lecture 4 : the neuromuscular junction Flashcards
what is a neuromuscular junction?
a specialised synapse between a motor neurone and a muscle fibre
what are the main structures in the NMJ
- the pre synaptic nerve terminal
- the synaptic cleft
- the post synaptic nerve terminal
what is present on the primary synaptic cleft?
what is present on the secondary synaptic cleft?
- ACh receptors are present
- acetylcholinesterases
what is the nature of the chemical communication?
unidirectional
what is the neurotransmitter for voluntary started muscle?
it is acetylcholine
how is the dominant motor neurone chosen?
- in development the motor neurones compete to be the dominant motor neurone
what is the mechanism of a neuromuscular junction?
- action potential opens the voltage gated calcium channels
- Ca2+ enters
- Ca2+ triggers the exocytosis of the vesicles
- ACh diffuses into the cleft
- ACh binds to the post synaptic receptors and opens the channels
- local currents flow from the depolarised region and the action potential is triggered and spreads across the surface membrane
- ACh is broken down by acetylcholinesterase enzyme
- the post synaptic response to the ACh ceases
what happens at the NMJ at rest?
- at rest individual vesicles release ACH at a very low rate which causes ‘miniature end plate potentials’
what are the potentials called when they don’t make an AP?
these are graded potentials
what are myofibrils?
- they are covered by the plasma membrane (the sarcolemma)
- T tubules in the centre
- the cytoplasm is called the sarcoplasm
- there is a network of fluid filled tubules
( the sarcoplasmic reticulum) - they extend along the whole length of the myofibre
what is the sarcomere?
the cytoplasm of the myofibril
- composed of two parts
- actin and myosin
what does the sarcomere look like?
- looks striated due to light and dark bands
what do myofilaments consist of?
- dense protein Z discs separate sarcomeres
- dark bands known as A bands (THICK MYOSIN)
- light bans are I bands
(THIN MYOSIN) - the myosin and actin filaments overlap
- H zone which is where only thick myosin is present
- M line which is a line of myosin
show a diagram of a myofilament:
INSERT PIC
show a diagram of a whole muscle fibre :
INSERT PIC
what happens to the bands when contraction happens?
sliding filament theory
- during contraction I bands get shorter
- A bands remain the same length
- H zone disappears
what is the activation/ relaxation process?
- the action potential propagates along the surface membrane and into the T tubules
- DHPR in the t tubule senses the change in potential and changes shape and links onto the RyR
- this triggers Ca2+ to be released from the sarcoplasmic reticulum
- Ca2+ binds to troponin and this causes tropomyosin to move
- this means cross bridges attach to the actin
- Ca2+ is then actively transported into the sarcoplasmic reticulum whilst the action potentials continue
- when the Ca2+ has declined it dissociates from the troponin and then tropomyosin blocks new cross bridge attachments from being made
what are examples of neuromuscular junction disorders?
- botulism
- myasthenia gravis
- Lambert-Eaton myastenic syndrome.
what happens in ?botulism
- Botulinum toxin produces an irreversible disruption in stimulation-induced
acetylcholine release by the pre-synaptic nerve terminal.
what happens in myasthenia gravis?
An auto-immune disease caused by production of antibodies against the ACh receptor
what happens in Lambert-Eaton myastenic syndrome?
(Associated with lung cancer) An autoimmune disease caused by antibodies directed
against the VGCCs.
