Neurotransmission II Flashcards
What is myelination?
A fatty type placed around the axon by either an oligodendrocyte or a Schwann cell. In between the myelin sheath have Node’s of Ranvier.
What are Oligodendrocytes and Schwann Cells?
Oligodendroglial cell = forms myelin around axons in the brain and spinal cord.
Schwann cell = wraps around peripheral nerves to form myelin.
Basically do the same job, but are different cells.
What are differences in the propagation of the AP along the axon in unmyelinated compared to myelinated neurons?
Unmyelinated - get the same response along the neuron - all or nothing response.
Myelinated - no ion exchange in the fatty sheath - AP only happens at the Node of Ranvier, and is therefore much more efficient - AP jumps.
What are the advantages of a myelinated axon?
The action potentials jump and impulses are quicker. It doesn’t have to occur at many places.
Communication can become more synchronous. Your movements become fast. So it’s important for coordination.
When sodium pump is working hard it uses lots of energy – therefore want this to be efficient. Myelinated neurons are more efficient than unmyelinated neurons; get less APs, signal jumps.
What happens with multiple sclerosis?
Get damage in the myelin, therefore won’t be as efficient. Symptoms are loss of sensitivity, muscle weakness, difficulty with coordination and balance.
What toxins affect AP?
Tetrodotoxin - blocks voltage gates Na+ channels, results in paralysis.
Dendrotoxin - blocks voltage gated K+ channels, results in convulsions.
What are the different types of synapse?
- Electrical synapse - very rare in adult mammalian neurons, junction between neurones is small, gap is spanned by proteins which are used to communicate between the neurons (ions move freely).
- Chemical synapse - common in adult mammalian neurons, junction between the neurons 20-50nm (synaptic cleft), chemicals (neurotransmitters) are released from the presynaptic neuron to communicate with the postsynaptic neurons.
What kind of synapses do we have?
Androdendritic - axon controls the postsynaptic neuron
Axosomatic - axon controls the soma
Axoaxonic - axon controls another axon, and modulates the signal.
Majority are androdendritic.
Why does synapse location matter?
Activation of an excitatory synapse leads to local and small (1mV) depolarisation of the postsynaptic cell known as an EPSP (excitatory postsynaptic potential). EPSP decays over the length of the dendrite (decremental decay). Therefore the closer the synapse is to the soma the greater its influence on the production of an action potential in the axon. All inputs are summated at the soma (cell body). If there is enough excitation then an action potential is generated at the axon hillock.
What are the processes at the chemical synapses?
- Action potential travels down the axon
- When it gets to the synapse, depolarisation opens voltage-dependent calcium channels
- Influx of calcium leads to neurotransmitter release
- Neurotransmitter binds to and activates receptors on the dendrites of the postsynaptic cell
- This leads to depolarisation or hyperpolarisation of the postsynaptic cells.
- This spreads to the postsynaptic soma, where summation occurs.
- If there is enough depolarisation, then an action potential is generated at the axon hillock.
What are neurotransmitters?
A chemical that is used to transmit information from the presynaptic neuron to the postsynaptic neuron.
What is the criteria for a neurotransmitter?
- Chemical synthesised presynaptically.
- Electrical stimulation leads to the release of the chemical.
- Chemical produces physiological effect e.g. switch on or off the neuron.
- Terminate activity.
What is Dales Law
If a particular neurotransmitter is released by one of a neuron’s synaptic endings, the same chemical is released at all synaptic ending of that neuron.
(But the dendritic field can receive signals from multiple transmitters).
What happens during neurotransmitter release?
- Synaptic vesicle containing neurotransmitter ‘docked’ at the synaptic membrane
- Depolarisation of the presynaptic neurone leads to the opening of
calcium channels and calcium influx (concentration gradient) - Vesicles fuse with the synaptic membrane and releases neurotransmitter
into the synapse - Vesicle detaches from the docking zone
What is the postsynaptic action of the neurotransmitter?
Neurotransmitter binds to receptors on the postsynaptic membrane, which affects the activity of the postsynaptic cell. The configuration of the receptors make them specific for different neurotransmitters.
Ionotropic receptor - opening of an ionic channel, e.g. sodium and potassium channel.
Metabotropic receptor - Activates an internal 2nd messenger systems that goes on to affect the functioning of the postsynaptic cells. Produces an amplified signal.
