Muscles and Movement: Topic 11.2

Question 1

Label a diagram or x-ray of the human elbow joint (and know the function of each structure, including the movements of flexion and extension).

Answer 1

LOOK AT NOTES - Muscles and Movement slide 5-7: https://docs.google.com/presentation/d/1zSmgUCOdpqvUUWLLZMuVysbrHRaw2usPf0H6gPR9FOs/edit#slide=id.p8

Cartilage absorbs compression; reduces friction between bones
Synovial fluid provides nutrients to cartilage; reduces friction
Joint capsule surrounds and seals joint cavity, limits range of motion, promotes stability
Tendons attach muscles (triceps and biceps) to bones
Ligaments connect radius, ulna and humerus (bone to bone)
Biceps are muscles that contract to provide flexion (bending) of the arm
Triceps are muscles that contract to provide extension (straightening) of the arm (note that biceps and triceps are antagonistic)
Humerus is upper arm bone that provides leverage and attachment for upper portions of muscles of the elbow
Radius (forearm) is lower arm bone (smaller) that acts as a lever for the biceps (biceps attach to it)
Ulna (forearm) is lower arm bone (larger) that acts as a lever for the triceps (triceps attach to it)

• FLEXION decreases the angle between connecting bones (bending the joint)
• EXTENSION increases the angle between connecting bones (straightening a joint)

Question 2

Outline the structure of skeletal muscle.

Answer 2

LOOK AT NOTES - Muscles and Movement slide 11: https://docs.google.com/presentation/d/1zSmgUCOdpqvUUWLLZMuVysbrHRaw2usPf0H6gPR9FOs/edit#slide=id.p14

Understand that skeletal muscle fibres are multinucleate and contain specialized endoplasmic reticulum

Each muscle cell is a LONG fiber, (muscle fibre) formed from many cells that have fused together during development; as a result, each has many nuclei.

Muscle fibers (cells) are covered by a cell membrane called the sarcolemma. This is the same as the cell membrane in other cells.
The sarcolemma has many tube-like invaginations into the muscle cell called T(transverse)-tubules
The cytoplasm of muscle cells (called the sarcoplasm) contains many organelles:
* Myofibrils – parallel filaments within muscle cells that are made up of the contractile proteins actin and myosin (responsible for the banded look of striated muscles)
- Note: myofibrils are made up of structures called sarcomeres – the functional unit of muscle cells!
* Sarcoplasmic reticulum – fluid filled membranous sacs that store calcium ions
* Numerous Mitochondria – positioned between myofibrils to provide ATP
* Glycosomes – store glycogen (broken down to provide glucose for cellular respiration in mitochondria)
* Myoglobin – oxygen-storing protein (red pigment)

Smallest to largest:
Sarcomere, myofibril, sarcolemma, muscle fiber, muscular bundle/fascicle, muscle

Question 3

Draw and label a sarcomere (and be able to identify the structures of a sarcomere in diagrams or micrographs).

Answer 3

LOOK AT NOTES - Muscles and Movement slide 12 and 13: https://docs.google.com/presentation/d/1zSmgUCOdpqvUUWLLZMuVysbrHRaw2usPf0H6gPR9FOs/edit#slide=id.p16
LOOK AT WORKSHEET - #15 from the Muscles and Movement worksheet

• Sarcomere (LABEL THE WHOLE THING, Z LINE TO Z LINE, AS A SARCOMERE… THANKS IB)
• Z lines (LABEL BOTH BECAUSE IB SUCKS)
• Myosin (thick filament)
• Actin (thin filament)
• Myosin heads
• A band, I band, H zone… these get marks if their STRUCTURAL COMPONENTS (whether they’re made up of myosin or actin or both) are described/addressed

Question 4

Explain the process of muscle contraction.

Answer 4

From the Notes:

1. Nerve impulse/ action potential arrives at the neuromuscular junction (between a motor neuron and a muscle cell) and acetylcholine (Ach) is released into the synaptic cleft
2. Ach binds to protein receptors on the sarcolemma (muscle fibre cell membrane)
3. Sodium channels open (on sarcolemma and in t-tubules) and sodium ions rush into the muscle cell, causing calcium channels (on sarcoplasmic reticulum) to open and release calcium ions
4. Calcium ions bind to troponin (on actin filaments), causing troponin to change shape, moving tropomyosin and exposing the myosin-binding sites on the actin filaments
5. ATP is hydrolyzed to ADP + Pi, providing energy for the myosin heads to bind to (form “cross bridges” with) the actin filaments
6. Myosin heads move/ change shape/ bend, which pulls the actin filaments toward the center of the sarcomere (myosin filaments and actin filaments slide past each other) - this is called the “power stroke” - and the muscle contracts (shortens the sarcomere)
7. ATP binds to myosin, releasing it from actin/ breaking “cross bridges”
   Note: myosin can then hydrolyze ATP and bind to the next myosin binding site on actin to pull it in even farther in/ shorten the sarcomere even more
   Note: actin and myosin fibers do NOT change their length during muscle contraction – they merely slide past each other to bring the Z lines of the sarcomere closer together.

• This cycle (myosin heads attaching to actin, power stroke, and detachment) continues as long as ATP and calcium levels remain high in the sarcoplasm.
  Note: each “cycle” shortens the sarcomere 1%, and hundreds of these cycles occur each second during muscle contraction!

From a Worksheet Markscheme:
a. myofibrils «in muscle fibers/cells»
b. sarcomeres «are the repeating units in muscle/myofibrils»
c. sarcomeres arranged end to end / sarcomeres shorten during muscle contraction
d. actin and myosin/overlapping protein filaments/ diagram to show sarcomere with actin and myosin overlapping
e. dark and light bands in sarcomeres»/ diagram to show this/light bands narrower when muscle is contracted
f. thick filament is myosin and thin filament is actin/diagram to show this
g. nerve impulses stimulate contraction/cause depolarization of sarcolemma/T-tubules/trigger release of calcium from sarcoplasmic reticulum
h. calcium ions released from sarcoplasmic reticulum
i. calcium ions bind to troponin
j. troponin causes tropomyosin to move/exposes binding sites on actin
k. ATP hydrolyzed to ADP/ used to provide energy for myosin «heads» to form cross-bridges with/bind to actin
l. myosin heads move/change shape/swivel/cock / myosin heads cause the power stroke
m. myosin filaments pull actin towards center of sarcomere/more overlap between actin and myosin/Z-lines move closer
n. ATP is used «to provide energy»/cause cross-bridges to break
o. intercostal/abdominal/diaphragm muscles contract «to cough»
Marks can be awarded for any point made clearly on an annotated diagram.

Question 5

Analyze electron micrographs of sarcomere structure to determine if muscle is relaxed or contracted.

Answer 5

LOOK AT NOTES - Muscles and Movement slide 18 AND 19: https://docs.google.com/presentation/d/1zSmgUCOdpqvUUWLLZMuVysbrHRaw2usPf0H6gPR9FOs/edit#slide=id.p21

• The length of the sarcomere shortens during muscle contraction. (Z lines get closer together)
• The length of the actin and myosin filaments does not change during muscle contraction (they simply slide past each other).