Neurones and nervous impulses

The nervous system overview

• The human nervous system is made up of the brain and spinal cord, which form the central nervous system, and nerves, which form the peripheral nervous system. 

• Information is transferred along neurones in the form of action potentials, sometimes known as nerve impulses. These are fleeting changes in the electrical charge on either side of the plasma membranes. 

Dendrites and dendrons

Dendrite: extensions that receive and integrate signals from neighbouring cells, conducting impulses towards the cell body 

Dendron: responsible for receiving signals from other neurones 

Myelin sheath

a protective layer of lipids and protein coating the neurone which allows the rapid conduction of electrical signals along the axons. It not only insulates the axon but also increases the speed of signal transmission for efficient nerve communication 

Cell body/soma

houses the nucleus and organelles e.g. ribosomes and mitochondria. Proteins and neurotransmitters are made here 

Axon

used to transmit electrical impulses away from the cell body, enabling the transmission of signals to other neurones, muscles or glands 

Nodes of Ranvier

uninsulated gaps in the myelin sheath, the gaps between the Schwann cells. These contain a high concentration of voltage-gated sodium channels, essential for regenerating action potential. Nodes of Ranvier are therefore crucial for the rapid transmission of nerve impulses in neurones 

Schwann cells

wrap around the axon along its length, providing insulation (because they are lipids), support and protection of the neurone. They are responsible for the formation of the myelin sheath 

What are the different structures between a sensory neurone and a motor neurone

Sensory neurone

• myelin sheath, dendrons and receptor cell

Motor neurone

• schwann cells and nodes of Ranvier

How is a resting potential established?

• During resting potential, 3Na⁺ ions are pumped out of the axon for every 2K⁺ ions that are pumped into the axon via active transport using a sodium-potassium pump.

• In addition, some potassium channels are open meaning the membrane is more permeable to potassium ions - therefore some potassium ions move out of the axon by facilitated diffusion. (The membrane is impermeable to sodium ions.) 

• The result is an imbalance of ions, with more positive ions outside of the axon than are inside and so the membrane potential is negative, at -70mV. 

Acronym to use when describing action potentials

CHIIMP

• CH – state the type of ion CHannel that either opens or closes 

• I – state which Ions move as a result  

• I – state whether the Imbalance of (positive) ions is inside or outside of the axon  

• MP – state how this affects the Membrane Potential 

Process of depolarisation

• When the post-synaptic neurone is at rest, there is a higher concentration of sodium ions outside the membrane than inside

• A neurotransmitter (acetylcholine) binds to receptor sites on the post-synaptic membrane

• When this neurone is stimulated, permeability to sodium changes, more gated sodium channels open, causing more sodium ions to move into the axon via facilitated diffusion. (membrane potential become more positive than outside)

• This produces a small depolarisation called a generator potential, and many generator potentials can combine to produce a larger one

• This causes the resting potential to become less negative. It changes from roughly –70mV to +40mV.

• This reversal in the potential difference is called action potential, and we say that the membrane has been depolarised. 

Action potential

• when the neurone’s voltage increases beyond a set point from the resting potential, generating a nervous impulse

• An action potential occurs when the membrane depolarises to a certain threshold, if this threshold (-55mV) is not reached, the action potential will not be triggered and no impulses are produced.

The all-or-nothing principle

• The power of a stimulus is not proportional to the power of the action potential. In simpler terms, the action potential will either occur, or it won’t, it will not be a graded response.

• Any stimulus that triggers depolarisation will therefore always peak at the same maximum voltage. Bigger stimuli will instead increase the frequency of action potentials.

• This principle is important as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment which would overwhelm them 

Wave of depolarisation

• Once an action potential occurs in one part of the neurone, it will stimulate an action potential in the adjacent part of the neurone, creating a ‘Mexican wave’ of depolarisation.

• This wave of depolarisation occurs because the sodium ions which diffuse into the neurone diffuse sideways, causing voltage-gated ion channels in the next portion of the neurone to open, so sodium ions move into the neurone further along the membrane.

• Action potential is conducted without loss of amplitude along the length of the axon, remaining equally intense and not losing energy 

Repolarisation

• At a potential difference of around +40mV, the voltage-gated sodium ion channels close and voltage-gated potassium channels open.

• Potassium will now diffuse out of the neurone down the potassium ion concentration gradient. The cell becomes more negative again. This is called repolarisation 

Refractory period/hyperpolarisation 

• Potassium ion channels are slow to close so there is a slight ‘overshoot’, in which the neurone becomes more negative than the original resting potential. This happens because too much potassium leaves the cell 

• During repolarisation and hyperpolarisation the neurone cell membrane cannot receive another stimulus.  
  - means that the action potential can only travel in one direction and not backwards, which would prevent a response 
  - produces discrete impulses so action potentials do not overlap immediately one after another and can be separated from each other  
  - also limits the number of action potentials in a given amount of time, preventing over reaction to a stimulus overwhelming the senses 

During an action potential, the membrane potential rises to +40mV and then falls. Explain the fall in membrane potential [3]

1. Potassium channels open

2. Potassium ions move out of axon (via facilitated diffusion)

3. Sodium channels close

After exercise, some ATP is used to re-establish the resting potential in axons. Explain how the resting potential is re-established [2]

1. Pump/active transport

2. Of sodium out of the axon/of potassium in

Explain how a resting potential is maintained across the axon membrane in a neurone [3]

1. Higher concentration of potassium ions inside and higher concentration of sodium ions outside of the neurone

2. Membrane is more permeable to potassium ions leaving (than sodium ions entering)

3. Sodium ions actively transported out of axon and potassium ions actively transported in

Explain why the speed of transmission of impulses is faster alone a myelinated axon than along a non-myelinated axon [3]

1. Myelinated provides (electrical) insulation

2. In myelinated there is saltatory conduction/depolarisation at nodes of Ranvier

3. In non-myelinated depolarisation occurs along the whole/length of the axon

A scientist investigated the effect of inhibitors on neurones. She added a respiratory inhibitor to a neurone. The resting potential of the neurone changed from -70mV to 0mV. Explain why [3]

1. No/less ATP produced

2. No/less active transport (or) sodium-potassium pump inhibited

3. Electrochemical gradient not maintained
   (or)
   No net movement of sodium and potassium ions (same concentration on either side of the membrane)

What are synapses?

The junction between two neurones, which an electrical impulse cannot pass across. This can also be the gap between a neurone and an effector

What is a cholinergic synapse?

The connection between two or more neurones where the neurotransmitter that diffuses across the synaptic cleft is acetylcholine  

Factors affecting the speed at which action potential moves along the neurone

• myelination

• diameter

• temperature

What effect does myelination have on speed of transmission?

• known as saltatory conduction

• Myelinated neurones increase the speed of transmission. The myelin sheath (made up of Schwann cells) acts as an electrical insulator, which means that ions cannot move into or out of the myelinated portions of the neurone 

• However, there are gaps in the myelin sheath called nodes of Ranvier, where sodium ion channels and potassium ion channels are concentrated (ion exchange can only occur where there is no myelin sheath present) 

• Action potentials occur only at the nodes of Ranvier – when one node is stimulated this triggers depolarisation of the next node, causing the action potential to ‘jump’ from node to node 

• Saltatory conduction is much faster than transmission along non-myelinated neurones 

What impact does diameter have upon the speed of transmission?

• An impulse will be conducted at a higher speed along neurones with thicker axons compared to those with thinner axons  
  Thicker axons have  

• Membrane has a greater surface area over which the diffusion of ions can occur 

• Greater volume of cytoplasm (which contains ions). This reduces their electrical resistance so that an action potential can push into the next section faster  

Temperature

• Hotter conditions can speed up the conduction of nerve impulses  

• The colder temperatures mean there is less kinetic energy available for the facilitated diffusion of potassium and sodium ions  

What occurs at a cholinergic synapse (step-by-step)

1. The arrival of an action potential depolarises the pre-synaptic membrane. This causes voltage-gated calcium channels in the pre-synaptic membrane to open, and as a result, Ca2+ enter the cells by diffusion 

2. Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane. This results in the exocytosis of the neurotransmitter acetylcholine 

3. The acetylcholine neurotransmitter diffuse across the synaptic cleft and bind to receptor molecules in the post-synaptic membrane 

4. Binding of neurotransmitters causes voltage-gated sodium ion channels to open, allowing sodium ions to diffuse into the post-synaptic neurone 

5. A generator potential is established in the generator region of the neurone 

6. If the combination of generator potentials is sufficient to take the membrane potential above the threshold potential, the change in potential difference causes voltage-gated sodium channels to open 

7. The rapid influx of sodium ions causes the depolarisation to peak at +40mV on the inside of the cell. At this point, the post-synaptic neurone transmits the action potential 

8. Effects are short-lived because the neurotransmitter is quickly hydrolysed by enzymes. This causes ion channels to close, and the synaptic response is terminated. Acetylcholinesterase hydrolyses acetylcholine --> acetate + choline. Choline is actively reabsorbed into the presynaptic neurone where it reforms acetylcholine  

Function of synapses

Synapses slow down the transmission of action potentials by 0.5ms, due to the time taken for: 
- vesicles to move to the presynaptic membrane  
- neurotransmitters to diffuse across the synaptic cleft 

Unidirectionality

The flow of impulses is only in one direction because -  
- the neurotransmitter is stored in vesicles in the presynaptic bulb and is released from the presynaptic membrane  
- the receptor molecules are only on the post-synaptic membrane 

Spatial summation

• One presynaptic neurone does not release sufficient neurotransmitter to exceed the threshold value of the postsynaptic neurone 

• More than one presynaptic neurone simultaneously releasing neurotransmitter will cause enough to be released to exceed the threshold value of the post-synaptic neurone  

• (impulses from multiple pre-synaptic neurones at the same time) 

Temporal summation

• A low frequency of action potentials will not release sufficient neurotransmitter from one presynaptic neurone to reach the threshold value in the postsynaptic neurone  

• Two action potentials in quick succession combine to exceed the threshold in the postsynaptic membrane 

• (several impulses from one neuron over time) 

What is summation?

‘Filtering out’ continual, unnecessary or unimportant background stimuli. If a neurone is constantly stimulated, the synapses will not be able to renew its supply of transmitter fast enough to continue passing the impulse across the cleft. This ‘fatigue’ places an upper limit of the frequency of depolarisation 

Excitatory neurotransmitters

Excitatory neurotransmitters make an action potential more likely to fire by depolarising the post-synaptic membrane (by blocking the reabsorption of neurotransmitters or causing more to be released) 

Inhibitory neurotransmitters

Inhibitory neurotransmitters make an action potential less likely to fire by hyperpolarising the post-synaptic membrane (by having a similar structure to the neurotransmitter so binds to the receptor instead) 

Effect of GABA

Binding of GABA to receptors increases the influx of Cl- ions into the postsynaptic neurone, making membrane potential more negative. More sodium ions (+) are required to reach the threshold for depolarisation 

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse [6]

1. (Impulse causes) calcium ions/Ca2+ to enter axon

2. Vesicles move to/fuse with presynaptic membrane

3. Acetylcholine (released)

4. (Acetylcholine) diffuses across synaptic cleft

5. Binds with receptors on postsynaptic membrane

6. Sodium ions enter postsynaptic neurone

7. Depolarisation of postsynaptic membrane

8. If above threshold potential nerve impulse/action potential produced