Synaptic Transmission

Question 0

What is chemical transmission ?

Answer 0

• neurotransmitter substance released from presynaptic cell diffuses across the synaptic cleft producing an effect on the post synaptic neuron
• amine and amino acid neurotransmitter so synthesised in axon terminal and stored in small clear vesicles
• large peptide neurotransmitter are formed in the cell body and stored in large dense core vesicles
• release of neurotransmitter occurs due to a rise in intracellular calcium
• predominant form of communication in CNS
• occur between many presynaptic elements - mostly axon terminals but also the soma and dendrites
• require presynaptic machinery such as synaptic vesicles and neurotransmitters and calcium and it also need post synaptic elements such as receptors for neurotransmitters to bind and influx of ions such as sodium ions or chloride ions
• distance between pre and post neurons is about 30-50nm
• events that occur in the post synaptic neuron are dependent upon the ion channel present which can be ionotropic or metabotropic
• transmission is unidirectional
• huge flexibility due to many different neurotransmitters that can be released and the many different receptors
• they enable ongoing transmission of electrical info by aps

Question 1

What is electrical transmission ?

Answer 1

• direct flow of ions from one neuron to another via gap junctions
• there is cytoplasmic continuity between the pre and post synaptic neurones
• the agent of transmission is ionic current
• has virtually no synaptic delay
• the distance between the neurons is 3.5nm
• directions of transmission is usually bidirectional
• has practically no flexibility but can be modulTed by pH, calcium and some neurotransmitter a
• usually occur between the somata of neurons but some have been shown at dendrites
• normally continually open and the flux of ions flows through the electrical synapses
• roles: synchronising info across lots of neurons, organising synaptic circuits during development and communication between glials

Question 2

What are the similarities of chemical and electrical transmission ?

Answer 2

1- important means of communication between neurons in the CNS
2- they are both abundant in the CNS and allow transmission of electrical signals between neurons

Question 3

What are the criteria for a chemical messenger to be considered a neurotransmitter ?

Answer 3

1- must be synthesised in the neurone
2- has to be present in the presynaptic terminal in sufficient quantities to exert its supposed effects
3- when applied exogenously in reasonable concentration it must mimic exactly the actions of the endogenous transmitter
4- has to have a specific mechanism for removing it from its site of action

Question 4

What are the molecule involved in vesicles release and how does it work ?

Answer 4

The free vesicles are targeted to the active zones.

The synaptobrevin present on the vesicles membrane is able bind to snap-25 on the plasma membrane
These 2 proteins then twist together and this causes fusion of the vesicle with the membrane allowing the neurotransmitter to be released by exocytosis
Once the synaptotagmin senses the rise in the calcium levels then the neurotransmitter can be released very rapidly

Question 5

What are metabotropic receptors ?

Answer 5

Coupled to intracellular proteins which transduce the signal to the interior of the cell
They can activate second messenger systems and cause more prolonged responses

Question 6

What are ionotropic receptors ?

Answer 6

Form ion channels which can depolarise or hyper polarise the post synaptic cell

Question 7

What are gray’s type 1 synapses?

Answer 7

Synapses where the membrane differentiation on the postsynaptic side is thicker= asymmetrical synapses

Question 8

What are gray’s type 2 synapses ?

Answer 8

Synapses where the membrane differentiation is similar on both sides= symmetrical synapses

Question 9

What are axo-dendritic synapses ?

Answer 9

Occur on dendritic shaft and dendritic spines

Commonly excitatory

Question 10

What are axo-somatic synapses ?

Answer 10

The synapse is on the initial segment of the axon which influences firing rates
Often inhibitory

Question 11

What are axo-axonic synapses ?

Answer 11

These are modulatory and control transmitter release
There is pre synaptic facilitation and presynaptic inhibitions
Their ability to control the release of neurotransmitter is due to controlling the amount of calcium released

Question 12

What is temporal summation ?

Answer 12

Summation of all the PSPs over a short period of time

Question 13

What is spatial summation ?

Answer 13

Summation of all the PSPs over the membrane surface