Ch9: Muscles and muscle tissue

1. Voluntary

   1. Skeletal :

      1. Striated

      2. Elongated cylindrical

      3. multinucleated

2. Involuntary:

   1. Cardiac :

      1. Striated

      2. Branches at intercalating discs

      3. Mononucleated

   2. smooth :

      1. Non-striated

      2. Sheet like

      3. Elongated

      4. Spindle shaped cells

      5. mono nucleated

1. Muscle cell is called muscle fiber in smooth and skeletal muscle but not in cardiac because they are elongated

2. Myo and mys and sarco

1. Excitability /responsiveness,

   1. Ability to receive and respond to a stimulus by changing membrane potential

2. Extensibility : Ability to extend / stretch

3. Elasticity : ability of muscle to recoil to resting length after stretch

4. contractility : ability to shorten forcibly when stimulated

1. Produce movement

2. Maintain posture and body position

3. Stabilizes joints

4. generate heat

1. The muscle is served with :

   1. One nerve

   2. One artery

   3. One / more veins

2. They pass through the central part of the muscle

3. They branch through connective tissue sheaths

4. Each muscle fiber of skeletal muscle is served with a nerve ending to control it’s voluntary action unlike cardiac and smooth which can sometimes not have one

1. Supporting each cell

2. holding together the muscle

3. prevent the muscles from bursting during strong contractions

4. Transmit pulling force to bone to be moved

5. Provide routes for entery and exit of Blood vessles and nerve fibers

1. they are continuous with each other

2. They are continuous with aponeurosis and tendons connecting to the muscle

1. Epimysium :

   1. Surrounding the whole muscle

   2. Dense irregular CT

   3. Sometimes blends with other neighboring facia

2. Perimysium :

   1. Surrounding each fascicle

   2. Dense irregular CT

3. Endomysium :

   1. Surrounding each muscle fiber

   2. Areolar CT

A fascicle group of muscle fibers surrounded by perimysium

1. At least at two points

2. Two types :

   1. Insertion :

      1. The attachment to a bone that can move

      2. Distal

   2. Origin :

      1. The attachment to a bone that can’t move or moves less

      2. proximal

3. The muscle’s insertion moves towards the muscle’s origin

1. Muscle attachment:

   1. Direct/ fleshy attachment :

      1. Epimysium of muscle is fused to the periosteum of the bone or the perichondrium of the cartilage

   2. Indirect attachments :

      1. Epimysium of muscle fuses to Dense Regular which connects to periosteum (all of which are CT)

      2. Through two types :

         1. Tendons : rope like

         2. Aponeurosis: Sheet like

2. Occurs generally between connective tissues one that belongs to the muscle (epimysium) and other that belongs to bone (periosteum) and mostly something in between (tendon / aponeurosis ) which are also connective tissues

3. Indirect attachment is better because

   1. It doesn’t require much space

   2. can withstand abrasion from bones unlike muscles

1. Organelles :

   1. Sarcolemma : Plasma Membrane of a muscle fiber

   2. Sarcoplasm : Cytoplasm of a muscle fiber

   3. Sarcoplasmic reticulum : endoplasmic reticulum

   4. Glycosomes : Granules that store glycogen which produces glucose when muscle needs

   5. Myoglobin : Red pigment that Stores Oxygen

2. hundreds of embryonic cells fuse to form one muscle fiber

1. Sarcoplasmic reticulum

2. T-tubules

3. Myofibrils

1. myofibrils are made of a chain of sarcomeres that are linked end to end

   1. They are made of myofilaments :

      1. Thick filaments

      2. Thin filaments

2. Many accounting up to 80 % of muscle fiber that are densely packed

1. Repeating series of dark and light bands evident along the length of each muscle fiber :

   1. A Bands :

      1. Extends along the thick filaments

   2. I Bands :

      1. The region with only the thin filaments

   3. H-zone :

      1. The central less dark region of thick filaments

   4. M-line :

      1. The line at the center of a thick filament formed by myomesin protein

   5. Z discs : Zig zag lines at the center of I bands

2. A sarcomere is between Z-line to the adjacent Z-line

1. a sarcomere

2. Between two successive Z-discs they have :

   1. Half I band at each side next to Z discs

   2. A band in center along thick filaments

   3. H zone and M line at the center of A bands

1. 300 Myosin molecules bundled together the tails facing inward and the heads facing outwards

   1. Made of Six polypeptide chains :

      1. 2 heavy : twist to form the tail

      2. 4 light : form the 2 globular heads

      3. attached to each other by a flexible hinge

   2. The globular head acts as ATPase

   3. This structure causes A band (thick filament with myosin heads) and the H zone (thick filament without myosin heads/thin filaments)

   4. The H zone is caused by lack of myosin heads at the center of the thick filaments

   5. Myomesin connects thick filaments of one myofibril to thick filaments of another and they form the M line which helps in alignment of myofibrils

1. Made of protein actin and regulatory proteins:

   1. Actin:

      1. The subunits are G-actin (globular) :

         1. Kidney shaped

         2. Has a myosin binding site where myosin heads attach to during a contraction

         3. They polymerize to form F-actin (filamentous)

         4. One thin filament has two F-actin interwinding each other to form a helix

   2. Regulatory proteins :

      1. Troponin :

         1. Is a globular protein

         2. Has three subunits :

            1. 1 Binds to Actin filament : to bind

            2. 1 binds to Ca

            3. 1 Binds to tropomyosin : to help position tropomyosin on actin

      2. Tropomyosin:

         1. Long rod

         2. Covers the actin filaments to block it’s binding sites and prevent contraction when not needed

1. Six thin filaments surround each thick filament

2. Three thick filaments surround each thin filament

1. Elastic filaments :

   1. Made of Titin protein

   2. Extends from Z discs through the center of the thick filament to connect to the M-line

   3. The part of the titin that spans the I bands is extensible,

      unfolding when the muscle stretches and recoiling when the tension is released

1. Dystrophin: Links thin filaments to integral proteins of sarcolemma which are anchored to the ecm

2. Nebulin:

3. myomesin

4. C-protiens

5. Intermediate (desmin) filaments extend from Z disc and connect each myofibril to the next one through the muscle fiber

Their general functions is to bind to filaments or sarcomeres and maintain their alignment

1. DMD : Duchenne muscle dystrophy

2. belongs to Muscular dystrophy family

3. appear during childhood (2-7)

4. sex linked recessive mostly to males

5. Caused by a defective gene for dystrophin which causes the sarcolemma to tear and allow Ca2+ to enter which causes muscle cells to go through apoptosis

6. Death

1. 2 types of sets :

   1. Sarcoplasmic Reticulum

   2. T-tubules

2. They form a triad

3. because it houses two terminal cisterns with a T-tubules in the center

1. Functions include :

   1. Stores and regulates intracellular levels of Ionic Ca²⁺

   2. Releases Ca²⁺on demand when muscle fiber is stimulated

2. They run longitudinally (same direction as myofilaments) along a Myofibril to surround it

3. they communicate with each other at two points :

   1. H zones (forms a network of interconnecting tubules)

   2. A–I band junction (forms the Terminal cisterns of Triad)

1. they are invaginations of the Sarcolemma going down along the circumference of each microfibril which is continuous with extracellular space

2. They increase surface area which allows changes in membrane potential to occur fast so they act as voltage sensors

1. Formation of cross bridges (myosin heads binding to actin filaments)

2. only occurs if the force generated is greater than the opposite tension force

3. cross bridges become inactive (relaxed)

4. sliding filament model of contraction

   1. It says that the myosin heads cause the thin filaments to overlap to a greater degree which causes shortening

5. Striations :

   1. A band : length Doesn’t change but move closer to each other

   2. I band : shortens

   3. H zone : shortens / disappears

   4. Z discs: distance between them decrease

1. cells that respond to external stimuli by changing their resting membrane potential.

2. two :

   1. Muscle fibers

   2. Neurons

3. Two types :

   1. Electrical signal :

      1. AP (action potential / nerve impulse) :

         1. Travels long distance

         2. cannot pass from cell to cell

   2. Chemical signal:

      1. Neurotransmitters (Acetyl-choline)

         1. Can pass from cell to cell

         2. Travels short distance

1. Chemically-gated :

   1. Opened by chemical messengers (AcH)

   2. cause a small local depolarization which triggers voltage gated channels

   3. ACh receptor

2. Voltage gated:

   1. open or close in response to membrane potential changes (Action potential)

   2. They cause the next voltage gated to open as they carry the message

   3. Voltage gated K⁺ and voltage gates Na⁺

1. Somatic motor neurons

2. They are present in the spinal cord except for brain and neck which is in the brain

3. The neurons have axons which branch alot as they enter the muscle then they branch to form a neuromuscular junction / motor end plate

4. near the center if the muscle fiber

5. each muscle fiber has 1 neuromuscular junction no more no less

1. Axon terminal : end of the neuron

2. synaptic cleft : Fluid filled area rich in glycoproteins and collagen fibers

3. Synaptic vesicles : membrane bound granules that contain acetylcholine

4. junctional folds of sarcolemma : increase surface area

5. Note : Axon never touches the muscle

1. Events at the neuromuscular junction

2. Excitation of a muscle fiber

3. Excitation contraction coupling

4. Cross bridge cycling

1. Action potential arrives at the axon terminal

2. Ca²⁺ ion channels are opened which allows it to move into the axon terminal down it’s electrochemical gradient

3. Ca2+ entry causes ACh to be released by exocytosis.

4. ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.

5. ACh binding opens chemically gated ion channels which allow K⁺ to move out and Na⁺ to move in at the same time which creates a local depolarization called (End plate potential)

6. ACh effects are terminated by acetylcholinesterase which breaks down acetylcholine to acetic acid and choline

1. opening of chemically gated channels causes Endplate potential which is a local depolarization

2. Depolarization: opens voltage gated sodium channel which allows sodium to move in

3. sodium moving in causes depolarization by which + charge is increasing in the cell which also triggers the same gated channel to close

4. depolarization triggers potassium voltage gated channels to open

1. Myasthenia gravis

2. signs include drooping eyelids and muscle weakness

3. it is caused when immune system destroys ACh receptors

1. Refractory period is the period of repolarization

2. it is important because no matter how strong the stimulation is the muscle cannot be stimulated until the refractory period has ended because the Na+ ion channels cannot be opened

3. repolarization only restore electrical conditions not the ionic conditions which is restored by the ATP-Na-K-pump

4. ionic imbalances ain’t a problem until thousand of contractions

5. Ionic imbalances are fixed with Na K+ ATPase

6. once started it cannot be stopped

1. The AP is carried along the sarcolemma down the T-tubules

2. AP reaches the area of T-tubules that has terminal cisterns which causes the the voltage sensitive tubule proteins on the T-tubules to change shape

3. This shape change opens the Ca²⁺ release channels in the terminal cisterns, allowing Ca²⁺ to flow into the cytosol.

4. Ca²⁺ binds to troponin

5. troponin rolls tropomyosin to groove of actin helix exposing it’s binding sites which allows them to bind to thick filaments

6. myosin binds and the next step starts

After math :

1. The voltage sensitive tubules return to normal shape which closes the Ca²⁺ channels

2. Ca²⁺ is actively pumped back to sarcoplasmic reticulum

3. without Ca²⁺ troponin returns to normal and the tropomyosin blocks the binding sites of acting

2 Ca²⁺ molecules are required to bind to troponin

1. upright myosin binds to actin

   1. It has ADP and P_i

2. ADP and P_i leave causing myosin and the actin to move

3. ATP binds which cause the Myosin to detach from actin

4. ATP is hydrolyzed to ADP and Pi and the myosin head becomes upright ready to bind to another actin

1. rigor mortis

2. No ATP is present which means that the myosin head will not separate from the actin filament

3. dead people have fully contracted muscles even tho they are dead

4. after 3 days because the muscle proteins are broken down after death

1. Muscle tension : the force exerted by the muscle on the opposing force

2. Load: the weight of the object to be moved

1. A motor unit consists of :

   1. One motor neuron

   2. all the muscle fibers it innervates

2. Differences in number of muscle fibers innervated in one motor unit :

   1. Larger motor units focus on larger force production (calves)

   2. smaller motor units focus on more precise movement (fingers)

1. a myogram

2. Muscle twitch: Muscle response to a single stimulation by which the muscle contracts quickly then relaxes

3. Three phases :

   1. Stimulus

   2. Latent period: Cross bridges begin to cycle but muscle tension not yet measurable

   3. Period of contraction: number of active Cross bridges is increasing

   4. period of relaxation: number of active Cross bridges is Decreasing

   5. Note: different type of muscles have different period legnth

4. period of contraction is generally faster / shorter than the period of relaxation

1. because it is sudden and produces robot movements

2. graded muscle contractions

   1. Temporal summation: Rate of firing of action potentials

   2. Recruitment: Number of motor neurons that are activated

1. is Increase in frequency of stimulation = stronger contraction by same muscles

2. because the second stimulus arrives before the Ca2+ is fully pumped back which means that the tension is still there which allows the second stimulus to ride on its back and produce stronger contraction

3. types are : in the picture

4. fused and unfused tetanus are rare because physiological mechanisms prevent it

5. To produce smooth, continuous muscle contractions

1. Multiple motor unit summation

2. Controls the force of contraction by :

   1. Different motor units have different thresholds by which

      1. According to stimulus intensity different motor units are recruited

3. Achieved by increasing voltage

4. Threshold is the point at which amount of depolarization required for the voltage gated channels to start an AP in muscle cells and start a contraction

5. Three types :

   1. Subthreshold are APs that don’t cause contraction as they don’t cause depolarization enough to reach to threshold

   2. threshold stimulus is the one that causes the first observable contraction

   3. maximal stimulus is the strongest that causes all the muscle’s motor units to be recruited

6. because some acts don’t require that much force like muscles that maintain posture

7. Size principle :

   1. Larger motor units require larger stimulus due to higher threshold stimuli

   2. Smaller motor units are recruited first then the larger then the largest

1. Different motor units are activated at different times and not at the same time with different muscle fibers in the same muscle belonging to different motor units

1. Muscle tone

2. Muscles are always slightly contracted but doesn’t produce active movements

3. spinal reflexes which activate some motor units then another continuously

4. 4

   1. helps maintain muscles healthy

   2. ready to respond to stimulation

   3. stabilize joints

   4. maintain posture

1. Types:

   1. Isotonic : generates enough force (more than the load) to shorten (thin filaments move)

      1. Concentric:

         1. Shorten and does work like picking up a book

      2. Eccentric:

         1. Generation of force when it is lengthening like walking down a steep hill

         2. Eccentric is stronger by 50 %

   2. Isometric : generates force but doesn’t shorten or lengthen because the load needs more force than the muscle can generate (no movement of thin filaments)

      1. Isometric is important for maintaining posture and holding joints stationary while movement occurs at other joints

2. Classification depends if muscle changes length or not

1. hydrolysis of stored ATP which lasts for only 4- 6 seconds

2. ATP hydrolysis should be = to ATP regeneration

3. Regeneration occurs by :

   1. Direct phosphorylation of ADP by creatine phosphate

      1. Creatine phosphate transfers a phosphate group to ADP to form ATP

      2. Fast

      3. 10 seconds (2 to 3 times more than ATP)

      4. Enzyme is creatine kinase

   2. Anaerobic pathway :

      1. Glycolysis occurs to break down glucose without the presence of oxygen to 2 pyruvate molecules to produce 2 ATP and if there is no oxygen the pyruvate is converted to lactic acid

      2. Fast

      3. Lactic acid diffuses to blood stream into liver so that it can be converted to energy by liver and kidneys

      4. lasts for 30-40 seconds

   3. Aerobic respiration :

      1. Glucose is broken down to pyruvate which is then broken down to CO2 and Water and 30 ATP

      2. really slow

1. aerobic endurance

2. Anaerobic threshold (when ATP is needed quickly and aerobic respiration cannot provide enough in that time)

1. Creatine phosphate

2. Aerobic damn u gettin a pump

3. anaerobic (u will never do it anyway)

1. when the muscle is unable to contract even though the muscle is still receiving a stimulus

2. preventative mechanisms stop contraction in order not to cause complete depletion (rigor mortis and muscle death)

3. many causes but include :

   1. Ionic imbalances : no more K+ in and too many Na+ in cause no AP to happen

   2. increased organic pi caused by ATP and CP breakdown : interferes with Release of other pi from Myosin heads and release of Ca2+ from SR

   3. Decreased ATP more Mg2+ : Mg2+ normally binds to ATP but with low ATP they bind to voltage sensitive proteins of T-tubules which cause Ca2+ release to decrease

   4. Decreased glycogen : no more energy

4. Lactic acid is assumed as a major cause but it isn’t it just causes pain

5. two types of activities :

   1. Strenuous but fast : fatigue fast : heal fast

   2. Slow developing fatigue : fatigue slow : heal slow

1. Excess postexercise oxygen consumption returns muscle chemistry back

   1. Muscle’s myoglobin are reoxygenated

   2. muscle’s glycogen stores are refilled

   3. lactic acid is reconverted to pyruvate by liver

   4. ATP and CP reserves are resynthesized

2. also called paying back the debt to muscles for their huge favor (oxygen debt)

1. Depends on number of myosin cross bridges that are attached to actin which is affected by:

   1. Frequency of stimulation : more = temporal summation = stronger

   2. number of muscle fibers recruited

   3. size of muscle fibers

   4. degree of muscle stretch in picture:

      1. More stretch = too far away from myosin heads to cause force

      2. Less stretch = no where for actin filaments to go in center

2. Hypertrophy is caused by resistance exercises which causes enlargement of muscles

1. Muscle fiber type

2. size of the load

3. recruitment

1. Speed of contraction :

   1. How fast does ATPase of myosin break down ATP

   2. How quickly Ca²⁺ moves from cytosol to SR

   3. it also show which pathway is used for generating ATP

2. Major pathways of forming ATP :

   1. Glycolysis and CP or aerobic or both

In the picture

1. they have a mixture between all three types but they depend on the person’s activity

2. they only have 1 muscle type

1. Aerobic exercise (running to Qatar) :

   1. More capillaries

   2. More mitochondria

   3. More myoglobin

   4. in general they turn convert fast glycolytic to fast oxidative fibers

2. Resistance exercise (500kg / 3 sets) :

   1. Fast oxidative to fast glycolytic

   2. causes hypertrophy

   3. return back to normal if stopped

1. disuse atrophy (degeneration of muscles)

2. muscle fibers are replaced by fibrous connective tissue

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