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Muscle tissue (textus muscularis) 99

Fibre types invested by a sheath of connective tissue, the epimysium.
VetBooks.ir Skeletal muscle cells, or muscle fibres, can be divided into Primary, secondary and tertiary fasciculi are surrounded
by connective tissue termed the perimysium. Like the epi-
three types:
· type I (slow oxidative) fibres (red muscle): mysium, the perimysium is composed of dense connective
tissue. It contains blood and lymphatic vessels as well as
− ample sarcoplasm, relatively few myofibrils, nerve fibres.
− high myoglobin (muscle pigment) content (dark The smallest functional unit of the muscle, the indi-
red in colour), vidual muscle fibre, is enclosed by a delicate network of
− abundant mitochondria with high levels of enzymes reticular fibres. Referred to as the endomysium (Figure
involved in cellular respiration (cytochrome), 4.7), this layer contains fibroblasts, histiocytes and mast
− high density of adjacent capillaries and cells as well as capillaries and nerve fibres.
− contract slowly, suited to sustained effort.
· type II (fast glycolytic) fibres (white muscle): Microvasculature
− little sarcoplasm, large numbers of myofibrils, At particular locations, blood vessels and nerves penetrate
− little myoglobin (‘white’ in colour), the connective tissue covering the muscle and ramify in
− relatively few mitochondria and the perimysium. Within the endomysium, they loop
− contract quickly, fibres fatigue rapidly. around the individual muscle fibre forming a network
· intermediate (fast oxidative glycolytic) fibres: that can adapt in width, according to the degree of muscle
− functional characteristics intermediate between contraction.
those of type I and II fibres.
Cardiac muscle (textus muscularis striatus
Satellite cells cardiacus)
Satellite cells are small cells with heterochromatic nuclei. Cardiac muscle (musculus cardiacus) is similar in struc-
They lie adjacent to skeletal muscle cells between the plas- ture to skeletal muscle (Figures 4.15 to 4.18). However, in
malemma of the muscle fibre and the basal lamina. These accordance with the functional demands on the heart, the
cells can undergo mitotic division and are responsible for structure of the muscular walls is specialised and cardiac
muscular regeneration. muscle tissue has the capacity for spontaneous genera-
Skeletal muscle is capable of only limited regeneration. tion and independent intracardiac conduction of nerve
Tissue that is damaged – for example due to injury or sur- stimuli. This, together with other morphological features,
gical intervention – is generally replaced by fibrous scar distinguishes cardiac muscle from skeletal striated muscle.
tissue. When the basal lamina surrounding the muscle cell
is preserved and the blood supply and innervation remain Structure of cardiac muscle
intact, satellite cells undergo myogenic differentiation and Cardiac muscle comprises the contractile portion of the
new muscle fibres are formed. walls of the organ, the myocardium.

Innervation FINE STRUCTURE OF CARDIAC MUSCLE (MYOCYTUS
Nerve impulses are transmitted to skeletal muscle at the STRIATUS CARDIACUS)
neuromuscular junction (motor end plate). Each skeletal In contrast to skeletal muscle cells, cardiac muscle fibres
muscle fibre is innervated by at least one axon of a motor reach lengths of only 50–100 μm (Table 4.1). They gener-
nerve arising from the spinal cord or brain stem. The ally contain a single, centrally located nucleus although
neurotransmitter (acetylcholine) released at the point of two nuclei are occasionally present. Cardiac muscle cells
contact between the nerve fibre and the muscle cell (syn- are connected end to end by tissue-specific regions known
apse) stimulates receptors on the muscle cell surface. This as intercalated discs (Figures 4.15 and 4.18). Some cells
opens Na channels in the sarcolemma, triggering an influx are connected with more than one other cell. This branch-
+
of sodium into the muscle cell. The cell membrane depo- ing configuration forms the basis of the three-dimensional
larises and the process of muscular contraction is initiated framework of the muscle of the heart.
(see Chapter 5, ‘Nervous tissue’). Skeletal muscle tissue
also contains stretch receptors associated with propriocep- SARCOPLASM
tion (see Chapter 16, ‘Receptors and sense organs’). In accordance with their high metabolic rate, cardiac
muscle cells are richly endowed with metabolically active
Connective tissue associated with skeletal organelles (Golgi apparatus, endoplasmic reticulum,
muscle ribosomes). Mitochondria are plentiful and large, reflect-
Each skeletal muscle is composed of numerous bundles of ing their degree of activity. Abundant glycogen granules
muscle fibres (fasciculi). The outer surface of the muscle is supply the cell with energy. In addition, lipid droplets and

Vet Histology.indb 99 16/07/2019 14:56