Nerve Cells and Excitability: The Action Potential

Question 1

Nerve Cell Communication

Answer 1

Neuron can communicate between each other through the dendrites and axon.

Neurons are polarized, resting membrane potential (RMP).

RMP is an electrical charge acrosstheplasma membrane, withtheinterior ofthe cellnegative with respect totheexterior.

The action potentials is defined as a brief change in the voltage across the membrane due to the flow of certain ions into and out of the neuron.

Question 2

ALL OR NONE” law

Answer 2

If a stimulus is strong enough, anaction potentialoccurs and a neuron sends information down an axon away from the cell body and toward the synapse. Changes in cell polarization result in the signal being propagated down the length of the axon.

The action potential is always a full response. There is no such thing as a “strong” or “weak” action potential. Instead, it is an all-or-nothing process.

Question 3

Phases of Action Potentials

Answer 3

Hypopolarisation is the initial increase of the membrane potential to the value of the threshold potential.

Depolarisation is the potential moving from RMP to less negative values.

Overshoot is the peak of the action potential is reached at about +40 millivolts.

Repolarisation is the potential moving back to the RMP (-70mV).

Hyperpolarisation/undershoot is the potential moving away from the RMP in a more negative direction.

Question 4

Sequence of events in an action potential

Answer 4

Resting potential VGNC and VGKC closed
Stimulus causes depolarization to threshold - VGNC open
Na+ flows in - membrane rapidly depolarises - more VGNC open - VGKC still closed
VGNC inactivated - Na+ entry slows - VGKC open - K+ flows out membrane repolarisation begins
VGNC move to closed state VGKC remain open - delayed hyperpolarisation
Resting potential restored

Question 5

Properties of voltage-gated Na+ and K+ channels

Answer 5

Na+ open rapidly with depolarisation
Inactivation gate rapidly blocks Na+ permeability
Inactivated channels blocked during continued depolarisation
Inactivated channels move to closed state on repolarisation

K+- open slowly during depolarisation
Channels remain open during depolarisation
Close slowly on repolarisation

Question 6

Refractory periods

Answer 6

Absolute refractory period
AP cannot be evoked as VGNC channels are inactivated and cannot be activated again until the membrane is repolarised and resting state restored

Relative refractory period
Membrane potential hyperpolarised by VGKC. AP can be generated if the stimulus strength is strong enough overcome hyperpolarisation to reach threshold

Question 7

saltatory conduction

Answer 7

Myelin electrically insulates large areas of axon allowing local currents to spread further and faster increasing speed of propagation

Saltatoryconduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. Therefore, the conduction velocity of the message increases as it directly goes from one node of Ranvier to another, without having to pass through the entire length of the axon. This, not only betters the nervous system function by decreasing the time spent in passing messages, but also reduces energy expenditure in the nerve.

Question 8

Guillain-Barre syndrome

Answer 8

is the destruction of Schwann cells (in the peripheral nervous system).

Question 9

Multiple Sclerosis (MS)

Answer 9

is caused by a loss of oligodendrocytes (in the brain and spinal column).