1.10 Control of muscle

Question 1

What are the two subsections of the nervous system?

Answer 1

Central (CNS) - Brain and spinal cord 
Peripheral (PNS) 
- efferent (motor)
-- somatic (skeletal muscle)
-- autonomic (cardiac/smooth muscle, exocrine glands)
- afferent (sensory)
-- somatic 
-- visceral

Question 2

Describe the reflex arc

Answer 2

• Afferent sensory neuron receives a stimulus
• Sends a signal to CNS
• Synapses with an efferent motor neuron
• Sends signal to muscle
• Triggering muscle contraction

Question 3

What is a motor unit?

Answer 3

One motor neuron and all the muscle fibres it innervates

- ratio of nerve to muscle fibres varies depending on function of the muscle

Question 4

How does a motor unit control muscle force?

Answer 4

By recruiting more/fewer motor units or by controlling force within a motor unit

Question 5

What are some specialised features of the neuromuscular junction?

Answer 5

• Synaptic vesicles containing neurotransmitter (ACh)
• Junctional folds (folded sarcolemma) - increase SA
• Receptors at ‘mouth’ of folds
• acetylcholinesterase - removes any extra ACh and eventually turns the signal from the nerve off

Question 6

Describe the events that occur at the NMJ

Answer 6

1. Action potential propagates down axon
2. Depolarised pre-synaptic membrane causes opening of voltage gated Ca channels, ions diffuse into presynaptic nerve cytoplasm
3. Ca ions cause synaptic vesicles to move and fuse to presynaptic membrane. Ach released into synaptic cleft
4. Acetylcholine diffuses across cleft and binds with receptors on post synaptic membrane
5. Binding of Ach causes ligand - gated ion channels to open. Na ions move into muscle cell causing depolarisation of motor end plate (end plate potential)
6. EPP causes opening of further voltage gated ion channels along muscle fibre - action potential is propagated
7. Acetylcholinesterase breaks down Ach to acetyl and choline

Question 7

How is the excitation-contraction coupling produced?

Answer 7

1. Stimulus produces an action potential in motor neurone which goes to the motor end plate. Motor neurone splits and these branches make contact with the muscle fibre
2. Terminal of motor end plate contains mitochondria and Ach
3. Ca channels open and Ca enters and fuses with vesicles with the edge of the terminal releasing neurotransmitter from vesicles into synaptic cleft and binds to Ach receptors on muscle membrane
4. Binding of Ach to receptors causes a depolarisation stimulating an action potential which travels along the sarcolemma and deep within a muscle fibre through a system of T tubules arranged closely to endoplasmic reticulum pockets
5. Endoplasmic reticulum and tubules are arranged very closely relative to contractile apparatus and importantly triad overlaps with actin and myosin

Question 8

What occurs after contraction?

Answer 8

1. Ca ions in myoplasm are pumped back into sarcoplasmic reticulum
2. Pump = CaATPase, requires energy
3. Absence of Ca ions into the sarcoplasm means muscle no longer contracts