Electrophysiology of Neurons - 11.3

a little bit of a review on electrophysiology from Unit 3 as well

Excitability (relative to neurons)

all neurons are responsive in the presence of various stimuli:

• chemical signals, local electrical signals, and mechanical deformation

What is conductivity?

stimuli generate electrical changes (action potentials) and are conducted/spread throughout the whole membrane

What are the 2 forms of electrical changes that occur in neurons?

1. Local Potentials: electrical changes across a neuron’s plasma membrane that travel only short distances

2. Action Potentials: electrical changes that travel the entire length of an axon

Electrophysiology

the study of electrical changes across the plasma membrane and the accompanying physiological processes

resting membrane potential (RMP)

charge difference across a plasma membrane caused by unequal distributions of ions on either sides of the plasma membrane

• (-) inside the membrane

• (+) outside the membrane

What is voltage and how is it established?

separation of charges, which forms an electrical gradient

Membrane potential?

the electrical gradient (voltage difference) across the plasma membrane, which is a source of potential energy

What is the resting membrane potential of a neuron?

-70 mV

When is a cell polarized?

when the membrane potential is not at 0 mV. it can either be positive or negative

Ion concentration gradients favor the diffusion of potassium (K+) ions __ of the cell through leak channels and sodium (Na+) ions _ the cell

out, in

• potassium is the only one that goes outside of the cell

What are electrochemical gradients and what does it determine?

• concentration and electrical gradients collectively

• determines direction of diffusion of an ion across the membrane

How is RMP generated?

1. a cell starts with a negative RMP

2. there is an unequal distribution of ions across the plasma membrane, Na+, K+, Cl, and Ca, which cause gradients

3. gradients are also maintained by the Na+/K+ ATPase pump

*big idea: Pumps and the combination of unequal distributions of ions create RMP*

Na+/K+ pump

requires ATP, primary active transport

makes sure more Na+ is outside and more K+ is inside

Local Potentials (graded)

small changes in the membrane potential of a neuron’s plasma membrane that may have caused either depolarization or hyperpolarization

Why are local potentials called graded potentials?

they can vary in size; some produce larger change in MP than others

The degree of the change in MP depends on:

• length of stimulation

• # of ion channels open

• type of ion channels open

What does it mean when local potentials are reversible?

when the stimulus that caused the ion channels to open stops, the neuron quickly returns to its RMP

What does it mean when local potentials are decremental?

local potentials weaken over long distances

• changes in MP caused by local potentials are small and lost across the membrane over distance as they travel away from the site of stimulation

Action Potential

uniform, rapid depolarization and repolarization of the membrane potential of a cell

• these changes in MP causes a response or action

What can trigger action potentials and local potentials?

• AP’s: only axons

• LP’s: dendrites and soma

Action Potentials are only generated in trigger zones that include:

• axon hillock

• node of ranvier (myelin sheath gap)

• initial segment of axon

Voltage-gated K+ channels have two possible states:

• resting (closed)

• activated (open)

Voltage-gated Na+ channels have two gates; an activation gate and an inactivation gate along with three states:

1. Resting: inactivation gate open, activation gate closed

2. Activated: BOTH inactivation and activation gates open

3. Inactivated: inactivation gate closed, activation gate open

Depolarization phase

the membrane potential rises towards 0 and becomes briefly positive

Repolarization phase

the membrane returns to a negative value and approaches RMP, but does not settle at RMP

Hyperpolarization phase

the MP temporarily becomes more negative than the RMP during this phase

What mV is threshold at?

usually at -55 mV

What are the steps for action potential?

1. A local potential must be able to depolarize the axon to reach threshold (-55 mV)

2. Once threshold is reached, voltage-gated Na+ channels activate and Na+ ions are able to flow into the axon causing it to depolarize

3. Na+ ion channels inactivate and voltage-gated K+ ion channels activate. Na+ ions stop flowing into the axon and K+ begins exiting the axon as repolarization begins

4. The axolemma may hyperpolarize before potassium ion channels return to the resting state; after this, the axolemma returns to its RMP

Big Idea:

• *Sodium channels = depolarization*

• *Potassium channels = repolarization*

What is the Refractory Period?

period of time, after a neuron has already generated an AP, when the neuron cannot produce another AP, and the membrane cannot be stimulated to fire another one

What are the 2 phases of the refractory period?

1. Absolute Refractory Period

2. Relative Refractory Period

What is the absolute refractory period?

no additional stimulus, no matter how strong, can produce another AP

• occurs during the period when the voltage-gated Na+ channels are activated and inactivated

• cannot occur so long as an AP is in motion

What is the relative refractory period?

only a strong stimulus will produce an action potential

• occurs when voltage-gated Na+ channels return to resting state (inactivated gate), despite voltage-gated K+ channels being open

• a stronger stimulus is required to stimulate another AP at this point because the MP is hyperpolarized

What is the all-or-none principle?

an event can either happen completely or not at all

• when applied to action potentials, if the neuron cannot depolarize the axolemma to a certain threshold, the action potential does not occur

Graded vs Action Potentials

1. Change

• graded - variable change

• action - maximum depolarization change to +30 mV

2. Reversible

• graded - reversible; neuron returns to RMP when stimulus ends

• action - irreversible; all or nothing principle

3. Distance Traveled

• graded - decremental

• action - nondecremental

Propagation

an AP must be conducted or propagated along the entire length of the axon to serve as a long-distance signaling service

Current

a flow of charged particles (ions) created by the propagation of an AP across the axon

An action potential is _ and travels in one direction at a constant speed from the trigger zone to the axon terminals

Self propagating

Events of Propagation down the axon

1. Depolarizing current from an AP propagates through a section of the axolemma

2. The depolarizing current triggers adjacent voltage-gated Na+ ions to open, firing another AP. the previously depolarized section of the membrane repolarizes and is refractory

3. This process repeats as the next section of the axolemma depolarizes to threshold and fires an AP, and the previous section repolarizes

4. The current propagates down the axon until the axon terminal is reached

Big Idea: AP’s continue to propagate down the axon by the continuous triggering of depolarization down the membrane, generating more AP’s

Why can’t AP’s go in both directions?

because of the refractory period

• voltage-gated Na+ ions are activated

Conduction speed

the rate of propagation that is influenced by both the axon diameter and the presence/absence of myelination

Diameter of the axon in conduction speed

larger diameter: faster conduction speed, less resistance to conduction of the current

Presence/Absence of Myelin Sheath relative to conduction speed, what are the 2 types?

1. Saltatory Conduction

2. Continuous Propagation (Conduction)

What is Saltatory Conduction?

Myelin Sheath is present

• myelin acts as an insulator of electrical charges, creating a more efficient flow of the current, resulting in faster conduction speeds

• myelin being an insulator means that it prevents currents from leaking out through the axolemma

Big idea: myelin present, insulation provided, signal decreases in strength very little because it travels through internodes

What are the steps of Saltatory Conduction?

1. The myelin sheath gaps are the only unmyelinated segments that must be depolarized to threshold

2. When the gap is depolarized via voltage-gated Na+ ion channels, an AP is triggered

3. The AP generates a current that flows passively with little loss of charge through the myelinated segment (internode)

4. When the current reaches the next myelin sheath gap, another AP is generated

5. This cycle repeats down the entire axon

6. The current potential jumps from one gap to the next

What is Continuous Propagation (conduction)

Myelin sheath is absent

• each section of the axolemma must be depolarized to threshold

• AP’s must be generated in a continuous sequence along the entire axolemma for the current to spread down the axon

• the axolemma is very leaky with respect to current, so the current flows easily from the axoplasm out to the ECF

Big Idea: myelin present, continuous firing of AP’s, signal loses strength as it travels through the axon

Big Idea: Saltatory conduction has a more faster and efficient propagation than continuous conduction because it has a myelinated axon and continuous conduction doesn’t have an axon

N/A

How are Axon fibers classified and what are the 3 types?

classified by their diameter and myelin sheath

• 3 types are:

  • Type A fibers

  • Type B fibers

  • Type C fibers

What are Type A fibers?

• largest diameters (5-20 micro meters)

• myelinated

• fast → ~250mph

• found in parts of body where CNS must communicate rapidly: certain sensory axons from joints or fibers, as well as motor axons to skeletal muscles

What are Type B fibers?

• intermediate diameter (2-3 micro meters)

• myelinated/unmyelinated (mostly myelinated)

• ~32 mph

• includes certain efferent fibers of the autonomic NS and certain sensory axons coming from organs

What are Type C fibers?

• smallest diameter (0.5-1.5 micro meters)

• slowest fiber - ~1-5 mph

• unmyelinated

• includes other efferent fibers of the autonomic NS and sensory axons that transmit pain, temp, and certain pressure sensations