M2 Topic 5: Synapses Flashcards
(19 cards)
What is a synapse
Site where two neurons communicate with each other
- Usually use neurotransmitters as chemical messengers between neurons
It is where the axon potential from one neuron is transferred to the dendrites of another neuron
- Presynaptic neuron (sends information)
- Postsynaptic neuron (receives information)
What are the two types of synapses
- Electrical synapses = gap junctions, (e.g. smooth muscle)
- Chemical synapses
Electrical synapses
Neurons directly connected by channels that allow rapid transfer of electrical signals and small molecules
- Very fast conduction
- Signal can be bi-directional
- Synchronises activity of a network of cells, e.g. CNS development
Chemical synapses
Specialised junctions where neurons communicate through release of neurotransmitters
Consists of…
Pre-synaptic terminal
- Synthesis of neurotransmitters
- Ca2+ releases neurotransmitters
Synaptic cleft
- Gap/space that separates neurons
Postsynaptic cell
- Neurotransmitter receptors generate graded potentials called…
- Excitatory postsynaptic potential (EPSP)
- Inhibitory postsynaptic potential (IPSP)
Events at the synapse
- AP arrives at axon terminal, depolarising it
- Ca2+ channels open, Ca2+ enters presynaptic terminal
- Synaptic vesicles dock and fuse with presynaptic membrane and release neurotransmitters
- Neurotransmitter crosses synaptic cleft, activates receptors on the postsynaptic membrane
- This generates an EPSP which, if big enough when reaches trigger zone, can trigger AP in the postsynaptic neuron, starting the process over again
Neurotransmitters (NT)
Chemicals that allow neurons to communicate at chemic synapse
- Made from small molecules found in diet
- CNS and PNS use number of different NT’s
- Can have either excitatory (depolarising) or inhibitory (hyperpolarising) effects on neurons
Main neurotransmitters in the PNS
- Acetylcholine (ACh)
- Adrenaline
- Noradrenaline
Main neurotransmitters in the CNS
- Serotonin
- Dopamine
- Glutamate
- GABA
Acetylcholine (ACh)
Located in parasympathetic nerves, prepares body to ‘rest’
- Receptors that bind to ACh are called cholinergic receptors
- Important for smooth and skeletal muscle contraction, gland secretion, heart rate reduction
- Used to communicate between neurons in the autonomic nervous system
Adrenaline and noradrenaline
Located in adrenal glands and sympathetic nerves respectively, prepares body for action
- Receptors that bind to these are called adrenergic receptors
- Noradrenaline (norepinephrine) in PNS released by noradrenergic receptors
- Adrenaline (epinephrine) secreted mainly by adrenal glands
Serotonin
Located in brain, controls mood and behaviour
- Mood, behaviour (e.g. aggression, depression)
- Memory, pain, sleep, eating patterns
Dopamine
Located in brain, controls reward and movement
- Important for motor systems, initiation of movement
- Important for reward systems, linked to addictive behaviours
Glutamate
Located in brain, major excitatory neurotransmitter
- Important for learning and memory
GABA
Located in brain, major inhibitory neurotransmitter
- Important for suppressing activity
Synaptic integration
The fact that the signal can ‘change’ as it passes from neuron to neuron
At each synapse…
- Signals can be lost
- Signals can be enhanced
Divergence of signals
In divergent pathway, one presynaptic neuron branches to affect a larger number of postsynaptic neurons
- One neuron to many
Convergence of signals
In convergent pathway, may presynaptic neurons converge to influence a smaller number of postsynaptic neurons
- Many neurons to one
Dendrites mostly receive excitatory signals
Cell body of neuron mostly receives inhibitory signals
Convergence in an excitatory synapse (additive)
- 3 excitatory neurons fire, all graded potentials separately are below threshold
- Graded potentials arrive at trigger zone together, sum to create suprathreshold signal
- Action potential generated
EPSP’s can add strength to make bigger signal
Convergence in an inhibitory synapse (subtractive)
- One inhibitory and two excitatory neurons fire
- Summed potentials below threshold
- No action potential generated
IPSP can cancel/subtract from strength of EPSP
