Muhammad Sohaib Shahid

(Lecturer & Course Co-ordinator BS-MIT)

University Institute of Radiological Sciences & Medical Imaging Technology

(UIRSMIT)

MUSCULAR SYSTEM

• Muscles maintain our posture, allow us to move, breath, circulate our blood and even close our eyes.

• Muscle structure

• There are three different types of muscle;

Muscle Type Description

Cardiac Muscle (striated, involuntary)

This makes up the wall of the heart.

Smooth Muscle (unstriated, involuntary)

This is contained in structures which we do not have control over such as blood vessels, stomach and intestine, urethra, uterus, internal muscles of the eye.

Skeletal Muscle (striated, voluntary)

This is the muscle attached to our skeletons and allows us to move our bodies.

• Muscle is composed of cells which can contract to cause movement.

• Cardiac muscle Cardiac muscle is the muscle found in the walls of the heart. It

contracts to force the blood around the body. Cardiac muscle contracts without stimulation, however, the strength and rate of the contraction is modified by the autonomic nervous system. Cardiac muscle does not tire. The cardiac muscle fibers are short with a single central nucleus; they are striated. The cells join directly together and are connected by connective tissue.

• Smooth muscle Smooth muscle is found in the walls of the internal organs, the

walls of blood vessels and the intrinsic (internal) muscles of the eye. Smooth muscle contracts without stimulation, however, the strength and rate of the contraction is modified by the autonomic nervous system. Smooth muscle does not tire. Smooth muscle cells are spindle shaped and contain a single nucleus; they are unstriated. They have no sheath but are connected by connective tissue.

• Skeletal muscle • Skeletal or 'voluntary' muscle is the muscle that moves our bodies and is

attached to the skeleton or connective tissue via tendons. It is under voluntary control but can tire quickly. The origin of a muscle is normally the end attached to the less movable bone. The insertion of a muscle is usually the end that is attached to the most movable bone. Between the origin and insertion of a muscle is the muscle belly. Muscles can have multiple origins, insertions and bellies.

• Skeletal muscle is able to contract, respond to stimulation from the nervous system and hormones, stretch beyond its normal resting length and recoil back to its original resting length. It is composed of long thin cylindrical cells known as muscle fibers. These cells contain multiple nuclei near the surface of the cell and two types of myofilaments; actin and myosin. The actin and myosin filaments are organized in units called sarcomeres which are joined end to end to form a myofibril. The arrangement of the myofilaments inside the myofibrils are the reason that muscle cells appear striped under magnification.

• Each muscle cell/fibre is surrounded by an external lamina called a sarcolemma. Groups of muscle fibres are surrounded by a loose connective tissue called endomysium, this contains capillaries which supply the muscle cells with blood. The cells with their surrounding endomysium are bundled together into fasciculae and surrounded by a strong connective tissue called perimysium. A muscle is made up of many fasciculae bound together by a dense connective tissue called epimysium.

• Muscle Movement • To understand how a muscle contracts you must be able to understand

the structure of a sarcomere. Sarcomeres are regular contractile units which divide up a myofibril. Sarcomeres are easily identified on micrographs as transverse lines (Z-lines) that intersect the myofibril.

• Each sarcomere is made up of two types of protein filaments, actin and myosin which overlap each other. Actin is thin and is made up of two chains of proteins which resemble two chains of pearls twisted around each other (helix). They contain binding sites for myosin and are anchored to the Z-line at the end of a sarcomere. Myosin molecules consist of a tail and a specialised binding head and a myosin fibre is made up of many of these molecules bunched together to form a thick fibre. Myosin fibers lay in the middle of the sarcomere and are connected to each other along the M-line.

• Z-line - transverse lines at the end of each sarcomere connecting the actin filaments together.

• A-band - where myosin and actin overlap. • H-zone - where only myosin is found. • M-line - found in the middle of the H-zone and is where the myosin

filaments are transversely linked together. • I-band - where only actin is found.

• Sliding filament theory

When a muscle contracts the myosin filaments bind onto the actin filaments by forming chemical bonds called cross-bridges. Once bound the myosin filaments pull the actin filaments towards the centre of the sarcomere. Because the actin filaments are attached to the Z-line, this sliding movement shortens the length of the entire sarcomere and the H-zone becomes almost non-existent. The combined shortening of the sarcomeres along a number of myofibrils causes a muscle contraction.

Clinical Considerations

• Muscular dystrophy This is a group of

inherited disorders which causes the degeneration of skeletal muscle, causing muscle weakness and wasting (atrophy). It is caused by a defect in the gene that controls the production of a protein called dystrophin which is essential for the normal functioning of muscle.