Synapses and neurotransmission

Question 1

Criteria for iedntification of a synapse

Answer 1

A point of contact between one neuron and another neuron which allows a nervous impulse to pass over from one neuron to the other.
Synaptic cleft
Pre- and post- synaptic specialisations
Vesicles

Question 2

Chemical synapses and specialisations

Answer 2

Synaptic cleft 20-30nm wide, can be seen on electron micrograph.
Post-synaptic density; a protein dense specialisatikn attached to the postsynaptic membrane. Ensures that receptors are in close proximity to presynaptic neurotransmitter release sites.
Most obvious presynaptic specialisations are the large cluster of synaptic vesicles and an active zone or presynaptic density which is associated with the site of vesicle fusion.
Mitochondria and endosomes are often associated with presynaptic specialisation.

Question 3

Synaptic vesicles - shapes and sizes

Answer 3

1. Agranular - (clear) 40nm, can be round or elliptical/pleomorphic
2. Granular - dense core, 70-150nm, round.

Round and elliptical agranular vesicles are not found in the same axon terminal
Granular and agranular vesicles sometimes co-exist in the same axon terminal.

Question 4

Agranular vesicles and their neurotransmitters

Answer 4

L-glutamate - round
GABA - elliptical
Glycine - elliptical
Acetylcholine - round
Serotonin
Dopamine
NA

Question 5

Granular vesicles and their neurotransmitters

Answer 5

Substance p
Enkephalin
Serotonin
Dopamine
NA

Question 6

Criteria for identification Of a neurotransmitter

Answer 6

Synthesis
Storage
Release
Action on receptor
Inactjvation
Action of agonists/antagonists

Question 7

Are synapses necessary for chemical transmission?

Answer 7

Not all chemical transmissionin the CNS is mediated via classical synapses.
Some transmitters, released from varicosities, diffuse and interact with high affinity receptors located at non-synaptic regions of plasma membranes.

Question 8

Volume transmission

Answer 8

The diffusion through the brain ECF of neurotransmitters released at points that may be remote from the target cells, with the resulting action being activation of extrasynaptic receptors.
Mostly involves peptides, some monoamine and amino acid involvement

Question 9

Classification of synapses

Answer 9

Synapses are classified by -

1. Morphology
2. Components

Question 10

Morphology of synapses

Answer 10

Grey type I - round vesicles, 30nm cleft, assymetric junction, excitatory, usually contains glutamate.

Grey type II - elliptical vesicles, 20nm cleft, symmetrical junction, inhibitory, usually contains GABA or glycine.

Limitations - does not take granular vesicles into account but is still of functional significance.

Question 11

Components of synapses

Answer 11

Classical - axo-dendritic (95%), axo-somatic

Axo-axonic - axon terminal, initial axon segment

Unusual - presynaptic dendrites, somato-somatic

Question 12

Electrical transmission

Answer 12

Electrical synapses are low resistance junctions.
Gap junction (2-4nm)
Two connexons with gap junction to allow membrance interaction.
Channel is 1.5nm diameter
Connexon composed of 6 subunits.

Question 13

Advantages of electrical transmission

Answer 13

Fast
Bidirectional
Often fail safe
Sub-threshold events can be transmitted

Question 14

Disadvantages of electrical transmission

Answer 14

Not versitile
Bidirectional
Usually cannot converg excitation to inhibition
Cannot modulate

Question 15

Mixed synapses

Answer 15

Best of both worlds
Speed of electrical transmission
Versatility of chemical transmission