Muscle

Question 1

What is a muscle?

Answer 1

Tissue specialized to convert biochemical reactions into mechanical work

Question 2

Muscle Function

Answer 2

• Two main functions are to generate:
  i. Motion
  ii. force
• Can only contract
• Cannot expand → except when physically pulled by antagonistic muscle groups
• Also generate heat & contribute to body temperature homeostasis

Question 3

Skeletal muscles

Answer 3

• Attached to bones of the skeleton → control body movement
• Contract in response to signal from somatic motor neuron → can NOT initiate contraction on its own or influenced by hormones

Question 4

Smooth muscles

Answer 4

• Primary muscle of internal organs & tubes
• e.g. stomach, blood vessels, urinary bladder
• Influences movement of materials through the body
• No striations (red-white stripes)

Question 5

Cardiac muscles

Answer 5

• Found only in the heart → pump to move blood around the body
• Striation

Question 6

Skeletal muscle charactersistic

Answer 6

• Responsible for positioning and movement of skeleton (skeletal muscles ~40% of body weight)
• Attached to bones via tendons
• Tendons are composed of dense regular connective tissue → collagen (a protein arranged into cable-like fibers)

Question 7

Muscle structure

Answer 7

• Outer connective tissue → epimysium
• Contains bundles of muscle tissue called fascicles
• Fascicles are covered by the perimysium → a connective tissue sheath
• Nerves and blood vessels
• Muscle fibres (muscle cells) are found within each fascicle
• Covered by an innermost connective tissue sheath endomysium
• Within the muscle fibres are the functional units of skeletal muscle → myofibrils
• Contain so many myofibrils that there is little room for other organelles
• Cytosol contains many glycogen granules (energy storage) & mitochondria
  (ATP synthesis)

Question 8

Structure of muscle fibre

Answer 8

• Long, cylindrical cell
• Several hundred nuclei on the surface of the fibre
• Cell membrane is called the sarcolemma
• Majority of space is taken up by the myofibrils
• Myofibrils are contractile and elastic protein bundles
• Contain a specialized endoplasmic reticulum → sarcoplasmic reticulum (SR)
• Associated with this SR are a series of branching tubes called T-tubules → transverse tubules → lumen continuous with ECF
• Closely associated with the terminal cisternae → Sequester Ca2+
• One T-tubule with flanking terminal cisternae → Triad
• T-tubules allow for rapid action potential diffusion into the muscle fibre

Question 9

Muscle equivalent term:

Answer 9

Muscle cell → Muscle fibre
Cell membrane → sacrolemma
Cytoplasm → sacroplasm
Modified endoplasmic reticulum → sacroplasmic reticulum

Question 10

Characteristics of myofibrils

Answer 10

• Myofibrils occupy most of the space in a muscle fibre
• They are highly organized → and consist of bundles of contractile elastic proteins
  A. Contractile proteins → actin & myosin
  B. Regulatory proteins → troponin & tropomyosin
  C. Accessory proteins → nebulin (aligns thin filament) & titin (elastic protein that returns stretched muscle to relaxed state)

Question 11

Contractile proteins: Myosin

Answer 11

• A motor protein that consists of two coiled protein molecules (chains) that have two important parts → head & tail region
• These two regions are joined by a flexible hinge
• About 250 myosin molecules join → thick filament
• Arranged so that the heads are at the ends and the tails are together

Question 12

Contractile proteins: Actin

Answer 12

• Subunits G-actin (globular actin)
• The G-actin subunits polymerize to form chain (F-actin) → filamentous
• Two F-actin chains twist together - this forms the basis of the thin filament
• The coiled F-actin associates with regulatory proteins → troponin an tropomyosin
• These proteins regulate muscle contraction
• Forms the completed thin filament

Question 13

The sacromere

Answer 13

• When observed under the light microscope, the myofibrils have stripes called striations
  (skeletal muscle = striated muscle)
• One repeated pattern of this unit forms a sarcomere
• A sarcomere is made up of organized thick and thin filaments that results in the striations seen in skeletal muscle

Question 14

Z line (disks)

Answer 14

• Z-line (disks) → this is the site of attachment for thin filaments
• One sarcomere is made of 2 Z discs & the filaments between them

Question 15

I band

Answer 15

• I band (isotropic) → this is a region containing only thin filaments
• A Z disc runs through the middle of an I band → thus each ½ of the I band is part of a different sarcomere

Question 16

A band

Answer 16

• A band (anisotropic) → this is a region containing thick and thin filaments
• The thick and thin filaments overlap at the outer edges of the A band
• The center is occupied by thick filaments only

Question 17

H zone (part of the A band)

Answer 17

this is a region containing only thick filaments
- The central region is lighter than the outer edges

Question 18

M line

Answer 18

• M line → this is a site of attachment for the thick filaments
• The M line is the center of the sarcomere