MUSCLE CONTRACTION MECHANISM

Question 1

There are three types of muscles in the human body:

Answer 1

1. Cardiac muscles – muscles of the heart. They are involuntary, meaning we have no control over them.
2. Smooth muscles - muscles that are scattered around our body’s organs to allow their function.
   They are involuntary too; we have no control over them.
3. Skeletal muscles – muscles that are bounded to our skeleton and allow its movement. These are voluntary muscles and we do have control over them

**These three types of muscles differ from each other by their microscopic anatomy (structure), location
in the body and the type of control we have over them (voluntary/involuntary).

Question 2

Skeletal Muscles

Answer 2

Skeletal muscles are striated muscles with multinuclear cells under the control of the somatic nervous
system. This is the reason why we have voluntary control over them.

Question 3

Types of skeletal muscles:

Answer 3

1. Red muscles – made of thin fibres rich with myoglobin and mitochondria (so they can generate
   power overtime) they are used for slow strong and strenuous construction. These are anaerobic
   muscles.
2. White muscles – made of thick fibres which contain less myoglobin and mitochondria. They are
   used for fast and short contractions. They get tired very soon, because they generate burst of
   power for short times. Thus, losing their energy quickly.
3. Mixed – made of both white and red muscle. Their performance and usage is in between the two
   other kinds.

Question 4

Structure(From largest to smallest)

Answer 4

Muscle ← Fascicle ← myocyte (muscle fibre) ← myofibril ← myofilaments.

Question 5

Myofibril

Answer 5

The basic unit of muscle tissue composed of repeating units of sarcomeres.

Question 6

Sarcomeres

Answer 6

a contractile unit of a muscle composed of thick and thin filaments that interact with each other to allow contraction.

Question 7

𝑍 line

Answer 7

divides the border of the contraction units.

Question 8

𝑀 line (𝐻 zone)

Answer 8

The middle of a contraction unit.

Question 9

Thick myosin filaments

Answer 9

made of protein myosin. It is a protein that has a gulf club shape withtwo heads containing two binding sites. One binding site for acting (to allow the contraction), whilethe second binding site is for the binding of 𝐴𝑇𝑃 (to get the energy required for contraction). They
are connected to the 𝑀 line.

Question 10

Thin actin filaments

Answer 10

These filaments are arranged in a double helix, and have specific site for the
myosin heads to allow the bonding. They are connected to the 𝑍 lines.

Question 11

Thin actin filaments are made out of three proteins:

Answer 11

• Actin – the protein that makes the filament itself. It is also the protein that binds with myosin to allow the contraction.
• Tropomyosin – covers the actin fibres to deny contraction. This happens when the muscle is relaxed.
• Troponin – bounded to tropomyosin. When the muscle needs to contract, troponin will tighten and change the shape of tropomyosin (using calcium ion), to expose the actin binding site and allow contraction.

Question 12

Contraction Mechanism

Answer 12

Contraction occurs when a muscle fibre consisting thousands of sarcomeres is innervated, causing
filaments to slide against one another, thereby shortening and lengthening the muscle.

Question 13

Initiation (Innervation) of Contraction Mechanism:

Answer 13

1. Nervous system sends an action potential to contract the muscle via a motor neuron.
2. The signal reaches the axon terminal and the neurotransmitter acetylcholine is released into the
   synapse.
3. Acetylcholine binds to is receptor on the muscle, causing 𝐶𝑎+2 channels to open and an influx of
   𝐶𝑎+2 allowing the muscle to contract.

Question 14

Sarcomere Shortening and the Rule of 𝐶𝑎+2

Answer 14

1. 𝐶𝑎+2 binds to troponin causing tropomyosin to shift and thereby exposing the myosin biding site on the actin filaments.
2. Myosin heads bind to the exposed site on actin.
3. Myosin then pulls on acting, drawing the thin actin filament to the centre of the sarcomere, and thereby shortening it.
4. 𝐴𝑇𝑃 activity with the myosin heads provides the energy for the power stroke and results in the dissociation of myosin from actin’s binding sites (𝐴𝑇𝑃 hydrolysed into 𝐴𝐷𝑃 and 𝑃).

Question 15

Process of Contraction Mechanism

Answer 15

1. At rest, 𝐴𝑇𝑃 is hydrolysed to 𝐴𝐷𝑃 + 𝑃, which are attached to myosin head (the myosin head
   angle is 90 degrees).
2. Once myosin binds to acting, 𝐴𝐷𝑃 + 𝑃 dissociates from the myosin head, and myosin undergoes
   a power stroke, pulling the acting filaments and shortening the sarcomere ( the myosin head’s
   angle is now 45 degrees).
3. The loss of 𝐴𝐷𝑃 + 𝑃 allows room for 𝐴𝑇𝑃 to bind to the myosin head, which causes myosin to
   dissociate from actin (the head’s angle is now 180 degrees to the other side, to allow another
   cycle of contraction continue the process)
4. These cycle are continuing till the muscle fully contracts.
5. After the contraction is finished, the actin and myosin will go back to their place and the muscle
   will relax (tropomyosin will cover the acting binding site to deny contraction).

Question 16

Important to remember Muscle contraction mechanism

Answer 16

• The main source of energy for muscle contraction is glucose.
• Rigor mortis – the third stage of death, is one of the recognizable signs of death. Caused by chemical
  changes in the muscles post mortem, which cause the limbs of the corpse to stiffen. It happens due to
  lack of 𝐴𝑇𝑃.
• The role of oxygen in the muscle contraction is in aerobic respiration – final electron acceptor in the
  electron transport chain (in production of 𝐴𝑇𝑃).
• The binding between myosin and actin is called cross bridges.
• 𝐴𝑇𝑃 binding is required for the dissociation of actin and myosin.
• Hydrolysis of 𝐴𝑇𝑃 (creation of 𝐴𝐷𝑃 + 𝑃) energies and reorienting myosin in order to pull actin.
  *The basic unit of the muscle is called myofibril.