6.5 - Neurons and Synapses

Question 1

How are the various parts of our bodies able to to respond to the environment?

Answer 1

communication between systems

Question 2

there are 2 ways the body relays information:

Answer 2

• the nervous system: neurotransmitters, nerve cells, neurons
• the endocrine system: glands, horomones

Question 3

the myelination of nerve fibres allows for saltatory conduction

Answer 3

• Nerves are able to transmit information rapidly from one part of your body to another by electrical signals called nerve impulses
• This type of nerve conduction only occurs in non myelinated neurons like that of the giant squid

humans have myelinated neurons - the conduction is different

Myelinated neurons:

• This is much faster than non-myelinated signal propagation
• speed: ~200m/s (vs. ~2m/s in non-myelinated neurons
• one estimate states that it would take 5000x more ATP for a non-myelinated neuron to keep pace with a myelinated one

Saltatory conduction

• from the latin saltare, meaning ‘to jump”
• Propagation along myelinated axons from one node of ranvier o the next node
• increases conduction velocity of action potentials

Question 4

Resting potential

Answer 4

the negative charge registered when a nerve is at rest and not conducting an electrical impulse

Question 5

Action potential

Answer 5

the positive charge generated at the nerve impulse

Question 6

depolarization

Answer 6

a change from the negative resting potential to the positive action potential

Question 7

repolarization

Answer 7

the change in the electrical potential from the positive action potential back to the negative resting potential

Question 8

A nerve impulse is only initiated if the threshold potential is reached

Answer 8

• the nerve impulse occurs when a stimulus is strong enough to depolarize the neuron and pass the threshold potential, after which an action potential will be generated
• it is an all or nothing situation (if the threshold is pass, the action potential will occur, and it will be the same magnitude each time, regardless of the strength of the stimulus.)

Question 9

Setting up a nerve impulse p1

Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Answer 9

Resting potential:

• resting because it isn’t transmitting an impulse
• it is not inactive - ie homeostasis takes energy
• it spends a lot of energy to maintain the potential difference (-70 mv)
• the inside is negative in comparison to the outside due to:
  - unequal distribution of charged ions
  - Na and K

Question 10

setting up a nerve impulse p2

Answer 10

• Potential is caused by a combination of active transport and diffusion of Na and K, and the presence of negatively charged organic compounds inside the neuron.
• Na - K pump actively transports 3 Na out and only 2 K in, allowing for a slight increase in potential
• BUT, Na cannot easily re-enter the cell by diffusion (Na channels are closed), whereas K can freely diffuse across the membrane down the concentration gradient. The membrane is ~50 times more permeable to K+ than Na+

Question 11

Depolarization explanation

Answer 11

• stimulus cause the membrane potential to change a little
• the voltage gated ion-channels can detect this change, and when the potential reaches the threshold the sodium channels open causing sodium to rush in, making the inside of the cell more positive
• It is referred to as depolarisation because the normal voltage polarity is reversed.

Question 12

Repolarization explanation

Answer 12

• the membrane potential passes 0v
• the potassium channels open which allows potassium ions to rush out
• this makes the inside more negative again
• since this restores original polarity, it is called repolarisation

Question 13

hyperpolarization explanation

Answer 13

• The K channels close slowly (membrane becomes more negative than resting potential)
• This makes a refractory period, where the neuron cannot be further stimulated

Question 14

The whole process of action potentials and depolarization/repolarization of membrane

Answer 14

1) resting potential is maintained by active transport (na and k pump)

2) arrival of AP causes depolarisation of adjacent sections of the neuron.
- this causes local Na diffusion and a current
- If the current is enough to rise above the threshold, voltage gated Na channels open and Na rushes in. Internal potential is depolarized.

3) K channels are opened and K diffuses out. Internal cell is repolarised

4) Refractory period is when the channels rest between openings, ensuring one-way impulse flow.
- resting potential is then returned by active transport

Question 15

Summary of synaptic transmission

Answer 15

1) nerve impulse reaches terminal end of pre-synaptic neuron
2) depolarization causes voltage gated calcium channels to open. Ca rushes in
3) Ca causes synaptic vesicles to move to the membrane and fuse
4) neurotransmitter that were stored in the synaptic vesicle now diffuse across the synaptic gap
5) Neurotransmitters bind with post-synaptic receptors. (NTs are specific to receptors)

6) Sodium channels open, causing Na to enter, leading to depolarisation of the post synaptic neuron. An action potential is initiated.
The nerve impulse is then propagated along the post-synaptic neuron

Question 16

Secretion and reabsorption of acetylcholine by neurons at synapses

Answer 16

• Acetylcholine : a neurotransmitter found between neurons and muscle cells
• motor neurons release acetylcholine in order to activate muscles
• cholinergic : synapses that use a ACH
• It travels across the synapse to bind to its receptor
• The enzyme acetylcholinesterase rapidly breaks down acetylcholine in the synapse into (choline + acetate)
• Choline is absorbed by pre-synaptic neuron and re-used to make more acetylcholine

Question 17

An insecticide example

Answer 17

• Neonicotinoids is an insecticide (synthetic chemical similar to nicotine)
• it binds to the acetylcholine receptor irreversibly
• leads to paralysis and death in insects
• neonicotioids are very widely used (because they are safe for mammals)
• But its non-specific so it kills helpful insects like bees

Question 18

Action Potential full definition

Answer 18

• it is the depolarization and repolarization of the electrical potential across a plasma membrane as a nerve impulse passes along a neuron

Question 19

Explain how a nerve impulse passes along a non-myleniated neuron

Answer 19

• Action potential activates voltage gated sodium channels
• sodium ions rush into the axon
• potential increases;
• If it increases beyond threshold, more sodium channels open
• axon depolarises, stimulating adjacent sections
• Potassium channels open, potassium rushes out
• potential is reduced (repolarisation)
• refractory period ensures one-way conduction of action potential
• sodium potassium pump returns axon to resting potential

Question 20

Explain the principles of synaptic transmition

Answer 20

• nerve impulse/action potential travels to the end of the presynaptic neuron
• triggers influx of Ca
• causes synaptic vesicles to fuse with the membrane
• releases neurotransmitters into the synaptic cleft
• neurotransmitter diffuses across cleft
• nt binds to receptors on postsynaptic neuron
• causes ion channels to open on the post synaptic neuron
• Ca pumped back into the synaptic cleft by active transport

Question 21

Outline the use of 4 methods of membrane transport in nerves and synapses

Answer 21

active transport:

• Sodium Potassium pump resets resting potential
• re-uptake of neurotransmitters to the pre-synaptic neuron following synaptic transmission
• removal of Ca from presynaptic neuron following synaptic transmission

Simple diffusion:

• diffusion of NT across synaptic cleft
• diffusion of K ions out of axon in resting potential

facilitated diffusion:

• opening of voltage gated Na and K channels in action potential
• opening of voltage gated Ca channels at pre-synaptic terminal
• Na channels at post-synaptic terminal to propagate AP

Vesicle transport:

• influx of Ca activates vesicles of neurotransmitters
• exocytosis of NT from pre synaptic neuron to synaptic cleft

Question 22

myelin sheath

Answer 22

a myelin sheath, which is an insulating layer that speeds up the transmission of a nerve impulse

Question 23

saltatory conduction short explanation

Answer 23

The myelin sheath forces the nerve signal to jump from one node of Ranvier to the next, which accounts for the faster speed of impulse transmission.