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Muscles And The Sliding Filament Model
Types of muscle
There are three different types of muscle:
- Involuntary (smooth) muscle
- Voluntary (skeletal) muscle
- Cardiac muscle
All muscles are made up of elongated cells that form fibres. Muscles are able to contract because they contain fibres that are made if myosin and actin (proteins).
Involuntary (smooth) muscle
Involuntary muscle is innervated (supplied with nerves) from the ANS (autonomic nervous system) which means that you cannot control the muscle contraction i.e it is not voluntary.
Some examples of involuntary muscle include the iris of the eye which are arranged in circular bundles and control the intensity of light that enters the eye. When the iris muscle contracts it dilates the pupils and allows more light to enter and when it relaxes it constricts the pupil and allows less light to enter.
Another example is the wall of the arteries and arterioles which are also arranged in circular bundles and are important in processes such as temperature regulation and redirecting blood to voluntary muscles during exercise. When the muscles contract the arteries/arterioles narrow and reduce blood flow and when they relax the dilate and increase blood flow.
The muscle cells in involuntary muscle are described as being 'spindle-shaped', only have one nucleus per cell and contain small bundles of the aforementioned proteins 'myosin' and 'actin'.
Voluntary (skeletal) muscle
Voluntary muscles control the movement f the skeleton at the joints (i.e. moves the limbs). Skeletal muscle is joined to bone by inelastic tendons. Muscle contraction pulls on tendons and as a result the bone moves. Voluntary muscle tends to fatigue very quickly compared to involuntary muscle but also contracts quickly and powerfully.
The involuntary muscle cells contain several nuclei form fibres and each fibre is surrounded by a cell surface membrane called the sarcolemma. The cytoplasm of the muscle cell is called the sarcoplasm and contains mitochondria, sarcoplasmic reticulum (specialised endoplasmic reticulum) and microfibrils. Microfibrils are contractile elements and each consists of a chain of smaller contractile units called sacromeres. The microfibrils contain two types of microfilaments made of protein - thin actin microfilaments and thick myosin microfilaments. These microfilaments run through the length of the cell.
There are three types of cardiac muscle; atrial, ventricular and exciatory and conductive muscle fibres and it forms the muscular part of the heart.
The atrial and ventricular muscle contracts in a very similar way to the voluntary (skeletal) musclebut contracts for a lot longer (i.e. your whole life!). The excitatory and conductive fibres conduct electrical impulses and control the heart beat.
Nerves of the autonomic nervous system carry impulses to the heart to regulate the rate of contraction. The sympathetic nervous system increases the rate and the parasympathetic system slows it down. However, some cardiac muscle fibres can stimulate contraction without nerve impulsion - this is called myogenic.
The cardiac muscle fibres are made of rows of individual connecting cells with cell membranes called intercalated disks. The intercalated disks fuse in such a way to allow ions to freely diffuse through and thus allowing quick and efficient action potentials to spread throughout the cardiac muscle. When viewed under the microscope cardiac muscle appears to be striated (i.e. striped).
To read more about the heart and it's structure, click here.
The two types of protein filament involved in the contraction of muscles are thin and thick filaments:
- Thin filaments which consist of two strands of F actin proteins wound around each other each consisting of many globular G actin subunits. This is reinforced with tropomyosin molecules which wind around the filament. A troponin complex consisting of three subunits is attached to each tropomyosin molecule. One subunit in troponin binds to calcium ions, the other to actin and the other to tropomyosin.
- Thick filaments consist of bundles of myosin. Each myosin molecule has a tail and two heads.
The power stroke - how muscle contraction works
When an action potential reaches the muscle calcium ions are released from the sarcoplasmic reticulum into the neuromuscular junction. The calcium ions bind to the troponin molecules and cause them to change shape and move the trypomyosin molecules away from the binding site for the myosin head. The myosin head will then bind to surrounding actin filaments and pull them along causing more of an overlap with the thick filament - during this process ADP and P are released. The ATP then binds to the myosin head and causes the cross bridges to break, the ATP is then hydrolysed and the myosin head moves backwards where it is able to bind with an actin filament further along.