These fast fatigue-resistant (FR) motor units are of intermediate size and are not quite as fast as FF units. A third class of motor units has properties that lie between those of the other two. These units are called fast fatigable (FF) motor units and are especially important for brief exertions that require large forces, such as running or jumping. They have sparse mitochondria and are easily fatigued. Larger α motor neurons innervate larger, pale muscle fibres that generate more force. These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction eg maintenance of an upright posture. They are rich in myoglobin, mitochondria, and capillary beds, such small red fibres are resistant to fatigue. In most skeletal muscles, the small motor units innervate small “red” muscle fibres that contract slowly and generate relatively small forces. Motor units also differ in the types of muscle fibres that they innervate. The force of contraction produced by a muscle is increased in two ways: multiple motor unit summation, which involves increasing the number of muscle fibres contracting, and multiple-wave summation, which involves increasing the force of contraction of the muscle fibres. ![]() The force generated by muscles to lift a pen is much less than the force required to lift a car wheel. In contrast, the gastrocnemius, a muscle that comprises both small and larger units, has an innervation ratio of 1000–2000 muscle fibres per motor neuron, and can generate forces needed for sudden changes in body position. eg the eyes require rapid, precise movements but little strength in consequence, extraocular muscle motor units are extremely small (with an innervation ratio of only 3!) and have a very high proportion of muscle fibres capable of contracting with maximal velocity. The size of the unit can involve only a few fibres for fine movement to huge numbers for gross movement such as what occurs in walking. A motor unit consists of a single motor neuron and all of the muscle fibres it innervates. Within a muscle, the muscle fibres are functionally organised as motor units. The interaction of myosin and actin is responsible for muscle contraction. Contraction of the sarcomere occurs when the Z-lines move closer together, making the myofibrils contract, and therefore the whole muscle cell and then the entire muscle contracts. The Z-line defines the lateral boundary of each sarcomere. The characteristic 'striations' of skeletal and cardiac muscle readily observable by light microscopy are the thin filaments (light) the thick filaments (dark). Myofibrils are contractile units (within the muscle cell) that consist of an ordered arrangement of longitudinal myofilaments (thin actin filaments and thick myosin filaments). Transverse (T)-tubules invaginate the sarcolemma and form a network around the myofibrils, storing and providing the Ca2+ that is required for muscle contraction. ![]() The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. It forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell. The sarcolemma is the cell membrane of a striated muscle cell. The below video gives a good brief illustration of this ![]() Connective tissue is present in all muscles as fascia. It also protects muscles from friction against other muscles and bones. The epimysium anchors muscle tissue to tendons at each end, where the epimysium becomes thicker and collagenous. Enclosing each fascicle is a layer called the perimysium which contains many muscle fibres Enclosing each muscle fibre is a layer of connective tissue called the endomysium. Skeletal muscles are sheathed by a tough layer of connective tissue called the epimysium. Because the contracting fibres are pulling at an angle to the overall action of the muscle, the change in length is smaller, but this same orientation allows for more fibres (thus more force) in a muscle of a given size. These are known as pennate muscles having individual fibers oriented at an angle relative to the line of action. In muscles were force is more important than length change eg rectus femoris. In most muscles the fibres are oriented in the same direction, running in a line from the origin to the insertion.
0 Comments
Leave a Reply. |