requires the presence of voltage-gated Na+ channels
• Inputs add up (+,-)
• small changes in membrane potential can trigger a much larger, all-or-nothing response (the action potential) if they reach the threshold

Question 25

Voltage Gated Na+ Channels

Answer 25

**Resting State:** - When the membrane potential (Vm) is at its resting level of about -65 mV, the Na+ channels are closed. **Activation:** - If the membrane potential reaches the threshold of about -40 mV, these channels rapidly open. - This allows Na+ ions to flow into the cell, creating an inward current. - This influx of positive ions further depolarizes the membrane, causing more Na+ channels to open. **Inactivation** - Even if the membrane stays depolarized, the Na+ channels will close after about 1 millisecond. - This state is called inactivation, and it's different from the initial closed state. - During this period, the channels cannot be reopened regardless of the membrane potential. This is known as the absolute refractory period. **Recovery:** - After the membrane potential returns to its resting level, the Na+ channels recover from inactivation. - This process is called deinactivation. - The channels are now ready to be activated again if the threshold is reached. - The period when the channels are recovering but can be activated with a stronger stimulus is called the relative refractory period.

Question 26

Na+ Channel Properties in AP

Answer 26

* AP requires 1000s of channels * Channels need to reach -40 mV to open (Threshold) * They respond rapidly to depolarization * Steep incline of rising phase * They only stay open for about 1 millisecond * Brevity of the AP > very fast These properties ensure that action potentials are rapid, all-or-nothing events that can quickly propagate along the axon.

Question 27

Voltage-Gated K+ Channels in AP

Answer 27

* These channels are responsible for rapid repolarization of the membrane. * Also due to K+ channels * They are called "delayed rectifier" channels because they open slightly later than the Na+ channels. * Delayed rectifier > opens after 1 msec * There are multiple types of K+ channels, but most of them function to counteract depolarization. Explanation: The delayed opening of K+ channels is crucial for the shape of the action potential. They allow the membrane to become highly positive before initiating repolarization. This delay ensures that the action potential can fully develop and propagate before the neuron returns to its resting state. - sodium is near the walls - nuerons that dont have myelin are very leaky

Question 28

AP Conduction

Answer 28

* action potentials are conducted along the axon * AP > Positive charge depolarizes the membrane ahead of it * Match > The heat of the flame ignites the wood ahead of it * Importantly, neither the action potential nor the flame can go backwards. Explanation: This analogy helps visualize how an action potential self-propagates along an axon. The depolarization at one point causes the next segment of the axon to reach threshold, triggering an action potential there. This process continues, allowing the signal to travel long distances without diminishing.

Question 29

AP Conduction (threshold)

Answer 29

* Threshold is achieved one segment of the axon (one directional) * Voltage-gated Na+ channels open in this segment * The influx of Na+ depolarizes the adjacent segment of membrane. * Simultaneously, the previous segment undergoes hyperpolarization. Explanation: This process creates a "wave" of depolarization that moves along the axon. The hyperpolarization of the previous segment is crucial as it prevents the action potential from moving backwards, ensuring unidirectional propagation.

Question 30

AP Conduction Velocity

Answer 30

* APs typically travel from 10-100 m/s (2-225mph) * This wide range of speeds reflects the diversity of axon types in the nervous system. Factors such as axon diameter and myelination can greatly affect conduction velocity. This variation in speed is important for coordinating timing of signals from different parts of the body. **Narrow axon with many pores:** * Results in slow conduction * More interference from adjacent areas * More leakage of current **Wide axon with few pores:** * Results in fast conduction * Less interference from adjacent areas * Less leakage of current **Longer = less myelin** * axon has more space to move around The diameter of an axon significantly impacts its conduction velocity. Wider axons conduct signals faster because they have less internal resistance to current flow. Additionally, having fewer pores (ion channels) per unit area reduces the leakage of current, allowing the signal to travel further before needing regeneration. This is why large, fast-conducting axons like those involved in reflexes tend to have larger diameters.

Question 31

Saltatory Conduction

Answer 31

**saltatory conduction in myelinated axons** * "Saltatory" comes from the Latin word meaning "to leap" * Myelination of axons significantly increases the speed of AP propagation * Action potentials "leap" from one Node of Ranvier to the next * Nodes of Ranvier are gaps in the myelin sheath where ion channels are concentrated * Explanation: Saltatory conduction is a specialized form of action potential propagation in myelinated neurons. The myelin sheath acts as an insulator, preventing ion flow except at the Nodes of Ranvier. This forces the action potential to "jump" from node to node, significantly increasing conduction velocity. ** At the Nodes of Ranvier, there's a high concentration of voltage-gated sodium channels.** * When an action potential reaches a node, it depolarizes the membrane enough to trigger the opening of sodium channels at the next node, effectively "jumping" the signal forward. * This method of conduction has two major advantages: * It greatly increases the speed of signal transmission, allowing for faster neural communication. * It's more energy-efficient than continuous conduction along an unmyelinated axon, as fewer ion pumps are needed to restore the resting potential.

Question 32

Actiona Potiental (All)

Answer 32

**Resting State:** - Voltage: Typically -70 mV (inside negative relative to outside) - Concentration: High K+ inside, high Na+ outside - Electrical gradient: Inside negative, outside positive - Gates: Voltage-gated Na+ and K+ channels closed - Pumps: Na+/K+ pump active, maintaining concentration gradients - Ions: K+ slowly leaking out, Na+ slowly leaking in - Channels: Leak channels allow slow ion movement - Location: Entire neuron - Distribution: Thousands of pumps and channels across the membrane **Stimulus and Threshold** - Voltage: Rises from -70 mV to about -55 mV (threshold) - Concentration: Slight Na+ influx - Electrical gradient: Slightly less negative inside - Gates: Some voltage-gated Na+ channels begin to open - Pumps: Na+/K+ pump continues normal activity - Location: Usually starts at dendrites or soma, culminates at axon hillock - Timing: Can take milliseconds to seconds, depending on stimulus strength **Depolarization:** - Voltage: Rapidly rises from -55 mV to about +40 mV - Concentration: Rapid Na+ influx - Electrical gradient: Reverses, inside becomes positive - Gates: More voltage-gated Na+ channels open - Channels: Na+ channels fully open, K+ channels begin to open - Location: Begins at axon hillock, propagates down axon - Timing: Less than 1 millisecond - Distribution: Thousands of Na+ channels activate in a cascading fashion **Action Potential Peak:** - Voltage: Reaches about +40 mV - Concentration: High Na+ influx - Gates: Na+ channels begin to inactivate, K+ channels fully open - Location: Occurs as a wave along the axon - Timing: Moment of peak is almost instantaneous **Repolarization:** - Voltage: Falls from +40 mV back towards -70 mV - Concentration: K+ efflux, Na+ influx stops - Electrical gradient: Returns to negative inside - Gates: Na+ channels inactivated, K+ channels open - Pumps: Na+/K+ pump activity increases - Location: Follows the wave of depolarization along the axon - Timing: 1-2 milliseconds **Hyperpolarization:** - Voltage: Overshoots to about -75 mV - Concentration: Continued K+ efflux - Gates: K+ channels still open, Na+ channels inactivated - Location: Follows repolarization along the axon - Timing: 2-3 milliseconds after peak **Refractory Period:** a. Absolute Refractory Period: The period during which it's impossible to generate another action potential, regardless of stimulus strength. - Voltage: From peak (+40 mV) to near resting (-70 mV) - Gates: Na+ channels inactivated and cannot be opened - Timing: 1-2 milliseconds b. Relative Refractory Period: The period during which it's possible to generate another action potential, but only with a stronger-than-normal stimulus. - Voltage: From near resting (-70 mV) to full resting - Gates: Na+ channels recovering, K+ channels closing - Timing: Up to several milliseconds after absolute refractory period **Return to Resting State:** - Voltage: Settles back to -70 mV - Concentration: Na+/K+ pump restores original ion concentrations - Gates: All voltage-gated channels closed - Pumps: Na+/K+ pump returns to normal activity - Location: Entire neuron - Timing: Can take several milliseconds Throughout this process, there are thousands of ion channels and pumps involved, distributed across the neuron's membrane. The exact numbers can vary depending on the type and size of the neuron. The action potential typically starts at the axon hillock and propagates down the axon, with each segment undergoing this cycle slightly later than the previous one, creating a wave of depolarization that travels along the axon.