Lecture 9

Question 1

Role of myelin

Answer 1

membranous wrapping of insulation around axons. Not all neurons are myelinated. Myelination status affects conduction velocity of action potentials

Question 2

continuous propagation

Answer 2

occurs in unmyelinated axons. Channels open sequentially-depolarisation of one regio then depolarisation of adjacent region and so on. slow

Question 3

saltatory propagation

Answer 3

occurs in myelinated axons. myelin sheath prevents leak (insulator). Action potentials ‘jump’ between Nodes of Ranvier fast (>100m/s)

Question 4

Myelin as insulator

Answer 4

myelin blocks leaky channels = decrease ion leakage = less current loss = current travels further before dissipating below threshold levels = action potential regenerated at next Node of Ranvier

Question 5

axon diameter and conduction velocity

Answer 5

increase axon diameter = decrease resistance (from membrane) = increase conduction velocity

Question 6

factors influencing conduction velocity

Answer 6

myelination = increase conduction

increase axon diameter = increase conduction

increase temperature = increase conduction

Question 7

type A axon fibres

Answer 7

myelinated, large diameter (4-20 um), fast conduction (>100m/s)

Question 8

type B axon fibres

Answer 8

myelinated, smaller diameter (2-4 um), moderate conduction (18m/s)

Question 9

type c axon fibres

Answer 9

unmyelinated, smaller diameter (<2um), slow conduction

Question 10

different sizes

Answer 10

not all fibres are fast to compromise between speed and space. increase diameter and myelination increase bulk. type A fibres reserved for essential motor and sensory signalling

Question 11

electrical synapse

Answer 11

pre and post synaptic membranes ‘locked together’. AP always propagated. Rare (vestibular nuclei, eye)

Question 12

chemical synapse

Answer 12

cells not directly coupled, use neurotransmitters. AP not always propagated. Most common synapse

Question 13

neurotransmitter storage in vesicles

Answer 13

vesicles of neurotransmitter stored in pre-synaptic terminals that are released on demand. terminals also contain many mitochondria to fuel metabolism and transport. High mitochondria produce energy to maintain Na+ pump for membrane potential.

Question 14

small neurotransmitter synthesis

Answer 14

e.g ACh, amines are synthesised and packaged in the synaptic terminals. In synaptic vesicles

Question 15

Peptide neurotransmitter synthesis

Answer 15

synthesised and packaged in cell body. travels to synaptic terminals as protein synthesis is more efficient in cell body and also, to stop them diffusing all over the cell, they are packaged there and then transported. Less efficient.

Question 16

Neurotransmitter release

Answer 16

When an action potential reaches the end of the axon Ca2+ moves in allowing neurotransmitter release.Neurotransmitters diffuse across the synaptic cleft to bind with receptors on the post synaptic membrane. Neurotransmitters are broken down by enzymes or undergo re-uptake into the presynaptic neuron.

Question 17

ligand-gated ion channel

Answer 17

ionotropic, fast response, ion channels open and ions immediately flood in/out of cells

Question 18

G-protein coupled receptor

Answer 18

Metatropic. slow response, ion channels opening relies on an intermediary 2ndary messenger

Question 19

Acetylcholine

Answer 19

most common neurotransmitter. Found in CNS, PNS, between neurons and muscle cells. Receptors include nicotinic (ligand-gated) and muscarinic (g-protein)

Question 20

Cholinergic

Answer 20

Synapses that release ACh

Question 21

Cholinergic signalling at NMJ

Answer 21

occurs in skeletal muscle fibres. Inactivation gives muscles ability to turn on and off quickly

Question 22

Cholinergic signalling at NMJ steps

Answer 22

1. An arriving action potential depolarises the synaptic knob
2. Ca2+ enters the cytoplasm and after a brief delay. ACh is released through the exocytosis of synaptic vesicles.
3. ACh binds to Na+ channel receptors on the postsynaptic membrane producing a graded depolarisation
4. Depolarisation ends as ACh is broken down into acetate and choline by ACh enzyme.
5. Synaptic knob reabsorbs choline from the synaptic cleft and uses it to synthesise new ACh molecules.

Question 23

noradrenaline

Answer 23

brain and ANS, GPCR, usually excitatory

Question 24

dopamine

Answer 24

CNS, GPCR, excitatory and inhibitory

Question 25

serotonin

Answer 25

CNS, GPCR and ligand-gated ion channels

Question 26

glutamate

Answer 26

primary excitatory neurotransmitter of CNS

Question 27

GABA

Answer 27

Primary inhibitory neurotransmitter of brain

Question 28

Post synaptic effect

Answer 28

neurotransmitter binding produces a graded potential in post-synaptic membrane (neuron-neuron synapses) Excitatory post-synaptic potentials (EPSPs) depolarisation Inhibitory post-synaptic potentials (IPSPs) hyperpolarisation A single neuron receives many post-synaptic potentials (PSPs). Net effect at axon hillock determines response.