Question

Identify the components in a muscle unit and explain how they are recruited during a contraction and how muscle tension and fatigue are generated in skeletal muscle.

Answer 1

Components in a muscle unit:

* Skeletal muscle is one of the three major muscle types.

* It is a form of striated muscle tissue is in voluntary control of somatic nervous system.

* Most muscle units are attached to bones by bundles of collagen fibres known as tendons.

* skeletal muscle made up of fasciles of cells joined together known as muscle fibres.

* The fibres and muscles are surrounded by connective tissue layers called fasciae.

* Muscle fibres are formed from the fusion of developmental myoblasts know as biogenesis.

* Muscle fibres composed of myofibrils which contains actin and myosin filaments which are repeated in muscle units called sarcomas.

* Sarcomere is the basic functional unit of muscle fibre.

* In skeletal muscle enclosing each muscle is a layer of connective tissue known as epimysium and enclosing each fascile is a layer called perimysium and enclosing each muscle fibre is a layer of connective tissue called the endomysium.

Muscle units recruited during a contraction:

* Muscle unit or motor unit recruitment refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle.

* Motor units generally recruited inorder of smallest to largest (i.e. smallest motor units to largest motor neurons and thus slow to fast twitch) as contraction increases.

* when a motor or muscle unit is activated all of its fibres contract.

* The force of muscle contraction is controlled by the number of activated motor units.

* Force of muscular contraction: the number of muscles, the size or length of the muscles involved the frequency or strength of stimulus and the degree of muscle stretch.

* The smaller units are recruited first, the size principle states that as more forced is needed, motor units are recruited in a precise order according to the magnitude of their force output, while small units recruited first.

* The CNS can increase the strength of muscle contraction by as follows: Incresing the number of active motor units ( spatial recruitment), increasing the firing rate( firing frequency) at which individual motor units fire to optimize the summated tension generated ( temporal recruitment).

* Tetanus is the fusion of contractions to produce a continuous contraction.

* Increasing the number of motor neurons involved increses the amount of motor units activated in a muscle which is called recruitment.

Muscle tension and fatigue are generated in skeletal muscle:

* Intense use of muscles leads to decline in performance known as muscle fatigue.

* Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions and many intracellular metabolites.

* There are two main causes of muscle fatigue

(i) The limitations of a nerve's ability to generate a sustained signal which is neural fatigue.

(ii) Reduced ability of the muscle fibre to contract which is metabolic fatigue.

* The ideal length of a sarcomere to produce maximal tension occurs at 80% to 120% of its resting length, with 100% being the state where the medical edges of the thin filaments are just at the most medial myosin heads of the thick filaments.

* Muscle fatigue can be associated with a state of exhaustion i.e. strenuous activity or exercise.

* During muscle fatigue the force behind muscle movements decrease and feel weaker.

* This is how muscle tension and muscle fatigue are generated in skeletal muscle.