Fig Spinal cord Motor neuron cell body Motor neuron axon Nerve Muscle Muscle fibers Synaptic terminals Tendon Motor unit 1 Motor unit 2 All motor neurons leading to skeletal muscles have branching axons, each of which terminates in a ____________ ____________ with a single muscle fiber. Nerve impulses passing down a single motor neuron will thus trigger contraction in ___the muscle fibers at which the branches of that neuron terminate. __________: minimum unit of contraction

Put it all together… 1 ATP

Botox Bacteria Clostridium botulinum toxin –blocks release of _______________ –botulism can be fatal muscle

Motor Units Size of the motor unit = ______________ a single motor neuron triggers fewer than 10 fibers in the muscles controlling eye movements the motor units of the muscles controlling the larynx are as small as 2–3 fibers per motor neuron In contrast, a single motor unit for a muscle like the gastrocnemius (calf) muscle may include 1000–2000 fibers (scattered uniformly through the muscle).

NS has 2 mechanisms for controlling graded whole muscle contractions 1.________________: strength is determined by the number of motor units participating and/or by selecting large or small motor units 2. _______ _______ of muscle fiber stimulation Don’t forget a nerve impulse is ______________!!!

Do Muscles ever REST? _____________: skeletal muscles are in a state of partial contraction, The state of activity or tension of a muscle beyond that related to its physical properties, that is, its ______ ____________ to stretch. In skeletal muscle, tonus is dependent upon efferent ________. (Stedman, 25th ed) is maintained by the activation of a few motor units at all times even in __________muscle. –As one set of motor units relaxes, another set takes over.

Tension Force (tension) developed by a muscle progressively increases as more and ______________ _________________ _________________ _________________

__________ ___________ Posture muscles are always partially contracted NS may alternate activation of motor units –Reducing time any one set of fibers is contracted

Fig. 50-UN2 Sensory receptors More receptors activated Low frequency of action potentials (b) Multiple receptors activated Fewer receptors activated (a) Single sensory receptor activated High frequency of action potentials Gentle pressure More pressure Gentle pressure Sensory receptor

Varying rate of muscle fiber stimulation 1 action potential (twitch) lasts <100msec If another ap arrives before fiber is relaxed, the two twitches add together = ________ _______ Further ___________ occurs as the rate of stimulation increases When the rate is high enough and the muscle fiber cannot relax at all between stimuli the twitches _____ into one smooth sustained contraction called - ____________

_____________ _____________: Time for Ca2+ to be released from SR to bind to troponin Why do we need this step? _______________ ______________: Repetitive power strokes Generating force /tension _______________ ______________: power strokes are decreasingLevels of Ca2+ decreasing How is this happening? _________: graph of twitch contraction stimulus

Tetanus When shocks are given at 1/sec, the muscle responds with a single twitch. At 5/sec and 10/sec, the individual twitches begin to fuse together, a phenomenon called _________ At 50 shocks per second, the muscle goes into the smooth, sustained contraction of ____________ Clonus and tetanus are possible because the __________________ is much briefer than the time needed to complete a cycle of contraction and relaxation. Note that the amount of contraction is greater in clonus and tetanus than in a single twitch. As we normally use our muscles, the individual fibers go into tetanus for brief periods rather than simply undergoing single twitches.

Fig Summation of two twitches Tetanus Single twitch Time Tension Pair of action potentials Action potential Series of action potentials at high frequency

Heartbeat ANS does run to the heart = __________ increase or decrease the _________ _________ the strength of the heartbeat Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat. Gap junctions __________ = _____________ Significance: All the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart.

Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random — called ________________. Fibrillation is the __________ cause of most deaths and its reversal is the function of defibrillators that are part of the equipment in ambulances, hospital emergency rooms, and — recently — even on U.S. airlines

Does the heart go into tetanus? The refractory period in heart muscle is ___________ than the period it takes for the muscle to contract (systole) and relax (diastole). Thus ______________ is not possible (a good thing, too!). Cardiac muscle has a much richer supply of ___________ than skeletal muscle. This reflects its greater dependence on _______________ for ATP. _____________________________ Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. –Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage — even death — of the affected part. This is what happens in heart attacks.heart attacks

Isotonic versus Isometric Contractions _______________: the muscle is allowed to shorten ________________: a stimulated muscle is held so that it cannot shorten, it simply exerts tension.

Fig a Bundle of muscle fibers Muscle Single muscle fiber (cell) Nuclei Z lines Plasma membrane Myofibril Sarcomere OK now we know how to get a muscle to contract and how to get gradients of strength…. Now how do we get stronger? __________= size of contractile force (cross section of the muscle’s belly) __________ : increases in CP MB Glycogen Lung capacity RBC count

How muscles grow initially Skeletal muscle forms by _________ of mononucleated myoblasts to form ________ _____________. If it tried to divide, it would have several dozen or hundreds of spindles = _____________ Some Recent Findings Satellite cells - Day K, Shefer G, Richardson JB, Enikolopov G, Yablonka-Reuveni Z. Nestin-GFP reporter expression defines the quiescent state of skeletal muscle satellite cells. Dev Biol Dec 15;Day K, Shefer G, Richardson JB, Enikolopov G, Yablonka-Reuveni Z. "Repair of adult skeletal muscle depends on satellite cells, quiescent myogenic stem cells located beneath the myofiber basal lamina....

The number of fibers is probably fixed early in life. This is regulated by __________, a cytokine that is synthesized in muscle cells (and circulates as a hormone later in life).cytokine Myostatin ____________ skeletal muscle development.. Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene Growing illicit market in drugs supposedly able to suppress myostatin. Inactive Oncogene?

Interesting note: In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.IL-4 In adults, increased strength and muscle mass increase in the thickness of the ________ _______ increase in the amount of ____________ ________ Increase of _______ _______ = bulking up

____________ is the partial or complete wasting away of a part of the body.wastingbody Causes : poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply, disuse or lack of exercise disease intrinsic to the tissuenourishmentcirculationhormonalnerveexercise Atrophy is a general physiological process of reabsorption and breakdown of tissues, involving apoptosis on a cellular level.physiologicaltissues apoptosiscellular Webbed feet due to lack of apoptosis wikipedia

Diseases of Muscle tissue ALS –amyotrophic lateral sclerossi –Lou Gehrig’s disease –motor neurons degenerate Myasthenia gravis –auto-immune –antibodies to acetylcholine receptors Stephen Hawking