Skeletal, smooth and cardiac muscle

Question 1

What are the three different types of muscle

(only slide not about skeletal muscle)

Answer 1

skeletal: for bone movement and maintaining posture
- voluntary, multinucleated, striated

cardiac: only in the heart, no fusion of cells
- involuntary, striated appearance, intercalated discs

smooth: in the wall of blood vessels and lining of hollow organs
- involuntary, not striated, single central nuclei

Question 2

What makes up muscle fiber

Answer 2

• myofibrils
• mitochondria for energy
• surrounded by sarcolemma (membrane)
• nucleus at the edge of muscle

endomesium: surrounds individual muscle fibers
perimesium: surrounds bundles of fibers
epimesium: surrounds entire muscle

Question 3

What components make up myofibril

Answer 3

• made of myofilaments (actin-thin and myosin-thick)
• actin forms the thin filament (I band)
• myosin forms the thick filament (A band - entire filament with overlap)
• contractile unit is the sarcomere (Z to Z line)

Question 4

What is actin

Answer 4

• the main component of the thin filament
• globular actin (G actin) combines to form filamentous actin (F actin) then 2 F-actin make double helical actin strands
• this double helix forms a complex with tropomyosin and troponin

Question 5

What is myosin

Answer 5

• the main component of the thin filament
• the head region known as the crossbridge contains actin binding sites
• tails of myosin always come together in the middle with heads facing out

Question 6

What is the sliding filament theory

Answer 6

actin and myosin can only bind if Ca2+ is present, Ca2+ binds to troponin which pulls tropomyosin away from myosin binding sites, this allows the myosin binding sites to be exposed

Question 7

How is the sliding filament theory carried out (crossbridge cycle)

Answer 7

• ATP binds to the site on myosin
• actin and myosin then dissociate
• ATP is hydrolysed (into ADP + Pi) and the myosin head is pulled back (in energetic state)
• myosin binds to actin and the crossbridge forms
• ADP + Pi is then released
• the myosin head moves back = POWER STROKE

this causes the thin filament to move towards the center of the sarcomere in both directions, shortening the sarcomere, when this happens in every muscle fiber it causes the entire muscle to shorten

Question 8

What is the evidence for the sliding filament theory

Answer 8

research led by Gordon et al ,1966, on isolated muscle fibers from frog skeletal muscle found the shorter the fiber the more overlap occurred, therefore more cross-bridges and tension produced, also found when decreasing thin filaments can overlap reducing tension

Question 9

What is excitation-contraction coupling

Answer 9

• An impulse is sent by the CNS down the motor neuron and crosses the neuromuscular junction into t-tubules.
• The t-tubules transmit the nerve impulse, and the depolarisation of the t-tubules triggers the release of a small amount of Ca2+ from the SR (sarcoplasmic reticulum), this triggers the release of more calcium
• The increase in Ca2+ is detected by the voltage-gated protein (DHRP)
• The DHRP then opens the ryanodine receptor (RyR) which releases enough Ca2+ to trigger muscle contraction

Question 10

What is the motor unit

Answer 10

• muscle fibers are innervated by motor neurons
• the motor neuron + the muscle fibers it innervates is a motor unit
• an action potential in a motor neuron causes muscle fiber contraction
• the ‘all or nothing’ law applies to this

Question 11

How can contraction size be increased

Answer 11

• recruiting more motor units
• increase stimulation frequency - greater force by increasing firing frequency (SUMMATION)