11. Muscular System +

Question 1

Skeletal muscle consists of numerous muscle cell called muscle fiber. Distinguishing characteristics:

Answer 1

1. Sarcolemma (plasma membrane). Highly invaginated by transverse tubules (T tubules) that permeate the cell.
2. Sarcoplasm (cytoplasm). Contain calcium-storing sarcoplasmic reticulum, specizlied endoplasmic reticulum of a muscle cell.
3. Skeletal muscle cells are multinucleate. The nuclei lie along the periphery of the cell, forming swelling visible through the sarcolemma.
4. Entire volume of muscle cell is filled with numerous, long myofibrils. Myofibrils consist of two types of filaments —> 1. Thin filaments: two strands of globular protein actin arranged in a double helix, along length of helix are troponin and tropomyosin molecules that cover special binding sites on actin; 2. Thick filaments: groups of filamentous protein myosin, array of myosin filaments possesses protruding heads at numerous positions at both ends.

Question 2

Sarcomere

Answer 2

• each repeating unit within a myofibril
• from z-line to z-line
• actin attached to z-line. myosin located between actin, unattached to z-lines.

Question 3

Sliding Filament Model of Muscle Contraction

Answer 3

1. ATP binds myosin head –> ADP + Pi. Remain attached to myosin head.
2. Ca+2 binds troponin —> tropomyosin exposes myosin head attachment on actin filament
3. When attachment sites on actin are exposed –> myosin heads bind to actin to form cross bridges.
4. Attachment of myosin to actin —> release of ADP and Pi —> change in shape of myosin head —-> sliding movement pulling actin to center of sarcomere —> z-lines pulled together contracting the muscle fiber.
5. When a new ATP molecule attaches to myosin head, cross bridge between myosin head and actin breaks returning myosin head to its unattached position.

*** without addition of a new ATP, cross bridges remain attached to actin —> reason corpses are stiff (new ATP unavailable)

Question 4

Neurons form specialized synapses with muscles called…

Answer 4

Neuromuscular junctions

Question 5

Muscle contraction is STIMULATED through the following steps:

Answer 5

1. When action potential of a neuron reaches neuromuscular junction —> neuron secretes neurotransmitter acetylcholine, which diffuses across synaptic cleft
2. Receptors on sarcolemma initiate a depolarization event and action potential –> action potential travels along sarcolemma throughout the transverse system of tubules.
3. —-> sarcoplasmic reticulum releases Ca+2
4. —> myosin cross bridges form. {Ca+ from sarcoplasmic reticulum binds troponin (on actin), causing tropomyosin to expose myosin binding sites -> if ATP is available, muscle contraction begins.}

Question 6

Human and other vertebrates have three kinds of muscles:

Answer 6

1. Skeletal muscle. Attached to bones. Causes movement of body.
2. Smooth muscle. Lines walls of blood vessel and digestive tract. No striated appearance due to its arrangement of actin and mysoin. Sarcolemma doesn’t form system of T-tubules –> contraction is controlled and slow.
3. Cardiac muscle. Rhythmic contractions of heart. Striated. Differ from skeletal muscle in that it is highly branched w/ cells connected by gap junctions. In addition, cardiac muscle generates its own action potential which spreads rapidly by electrical synapses across gap junction.

Question 7

Organization of Vertebrate Skeleton:

Answer 7

• Axial skeleton: basic framework (skull, vertebral column, rib cage)
• Appendicular skeleton: bones of appendages, pectoral and pelvic girdles.
• Bone organization:
  a. sutures - immovable joints (holds bones of skull)
  b. Moveable joints - bones that move relative to each other. 1. Ligament (bone to bone) 2. Tendon (muscle to bone)
• Origin: point of attachment of muscle to stationary bone.
• Insertion: point of attachment of muscle to bone that moves
• Extension: straightening of joint
• Flexion: bending of joint

Question 8

Joint Types

Answer 8

1. Fibrous - connect bones w/o allowing movement (ex. skull, pelvis, spinous process and vertebrae)
2. Cartilaginous: bones attached by cartilage. allow little movement. ex. spine and ribs
3. Synovial - allow for much more movement. Most common type. Filled w/ synovial fluid as a lubricant. ex. carpals, wrist, elbow, humerus and ulna, shoulder and hip joints, knee joint.

Question 9

Myofibrils are filament divided into…

Answer 9

Sarcomeres

Question 10

Individual contractile units

Answer 10

Sarcomere

Question 11

Wraps several myofibrils together to form a muscle cell/muscle fiber?

Answer 11

Sarcolemma (plasma membrane)

Question 12

Present in large amount in myofibrils?

Answer 12

Mitochondria

Question 13

Sarcomere Features:

Answer 13

• composed of thin (actin) and thick (myosin) filaments
• z-line: boundary of a single sarcomere. anchor actin
• M-line: center of sarcomere
• I band: region of actin only
• H zone: region containing myosin only
• A band: overlapping of actin and myosin

** H and I reduce during contraction, while A does not

Question 14

All or nothing!

Answer 14

• Strength of contraction of single muscle fiber cannot be increased, but strength of overall contraction can be increased by recruiting more muscle fibers.

Question 15

Type of Muscle Response

Answer 15

1. Simple Twitch - response of single muscle fiber to a brief stimulus; latent, contraction, relax
   a. latent period - time btw stimulation and onset of contraction. Action potential spreads on sarcolemma and Ca+ ions released
   a. Contraction
   c. Relaxation (absolute refractory period) - unresponsive to stimulus
2. Summation and Tetanus -
   a. Summation: contractions combine and become stronger and more prolonged (repeated AP)
   b. Tetanus: continues sustained contraction; muscle can’t relax.
3. Tonus - state of partial contraction; muscle never completely relaxed.

Question 16

Skeletal Muscles

Answer 16

• Striated, multinucleated

Question 17

Cardiac Muscles

Answer 17

• Striated (sarcomeres), one or two central nuclei.
• cells separated by intercalated discs that have gap junctions to allow AP’s to chain flow via electrical synapse.
• Involuntary

Question 18

Smooth Muscles

Answer 18

• mainly Involuntary
• once central nucleus
• Lack striations (smooth)
• No sarcomere organization
• Two types of smooth muscle:
  a. single unit: aka visceral, connected by gap junction, contract as single unit (stomach uterus, urinary bladder)
  b. multiunit: each fiber directly attached to neuron, can contract independently (iris, bronchioles, etc.)
• In addition to neuronal response, can respond to: hormones, change in pH, CO2 levels, temperature.

Question 19

Myogenic

Answer 19

• capable of contracting without stimuli from nerve cells.

- Smooth and Cardiac muscle.

Question 20

Motor Unit

Answer 20

• muscle fibers of single muscle don’t all contract at once. Single neuron innervates multiple muscle fibers (collectively called motor unit). Usually: smaller motor units activated first, then larger ones as needed –> smooth increase in force. Fine movement uses smaller motor units.
• Skeletal muscle types: Type I (slow-twitch): lots of myoglobin, lots of mitochondria, aerobic endurance. Type IIA: fast-twitch, endurance but not as much as type I (anaerobic endurance), Type IIB: fast-twitch, low myoglobin, lots of glycogen, power.

Question 21

Skeletal muscle generally doesn’t undergo mitosis to create new muscle cells (1………), but will increase in size (2………)

Answer 21

1. hyperplasia

2. hypertrophy

Question 22

Movement in lower forms:

Answer 22

Unicellular locomotion:

• protozoans & primitive algae - cilia or flagella by means of power stroke and recovery stroke
• amoeba - extend pseudopodia, advancing cell membrane extends forward

Invertebrate locomotion:

• hydrostatic skeletons:
  a. flatworms: bi-layered muscles, longitudinal and circular, contract against hydrostatic skeleton. contraction causes hydrostatic skeleton to flow longitudinally, lengthening animal
  b. segmented worms (annelids): advance by action of muscles on hydrostatic skeleton. Bristles in lower part of each segment, SETAE, anchor worm in earth while muscles push ahead.