Question

1. Explain how force generation by human skeletal muscle can be increased to pick up increasingly heavy loads. In your answer, include the terms summation and motor unit.

Answer 1

Answer :

The skeletal muscle is one of three major muscle type, the other being cardiac muscle and smooth muscle. A skeletal muscle refers to multiple bundles of cells joined together called muscle fibers. The fibers and muscles are surrounded by connective tissue layers called fasciae. Enclosing each muscle is layer of connective tissue known as the epimysium,enclosing each fascicle is a layer called the perimysium and enclosing each uscle fibre is a layer of connective tissue called the endomysium.

To move an object,refferd to as load ,the sarcomeres in the muscle fibers of the skeletal muscle must shorten.The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. Muscle tension also is generated when the muscle is contracting against load that does not move, resulting in two main types of skeletal muscle contraction:

• Isotonic contraction : Where the tension in the muscle stays constant, a load is moved as the length of the muscle changes (shortens). There are two types of isotonic contraction concentric contraction and eccentric contraction.

Concentric contraction :

It is a type of muscle contraction in which the muscles shorten while generating force,overcoming resistance. For example, when lifting a heavy weight , a concentric contraction of the biceps would cause the arm to bend at the elbow,lifting the weight towards the shoulder.Crossbridge cycling occurs , shortening the sarcomere , muscle fiber and muscle.

Eccentric contractions :

An eccentric contractions results in the elongation of a muscle while the muscle is still generating force; in effect resistence is greater than force genarated. It can be voluntary and involuntary. For example , a voluntary eccentric contraction would be the controlled lowering of the heavy weight raised during the above concentric contraction. An involuntary eccentric contraction may occur when a weight is too great for a muscle to bear and so it is slowly lowered while under tension. Crossbridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.

• Isometric contraction : It occurs as the muscle produces tension without changing the aankle of a skeletal joint. Isometric contractions involves sarcomere shortening and increasing muscle tension, but do not move a a load, as the force produce can overcome the resistance provided by the load. For example, if one attempt to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever increasig muscle tension, but no change in the angle of the elbow joint.

Motor Unit :

Every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order tocontract. Each muscle fiber is innervated by only one motor neuron.The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit. The size of a motor unit is variable depending on the nature of the muscle.

Small Motor Unit :

A small motor unit is an arragement where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permits very fine motor control of the muscle. The best example in the humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscles, but every six or so fibers are supplied by a single motor neuron.

Large Motor Unit :

A large motor unit is an arrangement where a single motor neron supplies a large number of muscle fibers in a muscle. Largemotor units are concerned with simple or 'gross', movements, such as powerfully extending the knee joint. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle , as its axon splits into thousands of branches.

More motor units are recruited , the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractie force of 50 times more than the smallest motor units in the muscle. This allows a feather to be picked up using the biceps brachii arm muscle with minimal force, and a heavy weight to be lifted by the same muscle by recruiting the largest motor units.

Summation :

Summation occurs as successive stimuli are added together to produce a stronger muscle contraction . Tetanus is the fusion of contractions to produce a continuous contraction . Increasing the number of motor neurons involved increases the amount of motor units activated in a muscle , which is called recruitment. The increase in force that occurs with increased firing rate reflects the summation of successive muscle contractions: the muscle fibers are activated by the next action potential before they have had time to completely relax, and the forces genarated by the temporarlly overlapping contractions are summed.