Synapses and the Role of Neurotransmitters Flashcards
what does the CNS consist of?
brain and spinal cord
what are the different roles of the PNS?
sensory nervous system
motor systems
autonomic nervous system
- sympathetic
- parasympathetic
- enteric
what specialisations do neurones possess in order to communicate?
dendrites (form the dendritic tree) - receiving input from other neurones
axon hillock - (junction of the axon from the cell body) AP generation
axon - impulse conduction
axon terminal (or synaptic bouton) - release of neurotransmitter triggered by AP
at a sensory neurone, where is the AP generated?
not all AP are generated at the axon hillock
in sensory neurones its at the nerve endings
what are synapses?
site of communication between two neurones, two cells or a cell (muscle cell) and a neurone
what are the 2 different types of synapses?
electrical
chemical
what is the electrical synapse?
known as ‘gap junction’
fastest and most primitive
between adjacent cells or neurites
direct transfer of ionic current (also small molecules)
bi-directional
what is the structure of an electrical synapse?
connexons (protein) line up physically connecting the two cells by having half of each connexon in either cell.
there is a pore in the connexon allowing the passage of ions
information (ions) can travel in both directions - allows synchronous activity between cells
what is the difference between electrical and chemical synapses
electrical allows bi-directional transfer of info
chemical is uni-directional- transfer of information from pre-synaptic –> post synaptic cell
what does the electrical synapse do?
- allows synchronous activity
- relatively rare between neurones in the CNS, although important in development
- glia-neurone, glia-glia communication - much more common than neurone-neurone
- also, cardiac myocytes are connected via gap junctions - allows cardiac muscle to depolarise to cause contraction
what is the chemical synapse?
- close association between the presynaptic (axon terminal) and the postsynaptic membrane
- presynaptic terminal releases neurotransmitter
- diffuses across the synaptic cleft
- interacts with receptor on the postsynaptic membrane
- plenty of vesicles to allow for the transport of the neurotransmitter.
what is the synaptic cleft?
space that separates two neurones
explain the critical process of neurotransmission
- Action potential (initiated in the cell body) propagates down the axon, invading the nerve terminal (The AP is dependent on membrane depolarisation which is dependent on the influx of Na+ through voltage-gated Na+ channels)
- Depolarisation at the nerve terminal triggers a conformation change in Ca2+ voltage gated ion channels causing them to open
- Ca2+ influx into the nerve terminal triggers the release of neurotransmitter
- Vesicle will move and fuse with the membrane of the presynaptic terminal -this portion is called the active zone
- Docks with the membrane and the vesicle contents (neurotransmitter) are released into the synaptic cleft via exocytosis
- Neurotransmitter diffuses across the synaptic cleft (down its concentration gradient)
- Neurotransmitter then binds to the receptor on the post synaptic membrane. (The receptor is very specific so only one type of neurotransmitter will bind to it)
- A signal is communicated and this causes depolarisation - EPSP in post synaptic neurone –> lead to AP.
- Signal must be quickly terminated after the signal is processed. The mechanism to do this, depends upon:
- the synapse itself and what neuron is involved
- the neurotransmitter released
explain the 1st mechanism used to terminate a signal in the synapse
Mechanism 1: The neurotransmitter is taken back up into the nerve terminal by a specific re-uptake mechanism. This is done rapidly:
1. Protein on the cell surface membrane (re-uptake sites) transport neurotransmitter back in pre-synaptic terminal.
2. Once the neurotransmitter is taken up, it can be broken down by enzymes present in the presynaptic membrane. Many (not all) of the enzymes which break down the neurotransmitter are expressed via the mitochondria
3. However, it can also be re-packaged into vesicles
explain the 2nd mechanism used to terminate a signal at the synapse
Mechanism 2:
Here mitochondria are in the extracellular fluid.
1. Enzymatic activity in the extracellular fluid breaks down the neurotransmitter released in the synaptic cleft without reuptake
2. This mechanism is used to terminate signalling at certain synapses e.g. those that use acetylcholine
define neurotransmitters
chemical messengers at the synapse
how does an inotropic receptor behave at an excitatory synapse?
1) An excitatory neurotransmitter binds to the receptor
2) This causes a conformational change in the Na+ ion channels causing it to open
3) This allows influx of Na+ (due to ionic driving force) causing depolarisation
4) This causes small excitatory post synaptic event (see in the graph) (EPSP - excitatory post synaptic potential)
5) This doesn’t last long as the Na+ is pumped out after
5) So excitatory neurotransmitters cause excitation in the post synaptic neuron
how does an inotropic receptor behave at an inhibitory synapse?
1) An inhibitory neurotransmitter is released and interacts with a specific receptor
2) This causes conformation change in Cl- ion channel –> opens
3) This causes an influx of Cl-ions into the cell
4) The movement of Cl-is due to its concentration gradient (against electrical attraction)
5) This causes membrane hyperpolarisation
6) The membrane potential becomes more negative than the resting membrane potential = small inhibitory post synaptic potential (IPSP)
how does the metabotropic receptor mechanism work?
1) Conformational change in the receptor protein itself
2) This activates a G-protein – this is tethered (connected to) on the intracellular side of the membrane
3) The G protein then activates ‘effector systems’ - this can be excitatory or inhibitory depending on the particular receptor and G-protein
4) Slower transmission but longer lasting effects on the excitability of the post synaptic cell
how does spatial summation allow threshold to be reached?
- One small EPSP is not enough for the threshold value to be reached.
- Multiple simultaneous inputs from several synapses will cause the membrane potential to reach the threshold value (causing VG Na+ ion channels to open –> Na+ influx) resulting in the generation of an AP.
- An input closer to the axon hillock will have a much larger effect on the excitability of the cell compared an input on the dendritic tree.
how is threshold reached at a mixed synapse (both excitatory and inhibitory neurotransmitters)?
- At a mixed synapse you can have both inhibitory and excitatory sypatic imputs
- An excitatory and inhibitory input, coming in at the same time can lead to no change at all for the excitability of the cell - membrane level of polarisation remains the same.
- If the level of the excitatory input = level of the inhibitory input, then they cancel each other out.
how does temporal summation work?
- one EPSP is quickly followed by another, so they can sum up because each one does not have time to decay away before another adds on top
- This can result in the threshold value being reached and an action potential being generated
- If EPSP are too slow, the first one will decay away!
