Neuromuscular Junction Flashcards
Describe nicotinic receptors at the neuromuscular junction?
They are transmembrane channels that open to allow Na+ ions to flow through in response to activation by ACh.
Each receptor contains 5 subunits, 2 alpha, 1 beta, 1 delta and 1 epsilon (epsilon is replaced by gamma in fetal muscle).
ACh binds to the alpha subunits.
This is an example of a ligand gated ion channel
Describe the muscarinic receptors at the neuromuscular junction?
Muscarinic receptors occur at parasympathetic postganglionic junctions and work by G-protein linkage.
Where are nicotinic receptors found within the body?
Autonomic ganglia
Adrenal medulla
Neuromuscular junction
Describe the steps involved in voluntary muscle contraction?
An action potential is propagated through the axon.
The action potential reaches the terminal end of the axon. This causes depolarisation, which opens voltage gated Ca2+ channels and there is an influx of Ca2+.
This causes ACh vesicles to be released, which causes the nicotinic receptors to open and Na+ to flow through.
This causes a further action potential to be propagated down the T tubule causing release of the calcium ions from the terminal cisternae into the cytosol.
Calcium ions trigger a contraction of the muscle cell.
Describe the structure of motor neurones?
Each motor neurone has a cell body which contains a nucleus, has a number of short dendrites which extend outwards and a single long fibrous axon which extends from the cell body to NMJ. As the axon approaches the muscle it branches and ends as axon terminals at the NMJ.
The axon is surrounded by a myelin sheath, there are regular gaps in the myelin sheath known as Nodes of Ranvier.
Action potentials jump from one node to the next this is known as saltatory conduction.
Name the different type of nerve fibres and the information they relay, there diameters and there conduction speed?
Aα Proprioception, somatic motor. 15μm. 100ms
Aβ Touch, pressure. 10μm. 50ms
Aγ Muscle spindles 5μm 25ms
Aδ Pain, temperature, touch 3μm 20ms
B Preganglionic autonomic 2μm 10ms
C Pain, post ganglionic sympathetic 1μm 1ms
What is the relationship between diameter of nerves and conduction?
The wider a nn the faster the conduction
What is myelin?
Myelin is a protein-lipid complex, and is formed by the layers of plasma membrane derived from Schwann cells.
What is the resting membrane potential of neurones and how is it achieved?
The RMP around neurones is -70mv
It is achieved by the active transport of Na+ out of cells in exchange for K+ against there concentration gradients. 3 Na ions are transported out in exchange for 2 K ions moving in. This contributes to the negatively charged RMP.
The cell membrane contains K+ ion gates but it is impermeable to Na+ ions. This leads to a slow efflux of K+ ions which helps maintain the -ve RMP.
Describe the all or nothing law with regards to an action potential at a neurone?
A nerve stimulus occurs which will alter the RMP.
The stimulus must be sufficient to reach the threshold potential (~-55mV), if it is not reached then no action potential (AP) will occur.
Once the threshold has been reached then an action potential will be generated, the amplitude and duration of the AP will be the same regardless of the stimuli.
Describe the events that occur when an action potential is generated at a neurone?
A stimulus occurs and raises the RMP sufficiently to reach the threshold potential (~-55mV).
This causes voltage gated Na channels to open. This causes an influx of Na+ ions which causes the MP to become positive ~40mV (depolarisation).
At this point voltage gated K+ ion channels open causing an efflux of K+ions causing the MP to become -ve again (repolarisation). The MP becomes more -ve than the RMP.
At this point there is a refractory period while the Na+/K+ ATP pump re-establishes the RMP.
What is a motor unit?
A motor unit describes a motor neurone an the muscle fibres it controls.
Muscles which need fine control have a smaller ration of motor neurone to muscles fibres e.g extrinsic muscles of the eye 6-10 muscle fibres are supplied by a single motor neurone.
What is the motor end plate and how does it function?
The motor end plate is the specialised area at the end of a motor neurone as it approaches the muscle.
It contain vesicles which contain ~10000 molecules of ACh.
As an action potential reaches the motor endplate, Ca2+ ions influx in to the motor end plate. This triggers the release of ACh.
The ACh crosses the synaptic cleft and bind to nicotinic receptors which are found on the post synaptic cleft.
These receptors are concentrated on crests which are opposite the active zones (areas containing the vesicles) on the motor end plate.
In the junctional folds of the post synaptic membrane there are high concentrations of acetylcholinesterase’s.
Describe the Acetylcholine cycle?
Choline reacts with Acetyl Coenzyme A (Acetyl CoA) to form Acetylcholine. This reaction is catalysed by choline acetyltransferase.
When ACh gets secreted from the motor endplate (after depolarisation causing Ca2+ influx) it binds to the post synaptic nicotinic receptors. This opens the transmembrane channel allowing influx of Na+ (and a few K+) causing depolarisation.
The ACh needs to be rapidly broken down to allow the membrane to repolarise. Acetylcholinesterase rapidly breaks down the bound ACh to produce acetate and choline.
The choline is then recycled into the motor end plate terminal.
Note: Choline can also be produced de novo in the neurone.
Describe the structure of skeletal muscle?
If you take a cross section of skeletal muscle it is made up of bundles of muscle fibres known as fasiculi.
Fasiculi contain anywhere between 10 to 100 muscle fibres (larger muscles contain more fibres per fasiculi)
Each muscle fibre is surrounded by a cell membrane known as the sarcolemma.
The muscle fibres are made up filamentous bundles called myofibrils. Each muscle fibre contains 100-1000 myofibrils. Each myofibril is made from actin and myosin filaments.
A network of tubules and channels called the Sarcoplasmic Reticulum surrounds the Myofibril. It is here in the structure of skeletal muscle that we store Calcium.
Transverse tubules pass inwards from the Sarcolemma throughout the Myofibril, through which nerve impulses travel.
Myofibrils can split into functional units called sarcomeres.
