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Muscle system. Characteristics of the muscle system Excitable: muscles will respond to stimuli (neural stimuli) Contractile: muscles can contract (shorten)

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Presentation on theme: "Muscle system. Characteristics of the muscle system Excitable: muscles will respond to stimuli (neural stimuli) Contractile: muscles can contract (shorten)"— Presentation transcript:

1 Muscle system

2 Characteristics of the muscle system Excitable: muscles will respond to stimuli (neural stimuli) Contractile: muscles can contract (shorten) –Will do so when excited Extensible: muscles can be stretched Elastic: muscles, when relaxed, will recoil or return to their original position Think…rubber that will shrink on command

3 Functions of the muscle system Act to control of move the skeleton –Mobility, actions, communication Plays a major role in body heat production – 85% of your body heat is generated by skeletal muscle All your muscle tissue amounts to the second largest user of glucose and oxygen in your body Blood flow –Muscle contractions help to push blood to the extremities (appendages)

4 Muscle cells Also called muscle fibers or myocytes –Myo = muscle Plasma membrane is unique –Sarcolemma (Flesh lining) Plays a role in excitability (similar to a neuron) –Cytoskeleton is a major constituent of skeletal muscle cells Actin (microfilaments) arranged into bundles Normally covered by control proteins that limit contraction

5 Muscle cells Actin = round balls (in reality, it looks like a bunch of arrowheads stuck in a line) Tropomyosin = block or plug protein that covers the actin filament Troponin = regulator for tropomyosin

6 Muscle cells Interspersed between the actin bundles are thicker bundles of motor proteins = myosin –Looks like a golf club or music note head will grab onto actin and pull itself along the actin filament

7 Muscle cells Thick (myosin) and thin (actin) filaments are arranged into a bundle = sarcomere

8 Muscle cells

9 Sarcomere = the bundles of actin & myosin between Z-discs –Z-disc is what holds the actin microfilaments together –Z-disc is also what is attached to the sarcolemma (muscle cell plasma membrane) Z-disc attached to sarcolemma using dystrophin protein –There are 1000s of sarcomere structures along the length of a muscle cell Why skeletal muscle also called striated muscle…striations = part of the sarcomere



12 Muscle cells Dystrophin = muscular dystrophy –2 kinds of muscular dystrophy Dystrophy = muscular weakness –Most common forms = genetic disorders Duchenne muscular dystrophy –Nucleotide sequence for dystrophin often has an extra or missing nucleotide »out of reading frame….remember the triplet codon system on DNA? Becker muscular dystrophy –Protein primary sequence off by 1 or 2 amino acids »Protein 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 out Cytoskeleton contracts, but isnt attached to sarcolemma –Repeated back & forth motion within the cell leads to tears in the cell etc.

14 Muscle cells Wrapped around the sarcomere (actin & myosin bundles) is a specialized form of endoplasmic reticulum –sarcoplasmic reticulum: holds calcium Calcium is the key to contraction –Ca 2 = 2+ charge or very strong ionic nature –Doesnt take very much to change the electrical gradient When stimulated, sarcoplasmic reticulum releases calcium into the sarcomere –ONLY for a fraction of a second (a pulse of calcium) –THEN, it quickly re-absorbs the calcium


16 Muscle cells When 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 contraction Muscle cells will ONLY contract when told to do so by a NEURON –Have to be conscious for a muscle to contract Without 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 dont use it, youll eventually lose it

21 Muscle contraction Neuron that controls a muscle cell = somatic motor neuron –1 neuron can control a number of muscle cells Stronger muscles (gastrocnemius & soleus of the calf) = 1 neuron controls 1000s of muscle cells control muscles (eye, hands) have lower ratio: 1 neuron controls 3-6 muscle cells

22 Muscle contraction When you want to contract a muscle, you think if first –Conscious 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 contraction At the motor end plate, the neuron does not physically touch the muscle cell –Neuron releases acetylcholine (chemical neurotransmitter…made from amino acid choline) –Acetylcholine then triggers an electrical change in the sarcolemma –Neuron releases pulses of acetylcholine Doesnt drop a whole ton of it at once

25 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 contraction After twitch (if the muscle cell only receives 1 stimulatory pulse), the sarcolemma will recoil back to its original length –Recall the elastic fibers that hold the actin microfilament bundles together on the Z-disc

27 Muscle contraction If a muscle cell only twitches and the motor neuron only releases pulses of acetylcholine…how do you contract your muscle so much? –Deliver more pulses But, neurons have only a limited number of acetylcholine pulses they can deliver –Takes a great deal of energy to make and release acetylcholine –If 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 1000s or more muscle cells –Each muscle is therefore fed by 100- 1000s of neurons –ALL the neurons that feed a muscle NEVER fire at the same time (youd break a bone if that happened) –You fire groups of neurons –You also fire groups of neurons in succession

29 Muscle contraction Firing groups of neurons helps to spread the metabolic load throughout a number of neurons and a number of muscle cells that make up a muscle –For a given contraction (ie. lifting a book), only about 20-70% of the muscle cells are involved The neurons trigger muscle cells to contract in succession –Like 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 next For each sarcomere, there are 1000s of myosin heads –1000s of ATP molecules per sarcomere… 1 muscle cell can have 1000s of sarcomeres!!!! –Each neuron needs a great deal of ATP to perform the exocytosis to release acetylcholine –The sarcoplasmic reticulum needs a great deal of ATP to vacuum up the calcium released –This 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 ATP About 10 seconds worth can be recycled using the creatine phosphate system Muscle cells then revert to anaerobic fermentation to produce 2 ATPs from 1 glucose (they have about 30-40 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 Roughly 6 seconds of stored ATP –Once used up, have to make more For about 4 seconds after all the ATP is used up, your muscle cells can recycle ADP into ATP –Uses Creatine phosphate system Creatine phosphate has PHOSPHATE that can be quickly attached to ADP –Only have about 4-5 seconds worth of creatine phosphate Muscle metabolism

34 After creatine phosphate system is exhausted, muscle cell relies on anaerobic fermentation (simply hydrolysing glucose into 2 pyruvate halves) –Muscle cells store glucose About 30-40 seconds of glucose for anaerobic fermentation Unlike liver cells, muscle cells store glucose as glycogen, but DO NOT release it into the blood Ideally, some time during anaerobic fermentation, your heart rate will increase enough to deliver oxygen to your muscle cells to permit aerobic respiration Muscle metabolism

35 Muscle training You can train your muscles to increase their strength & endurance –Endurance training = increasing the amount of glucose (glycogen) stored in each muscle cell Increasing bloodflow to the muscles Increasing red blood cell count (increased oxygen in blood) –Strength training = increasing the creatine phosphate (ATP recycling mechanism) supply Building 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 elicit –Isometric contraction: contraction without a change in muscle length When 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 length –Isotonic contraction: contraction resulting in a change in muscle length, but no alteration in muscle tension When you increase the internal tension in the muscle to overcome the resistance

38 Types of muscle contractions Isotonic contraction: –2 forms of isotonic contractions Concentric contraction: muscle shortens, tension remains the same –When you lift something with your arm, you concentric contract your biceps brachii Eccentric contraction: muscle lengthens, tension remains the same –When you gently place something down with your arm, you eccentrically contract your biceps brachii –You have more strength with this form of contraction


40 Muscle actions Remember! Its 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 calcium –Muscle cells release that calcium in response to neural stimulation

42 Rigor mortis Myosin requires ATP to release from the thin actin myofilament –In death, muscle cells cannot regulate calcium release and re-uptake Causes myosin to bind to the thin myofilament, but cant let go (no more ATP production) –Stiff muscles (often contracted…why fresh carcasses often look like theyre in pain) Starts 3-4 hours post-mortem Peak contractility/rigor 12 hours post-mortem Dissipates 48-60 hours post-mortem

43 Tetanus (lock jaw disease) The disease tetanus is actually an infection by Clostridium tetani –Often found in soil (rusty nails are usually found in soil…rust itself doesnt harbor the bacterium) –Once they enter the body (via an incision etc.), they release tetanospasmin toxin Inhibits the relaxation of muscle –Neurons 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 system –The further the insertion/infection site is located from the central nervous system, the longer the incubation period You can see signs of tetanus infection prior to the locked jaw phase –Muscles around the infection site will tense up

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46 Tetanus (lock jaw disease) Tetanus vaccine: usually combined with diptheria and pertussis vaccination in 1 shot –A vaccination, especially tetanus vaccine shot, is no good for you if you have just been infected Vaccine takes at least 2 weeks to provide protection (takes that long to make antibodies)…if youve been infected, its almost too late The tetanus vaccine helps you make antibodies against the tetanospasmin toxin, not the Clostridium tetani bacteria –If 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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