L12 Neurophysiology and Neurosecretion

Question 1

Where do the axons arise from?

Answer 1

Axons arise from the axon hillock

Question 2

What are telodendria?

Answer 2

The branches at the end of axons.

Question 3

What are presynaptic terminals (boutons)?

Answer 3

Small swellings which indicate the termination of telodenrias.

Presynaptic terminals contain small vesicles which contain neurotransmitters.

Question 4

Which statement(s) about resting membrane potentials are correct?

1) When there is a low [Na] and a high [K] outside the cell, K flows out of the cell
2) When there is a low [Na] inside the cell, K flows into the cell
3) When there is a high [Na] and a low [K] outside the cell, K flows out of the cell
4) When there is a high [Na] inside the cell, K flows into the cell

Answer 4

2) When there is a low [Na] inside the cell, K flows into the cell
3) When there is a high [Na] and a low [K] outside the cell, K flows out of the cell

Question 5

What are the key components of a resting membrane potential?

Answer 5

Inside the cell has a negative potential.

Na/K pump

Question 6

How is an electrical gradient created in a nerve cell?

Answer 6

It has a high concentration of K inside the cell and a high concentration of Na outside the cell, producing an electrical gradient

Question 7

True or false: Nerve cell membranes are permeable to sodium

Answer 7

False. The membranes are impermeable to sodium ions and require voltage gated channels for the movement of the ions.

Question 8

True or false: Nerve cell membranes are permeable to potassium

Answer 8

True. Potassium moves down an electrical gradient and a concentration gradient.

Question 9

The resting membrane potential is maintained at:

Answer 9

-70 to -90mV

Question 10

In the Nernst Equation, what do the letters represent?

V = (RT/ZF).ln(C0/C1)

Answer 10

V = equilibrium potential

C0 and C1 = outside and inside concentrations of potassium

R = gas constant

T = absolute temperature

F = Faradays constant

Z = charge of the ion

Question 11

What happens during an action potential?

Answer 11

Na channels open, Na enters the cell = depolarisation.

The Na/K pump opposes it by opening voltage gated K channels = 3Na out, 2K in.

Question 12

Function of intrinsic (local) currents:

Answer 12

Open downstream voltage gated Na channels = initiates another action potential

Question 13

A period of A.P inactivation is known as:

Answer 13

Refractory period. This prevents the back flow of excitation

Question 14

Function of myelinated sheaths:

Answer 14

Increases conduction of the axon, which increases the intrinsic currents and the conduction velocity

Question 15

Where are myelin sheaths formed?

Answer 15

In Schwann cells

Question 16

What are nodes of Ranvier?

Answer 16

Short unmyelinated stretches along the axon

Question 17

True or false: Local anaesthetics have a high affinity for open sodium ion channels

Answer 17

True.

Local anaesthetics target voltage gated channels - especially open sodium ion channels (the more channels open, the greater the effect of the anaesthetic).

The LA maintains ion channel in inactivated state and so the cell cannot be re-stimulated.

Question 18

What is the difference between an electrical and chemical synapse?

Answer 18

• Electrical synapse: Very quick as sends transmissions via pores (i.e. gap junctions). Found in smooth and cardiac muscle.

Chemical synapse: AP causes pre-synaptic membrane to release neurotransmitters which diffuse across the synaptic cleft and bind to receptors on the pots-synaptic membrane. This causes ligand-gated Na channels to OPEN, allowing the AP to continue its journey.

Question 19

What are the 3 amino acid neurotransmitters?

Answer 19

Glutamate, GABA and Glycine.

Question 20

What are the 6 monoamine neurotransmitters?

Answer 20

Ach, Dopamine, NA, adrenaline, serotonin (5-HT), histamine.

Question 21

What are the 2 purine neurotransmitters?

Answer 21

Adenosine and ATP

Question 22

What are the 6 peptide neurotransmitters?

Answer 22

Substance P

Vasoactive intestinal peptide

Somatostatin

Cholecystokinin

B-endorphin

Met-enkephalin

Question 23

What are the 3 gas neurotransmitters?

Answer 23

Nitric oxide

Hydrogen sulphide

Carbon monoxide

Question 24

What causes the release of neurotransmitters?

Answer 24

AP at the synaptic bouton opens Ca channels, so Ca enters.

Synaptotagmin detects the Ca ions, which promotes the formation of SNARE complex between synaptobrevin (embedded in the vesicle membrane) and the SNARE proteins (syntaxin and SNAP-25, in the pre-synaptic membrane).

The vesicle and pre-synaptic membrane fuse and the neurotransmitter is released into synapse.

Question 25

How are neuropeptides released?

Answer 25

They are released from any location of the terminal membrane.
More than one can be released from a single synapse.
It requires a higher concentration of Ca ions.

Question 26

What is the function of autoreceptors?

Answer 26

Monitor self secretion. - Part of localised negative feedback system

Question 27

Function of heteroreceptors:

Answer 27

Monitor transmissions from other synapses. - Part of negative feedback system

Question 28

When neurotransmitters released at a single synapse are too small to trigger a full action potential they are known as:

1) Threshold potential
2) Sub-threshold potential

Answer 28

Sub-threshold potential.

These can be Excitatory post-synaptic potentials (EPSPs) or inhibitory post-synaptic potentials (IPSPs)

Question 29

How does a neurotransmitter become inactivated?

Answer 29

1) Reuptake: secondary active transport
2) Enzymes: acetylcholinesterase breaks Ach down into acetate and choline
3) Diffusion: neurotransmitters diffuse away slowly, prolonging effects

Question 30

What is a resting membrane potential?

Answer 30

The inside of a nerve cells is less positively charged than the extracellular fluid.

Maintained because sodium is actively pumped out of the cell and cannot reenter across the membrane. Potassium is pumped into the cell but can diffuse back out across the cell membrane (to balance K+ concentrations) and so effect is that the outside has both Na+ and K+, inside only has K+ making the inside more -ve.

Question 31

Sodium channels in a nerve cell membrane are normally closed. How can they be opened?

Answer 31

Localised changes in resting-membrane potential (voltage-gated), or by neurotransmitter activation (ligand-gated).

Question 32

What is a ‘polarised nerve cell’?

Answer 32

When the resting membrane potential is maintained in balance.

Question 33

What is a ‘depolarised nerve cell’?

Answer 33

When sodium channels are activated (by local change in resting membrane potential or a neurotransmitter) and an influx of Na+ causes the inside of the cell to becomes more positively charged.

Question 34

What is a refractory period and why is it important?

Answer 34

Following cell depolarisation the voltage-gated sodium channel goes through a period of inactivation while the cell re-polarises. This prevents back flow of excitation to maintain a unidirectional propagation of the action potential.

Question 35

What is a node of Ranvier?

Answer 35

A gap in the myelin sheath between two Schwann cells.

Question 36

What is ‘saltatory conduction’?

Answer 36

Conduction of an action potential between nodes of Ranvier.

Question 37

Which protein is embedded in the membrane of a vesicle?

a) Syntaxin
b) SNAP-25
c) Synaptobrevin
d) Synaptotagmin

Answer 37

c) Synaptobrevin

This protein binds to the SNARE receptors in the presynaptic membrane to allow fusion of the vesicle and membrane and ultimately the dispersal of a neurotransmitter into the synapse.

Question 38

Which protein is embedded in a presynaptic membrane?

a) Syntaxin
b) SNAP-25
c) Synaptobrevin
d) Synaptotagmin

Answer 38

both a and b

a) Syntaxin
b) SNAP-25

These are known as SNARE proteins (SNAP-receptor proteins) and allow the vesicle to fuse with the presynaptic membrane to facilitate the release of a neurotransmitter into a synapse.

The vesicle membrane protein is synaptobrevin.

Question 39

What types of receptor do neurotransmitters bind to?

Answer 39

Type 1 (ionotropic) and type 2 (metabotropic, G protein-coupled)

Question 40

True or false: Neuropeptide release requires high concentrations of calcium?

Answer 40

True.

Question 41

What does co-transmission mean for neuropeptides?

Answer 41

More than one neuropeptide can be released from a single synapse