Neuronal conduction and neurotransmission Flashcards
how does an AP propagate/conduct down an axon?
- voltage-gated sodium channels are open either side of depolarisation site
- Na+ passive spreads into the axon in each direction
- behind the site of depolarisation, K+ channels open and Na+ channels close (refractory period)
- ahead of the depolarisation site, Na+ channels keep opening, so influx of Na+ spreads down the axon to trigger APs
- therefore APs only move forward as the Na+ channels in the next section have opened
- the wave cannot move backwards as Na+ channels behind are inactivated, and K+ channels are opened to hyperpolarise the membrane
what determines the speed of an action potential?
- how fast the next segment of the membrane gets depolarised to threshold
- space constant
- time constant
what is the space constant?
- how far the current/depolarisation spreads passively along the axon before it decays to 37% of its initial value
how is an axon like a leaky water hose?
- current (water) flows down the axon and leaks out through channels in the membrane
what is membrane resistance?
- how much perforation is in the membrane - how many open ion channels
- if the membrane is less leaky, so less ion channels are open, then the depolarisation will spread further
- the greater the number of open ion channels, the lower the membrane resistance
- the greater the membrane resistance, the longer the space constant
what is internal resistance?
- how big the diameter of the axon is
- the larger the axonal diameter is, the lower the internal resistance (vice versa)
- neurons with low internal resistance have a longer space constant
what is the equation for the space constant?
sqrt(Rm/Ri) = space constant
how does the size of the axon influence the space constant?
- membrane resistance is inversely proportional to surface area of the membrane (the greater the area, the more leaks)
- internal resistance is inversely proportional to the cross-sectional area of the axon (the wider the axon, the less resistance to flow)
therefore:
- Rm depends on circumference (2 x pi x radius)
- Ri depends on area (pi x radius^2)
space constant is proportional to sqrt(radius)
wider axons have a longer space constant
what is a capacitor?
- two conducting plates with a non-conducting gap in between them
- charge can build up on one side to create a voltage
- can store and separate charges
how is a cell membrane both a resistor and a capacitor?
resistor: current can pass through, but not easily
capacitor: charge can build up on one side
what is the time constant?
- the time it takes for the change in voltage to reach 63% of its final value
- depends on Rm (how leaky is the neuron) and membrane capacitance (Cm) (how stretchy is the axon)
what is the equation for the time constant?
Rm x Cm = time constant
how does myelin affect Rm and Cm?
increases Rm:
- oligodendrocytes/schwann cell wrap around axons and insulate them
- many layers of membrane
decreases Cm:
- increases the distance between extracellular and intracellular solution
- moves capacitors further apart
how does myelination affect space constant and time constant?
increases space constant:
- myelin increases membrane resistance so current can spread further down axon
keeps time constant the same:
- decreases membrane capacitance so counteracts affect of increased Rm
- membrane can still charge up as quick as normal
does myelin speed up AP conduction?
yes - myelinated axons can conduct over 100m/s
- squid giant axon is unmyelinated, so despite being so big, it only conducts at 25m/s
what are nodes of Ranvier?
- nodes of ranvier are short spaces of bare axon which are packed with Na+ channels to allow APs to be conducted from node to node
what is the process of saltatory conduction?
- current enters via Na+ channel at a node of Ranvier
- depolarisation spreads passively down axon
- long space constant speeds up conduction
- passive as axon is insulated at this point (no Na+ channels)
- charge decays with distance
- at next node, depolarisation triggers Na+ channels to open to regenerate the decayed AP
- the next node is just close enough before the AP has fully decayed
how does saltatory conduction save energy?
- means Na+ enters only at nodes, not whole axon lenght, meaning there is less work for Na+/K+ pump to restore the Na+ gradient
how does myelination save space?
- speed of conduction is increased without needing to widen the axon
- to increase speed 10x, axon radius would need to be increased 100x and axon volume would be increased 10000x
why aren’t all axons myelinated?
- myelin is costly
- only myelinate axons that need to carry info quickly e.g. proprioceptors and motor axons
- unmyelinated axons = nociceptors, thermoreceptors
how do demyelinating diseases impair neuronal conduction?
- distribution of Na+ channels was designed with respect to myelination (they/re only at nodes of Ranvier)
- if myelin disappears, signals won’t travel correctly
what causes ectopic spikes?
maladaptive homeostatic compensation:
- axons form more Na+ channnels to compensate for lack of APs, but the Na+ channels are positioned in random places, so AP generation is random and aberrant
give 2 examples of demyelinating diseases:
- Multiple sclerosis
- autoimmune disease where immune system attacks myelin
- episodic as symptoms get worse - CNS myelin cannot regenerate
- vision issues, numbness. muscle spasms and weakness
- symptoms worsen in high temperatures or in stress as Na+ channels inactive rapidly, not with delay - Guillian Barre syndrome
- autoimmune disease affecting PNS myelin
- numbness, tingling, muscle weakness
- patients can recover as PNS myelin can be regenerated unlike CNS myelin
what is a synapse?
- a junction between 2 neurons to allow signals to pass from one neuron to another
- the process of signalling via synapses is called synaptic transmission
- brain has 100 trillion synapses compared to 100 billion neurons
what was the neuron doctrine vs reticular theory?
- disagreement over whether there are discrete neurons or a continuous net
- separate neurons = neuron doctrine, continuous network = reticular theory
what evidence was there for neurons and synapses?
- golgi stain - random impregnation of neurons with a dark stain to see individual neurons
- physiological evidence from study of reflexes - reflexes explained by one neuron inhibiting another, so must be continuous
- electron microscopy - identified synapses
what do synapses enable?
flexible processing:
- complex organism involves neuron being split into sensory and motor neurons which can both be modified to produce a response. interneurons can modify how sensory neuron signals to motor neuron
what are electrical synapses?
- gap junctions which allow current to pass directly between neurons
- hole directly connects the cytoplasm of two neighbouring cells
- connexins join up the two cells and open a hole to connect them
- allows currents and small molecules to flow between the two cells
how can we see if neurons are connected by gap junctions?
small molecules like dyes can diffuse from one neuron to the other:
- can fill a GFP neuron with red dye (shows as yellow) and can see the next neuron being filled with dye
can stimulate on neuron and then record the next neuron to see if the echo of depolarisation appears
- both hyperpolarising and depolarising stimuli are passed from one neuron to the other
- this is blocked by deleting a connexin gene (shakB2 mutant) - if gap junctions aren’t present, then transmission cannot occur
what are electrical synapses good for?
- fast communication - pass directly from one cell to another (continuous)
- synchronising neurons - a whole population of neurons can fire simultaneously
how were chemical synapses first discovered?
- Loewi demonstrated using 2 isolated frog hearts that nerves release a chemical which slows heart rate
- electrical stimulation of Vagus causes heart rate to slow down
experiment:
- stimulate vagus on one heart, collect the fluid produced and apply to another heart
- the same effect of slowing heart rate is seen on the second heart
what are examples of postsynaptic cells?
- another neuron
- motor neuron -> skeletal muscle
- autonomic neuron -> gland, smooth muscle
what are the general steps in synaptic transmission?
- Neurotransmitters are packaged into vesicles and transported to the presynaptic terminal
- AP arrives, causing voltage-gated Ca2+ channels to open
- Ca2+ influx depolarises the synapse and causes vesicles to fuse to the presynaptic membrane. Neurotransmitters are released
- neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic membrane, causing further signalling
- neurotransmitters are recycled from the cleft back to the presynaptic terminal
what are the two types of vesicles that neurotransmitters are packaged into?
- synaptic vesicles
- dense-core secretory granules
what are synaptic vesicles and what type of transmitter do they release?
- clear/small (40-50nm)
- filled by transporter proteins at the presynaptic terminal
- recycled by endocytosis
- contain small molecule neurotransmitters
what are dense-core secretory granules and what type of transmitter do they release?
- dense/large (100nm)
- created and filled by ER/golgi secretory apparatus
- one and done
- release peptide neurotransmitters
how do vesicles fuse to the presynaptic membrane?
- via SNARE proteins
- v-SNARE docks to t-SNARE
- when Ca2+ binds to synaptotagmin, a conformational change makes the SNAREs zip together, forcing the vesicles to fuse to the plasma membrane
- SNAREs are targets for the botulinum toxin and tetanus toxin
what kind of receptors can neurotransmitters bind to?
- ligand-gated ion channels (ionotropic) - directly depolarise/hyperpolarise postsynaptic cell when bound to specific neurotransmitter
- G-protein coupled receptors (metabotropic) - indirect complex effects by G-protein inducing a signal transduction cascade involving second messengers
what are the 3 ways in which neurotransmitters can be removed from the cleft?
- they diffuse away
- they are actively taken up by transporters for recycling
- they are destroyed by enzymes in the cleft
what are the differences between electrical and chemical synapses?
electrical:
- signals pass in both directions
- signals are passed directly and can only be attenuated
- fast (< 0.3ms)
- can be strengthened/weakened by addition/deletion of connexins
chemical:
- signals pass in one direction
- signals can be radically transformed: inverted, amplified, modulated
- slower (0.3-0.5ms)
- allow summation of inputs for modification
what are the similarites between chemical and electrical synapses?
- both are plastic (can be modified)
what is the neuromuscular junction (NMJ)?
- fast and reliable neurotransmittion from motor neuron to skeletal muscle
- uses ACh as transmitter (cholinergic synapse)
why does NMJ have such efficient transmission?
- synpase has large SA
- large number of active zones in presynapse
- postsynapse contains junctional folds which are densely filled with neurotransmitter receptors that are posiitioned directly opposite the active zones
how were vesicles discovered?
- neurotransmitter was showed to come from quantal packets
- each quantum is one vesicle full of neurotransmitter
- peaks of integer mutliples of spontaneous potentials
- neurons release quanta - discrete packages of neurotransmitter
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
what criteria is there for neurotransmitters?
- should be present in presynaptic terminals
- should be released in response to stimulation
- should act on postsynaptic neuron
- blocking the neurotransmitter should prevent synaptic transmission
how do we experimentally identify a neurotransmitter?
- use immunostaining to see if the molecule is present
- use immunostaining/in situ hybridisation to see if the cell expresses enzymes to synthesise it and transporter proteins to store it
- collect fluid around neurons after stimulating them to see if it is released
- test if the molecule mimics the effect of stimulating the presynaptic cell
- block the molecule by applying drugs to see if the response is stopped
what are the 3 types of neurotransmitters?
- amino acids: glutamate, glycine, GABA
- amines (contain amine group): ACh, monoamines (dopamine, epinephrine, norepinephrine)
- peptides
what are the differences between amino acids/amines and peptides?
amino acids and amines:
- small molecules (100-200Da)
- stored in synaptic vesicles
- can bind to ionotropic and metabotropic receptors
peptides:
- large molecules (1000-3000Da)
- stored in secretory granules
- only binds to metabotropic receptors
- when released, they also release a small molecule co-transmitter
how do convergence and divergence allow flexibility?
divergence:
- each transmitter can activate multiple different receptors
- each receptor can activate different downstream receptors
convergence:
- different transmitters or receptors can activate the same downstream effector
divergence and convergence can be combined to produce a variety of effects from few molecules
how do convergence and divergence allow flexibility?
divergence:
- each transmitter can activate multiple different receptors
- each receptor can activate different downstream receptors
convergence:
- different transmitters or receptors can activate the same downstream effector
divergence and convergence can be combined to produce a variety of effects from few molecules
what is glutamate?
- the most common excitatory neurotransmitter in the CNS
- an amino acid, so found in all neurons
- 3 ionotropic glutamate receptor subtypes based on drugs which act as selective agonists:
NMDA, AMPA, kainate - action is terminated by selective uptake into presynaptic terminals and glia
what are glutamate AMPA receptors?
- ionotropic receptor with 4 subunits
- mediates fast excitatory transmission
- allows Na+ and K+ through, but not Ca2+
glutamate binding triggers influx of Na+ and K+ currents, resulting in an EPSP:
- at rest, membrane is more permeable to K+ so closer to nernst potential of K+
- opening causes increased permeability to Na+ so membrane potential shifts closer to nernst potential of Na+
- causes depolarisation and EPSP
what are glutamate NMDA receptors?
- ionotropic receptor with 4 subunits
- have a voltage-dependent Mg2+ block: when cell is at rest, Mg2+ is attracted to negative potential, so moves to block the NMDA receptor
- receptor only opens when neuron is already depolarised as Mg2+ is pushed out, allowing glutamate to bind and open
- let Na+, K+ and Ca2+ enter
- coincidence receptor due to duel gating
can glutamate activate metabotropic receptors?
yes - mGluRs allow glutamate to sometimes be inhibitory e.g. in the retina by opening K+ channels
mGluRs are slower than iGluRs
what is GABA?
- major inhibitory neurotransmitter in CNS
- amino acid
what is GABA?
- major inhibitory neurotransmitter in CNS
- amino acid
- synthesised from glutamate by enzyme glutamic acid decarboxylase
- action is terminated by selective uptake into presynaptic terminals and glia
- produces IPSPs via GABA-gated chloride channels (GABAa receptors) if the membrane potential is above chloride’s nernst potential (-60 to -65mV
- the right amount of GABA inhibition is critical: too much = coma, too little = seizures
what are GABAa receptots and how are they modulated?
- ionotropic GABA-gated chloride channels
- other chemicals can bind to the GABAa receptor to modulate the response of GABA binding
- these chemicals are allosteric drugs as they have no effects without GABA binding
give examples of GABA allosteric drugs:
- ethanol - enhance inhibition by GABA
- benzodiazepines e.g. diazepam used to treat anxiety (enhance inhibition by GABA)
- barbiturates - sedatives and anticonvulasants (enhance inhibition by GABA in moderation to prevent coma)
- neurosteroids - metabolites of steroid hormones e.g. progesterone
what are GABAb receptors?
- metabotropic receptors
- open K+ channels (hyperpolarisation)
- close Ca2+ channels
- trigger second messengers like cAMP
- often presynaptic receptors and autoinhibitory - automatic feedback loop where GABA can bind to itself and influence the channel
what is glycine?
- amino acid
- inhibits neurons via glycine-gated chloride channel (ionotropic)
- can activate NMDA glutamate receptors
- can be excitatory or inhibitory
what is dendritic integration?
- just one synapse activation isn’t enough to activate a neuron as it only causes a small EPSP, so postsynaptic neuron cannot fire an AP
- if 5 or 6 synapses fire in quick succession, EPSPs can summate at the axon initial segment containing many Na+ channels, causing an AP to be triggered
why is the arrangement of excitatory and inhibitory synapses important? what is shunting inhibition?
- an inhibitory synapse can block the propagation of an EPSP towards the soma
- GABAa receptors do not always produce IPSPs, e.g. when membrane potential is close to chloride nernst potential - shunting inhibition
- opening chloride conductance decreases membrane resistance - so current leaks out of the membrane
how does inhibition occur presynaptically?
- GABAb receptors close Ca2+ channels
- when a GABAergic neuron synapses to another neuron, it blocks the neurons presynaptic release, due to the inactivation of Ca2+ channels
- less neurotransmitter is released so there is a reduced effect on postsynaptic membrane
why is inhibition important?
- inhibitory neurons modulate the specificity of excitatory neurons
- they gate the activity of excitatory neurons, as they would just continue to excite each other excessively
- inhibitory neurons sculpt activity patterns in the brain
what is acetylcholine?
- major neurotransmitter in PNS
- mainly excitatory (at the neuromuscular junction, at autonomic ganglion, at certain glandular tissues and in the CNS) but inhibitory in certain smooth muscles and at cardiac muscle
ChAT = good marker for ACh as it lives in cytoplasm of cholinergic neurons
what is the metabolism of ACh?
- acetyl CoA is produced by mitochondria and acetyl group combines with choline to form ACh, catalysed by choline acetyltransferase (ChAT)
- ACh is then transported by vesicle to synaptic cleft
- after binding to postsynaptic receptors, ACh is hydrolysed by acetylcholinesterase into choline and acetic acid
- choline is recycled in presynaptic terminal by a choline transporter to form more ACh with acetyl CoA
what kind of receptors does ACh act on?
ionotropic receptors: nicotinic ACh-gated Na+/Ca2+ channels (nAChRs)
- found at NMJ and CNS
metabotropic receptors: muscarinic (mAChRs)
- found in CNS and ANS
- M1, M3, M5 = excitatory via Gq
- M2, M4 = inhibitory via Gi/o
brain has 10-100x more mAChRs than nAChRs
how does ACh work at the NMJ?
- nAChRs receive signal on muscles to quickly excite the muscle and cause contraction
- quick due to many nAChRs being positioned in junctional folds in relation to the active zones on the presynaptic terminal
how can release of ACh be blocked?
botulinum toxin - prevents vesicle fusion by destroying SNAREs
latrotoxin (black widow venom) - increases ACh release at NMJ and then eliminates it by allowing a big Ca2+ influx to induce paralysis
how can AChE be blocked?
Nerve gas - AChE inhibitor which prevents parasympathetic signalling as ACh cannot get broken down and accumulates in cleft
Organophosphate pesticides - causes overexcitation in insects as ACh accumulates in cleft for too long
Alzheimer’s treatments - first neurons to die in brain are cholinergic, so enhanced signalling of cholinergic neurons via accumulation of ACh in cleft is beneficial
how can ACh receptors be activated, aside from by ACh itself?
Nicotine - agonist
Muscarine - agonist
Neonicotinoid pesticides - overactivate ACh receptors, causing overexcitation in insect nervous system
how can ACh receptors be blocked?
nAChR inhibitors:
- curare - antagonist used in poison arrow darts
- alpha-bungarotoxin - from snake venom, binds to nAChRs and takes days to unbind
mAChR inhibitors:
- atropine - anatgonist which can be used as an antidote for nerve gas, causes pupil dilation and increase in heart rate
what are monoamines? give examples
- they are synthesised from amino acids
catecholamines:
- catechol group + amine group
- common biosynthetic pathway: tyrosine amino acid forms L-DOPA by tyrosine hydroxylase (TH), which forms dopamine which can be converted to norepinephrine by dopamine beta-hydroxylase, then epinephrine
- rate limiting step = TH
serotonin (5-HT)
- synthesised from tryptophan amino acid to form 5-hydroxyltryptophan (5-HTP) via tryptophan hydroxylase. 5-HTP is converted to serotonin y 5-HTP decarboxylase
- rate limiting step = tryptophan
how are monoamines stored in terminal and removed from synaptic cleft?
- packed into vesicles by Vesicular Monoamine Transporters (VMAT)
- removed from cleft by reuptake transporters
- destroyed by Monoamine Oxidase (MAO) and catechol-O-methyltransferase (COMT)
(COMT only works for catecholamines)
are monoamine receptors ionotropic or metabotropic?
metabotropic:
- different receptors activate different G-proteins
- different receptors are expressed in different neuron types and parts of brain
dopamine: D1-like: D1, D5 receptors, D2-like: D2, D3, D4 receptors
epinephrine, norepinephrine: alpha and beta adrenergic receptors
serotonin: 7 receptors (one is ionotropic Na+/K+ channel however)
how is dopamine involved in motor control?
- in the substantia nigra within 2 nuclei in the ventral tegmental area
- project to the striatum
- follow nigrostriatal pathway to facilitate initiation of voluntary movement
these neurons die in Parkinson’s disease
how can Parkinson’s be treated by increasing dopamine?
- dopamine signalling is enhanced by increasing the product of TH (L-DOPA)
- dopamine does not cross the blood-brain barrier, but L-DOPA does, so more dopamine can be generated by L-DOPA in the brain
MAO-B inhibitors block MAO (enzyme which destroys dopamine) so that dopaminergic activity in the brain is increased
how is dopamine involved in reward?
- dopaminergic neurons in the ventral tegmental area project to the cortex and limbic system
- this mesolimbic pathway mediates reward/motivation
- intera-craniall self-stimulation of the mesolimbic pathway is extremely rewarding - likely target for addiction
what do noradrenergic neurons regulate?
- small number in the locus coeruleus innervate the whole brain
- control sleep/wake, attention, arousal, mood, memory, anxiety, pain
- role of norepinephrine in ANS
what do serotonergic neurons regulate?
- sleep/wake and mood
- these neurons live in the Raphe nuclei and project all over the brain
how does cocaine affect monoamines?
- blocks reuptake of dopamine and norepinephrine by blocking the dopamine reuptake transporter
- dopamine stays in cleft for longer so has greater signalling and increases the reward pathway
- overdose increases heart rate so can be fatal
how do cocaine and amphetamines affect monoamines?
- blocks reuptake of dopamine and norepinephrine by blocking the dopamine reuptake transporter
- dopamine stays in cleft for longer so has greater signalling and increases the reward pathway
- overdose increases heart rate so can be fatal
- amphetamines act as stimulants so can increase focus
how do antipsychotics affect monoamines?
- block dopamine receptors
- can have Parkinson-like side effects due to lack of dopamine signalling - impair motor control pathway
- treat schizophrenia
how do antidepressants affect monoamines?
- tricyclics block reuptake transporters of norepinephrine and serotonin so they are increased in cleft
- SSRIs (selective serotonin reuptake inhibitors) e.g. fluxoetine (prozac) (dont block norepinephrine, only serotonin)
- MAO-A inhibitors prevent destruction of serotonin so more innervation
what is the action of opioid peptides (endorphins)?
- bind to opioid receptors (GPCRs)
- regulate pain, coughing and GI tract
- opioid receptors are targets of morphine and heroin
what is the action of endocannabinoids?
- lipid-soluble
- Ca2+ triggers synthesis of endocannabinoids which then pass through the membrane
- retrograde signalling (postsynaptic neuron to presynaptic neuron)
- bind to GPCRs on postsynaptic neuron
what is the action of ATP?
- co-transmitter
- P2X2: ATP-gated ion channels (ionotropic)
- P2Y2: GPCRs (metabotropic)
what is the action of nitric oxide?
- gas = membrane permeable
- acts on soluble guanylate cyclase - not a membrane receptor
- this enzyme turns GTP to cyclic GMP which has downstream effects
