The neuromuscular junction

Question 1

what is a neuromuscular junction

Answer 1

specialised synapse between neuron and muscle fibre
neuron’s branched are set out across the fibre
see the nerve terminal in a light micrograph
the point where things go wrong meaning muscles don’t contract eg diaphragm - fatal

Question 2

describe the synapse

Answer 2

upstream and downstream element 
presynaptic terminal - 'BOUTON' 
synaptic cleft - 10-50nm 
ion channels - separation of charge 
allow for unidirectional contact from neuron-neuron/muscle

Question 3

describe the use of electrodes

Answer 3

on skin measure action potential non-invasively
activity measured in muscle
see if disorders/progression/differentiate between causes
measure activity at contraction/rest

Question 4

describe the features of the NMJ

Answer 4

distal axon terminal
myelin sheath - disorder MS
V gated calcium channels
synaptic vesicles - Acetylcholine
acetylcholine receptors on the post synaptic membrane
end of synapse - motor end plate, larger on micrograph
acetylcholinesterase on post-synaptic membrane - break down Ach
synaptic cleft - space between motor neuron and muscle fibres

Question 5

describe the organisation of neurons and muscles

Answer 5

upper motor neuron - brain
lower motor neuron - brainstem and spinal chord - arms and trunk
each nerve highly branched and only contacts many muscles but each muscle only contacted by 1 type of nerve
in development lots of nerves innervate different muscle fibres - selective innerbition and breakdown so 1 fibre only receive innervation from 1 branch
In regeneration more than 1 nerve reaches each fibre - used in diagnostics - signal from injured is different from non-injured nerves

Question 6

describe muscle contraction at the NMJ

Answer 6

AP open V gated Ca channels
Ca enters cell vesicles move to membrane and triggers Ach vesicles to exit the cell by exocytosis
vesicle membrane fuse with synaptic membrane - release Ach
Ach diffuses in cleft
binds to receptor cation channel and opens channel
localised current flow from depol region - AP triggered and spread along surface, T-tubules - go inside muscle, localised current trigger actin and myosin - pull of muscle ends together
Ach broken down by acetylcholinesterase - muscle respond to lack of signal

change from neuronal to muscular event

Question 7

What does Ach do at rest

Answer 7

membrane potential of muscle fibres fluctuates - no contraction
record end plate potential/muscle fibre membrane potential
some Ach is released in vesicles - a discrete amount, similar magnitude every time
not enough to make an AP - don’t activate the muscle

Question 8

where is smooth muscle

Answer 8

around circular structures eg blood vessels

Question 9

describe skeletal muscle structure

Answer 9

muscle made of fascicles - made of fibres and connective tissue
muscle attached to skeleton - voluntary movement
myofibrils - proteins in discrete organised sections

Question 10

describe myofibres

Answer 10

covered by sarcolemma
T-tubules into the centre - AP to individual proteins
sarcoplasm - myoglobin for oxygen usage and mt - energy dependant
sarcoplasmic reticulum - fluid filled tubules
composed of myofibrils

Question 11

effects of isotonic, eccentric and isometric contractions

Answer 11

isometric - muscle stay the same length
isotonic - force stay constant but length changes
eccentric - muscle lengthens with force - more force than isotonic and is more painful/dangerous - if fast it can rip myofibres

Question 12

describe myofibrils

Answer 12

1-2um in diameter - extend the length of myofibres
sarcomere - length of muscle, between 2 Z disks, shortens in contraction
proteins - many components eg troponin, titin
striped down repeating lengths of proteins
in a L micrograph see what happens to discrete element of muscle - sliding filament model

Question 13

describe myofibrils

Answer 13

light and dark bands give muscle the striated appearance 
don't extend along length of myofibres 
overlap - arranged in sarcomeres 
dense Z disks separate sarcomeres 
dark A band - thick myosin 
light I band - actin 
myosin and actin filaments overlap 
Z discs get closer 
actin alone can disappear - pulled completely over myosin 
myosin stays the same length

Question 14

how are the components of the sarcomere named

Answer 14

Z M/H - german language

A I - effect on polarised light

Question 15

what were the observations of the sliding filament model

Answer 15

I band shorter
A band same length
H zone narrowed/disappeared

Question 16

describe the activation and relaxation process

Answer 16

AP - Ca out of SR into cell - bind to troponin - move tropomyosin - myosin bind to actin [cross bridges] - myosin head energised - ATP bind - allow myosin head release - Ca AT into SR (always just in activation it is also being released) - Ca released from troponin - tropomyosin recovers and blocks binding site

Question 17

describe botulism

Answer 17

botulinum toxin - stops Ach release into synaptic cleft
muscle can’t contract
paralysis
used in botox

Question 18

describe myasthenia gravis

Answer 18

autoimmune
antibodies directed against Ach receptor - receptor not receptive
muscle not contract
may be family history of other autoimmune
fatigable weakness
affect ocular, bulbar (brainstem), respiratory or limb muscles
corticospinal tract - cortex - spinal cord - limbs
corticobulbar tract - brainstem - face muscles and eyes
difficulty talking, fatigue, drooping eyelid
related to thymus - make Ab
only some receptors blocked
stop acetylcholinesterase - bind to receptors that aren’t blocked
to diagnose - inject to stop Achesterase - symptoms disappear
or remove Ab and return plasma to body

Question 19

Lambert-eaton myastenic syndrome

Answer 19

autoimmune 
Ab directed against V gated Ca channels 
associated with lung cancer 
in presynaptic terminal 
Ca can't enter so Ach cant be released
supply Ach instead so can function