Electrical and Chemical Signalling; propagation of the action potential and synapses

Question 1

The action potential can…

Answer 1

travel in one direction only

Question 2

how do unmyelinated neurones conduct action potentials

Answer 2

At axon hillock depolarisation reaches a threshold and a 1st action potential is generated, the sodium diffuses passively into the adjacent site

• when the threshold potential reached voltage gated sodium ion channels open, sodium influxes and secondary action potential is generated
• sodium ions can either diffuse backward or forward but previous site still repolarising this is the refractory period
• sodium can only diffuse forward
• threshold is reached - sodium influx third action potential is generated
• Action potentials are spread by passive voltage changes along axon membrane
• When action potential occurs at trigger site (axon hillock) positive charges rush into the cell
• Creates a local zone in both extracellular and intracellular fluid with a sudden change in charge
• ions move to a path of least resistance to the positive ions tend to move towards the negative areas, when the 1st site is repolarising it means there are still positive ions there so the sodium in the 2nd site tends to move away from that and move forward where there are more negative ions

Question 3

how does action potentials happens in the myelinated neurones

Answer 3

• Myelin provides high resistance to ion flow across the membrane as it prevents ions leaking across plasma membrane
• Resistance is lost at node of Ranvier (gap in myelin sheath)
• Voltage gated sodium and potassium channels concentrated at node
• when sodium diffuses through the node of Ranvier it goes along the axon to the next node of Ranvier
• 2nd action potential is generated when threshold is reached
• Local current that flows to the next node is strong enough to initiate an action potential
• continues down the myelinated axon with the action potentials jumping from one node to the next
• this is called saltatory conduction

Question 4

what is the gap between internodes

Answer 4

1.5mm between nodes

1um nodes

Question 5

why does the action potential jump down the axon

Answer 5

1. As the charge spreads down an axon, myelination (via Schwann cells) prevents the ions from leaking out across the plasma membrane
2. Charge spreads unimpeded until it reaches an unmyelinated section of the axon called the node of Ranvier which is packed with sodium channels
3. In this way electrical signals continue to jump down the axon much faster than they can move down an unmyelinated cell

Question 6

in Unmyelinated neurones what happens to the current

Answer 6

• current flows along an unmyelianted axon can leak across the cell membrane or travel through the cytoplasm depending on the site of least resistance this is because the membrane is not impermeable

Question 7

how do you decrease the current leakage from the unmyelianted neurones

Answer 7

increase the diameter of the axon

Question 8

how do you increase conduction velocity

Answer 8

increase the diameter

increase the insulation

Question 9

how do you reduces the resistance in myelinated neurones

Answer 9

increase myelination

Question 10

how do you reduce cytoplasmic resistance

Answer 10

increase the diameter

Question 11

what does the myelin sheath do in the myelinated axon

Answer 11

Axon diamter
- Large diameter axons offer less resistance to current flow this is because there is more volume and space for sodium to move to the next site so sodium can move faster and action potential is generated faster

Myelin
- layers of lipid rich cell membrane wrapped around axon - this repels ions
- reduces ion leakage across axon membrane
more sodium spreads to the next site therefore threshold is reached faster and action potential is generated faster
- Myelin limits the amount of membrane in contact with extracellular fluid
- Current leakage out of the axons is minimised

Question 12

how are unmyelinated axons myelinated

Answer 12

• They are not unmyelinated – rather than having one Schwann cell wrap around the axon, they have one Schwann cell with multiple axons in it, has a single layer of Schwann cell membrane in it

Question 13

what are the difference between the electrical and chemical synapse

Answer 13

chemical

• presynaptic cell; electrical action potential which produces a chemical neurotransmitter which is released into the synaptic cleft
• postsynaptic cell - chemical neurotransmitter causes the electrical signal
• slow transmission
• more common

electrical

• electrical signal directly transmitted to the next cell via gap junctions
• rapid transmission

Question 14

What happens when the action potential reaches the synapse

Answer 14

• The digital signal (action potential) is converted into an analogue signal (chemical neurotransmitter)
• Different neurotransmitters are excitatory or inhibitory
• The addition of all the excitatory and inhibitor signals is called summation

Question 15

what happens when the action potential reaches the synapse

Answer 15

• The digital signal (action potential) is converted into an analogue signal (chemical neurotransmitter)
• Different neurotransmitters are excitatory or inhibitory
• The addition of all the excitatory and inhibitor signals is called summation

Question 16

describe the voltage activated calcium channels at the synapse (synaptic transmission)

Answer 16

• Action potential arrives at a synaptic terminal
• Depolarisation opens voltage activated calcium ion channels
• [Ca 2+]i very low 10-8M,
• [Ca2+]o 10000 time higher 10-3M
• I == inside
• O = outside
• Large inward gradient concentration and electrical

Question 17

describe the neurotransmitter release at the synapse

Answer 17

• Active zones – arrays of calcium channels
• Vesicles – dock
• SNARE complex – anchors vesicles to cytoskeleton, they wrap around each other tighten and pull the vesicle towards the cell membrane so the vesicle and membrane touch
• Calcium ions 2+ enter – vesicles fuse with membrane to release contents into the synaptic cleft
• Floating vesicles move towards the cell membrane so they are ready to attach for the next wave of neurotransmitter release

Question 18

what are the type of neurotransmitters

Answer 18

Acetylcholine – learning
Amino acids 
-	Glutamate – memory
-	Y-aminobutyric acid (GABA) – calming 
-	Glycine
Monoamines 
-	Noradrenaline (adrenaline) – autonomic but used in CNS
-	Serotonin (5-HT) - mood
-	Dopamine – pleasure
Neuropeptides including opiates – endorphins – take pain away 
Others 
-	Purinergic – adenosine, ATP, ADP, AMP
-	Gasses (NO)

Question 19

What does the EPSP result in
- what is the neurotransmitter that it causes
what is it linked to

Answer 19

• Postsynaptic memrbane potential is more likely to resultin carrying an action potential
• Glutamate – gated channels this casue an net influx of sodium ions and depolarization of postsynaptic neurone
• Linked to ligand gated ion channel for sodium
• more likely for action potential to be generated

Question 20

what does the inhibitory postsynaptic potential do

Answer 20

• Decreases the ability of the membrane to reach threshold and carry an action potential
• caused by binding of inhibitory neurotransmitters such as GABA/glycine
• causes a chlorine influx this hyper-polarises the postsynaptic neurone

Question 21

what is temporal summation

Answer 21

• Post synaptic potentials at the same synapse occur in rapid succession
• Because 1st potential does not have time to dissipate the next potentials add to previous one and increase the change in potential

Question 22

what is spatial summation

Answer 22

• Multiple post synaptic potentials from different synapses occur at about the same time and add
• EPSP’s from the different synapses are not strong enough to generate an action potential but by reinforcing one another they may trigger an action potential

Question 23

how are small neurotransmitters synthesised

Answer 23

• made/packaged in the axon terminal
• reuptake into axon terminal via transporter
• such as ACh, monoamines (dopamine, adrenaline, noradrenaline, serotonin) and amino acids (glutamate, GABA, glycine)

Question 24

how are large (peptide) neurotransmitters synthesised

Answer 24

• made and packaged in the cell body
• vesicles are transported along axon to the presynaptic terminal
• degraded in synapse by protases
  e

Question 25

how are the vesicles recycled

Answer 25

• Ultrafast endocytosis
• Kiss and run
• Clathrin mediated endocytosis

Question 26

how are vesicles recycled

Answer 26

endocytosis

Question 27

what happens if vesicles are not recycled

Answer 27

If vesicles not recycled most neurons would completely deplete their internal storage mechanism in a few seconds.

Question 28

what is acetylcholine

Answer 28

• Small neurotransmitter

- Made in the synapse

Question 29

what happens to small neurotransmitters

Answer 29

• All small Neurotransmitters are transported back into the neurons and usually recycled

Question 30

what happens to protein neurotransmitters

Answer 30

• Protein Neurotransmitters are degraded in the extracellular fluid by non-specific proteases

Question 31

Whats the difference between a myelinated axon and an unmyleinated axon

Answer 31

myelinated

• has myelin sheath and nodes of Ranvier
• action potential generated only at nodes of Ranvier
• saltatory conduction(fast)

unmyelinated axon
- no myelin sheath (so no NoR)
- action potentials all along the axon membrane
continuous conduction (slow)

Question 32

Receptors

Answer 32

Aα - proprioceptors and motor neurones - these are receptors that detect change in muscle length or tension, this is relayed to motor neurones to cause muscle contraction or relaxation
Aβ - mechanoreceptors (touch, pressure)
Aδ - fast pain fibres
C - slow pain fibres

Question 33

Name some examples of proprioceptors

Answer 33

• muscle spindle (found within skeletal muscle)

- Golgi tendon organ (found in tendons attaching muscle to bone)

Question 34

what does calcium do in the synapse

Answer 34

• calcium acts as a 2nd messenger
• stimulates the translocation of free vesicles in the presynaptic knob to presynaptic membrane
• triggers fusion of docked vesicles with the presynaptic membrane
  SNARE proteins anchor synaptic vesicles to the presynaptic membrane

Question 35

What is partial fusion

Answer 35

only a small part of vesicle fuses with membrane, so a pore is formed through which neurotransmitters are released

Question 36

What is full fusion

Answer 36

vesicle fuses completely with presynaptic membrane

Question 37

What is ultrafast endocytosis

Answer 37

Full fusion
Immediately after fusion, portions of cell membrane internalised to form vesicles
- less than a 0.1s

Question 38

what is kiss and run

Answer 38

Partial fusion
As soon as vesicle is emptied, the pore closes and the vesicle moves away
- less than 2s

Question 39

what is Cathrin mediated endocytosis

Answer 39

Full fusion
Clathrin then coats portions of the cell membrane to form a vesicle and internalises it
- 10 -20s