Lecture 2 - The Neuron and the action potential

Question 1

What is the neuronal membrane?

Answer 1

• it keeps fluid inside the cell (intracellular) separate from fluid outside the cell (extracellular)
• contains proteins = ion pumps and ion channels that control the movement of ions into and out of the cell
• movement of ions across the neuronal membrane causes electrical signals
• ion channels can be resting (open); voltage gated, ligand gated or mechanically gated

Question 2

What ions are in the intracellular and extracellular fluid?

Answer 2

• sodium (Na+)
• potassium (K+)
• chloride (Cl-)
• large negative ions (A-)

Question 3

What controls the movement of ions?

Answer 3

• 2 forces determine the movement of ions into and out of the cell:
  1. concentration (hight to low density)
  2. electrical (negative <-> positive)
• ion channels only allow specific ions to move through the neuronal membrane
• different ions have different channels

Question 4

Ion channels and the resting membrane potential?

Answer 4

• at rest sodium ion channels are closed = sodium is not free to move across the membrane
• however some potassium ion channels are open = potassium can move in and out of the cell freely
• potassium is attracted into the cell because the cell is more negative inside compared to the outside (electrical force)
• potassium is attracted out of the cell because there is less potassium outside the cell (concentration force)
• the 2 forces are in equilibrium - the equilibrium potential of k+ is -90mv

Question 5

The sodium and potassium pump?

Answer 5

• at rest a neuron has more positive ions outside the cell than inside the cell which means it is negatively charged
• the Na/K pump causes this imbalance by pumping 3 positive sodium ions out the cell and 2 positive potassium ions into the cell
• this causes increased potassium concentration inside the cell and increased sodium concentration outside the cell

Question 6

The action potential?

Answer 6

• at rest a neuron is negatively charged (-65mV)
• this is caused by more negative ions inside the cell than outside = resting membrane potential
• if the neuron is stimulated the equilibrium will be upset and the resting potential will become an action potential
• an action potential is generated at the axon hillock if the net change is above threshold (-50mV)
• the action potential is then propagated down the axon

Question 7

Sodium and the rise of action potential?

Answer 7

• when a cell is stimulated above threshold (-50mV), voltage-gated sodium ion channels open
• sodium will be attracted into the cell because there is more Na+ outside than inside (concentration force)
• it is also attracted into the cell because it is negatively charged (electrical force)
• the influx of positive Na ions causes the cell to become more positive - and the neuron is depolarized

Question 8

Potassium and the fall of the action potential?

Answer 8

• when the action potential reaches its peak (+40mV) the cell becomes positively charged
• positive potassium ions are therefore attracted to the negative outside the cell
• there are more potassium inside the cell which means the concentration force forces the potassium out of the cell
• the loss of potassium causes the cell to become more negative and the neuron is repolarized

Question 9

What happens when an action potential occurs in a neuron?

Answer 9

• it is quickly transmitted down the axon to the presynaptic terminals

Question 10

What is myelin sheath?

Answer 10

• a fatty substance which is produced by glial cells known as oligodendrocytes (central NS) or schwann cells (peripheral NS)
• myelinated axons conduct action potentials faster than unmyelinated axons by saltatory conduction

Question 11

The refractory period?

Answer 11

• An action potential is an all or nothing event
• Another action potential cannot be generated until the preceding potential has finished
• The strength of the stimulus is denoted by increased firing rate
• Neurons can fire many action potentials per second

Question 12

What happens at the synapse?

Answer 12

• When the action potential reaches the pre synaptic terminal a neurotransmitter is released into the junction between the neurons (synapse)
• Neurotransmitters open ligand-gated ion channels in the postsynaptic membrane
• The neurotransmitter will have either an excitatory or inhibitory effect
• The cell body of the next neuron collects up the excitatory and inhibitory postsynaptic potentials (PSP’s) to determine if threshold is reached to trigger an AP
• The correct balance of excitatory and inhibitory signals is necessary for the proper working of the nervous system
• The balance breaks down in conditions such as epilepsy = uncontrollable patterns of electrical activity