Membrane potentials

Question 1

What are the concentrations of sodium and potassium ions inside and outside the cell?

Answer 1

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Question 2

Which ions are pumped out of the cell and which ions are pumped into the cell. How many ions of each are there being pumped?

Answer 2

3Na+ out of the cell and 2K+ into the cell

Question 3

What does potassium ions want to do?

Answer 3

Diffuse out of the cell

Question 4

Describe potassium ion leak

Answer 4

• Potassium moves out of the cell DOWN its’ concentration gradient
• Potassium can leave the cell through K+ channels in the cell membrane
• This leaves behind the negatively charged anions

Question 5

Describe the electrical gradient

Answer 5

• There is a separation of charge across the membrane with a more negative charge INSIDE the cell
• We have a separation of positive and negatively charged ions – this leads to a POTENTIAL DIFFERENCE
• There is an electrochemical gradient across the membrane that results in a membrane potential
• Membrane potential - Voltage (difference in electrical charge) across the plasma membrane
• Resting potential - The membrane potential of a cell not sending signals

Question 6

What produces the concentration gradient? / What produces the electrical gradient and how can we calculate membrane potential?

Answer 6

• The sodium and potassium pumps
• Potassium ions
• We can use an equation

Question 7

What is the Nernst Equation?

Answer 7

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Question 8

What is the Goldman equation?

Answer 8

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Question 9

What is the equilibrium potentials of sodium and potassium ions and what happens if the membrane becomes too permeable to sodium ions?

Answer 9

Ena = 9mv

Ek= -90mv

resting potential = -70mv

Question 10

What is depolarisation?

Answer 10

Depolarisation is the membrane potential is MORE POSITIVE than the resting potential - Na+ in

Question 11

What is repolarisation?

Answer 11

Repolarisation is when the membrane potential RETURNS to resting potential after depolarisation - K+ out

Question 12

What is hyperpolarisation?

Answer 12

Hyperpolarisation is when the membrane potential is MORE NEGATIVE than the resting potential

Question 13

Why does membrane potentials change?

Answer 13

Membrane potentials change due to the opening or closing of gated ion channels in response to stimuli

Question 14

How is an action potential produced?

Answer 14

If the wave of depolarization is significant and reaches the threshold value then an action potential is produced

Question 15

Describe the cardiac action potential graph

Answer 15

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Question 16

Describe cardiac action potentials

Answer 16

action potentials in the heart underlie the electrical conduction system of the heart. This allows for rhythm and synchronously of the contractions of the heart

Question 17

Describe the role of sensory neurons

Answer 17

• Sensory receptors are specialised cells that detect changes in our surroundings.
• They often convert energy from one form to electrical or detect the presence of chemicals.
• Pacinian corpuscles detect pressure changes in the skin: oval structure consisting of concentric rings of connective tissue wrapped around the ending of a nerve cell.
o When pressure is placed on the skin, the rings of connective tissue are pushed against the nerve ending. Only sensitive to changes in pressure; when constant pressure stop responding.
• Cell surface membranes of nervous system contain specialised protein channels (sodium, potassium, and gated channels).
• The membranes also contain sodium/potassium pumps that actively pump sodium ions (3) out of the cell and potassium ions (2) into the cell - creating a concentration gradient (-70mV).
• When the membrane is deformed by pressure changes, sodium channels are opened, so sodium ions diffuse into the cell, which depolarises the membrane as the inside of the cell becomes less negative relative to the outside - a generator potential (receptor potential) has been created.
• The larger the stimulus, the more gated channels that will open. If enough open, a significant pD change is created which may initiate an impulse or action potential.

Question 18

Describe the structure and functions of neurons

Answer 18

• Neurones transmit impulses as action potentials (rapid depolarisation of membrane).
• Sensory neurones carry action potentials from the sensory receptor to the central nervous system.
• Motor neurones carry action potentials from the CNS to an effector (muscle or gland).
• Relay neurones connect sensory and motor neurones.
• Neurones are very long as they must transmit action potentials over long distances.
• Plasma membranes contain many gated ion channels and sodium/potassium pumps (use ATP).
• Cell body contains nucleus, many mitochondria and ribosomes.
• Dendrites connect to other neurones that carry impulses towards the cell body.
• Axon carries impulses away from cell body.
• Neurones are surrounded by a fatty layer called the myelin sheath (made up of Schwann cells) that prevent electrical activity being passed to other nerve cells nearby.
• Sensory neurones have a long dendron that carries the action potential from the sensory receptor to the cell body (positioned just outside the CNS). Short axon carry action potential to CNS.
• Relay neurones have many short dendrites (connect to many motor and sensory) and short axon.
• Motor neurones have cell body in the CNS and long axon that carries action potential to the effector.
• Myelin sheath prevents ions moving across neurones, so they can only move at the nodes of Ranvier (occuring every 1-3mm) which means the impulse/action potential jumps from node to node (fast).
• Non-myelinated neurones are also associated with Schwann cells, but many neurones may

Question 19

Describe nerve impulses: action potentials

Answer 19

• When at rest, neurone pumping (3) Na+ out of and (2) K+ into cell (use ATP).
• Some gated K+ channels open, so plasma membrane more permeable to K+.
• Cell cytoplasm also contains large organic anions, helping to maintain the resting potential difference of -70mV, as the interior of cell has a more negative potential than the exterior.
• If sodium ion channels are opened (eg due to pressure changes) then sodium ions diffuse into the cell, causing the membrane to become depolarised (less negative) and reaches threshold of -55mV.
• Change in potential difference stimulates opening of sodium gated ion channels, so more sodium ions flow into the membrane (positive feedback) - cell becomes more charged and reaches +40mV.
• Voltage change causes voltage gated sodium ion channels close and voltage gated potassium channels open, so potassium ions diffuse out of the cell bringing the potential difference back to negative (compared with inside) - repolarisation.
• Potential difference overshoots slightly, and the cell becomes hyperpolarized.
• Original potential difference restored so cell returns back to its resting state.
• After an action potential, sodium and potassium ions are in the wrong places, so sodium potassium pumps must restore the correct concentrations of these ions before another action potential can be stimulated. Refractory period allows cell to recover and ensures transmitted in one direction.
• All or nothing response, as an action potential is either generated (if membrane potential reaches threshold value) or it is not generated (if membrane potential does not reach threshold value).

Question 20

Describe nerve impulses: transmission

Answer 20

• The diffusion of sodium ions into the neurone increases their concentration, and so they diffuse down (sideways) the axon to where there are lower concentrations (local currents).
• The sodium ions which are slightly further down the membrane cause a slight depolarisation, which causes voltage gated sodium ion channels to open (due to the voltage changing as sodium ions are present) and so more sodium ions enter the neurone causing action potential if threshold reached.
• The sodium ions continue to move down the membrane to where their concentration is lower, generating more action potentials. Cannot move backwards as Na+ concentration still high.
• Saltatory conduction occurs in myelinated neurones. Myelin sheath prevents ion exchange, except at nodes of Ranvier, therefore sodium ions diffuse (local currents) along axon until node of Ranvier, where the next action potential can be generated, so impulse jumps from node to node (faster).
• All action potentials are the same intensity (+40mV), so a more intense stimulus produces a higher frequency of action potentials as opposed to higher intensity action potentials