Lecture 8 - Neurons and Action Potentials

Neurons

Specialized nerve cells → transmit messages

Structure of Neurons

1. Cell body (soma)

2. Processes

   1. “an extension”

Cell Body (Soma)

• Contains nucleus

• Site of metabolism

  • “generally lots of NTs here and a lot of other metabolic processes”

Processes

Fibers that extend from the soma

Types of Processes

1. Dendrites: receives information

2. Axons: sends information

Functional Properties of Neurons

1. Irritability

   1. Ability to respond to a stimulus

2. Conductivity

   1. Ability to transmit (“conduct”) an impulse

Resting Neuron

• Plasma membrane polarized

  • “a difference of charge across the membrane”

• High concentration of Na+ outside

• High concentration of K+ inside

• Fewer + ions inside vs. outside → Resting membrane potential (RMP = -70 mV)

  • “More Na+ outside than K+ inside = more positive outside and more negative inside”

How is the RMP maintained?

1. Na⁺ / K⁺ pumps

2. Leak channels

Na+/K+ Pumps

Generate concentration gradient

• 3 Na+ out

• 2 K+ in

• Cost: 1 ATP

“outside is more positive b/c of Na+, less positive inside = RMP = -70 mV”

Leak Channels

Small Na+ leak at rest

• Why?

  • High force (large gradient), low permeability (less channels)

Small K+ leak at rest

• Why?

  • Low force (less of a gradient), high permeability (more channels)

Channels on Neurons

1. Leak channels

2. Ligand-gated channels

   1. “ligands = chemical messengers”

3. Voltage-gated channels

Leak Channels

ALWAYS OPEN

Location: throughout neuron

“help establish membrane potential”

Ligand-gated Channels

Open or close when ligand (chemical messenger) binds to receptors on the membrane

• Location: usually dendrites and soma

  • “receive chemical messengers”

Voltage-gated Channels

Opens or close when membrane potential changes

• “difference of charge across the membrane”

Location: mostly on axon, but throughout

Action Potential (AP)

Rapid, large depolarization for communication

“electrical impulse”

Stages of Action Potential

1. Depolarization

2. Repolarization

3. Hyperpolarization

Depolarization

• Na+ gates open → Na+ rushes in

  • “large concentration of Na+ outside cell, so high → low = Na+ will rush in”

• RMP: increases from -70 mV → +30 mV

Repolarization

• K+ gate open → K+ rushes out

  • “concentration of K+ is high inside, low outside, so K+ goes out = cell becomes more negative”

• Na+ gates close

• RMP: drops from +30 mV → -70 mV

Hyperpolarization

• K+ closing after RMP

  • “slow to close!”

• RMP: drops below -70 mV

  • “What gets you back to -70 mV = Na+/K+ pumps and leak channels”

How long does an AP take?

Time: 1-2 ms

“milliseconds, very fast”

Properties of AP

1. All-or-nothing principle

2. Unidirectional propagation

   1. “one direction down the axon”

All-or-nothing Principle

• Threshold (-55 mV threshold)

  • Minimum depolarization necessary to open Na+ channels → AP

• If stimulus is at or above threshold → AP of same magnitude (100 mV)

  • “-70 → +30 = 100 mV”

• Terminology:

  • Subthreshold depolarization → no AP

    • “initial depolarization doesn’t get you above -55 mV threshold”

  • Threshold depolarization → AP

    • “@ -55 mV”

  • Suprathreshold depolarization → AP

    • “above -55 mV”

    • “always same size (magnitude)

Unidirectional Propagation

AP moves down axon toward axon terminal

Depolarization in one patch → depolarization in adjacent patch

“One direction!”

“domino effect”

Voltage-gated Na+ Channels

2 gates:

1. Activation gate

2. Inactivation gate

Activation Gate

• Voltage dependent

  • “ex: gate in Na+ channel”

  • “open or close based off of changes in membrane potential”

• Opened: threshold (“@ -55 mV”) and (“throughout”) depolarization

Inactivation Gate

• Voltage and time dependent

  • “dependent on RMP and time”

• Opened: first part of depolarization

  • “majority”

• Closed: second part of depolarization

  • “small portion, right before repolarization; allow for Na+ channels to close as K+ channels open”

3 Stages of Voltage-gated Na+ Channels

1. Activation closed - Inactivation opened

   1. During resting state (-70 mV)

   2. Activation gate capable of opening with stimulus

   3. “Na+ can’t go thru”

2. Activation opened - Inactivation opened

   1. During depolarization

   2. Allows Na+ to rush into cell

3. Activation opened - Inactivation closed

   1. During 1 msec following depolarization

      1. “time dependent”

   2. Will not open until returned to resting state

      1. “-70 mV → then flip from Stage 3 → Stage 1”

   3. “Na+ can’t go thru”

What do the stages of voltage-gated Na+ channels result in?

Refractory period

Types of Refractory Period

• Absolute

• Relative

Absolute Refractory Period

• Second AP cannot be generated

  • “if you’re already having an AP, you can’t have another one b/c its already occuring”

• Na+ gates: inactivated (“not capable of opening [Stage 3]”) or already opened (“Stage 2”)

• Time: all of depolarization (“when everything is opened”) and part of repolarization (“1st part”)

Relative Refractory Period

• Second AP can be generated, but with stronger stimulus

• Na+ gates: closed (“Stage I, some closed but capable of opening = possibility of another AP”), some inactive

• Time: end of repolarization, and hyperpolarization

  • “close to -70 mV”

Refractory Periods and Stages

1. At rest

   1. STAGE 1

2. Absolute Refractory Period

   1. Depolarization

      1. STAGE 2

   2. Repolarization

      1. STAGE 3

3. Relative Refractory Period

   1. Hyperpolarization

      1. STAGE 1

*NOTE: “Hyperpolarization = you will need a bigger stimulus to occur”

Consequences of Refractory Periods

1. All-or-nothing principle

2. Unidirectional propagation

3. Frequency coding

Frequency Coding

• Intensity of stimulus is coded in AP frequency

  • Higher intensity = higher frequency

  • Lower intensity = lower frequency

NOTE: “AP always same size”

The RMP of a neuron is…

A) -55 mV

B) 30 mV

C) 70 mV

D) -70 mV

During repolarization…

A) The K+ gates close

B) The Na+ gates open

C) Na+ rushes out

D) K+ rushes out

“Na+ gates close, K+ opens”

When a neuron is at rest, the Na+ channel activation gate is ________ and its inactivation gate is ________.

A) opened; closed

B) opened; opened

C) closed; opened

D) closed; closed

Which of the following is not true regarding an action potential?…

A) It displays an all-or-nothing principle

B) It is unidirectional

C) It consistently has a 100 mV amplitude

D) All of the above are true

True or False?

The intensity of a stimulus is coded in action potential frequency.

TRUE

“High frequency = high intensity; low frequency = low intensity”