The Muscular System

Question 1

What are some features of skeletal muscle?

Answer 1

• voluntary movement so somatic control
• actin and myosin arranged in repeating sarcomere units
• appears striated
• multi-nucleated - forms by fusing of individual muscle cells

Question 2

What are the 2 different types of skeletal muscle fibers, and where are they each most dominant?

Answer 2

1. Red / slow-twitch fibers: high myoglobin (Fe binding O2) content; derive energy aerobically; many mitochondria
2. White / fast-twitch fibers: much less myoglobin; lighter color
   - fiber types can be mixed in a muscle
   - muscles that contract slowly but sustain activity contain mostly red
   - muscles that contract rapidly but fatigue quickly contain mostly white

Question 3

What are some features of smooth muscle?

Answer 3

• involuntary, autonomic control; single nucleus, no striations though they have actin/myosin
• found in many organs and walls
• capable of more sustained contractions (tonus: constant state of low-level contraction like in vessels)
• myogenic activity: can contract without nervous system input (in response to stretch, etc)

Question 4

What are some features of cardiac muscle?

Answer 4

• mostly uninucleated (sometimes bi); striated
• involuntary, autonomic control: nervous and endocrine systems
• vagus nerve - parasympathetic signals; (nor)epinephrine from adrenal medulla
• cells connected by intercalated discs that contain gap junctions (connections between cytoplasms, allowing for ion flow + rapid and coordinated depolarization and contraction)
• myogenic activity: SA node to AV node to bundle of His to bundle branches to Purkinje fibers

Question 5

What do all muscle cells rely on for contraction?

Answer 5

Ca2+ ions

Question 6

How are proteins arranged in a sarcomere?

Answer 6

• thick filaments: organized bundles of myosin
• thin filaments: actin, troponin, tropomyosin (help to regulate interaction between actin/myosin)
• titin protein anchors actin/myosin together to prevent excessive stretching

Question 7

What the the different lines, zones, and bands of the sarcomere?

Answer 7

1. Z-lines: define boundaries of each sarcomere
2. M-line: runs down center of sarcomere, thru the middle of the myosin filaments
3. I-band: region containing only thin filaments
4. H-zone: only thick filaments
5. A-band: contains thick filaments in their entirety, including overlap with thin

Question 8

Which distances in the sarcromere change during contraction?

Answer 8

• H-zone, I-band, distance between Z-lines, and distance between M-lines, all become smaller
• A-band’s size remains constant

Question 9

What is the structure of a skeletal myocyte?

Answer 9

• sarcomeres attached end-to-end to form myofibrils
• sarcoplasmic reticulum (modified ER with high conc of Ca2+) covers myofibrils
• sarcoplasm (modified cytoplasm) right outside SR
• sarcolemma: cell membrane - can propagate an AP thru a system of T-tubules
• T-tubules: orientated perpendicular to myofibrils
• each myocyte has many parallel myofibrils and can be called a muscle fiber
• has nuclei at the periphery
• many myocytes in parallel is a muscle

Question 10

How is muscle contraction initiated?

Answer 10

1. starts at neuromuscular junction, where nervous system communicates with muscles using motor (efferent) neurons
   - each nerve terminal controls a group of myocytes; myocytes + terminal = motor unit
2. neuron releases ACh at nerve terminal aka motor end plate
3. ACh binds to sarcolemma receptors and causes depolarization
4. Depolarization triggers AP, which spreads down sarcolemma to T-tubules, into muscle tissues to SR
5. At SR, Ca2+ is released
6. Ca2+ ions bind to troponin, triggering a conformational change in tropomyosin, exposing the myosin-binding sites on actin

Question 11

What occurs during the sarcomere shortening phase of muscle contraction?

Answer 11

1. Myosin heads carrying ADP + P bind to exposed sites on actin, forming cross bridge
2. ADP+P dissociate from myosin and a powerstroke occurs, drawing actin toward M-line and shortening the sarcomere
3. ATP binds to myosin, releasing it from actin
4. This ATP is hydrolyzed to ADP+P to recock the myosin head so it’s ready to start another crossbridge

Question 12

What occurs during the relaxation phase of muscle contraction?

Answer 12

• ACh is degraded in synapse by acetylcholinesterase, resulting in termination of signal at NMJ and allowing sarcolemma to repolarize
• Ca2+ release stops and SR takes up Ca from the sarcoplasm
  3. ATP binds to myosin to disconnect it from actin, and tropomyosin covers binding sites again

Question 13

How do nerves control force of contraction?

Answer 13

• muscle cells are all-or-nothing with their responses; must be over a threshold
• nerves control force by # of motor units they recruit to respond
• maximal response when all fibers in a muscle are stimulated

Question 14

What is a simple twitch? What are its stages?

Answer 14

• response of a single muscle fiber to a brief stimulus at or above threshold
  1. latent period: time between reaching threshold and onset of contraction; AP spreads along muscle and Ca2+ releases from SR
  2. contraction
  3. relaxation

Question 15

What is summation and tetanus?

Answer 15

• frequency summation: if there is insufficient time to relax, contractions will combine, becoming stronger and more prolonged
• tetanus: if contractions become so frequent that the muscle is unable to relax at all; leads to muscle fatigue

Question 16

What is creatine phosphate, and what is its purpose?

Answer 16

• supplemental energy reserve
• created by transferring a P from ATP to creatine during rest
• during muscle use, body can quickly generate ATP by reversing this
• creatine + ATP –> creatine P + ADP

Question 17

What do muscles rely on when in oxygen debt?

Answer 17

• creatine phosphate, myoglobin reserves
• glycogen and fermentation by fast twitch fibers
• heart rate and respiratory rate increase (oxyhemoglobin dissoc curve shifts right, increased CO2 conc, increased H+ conc, increased temp)
• even red fibers switch to anaerobic + make lactic acid

Question 18

What occurs with lactic acid once strenuous exercise is over?

Answer 18

• lactic acid must be metabolized
• converted back to pyruvate, which can enter citric acid cycle
• process requires oxygen, and the amount of oxygen required = oxygen debt

Question 19

What are exoskeletons vs. endoskeletons?

Answer 19

• exoskeleton: encases whole organisms, usually in arthropods
• endoskeletons: internal, but don’t protect soft tissue structures, but can accommodate growth better

Question 20

What is the axial vs. appendicular skeleton?

Answer 20

• axial skeleton: skull, vertebral column, ribcage, hyoid bone
• appendicular: bones of limbs, pectoral girdle (scapula and clavicle), pelvis

Question 21

What are the two types of bone, and what germ layer does bone come from?

Answer 21

• derived from mesoderm
  1. compact bone: dense and strong
  2. spongy/cancellous bone: lattice structure with trabeculae
• cavities between trabeculae are filled with bone marrow
• red bone marrow: hematopoietic stem cells
• yellow bone marrow: fat

Question 22

Describe long bones.

Answer 22

• found in appendicular skeleton
• cylindrical shafts called diaphyses that swell at each end to form metaphyses and terminate in epiphyses
• internal spongy core with outer compact bone
• diaphyses and metaphyses full of bone marrow
• epiphyseal plate - cartilage site of longitudinal growth; filled with mitotic cells before adulthood, these plates close during puberty
• periosteum - fibrous sheath encloses long bone; some cells can become bone-forming cells for growth and repair; serves as attachment point for muscle

Question 23

What are tendons and ligaments?

Answer 23

• tendons attach muscle to bone

- ligaments hold bones together at joints

Question 24

What about compact bone’s structure makes it so strong?

Answer 24

• bone matrix
• organic components: collagen, glycoproteins, peptides
• inorganic: calcium, phosphate, hydroxide ions, which harden together to make hydroxyapatite crystals, plus minerals

Question 25

How is bone matrix organized?

Answer 25

• ordered into structural units called osteons or Haversian systems
• each osteon is concentric circles of matrix called lamellae surrounding central channel
• longitudinal channels are Haversian canals, transverse channels are Volkmann’s canals; contain blood vessels, nerves, lymph vessels
• between rings are lacunae, small spaces that house mature bone cells called osteocytes
• lacunae connected by tiny channels called canaliculi that allow for nutrient/waste exchange with canals

Question 26

How is bone remodeled?

Answer 26

• osteoblasts build bone, osteoclasts (polynucleated resident macrophages) resorb it
• Ca and PO4 are obtained from blood during building; released back to bloodstream during resorption

Question 27

What are the influences for bone remodeling?

Answer 27

• in response to repetitive stress
• endocrine hormones: PTH promotes bone resorption; Vit.D promotes resorption for newer, stronger bone; calcitonin promotes bone formation

Question 28

What are some features of cartilage?

Answer 28

• consists of a firm but elastic matrix called chondrin secreted by chondrocytes
• avascular and not innervated

Question 29

What are the two processes for bone formation?

Answer 29

1. endochondral ossification: hardening of cartilage into bone; in long bones
2. intramembranous ossification: undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into bone; in skull

Question 30

What are the two types of joins?

Answer 30

1. immovable joints: bones that are fused together to form sutures, like in skull bones
2. Movable joints: bones shift relative to one another; hinge (like elbow/knee), ball-and-socket (like hip/shoulder)
   - strengthened by ligaments
   - consist of synovial capsule - encloses actual joint (articular) cavity; layer of soft tissue called synovium secretes synovial fluid to lubricate movements
   - articular cartilage coats articular surfaces of bones so impact is restricted to lubricated joint cartilage rather than bones

Question 31

What are origins and insertions?

Answer 31

• when muscle is connected to two bones
• origin: end of muscle with a larger attachment to bone (usually proximal)
• insertion: end with smaller attachment (usually distal)

Question 32

How do muscles work together in pairs?

Answer 32

• can be antagonistic - one relaxes while the other contracts (like biceps brachii and triceps brachii)
• can be synergistic - working together

Question 33

What are the types of muscles classified by their movements?

Answer 33

1. flexor: decreases angle across a joint (like biceps brachii)
2. extensor: increases or straightens this angle (like triceps brachii)
3. abductor: moves a part of the body away from the midline (like the deltoid)
4. adductor: moves a part of the body toward the midline (like the pectoralis major)
5. medial rotator: in limbs; rotates axis of limb toward midline (like subscapularis)
6. lateral rotator: rotates axis of limb away from the midline (like infraspinatus)