2 Characteristics of the muscle system Excitable: muscles will respond to stimuli (neural stimuli)Contractile: muscles can contract (shorten)Will do so when excitedExtensible: muscles can be stretchedElastic: muscles, when relaxed, will recoil or return to their original positionThink…rubber that will shrink on command
3 Functions of the muscle system Act to control of move the skeletonMobility, “actions”, communicationPlays a major role in body heat production 85% of your body heat is generated by skeletal muscleAll your muscle tissue amounts to the second largest user of glucose and oxygen in your bodyBlood flowMuscle contractions help to push blood to the extremities (appendages)
4 Muscle cells Also called muscle fibers or myocytes “Myo” = musclePlasma membrane is uniqueSarcolemma (“Flesh lining”)Plays a role in excitability (similar to a neuron)Cytoskeleton is a major constituent of skeletal muscle cellsActin (microfilaments) arranged into bundlesNormally “covered” by “control” proteins that limit contraction
5 Muscle cellsActin = round balls (in reality, it looks like a bunch of arrowheads stuck in a line)Tropomyosin = “block” or “plug” protein that covers the actin filamentTroponin = regulator for tropomyosin
6 Muscle cellsInterspersed between the actin bundles are thicker bundles of motor proteins = myosinLooks like a golf club or music note“head” will grab onto actin and pull itself along the actin filament
7 Muscle cellsThick (myosin) and thin (actin) filaments are arranged into a bundle = “sarcomere”
9 Muscle cellsSarcomere = the bundles of actin & myosin between “Z-discs”Z-disc is what holds the actin microfilaments togetherZ-disc is also what is attached to the sarcolemma (muscle cell plasma membrane)Z-disc attached to sarcolemma using “dystrophin” proteinThere are 1000’s of sarcomere structures along the length of a muscle cellWhy skeletal muscle also called “striated muscle”…striations = part of the sarcomere
12 Muscle cells Dystrophin = muscular dystrophy 2 kinds of muscular dystrophyDystrophy = muscular weaknessMost common forms = genetic disordersDuchenne muscular dystrophyNucleotide sequence for dystrophin often has an extra or missing nucleotide“out of reading frame”….remember the triplet codon system on DNA?Becker muscular dystrophyProtein “primary sequence” off by 1 or 2 amino acidsProtein is made with correct number of amino acids, but 1-2 have been “substituted” due to mutation of change in the DNA sequence
13 Muscle cells Muscular dystrophy Without the correct “connection” between the contracting cytoskeleton and the outer plasma membrane (sarcolemma), the cell will rip itself from the inside outCytoskeleton contracts, but isn’t attached to sarcolemmaRepeated back & forth motion within the cell leads to tears in the cell etc.
14 Muscle cellsWrapped around the “sarcomere” (actin & myosin bundles) is a specialized form of endoplasmic reticulum“sarcoplasmic reticulum”: holds calciumCalcium is the key to contractionCa2 = 2+ charge or very strong “ionic” natureDoesn’t take very much to change the electrical “gradient”When stimulated, sarcoplasmic reticulum releases calcium into the sarcomereONLY for a fraction of a second (a pulse of calcium)THEN, it quickly re-absorbs the calcium
16 Muscle cellsWhen you release calcium from the sarcoplasmic reticulum, the calcium will then bind to troponin, and move the tropomyosin OFF of the actin microfilament.This will then permit the myosin head on the THICK filament to interact with the actin microfilament.
20 Muscle contractionMuscle cells will ONLY contract when told to do so by a NEURONHave to be conscious for a muscle to contractWithout neural connection or stimulation, muscle will remain “dormant”Sever the neural connection (injury etc.), muscle will cease to “function” or contract“denervation atrophy” if you don’t use it, you’ll eventually lose it
21 Muscle contractionNeuron that controls a muscle cell = somatic motor neuron1 neuron can control a number of muscle cellsStronger muscles (gastrocnemius & soleus of the calf) = 1 neuron controls 1000’s of muscle cells“control” muscles (eye, hands) have lower ratio: 1 neuron controls 3-6 muscle cells
22 Muscle contractionWhen you want to contract a muscle, you “think” if firstConscious thought triggers somatic motor neurons in brain to send stimulatory impulses to the target muscle group (electrical impulse)At the muscle cell, the final “interface” = motor end plate
24 Muscle contractionAt the motor end plate, the neuron does not physically “touch” the muscle cellNeuron releases acetylcholine (chemical neurotransmitter…made from amino acid choline)Acetylcholine then triggers an electrical change in the sarcolemmaNeuron releases “pulses” of acetylcholineDoesn’t drop a whole ton of it at once
25 The motor neuron will release pulses of stimulatory acetylcholine The motor neuron will release pulses of stimulatory acetylcholine. Therefore, muscle cells “contract” or respond to the neural pulses by “twitching” or “transiently contracting” rather than a constant action.
26 Muscle contractionAfter twitch (if the muscle cell only receives 1 stimulatory pulse), the sarcolemma will “recoil” back to it’s original lengthRecall the elastic fibers that hold the actin microfilament bundles together on the “Z-disc”
27 Muscle contractionIf a muscle cell only “twitches” and the motor neuron only releases “pulses” of acetylcholine…how do you contract your muscle so much?Deliver more pulsesBut, neurons have only a limited number of acetylcholine pulses they can deliverTakes a great deal of energy to make and release acetylcholineIf you fire 1 neuron fast enough, it will exhaust itself (work itself to DEATH)
28 Muscle contraction Remember this slide: Each muscle in your body is made of 1000’s or more muscle cellsEach muscle is therefore “fed” by ’s of neuronsALL the neurons that feed a muscle NEVER fire at the same time (you’d break a bone if that happened)You fire “groups” of neuronsYou also fire groups of neurons in succession
29 Muscle contractionFiring groups of neurons helps to spread the metabolic load throughout a number of neurons and a number of muscle cells that make up a muscleFor a given contraction (ie. lifting a book), only about 20-70% of the muscle cells are involvedThe neurons trigger muscle cells to contract in successionLike a flip comic book…the faster you flip the pages, the more fluid the motion
30 You normally have a number of somatic motor neurons that control a number of muscle cells. These neurons trigger contractions in succession.Since all the muscle cells are connected to the same tendon, you can adjust the strength of the overall muscle contraction by adjusting how many muscle cells contract at any one time.Note that you never have EVERY muscle cell contracting at the same time.
31 Muscle contraction Spreading the “metabolic load” Each myosin head needs 1 ATP molecule in order to attach to the actin microfilament and “move” to the nextFor each sarcomere, there are 1000’s of myosin heads1000’s of ATP molecules per sarcomere…1 muscle cell can have 1000’s of sarcomeres!!!!Each neuron needs a great deal of ATP to perform the exocytosis to release acetylcholineThe sarcoplasmic reticulum needs a great deal of ATP to “vacuum” up the calcium releasedThis is why you never trigger EVERY muscle cell in a muscle to contract at once (they would all wear out at the same time)
32 There is a limit to how much activity your muscles can carry out About 6 seconds of “burst” energy is stored as ATPAbout 10 seconds worth can be “recycled” using the creatine phosphate systemMuscle cells then revert to anaerobic fermentation to produce 2 ATP’s from 1 glucose (they have about seconds worth of glucose stored for this)After this, (hopefully), your heart has started to pump more blood (with oxygen and glucose) to your muscles to permit them to undergo aerobic respiration
33 Muscle metabolism Roughly 6 seconds of “stored ATP” Once used up, have to make moreFor about 4 seconds after all the ATP is used up, your muscle cells can “recycle” ADP into ATPUses Creatine phosphate systemCreatine phosphate has PHOSPHATE that can be quickly attached to ADPOnly have about 4-5 seconds worth of creatine phosphate
34 Muscle metabolismAfter creatine phosphate system is exhausted, muscle cell relies on anaerobic fermentation (simply hydrolysing glucose into 2 pyruvate halves)Muscle cells store glucoseAbout seconds of glucose for anaerobic fermentationUnlike liver cells, muscle cells store glucose as glycogen, but DO NOT release it into the bloodIdeally, some time during anaerobic fermentation, your heart rate will increase enough to deliver oxygen to your muscle cells to permit aerobic respiration
35 Muscle “training”You can “train” your muscles to increase their strength & enduranceEndurance training = increasing the amount of glucose (glycogen) stored in each muscle cellIncreasing bloodflow to the musclesIncreasing red blood cell count (increased oxygen in blood)Strength training = increasing the creatine phosphate (ATP recycling mechanism) supplyBuilding muscle mass is largely through “micro-tears” in the muscle cells that heal
36 Muscle actions 1 muscle rarely acts alone In order to move your skeleton (“articulate” a joint), many muscles must combine their efforts
37 Types of muscle contractions Knowing that twitches are “added” or “wave summated” in order to elicit a contraction, we now want to know what kind of contractions we can elicitIsometric contraction: contraction without a change in muscle lengthWhen you hold in one place…it takes muscles to hold your book in front of you, even if none of the muscles in your arm are changing lengthIsotonic contraction: contraction resulting in a change in muscle length, but no alteration in muscle tensionWhen you increase the internal tension in the muscle to overcome the resistance
38 Types of muscle contractions Isotonic contraction:2 forms of isotonic contractionsConcentric contraction: muscle shortens, tension remains the sameWhen you lift something with your arm, you concentric contract your biceps brachiiEccentric contraction: muscle lengthens, tension remains the sameWhen you gently place something down with your arm, you eccentrically contract your biceps brachiiYou have more strength with this form of contraction
40 Muscle actions Remember! It’s not a simple “contraction-only” process You need to control that contraction by resisting with another muscle
41 Muscle pathophysiology Some key points to “ponder”Muscle cells contract in response to calciumMuscle cells release that calcium in response to neural “stimulation”
42 Rigor mortisMyosin requires ATP to release from the thin actin myofilamentIn death, muscle cells cannot regulate calcium release and re-uptakeCauses myosin to bind to the thin myofilament, but can’t let go (no more ATP production)Stiff muscles (often contracted…why “fresh” carcasses often look like they’re in “pain”)Starts 3-4 hours post-mortemPeak contractility/rigor 12 hours post-mortemDissipates hours post-mortem
43 Tetanus (lock jaw disease) The disease tetanus is actually an infection by Clostridium tetaniOften found in soil (rusty nails are usually found in soil…rust itself doesn’t harbor the bacterium)Once they enter the body (via an incision etc.), they release “tetanospasmin” toxinInhibits the relaxation of muscleNeurons inhibit muscle contraction by releasing glycine and/or gamma aminobutyric acid (GABA)Tetanospasmin inhibits release of these agents
44 Tetanus (lock jaw disease) The “lock jaw” aspect occurs when the infection spreads to the central nervous systemThe further the insertion/infection site is located from the central nervous system, the longer the “incubation” periodYou can see signs of tetanus infection prior to the “locked jaw” phaseMuscles around the infection site will tense up
46 Tetanus (lock jaw disease) Tetanus vaccine: usually combined with diptheria and pertussis vaccination in 1 shotA vaccination, especially tetanus vaccine shot, is no good for you if you have just been infectedVaccine takes at least 2 weeks to provide protection (takes that long to make antibodies)…if you’ve been infected, it’s almost too lateThe tetanus vaccine helps you make antibodies against the tetanospasmin toxin, not the Clostridium tetani bacteriaIf you show signs of tetanus infection, usually try to vaccinate AND provide anti-tetanospasmin antibody I.V. (to give you an immediate “shot” of antibodies against the toxin)
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