Muscle

Skeletal Muscle

A skeletal muscle consists of muscle fibres. One muscle fibre is approximatels 100 µm in diameter and consists of several nuclei and many mitochondria. Each muscle fibre contains myofibrils. These are approximately 1 µm in diameter. Muscle fibres are made up of many muscle cells.

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 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.

Contraction in a muscle cell is produced by an action potential travelling along a motor neurone and arriving at a Synapse. 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 sarcoplasmic reticulum to become depolarised, releasing calcium ions and triggering muscle contraction to take place by the sliding filament theory . 

Smooth Muscle

A smooth muscle cell's location are mainly on the walls of hollow organs such as the urinary, reproductive, intestinal and respiratory tracts of both females and males. 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 Multiunit smooth muscles and Single unit smooth muscles. 

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. 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 mode of control is 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.