contractile units of the muscle

threadlike structures that create a Myofibril

• A muscle twitch is a muscle contraction in response to a single stimulus of adequate strength.

• the sarcolemma and the T tubules depolarize

• calcium ions are released into the cytosol

• cross bridges begin to cycle but there is no visible shortening of the muscle

• myosin cross bridge cycling causes sarcomeres to shorten

• calcium ions are actively transported back into the terminal cisternae

• cross bridge cycling decreases and ends

• muscle returns to its original length

• The frequency of stimulation was so slow here that relaxation was complete between contractions. Note that the curve goes down to the baseline after each contraction

• The strength of contraction did increase, because muscle contraction causes heat to build up in the muscles and muscles work better when they are warmer. Enzymes can work faster and more efficiently when a muscle is "warmed up."

an increase in the frequency with which a muscle is stimulated increases the strength of contraction. This is illustrated in (b). With rapid stimulation (so rapid that a muscle does not completely relax between successive stimulations), a muscle fiber is re-stimulated while there is still some contractile activity. As a result, there is a 'summation' of the contractile force. In addition, with rapid stimulation there isn't enough time between successive stimulations to remove all the calcium from the sarcoplasm. So, with several stimulations in rapid succession, calcium levels in the sarcoplasm increase. More calcium means more active cross-bridges and, therefore, a stronger contraction.

• We need to recruit fewer motor units to move a light object compared to a heavy object.

occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency the twitches will overlap resulting in tetanic contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal possible contraction.

• When the frequency of stimulation becomes fast enough, the contractions fuse into a smooth, continuous, total contraction with no apparent relaxation.

• This state is due to a continual depositing of calcium ions in the cytosol. As a result, the binding sites on actin continually stay exposed.

• Now the frequency of stimulation is increased to the point where the muscle exhibits even shorter contraction-relaxation cycles, but there is still some degree of relaxation after each contraction.