Ross & Wilson Anatomy and Physiology in Health and Illness (190 page)

describe the nature of muscle tone and fatigue

discuss the factors that affect the performance of skeletal muscle.

Muscle cells are specialised contractile cells, also called
fibres
. Three types of muscle tissue are identified:
smooth
,
cardiac
and
skeletal
, each differing in structure, location and physiological function.

Smooth muscle

Smooth (involuntary or non-striated) muscle is not under conscious control. The cells are small, have one nucleus and are spindle shaped (see
Fig. 3.22, p. 39
). Unlike skeletal and cardiac muscle, smooth muscle cells do not have a striped appearance under the microscope. Smooth muscle forms sheets in the walls of hollow organs and tubular structures to regulate diameter and propel substances through tracts. Some smooth muscle units have the ability to initiate their own contraction independently of nerve stimulation (automaticity); cardiac muscle has this property too. Both are normally innervated by branches of the autonomic nervous system (
p. 167
). In addition some hormones and local metabolites may influence contraction; for example, adrenaline (epinephrine) from the adrenal medulla dilates the airways.

Cardiac muscle

Cardiac muscle is found exclusively in the wall of the heart (see also
p. 39
and
Fig. 5.12, p. 80
).

Skeletal muscle

This type of muscle is also called
voluntary muscle
because there is conscious control over it; these muscles are attached to bone via tendons, and are used to move the skeleton. It is also referred to as
striped
or
striated
muscle because of the characteristic banded pattern of the cells seen under the microscope.

Organisation of skeletal muscle (
Fig. 16.54
)

A muscle consists of a large number of muscle fibres. The entire muscle is covered in a connective tissue sheath called the
epimysium
. Within the muscle, the cells are collected into separate bundles called fascicles, and each
fascicle
is covered in its own connective tissue sheath called the
perimysium
. Within the fascicles are the individual muscle cells, each wrapped in a fine connective tissue layer called the
endomysium
. Each of these connective tissue layers runs the length of the muscle. They bind the fibres into a highly organised structure, and blend together at each end of the muscle to form the
tendon
, which secures the muscle to the bone. Often the tendon is rope-like, but sometimes it takes the form of a broad sheet called an
aponeurosis
. The multiple connective tissue layers throughout the muscle are important for transmitting the force of contraction from each individual muscle cell to its points of attachment to the skeleton.

Figure 16.54 
Organisation within a skeletal muscle. A.
A skeletal muscle and its connective tissue.
B.
A muscle fibre (cell).
C.
A myofibril, relaxed and contracted.

The fleshy part of the muscle is called the
belly
, and when the muscle contracts it bulges and becomes shorter.

Skeletal muscle cells (fibres)

Contraction of a whole skeletal muscle occurs because of coordinated contraction of its individual fibres.

Structure

When skeletal muscle is examined microscopically, the cells are seen to be roughly cylindrical in shape, lying parallel to one another, with a distinctive banded appearance consisting of alternate dark and light stripes (
Figs 16.54B
and
16.55
). Individual fibres may be very long, up to 35 cm in the longest muscles. Each cell has several nuclei (because the cells are so large), found just under the cell membrane (the sarcolemma). The cytoplasm of muscle cells, also called sarcoplasm, is packed with tiny filaments running longitudinally along the length of the muscle; these are the contractile filaments. There are also many mitochondria (
Fig. 16.55
), essential for producing adenosine triphosphate (ATP) from glucose and oxygen to power the contractile mechanism. Also present is a specialised oxygen-binding substance called
myoglobin
, which is similar to the haemoglobin of red blood cells and stores oxygen within the muscle. In addition, there are extensive intracellular stores of calcium, which is released into the sarcoplasm by nervous stimulation of muscle and is essential for the contractile activity of the myofilaments.

Figure 16.55 
Coloured transmission electron micrograph of part of a skeletal muscle cell, showing the characteristic banding pattern and multiple mitochondria.

Actin, myosin and sarcomeres

There are two types of contractile myofilament within the muscle fibre, called thick and thin, arranged in repeating units called sarcomeres (
Figs 16.54C
and
16.55
). The thick filaments, which are made of a protein called
myosin
, correspond to the dark bands seen under the microscope. The thin filaments are made of a protein called
actin
. Where only these are present, the bands are lighter in appearance.

Each sarcomere is bounded at each end by a dense stripe called the Z line, to which the myosin fibres are attached, and lying in the middle of the sarcomere are the actin filaments, overlapping with the myosin.

Contraction

The skeletal muscle cell contracts in response to stimulation from a nerve fibre, which supplies the muscle cell usually about halfway along its length. The name given to a synapse between a motor nerve and a skeletal muscle fibre is the
neuromuscular junction
. When the action potential spreads from the nerve along the sarcolemma, it is conducted deep into the muscle cell through a special network of channels that run through the sarcoplasm, and releases calcium from the intracellular stores. Calcium triggers the binding of myosin to the actin filament next to it, forming so-called cross-bridges. ATP then provides the energy for the two filaments to slide over each other, pulling the Z lines at each end of the sarcomere closer to one another, shortening the sarcomere (
Fig. 16.54C
). If enough fibres are stimulated to do this at the same time, the whole muscle will shorten (contract). This is called the
sliding filament theory
.

The muscle relaxes when nerve stimulation stops. Calcium is pumped back into its intracellular storage areas, which breaks the cross-bridges between the actin and myosin filaments. They then slide back into their starting positions, lengthening the sarcomeres and returning the muscle to its original length.

The neuromuscular junction