5 2 Muscle Diversity As you saw in the last lesson and your activity muscle cells display a remarkable range of diversity in their behavior and the mechanisms that underlie those behaviors They differ in their location shape the stimuli they respond to the ion channels that allow them to respond the electrical response of their membrane their speed and force of contraction and the time course of contraction among others This lesson will cover several specific examples of muscle diversity and the mechanisms that underlie their unusual functions First let s review how vertebrate skeletal muscles contract Vertebrate skeletal muscles contract in an all or none manner When the efferent motor neuron fires an action potential it releases ACh into the neuromuscular junction which is always excitatory ACh binds cholinergic receptors that cause the muscle cell membrane sarcolemma to depolarize This opens voltage gated Na channels and the myocyte fires an action potential This massive depolarization is conducted down the sarcolemma opening voltage gated Ca2 channels as it goes It is conducted down the T tubules where the sarcolemma contacts the SR membrane and opens SR Ca2 channels causing a flood of cytosolic Ca2 This massive fast increase in cytosolic Ca2 allows near maximal numbers of actin myosin crossbridges to form breaking and re breaking as the myosin heads pull the thin filaments inward The sarcomeres shorten contracting the muscle fiber until cytosolic Ca2 decreases due to Ca2 pumps and exchangers Skeletal muscles that can contract and relax quickly or twitch muscles are composed of multiple muscle fiber groups that can are innervated separately The animal is able to control how many of these motor units a motor neuron and the fibers it innervates to activate starting with the smallest units fewest fibers This process is called recruitment Recruitment of a few or many motor units allows a single skeletal muscle to produce graded contractions although contraction of each fiber is all or none In invertebrates the muscles are simpler but innervation of them is more complex In general vertebrates have highly variable muscles that are simply innervated invertebrates have fairly simple muscles with complicated innervation For example some invertebrate myofibers do not contract in an all or none manner Instead they can sum inputs like in the dendrites of a neuron When the innervating neuron fires it may cause a small depolarization in the myocyte called an EPSP just like in neurons which is coupled to a small contraction of the fiber If the innervating neuron fires multiple times the EPSPs of the sarcolemma will temporally sum resulting in a stronger contraction Unlike in vertebrates a single muscle fiber may be innervated by multiple neurons and some of them can be inhibitory The resulting EPSPs and IPSPs will spatially sum affecting the strength of contraction Many animals can produce sounds using their respiratory system as we do By controlling muscles that affect air flow animals can emit and control sounds Some animals are also able to produce sounds using sonic muscles which rely on rapid cycles of contraction and relaxation This is accomplished with large intracellular Ca2 stores in the SR that are rapidly released and withdrawn As fast as these muscles move the flight muscles in insects move 10x faster Some insects can beat their wings over 100 times a second This allows these insects to perform difficult flying maneuvers such as hovering and moving backwards The speed with which insect flight muscles contract and relax outpaces what Ca2 flow within the cell is capable of If we stimulate the insect flight muscles to contract we see the myocyte fire an action potential and the muscles rapidly contract and relax Because there is not a 1 1 correspondence between electrical activity and contraction this is called asynchronous flight muscle If we measure cytosolic Ca2 we see it go up and stay up How do the muscles relax and contract again etc when Ca2 is so high inside the cell The answer is similar to what we have seen in other rhythmic motor outputs feedback loops of stretching and flexing When an asynchronous flight muscle is stretched it responds with a delay and then activation contraction When it is contracted it responds with a delay and then deactivation relaxation The molecular mechanism of this cycle while cytosolic Ca2 remains high is unclear but it seems to involve multiple isoforms of troponin and tropomyosin Experiments looking at the shape and sequence of these proteins in asynchronous flight muscle suggest that they might have regions that are activated when the cross bridges they are associated with are stretched resulting in an oscillation of contraction and relaxation I have attached a review here https utexas instructure com courses 1099761 files 34789994 download verifier YBmsTk5WCZCOWay5eFHseRkJMM8Xwm0SH0DnPegG wrap 1 https utexas instructure com courses 1099761 files 34789994 download verifier YBmsTk5WCZCOWay5eFHseRkJMM8Xwm0SH0DnPegG wrap 1 https utexas instructure com courses 1099761 files 34789994 download verifier YBmsTk5WCZCOWay5eFHseRkJMM8Xwm0SH0DnPegG wrap 1 if you are interested in learning more There is a trade off because the motor output is largely independent of the neural input there is less control over the muscle movements When cells differentiate during embryogenesis some muscle cells undergo a process called trans differentiation where the embryonic myocyte turns into a modified non contractile tissue Billfish are a group of predatory marine fish To successfully pursue their prey in the ocean they must be able to maintain high visual acuity regardless of temperature even when they dive deep into colder water During development some of the embryonic eye muscle cells in the billfish trans differentiate to form a heater organ This is an organ located close to the eye that warms it allowing the billfish to maintain visual function even when pursuing prey in deep cold water These modified myocytes contain few myofibrils but they are abundant in SR and mitochondria What do muscle cells have to do with heat generation You know from your own experience that when your skeletal muscles are working hard your body gets hotter Recall that to continue to contract and relax your muscle cells need lots of ATP Muscle cells use ATP for the power stroke in actin myosin cross bridge cycling as well as during relaxation to pump Ca2 back out of the cytosol This ATP
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