The different types of muscle

Macroscopic and microscopic description

Skeletal striated muscle is the muscle that attaches to the skeleton through the tendon and allows it to move in a clearly defined direction, thanks to its essential contractile function. The first observations under the light microscope, on longitudinal sections of tissue, described it as a tissue showing both transverse and longitudinal striations. Muscle fibres are multinucleated cells of polygonal shape whose nuclei lie at the periphery of the fibre, against the sarcoplasmic membrane. A skeletal muscle fibre is a true syncytium, in other words a set of protoplasms that have fused together. The skeletal striated muscle fibre therefore has the functional characteristics of a giant cell (1-5 cm long, 10-100 µm in diameter).

Metabolism and blood supply

The blood supply to skeletal striated muscle fibres is provided, as in all tissues, by an anastomotic network of capillaries that is essential for tissue oxygenation. This network forms rectangular meshes around the muscle fibres. Its density depends on the type of fibre concerned. This point will be developed in another chapter.

Innervation

Striated muscle fibres are innervated by alpha motor neurons, whose cell bodies lie in the anterior horn of the spinal cord. Motor units (made up of the neuron that innervates the fibre, its axon and the fibres it innervates) have a characteristic feature: the fibres innervated by a single axon are randomly distributed throughout the muscle.

Contraction

Skeletal striated muscle is a voluntarily contracting muscle. Each of the fibres that make it up is a structural syncytium. The intensity of the contraction of the muscle as a whole depends on the number of motor units that have been activated.

Excitation-contraction coupling

Excitation-contraction coupling is acetylcholine-dependent. Depolarisation of the membrane of the skeletal striated muscle fibre triggers the release of calcium (stored in its ionic form in the sarcoplasmic reticulum) into the sarcoplasm.

Ultrastructure

Using the electron microscope to define the ultrastructure of skeletal striated muscle has made it possible to observe in detail the various organelles that make it up, to describe the arrangement of the contractile filaments and, finally, to deduce the role of these various structures in its contraction.

Macroscopic and microscopic description

Smooth muscles are found in the wall of many organs (all blood vessels except the smallest ones, the intestines, the uterus, etc.). They form dense layers that line the inner wall of vessels and hollow organs, and they show no transverse striations. They are made up of mononucleated spindle-shaped cells of variable size (20 to 200 µm) with a centrally placed nucleus: the smooth muscle fibres. These cells are either isolated within connective tissue or grouped into a muscular coat (vessels, digestive tract) or into muscles (the arrector pili muscle). The bundles of smooth fibres in the muscular coats are generally arranged in two superimposed layers: a circular layer and a longitudinal layer. The orientation of these layers is defined by the orientation of the smooth muscle fibres relative to the axis of the organ.

Metabolism and blood supply

Smooth muscle generally relies more on anaerobic metabolism. The smooth muscle of arterial walls is thus avascular, which is not the case for the smooth muscle of the digestive tract.

Innervation

Smooth muscles are controlled by the autonomic nervous system (also called the vegetative nervous system), which is not subject to voluntary control. Within smooth muscle, the nerve fibres of the autonomic nervous system display axonal varicosities in the form of bulb-shaped swellings. These varicosities release the neurotransmitters needed to stimulate the smooth muscle fibres into a relatively wide cleft: the diffuse junctions. This stimulation thus induces contraction of the smooth muscle fibres.

Contraction

Rhythmic contraction is characteristic of single-unit smooth muscles, or visceral muscles, which are not suited to producing fine movements. The smooth fibres of these muscles are electrically coupled to one another through “gap junctions”. These visceral muscles therefore behave as syncytia, even though there are no protoplasmic bridges between the cells: this is known as a functional syncytium. They spontaneously show irregular, continuous contractions that are independent of innervation. This state is called tone. These muscles are involved in peristalsis.

Graded contraction is characteristic of multi-unit smooth muscles, which are found for example in the iris of the eye. The smooth muscle fibres that make up these muscles are independent of one another and therefore do not form a functional syncytium. Unlike visceral muscles, however, they can produce fine movements.

Ultrastructure

The cytoplasm of smooth muscle cells contains a well-defined area holding the organelles of the cell (capping the two poles of the nucleus) and another area, which occupies most of the cell and contains the myofilaments. The actin myofilaments (thin myofilaments), visible under electron microscopy, are gathered into irregular bundles oriented along the long axis of the fibre. They are associated with tropomyosin molecules and lack troponin. The thick myosin myofilaments are not visible under electron microscopy and specific labelling techniques are required to reveal them. The contractile proteins (myosin and actin myofilaments) are attached to dense bodies made of alpha-actinin, which are either scattered through the cytoplasm or apposed to the inner face of the plasma membrane. Intermediate filaments made of desmin and vimentin are also attached to these dense bodies.

Excitation-contraction coupling

The molecular events underlying the contraction of smooth muscle fibres require the presence of calcium. There is an influx of calcium in its ionic form (Ca2+), coming either from the endoplasmic reticulum or from the extracellular space via the voltage-gated and/or ligand-gated calcium channels of the caveolar domain of the plasma membrane. The caveolar domain is the part of the plasma membrane that shows small invaginations, the caveolae or plasmalemmal vesicles. The calcium that flows into the smooth muscle fibre binds to calmodulin, a calcium-binding protein. The calcium-calmodulin complex thus formed activates an enzymeProtéine capable d’activer une réaction biochimique précise., myosin light chain kinase. Using ATPmolécule représentant la principale forme d’énergie immédiatement utilisable par la cellule pour son fonctionnement., this kinase phosphorylates one of the two myosin light chains of each myosin head. This phosphorylation unmasks the actin-binding site on the head of the myosin heavy chain. The binding of actin to myosin triggers contraction of the smooth muscle fibre.

Macroscopic and microscopic description

Cardiac striated muscle is a hollow muscle made up of myocytes with involuntary, rhythmic and automatic contraction, which form a three-dimensional network within the myocardium. Cardiomyocytes are cylindrical in shape, and their ends branch to meet the adjacent cells, forming this three-dimensional network. Each of them has a central nucleus elongated along the long axis of the cell. The striations seen in the sarcoplasm of cardiomyocytes are similar to those seen in skeletal striated muscle. The ends of adjacent fibres are apposed to one another at a structure called the intercalated disc.

Metabolism and blood supply

Cardiac muscle is highly dependent on oxidative metabolism and is continuously active. The blood flow it requires in order to work is therefore substantial, which is why numerous capillaries are seen in a section of cardiac muscle.

Contraction

Cardiac muscle contracts involuntarily: the myocytes that make it up contract rhythmically and automatically, so it can be described as a functional syncytium. Cardiac muscle is not, however, a syncytium in the strict sense. A cardiac cell has a single nucleus, and the functional syncytium is achieved through the gap junctions that allow the cells to communicate with one another.

Excitation-contraction coupling

The contractile response of cardiac muscle begins shortly after depolarisation of the membrane and lasts about 1.5 times the duration of the action potential. There is an absolute refractory period during which a cardiac muscle fibre cannot be excited again; this period extends from depolarisation of the membrane to the end of repolarisation, that is, about 200 ms after depolarisation.
The role of calcium (in its ionic form) in excitation-contraction coupling is similar to that in skeletal striated muscle. However, the influx of extracellular calcium that follows activation of the dihydropyridine channels in the T system is the precursor of the molecular events that trigger contraction.

Ultrastructure

The ultrastructure of cardiac muscle displays features of its own. A region free of material and containing various cytoplasmic organelles is present around the nucleus. Mitochondria are more numerous and glycogen granules more abundant than in skeletal striated muscle fibres. The transverse tubule (T-tubule) is located at the Z line rather than at the junction between the A band and the I band, as is the case in skeletal striated muscle. At the intercalated discs, the membranes of adjacent cardiomyocytes run parallel to one another along an extensive series of folds. This structure maintains strong cell-to-cell cohesion. These intercalated discs always occur at Z lines. The influence of one contractile unit (the sarcomere) can thus be transmitted to the next unit. Along the sides of the muscle fibres, close to the intercalated discs, the membranes of adjacent fibres fuse over long distances, forming gap junctions. These junctions provide low-resistance bridges for the propagation of excitation from one fibre to another. They allow cardiac muscle to function as a syncytium.