6-2 Nervous coordination

Question 1

What are the cells called in the nervous system?

Answer 1

• Neurones

Question 2

Describe the structure of neurones?

Answer 2

• Cell body composed of the nucleus as well as organelles such as mitochondria within the cytoplasm.
• Extensions called dendrites involved in conducting impulses towards the cell body.
• Axons which conduct them in the opposite direction, that is away from the cell body.

Question 3

Describe how the structure of neurones allows them to carry out their function?

Answer 3

• The structure of neurone, that is the length of axons as well as the polarised nature of the neurone membrane in the resting state, where the outside of the membrane is positively charged and the inside is negatively charged, enables that neurones to carry electrical impulses called action potentials.

Question 4

What is a resting state and why does it occur?

Answer 4

• As previously mentioned, nerve cells are polarised in their resting state.
• This occurs as a result of an imbalance between sodium ions and potassium ions, thus giving the inside of the axon a negative charge in comparison to the external environment.
• As a result of the polarisation, there is a difference in the voltage across the axon membrane, with a value of -70mV known as the resting potential.

Question 5

How is the resting potential generated and maintained?

Answer 5

• The resting potential is generated and maintained with the help of the sodium-potassium pump which moves sodium ions out of the axon thus creating an electrochemical gradient with a higher concentration of sodium ions outside the axon.
• This is because the membrane is not permeable to sodium ions.
• The sodium-potassium pump is also involved in transporting potassium ions into the axon.
• However, the potassium ions move back in by facilitated diffusion due to the presence of potassium ion channels which are mainly open, compared to sodium ion channels which are mainly closed.
• As a result, the outside of the axon is positively charged due to the imbalance of positively charged ions.
• For every three sodium ions that are pumped out of the axon, two potassium ions are pumped in.
• The pumping of ion requires the use of ATP for active transport.

Question 6

How is the action potential generated?

Answer 6

• Upon stimulation, the axon membrane becomes depolarised.
• This occurs as following,
  o Firstly, the excitation of the neurone cell, triggered by a stimulus, causes the sodium ion channels to open, as a result making it more permeable to sodium ions.
  o These subsequently diffuse into the axon down the electrochemical gradient, as a result making the inside less negative.
• Upon reaching the threshold of -55mV, even more sodium ion channels open eventually giving a potential difference of +40mV.
• This is the end of depolarisation and now repolarisation starts.
• Repolarisation is achieved as a result of sodium ion channels closing and potassium ion channels opening.
• The potassium ions diffuse out of the neurone down the concentration gradient and eventually restore the resting potential.
• However, as the closing of potassium ion channels is slightly delayed, this leads to hyperpolarisation i.e., when the potential difference becomes more negative than the resting potential.
• The resting potential is then achieved with the help of the sodium-potassium pump which returns the potential difference to the value of -70mV.

Question 7

Describe the passage of an action potential?

Answer 7

• If an action potential travels along an unmyelinated axon, the wave of depolarisation moves to the adjacent resting region where sodium ions go on to trigger a change in potential difference, thus stimulating another action potential.
• The presence of a myelin sheath speeds up the passage of an action potential down an axon.
• Due to the presence of a myelin, no action potential can be generated.
• Therefore, the action potential has to jump between gaps in the myelin called nodes of Ranvier.
• The mechanism they do this by is called saltatory conduction.

Question 8

What affects the speed of an action potential?

Answer 8

• Presence of absence of myelin sheath
  o If an axon is myelinated, then saltatory conduction can occur which is much faster than generating an action potential at every point along the axon.
• Diameter of the axon
  o The greater the diameter of the axon, the faster the conduction e.g. the giant squids axon is 1mm in diameter compared toa humans at 22um.
• Temperature
  o If the temperature is increased, then the ions will diffuse more rapidly.
  o It will also affect the rate of respiration and therefore the production of ATP needed in the sodium-potassium pump.

Question 9

What is the refractory period and why is it important?

Answer 9

• After an action potential’s passage, there is a short period during which the neurone membrane cannot be excited as the sodium channels enter a recovery stage.
• This period is known as the refractory period and serves an important role in ensuring that an action potential can only pass in one direction as discrete signals.

Question 10

What is the all or nothing principle and what is its purpose?

Answer 10

• Finally, the all-or-nothing principle means that either an action potential is produced, or it is not.
• A threshold value must be reached in order for an action potential to be create, with all action potentials being of the same strength.

Question 11

What are synapses?

Answer 11

• Synapses are junctions between two neurones.

Question 12

What roles do synapses have other than acting as a junction between two neurones?

Answer 12

• Prevent action potentials from going in the wrong direction.
  o They do this because the neurotransmitter is only made in the presynaptic neurone, with receptors only on the postsynaptic neurone.
• They can amplify the effects of low frequency action potentials using summation.
  o This can be temporal in which a single presumaptic neurone releases neurotransmitters manner times over a short period, causing threshold potential to be reached in the post synaptic neurone.
  o Or it can be spatial where multiple presynaptic neurones release neurotransmitters to reach the threshold value.
• Some synapses can be inhibitory and prevent the movement of action potentials.
  o Most though are excitatory.

Question 13

Describe how an action potential moves across a synapse?

Answer 13

1. Upon the arrival of an action potential, the presynaptic membrane depolarises therefore causing calcium ion channels to open subsequently allowing calcium ions to enter the presynaptic neurone.
2. The presence of calcium ions in the neurone causes the fusion of synaptic vesicles, filled with a particular neurotransmitter such as acetylcholine, with the presynaptic membrane.
3. The neurotransmitter is then released into the synaptic cleft, that is the gap between the two neurones.
4. The neurotransmitter then diffuses across the synaptic cleft towards the post synaptic neurone. Here the neurotransmitter binds to the receptors located on the postsynaptic membrane therefore stimulating the opening of sodium ion channels which enable sodium ions to enter the neurone down their concentration gradient by diffusion.
5. After the new action potential has been created the enzyme acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) which diffuses back across the synaptic cleft and back into the presynaptic neurone where it can be reassembled and reused. The benefit of this is both the recycling of the neurotransmitter as well as the fact that it prevents continuous generation of an action potential in the post synaptic neurone.