Graded and Action Potentials

Question 1

Neural Communication

Answer 1

-Nerves can undergo rapid changes in their membrane potentials
-Can change their resting potentials into electrical signals
-Electrical signals are critical to the function of nervous system and muscles
-RMP is voltage across the plasma membrane in a cell with no change in membrane permeability
-Neural communication is based on rapid changes in membrane permeability to ion
-SPEED is key!!

Question 2

Ion Channels involved in Graded and Action Potentials

Answer 2

1. Ligand-gated channels
2. Voltage-gated channels

Question 3

Graded Potentials

Answer 3

Step 1: Resting membrane exposed to chemical stimulus
Chemically-gated channels open
Membrane potentials changes de/hyperpolarization
Step 2: movement of ions through channel produces local current
This de/hyperpolarizes nearby regions of cell membrane
Change in potential is PROPORTIONAL to the STIMULUS

*Graded potentials can lead to action potentials

Question 4

Depolarization

Answer 4

Decrease in potential

Membrane less negative

Question 5

Repolarization

Answer 5

Return to resting potential after depolarization

Question 6

Hyperpolarization

Answer 6

Increase in potential

Membrane is more negative

Question 7

Postsynaptic Potentials

Answer 7

Graded potentials depend on the permeability changes induced by neurotransmitter in the postsynaptic region

EPSP: excitatory postsynaptic potentials
IPSP: inhibitory postsynaptic potentials

Question 8

Summation of postsynaptic potentials

Answer 8

1. Action potential reaches pre-synaptic terminal leading to neurotransmitter release
2. Released neurotransmitter binds to post-synaptic receptors leading to post-synaptic potentials
3. Integration of post-synaptic potentials at initial segment of axon triggers action potential if threshold is exceeded

Question 9

Temporal summation

Answer 9

When a single synapse receives many ESPSs in short period of time

Question 10

Spatial summation

Answer 10

When single synapse receives many EPSPs from many presynaptic cells

Question 11

Electrochemical Driving Force

Answer 11

RMP a big K concentration favors efflux and small electrical gradient favors influx = WEAK outward driving force for K associated with high permeability

Big Na concentration and electrical gradients favor influx of sodium = STRONG inward driving force for Na which is associated with low permeability

Question 12

Voltage-Gated Channels

Answer 12

Action potentials takes place as a result of a triggered opening and subsequent closing of two specific channels

**All voltage-gated channel gates are triggered to respond at threshold

Question 13

Voltage-Gated Na+ channels

Answer 13

Two gates: 1. activation gate & 2. inactivation gate

Question 14

Voltage-Gated K+ channels

Answer 14

Only have 1 gate that is either open or closed

Question 15

Conformation of V-G Sodium Channels

Answer 15

Resting: Closed but capable of opening at RP (-70mV)
Activated: Open from threshold to peak (-50mV to 30mV)
Opens fast - rapid opening
Inactivated: Closed and not capable of opening from peak to resting potential (30mV to -70mV)
Closes slowly - slow closing triggered at peak

Question 16

Conformation of V-G Potassium Channels

Answer 16

Closed: at RP, delated opening triggered at threshold; remains closed to peak potential (-70mV to 30mV)

Open: from peak potential through after hyperpolarization phase (30mV to -80mV)

Question 17

Refractory Period

Answer 17

Absolute RP: Interval during which NO stimulus can elicit an action potential; most V-G Na+ channels are inactivated

Relative RP: interval when a SUPRANORMAL stimulus is required to elicit an action potential; due to elevated gK coupled with residual inactivation of V-G Na+ channels

Question 18

Why is there a refractory period?

Answer 18

• Ensures one-way propagation of the action potential
• Limits the frequency of action potentials (energy conservation; prevents seizures)

Question 19

What determines the speed of Conduction?

Answer 19

1. Diameter of the fiber
   - the larger it is, the lower the internal resistance for current flow and the faster it conducts
   - Rapid fibers (large): motoneurons; Slow fibers: internal organs
2. Myelination: lipid sinsulator of nerve fibers that greatly increases the conduction velocity by decreasing the capacitance of the axon and restricting the action potential generation to the Nodes of Ranvier

Question 20

Types of Conduction

Answer 20

1. Contiguous: conduction is unmyelinated fibers; action potential spreads along every portion of the membrane
2. Saltatory (50x faster): Rapid conduction in myelinated fibers; impulse jumps over sections of the fiber covered with insulating myelin