L5 synaptic structure and function Flashcards
The Synapse
Refers to the place of connection together of two cells where an impulse is transmitted, usually referring to two neurones.
3 types of synapse
- electrical/electroic
- chemical
- mixed
Electrical Synapse
relatively simple
- Reduced extracellular space (2nm) cytoplasmic continuity between pre- and post synaptic cell
- Mediating agent is ionic current.
- Little or no synaptic delay
- Typically bidirectional - conc of ions dictates the direction of travel
- Examples:
- particularly in invertebrates and lower vertebrates. Lateral vestibular nucleus, inferior olive, molecular layer of the cerebellum, retina and a few junctions in the cerebral cortex
Chemical synapse:
more complicated than the electrical synapse
- Increased extracellular space (30-50nm); no cytoplasmic continuity.
- Mediating agent is a chemical messenger.
-
Significant synaptic delay (0.3-5msec). Contributions include:
- the time required for opening of Ca2+ channels
- secretory process at presynaptic terminal
- diffusion across synapse
- interaction with postsynaptic receptor
- in some cases production of second messengers.
- Unidirectional
- Examples:
- majority of synapses in the CNS
Gray Type 1 synapse
type 1:
- prominent post synaptic density
- wide synaptic cleft
- round vesicles
- large active zone
= mostly excitatory type
End on a shaft or spine
Gray’s Type 2 Synapse:
Type 2:
- flattened synaptic vesicles,
- narrow synaptic cleft
- less dense regions
- small active zone
= inhibitory
often end on Soma
Anatomical classification of synapses
Anatomical classification of synapses describes the part of the presynaptic cell in contact with the part of the postsynaptic cell thus:
- axodendritic - between axon and dendrite
- axosomatic - axon and cell body
- axoaxionic - axon to axon
- dendrodendritic - dendrite to dendrite
- somatosomatic -cell body to cell body
- somatodendritic - cell body to dendrite
Where can synaptic contact occur?
Can occur on the:
- cell body
- dendrites
- axon
If cell body is contacted then it is likely that the whole cell will be activated
Location of synapses is important
Summary of events occurring during neurotransmission at chemical synapses: I
- Action potential in presynaptic cell.
- Depolarisation of the plasma membrane of the presynaptic axon terminal. (activates voltage-gated Ca2+ chanels)
- Entry of Ca2+ into presynaptic terminal.
- Release of transmitter by the presynaptic terminal.
Summary of events occurring during neurotransmission at chemical synapses: II
- Chemical combination (binding) of the transmitter with specific receptors on the plasma membrane of the postsynaptic cell.
- Transient change in the conductance of the postsynaptic plasma membrane to specific ions.
- Transient change in the membrane potential of the postsynaptic cell.
- Points 6 and 7 are true for ionotropic receptors, metabotropic receptors have a diffferent mechanism
Ca 2+ dependent NT release
Synapsin 1 - imp in vesicle docking to presynaptic membrane
Phosphorylation of synapsin 1 leads to NT release
Termination neurotransmitter action
- Metabolism to non-active compounds eg acetylcholine is broken down to acetate and choline by acetylcholinesterase.
- Reuptake of neurotransmitter into the presynaptic terminal or glia in the case of glutamate.
- A combination of both in the case of 5-HT and dopamine.
Excitatory amino acids
Glutamate
Aspartate
Inhibitory Amino acids
GABA - gamma aminobutyruic acid
Glycine
Divergence
One transmitter acting on different receptors to cause different multiple effects
