Unit 4.1

Question 1

Label a diagram of a motor unit

Answer 1

Limit to dendrite, cell body, nucleus,
axon, motor end plate, synapse and
muscle

Question 2

Explain the role of neurotransmitters in
stimulating skeletal muscle contraction

Answer 2

Acetylcholine is released as the impulse nears the end of the axon. This is a neurotransmitter (a chemical that transmits signals from a neuron to a target cell across the synapse).It is stored in vesicles, which ‘pop’ when the membrane is depolarized (+charged).

Acetylcholine crossed the synapse and docks on receptors in the muscle membrane. This causes that area of the muscle fiber to become slightly more positive (“depolarized”)

Sodium channels open in response to this small depolarization, permitting a huge flow of positively charged sodium and ions to enter the muscle fibre. The depolarization is greatly amplified, and an electrical impulse spreads throughout the fibre causing muscle to contract.

After the charge has passed from the neuron to the muscle fibre, acetylcholine is released from the receptors and broken down by cholinesterase to be recycled in a continuous process, this immediately repolarizes the membrane and muscle relaxes

Question 3

Dendrites

Answer 3

Receive impulses from other neurons and pass them to the cell body

Question 4

Cell Body

Answer 4

Sorts out the information and sends an impulse down the axon.

Question 5

Axon

Answer 5

Acts like a wire, carrying the electrical impulse.

Question 6

Myelin Sheaths

Answer 6

Surround the axon, protecting it. They act as insulators, which speeds up the impulse.

Question 7

Motor End Plate

Answer 7

As the impulse reaches the end of the axon, it connects with muscle fibres at motor end plates.

Question 8

Synapses

Answer 8

are where one neuron terminal meet the dendrites of another neuron or where they meet the muscle fibre

Question 9

Neurotransmitters

Answer 9

chemicals that are used for communication between a neuron at the
synapse and another cell

Question 10

Acetylcholine

Answer 10

a chemical that transfers the action potential from the motor unit to the
muscle cells allowing the muscle to contract

Question 11

Cholinesterase

Answer 11

an enzyme that breaks down acetylcholine

• The act of breaking down acetylcholine will depolarize the membrane which will stop
  the muscle from contracting

Question 12

Explain how skeletal muscle contracts by the sliding filament theory

Answer 12

1. Each muscle contraction begins with an action potential (electrical impulse) arriving at the neuromuscular junction (cell membrane)
2. The cell membrane is depolarised (positive inside/negative outside) and acetylcholine (neurotransmitter) is released. This allows sodium into the cell.
3. Calcium ions are released from the sarcoplasmic reticulum into the muscle cell
4. Calcium binds to troponin on the tropomyosin which causes it to move and reveal the myosin binding sites on the actin. ATP on the Myosin head is hydrolysed to form ADP + Phosphate
5. The myosin head binds to actin to form a Cross bridge. Myosin head remains bound until ATP molecule releases it. As long as there is calcium available cross bridge formation will continue until maximum contraction of the muscle fibre is reached
6. ADP is released and the activated myosin head pivots, sliding the actin filament towards the centre of the sarcomere. This is known as the ‘power stroke’.
7. The myosin heads continuously crawl up the actin, until both ends of the sarcomere (Z lines) are pulled towards the H zone in the middle.
8. Cross bridge formation is terminated when calcium ions are actively transported back the sarcoplasm. The myosin binding sites on the actin filament are covered by tropomyosin. Troponin returns to its original state, allowing tropomyosin to cover the myosin binding site on actin.

Question 13

Explain how slow and fast-twitch fibers differ in structure and function

Answer 13

Activity used for:
Type l: aerobic
Type lla: long term anaerobic
Type llx: short term anaerobic

Contraction speed:
Type 1: Slow
Type lla: Fast
Type llx: Very fast

Size of motor neuron:
Type l: small
Type lla: large
Type llx:very large

Force production:
Type l: low
Type lla: high
Type llx: very high

Number of mitochondria:
Type l: High
Type lla: High
Type llx: Low

Capillary density:
Type l: High
Type lla: Intermediate
Type llx: Low

Oxidative capacity:
Type l: High
Type lla: intermediate
Type llx: low

Resistance to fatigue:
Type l: high
Type lla: intermediate
Type llx: low