Muscle Contraction The role of skeletal muscle is to aid in movement and maintain posture. In order to do this it must contract.  Twitch: a brief mechanical contraction of a single muscle fiber produced by a single action potential at low frequency stimulation is known as single twitch.  A twitch contraction may be --isometric or -- isotonic in nature.

Isometric contraction The muscle develops force but does not shorten The muscle (and tendon) has what are called the series elastic components (tendons, connective tissue and the sarcolemma) and the viscous components (muscle fluids). Before a muscle can shorten as a whole it must stretch these components which are relaxed with the muscle at rest. If the muscle is fixed at both ends then the sarcomeres will shorten stretching the series elastic and viscous components). The stretching of these components will generate tension within the muscle

Preload The preload is the resting force on the muscle (amount of force within the muscle prior to contracting) – Is determined by the length of the muscle – An increase in length  increase in preload

Figure 14 Increasing preload

Why is this important? As you stretch the muscle up to a point you increase the overlap between the thick and thin filaments. This means more crossbridges are able to bind to the binding site on the thin filament, increasing crossbridge cycling. An increase in crossbridge cycling, increases the force developed by the twitch.

Fig 13.

length tension relationship in skeletal muscle in vivo most skeletal muscles are found at a length just short of the optimal length (the resting length) meaning that our muscles in our body are set up to perform maximally if required.

Afterload The afterload is the force the muscle must generate above the preload in order to shorten Isotonic contraction The muscle shortens while lifting a load. The muscle initially develops force until it equals that generated by the load. Then it lifts the load by shortening

Temporal Summation If all muscles could perform were single twitches we would be in a sad state indeed. Fortunately due to the relatively short time of the muscle action potential compared to the time course of contraction you can stimulate a muscle many times producing many twitches on top of each other. This is called temporal summation

Temporal Summation

tetanus (tetanic contraction)

motor unit A whole muscle is innervated by many motorneurones. All motorneurones branch as they enter the muscle and each branch innervates a single muscle fibre All the muscle fibres and branches associated with the one motorneurone is called the motor unit

Recruitment (Spatial Summation) Muscle force is dependent upon the number of fibres contracting Increasing the number of motor units activated at any one time increases force This is called recruitment

A 21-year-old man presents to a rural emergency center with a 1-day history of progressive stiffness of the neck and jaw, difficulty swallowing, stiff shoulders and back, and a rigid abdomen. Upon further questioning, the patient reports that the stiff jaw was the first symptom, followed by the stiff neck and dysphagia. On examination he is noted to have stiffness in the neck, shoulder, and arm muscles. He has a grimace on his face that he cannot stop voluntarily and an arched back from contracted back muscles. The physician concludes that the patient has “tetanic” skeletal muscle contractions. A 3-cm laceration is noted on his left foot. The patient reports sustaining the laceration about 7 days ago while he was plowing the fields on his farm. He has not had a tetanus booster. He is diagnosed with a tetanus infection, and an injection of the tetanus antitoxin is given. ◆ On which skeletal muscle filament is troponin located? Thin filaments ◆ What is the function of the sarcoplasmic reticulum (SR)? Storage and release of calcium ◆ What is the molecular basis for initiation of contraction in skeletal muscle? Calcium-troponin-C binding

CLINICAL CORRELATION Tetanus is a neurologic disorder caused by the toxin produced in the bacterium Clostridium tetani. Clostridium tetani is an anaerobic gram-positive motile rod that is found worldwide in soil, inanimate environments, animal feces, and occasionally human feces. Contamination in wounds with spores of C. tetani is seen commonly, but germination and toxin production occur only in devitalized tissue, areas with foreign bodies, and active infection. The toxin that is released blocks the release of several inhibitory neurotransmitters, including γ-aminobutyric acid (GABA), altering the synaptic vesicle release apparatus. With diminished inhibition,the resting firing rate of motor neurons increases. Because of the increased repetitive stimulation of the motor neuron, the calcium released from the SR remains bound to troponin and extends the time for cross-bridge cycling, resulting in muscles that do not relax-tetanic contraction. Symptoms of tetanus often begin in facial muscles such as those in the jaw (“lockjaw”) and then progress down the neck, shoulder, back, and upper and lower extremities. Generalized spasms may jeopardize breathing. Antitoxin is administered to bind and neutralize circulating and unbound toxin. Wounds should be explored, cleaned, and debrided. Muscle spasm can be controlled with medications such as diazepam (GABA agonist). Protection of the airway is essential.