Muscle and movement

Muscle types

Muscle may be roughly divided into two types: striated muscle and smooth muscle. Striated muscle itself may be further subdivided into skeletal muscle and cardiac muscle.

Skeletal muscle is under voluntary control. The fine structure of skeletal muscle will be described later. Cardiac muscle, as the name suggests is muscle which makes up the heart. One of its characteristic features is the presence of junctions between individual cells called intercalated discs, sometimes called gap junctions. Intercalated discs represent regions of low electrical resistance between the individual muscle cells which constitute the heart. They are important in ensuring that action potentials are rapidly transmitted across the heart, so that all the muscle cells contract in unison and the heart beats synchronously.

Smooth muscle is so called because it lacks the striations that give striated muscle its characteristic appearance. On the basis of its neural innervation, it is possible to distinguish two different types of smooth muscle, visceral, or single-unit smooth muscle, and multi-unit smooth muscle. Single-unit smooth muscle is characterized by the appearance of gap junctions ― cellular junctions rather like the intercalated discs found in cardiac muscle ― between individual cells. These junctions allow action potentials to be transmitted between groups of individual cells very rapidly. Thus, a wave of contraction may appear to pass over such smooth muscle as an action potential is transmitted from one cell to the next. Multi-unit smooth muscle, on the other hand, requires each smooth muscle cell to be innervated in order to generate a contractile response.

In the higher vertebrates, smooth muscle surrounds internal organs, such as the gastrointestinal tract and the airways, and, generally, it is not under voluntary control. However, it is important not to make such generalizations across the animal kingdom as a whole. For example, the muscle associated with the gut in arthropods is striated in appearance, but it is not under voluntary control, unlike striated muscle in vertebrates.

Contraction of vertebrate skeletal muscle

Many of the earliest studies investigating the mechanism of muscle contraction were performed using skeletal muscle. Muscles are made up of many muscle fibers, equivalent to muscle cells. The cell membrane which surrounds the muscle cell is called the sarcolemma and it has a number of invaginations into the cell called transverse (T) tubules. The cytoplasm of muscle cells is called the sarcoplasm whilst the endoplasmic reticulum of muscle cells is called the sarcoplasmic reticulum. Muscle fibres in turn are composed of myofibrils. The myofibrils are seen to be banded (or striated, hence the name striated muscle) in appearance. Some of the bands (I bands) appear to be lighter than other bands (A bands). This is because I bands contain only thin filaments (actin), whereas A bands are regions where thin filaments overlap with thick filaments (myosin). Hence, the A regions appear darker. Each I band is split into two by the presence of the Z line, a region where the thin filaments of adjacent sarcomeres are connected by a protein called α-actinin. The sarcomere is the basic unit of contraction and extends from one Z line to the next. In turn, the myofibrils {and therefore the sarcomeres) are composed of myofilaments. There are two types of myofilaments present in striated muscle. The first is a group of thick filaments, which are composed of the protein myosin. Myosin consists of two polypeptide chains which wrap around each other to form a coil. Several of these coils group together in a bundle with their myosin heads facing outwards to form a thick filament. In addition, there is a group of thin filaments, which consist mainly of the protein actin, together with two other proteins called tropomyosin and troponin. These thin filaments are formed from two actin molecules coiled together with two tropomyosin molecules wrapped around them. Pairs of troponin molecules occur at intervals along the actin molecule. Tropomyosin and troponin are referred to as regulatory proteins. It is the overlap of the myofilaments in the myofibrils which gives skeletal muscle its characteristic banded appearance. Actin and myosin overlap in the A band. The region where there is no overlap is called the H band.

How, then, does contraction occur? The mechanism by which muscle contracts is known as the sliding filament model. With this model, the actin and myosin filaments slide past each other without physically shortening. At rest, when the muscle is relaxed, the process of contraction is prevented by the regulatory proteins troponin and tropomyosin, which prevent the formation of cross-bridges that are necessary for contraction (see later). The result of the filaments sliding past each other is that the Z lines move closer to each other and the muscle shortens.

The contraction of skeletal muscle is initiated by the release of a neurotransmitter from the nerve which innervates the muscle. In the case of vertebrates, this is the release of acetylcholine from the axon terminals of motor neurons. A single motor neuron and all the muscle fibers which it innervates is termed a motor unit. A single neuron may innervate a single muscle fiber or anything up to 100 muscle fibers. In terms of overall muscular activity, the fewer the number of muscle fibers in an individual motor unit, the more precise and fine is the movement of that particular muscle. Equally important are the total number of motor units which serve a particular muscle. In primates, for example, the muscles of the hands and fingers are composed of many motor units. Thus, very intricate movements of these appendages may be performed. This contrasts with the muscles of, say, the trunk. These are composed of fewer, larger motor units and the movements achieved by such muscles are much broader.

The interaction of the neurotransmitter with the muscle results in depolarization of the muscle cell. This initial depolarization spreads across the entire muscle cell membrane including transmission down the T tubule system deep into the myofilaments. The effect of depolarization is to release Ca2+ from the sarcoplasmic reticulum where it is normally stored at rest. The release of Ca2+ into the sarcoplasm is the trigger for contraction. Ca2+ released into the sarcoplasm binds to troponin, inducing a conform a tional change in the shape of the troponin molecule. This in turn causes a change in the orientation of the tropomyosin molecule, which moves exposing a binding site for myosin on the actin filament. The exposure of the myosin binding site on the actin filament allows the two myofilaments to bind to each other ― this is known as cross-bridge formation. The head of the myosin filament which binds to actin also possesses ATPase activity, and it converts ATP to ADP and inorganic phosphate (P;). In doing this, energy is released, which is stored in the head region. The ATPase activity described above occurs before cross-bridge formation takes place. Upon the formation of cross-bridges, the energy stored in the myosin is released. The effect of this is to pull the thin filaments towards each other, thus shortening the length of the sarcomere and causing the muscle to contract. During cross-bridge formation, the ADP and Pi ,which are bound to the myosin head region, become detached. Their removal allows the bonding of another molecule of ATP. This causes the cross-bridge binding between actin and myosin to cease. The molecule of ATP is broken down to ADP and Pi, leading to the reformation of cross-bridges which pull the thin filaments further towards each other. The cycle continues and the microfilaments are pulled past each other causing the muscle to contract. The whole process is terminated when repolarization of the muscle cell occurs. During repolarization, Ca2+ is removed from the muscle cell and is stored in the sarcoplasmic reticulum from where it was initially released. This allows the tropomyosin molecule to return to its original position on the actin molecule thereby blocking the myosin binding site. Given the role that troponin and tropomyosin play in muscle contraction, it is understood why they are called regulatory proteins.

Although the above description relates to striated muscle, much the same sort of process occurs in other types of muscle. However, there are also significant differences. For example, cardiac muscle cannot enter into the state of maintained contraction known as tetany because it has an extremely long refractory period of 250 ms. This time period is almost as long as it takes for cardiac muscle to contract and fully relax. Therefore, it is impossible for individual contractions to summate to give a sustained contraction. Remember that during the refractory period, a cell is unresponsive to further stimulation. Obviously, if this were allowed to happen, then the heart would cease to function as a pump. Another difference is that in smooth muscle, Ca2* entry from the extracellular fluid is important in initiating the contractile response, in contrast to the role of internally released Ca2+ seen in skeletal muscle.

Types of vertebrate skeletal much

It is possible to distinguish two main types of vertebrate skeletal muscle ― fast for twitch) fibers and slow (or phasic) fibers. Fast fibers represent those muscles which are utilized for rapid bursts of activity. Such fibers possess few mitochondria, little myoglobin and a poor blood supply. Consequently, much of their metabolism is anaerobic. This type of muscle is found, for example, in frogs’ legs and is used to help the animal to jump. Slow fibers, on the other hand, are rich in mitochondria and myoglobin and have a good blood supply. These muscles are used where contraction needs to be maintained for longer periods, such as the maintenance of posture in terrestrial vertebrates.