4- Nmj, Mucle Contraction And EMG

Question 1

What is the neuromuscular junction

Answer 1

A specialised synapse between a motor neuron and a muscle fibre.

Question 2

Describe the structure of the NMJ

Answer 2

A specialized structure incorporating the distal axon terminal and the muscle membrane that allows for the unidirectional chemical communication between peripheral nerve and muscle

•Main structures constituting the NMJ:
–presynaptic nerve terminal
–synaptic cleft
–postsynaptic endplate region on the muscle fibre which has folds

Question 3

Describe the 7 steps in muscle contraction

Answer 3

1. Action potential open V-gated Ca2+ channels
2. Ca2+ enters
3. Ca2+ triggers exocytosis of vesicles
4. Acetylcholine diffuses in cleft
5. Acetylcholine binds to receptor-cation channel & opens channel
6. Local currents flow from depolarized region and adjacent region; action potential triggered and spreads along surface membrane
7. Acetylcholine broken down by acetylcholine esterase (enzyme). Muscle fibre response to that molecule of Acetylcholine ceases

Question 4

Describe myasthenia gravis

Answer 4

An autoimmune disorder where antibodies are directed against the acetylcholine receptor.
•There may be a personal or family history of other autoimmune diseases. It cause fatigable weakness (i.e. becomes more pronounced with repetitive use) and may affect the ocular, bulbar, respiratory or limb muscles.
•Antibodies are detected in nearly 90% of cases and EMG examination will confirm the diagnosis.
•In severe cases the antibodies in the blood can

Question 5

Describe Lambert-Eaton myastenic syndrome

Answer 5

•an autoimmune disease caused by antibodies directed against the voltage-gated calcium channel (VGCC) - associated with lung cancer.

Question 6

What is the neurotransmitter in voluntary striated muscle

Answer 6

Acetylcholine

Question 7

Describe muscular activation and relaxation

Answer 7

“Action potential propagates along surface membrane and into T-tubules
DHP (dihydropyridine) receptor in T-tubule membrane: senses V & changes shape of the protein link to Ryanodine receptor, opens the Ryanodine receptor Ca2+ channel in the SR membrane; Ca2+ released from SR into space around the filaments
Ca2+ binds to Troponin & Tropomyosin moves allowing
Crossbridges to attach to actin
Ca2+ is actively transported into the SR continuously while action potentials continue. ATP- driven pump (uptake rate < or = release rate).
Ca2+ dissociates from TN when free Ca2+ declines; TM block prevents new crossbridge attachment; Active force declines due to net crossbridge detachment

Question 8

Describe botulism

Answer 8

Botulinum toxin produces an irreversible disruption in stimulation-induced acetylcholine release by the presynaptic nerve terminal

Question 9

How are MEPPS caused

Answer 9

At rest vesicles release ACh at a low rate causing membrane end plate potentials, causing small changes in membrane potential

Question 10

Describe what happens to the bands in the myofibril during muscle contraction

Answer 10

I band shortens( actin)
A band doesn’t shorten (actin and myosin)
H band narrows and disappears (myosin)

Question 11

What is an electrocardiography used for

Answer 11

Recording a.p. In skeletal muscles

Question 12

Describe ECG and EEG

Answer 12

ECG: a.p. In heart, electrodes on limbs or chest

EEG (electroencephalogram): a.p. In brain, electrodes on scalp

Question 13

What happens as stimulus is increased up to 25hz

Answer 13

Twitch force plateaus

Question 14

What is tetanus

Answer 14

TETANUS = the prolonged contraction of a muscle caused by rapidly repeated stimuli
• In other words, it is the fusing of lots of twitches such that the
time between one twitch and the next is too short for the contraction to have died off before the next one comes
• If you increase the frequency of the stimuli, you can build up the force because the force of contraction doesn’t return to zero before the next stimulus arrives
• This leads to a build up of force and it is called an UNFUSED TETANUS
• It is unfused because you can still see the ripples where the force decreases slightly before the next stimulus comes
• If you further increase the frequency (around 30 Hz) you can start to see
FUSED TETANUS - the muscle will contract and continue to contract with no decrease in force
• Fused Tetanus is how people die when they get electrocuted
• The force summates but the action potentials DO NOT