The Muscular System PDF

THE MUSCULAR SYSTEM Dr. Patricia De Gabriele MD MMCFD DipWH(ICGP)  Types of Muscle Skeletal Muscle Cardiac Muscle Smooth Muscle All three types have similarities: Skeletal and smooth muscle cells are elongated and are thus called muscle fibres (not cardiac muscle). The...

THE MUSCULAR SYSTEM Dr. Patricia De Gabriele MD MMCFD DipWH(ICGP)  Types of Muscle Skeletal Muscle Cardiac Muscle Smooth Muscle All three types have similarities: Skeletal and smooth muscle cells are elongated and are thus called muscle fibres (not cardiac muscle). The ability to contract depends on two types of myofilaments.  Skeletal Muscle Fibres attach to the body’s skeleton The fibres are cigar-shaped, multinucleate cells. They are the largest of the muscle fibre types. Striated muscle is voluntary. It is subject to conscious control. However, skeletal muscle is also activated by reflexes. Smooth Muscle Found mostly in walls of hollow visceral organs such as the stomach, urinary bladder and respiratory passages Smooth muscle has no striations. It is involuntary. Smooth muscle cells are spindle-shaped and have a single nucleus. They are arranged in sheaths or layers. Most often there are two such layers, one running circularly and the other longitudinally. Cardiac Muscle Found only in the heart. Cardiac muscle is striated. Involuntary Cardiac muscle fibres are branching cells joined by special junctions called intercalated discs. FUNCTIONS OF MUSCLES Producing movement Maintaining posture Stabilizing joints Generating heat – As ATP is used to power muscle contraction, energy escapes in the form of heat. This heat is vital in maintaining normal body temperature. MICROSCOPIC STRUCTURE OF SKELETAL MUSCLE Myofibrils nearly fill the cytoplasm. Light (I) & dark (A) bands The light I band has a midline interruption – a darker area called the Z disc. The dark A band has a lighter central area called the H zone. The M line in the centre of the H zone contains tiny protein rods that hold adjacent thick filaments together.  Myofilaments There are two types of myofilaments – actin and myosin filaments. – The actin filaments are thin filaments and they are anchored to the Z disc. The light I band contains only actin filaments. – The myosin filaments are thick filaments and they extend the entire length of the dark A band. Myosin heads protrude all around he ends of the thick filaments. – The thin filaments overlap the ends of he thick filaments, they do not extend into the middle of a relaxed sarcomere, and thus the central region, which lacks actin filaments is sometimes called the bare zone.  The Sliding Filament Theory Nervous system activates muscle fibres. The myosin heads attach to the binding sites on the thin filaments, and the sliding begins. Energized by ATP, each cross bridge attaches and detaches several times during a contraction. In this way, the thin filaments are pulled towards the centre of the sarcomere. In this way the muscle cell shortens.  Types of Muscle Contraction Isotonic contractions – Contractions where the muscle shortens – The myofilaments are successful in their sliding movements, the muscle shortens and movement occurs. e.g. Bending the knee, rotating the arms and smiling are all examples of isotonic contractions. Isometric contractions – Contractions in which muscles do not shorten – The myosin myofilaments are ‘skidding their wheels’ and the tension in the muscle keeps increasing. They are trying to slide but the muscle is pitted against some more or less immovable object. e.g. When you push against a wall with bent elbows, the wall doesn’t move, and the triceps muscles, which cannot shorten to straighten the elbows, are contracting isometrically. THE 5 GOLDEN RULES OF SKELETAL MUSCLE ACTIVITY 1. With a few exceptions, all muscles cross at least one joint. 2. Typically, the bulk of the muscle lies proximal to the joint crossed. 3. All muscles have at least 2 attachments – the origin and the insertion. 4. Muscles can only pull; they never push. 5. During contraction, the muscle insertion moves towards the origin. Skeletal Muscle Types of Body Movements (1) FLEXION – A movement in the sagittal plane that decreases the angle of the joint and brings two bones closer together. Flexion is typical of hinge joints and is also common at ball-and-socket joints. EXTENSION – A movement that increases the angle or the distance between two bones or parts of the body. ROTATION – A movement of a bone around its longitudinal axis. This is a common movement of ball- and-socket joints. ABDUCTION – A movement where the limb is moved away from the midline of the body ADDUCTION – The movement of a limb toward the body midline. Types of Body Movements (2) CIRCUMDUCTION – A combination of flexion, extension, abduction and adduction commonly seen in ball-and-socket joints such as the shoulder. DORSIFLEXION or PLANTAR FLEXION – Up and down movements of the foot at the ankle. INVERSION and EVERSION – To invert the foot turn the insole medially. SUPINATION and PRONATION – Movements of the radius around the ulna.