Chemical Synapses Flashcards
1
Q
Chemical Synapse
A
- Specialised junction when an axon reaches its target cell
Presynaptic neuron - nerve cell that gave rise to axon
Postynaptic cell - target cell (e.g neuron, muscle or gland cell)
- Function is to transmit information coded in a sequence of action potentials, to the postynaptic cell for an appropriate response
- synaptic transmission - one-way signalling mechanism
2
Q
Types of Synapses
A
Excitatory synapse - when activity of postsynaptic cell is increased
Inhibitory synapse - when activity in postsynaptic neuron leads to a decrease in activity of the postynaptic cell
Chemical synapse - operate by the secretion of a chemical (neurotransmitter) from the nerve terminal
Electrical synapse - operates by transmitting electrical current generated by an action potential to the postynaptic cell via gap junctions (e.g in retina or brain)
3
Q
Structure of Chemical Synapses
A
- When an axon reaches its target cell, it looses it myelin sheath & ends in a small swelling known as a nerve terminal or synaptic bouton
- A nerve terminal + the underlying membrane on its target cell = synapse
- Postynaptic thickening - post-synaptic membrane contrains electron dense material that makes it appear thicker than the plasma membrane outside the synaptic reigon
- Postsynaptic membrane contains specific receptor molecules for neurotransmitters released by the nerve terminal
- Synaptic cleft is approx 20nm
- neurotransmitters from nerve cells include: ACh, dopamine, norepinepherine (noradrenaline), glutamate, seretonin, GABA & many peptides such as substance P and enkephalins
- Synaptic vesicles contain nearly all the neurotransmitter present in the terminal
4
Q
Mechanism of a Chemical Synapse
A
- Action potential reaches the presynaptic nerve terminal so it depolarises
- Depolarisation causes the voltage-gated calcium channels in the presynaptic membrane to open
- Calcium ions flow into the nerve terminal down their electrochemical gradient
- Influx of free calcium causes the fusion of one or more synaptic vesicles, resulting in secretion of a neurotransmitter into the synaptic cleft
- Secretion process is extremely quick - happens within 0.25ms of action potential reaching terminal
- Secreted transmitter diffuses across the cleft & binds to receptors on the postsynaptic membrane
- If the transmitter activates a ligand-gated ion channel, synaptic transmission is rapid & short-lived - fast synaptic transmission
- If the transmitter activates a G protein linked receptor, change in postsynaptic cell is slower in onset & lasts longer - slow synaptic transmisssion
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5
Q
Formation of an EPSP
A
- Fast excitory synaptic transmission - neurotransmitter binds to & opens a non-selective cation channel
- Opening of these channels causes a brief depolarisation of the postynaptic cell
- Shifts the membrane potential closer to the threshold for action potential generation & renders the postsynaptic cell more excitable
- When the postynaptic cell is a neuron, depolarisation is called an EPSP
- Multiple EPSPs are needed to reach the threshold voltage & cause an action potential
- A single EPSP will reach its peak value within 1-5ms of the arrival of the action potential in the nerve terminal & decays to nothing over the ensuing 20-50ms
6
Q
Formation of an IPSP
A
- Fast inhibitory synaptic transmission - a neurotransmitter such as GABA or glycine is able to activate chloride channels in the postynaptic membrane
- Opening of chloride channels causes postsynaptic cell to become hyperpolarised (e.g more negative) for a brief period
- Negative shift in membrane potential = inhibitory postsynaptic potential (IPSP), as the membrane potential is moved farther away from the threshold
- A single IPSP will reach its peak value withub 1-5ms of the action potential reaching the nerve terminal & will decay to nothing in milliseconds
7
Q
Locations of Synapses
A
Locations of Synapses:
- axo-somatic - between nerve terminal & cell body of post-synaptic cell
- axo-dendritic - between a nerve terminal & a dendrite on a postsynaptic cell
- axo-axonic - between the nerve terminal & the terminal reigon of another axon
8
Q
Presynaptic Inhibition
A
- Impairment of synaptic transmission
- Inhibitory inputs connecting to excitatory nerve terminals, leading to a reduction in synaptic excitation
- Permits selective blockade of a specific synaptic connection without altering the excitability of the postsynaptic neuron
- Different from postynaptic ihibtion - results from IPSPs occuring & changes permeability
9
Q
Classes of Neurotransmitters
A
- Esters - acetylcholine (ACh)
- Monoamines - noradrenaline, dopamine, seretonin
- Amino acids - glutamate, GABA
- Purines - adenosine, ATP
- Peptides - enkephalins, substance P, vasoactive intestinal peptide (VIP)
- Most transmitters can activate both an ion channel & G protein linked receptor
- A particular synapse may utilize more than one neurotransmitter (co-transmission)
10
Q
Fast Transmisson
A
- Fast transmisson - ligand-gated channels are rapidly activated by high concentrations of neurotransmitter
- High concentration achieved by the release of a small amount of neurotransmitter into narrow synaptic cleft
- As neurotransmitter diffuses away from synaptic cleft, concentration falls rapidly
- Concentration is too low to have an affect on neighbouring cells
- Type of transmission is highly specific for contact between a pre-synaptic neuron & its target cell
- Well adapted for serving a role in rapid processing of sensory information & control of locomotion
11
Q
Slow Synaptic Transmission
A
- Neurotransmitters are not secreted into a specific point on the post-synaptic cell
- Nerve fibres have a number of swellings (varicosities) along their length that secrete neurotransmitters into extracellular fluid close to a number of cells
- Synapses of the autonomic nervous system (ANS) have slow synaptic transmission
- Importance for control of cardiac output, calibre of blood vessels & secretion of hormones
- Within CNS, slow synaptic transmission may underlie changes in mood & control of appetite e.g hunger/thurst
12
Q
Cotransmission
A
- Some nerve terminals contain two different neurotransmitter
- When the nerve ending is activated, both neurotransmitterss may be released
- Example - parasympathetic nerves of the salivary gland release both acetylcholine & VIP when activated
- ACh will then act on acinar cells to increase secretion & VIP acts on the smooth muscle of the arterioles to increase blood flow
13
Q
Limiting Factors for Duration of Action of a Neurotransmitter
A
- Neurotransmitters are very potent chemicals secreted in response to very specifc stimuli
- If a particular neurotransmitter needs to be restricted to a particular synaspe & timing, there needs to be ways to terminate action
This can be achieved by:
- rapid enzymatic destruction
- uptake either into the secreting nerve terminals or into neighbouring cells
- diffusion away from the synapse followed by enzymatic destruction, uptake or both
- ACh is innactivated by rapid enzymatic destruction - hydrolysed by acetelycholinesterase
- monoamides (e.g noradrenaline) are innactivated by uptake into the nerve terminals where they may be reincorporated into synaptic vesicles for subsequent release
- Any monomide not removed is metabolised by monoamine oxidase or catechol-O-methyl transferase - present in nerve terminals, or other tissues like liver
- Peptide neurotransmitters become diluted in extraceulllular fluid as they diffuse away from site of action & get destroyed by extraceullar pepsidases
- Amino acids released in the process are taken up by surrounding cells & enter normal metabolic pathways
14
Q
Neurotransmitters & Receptors
A
