Lecture 9: Nerve conduction and neurotransmitter release

Question 1

What is Resting State?

Answer 1

The membrane is at the RMP (–65 mV)

Question 2

What is Rising Phase?

Answer 2

a rapid depolarisation of Vm

Question 3

What is Overshoot?

Answer 3

inside of the neuron is now positive relative to the outside.

Question 4

What is Falling?

Answer 4

rapid repolarisation of Vm.

Question 5

What is Undershoot?

Answer 5

inside of the neuron is now more negative than at rest (hyperpolarised)

Question 6

Where do action potentials start?

Answer 6

Axon hillock due to the high proportion of voltage-gated Na+ and K+ channels there

Question 7

What happens to the high proportion of Voltage gated channels down the axon?

Answer 7

They continue down the axon to the terminal

Question 8

How is the rising phase initiated?

Answer 8

by the opening of hundreds or even thousands of Na+ channels simultaneously.

Question 9

Where is an action potential initiated?

Answer 9

at the hillock propagates in one direction towards the terminal

Question 10

How is diameter an important factor in axons?

Answer 10

The wider the diameter, the more ions flow through the axons

Question 11

How can an action potential be regenerated?

Answer 11

• The first action potential brings some Na+ into the neuron.
  
  • The segment of membrane immediately in front is still at rest (–65 mV).
  • The Na+ is electrically attracted to this region and depolarises the membrane.
  • Another action potential is then generated.

Question 12

Can action potentials travel in both directions?

Answer 12

yes

Question 13

What do Oligodendrocytes do?

Answer 13

help wrap myelin sheath around the axons, which speeds up the conduction of an action potential down the axon

Question 14

What do glial cells provide in regard to myelin?

Answer 14

Myelin Sheath

Question 15

What is action potential regeneration at each node called?

Answer 15

Saltatory Conduction

Question 16

Does myelin sheath affect the rate of an action potential?

Answer 16

Saltatory conduction in a myelinated axon is significantly faster

Question 17

Where are action potentials only generated?

Answer 17

node of Ranvier

Question 18

What is the node of Ranvier?

Answer 18

spaces between divisions of myelin- exposed allow sodium and potassium to move

Question 19

How is action potentials generated at the nodes?

Answer 19

Voltage-gated Na+ and K+ channels gather in high density at the nodes, to trigger a spike

Question 20

Do the nodes have any leakage when action potentials are transmitted?

Answer 20

Very minimal leakage

The conduction of the action potential between any two nodes is completely passive

Question 21

What does myelin provide Na and K+?

Answer 21

myelin provides the pumps with a rest

Question 22

How does size affect conduction velocity?

Answer 22

Large diameter axons can pass more current

Question 23

How does myelination affect conduction velocity?

Answer 23

The myelin sheath insulates axons.

Question 24

How does temperature affect conduction velocity?

Answer 24

This affects the diffusion/ movement of ions in the axon.

Question 25

What is the Golgi stain?

Answer 25

Golgi developed a method to impregnate nervous tissue with metal (silver).
In this way, a limited number of random cells were stained black.

Question 26

How did Cajal relate to the Golgi stain?

Answer 26

Using Golgi’s stain, Cajal described the morphology of cells located within many different structures. He also produced exquisite drawings of neurons and their various connections

Question 27

What is the Reticular Doctrine? (Golgi)

Answer 27

Neurons being hard-wired together to form a continuous network, similar to the arteries and veins of the circulatory system

Question 28

What is the Neurones Doctrine? (Cajal)

Answer 28

The neurone being the functional unit in the brain

neurones communicate through chemicals and not continuity.

Question 29

What is the process of information transfer at the synapse called?

Answer 29

Synaptic Transmission

Question 30

What is an electrical synapse?

Answer 30

information can pass between cells in either direction. (Golgi)
Hard wiring between two physical thing sin the brain. Direction can go both ways

Question 31

What is a chemical synapse?

Answer 31

information flow is unidirectional.

Question 32

What are transmissions at electrical synapses like?

Answer 32

Fast
neurons are coupled together by gap junction
The movement of ions and small molecules can move across an electrical synapse is bidirectional

Question 33

How do ions and molecules pass through a gap junction?

Answer 33

Gap junctions allow them to pass directly across the membrane

Question 34

Is there a delay at an electrical synapse?

Answer 34

There is no measurable delay in signal propagation across an electrical synap

Question 35

How does depolarisation spread across a gap junction?

Answer 35

Depolarisation can spread very quickly across gap junctions.

Question 36

Which important reflexes do electrical synapses mediate?

Answer 36

coordination of heart muscle contractions and other reflexes

Question 37

Where are electrical synapses rare?

Answer 37

between mammalian neurones

Question 38

Where are electrical synapses common?

Answer 38

glia, epithelial, smooth cardiac muscle, liver and glandular cells

Question 39

Which neurones are electrically coupled?

Answer 39

Pre- and post-synaptic

Question 40

How wide is the synaptic cleft?

Answer 40

20-50 nm

Question 41

What is the cleft filled with?

Answer 41

matrix of fibrous extracellular protein that anchors the pre- and postsynaptic regions together. The protein help maintain the synapses

Question 42

How is information flow transmitted through chemical synapses?

Answer 42

Information flow at a chemical synapse is unidirectional and involves the conversion of an action potential into a chemical message

Question 43

What does the presynaptic terminal consist of?

Answer 43

1. Synaptic vesicles (spherical) are small (50 nm in diameter) and contain neurotransmitter.
   
   2. Secretory granules contain 2 larger peptide transmitters.
   3. The active zone is the site of neurotransmitter release.
   4. Mitochondria provide energy for synaptic transmission.

Question 44

What are receptors in the postsynaptic density responsible for?

Answer 44

Receptors in the postsynaptic density are responsible for converting chemical messages back into electric or metabolic signals.

1. Postsynaptic receptors respond to transmitters by altering Vm or changing cell metabolism. 
2. The postsynaptic membrane is studded with receptor proteins (postsynaptic density)

Question 45

Why are chemical synapses more important?

Answer 45

Due to amplification

Question 46

What can a brief pre-synaptic action potential trigger?

Answer 46

a large postsynaptic response

Question 47

What is an axodendritic synapse?

Answer 47

this is where the axon makes contact on the dendrite of the next neuron

Question 48

What is a axon somatic synapse?

Answer 48

this is where the axon makes contact on the soma of the next neurone

Question 49

What is an axoaxonic synapse?

Answer 49

this is where the axon makes contact on the axon of the next neuron

Question 50

What does Gray’s type 1 mean?

Answer 50

this is a synapse with asymmetrical membrane differentiation (excitatory)

Question 51

What does Gray’s type 2 mean?

Answer 51

this is a synapse with symmetrical membrane differentiations (inhibitory)

Question 52

What are the three major classes of neurotransmitters?

Answer 52

Amino acids: small and fats acting, found widely in the brain
Amine neurotransmitters - intermediate in side, both fast and modulatory
Peptide neurotransmitter - large, require protein synthesis to be produced, specialised in actions, slow and specialised.

Question 53

Which NTs are made in the terminal bouton?

Answer 53

small- and intermediate-sized neurotransmitters are made directly in the axon terminal

1. Precursor molecule interacts with an enzyme in the cytosol at the terminal region and is converted into neurotransmitter. 
2. Neurotransmitter is packaged and stored in synaptic vesicles.

Question 54

How are neurotransmitters made in the soma?

Answer 54

1. Precursor peptide is synthesised in the rough ER.
   
   2. In the Golgi apparatus, the new peptide is sorted, activated and packaged.
   3. Secretory granules bud away from the golgi apparatus
   4. The granules are transported to the terminal via axoplasmic transport.

Peptide neurotransmitters are made in the soma and require protein synthesis

Question 55

What happens to the vesicle in producing an action terminal?

Answer 55

• Vesicle is docked along inside of presynaptic membrane.
• An action potential depolarises terminal opening voltage- gated Ca2+ channels.
• Ca2+ rapidly floods into the terminal (10,000 times greater concentration outside the neuron!).
• Ca2+ interacts with SNARE proteins causing the vesicle to fuse with the presynaptic membrane and spill its contents (exocytosis)
• The vesicle membrane is recycled (endocytosis).

Question 56

How does calcium play in the part of releasing the synaptic vesicle?

Answer 56

Calcium binds to synaptotagmin (a calcium sensor) which then stimulates the v- and t-SNAREs to combine into an α-helical-shaped complex.

The SNARE complex forces the two membranes together (fusion) and then pulls them apart to spill the vesicle contents into the synaptic cleft (exocytosis).

Question 57

What happens every time an action potential causes a neurotransmitter to be released?

Answer 57

more membrane is added to the presynaptic terminal
Unchecked, the terminal would expand to a very large size.
To prevent this, vesicle membrane is recycled via a process known as endocytosis.