Muscular system in vertebrates


Muscles are made up of muscular tissue. Muscular tissue is a group of muscle cells. The muscle cells are specialized to perform one unique function to generate a pulling force i.e. they shorten or contract. Muscles move the parts of skeleton. Muscle tissue has three important properties (i) excitability or irritability, the capacity to receive and respond to a stimulus, (2) Extensibility, the ability to be stretched and (3) elasticity, the ability to return to its original shape after being stretched or contracted.

Skeletal muscle or contraction striated muscle is a voluntary muscle because the nervous system consciously controls its constrictions. Skeletal muscle fibres are multinucleated and striated. Skeleton muscles attach to skeleton when skeletal muscles contract, they shorten. Thus muscles can only pull, they cannot push. Therefore, skeletal muscles work in antagonistic pairs. For example one muscle of a pair bends a joint and brings a limb close to the body. The other member of the pair straightens the joint and extends the limb away from the body.

Skeletal muscle contraction:
Electron microscopy and biochemical analysis show that in muscle fibres cell bends are due to the placement of muscle protein action and myosin with myofibrils. Myosin occurs as thick filaments and action as thin filaments. The lightest region of a myofibril contains only actin whereas the darkest region contains both actin and myosin.

Sarcomere:
The functional contractile unit of a myofibril is sarcomere each of which extends from one Z line to another z line. The actin filaments attach to the Z lines whereas myosin filaments do not.

Contraction:
When a sarcomere contracts the actin filament slide pas the myosin filaments as they approach one another. This process shortens the sarcomere. The combined decreases in length of the individual sarcomeres account for contraction of the whole muscle fibres and in turn, the whole muscle. A ratchet mechanism between two filament types produces the actual contraction. Myosin contains globular projections that attach to actin as specific active binding sites, forming attachments called cross bridges. Once cross bridges form, they exert a force on thin actin filament and cause it to move.

Control of Muscle Contraction:
When a motor nerve conducts nerve impulses to skeletal muscle fibres, the fibres are stimulated to contract via on a motor unit. A motor unit consists of one motor nerve fibre and all the muscle fibres with which it communicates. A space separates the specialized end of the motor nerve fibre from the membrane (sarcolemma) of the muscle fibre. The motor end plate is surrounding the terminal end of the nerve. This arrangement of structures is called neuromuscular junction. When nerve impulses reach the ends of the nerve branches, synaptic vesicles in the nerve ending release a chemical called acetylcholine. 

Acetylcholine diffuses across the neuromuscular junction and binds with acetyl chlorine receptors on sarcolemma. Sarcolemma is normally polarized; the outside is positive and the inside in negative. When acetylcholine binds to the receptors, ions are redistributed on both sides of the membrane and the polarity is altered. This altered polarity flows in a wave like progression into the muscle fibre by conducting paths called transverse tubules. Associated with transverse tubules is sarcoplasmic reticulum. The polarity causes the sarcoplasmic reticulum to release calcium ions (Ca2+), which diffuse into the cytoplasm. Calcium then binds with troponim that is on another protein called tropomysin. This binding exposes the myosin binding sites on the actin molecule that tropo myosin had blocked once the binding sites are open the myosin filament can form cross bridges with actin and power strokes of cross bridges result in filament sliding and muscular contraction.

During relaxation an active transport system pumps calcium back into sarcoplasmic reticulum. By controlling the nerve impulses that reach sarcoplasmic reticulum, the nervous system controls Ca2+ levels in skeletal muscle tissue, thereby extending control over contraction. Except skeletal muscles there are smooth muscles and cardiac muscles. Smooth muscles are non striated and consist of long spindle shaped uni-nucleated cells arranged in sheets that surround body’s hollow organs. Smooth muscles are involuntary i.e. their contraction is not under the control of animal, but are controlled by automatic nervous system they are present in urinary bladder blood vessels. Cardiac muscles are present in the wall of heart. They are involuntary and striated. Cardiac muscle fibres when contract, the entire chamber of heart squeezes to maintain the flow of blood.

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