Lecture 8: Modulation of Action Potential

How does neuronal activity travel through a neuron?

via the spread of electrical charges regulated by multiple ion channels along the membrane; similar to electrical cables

True or False: The neuronal membrane acts like a capactior.

true

What does a capacitor consist of?

2 conducting regions separated by an insulator

What is capactiance?

an ability to store and separate charge; the quantity of charge required to create a given potential difference between two conductors

Capacitance is proportional to what?

the membrane surface

What does capacitance decrease with?

the distance between the 2 conducting surfaces

What is longitduinal resistance?

r_i; the inability of a charge to migrate internally (the internal resistance of the axoplasm)

What is the equation for longitudinal resistance per 1 cm segment (ohm/cm)?

r_i = R_i / A where R_i is the specific internal axial resistance and A is the cross-sectional area

What is the membrane resistance?

r_m; the inability of a charge to migrate across the membrane

What is the equation for r_m?

r_m = R_m / c where R_m is specific membrane resistance and c is circumference

What is membrane capacitance (C_m)?

an ability to store and separate charge

What is E_r?

resting membrane potential

Overall longitudinal resistance does what with length?

increases

Overall membrane resistance does what with length?

decreases

True or False: If axoplasmic composition is similar, the longitudinal resistance (r_i) will be greater in a smaller fiber.

true

True or False: If two fibers have similar density of leak channels, R_m will be similar, but the smaller fiber will have a higher r_m.

true

What is a steady state change?

• a change induced by injected current

• a change in synaptic conductance

• a change in voltage-gated conductance

What can be ignored in regards to steady state?

transient charging or discharging due to C_m

True or False: Assume uniform E_r.

true

What is V₀?

the induced change in potential; peak potential change

What is the equation for lambda?

lambda = space / length constant = square root of r_m /r_i

What is lambda?

the length at which V spreading along the infinite cable decays to a value of 0.37

What is the equation for passive spread at steady state?

V_x = V₀e^-x/lambda

What is V_x?

peak change at distance x

What is X?

distance from the stimulus

What are the factors influencing conduction velocity?

• spread of AP along membrane

• path of the positive charge

• axonal excitability

What is the spread of AP along the membrane dependent on?

axon structure

What affects the path of positive charge?

• location of positive charge:

  • inside of the axon (faster)

  • across the axonal membrane

What affects axonal excitbaility?

• axonal diameter (bigger = faster)

• number of voltage-gated channels

• increase Na_v and decrease K_v

How long do action potentials last?

1-5 ms

How long do synaptic potentials last?

5-30 ms

True or False: The spread of rapid and transient signals depend on all the factors for passive spread and membrane capactiance (C_m).

true

What is the equation for I_pulse?

I_pulse = I_c - I_i

What is I_i?

current through membrane

What is I_c?

current through capacitance

Describe the 2 compartment model to define the basic properties of signal spread.

• the charge starts to flow through R_i onto the compartment B, where current again divides between C_m and R_m

• the charging and discharging transient in compartment A departs from membrane time constant tau of a single compartment and becomes faster because of the current sink of a compartment B

• currents spreads by means of localized conductance (an ability of a charge to migrate) changes across the membrane

How do APs propagate in unmyelinated axons?

by means of local current spread

What is orthodromic?

AP travels in one direction (down axon to the axonal terminal)

What is antidromic?

experimental, backward propagation

What is the typical conduction velocity and length of action potential for unmyelinated axons?

• typical conduction velocity: 10 m/sec

• length of action potential: 2 msec

True or False: Local current spreads through the internal resistance to the neighboring compartments.

true

True or False: The spread of electrical charges is attenuated rapidly in unmyelinated mammalian neurons.

true

Mammalian axons can be up to how long?

180 cm long

What is the typical lambda for unmyelinated axons?

2.2 mm

What does myelination increase?

• R_m of the axon

• the distance between the conducting, so decreases C_m

True or False: There is a high density of voltage-gated Na channels (10,000 micrometer^-2) at the Nodes of Ranvier compared to the internodal region (20 microm^-2).

true

True or False: Neurons in the mammalian brain exhibit various electrophysiological properties by altering frequency, amplitude, and shape.

true

What is the effect of I_c on AP?

reduces excitability by enhancing repolarization

What is the effect of I_a on AP?

reduces excitability by enhancing AP latency

What are the outward/hyperpolarizing currents that regulate AP? Describe each type of current.

• K+ current

  • rapid activation, no inactivation

• KC current

  • activated by Ca2+

• KA current

  • rapid activation and rapid inactivation

• KM current

  • slow activation, no inactivation

What are the inward/depolarizing currents that regulate AP? Describe each type of current.

• Na+ current

  • rapidly inactivating

• NaP current

  • persistent

• Ca2+L current

  • long-lasting

• Ca2+T current

  • transient

• Na+/K+ current