6.2.1 Nerve impulses

Question 1

What are the body’s two main co-ordination system?

Answer 1

The nervous system and the endocrine (hormone) system

Question 2

What is the nervous system?

Answer 2

• Uses nerve cells to pass electrical impulses
• Stimulate their target cells by secreting chemicals - neurotransmitters
• Rapid communication
  -Short lived, localised region of the body

Question 3

What is the endocrine (hormone) system?

Answer 3

• Produces chemical (hormones) that are transported in the blood plasma to their target cells
• Specific receptors on their cell-surface membranes and the change in the concentration of hormones stimulates them
• Slower, less specific
• Long-lasting and widespread

Question 4

Compare the endocrine and nervous system:

Answer 4

Endocrine:
- Communication is by chemicals (Hormones)
- Transmission is in the blood
- Transmission is usually relatively slow
- Hormones travel all over the body, but only effect their target organs
- Response is widespread
- Response is usually slow
- Response is usually long lasting
- Effect is long lasting, and can be permanent

Nervous:
- Communication is by nervous impulse
- Transmission is along neurones
- Transmission is very fast
- Impulses travel to specific parts of the body
- Response is localised
- Response is very fast
- Response is usually short lived
- Effect is usually temporary and reversible

Question 5

What are neurones (nerve cells)?

Answer 5

Specialised cells that carry electrochemical changes, called nervous impulses from one part of the body to another

Question 6

What does a motor neurone do?

Answer 6

Transmit nerve impulses from an intermediate or relay neurone to an effector
- Long axon and many short dendrites

Question 7

What do dendrites do?

Answer 7

Carry nerve impulses towards the cell body

Question 8

What is an axon?

Answer 8

Single long fibre that carries nerve impulses away from the cell body

Question 9

What is the axon of the motor neurone covered in?

Answer 9

Myelin sheath

Question 10

What is the myelin sheath made up of?

Answer 10

Formed from cells called schwann cells

Question 11

Give the function of schwann cells?

Answer 11

Surround the axon, protecting it and providing electrical insulation
- Carry out phagocytosis, nerve regeneration

Question 12

What do sensory neurones do?

Answer 12

Transmit nerve impulses from a receptor to an intermediate/motor neurone
- Long dendron
- Carries impulse towards the cell body and one axon that carries it away from the cell body

Question 13

What do intermediate/relay neurones do?

Answer 13

Transmit nerve impulses between neurones
Eg/ sensory to motor neurones
- Numerous short processes

Question 14

What do neurones have in their membrane?

Answer 14

Channels
Sodium-potassium pump

Question 15

What are the two types of channels and what do they in response to a change in the charge across the membrane?

Answer 15

• Specific to either sodium or potassium ions
• ‘voltage gated’ - this means that they open and close in response to a change in the charge across the membrane

Question 16

What is the membrane potential?

Answer 16

The distribution of ions on either side of the membrane causing a difference in charge across the membrane

Question 17

What is resting potential?

Answer 17

When there is no impulse passing along a nerve, then there is a potential difference between the charge outside the neurone and inside the neurone

Outside is slightly positive compared to the inside - due to the distribution of charged ions inside and outside of the neurone.
Difference is around -65mV

Question 18

What is action potential?

Answer 18

When an impulse is passed along a nerve the potential difference across the membrane is briefly reversed. The inside becomes positive compared to the outside. The action potential is around +40mV.

Question 19

What causes the resting potential?

Answer 19

• The Sodium-potassium ion pump actively transports: 3 Na+ ions out of axon and 2 K+ ions into axon
• Na+ channels are closed, so Na+ can’t move back into the axon by facilitated diffusion
• Membrane more permeable to K+ ions (open K+ channels) than Na+ ions (closed Na+ channels)
• Electrochemical (concentration) gradient created eg/ higher conc. of K+ ions inside axon than outside, higher conc. of Na+ ions outside axon. Prevents much K+ moving out of the axon
• An equilibrium is established with more Na+ outside the axon than there are K+ inside the axon
• Potassium ions move out of axon by facilitated diffusion. Inside of axon is negatively charged relative to outside; axon is polarised = resting potential

Question 20

How is resting potential controlled?

Answer 20

• Phospholipid bilayer prevents Na+ and K+ diffusing across it
• Channel proteins - have ‘gates’
• Sodium-potassium pump

Question 21

How is action potential generated?

Answer 21

• The stimulus causes some of the voltage gated sodium channels to open, therefore membrane more permeable to sodium ions. Sodium diffuse into neurone down electrochemical gradient
• Therefore making it less negative, relative to the outside - DEPOLARISED. This change in charge causes more voltage gated sodium channels to open and more Na+ to move into the cell
• Once an action potential is of +40mV is reached the voltage gated sodium channels close and potassium channels open
• K+ ions move out of the axon, reversing the depolarisation (more K+ channels opening therefore more K+ ions diffuse out) and causing the outside to become more positive again - REPOLARISATION
• The outward movement of K+ ions causes an overshoot of the electrochemical gradient outside temporarily becoming more negative than it needs to be - HYPERPOLARISATION
• Hyperpolarisation triggers the potassium gates to close
• The resting potential is now restored, - This is called repolarisation. But the Na+ and the K+ ions are in the wrong place. So the sodium potassium pump restores them to their correct place by active transport

Question 22

How is the impulse transmitted along the neurone?

Answer 22

• An action potential in a region of the neurone creates “local currents” around it
• These “local currents” stimulate the sodium channels ahead of the action potential to open

Question 23

Describe the passage of an action potential along an unmyelinated axon

Answer 23

1. Resting potential: conc of Na+ outside the axon membrane is high relative to inside. K+ is high inside the membrane. Overall conc of positive ions is greater on the outside. Axon membrane is polarised
2. Stimulus - sudden influx of sodium ions, reversal of charge on the axon membrane. Membrane depolarised
3. Localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gated channels. Causes depolarisation. Na+ close and K+ open. Depolarisation moves along the membrane
4. Action potential is propagated in the same way further along the axon. Outward movement of the potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state - repolarised
5. Sodium ions to be actively transported out, once again returning the axon to its resting potential. Ready for new stimulus

Question 24

Describe the passage of an action potential along an myelinated axon

Answer 24

Fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming. Action potentials occur when there’s a break in the myelin insulation. Localised circuits arise between adjacent nodes of the Ranvier and the action potentials in effect jump from node to node (saltatory conduction)

Question 25

What factors affect the speed of transmission?

Answer 25

• The temperature
• The diameter of the neurone
• The myelin sheath

Question 26

How does temperature affect the speed of transmission?

Answer 26

• Affects rate of diffusion therefore higher temperature = faster nerve impulse
• Energy for active transport comes from respiration

Question 27

How does the diameter of the neurone affect the speed of transmission?

Answer 27

• The greater the diameter, faster the speed of conductance
• Due to less leakage of ions from a large axon

Question 28

How does the myelin sheath affect the speed of transmission?

Answer 28

• Acts as an electrical insulator, preventing an action potential forming in the part of the axon covered in myelin
• But jumps from one node of Ranvier to another (saltatory conduction)
• Increase speed of conductance from 30m/s in an unmyelinated -> 90 m/ in myelinated

Question 29

What does the all-or-nothing principle suggest?

Answer 29

• If a stimulus reaches a certain level - called the threshold value - an action potential will be triggered
• All action potentials are the same size

ie. nothing = any stimulus that is below the threshold value will fail to generate an action potential

Question 30

How can organisms perceive the size of a stimulus?

Answer 30

• By the number of impulses passing in a given time
  (larger the stimulus, the more impulses that are generated in a given time)
• By having different neurones with different threshold values

Question 31

What is the refractory period:

Answer 31

• Once an action potential has been created in any region of an axon, there is a period afterwards when inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed
• During this time it is impossible for a further action potential to be generated which is the refractory period