Muscle Contraction and Excitation

Question 1

Describe how contraction of muscles is stimulated

Answer 1

• Signal from motor-neurone reaches the neuromuscular junction
• Acetylcholine is released from the motor neurone which crosses the synaptic cleft and binds to receptor on the muscle membrane
• This binding causes an influx of sodium into the cell which causes depolarisation
• This depolarisation causes an action potential to be generated
• The action potential travels across sarcolemma and down T tubules
• Calcium is released from sarcoplasmic reticulum
• Calcium binds to troponin on action filaments which changes the conformation of troponin and moves the tropomyosin complex to allow myosin heads access to the myosin binding site
• Myosin heads attach to the binding sites of the actin filament and create a power stroke
• ATP detaches the myosin heads and energises them for another contraction
• This will continue until the action potential ceases
• Without the action potential the calcium ions will go back to the sacroplasmic reticulum

Question 2

Define a motor unit

Answer 2

It is all the muscle cells that are controlled by one neurone cell

Question 3

Define muscle tonus

Answer 3

• Tightness of a muscle to keep the skeletal muscles firm but does not result in a force strong enough to produce movement
• Some fibres are always contracted

Question 4

Define muscle tetanus

Answer 4

Sustained contraction of a muscle

Results from rapid succession of nerve impulses

Question 5

Define the refractory period

Answer 5

Brief period of time where the muscle cells will not respond to stimulus

Question 6

Compare the refractory period of cardiac and skeletal muscle

Answer 6

The cardiac period has a longer period of tetany than skeletal muscle. This prevents tetany of the heart

Question 7

Define a twitch contraction

Answer 7

It is a brief contraction of all the muscle fibres in a motor unit in response to a single action potential in a motor neurone.

Question 8

Describe the latent, contraction and relaxation period

Answer 8

Latent period - where the action potential sweeps across the sarcolemma and releases calcium ions from the sarcoplasmic reticulum

Contraction period - This is where the calcium ions binds to troponin, changing its conformation and moving tropomyosin to expose the myosin binding sites allowing attachment of the myosin heads to the actin filament. Peak tension occurs in the muscle fibre

Relaxation period - Where calcium ions are actively transported back to the SR. The myosin binding sites are covered by tropomyosin and the myosin heads detach from action and the tension in the muscle fibre decreases

Question 9

Describe the summation of nerve impulses

Answer 9

If a second stimulus occurs after the refractory period but before the muscle is relaxed then the second contraction will be stronger than the first

Question 10

Describe the micro-anatomy of skeletal muscle

Answer 10

Each muscle cell is called a muscle fibre. Within each muscle fibre are many myofibrils.
Myofibrils are made up of smaller filaments or myofilaments called actin and myosin.

Two Z lines mark the boundary of the sacromere. The sacromere is the functional part of the muscle cell

The area of the sacromere that only has myosin is called the H band.
The A band is the part of the sacromere that contains both actin and myosin
The part of the sacromere that contains only actin is called the I band

Question 11

Describe the function of creatine

Answer 11

Creatine is a molecule that is capable of storing ATP
It can combine with ATP to produce Creatine phosphate and ADP.

Creatine phosphate can combine with ADP to release ATP and creatine.

This generates ATP faster than synthesis from food

Question 12

Define muscle atrophy and what causes it

Answer 12

Muscle atrophy is caused by weakening and shrinking of the muscle and is also known as muscle wasting.

Can be caused by immobilisation, loss of neural stimulation and other factors

Question 13

Define muscle hypertrophy and what causes it

Answer 13

It is the enlargement of muscle due to increased capillaries and mitochondria

It can be caused by strenuous exercise and steroid hormones

Question 14

Define an isometric contraction

Answer 14

Isotonic contractions produce no movement. The tension produced is not enough to exceed the resistance of an object and there is not change in the length of the muscle. It is used in standing, sitting and maintaining our posture

Question 15

Define an isotonic contraction

Answer 15

This is a contraction that produces movement. In an isotonic contraction the tension remains almost constant while the muscle changes its lengths.

Question 16

Describe concentric and eccentric isotonic contractions

Answer 16

Concentric contraction - If the tension generated is great enough to overcome the resistance
of the object to be moved, the muscle shortens and pulls on another structure, such as a tendon, to
produce movement and to reduce the angle at a joint

Eccentric contraction - When the length of the muscle increases during contraction. The tension exerted by myosin cross bridges resists movement of a load and slows the lengthening process