TrueFalse 1Control of muscle tension is by recruitment of mo

True/False

----------1-Control of muscle tension is by recruitment of motor neurons and the frequency of their stimulation.

_____ 2. The muscles of vertebrates for rapid ‘transient’ contractions use ‘catch’.

_____ 3.    Smooth muscles do not have Z lines per se but rather have dense bodies.

Solution

1) control of muscle tenison is by recruitment of motor neurons and the frequency of their stimulation

this one is true

every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. 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.

A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual NMJs. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc.

A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor 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.

There is a wide range of motor units within many skeletal muscles, which gives the nervous system a wide range of control over the muscle. The small motor units in the muscle will have smaller, lower-threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units, results in a relatively small degree of contractile strength (tension) generated in the muscle. As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile 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.

2) the muscles of vertebrates for rapid transient contractions use catch - true

3) smooth muscles do not have Z lines but rather have dense bodies - true

Smooth Muscle. The thick and thin filaments are attached to alpha-actinin in dense bodies (equivalent to Z-lines in skeletal muscle), which are attached to the plasma membrane by intermediate filaments. The thin filaments do not have troponin. This type of muscle responds to a increase in calcium, following nerve stimulation through a protein called calmodulin. Binding of calcium to calmodulin, results in the activation of an enzyme (myosin light chain kinase) that phosphorylates myosin, which activates it, enabling it to interact with actin.