6.5 - Neurons and Synapses

Question 1

Neuron

Answer 1

neuron: nerve cell that transmits nerve impulses (electrical signals) for internal communication of nervous system
- cell body: nucleus in cytoplasm with nerve fibres (outgrowths)
- Dendrites: short nerve fibres
- axons: long nerve fibres

Question 2

Unmyelinated vs. myelinated nerve fibres

Answer 2

Unmyelinated:

• simple nerve fibres
• continuous transmission of nerve impulses (1m/s)

myelinated:

• nerve fibres coated in myelin sheaths (many phospholipid bilayers) deposited by Schwann cells
• saltatory conduction allowing rapid transition of nerve impulses by jumping from node of Ranvier to another (100m/s)
  - node of ranvier: gap between myelin sheats

Question 3

Resting Membrane potential

Answer 3

• 70mV potential difference inside neuron relative to outside due to charge imbalance (more negative inside than out) from
  
  • sodium potassium pump: 3Na+ are pumped out of neuron
    while 2 K+ is pumped in
  • membrane is more permeable to K+ which diffuse out of
    neuron faster than Na+ in
  • some proteins inside nerve fibres are negatively charged

Question 4

Action potential

Answer 4

rapid change in membrane potential from depolarization and repolarization

depolarization:

• change from negative to positive inside the neuron
• sodium channels open moving Na+ from outside to inside the neuron increasing voltage

repolarization:

• change from positive to negative
• sodium channels close
• potassium channels open moving K+ inside to outside decreasing voltage
• sodium-potassium pump reestablishes resting potential

Question 5

Nerve impulse

Answer 5

propagation of action potentials across axons from one terminal to another

• depolarization of one axon triggers the depolarization of the neighbouring axon from local currents
  - inside depolarized, Na+ concentration is higher than non-polarized axon, Na+ moves down gradient increasing potential inside of neighbouring axon
  - Outside depolarized axon Na+ concentration is lower than outside non-polarized axon, Na+ moves down gradient decreasing outside charge which also increases potential inside neighbouring axon
• if threshold potential reached (-50mV): sodium channels open and full depolarization occurs
• Unidirectional nerve impulse:
  - refractive period after depolarization prevents backwards propagation of nerve impulse
  - only one terminal can initiate a nerve impulse, the other passes impulse to other neurons

Question 6

Synaptic transmission

Answer 6

pre-synaptic neuron passing a nerve impulse to a post-synaptic neuron through the release of neurotransmitters in the synapse between neurons

1. Ca2+ enter pre-synaptic neuron from opening of channels due to depolarization at the end-terminal membrane (from nerve impulse)
2. Ca2+ signals vesicles containing neurotransmitters to be released into the synaptic cleft by exocytosis
3. neurotransmitters diffuse through synaptic cleft
4. neurotransmitters bind to receptors on post-synaptic neuron membrane
5. triggers opening of sodium channels, moving Na+ into the post-synaptic neuron
6. post-synaptic neuron reaches threshold potential causing full-depolarization
7. neurotransmitters are broken down in synaptic cleft and return to pre-synaptic neuron

Question 7

Threshold potential

Answer 7

energy potential required to initiate a nerve impulse (-50mV)
if reached:
- sodium channels open increasing potential further causing more sodium channels to open
- positive feedback
- full-depolarization propagating nerve impulse
- may require several pre-synaptic neurons releasing neurotransmitters to reach threshold potential

if not reached:

• no nerve impulse
• sodium-potassium pump reestablishes resting potential

Question 8

neonicotinoides affect on cholinergic synapses in insects

Answer 8

• acetylcholine are the neurotransmitters used in cholinergic synapses
  - acetylcholinesterase is the enzyme that breaks down
  acetylcholine to return choline to pre-synaptic neuron
• neonicotinoids are compounds that block acetylcholine receptors on post-synaptic neuron
• acetylcholine cant bind to receptors preventing synaptic transmission
• insects are paralyzed and die
• humans are less affected because there are few cholinergic synapses