11 - Neuromuscular Transmission Flashcards
What should I know?
- motor units
- pre-synaptic and post-synaptic events at NMJ
- events at the NMJ
- nature of the EPC and EPP
- abnormal muscle contraction (myasthenia gravis)
What is the primary role of skeletal muscle?
To produce the force and movements necessary for life. It also provides support and produces heat
How much of the body does skeletal muscle make up?
35-55%
What does it mean to say skeletal muscle is able to make chemomechanical transduction?
Skeletal muscle transduces chemical energy in ATP bonds into mechanical movements
How is skeletal muscle voluntary muscle?
- contraction is controlled by input from the nervous system via alpha motor neurons
- a complex series of events links the nerve mediated ELECTRICAL excitation of a skeletal muscle to its mechanical contraction via a NMJ
What is the path of an alpha motor neuron?
- One alpha motor neuron branches with each branch synpasing at a single muscle fiber
- This means a single alpha motor neuron innervates many muscle fibres (motor unit)
What is a motor unit?
A single motor neuron and the muscle fibres it innervates
What is the physiological consequences of a motor unit?
Means that each motor unit can contract in symphony as the fibres are all innervated by the same one motor neuron so all the fibres contract
What is one way to increase the force of contraction?
- Increase the recruitment of motor units
- results in the electrical activity being amplified
- one action potential in one motor neuron results in action potentials in all the muscle fibres of the motor unit
- easier to record as amplified
What is the size principle that describes the recruitment of motor units?
- small oxidative fibres and motor units are recruited first as have a lower threshold (Type 1/slow)
- large glycolytic motor units are recruited last (Type 2/fast)
- this size principle ensures efficiency as slower fibres which there are more of are recruited first
- motor neurons innervate only the same type fibres so also applies to motor units
What is neuromuscular transmission?
The transduction of the electrical signal from the nerve to the muscle fibre at the neuromuscular junction/end plate
How is contractile function graded?
The force of contraction is graded by the recruitment of motor units with progressively increasing excitatory input
Where does most of our knowledge of synaptic transmission stem from?
Studies of the NMJ due to its large size, relative isolation, robust transmission
Describe the neuromuscular junction
- is where alpha motor neurons synapse at the motor end plate
Are the terminal ends of the motor neurons myelinated?
- No they are unmyelinated.
- the rest of the motor neurone is myelinated
- There is one ‘area’ of a terminal branch per muscle fibre
Describe the molecular organisation of the post-synaptic folds
- many pre-synaptic terminal boutons per fibre at the motor end plate each with schwann cell sheaths
- the membrane of the folds has 2 distinct domains
- At the CRESTS there is a high concentration of AChRs (acetylcholine receptors) and the the AChR clustering proteins rapsyn and utrophin
- the DEPTHS of the folds have a high conc of VGSCs (Voltage Gated Sodium Channels) to initiate the action potentials
What is found at the crests of the post synaptic folds?
- AChRs in a high concentration
- rapsyn
- utrophin
What is in direct apposition with the pre-synaptic nerve cells?
A high concentration of the AChRs in the post synaptic folds
Where is the motor end plate?
On the post synaptic MUSCLE CELL
What do the pre synaptic terminal boutons contain?
- synaptic vesicles (ACh)
- below this is the dense bar (active zone)
- pre synaptic membrane
- synaptic cleft
- basement membrane
- post synaptic membrane (with ACh receptors and VGSCs)
What was the fundamental question scientists were trying to find the answer to with regards to neuromuscular transmission?
How do action potentials in a pre-synaptic nerve cell influence the electrical activity of a post-synaptic muscle cell?
What were the 2 suggested answers to the fundamental question and who suggested them?
- John Eccles
> electrical transmission
> cells are continuous allowing direct current flow
> gap junctions do allow transmission electrical signals - Henry Dale
> chemical transmission
> that there are discrete pre synaptic cells and post synaptic cells. That the pre synaptic cells release a chemical that initiates current flow in the post synaptic cell
What was the experiment that provided the first reliable evidence that there was chemical transmission at a synapse (Henry Dale)
Otto Loewi’s Experiment
What was Otto Loewi’s experiment?
- Put a perfused heart in solution, connect it to a cannula and provide electrical stimulation from an electrode to the VAGUS nerve (slow heart)
- Perfuse the 2nd heart with the same solution that had the vagus nerve removed and he could slow the heart rate of the second heart
What did Otto Loewi’s experiment show?
- Shows when you stimulated the (vagus) nerve there was a release of a chemical (into solution) that acted to slow down the SA node
- It was the first reliable evidence for the existence of chemical transmission at a synapse
- That communication between nerve and muscle fibre must be chemical
- Para/sympathetic nerves released a different neurotransmitter
- It was the characteristics of the transmitter that altered the heart rate
What are the 3 Pre-Synaptic Events
- An action potential in the pre-synaptic cell reaches the nerve terminal
- Depolarisation of the nerve potential opens voltage gated Calcium chanels resulting in Calcium influx into the pre-synaptic cell
- The increase in calcium triggers fusion of vesicles to the pre-synaptic membrane, exocytosis of the pre-synaptic vesicles and quantal release (set number) of ACh into the synaptic cleft
- The vesicles are then taken up again/recycled
What is the benefit of vesicle recycling?
As the vesicles fuse to the pre-synaptic cell membrane, recycling them stops the pre-synaptic cell increasing in size
What are the 9 steps of vesicle cycling and release?
- The vesicle is filled with neurotransmitter at the pre-synaptic terminal
- Form a vesicle cluster at the terminal
- Filled vesicles then dock at the active zone
- The vesicles undergo priming where ATP IS CONSUMED
- Calcium triggers fusion-pore opening and recycle via several routes
- Local reuse
- Fast recycling (acidification)
- Clathrin mediated endocytosis (slow)
- Recycling via endosomes
At which step is ATP consumed at pre-synaptic vesicle cycling?
Priming to convert the vesicles to a state of competence for fusion and pore opening AT the active zone
What are the 3 recycling routes?
- Local reuse
- Fast recycling via acidification
- Clathrin mediated endocytosis with recycling via endosomes
How big is the synaptic cleft and how is it held together?
20nm and is held together by scaffolding proteins
Describe the post-synaptic events at the NMJ
- 2x ACh diffuses and binds to the AChR (2 binding sites)
- Non-specific channel opening
- Na inflow and K+ outflow (at -80mV)
- Depolarisation of motor endplate (EPP)
- Opening of voltage gated sodium channels at the endplate
- Na inflow
- Depolarisation of the entire muscle fibre
- Propagated muscle action potential in all directions
Describe the nicotinic AChR
- in the CRESTS
- 2 binding sites for ACh
- Is ligand gated (ACh) and is non-specific for cations so once it opens it allows the flux of both K+ and Na+ into the post synaptic cell (muscle fibre)
What is the motor endplate and endplate potential?
- The motor endplate is the area of the muscle fibre where the AChRs are and so the region where the muscle fibre depolarises
- the endplate potential is the depolarisation of this area
- opens VGSCs in the DEPTHS
What is the end plate current (EPC)?
- at the endplate an EPC is generated by the synchronous opening of 200 000 AChR channels each permitting 17 000 Na to MOVE and enter (and K+ leaves)
- each individual channel that opens contributes a discrete increment of EPC
When do the AChR channels close?
As ACh unbinds and is removed from the synapse by ACh-esterase
What is the End Plate Potential (EPP)?
- is always suprathreshold in muscle
- EPP is the local depolarisation of the post synaptic cell as a result of the ACh non-specific cation channels opening.
- the EPP travels passively along the post synaptic cell for a SHORT distance before it dissipates
- this is different from an action potential which is the result of this EPP reaching threshold which causes the VOLTAGE gated Na+ channels to then open, depolarise the entire cell and propagate an AP
How can you separate the EPP from the AP?
With the use of Curarae which binds to the receptor instead of ACh
What is the falling time course of the EPC determined by?
- The random closure of individual channels open for a RANGE of times
- is defined by the mean open time of of AChR channels
What is the falling time course of the EPP dependent on?
- longer than the EPC
- defined by both the AChR channel open time and passive electrical charging and discharging time of the muscle membrane
Describe the difference in the time course for the EPC vs the EPP?
- have the same neuromuscular transmission delay
EPC: - has a steep slope
- normally shorter time course
EPP: - has a gradual slope
- the voltage change is slower than the EPC due to having to overcome the membrane capacitance
What are alpha toxins and where are they found?
- they are specific blockers of the nicotinic AChRs
- found in snake venoms (eg. alpha-bungarotoxin)
- found in Curare; a plant extract used to paralyse prey
- act as muscle blocks
How can the EPP be isolated?
- using voltage clamp techniques with either decreasing calcium of the bathing solution
- or using a blocker of AChR like an alpha toxin
Using curare as a AChR block
- able to record the EPP
- eliminates a lot of the depolarisation so the voltage gated Na channels don’t open and the AP doesn’t occur so can look at isolated EPP
- can look at the time course of EPC and EPP
How did people know the EPP was NOT a propagated response?
- in order to record the change in EPP they had to place the electrodes very close to the end plate
- 1 mm away was too far the the membrane wouldn’t depolarise from the EPP
- in order for the electrical activity to be propagated to the muscle fibre the VGSCs had to open and propagate the AP
What does transmitter inactivation and recycling ensure?
- ensures the post synaptic cell remains SENSITIVE and is activated rapidly by ACh and that the response is NOT ongoing
- ensures ACh use is fast and short and so the cell has effective sensitivity
What allows transmitter inactivation and recycling?
- Acetylcholinesterase is anchored to the collagen fibrils of the basement membrane in the synaptic cleft
What does AChE do to ACh?
- hydrolyses it into choline and acetate
- choline diffuses back into the pre-synaptic terminal and is reabsorbed and acetate is recycled
- allows the repackaging of vesicles with neurotransmitter
What is AChE the physiological target of?
- insectisides and military nerve gases like sarin and VX as it means ACh does not break down
What are the 2 types of abnormal neuromuscular transmission?
- Pre-synaptic and post-synaptic
- these interfere with the transmission of the electrical impulse at the NMJ and can result in muscle weakness or complete block of transmission
Pre-Synaptic Abnormal NM Transmission
- Lambert-Eaton Syndrome
- Diabetes
- Toxins (alpha-latrotoxin in black widow spiders (triggers exocytosis), botulinum in botox, tetanus toxins
> prevent vesicles docking and so local muscle contraction
Name 3 toxins that affect pre-synaptic transmission of electrical impulses
- a-latrotoxin in black widow spiders that triggers exocytosis
- Botulinum
- Tetanus toxins
> prevent vesicles docking and so local muscle contraction
Post-Synaptic Abnormal NM Transmission
- Myasthenia Gravis
2. Alpha toxins (curare and a-bungarotoxin)
What is Myasthenia Gravis?
- most common primary disorder of neuromuscular transmission
- is an autoimmune disorder
- antibodies against the AChR are made and bind to the receptors
- is less functional receptors so action potential initiation is inhibited
- fine motor skills of the face are affected, weakness and fatigue of voluntary muscles
- treated with anti-AChEs and immuno-suppressive drugs
- less ACh receptors means the junctional folds with the voltage gated sodium channels aren’t as extensive so APs can only be triggered by LARGE amounts of ACh and when it isn’t broken down as fast therefore block this enzyme!
