Block 1 - Nervous System Flashcards
Describe the functions of the autonomic nervous system.
Sympathetic and parasympathetic divisions of the ANS often work simultaneously in a reciprocal and complementary manner maintaining homeostasis.
Sympathetic nervous system: orchestrates the stress response and energy consumption associated with ‘fight or flight’ reactions but also has very important ongoing activity.
Parasympathetic nervous system: regulates many functions, some of which are restorative and energy conserving ‘rest and digest’.
Outline the maintenance of homeostasis by various body structures.
Skin; thermoregulation by controlling contraction and relaxation of smooth muscle in the vasculature.
Liver/pancreas; metabolism of glucose and lipids.
Lungs; ventilation to control partial pressures and pH.
Heart and vasculature; blood pressure by contraction and relaxation of smooth muscle in the vasculature.
Kidneys; fluid balance.
Outline the components of the homeostasis negative feedback loop.
Involves three parts:
- a sensor
- a comparator/integrator
- an effector
Draw a labelled diagram of the neural pathway in the sympathetic nervous system.
[see notes for answer]
Draw a labelled diagram of the neural pathway in the parasympathetic nervous system.
[see notes for answer]
Outline the features of the sympathetic nervous system.
Short myelinated pre-ganglionic fibres.
Pre-ganglionic fibres synapse in paravertebral or prevertebral ganglia, basically as close to the central nervous system as possible.
Long unmyelinated post-ganglionic fibres.
From thoracic and lumbar regions of the spine.
Efferent nerves.
Outline the features of the parasympathetic nervous system.
Long myelinated pre-ganglionic fibres.
Pre-ganglionic fibres synapse in or on target tissues/organs.
Short unmyelinated post-ganglionic fibres.
From cranial or sacral regions or the spine.
Afferent nerves.
Outline neurotransmission in the sympathetic nervous system.
Preganglionic neurons
- neurotransmitter = acetylcholine
- post-synaptic receptor = nicotinic ACh receptor (type 2)
Post-ganglionic neurons (primary route)
- neurotransmitter = noradrenaline
- post-synaptic receptor = [alpha 1], [alpha 2], [beta 1], [beta 2] adrenoreceptors
Post-ganglionic neurons (sweat glands only)
- neurotransmitter = acetylcholine
- post-ganglionic receptor = muscarinic ACh receptor
Outline neurotransmission in parasympathetic nervous system.
Pre-ganglionic neurons
- neurotransmitter = acetylcholine
- post-ganglionic receptor = nicotinic ACh receptor
Post-ganglionic neurons
- neurotransmitter = acetylcholine
- post-ganglionic receptor = muscarinic ACh receptor
Describe non-adrenergic, non-cholinergic (NANC) transmission.
Sometimes the transmission in the autonomic nervous system is not caused by the classic neurotransmission but is caused by non-adrenergic, non-cholinergic transmission i.e. not a result of either noradrenaline or acetylcholine.
Outline NANC transmission.
Transmission might be a result of a NANC co-transmitter, primarily from the post-ganglionic fibres.
Rapid response
- parasympathetic = acetylcholine
- sympathetic = adenosine triphosphate
Intermediate response
- parasympathetic = nitric oxide
- sympathetic = noradrenaline
Slow response
- parasympathetic = vasoactive intestinal peptide
- sympathetic - neuropeptide Y
Describe cholinoreceptors.
Acetylcholine (ACh) is the endogenous agonist of cholinoreceptors that are nicotinic or muscarinic.
Nicotinic acetylcholine receptors (nAChR) are present in ganglia and are ligand gated ion channels.
Muscarinic acetylcholine receptors (mAChR) are present in effector cells and are g-protein coupled receptors.
Describe adrenoreceptors.
Noradrenaline and adrenaline are endogenous agonists of a family of adrenoreceptors that are all g-protein coupled receptors. They are classified by the rank order of agonists:
[alpha]-adrenoreceptor = noradrenaline -> adrenaline -> isoprenaline
[beta]-adrenoreceptors = isoprenaline -> adrenaline -> noradrenaline
Define ‘endogenous agonists’.
A naturally occurring substance within the body that binds to and activates a specific receptor, leading to a physiological response.
Define ‘exogenous agonists’.
A substance from out with the body that binds to and activates a specific receptor, leading to a physiological response.
Describe the neuromuscular junction (NMJ).
The neuromuscular junction is the connection between a motor nerve and a muscle. Its responsible for converting electrical impulses from the nerve into chemical signals that make the muscle contract.
Describe action potentials.
Action potentials can travel up to 120 metres/sec, triggering at the nerve terminal, the release of neurotransmitters. The human brain has 100 billion neurons many with >1000 synapses resulting in 100 trillion interconnections.
Describe and outline terminal buttons.
Terminal buttons = small knobs at the end of an axon that release neurotransmitters to send signals to other neurons. They are also known as synaptic boutons, end-feet or presynaptic terminals.
- when an electrical signal reaches their terminal buttons, they release neurotransmitters into the synapse.
- the neurotransmitters carry the signal across the synapse to other neurons.
- the other neurons receive the signal and respond.
Describe the synapse.
Communication occurs at synapses via the release of chemical messengers (neurotransmitters) from presynaptic nerve terminals to act upon receptors on the post synaptic membrane.
Outline the synaptic transmission at the neuromuscular junction.
the release of transmitters onto receptors involves 5 steps. Each step can be influenced by drugs and toxins, resulting in either an increase or decrease of transmission.
1) synthesis
2) storage (protect, package/quanta)
3) release - dependent upon the action potential coming along the axon
4) activation
5) inactivation
Describe the inactivation of acetylcholine receptors.
Acetylcholine is released at the neuromuscular junction and other synapses. AChE (acetylcholinesterase) is present in the synaptic cleft and breaks down acetylcholine through hydrolysis. The choline produced by AChE is then transported back to the presynaptic terminal, where it used to synthesise more acetylcholine later.
Outline how drugs can enhance synaptic transmission at the NMJ.
Direct stimulation of the receptors through the use of;
- natural neurotransmitters
- analogues
Indirect action via:
- increased transmitter release (like the use of the stimulant durgs such as amphetamine)
- inhibition of transmitter removal (in the case of acetylcholine, acetylcholinesterases)
Outline how drugs can inhibit synaptic transmission at the NMJ.
Blocking synthesis, storage or release from the pre-synaptic neuron.
Blocking postsynaptic receptors, preventing neurotransmitter binding.
Describe agonists.
Drugs, hormones or transmitters which bind to specific receptors and initiate a conformational change in the receptor resulting in biological response. Agonists have two properties: affinity and efficacy.
Define affinity.
The ability of an agonist to bind to receptors.
Define efficacy.
The ability of an agonist, once bound to a receptor, to initiate a biological response.
Outline antagonists.
Bind to receptors but do not activate them.
Block receptor activation by agonists.
Posses affinity but lack efficacy.
Describe competitive receptor antagonists.
Compete with the agonists for the binding site on the receptor.
The blockage can be reversed by increasing the agonist concentration, however they can also be irreversible.
Describe synapse classification.
Synapses are classified according to the transmitter released from the presynaptic neuron. In the neuromuscular junction acetylcholine is released and are therefore classified as cholinergic. Receptors upon which are acetylcholine acts are called cholinoreceptors.
Outline the two classes of cholinoreceptors.
Nicotinic cholinoreceptors (nAChRs) - ligand-gated ion channels.
Activated by ACh or the tobacco alkaloid nicotine but not by muscarine.
Muscarinic cholinoreceptors - g-protein coupled receptors.
Activated by ACh or the fungal alkaloid muscarine but not by nicotine.
Outline the common features of cys-loop superfamily receptors.
Fast synaptic transmission is mediated by ligand-gated ion channels (nAChRs). The nACHR is part of the cys-loop superfamily.
- integral ion channel
- agonist binding to the receptor induces a rapid conformational change to open the channel
- the channel is selective for certain ions
- signalling is extremely rapid (milliseconds)
Describe the features of the nAChR.
Made up of 5 subunits, arranged around a central pore, each subunit comprises of four transmembrane domains with both the N- and C- terminus located extracellularly.
Requires the binding of agonist or transmitters, binding site is located near the N- terminus, can be blocked by nicotinic antagonists.
When agonists bind, the subunits undergo a conformation change and the channel opens.
Cations (positive ions) cause depolarisation of the plasma membrane resulting in excitatory postsynaptic potential leading to the activation of voltage-gated ion channels.
Calcium entry acts in intracellular cascades, potentially leading to the activation of gene activity and the release of neurotransmitters.
Describe drug potency.
An expression of the activity of a drug, in terms of the concentration or amount needed to produce a desired effect such as an EC50.
Drug potency depends on both receptor (affinity and efficacy) and tissue (receptor numbers and drug accessibility) parameters.
Outline non-competitive antagonism.
The agonists acts by combining with a sperate inhibitory site on the receptor.
Agonists and antagonist molecules can be bound to the receptor at the same time.
The receptor can only become active when the agonist site alone is occupied not both sites.
The antagonism action can be reversible or irreversible.
Outline other forms of antagonism.
Chemical antagonism - the antagonist combines in solution directly with the chemical being antagonised.
Physiological antagonism - two agonists that produce opposing physiological actions and cancel each other out, each drug acts through their own receptors.
Pharmacokinetic antagonism - the ‘antagonist’ reduces the concentration of the active drug at its site of action.
Describe the patch-clamp technique and the steps involved in the process.
The patch-clamp technique is a laboratory technique that measures the ionic currents in living cells, tissue or cell membranes. It’s used to study how ions flow through biological membranes and ion channels such as the nAChRs.
1) A glass pipette containing an electrolyte solution is sealed onto the cell membrane.
2) The pipette creates a tight seal with the cell membrane, called a giga-ohm seal.
3) The pipette’s wire conducts ions into the pipette.
4) An electrode connected to a differential amplifier records the currents flowing through the channels in the patch.
Describe the generation of a miniature endplate potential in ACh receptors.
ACh released from a single vesicle activates many nicotinic ACh receptors. Upon activation the associated nicotinic cation channels open and Na+ ions flux into the muscle fibre to cause a local depolarisation at the endplate region i.e. a MEPP (miniature endplate potential).
Define exocytosis.
Vesicle fusion with the membrane.
Define endocytosis.
Recovery of vesicular membrane after fusion.
Describe the mechanism of [alpha]-latrotoxin.
Black widow venom, containing the toxin [alpha]-latrotoxin or [alpha]-LTX, influences spontaneous transmitter release.
[alpha]-LTX binds to receptor on the cell membrane such as latrophilins, neurexin la and receptor-like protein-tyrosine phosphate sigma receptors.
After binding [alpha]-LTX assembles into a tetramer (a molecule consisting of 4 structural subunits) which is shaped like a propeller.
This tetramer then inserts into a cell membrane, forming a pore which allows calcium to flow into the cell.
This calcium influx causes the cell to release neurotransmitters, such as acetylcholine and norepinephrine, potentially causing toxic effects on the central nervous system.
Outline the steps in simplified ‘fast’ synaptic transmission.
1) Presynaptic action potential.
2) A synchronous Ca2+ influx via voltage-gated ion channels.
3) Many vesicles undergo exocytosis releasing a large cloud of acetylcholine.
4) Activation of many nicotinic acetylcholine receptors.
5) This causes a large depolarisation of the endplate region of the muscle cell (an EPP).
6) If the depolarisation is large enough it activates postsynaptic voltage-gated sodium ion channels to initiate an action potential.
Explain the problem with the patch-clamp technique and describe the solution to this problem.
The muscle fibre contraction breaks the glass microelectrode when using the patch-clamp technique so to study neurally-evoked transmitter release the size of the EPP must be reduced. When studying the neuromuscular junction, nerve stimulation causes muscle contraction thus breaking the glass microelectrode.
Therefore we use a high Mg2+/low Ca2+ buffered solution. A high Mg2+/low Ca2+ extracellular solution reduces the EPP to below the threshold for firing an action potential.
Describe why calcium is essential for neurally-evoked neurotransmitter release.
Localised calcium entry via voltage-gated ion channels.
Calcium triggers vesicle fusion very quickly.
Magnesium blocks the voltage-gated calcium ion channels.
Describe the evidence that suggests neurotransmitter release at the neuromuscular junction is quantal.
The amplitude of the EPP is multiple the amplitude of the MEPP, with the smallest EPP amplitude being equal to that of the MEPP amplitude. This suggests that neurotransmitter release at the neuromuscular junction is quantal.
State how to find quantal content.
quantal content = number of vesicles / stimulus
OR
QC = mean EPP amplitude (mV) / mean MEPP amplitude (mV)
Describe quanta transmission.
Release of a vesicle gives a quanta of transmission. Each quanta gives rise to a MEPP via activation of nicotinic ACh receptors. MEPPs occur spontaneously (without nerve stimulation), EPPs occur is response to motor nerve stimulation. Upon nerve stimulation MEPPs summate to give an EPP, which initiates an action potential leading to muscle contraction.
Describe drug action at step 1 of cholinergic transmission at the neuromuscular junction. (Synthesis)
Choline acetyl transferase (CAT) synthesises ACh from precursors choline and acetyl coenzyme A (acetyl-CoA) from mitochondria.
Reuptake of choline is Na+ dependent and blocked competitively by hemicholinium 3 (not used clinically).
There will be less ACh in each vesicle.
Amplitude of the EPP and the MEPP are both decreased therefore there is no effect on QC.
Describe drug action at step 2 of cholinergic transmission at the neuromuscular junction. (Storage)
Transport of ACh int vesicle is blocked by inhibition of the ACh vesicular transporter by VESAMICOL (not used clinically).
Less ACh in each vesicle so amplitude of EPP and MEPP are both decreased, no change in QC.
Describe the mechanism of Tetrodotoxin (TTX).
Tetrodotoxin (TTX) – a powerful toxin produced by bacteria and concentrated in organs of certain organisms such as the puffer fish. It’s extremely toxic and causes the paralysis of the diaphragm leading to respiratory failure. It acts like the local anesthetic lidocaine but it’s much more potent.
Describe he use of Lidocaine.
Lidocaine acts as a local numbing agent by altering signal conduction in neurons by prolonging the inactivation of voltage-gated Na+ channels, with sufficient blockage the voltage-gated sodium channels will not open and an action potential will not be generated.
Describe drug action at step 3 of neurotransmitter release at the neuromuscular junction.
Tetrodotoxin (TTX) blocks Na+ channels therefore there is no action potentials, no vesicles are released resulting in no EPP.
Voltage-gated Ca2+ channels are blocked by conotoxins - decreased Ca2+ influx, decreased release.
The EPP amplitude decreases and there is no change to the MEPP amplitude therefore quantal content is decreased.
Describe dendrotoxin in green mamba venom.
Dendrotoxin blocks voltage-gated K+ leading to prolonged action potential resulting in increased Ca2+ influx and therefore increased vesicle release.
Increased EPP amplitude, MEPP amplitude doesn’t change therefore quantal content increases.
The synthetic drug 4-aminopyridine (4-AP) has a similar mechanism, it is used as a research tool in characterising the subtypes of potassium channel, can be used to manage some of the symptoms of multiple sclerosis.
Describe the mechanism of Botulinum toxin.
It is one of the most potent toxins known from the bacteria Clostridium botulinum. It binds to presynaptic surface of cholinergic neurons, once bound the neuron takes up the toxin into a vesicle by receptor-mediated endocytosis.
Once inside the cytoplasm the toxin cleaves SNARE proteins - proteins that mediate vesicle fusion with their target membrane bound compartments. Thus blocking the release of acetylcholine from the presynaptic terminal therefore less vesicles are released. This results in a decreased EPP amplitude but the MEPP does not change therefore QC decreases.
Outline the effects Botulinum toxin can produce.
Can lead to respiratory paralysis (flaccid paralysis).
Used clinically to treat a variety of muscle disorders, chronic migraines and neuropathic pain.
Used cosmetically in Botox.
Describe and outline non-depolarising neuromuscular blockers.
Non-depolarising neuromuscular blockers prevent acetylcholine from binding to its receptors while depolarising neuromuscular blockers open the acetylcholine receptor ion channels.
Non-depolarising neuromuscular blockers:
- drugs/toxins that prevent muscle contraction by blocking acetylcholine from binding to receptors.
- Compete with ACh for binding of skeletal nicotinic ACh receptors.
- Reduce the amplitude of the EPP to below the threshold for muscle fibre action potential generation.
Describe tubocurarine (curare).
It is a non-depolarising antagonist that acts as a muscle relaxant, so it binds to nicotinic acetylcholine receptors in skeletal muscles therefore blocking their function.
This causes paralysis as muscles cant contract (flaccid paralysis), death from respiratory paralysis can occur in 3-20 minutes.
- has little to no effect of quantal content
- used during surgeries (not as much nowadays)
- patient requires artificial respiration as the diaphragm muscle is paralysed.
- the patient is left to recover naturally or recovery can be sped up by an anticholinesterase (an antagonist for acetylcholinesterase) such as neostigmine.
Describe the mechanism of neostigmine.
Neostigmine inhibits acetylcholinesterase by binding to it thus blocking the enzyme from breaking down acetylcholine, thus ending the effects of non-depolarising neuromuscular blocking toxins.
Can also be used to treat diseases such as myasthenia gravis, Ogilvie syndrome and urinary retention eve when there is no blockage present.
Outline [alpha]-Bungarotoxin.
[alpha]-Bungarotoxin is a component of the Taiwan banded krait’s venom (another non-depolarising neuromuscular blocker).
- Cobras and other sea snakes has a similar [alpha] toxin
- It binds irreversibly to the agonist recognition site on the nicotinic receptor of the neuromuscular junction
- It decreases the amplitude of both the EPP and MEPP therefore no change in QC
Describe vecuronium.
Vecuronium is a general anesthesia that provides skeletal muscle relaxation during surgery or mechanical ventilation by acting as a non-depolarising neuromuscular blocker. Can be reversed by neostigmine but is better counteracted by sugammadex. Relatively slow acting so Suxamethonium is preferred.
Describe Succinylcholine/Suxamethonium.
Its a depolarising neuromuscular blocker.
nAChR agonist which causes skeletal muscle paralysis (can cause muscle rigidity).
Rapid onset (~30 secs) and has a short duration.
Used for rapid tracheal intubation and to prevent violent muscle contractions associated with electro-convulsant therapy.
Metabolised by plasma cholinesterase.
1 in 3000 individuals express a genetic variant of the enzyme which does not degrade Suxamethonium thereby causing prolonged paralysis.
Describe the steps involved in the phase 1 block of Suxamethonium.
1) Persistent activation of endplate nicotinic receptors.
2) Prolonged depolarisation of endplate.
3) Inactivation of voltage-gated sodium channels.
Describe the steps involved in the phase 2 block of Suxamethonium.
1) Desensitisation of endplate nicotinic receptors.
2) Repolarisation of endplate.
3) Receptor desensitisation maintains blockade.
ACh and nicotine are _______.
Agonists.
Suxamethonium is a _______.
Agonist.
Tubocurarine is a ____________.
Competitive antagonist.
[alpha]Bungarotoxin is an ________________.
Irreversible antagonist.
Describe the inactivation of acetylcholine stimulation.
ACh is terminated by an enzyme called acetylcholinesterase, which breaks down ACh into acetate and choline.
Drugs which inhibit AChE are called anticholinesterases, these are substances such as nerve gas and neostigmine, and they increase the effects of ACh.
Outline cholinesterase enzymes.
“True” acetylcholinesterase (AChE)
* Are present at cholinergic synapses
* Bound to the postsynaptic membrane in the synaptic cleft
Pseudo-cholinesterase
* Widely distributed and found in plasma
* Important in inactivating the depolarising neuromuscular blockers such as Suxamethonium
* Both “true” and pseudo cholinesterase are inhibited equally by most clinically relevant anticholinesterases
Describe anticholinesterases, making sure to mention quantal content.
Anticholinesterases are used to reverse non-depolarising skeletal muscle relaxants such as tubocurarine.
Anticholinesterase such as neostigmine increase the amplitude of both the EPP and the MEPP, so they do not effect quantal content.
Neostigmine prolongs the duration of the MEPP and the EPP due to the increased “life-time” of the ACh in the synaptic cleft, which permit’s receptor rebinding.
Outline organophosphates.
Organophosphates are anticholinesterases.
Nerve gas agents.
Poisoning by organophosphates occurs via phosphorylation of the hydroxy group of serine in the active site of cholinesterases.
Binds very stably to the enzyme.
Recovery requires synthesis of new enzyme which takes weeks.
Dyflos: volatile non-polar, lipid soluble, rapidly absorbed through mucous membranes and unbroken skin and across the blood brain barrier
- Includes sarin, Soman and Novichok
Sarin; an extremely toxic organophosphate, a colourless and odourless liquid, exposure can be very lethal even in small concentrations, death can occur within 1-10 minutes after direct inhalation of a lethal dose, respiratory paralysis.
Soman; when pure it is a volatile corrosive and colourless liquid with a faint odour, most commonly a yellow of brown colour.
Novichok; a family of nerve agents, is claimed to be the deadliest nerve agent ever made, some are solids with others are liquids, often dispersed as a ultrafine powder.
Outline the reversal or organophosphates.
Atropine; counteracts effects of excessive muscarinic receptor stimulation by ACh.
Oximes; antidote to nerve gas, reactivates the AChase.
Organophosphates are also used in insecticides as they rapidly absorbed through the insect cuticle.
Describe the use of organ baths.
Organ baths is an in vitro pharmacological technique that investigates nerve-muscle interactions. Isolated tissues are suspended in a fluid-filled, heated and gassed chamber and changes in tissue contractility are recorded via sensitive force transducers.
