Neuronal conduction and neurotransmission Flashcards
What 2 factors determine speed of depolarisation
The space constant
Time constant
What is the space constant
How far a current can spread passively along the axon
What is the space constant equation
Space constant = sqrt(membrane resistance/internal resistance)
Why is the cell membrane both a resistor and a capacitor
Current can pass through but not well and charge can build up on one side
What is myelin made of
Oligodendroytes/Schwann cells
Increases membrane resistance
Decrease membrane capacitance
What is saltatory conduction
Short stretch of bare axon which allows AP to conduct down the axon and jump from node to node
What type of disease is Multiple Sclerosis
Auto-immune disorder, Episodic
What neurons are myelinated in the body
In CNS and PNS including motor and proprioception
Why is MS symptoms better at low temperatures
Na+ channels inactivate more slowly
What is Guillain-Barre syndrome
Auto-immune disorder affecting PNS myelin
Symptoms - numbness, tingling and weakness
Why do GBS patients recover
PNS myelin can regenerate
Why are vertebrates bigger than any other organism
They have myelination
What does saltatory conduction do
Makes signal propagation more energy efficient
What is a synapse
A junction between two neurons allowing for signals to pass from one to the other
What is the name for the process in which synapses signal
Synaptic transmission
What do synapses allow for
Flexible processing
How are electrical synapses formed
Gap junctions that allow current to pass directly between neurons
What are electrical synapses good for
Fast communication and synchronizing neurons
What does a ShakB2 mutation do
Blocks hyperpolarising and depolarising stimulus (Connexin gene)
No gap junctions
What are the steps in chemical synaptic transmission
Package neurotransmitters in vesicles and place in pre-synaptic terminal
Action potential arrives –> voltage gated Ca2+ channels open
Ca2+ influx –> vesicles fuse to membrane and neurotransmitters released
Neurotransmitters diffuse across the synaptic cleft, activate receptors on the postsynaptic cell –> further signalling
Neurotransmitters are removed from the cleft
Differences in synaptic vesicles and dense core secretory granules
Synaptic:
Small (40-50nm), Small molecule neurotransmitters, filled by transporter proteins at the presynaptic terminal, recycled by endocytosis
Dense-core secretory granules:
Large, Peptide neurotransmitters, created and filled by the ER/Golgi secretory apparatus, one and done
How does The action potential causes Ca2+ channels to open
Calcium follows the electrical force and concentration gradient to move inside the cell
Motor neuron fills up with calcium
Triggered by depolarisation
How do vesicles fuse onto the membrane
SNARE proteins
How do the SNAREs bind the vesicle to the plasma membrane
Ca2+ binds to the synaptotogmin causing a conformational change, SNAREs to zip together so the vesicle is bound to membrane
What does BOTOX and tetanus target
SNARE proteins
How do ionotropic receptors depolarise the cell
Neurotransmitter binds –> channels open ions enter –> depolarise the cell
Difference between electrical and chemical synapses
Chemical: Pass in one direction Signals can be inverted (inhibitory), amplified (more EPSPs), modulated... Slower Electrical: Signals can pass in both directions Fast
Both are plastic and allow summing up inputs at the post-synaptic neuron
What neurotransmitter do NMJ use
ACh
Does a motor neuron AP always cause muscle cell APs
Yes
Why is neuromuscular junction have such efficient transmission
High surface area
Large number of active zones
Contains junctional folds which are densely filled with neurotransmitter receptors
Active zones and junctional folds are precisely aligned
What type of packet is neurotransmitter found in
Quantal
Who discovered that neurotransmitter was found in vesicles
Bernard Katz
What does axodentric mean
When an axon synapses to dendritic neuron
What should a neurotransmitter do
Be present in presynaptic terminals
Be released in response to stimulation
Act on postsynaptic neurons
What are ways to determine if there is a neurotransmitter present
Immunostaining
Collect fluid around neurons after stimulation
Does the molecule mimic the effect of stimulating the presynaptic cell
Can you block the neurotransmitter
What are the amino acid neurotransmitters
Glutamate, glycine and GABA
What are the amine neurotransmitters
Ach Monoamines (dopamine, epinephrine ....)
Difference between Amino acids/amines and peptides when acting as a neurotransmitter
Amino acids/amines:
Small molecules
Stored in synaptic vesicles
Can bind to ionotropic or metabotropic receptors
Peptides
Large molecules
Stored in secretory granules
Only bind to metabotropic
Why can neurons diverge
One neuron sending the same signal can cause different downstream effect
Why can neurons converge
Different transmitters can act on the same effector
What is the most neurotransmitter in the body
Glutamate (excitatory)
GABA (inhibitory)
What neurons in glutamate found in
Amino acid so found in all neurons
What are the 3 glutamate receptor subtypes
AMPA
NMDA
Kainate
What do AMPA receptors do
Mediate fast excitatory transmission
Glutamate binds to AMPA triggers Na+ and K+ currents result in ESPS
What are NMDA receptors
Co-exist with AMPA receptors
Voltage gated Mg2+ block
NMDA receptors only open when the neuron is already depolarised
Let Ca2+ in –> downstream signalling
Act as a coincidence detector which is important for learning
What do mGluRs do
Allow glutamate to sometimes be inhibitory
How many subunits in iontropic receptors
4
What is the most common inhibitory transmitter in the CNS
GABA
What type of channel is GABAa
Chloride channel
When are IpSp produced
When membrane potential is above chlorides Nernst potential
What happens when there is too much/little GABA
Too much - coma or loss of consciousness
Too little - seizures - too much glutamate
How does ethanol, benzodiazepines and barbitudes work (allosteric drugs)
Activates GABAa which causes inhibition of neurons
What is GABAa
Ionotropic receptor
Cl-
Inhibitory
What are GABAb receptors
Metabotropic GABA receptors
What does GABAb and mGluRs do
They can:
Open K+ channels, close Ca2+ channels and trigger other secondary messengers like cAMP
Cause hyperpolarisation
What does glycine do?
Inhibits neurons via glycine-gated chloride channel
Binds to NMDA glutamate receptors (excitatory)
What is dendritic integration>
5 or 6 EPSPs are needed for activation not just one
How do EPSPs work
Signal reaches the dendrites and slowly decays, before complete decay another signal arrives, process repeats until threshold is reached.
Once reached an AP is formed which sends downstream signals
What is shunting inhibition
Opening of chloride conductance decreases the membrane resistance so current leaks out the membrane
Inhibitory blocks EPSP towards soma
GABAa dont produce IPSPs if close to chlorides nernst potential
Where does inhibition usually occur
Presynaptic cleft
What is a axoaxonic synpase
Axon synapsed to another axon
What is a GABAergic neuron
A neuron which can block the presynaptic release from another neuron by releasing GABA
What do GABAergic neurons release
GABA which inactivates calcium channels so less Ca2+ enters so less neurotransmitter is released so reduced effect on postsynaptic membrane
Why are inhibitory neurons important
They modulate the activity of excitatory neurons
