muscle

• muscles are composed of cells arranged to form fibres. these fibres can contract to become shorter which produces a force
• Contraction is achieved by the interaction between two protein filaments called actin and myosin in the muscle cells. Muscle cannot elongate without an antagonist therefore muscles are usually arranged in opposing pairs so that one can contract as the other elongates.
• What are the three types of muscle?

  Involuntary (Smooth)

  Cardiac

  voluntary (skeletal or striated)
• involuntary (smooth) muscle consists of individual cells that are tapered at both ends hence they are spindle shaped. Each cell contains a nucleus and bundles of actin and myosin.
• smooth muscle contracts slowly and regularly,. it doesnt tire quickly. it is controlled by the autonomic nervous system
• smooth muscle is found in the walls of tubular structures like the digestive system or blood vessels. the muscle is usually arranged in longitudinal and circular layers which oppose each other.
• cardiac muscle forms the muscular part of the heart. Individual cells form long fibers that branch to form cross bridges between the fibres. These cross bridges help to ensure that electrical stimulation is evenly spread over the walls of the chambers. when muscle contracts, this arrangement also ensures that the contraction is a squeezing action rather than a one-dimensional action.
• muscle cells of the heart are joined by intercalated discs. these are specialised cell surface membranes fused to produce gap junctions that allow free diffusion of ions between the cells. action potentials can pass easily and quickly along and between cardiac muscle fibres.
• skeletal muscle occurs at joints in the skeleton. contraction causes movement of the skeleton by bending or straightening the joint. the muscles are arranged in pairs that are antagonistic, when one contracts the other elongates. The muscle cell fibres of about 100 micrometers in diameter, each fibre is multinucleate and is surrounded by a membrane called sarcolemma.
• muscle cell cytoplasm is known as sarcoplasm, it is specialised to contain many mitochondria and have an extensive sarcoplasmic reticulum.
• The contents of the muscle fibers are arranged into a number of myofibrils which are the contractile elements of the muscles. These myofibrils are divided into a chain of subunits called sarcomeres- these contain the protein filaments actin and myosin.
• actin and myosin are arranged in a particular banded pattern giving the muscle a striped or striated appearance. The darker bands are called A bands and the lighter bands are called I bands.
• skeletal muscle is under voluntary control. the somatic nervous system stimulates its contractions. The junction between the nervous system and the muscle is called the neuromuscular junction, similar to a synapse.