Muscle fibre types
Skeletal muscle is remarkably uniform amongst mammals. All muscles contain thick and thin filaments made up essentially of myosin and actin molecules. The packing of these proteins into filaments is identical across species so that their diffraction patterns are virtually indistinguishable. Myosin filaments each contain 294 myosin molecules. The distance between the filaments is also constant, and the range of sarcomere lengths remains the same. A cross-section of a muscle sarcomere in the overlap region between thick and thin filaments is shown below:
A fragment of an electron micrograph of a transverse section of a fibre from rabbit psoas muscle in the low temperature, relaxed state. Thick and thin spots correspond to myosin and actin filaments, respectively. Examples of elementary unit cells for individual filaments are shown by the circles: a hexagonal myosin-centred super-lattice cell (on the left) and a triangular actin-centred cell (on the right). Empty circles show the software calculated positions of myosin filaments in the cells relative to their centres of mass. Black circles cover the central filaments in the cells (Bershitsky et al., Myosin Heads Contribute to the Maintenance of Filament Order in Relaxed Rabbit Muscle, Biophysical
Journal (2010), doi:10.1016/j.bpj.2010.06.072, In Press)
Other vertebrates also share the same arrangement of proteins to form muscles as different as the body-wall muscle of fish and leg muscles of frogs. Remarkably, many invertebrates also share many similarities in their muscles, including actin and myosin, although here the protein packing is slightly different, and sarcomeres can be much longer.
Control of muscle contraction and relaxation: The control of contraction is also remarkably similar in all vertebrate skeletal muscles, requiring nerve impulses to cause depolarisation of the muscle cell. Depolarisation causes the release of calcium into the cytoplasm from internal stores, the sarcoplasmic reticulum. Cytosolic calcium binds to troponin on the thin filaments. This increases the affinity of myosin cross-bridges to bind to thin filaments, hence activating actomyosin interactions and causing contraction. In the absence of nerve impulses, the membrane potential returns to its resting state with a negative membrane potential of about -80mV. Calcium pumps in the sarcoplasmic reticulum membrane return calcium to the stores, calcium occupation of troponin decreases and muscle relaxes. Activation can take different forms in invertebrates. Some insects have 'indirect' flight muscles where activation is not synchronous with electrical activity, but where the chest and wings create a resonant cavity allowing the flight muscles to contract in tune with the wing frequency.
Myosin genes - slow and fast fibres: There are six different myosin heavy chain genes in mammalian skeletal muscles which are expressed in different muscles at different times in development. The genes share 95% sequence identity across mammalian species. Some appear in the foetus or peri-natally to disappear later in development. In adults, there are two predominant myosin heavy chains (HC) that are expressed in either slow or fast fibres, respectively called type I and type II. Type II fibres express one of several fast myosin isoforms: HC IIA and HC IIX are the fastest forms. Type I muscle fibres are found in muscle used for long-lasting activity. These can be postural muscles such as the soleus, in the lower leg. Slow muscles appear red because of their high myoglobin content and contain many mitochondria. These muscles have high oxidative metabolism as they metabolise sugars and fats to maintain ATP production, and are highly vascularised to maintain adequate O2 supply and to remove CO2. Type II fibres are found in muscles used for explosive or short lasting effort and have a fast glycolytic metabolism. The thigh muscle vastus lateralis contains a high proportion of fast fibres. Fast fibres have a whiter appearance, have fewer mitochondria than slow fibres and are less vascularised than the slow muscle. The fastest muscles are the peri-ocular muscles that make the eye-ball turn. They consist of 84% fast fibres.
The muscles are plastic - they respond quickly to demands made on them by changing the myosin heavy chain expression, and by growing or shrinking. In athletes, fibre composition of particular muscles depend much on the type of sport that is being performed. The vastus lateralis of sprinters can be made up of 74% fast fibres, but only 38% in cyclists. Exercise increases gene expression within minutes, and during development and training fibres may co-express several myosins. Strength training increases expression of the fast type I fibres suited for explosive movements. Eccentric exercise enhances this development. Focal electrical and magnetic stimulation of muscle can be used effectively to alter gene expression and to achieve changes otherwise requiring voluntary exercise.
In the untrained, or following bed rest, slow type I fibres tend to disappear. Loss of type I fibres and loss of muscle mass, sarcopenia, is a problem associated with chronic diseases such as heart failure, COPD, cancer, spinal chord injury and with ageing. Loss of muscle mass results in reduced quality of life and in falls that may lead to hospitalisation. It reduces the body's capacity to take up glucose in response to insulin stimulation and reduces the body's ability to deal with the cold.