5.3. Muscle and motility

Question 1

What organisms don’t move (locomotion)? Why is movement important for animals?

Answer 1

Sessile organisms
Foraging for food, escaping danger, finding a mate, migration

Question 2

What is the function of a skeleton?

Answer 2

1. Providing support
2. Protection (exo.)
3. Metabolic function (blood cell creation, storing vitamins and minerals)
4. Anchorage for muscles and levers

Question 3

Parts of muscle/bone (attachment points), on example of biceps and masseter muscle

Answer 3

Starts at origin/tip and ends at insertion (on the bone which the muscle moves), fulcrum/pivot point is the joint
biceps: origin on the humerus bone and insertion on the radius. Pivot point is the elbow. masseter muscle: origin on cheek bone and insertion on jawbone

Question 4

Parts of a skeleton/muscle complex

Answer 4

1. Skeletal/striated muscle – elongated, spindle-shaped; surrounded by a membrane which forms tendons at its ends.
2. Tendons – attach bones to muscles, (relatively) non-elastic structure which transmits the contractile force of muscles to the bone.
3. Synovial fluid – fluid inside the synovial joint (self-lubricating articular joints) that prevents friction by being incompressible. It contains nutrients supplied to the cartilage (which doesn’t have its own blood supply) and received waste products so it has to be regularly exchanged – the synovial membrane constantly produces new and removes old synovial fluid.
4. The joint is sealed with a capsula.
5. Cartilage – prevents friction and damaging of bone tissue.
6. Ligament – connects bones, stabilization.

Question 5

Types of joints:
How is the range of movement measured?

Answer 5

1. Ball and socket joint (three planes of movement: protraction/retraction, abduction/adduction, rotation)
2. Hinge joint (one plane of movement: flexion/extension)
   Using a goniometer.

Question 6

Muscle structure (until filaments):
What is a myocyte made of?

Answer 6

Muscles are made of many muscle bundles. Each muscle bundle is made of many muscle cells/fibres called myocytes. Each myocyte contains myofibrils. Each myofibril is made of sarcomeres (contractile units). Sarcomeres are divided by Z-discs/lines onto dark (myosin) and light (actin) bands. Only the thickness of light bond changes with muscular contraction.
sarcolemma (membrane), sarcoplasm (cytoplasm – multinucleated due to fusion of several embryonic muscle cells), mitochondria, and sarcoplasmic reticulum (modified ER, close to Mb)

Question 7

Myosin and actin structure in striated vs smooth muscle
Myocyte structure in skeletal vs myocardium

Answer 7

arranged regularly in striated muscles and irregularly in smooth
myocardium myocytes branch out to ensure powerful and uniform heart contractions

Question 8

What is a motor unit?

Answer 8

one motor neuron plus all muscle fibres it stimulates

Question 9

how does contraction happen on the level of filaments (in summary)

Answer 9

the contraction of sarcomeres is due to the sliding of actin and myosin filaments. Myosin has lateral extensions (“heads”) that can attach to binding sites on actin. These heads undergo a cycle of binding to form a cross-bridge, pulling the actin molecule towards the centre of the sarcomere and then detaching and swivelling to the next binding site on actin. The small force exerted by each myosin head is multiplied up and muscles can exert very powerful forces.

Question 10

how the nerve impulse travels and impacts the filaments, different conformations of the myosin head…

Answer 10

When a nerve impulse arrives at the end of a motor neuron at the neuromuscular junction (synaptic cleft) it is carried over to the myocyte by neurotransmitter acetylcholine. The nerve impulse causes the sarcoplasmic reticulum to release Ca2+ ions into myofibrils. During relaxation, myosin head binding sites on actin are covered with tropomyosin and troponin complex, however Ca2+ ions cause the complex to rotate, expose the binding sites to myosin heads and thus enable them to attach and form a cross-bridge, beginning a contraction.
Myosin head has different conformations based on how much energy it has. Its low energy shape is bent (muscle relaxed). If it gains energy after ATP hydrolysis, it straightens out to reach the myosin head bonding site. Once a cross-bridge forms, P group and ADP detach from the head, causing another conformational change of the head which’ll make the head push actin towards the centre of the sarcomere. Once ATP joins again (another hydrolysis), head will detach and change back to its original conformation (E needed for relaxation too). As long as there is a nerve impulse, ATP and Ca2+ ions, the process of sliding filaments continues and results in muscle contraction. Each sarcomere (light band) gets shorter when contracted and thus the whole muscle gets shorter.

Question 11

What is rigor mortis and why does it happen?

Answer 11

Muscles are stiff the first few minutes after death due to lack of ATP needed for muscle relaxation

Question 12

What is titin, what is its function?

Answer 12

Elastic (not contractile) protein in between six actin filaments and myosin, acts as a molecular spring. It is the largest protein in nature (34 000 a-a long) and the 3rd most abundant protein in muscle. Function:
1) Attaches myosin to the z-line
2) Prevents overstretching of muscle
3) Increases the F exerted by muscle contraction by storing potential E when stretched (relaxation) and releasing it when it recoils in contraction (study guide pg. 130)