Muscle

Skeletal Muscle

Skeletal muscle is the main muscle type in our body and makes up approximately 40% of our total body weight .

Structure

Multinucleated skeletal muscle cells

A skeletal muscle consists of muscle fibres. One muscle fibre is about 100 µm in diameter and consists of several nuclei and many mitochondria. Each muscle fibre contains myofibrils. These are approximately 1 µm in diameter. Each muscle fibre is one muscle cell.

The myofibril is organised in repeating units called sarcomeres. These contain thick and thin filaments; these may be viewed under a microscope, and for this reason they are also known as striated muscle cells. The thick and thin filaments are made up of two different contractile proteins called actin and myosin. The actin filaments are the thin and flexible filaments and the myosin filaments are the thick filaments. Thick filaments consist of the protein myosin II which forms a globular head and fibrous tail. The thin filaments are formed from G-actin monomers which polyermise to form F-actin .  Each G-actin has a myosin-heading binding site which is blocked during muscle relaxation by the protein tropomyosin. Tropomyosin winds around the F-actin in association with troponin. Troponin consists of 3 subunits; I, T and C. The I and T subunits bind to the tropomyosin blocking the myosin-head binding sites by holding the tropomyosin in position. The C subunit binds to calcium ions after their release from the sarcoplasmic reticulum during muscle stimulation. Muscle contraction occurs when the thin filaments slide along the thick filament by hydrolysing ATP  by what is known as the Sliding Filament Theory. The myofibrils also contain the elastic proteins Titin and Nebulin which help the actin fibres retun to their resting position in relaxation and keep the contractile proteins aligned.

Contraction

Contraction in a muscle cell is propagated by an action potential travelling along a motor neurone and arriving at a Synapse; it is mediated by sodium ions. The voltage gradient causes voltage-gated calcium ion channels in the presynaptic neurone to open, triggering vesicles containing neurotransmitters, specifically acetylcholine, to travel towards the sarcolemma; fusing with the membrane and releasing acetylcholine into the synaptic cleft . They diffuse across the cleft where they bind to specific receptors called nicotinic cholinergic receptors on the sarcolemma, where the depolarisation travels along the membrane and deep into the cell via T-tubules . Therefore it allows the terminal cisternae of the sarcoplasmic reticulum to become depolarised, releasing calcium ions.The calcuim ions bind to troponin on the actin filaments, the complex the moves tropomyosin therefore un blocking the myosin binding site, muscle contraction can then take place by the sliding filament theory . 

Sliding filament contraction of skeletal muscle

Skeletal muscles are able to undergo muscle hypertrophy during increased physical exercise, e.g. in athletes. As well as this, muscles are also able to undergo atrophy when the muscles are underused, such as in someone who is immobilized by paralysis or limb injury.

Types

There are three main types of skeletal muscle fibres; Type I, Type IIA and Type IIB. Our muscles have motor units of each of these types but some are found more in particular areas of our body than others. 

Type I are also known as slow oxidative fibres. Their contraction time is slow and they are highly resistant to fatigue. They can generate ATP by aerobic respiration and fat is their main energy source. They are red in colour due to their high myoglobin levels. Type I muscles are mainly found in bodies of marathon runners . In an average person's body, these muscle fibers are found in postural muscles (those that maintain posture), such as the neck . 

Type IIA muscle fibres are also known as fast oxidative fibres. Their contraction time is fast and they are quite resistant to fatigue. ATP is generated by aerobic respiration and both fats and glucose are used as their energy source. These fibres are also red due to the high concentrations of myoglobin. Such muscle fibres are not very prominent in humans, but do usually exist in bodies of athletes who do sports requiring a lot of stamina such as long distance running . 

Type IIB muscle fibres are also known as fast glycolytic fibres. They contract very fast and are highly irresistant to fatigue (so can only be used briefly). They obtain ATP from anaerobic respiration in which glucose is converted to lactic acid. Since they have minimal myoglobin, they are white in colour. Such types of muscle fibers are most useful for sprinters, as it requires a short burst of energy . They are usually found in our arms .

Cardiac Muscle

Cardiac Muscle is composed of smaller interconnection cells with single nucleus per cell instead of long multinucleate cells in skeletal muscle. Interconnection which appears as dark lines under microscope is known as intercalated discs. These interconnections make the cardiac muscle cells to form single functioning unit called myocardium. Some cardiac muscle cells generate electric impulses which spread across the gap junctions from cell to cell by itself this enables cell contractions in the myocardium.

Cardiac muscle fibers are electrically coupled to each other and consequently excitation of one cardiac muscle fiber triggers a series of action potentials throughout all of the muscle fibers in the cardiac muscle, hence allowing cardiac muscle to contract as one entity, much like single-unit smooth muscle cells. The strength of the cardiac muscle is further enhanced by the fact that action potentials are maintained in cardiac muscles cells considerably longer than in skeletal muscle fibers; cardiac muscle cells remain depolarized for several hunder milliseconds whilst a nerve or skeletal muscle fiber is depolarizaed for several milliseconds. The significantly longer depolarization span in cardiac muscles induces a longer refractory period which inhibits "circus movements" of constant re-excitation around the wall of the heart. 

Cardiac muscles are able to undergo muscle hypertrophy both as a result of increased physiological demand and as a result of some disease processes.

Smooth Muscle

Smooth muscle tissue is classed as non-striated due it's appearance and cells are located mainly in the walls of hollow organs such as the urinary, reproductive, intestinal and respiratory tracts of both females and males. contraction is much slower and can resist fatigue for much longer than other types of muscle. This is due to the lower rate of oxygen and energy consumption.They also contribute to other major functions such as peristalsis and vasoconstriction. Due to the smooth muscle cell having many different functions the cells are organised into two groups. These are catagorized as:  multi-unit smooth muscles or single-unit smooth muscles. The majority of smooth muscle is of the single-unit type for simultaneous contraction within organs.  

Single unit smooth muscle cells are electrically coupled, so that the action potential can pass from one cell to the adjacent cells via gap junctions. This results in a wave of contraction as can be evidenced in peristalsis. The fibres are therefore all stimulated at the same time so force of contraction is controlled by the calcium ion concentration, the higher the concentration the more force is generated. Some smooth muscles cells have pacemaker activity and can depolarise without external stimuli and these are the sort of cells that may initiate the wave of contraction.

Multi unit smooth cells, however, are not electrically coupled and hence the cells must be stimulated seperatley. Each cell is therefore situated close to an axon terminal or variscosity where it can easily make contact with a neurotransmitter, This structure allows specific selection of cells and therefore a greater control of the contractions.As the cells are not electrically coupled the force of contraction can be controlled by the number of contractile muscle fibres. Multiunit smooth cells can be found in the Iris of the eye and the Vas deferens in the male genital tract .

smooth muscle cell structure and its contraction

Unlike skeletal muscles they are 2 to 10 µm and have only one nuclei. They contain similar components to both cardiac and skeletal muscle cells; myosin, actin and tropomyosin but they do not have troponin. Instead, the myosin-head binding sites on the actin filaments are blocked by the protein calmodulin. When calcium ions are released from the extracellular fluid, 4 calcium ions bind to the protein calmodulin. This activates an enzyme - myosin light chain kinase - which phosphorylates the regulatory light chain myosin-heads. This activates myosin ATPase activity enabling cross-bridge formation and consequently muscular contraction. The non-striated cells contain more actin than myosin in the fibre composition. Therefore, there is a larger proportion of thin filaments than thick filaments in smooth muscles than striated muscles. The actin and myosin are arranged in a diagonal web like structure around the call and are attatched to the cell membrane via dense bodies and protein attatchment plaques. The contractile fobres are in les organised bundles rather than the sacromeres observer in skeletal muscles. The resulting contraction moves the cell in varios directions.Smoth muscle is often layered in many different directions.

The mode of control is mostly governed by the autonomic nervous system, meaning it is an involuntary control. Whereas, the skeletal muscles are innervated by the somatic nervous system control. The neuron can make contact with the smooth muscle cell at many points on the cell. This forms a swelling called a varicosity which contains the components for vesicular neurotransmitter release. The multiunit smooth muscle's cells each receive a nervous input and act independently to each other. The single unit muscle cells recieve a nervous input together and due to the many gap junctions electrical communication and take place. This allows the cells to act in unison.  Smooth muscle also respond to hormones and paracrines and consequently has to modulate multiple signals simultaneously, this results in differing electical behaviors. The variety of responce from these stimuli makes the muscle hard to work with.

Smooth muscles are able to undergo both muscle hypertrophy as well as muscle hyperplasia in response to increasing demands of heavier workloads. Hyperplasia is usually the major response. Muscle atrophy also occur in smooth muscles, as in the uterine smooth muscle after menopause, indicating that the status of uterine smooth muscle is under hormonal control .