Handbook of Food Science and Technology 3

Z-line
Z-line
H-zone
Sarcomere
A-band
I-band
I-band
RELAXATION
CONTRACTION
M-line

---

76 Handbook of Food Science and Technology 3 
Myofilaments 
Each myofibril consists of several parallel myofilaments. There are two 
types: thick and thin myofilaments. In I-bands (light regions), only thin 
filaments are present (
∅
= 6 nm). The dense regions of A-bands contain: 
– thin filaments found in I-bands; 
– thick filaments (
∅
= 15–17 nm) giving A-bands their characteristic 
birefringence.
Thick and thin filaments have a hexagonal arrangement, visible in a 
cross-section of muscle (Figure 2.6). A thick filament is surrounded by six 
thin filaments.
Figure 2.6.
Cross-section of a myofibril fragment
Thick filaments extend from one end of the A-band to the other. 
However, thin filaments start at the Z-line and continue into the A-band as 
far as the edge of the H-zone.
Within the dense regions of the A-bands, there are bridges between the 
adjacent thick and thin filaments. The bridges are made of thick filaments 
and are the only existing connections between these filaments.
These various elements of the muscle cell are held in place in the 
sarcoplasm by longitudinal and vertical elements belonging to the 
sarcoplasmic reticulum and the cytoskeleton. The latter consists primarily of 
two insoluble proteins: connectin and desmin.
Actin filament (Ø = 6 mm)
Myosin filament (Ø =16 mm)
Sarcoplasm
Sarcoplasmic reticulum

---

From Muscle to Meat and Meat Products 77 
Change in the sarcomere during muscle contraction 
During contraction and extension of the muscle, the size of the sarcomere 
varies considerably. During contraction, it can decrease in length by 50% 
and increase by up to 120% during extension.
Thick and thin filaments do not change in length, but rather the 
shortening of the muscle during contraction is due to the sliding of thick and 
thin filaments over each other. There is an interpenetration and overlapping 
of thick and thin filaments (Figure 2.5).
2.1.2.2.
 Red and white muscles 
Red and white muscles are characterized by varying proportions of red 
and white fibers, which, apart from their color, can de distinguished by their 
contraction rate and metabolic type. Red fibers have the following 
characteristics: 
– slow and prolonged contraction; 
– strong vascularization and rich in myoglobin; 
– significant respiratory processes and aerobic metabolism; 
– rapid oxidation of unsaturated fats (palatable meat).
White fibers have the opposite characteristics, namely, fast and short 
contraction, low vascularization and poor in myoglobin. 
Red and white fibers coexist in all muscles, thereby giving every muscle 
a different composition and metabolism. The relative proportion of both 
fibers determines the type of muscle, which also differs in morphology from 
other muscles.
2.1.2.3.
 Connective tissue 
Connective tissue has a complex composition. It consists of: 
– protein fibers (collagen, reticulin, elastin); 
– the ground substance surrounding these fibers and cells. It plays a 
crucial role in exchanges between the blood and the muscle cell; 
– characteristic cells, including fibroblasts that develop connective fibers, 
histiocytes (or macrophages) and fat cells; 

---

78 Handbook of Food Science and Technology 3 
– blood vessels; 
– nerves – the finer the movement, the greater the number of nerves (e.g. 
hand muscles).