B1. Neurones

Question 1

The resting membrane potential
In a neurone’s resting state (when it’s not being stimulated), the outside of the membrane is ____________charged compared to the inside. This is because there are ______ ___________ _____outside the cell than ___________. So the membrane is polarised there’s a difference in charge (called a potential difference or voltage) across it. The voltage across the membrane when it’s at rest is called the resting potential—it’s about -70 mV (millivolts).

Answer 1

In a neurone’s resting state (when it’s not being stimulated), the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside. So the membrane is polarised there’s a difference in charge (called a potential difference or voltage) across it. The voltage across the membrane when it’s at rest is called the resting potential—it’s about -70 mV (millivolts).

Question 2

Movement of sodium and potassium ions
The resting potential is created and maintained by _________-______________ _______and ______________ ____ ___________in a neurone’s _____________

• Sodium-potassium pumps use ________ ____________to move _________ _________ _____(___’) out of the neurone for every ____ ____________ _____ (__) moved in. ____is needed to do this.
• Potassium ion channels allow _____________ ____________of potassium ions (K’) out of the neurone, _______their concentration gradient.

Answer 2

The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone’s membrane

• Sodium-potassium pumps use active transport to move three sodium ions (Na+) out of the neurone for every two potassium ions (K) moved in. ATP is needed to do this.
• Potassium ion channels allow facilitated diffusion of potassium ions (K’) out of the neurone, down their concentration gradient.

Question 3

Figure 1: Movement of sodium and potassium ions across a resting cell membrane.

Answer 3

Question 4

Movement of sodium and potassium ions

1. The _________-______________ _________move __________ions out of the neurone, but the membrane isn’t ______________to sodium ions, so they can’t __________back in. This creates a _____________ ____ ________________ ______________because there are more positive sodium ions __________the cell than __________.
2. The sodium-potassium pumps also move ____________ions in to the neurone.
3. When the cell’s at rest, most _____________ion channels are open. This means that the membrane is _____________to potassium ions, so some ________back out through potassium ion channels.
   Even though positive ions are moving in and out of the cell, in total ______positive ions move _____of the cell than __________. This makes the _________of the cell positively charged compared to the __________.

Answer 4

1. The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in. This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside.
2. The sodium-potassium pumps also move potassium ions in to the neurone.
3. When the cell’s at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions, so some diffuse back out through potassium ion channels.
   Even though positive ions are moving in and out of the cell, in total more positive ions move out of the cell than enter. This makes the outside of the cell positively charged compared to the inside.

Question 5

Action potentials
When a neurone is stimulated, other ion channels in the cell membrane, called sodium ion channels, open. If the stimulus is big enough, it’ll trigger a rapid change in ___________ difference. This causes the cell membrane to become _______________(it’s no longer polarised). The sequence of events that happens is known as an action potential

Answer 5

When a neurone is stimulated, other ion channels in the cell membrane, called sodium ion channels, open. If the stimulus is big enough, it’ll trigger a rapid change in potential difference. This causes the cell membrane to become depolarised (it’s no longer polarised). The sequence of events that happens is known as an action potential

Question 6

Tip: These sodium ion channels are voltage: gated-they only open when the potential difference reaches a certain __________.

Answer 6

Tip: These sodium ion channels are voltage: gated-they only open when the potential difference reaches a certain voltage.

Question 7

Tip: The sodium-potassium pump, potassium ion channel and sodium ion channel are all types of ____________ __________.

Answer 7

Tip: The sodium-potassium pump, potassium ion channel and sodium ion channel are all types of transport protein.

Question 8

Figure 2: A graph to show the changes in potential difference across a neurone cell membrane during an action potential.

Answer 8

Question 9

Action potentials
1. Stimulus - this _________the neurone ____ _________, causing ____________ ___ ____________to _____. The membrane becomes more _____________ to ____________, so ___________ ____ _________into the neurone down the ___________ ___ __________________ _____________. This makes the inside of the neurone less ____________.

2. Depolarisation - if the potential difference reaches the threshold (around 55 mV), more __________ ___ ___________ ______. More __________ _____ _________into the ________.
3. Repolarisation - at a potential difference of around +30 mV the ___________ ____ ___________ ______ and ____________ ____ ___________ ______. The membrane is more _______________to ______________so ______________ions diffuse out of the neurone down the ______________ ion _______________gradient. This starts to get the membrane back to its _________ ___________.
4. Hyperpolarisation - _______________ ____ ____________ are _____to close so there’s a slight ‘overshoot’ where too many _______________ _____ ___________out of the neurone. The potential difference becomes more ____________than the resting potential (i.e. less than -70 mV).
5. Resting potential - the ion channels are reset. The _________-____________ ______returns the _____________to its __________ ___________by pumping ___________ ____out and ______________ions in, and maintains the ____________ ___________until the _____________-___________by another ___________.

Answer 9

1. Stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
2. Depolarisation - if the potential difference reaches the threshold (around 55 mV), more sodium ion channels open. More sodium ions diffuse into the neurone.
3. Repolarisation - at a potential difference of around +30 mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.
4. Hyperpolarisation - potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (i.e. less than -70 mV).
5. Resting potential - the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting potential until the membrane’s excited by another stimulus.

Question 10

This graph shows when the _____________ ion channels (blue) are open:

Answer 10

This graph shows when the potassium ion channels (blue) are open:

Question 11

Tip: During repolarisation the ____________channels have to close or the membrane will remain depolarised.

Answer 11

Tip: During repolarisation the sodium channels have to close or the membrane will remain depolarised.

Question 12

The refractory period
After an _________potential, the neurone cell membrane can’t be _________again straight away. This is because the ____ __________are _____________and they can’t be made to _______-_____________ion channels are closed during _______________and _______________ion channels are closed during ______________________. This period of recovery is called the refractory period
The refractory period acts as a time delay between one action potential and the next. This makes sure that action potentials don’t ___________but pass along as discrete (____________) impulses. The refractory period also means that there’s a limit to the _______________at which the _________ ______________can be transmitted, and that action potentials are _______________(they only travel in one direction).

Answer 12

After an action potential, the neurone cell membrane can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open -sodium ion channels are closed during repolarisation and potassium ion channels are closed during hyperpolarisation. This period of recovery is called the refractory period
The refractory period acts as a time delay between one action potential and the next. This makes sure that action potentials don’t overlap but pass along as discrete (separate) impulses. The refractory period also means that there’s a limit to the frequency at which the nerve impulses can be transmitted, and that action potentials are unidirectional (they only travel in one direction).

Question 13

Figure 3: The refractory period of an action potential.

Answer 13

Question 14

Waves of depolarisation
When an action potential happens, some of the ___________ions that enter the neurone __________ ____________. This causes ___________ion channels in the _____ __________of the neurone to ______and ___________ ions __________into that part. This causes a wave of depolarisation to travel along the neurone. The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an _______potential.

Answer 14

Waves of depolarisation
When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part. This causes a wave of depolarisation to travel along the neurone. The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential.

Question 15

Figure 4: The movement of ions across a neurone cell membrane during a wave of depolarisation.

Answer 15

Question 16

Tip: The electrical impulse can be said to ‘____________-along the neurone. This just describes the wave-like movement of the action potential.

Answer 16

Tip: The electrical impulse can be said to ‘propagate along the neurone. This just describes the wave-like movement of the action potential.

Question 17

Tip: A wave of depolarisation is like a Mexican wave travelling through a crowd- sodium ions rushing _________causes a wave of activity _______the ____________.

Answer 17

Tip: A wave of depolarisation is like a Mexican wave travelling through a crowd- sodium ions rushing inwards causes a wave of activity along the membrane.

Question 18

All or nothing principle
Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how ____the stimulus is. If the ____________isn’t reached, an action potential won’t fire. This is the all-or-nothing nature of action potentials.

Answer 18

Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is. If the threshold isn’t reached, an action potential won’t fire. This is the all-or-nothing nature of action potentials.

Question 19

Figure 5: An action potential only fires if the stimulus reaches the threshold.

Answer 19

Question 20

All or nothing principle
A bigger stimulus won’t cause a __________ ________ __________but it will cause them to fire _________ ____________

Answer 20

A bigger stimulus won’t cause a bigger action potential but it will cause them to fire more frequently

Question 21

Tip: The all-or-nothing principle stops the brain from getting _____-stimulated by not responding to _____ _______ _________.

Answer 21

Tip: The all-or-nothing principle stops the brain from getting over-stimulated by not responding to very small stimuli.

Question 22

Figure 6: A bigger stimulus causes more frequent action potentials.

Answer 22

Question 23

Speed of conduction
Three factors affect the speed of conduction of action potentials:

Answer 23

1. Myelination
2. Axon diameter
3. Temperature

Question 24

Speed of conduction - 1. Myelination
Some neurones, including many motor neurones, are myelinated—they have a myelin sheath. The myelin sheath is an __________insulator. In the ___________nervous system, the sheath is made of a type of cell called a ___________ ____. Between the ___________ ____are tiny patches of bare membrane called the _______ of ___________. ___________ ____ __________are concentrated at the _______of ___________.

Answer 24

Some neurones, including many motor neurones, are myelinated—they have a myelin sheath. The myelin sheath is an electrical insulator. In the peripheral nervous system, the sheath is made of a type of cell called a Schwann cell. Between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier. Sodium ion channels are concentrated at the nodes of Ranvier.

Question 25

Figure 8: Structure of a myelinated motor neurone.

Answer 25

Question 26

Speed of conduction - 1. Myelination - Saltatory Conduction
In a myelinated neurone, ______________only happens at the _______ __ _________(where ___________ions can get through the membrane). The neurone’s _________ ___________enough electrical charge to _________the next _____, so the impulse ‘jumps’ from node to node. This is called saltatory conduction and it’s really fast.
In a non-myelinated neurone, the impulse travels as a wave along the _______ __________of the ______ ______________-so you get depolarisation along the whole length of the membrane. This is _________than saltatory conduction (although it’s still pretty quick).

Answer 26

In a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane). The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node. This is called saltatory conduction and it’s really fast.
In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane-so you get depolarisation along the whole length of the membrane. This is slower than saltatory conduction (although it’s still pretty quick).

Question 27

Figure 10: Saltatory conduction along a myelinated neurone.

Answer 27

Question 28

Figure 11: Conduction along a non-myelinated neurone

Answer 28

Question 29

Tip: If you imagine a Mexican wave travelling through a crowd, then saltatory conduction is like every tenth person doing the wave instead of everyone doing the wave-so it travels much _________.

Answer 29

Tip: If you imagine a Mexican wave travelling through a crowd, then saltatory conduction is like every tenth person doing the wave instead of everyone doing the wave-so it travels much faster.

Question 30

Speed of conduction - 2. Axon diameter
Action potentials are conducted quicker along axons with ___________diameters because there’s _____ ____________to the ______ of ____ than in the ___________ of a ___________ _____. With less __________, __________________reaches other parts of the neurone cell membrane __________.

Answer 30

Action potentials are conducted quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker.

Question 31

Speed of conduction - 3. Temperature
The speed of conduction increases as the temperature _____________, because _____ ___________ __________. The speed only increases up to around ___°C though— after that the __________begin to ___________and the speed ____________.

Answer 31

The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to around 40 °C though— after that the proteins begin to denature and the speed decreases.

Question 32

Tip: The pumps and channels that move ions across the membrane are ___________, so these will ___________at high temperatures.

Answer 32

Tip: The pumps and channels that move ions across the membrane are proteins, so these will denature at high temperatures.