Nervous System 3

Question 1

What makes a neuron an excitable cell?

Answer 1

The ability of a cell to respond to electrical/chemical stimuli by producing an action potential

Question 2

Define action potentail

Answer 2

a brief change in electrical potential across the cell’s membrane involving the movement of ions (Na+ and K+) across the membrane – in response to stimulation

Question 3

What is the resting membrane potential of a neuron ?

Answer 3

-70mV

Question 4

What is the ionic basis of the resting membrane potential?

Answer 4

The difference in ion concentration between the inside and outside of the axon

Question 5

Where is the greater concentration of K+ and Na+?

On the inside or outside of the cell?

Answer 5

Na+= greater con. on the outside

K+= greater con. on the inside

Question 6

Which enzyme helps maintain the NA+/K+ concentrations in the cells?

Answer 6

Na+ /K+ ATPases

Ion pumps maintain this as well

Question 7

What toxin can block Na+ /K+ ATPases ?

Answer 7

Ouabain

Question 8

What is NA+ movement responsible for?

Answer 8

Action potential

Question 9

What is K+ movement mainly responsible for?

Answer 9

Resting Membrane potential

as membrane is more permeable to K+

Question 10

Describe the membrane potential graph that shows an action potential

Answer 10

x= time (msec)
y= membrane potential mV
Ek= lowest trough
E Na= almost peak

Question 11

Describe an action potential in 8 steps

Answer 11

1. Membrane at rest Pk>PNa+
2. Membrane is passively depolarised
3. Threshold for activation of voltage gated Na+ channels so Na+ depolaries membrane PNa> PK
4. Positive Feedback cycle of depolarisation. Em overshoots zero.
5. Em approaches ENa+. Driving force of the current decreases. Two time dependent events occur; Na+ channels inactivate, delayed rectifier K+ channels start to open ; PNa+ decreases and PK+ increases. membrane starts to depolarise
6. Na+ channels inactivate and K+ current repolarizes the membrane and Em returns towards the resting value.
7. Na+ channels are inactivated and K+ still opn so Pk>Pna and Em moves towards Ek. HYperpolarisation from A level
8. Em return to resting value as K+ channel close

Question 12

Describe the shape of the action potenial graph

Answer 12

Plateau then gradual increase then sharp peak then dip below plateau line before returning to the plateau

Question 13

Name the two types of refractory periods

Answer 13

Absolute and relative

Question 14

Define the relative refractory period

Answer 14

Na+ channels returning to their resting state BUT larger threshold for activation

Question 15

Define Absolute refractory period.

Answer 15

Immediately after AP spike all the Na+ channels are inactivated and AP cannot be generated

Question 16

Why are only a few ions needed to generate an Action Potential?

Answer 16

The extra and intracellular layers are conducting. The lipid membrane is non-conducting CAPACITOR ( stores electrical charge)

Question 17

What properties of a neuron allow for rapid conduction of signals?

Answer 17

1. Rapidly activated voltage-dependent channels that increase INWARD (Depolarising) current in response to DEPOLARISATION - positive feedback
2. Propagation of voltage-change over membrane surface
3. Myelination increases ratio of Rout/Rin resulting in increased conduction velocity of an axon. Faster AP

Question 18

In the CNS and peripheral nerves what forms the Myelin?

Answer 18

CNS= oligodendrocytes
Peripheral = Schwann cells

Question 19

What does a myelinated axon allow?

Answer 19

Rapid Action Potential transmission
Zero signal loss
Small axon diameters
Signal jumps from Node to Node (Node of Ranvier); signal can only occur at exposed axonal membrane
Saltatory conduction (~100m/sec +)
Lots of Na+ in meilein gaps

Question 20

Where are voltage-gated channels clustered?

Answer 20

At the Nodes

Question 21

Conduction Velocity in unmyelinated axons

Describe how this occurs
What is the typical velocity

Answer 21

Adjacent active and inactive regions exchange charge
This depolarizes the next section of the axon to threshold
The action potential does not travel in the reverse direction as the preceding section is in the refractory period
Typical velocity ~ 2 -5 m/sec (compare to 100m/sec for saltatory cond.)
Increase axon diameter increases velocity

Question 22

Define synapse

Answer 22

Electrochemical communication between two or more neurons occur via synapses between axons and dendrites

Question 23

Describe the first 2 steps of Synaptic transmission

Answer 23

1) The neurotransmitter must besynthesized and storedin vesicles so when an AP arrives at the nerve ending, the cell is ready to pass it along to the next neuron.

2) When an AP arrives at the terminal, the neurotransmitter must be quickly andefficiently released from the terminaland into the synaptic cleft.

Question 24

Name 2 examples of neurotransmitters

Answer 24

Acetylcholine, Noradrenaline, GABA, glutamate

Question 25

Describe the last 2 steps of Synaptic transmission

Answer 25

3) The neurotransmitter must then be recognized by selective receptors on the postsynaptic cell.  This allows the signal to initiate another AP (‘propagation’ of signal). {Or, the receptors act to block the signals of other neurons also connecting to that postsynaptic neuron (‘inhibition’ of signal).}   4) After recognition by the receptor, the neurotransmitter must be inactivated so that it does not continually occupy postsynaptic receptor sites. 

Question 26

State 2 reasons why neurotransmitter inactivation is beneficial

Answer 26

avoids constant stimulation of postsynaptic cell | frees up receptor sites to receive additional neurotransmitter molecules

Question 27

Describe how the neurotransmitter is released at the synapse

Answer 27

AP arrived at the Synapse Which triggers fusion of neurotransmitter containing vesicles to the plasma membrane and contents released into the synapse This happens because the AP depolarizes the nerve terminal and opens voltage-gated Ca++ channels triggering fusion of neurotransmitter containing vesicles with the plasma membrane.

Question 28

What happens to neurotransmitter release when extraceulluar Ca 2+ is removed and Na+ channels are closed?

Answer 28

No Ca2+ = no release | No Na+ channel = still release

Question 29

What happens on the postsynaptic membrane?

Answer 29

The postsynaptic current depolarises the postsynaptic membrane. The postsynaptic potentials summate until the membrane reaches the threshold for action potential generation.

Question 30

What are EPSPs?

Answer 30

Excitatory Postsynaptic Currents and Postsynaptic Potentials Xesicular release (only 1-2 vesicles released) EPSCs (excitatory post-synaptic currents) – current through ligand-gated channels / neurotransmitters Membrane depolarised (Na+ moves into the cell) Glutamate moves across the synapse Many little response

Question 31

What are IPSPs?

Answer 31

Inhibitory Postsynaptic Currents and Postsynaptic Potentials current flow through inhibitory neurotransmitter activated channels IPSPs can summate to reduce excitability, hyperpolarise cell (K+ moves out of the cell, Cl- ions moving into the cell) Uses GABA to move across the synapse leading to a reduced probability of AP generation

Question 32

what properties of a neuron allow for rapid conduction of signals?

Answer 32

large diameter or myelination