Muscular System

Functions of the Muscular System

• Movement of skeleton

• Circulation - smooth and cardiac

• Limited energy storage

• Body support and maintain posture

• Maintain body temperature

• Guard entrances and exits

  • Opening of GI and urinary tract (sphincters)v

Types of Muscle

• Skeletal

  • 40-50% of body mass

  • Multinucleated

  • Over 600

• Smooth

• Cardiac

Myoglobin

• Brownish red pigment

• Similar to hemoglobin

• Stores O₂ in the muscle

• Decrease the need for constant blood supply

• Oxygen storage for protein

• Provides muscle cells with oxygen

Fascicles

• Muscle is organized into many individual bundles to provide greater control 

Fascia

• Dense connective tissue that separates individual muscles and holds them in place

• bonds to the epimysium (CT) that surround each skeletal muscle fiber

Epimysium

• surround each skeletal muscle fiber

Perimysium

another layer of CT that extends into the muscle and separates it into sections called fascicles

endomysium

a thin layer of CT that each muscle fiber within the fascicle lies in

Tendons

The connective tissue layers may project beyond the muscles end to form this that attach the muscle to the bone.

Tendonitis

• when a tendon becomes painfully swollen following injury or repeated stress

  • Treated with rest, physical therapy and anti-inflammatory drugs

  • If this does not work, ultrasound can be applied to break up scar tissue

Skeletal Muscle Fibers

•  a single cell that contracts in response to stimuli

  • Long thin cylinder with rounded ends.  Just beneath the cell membrane (sarcolemma), the cytoplasm (sarcoplasm) has many small nuclei and mitochondria. 

Sacroplasm

contains many myofibrils that lie parallel to each other 

Myofibrils

• contractile elements of the muscle cell - made up of two kinds of protein filaments (thick and thin myofilaments)

Myosin

Thick filament

Actin

Thin filament, Associated with 2 additional thin filament proteins: Tropomyosin and Troponin

Sarcomere

• a packet of myofilaments (actin and myosin) and their regulatory proteins (troponin and tropomyosin) 

Sarcoplasmic Reticulum (SR)

• specialized smooth endoplasmic reticulum which stores, releases and retrieves calcium ions (Ca²⁺)

• T-tubules (Transverse Tubules)

• Part of the sarcolemma (muscle cell membrane) that forms tunnels which dip into the muscle fiber

•  This allows the action potential to pass easily to the sarcoplasmic reticulum

Terminal cisternae

• Expanded region of the SR near the T-tubule

• Stores and releases Ca²⁺ for contraction

Triad

T-Tubule + SR

Tropomyosin

• Twisted thread like protein

• Covers myosin binding sites when Ca²⁺ is low

Troponin

• Regulatory protein 

• Moves tropomyosin away from myosin binding sites on actin when calcium ions (Ca²⁺) are present

ATP (Adenosine Triphosphate)

• Required for the myosin head to detach from the actin and prepare to bind again 

• ATP → ADP + P_i + Energy 

  • The energy released is the energy used to do work

  • There are three ways the muscle cells can get more ATP:

    • 1. Creatine Phosphate

    • 2. Cellular Respiration

    • 3. Fermentation

Fermentation

• Glycolysis when oxygen is absent will produce lactic acid and 2 ATP’s per glucose

  • This will give about 1 minute of high intensity output

Muscle fatigue

• No longer contract even with neural stimulation

• This is due in part to a decrease in pH caused by a buildup of lactic acid

Creatine Phosphate

• Mitochondria store energy in the form of CP and ADP

• The one molecule of Phosphate in CP can be added back to ADP to power the first 5-10 seconds of muscle contraction 

Cellular Respiration

• Once the cells run out of CP, ATP is generated using glycolysis 

• Glycolysis in the presence of oxygen produces pyruvate

• Pyruvate will enter cellular respiration and the whole process will produce about 36 ATP’s per glucose

• This can sustain slow or low impact contraction over long periods of time

  • Myoglobin

1st step of Muscle Contraction

Impulse travels dow the axon of the motor neuron

2nd step of Muscle Contraction

The Motor neuron releases the neurotransmitter acetylcholine (ACh)

• At the Neuromuscular Junction

  • Action Potential reaches the end of the axon and Ca²⁺ channels in the axon open

    • Ca²⁺ enters the axon terminal form the surrounding fluids

    • Ca²⁺ in the axon terminal triggers the vesicles holding the ACh to move to the synapse and release the ACh 

      • The ACh crosses the synapse and binds to receptors on the motor end plate of the muscle cell

3rd Step of Muscle contraction

ACh binds to the ACh receptors in the muscle fibers

• When the acetylcholine binds the muscle, it binds to the ACh receptor allowing Na⁺ ions to enter the cell - this channel will also allow K⁺ ions to leave the cell for repolarization when contraction is completed 

  • Before the impulse, the membrane had a potential of -70mV (with [K⁺]>[Na⁺] inside the cell)

    • Having more + ions enter the cell makes the potential less negative (closer to zero)

    • This causes other voltage gated Na⁺ ions channels, in the sarcolemma, adjacent to the neuromuscular junction to open passing the impulse along/into the muscle cell. 

      • Voltage gated: when the charge reaches -55mV they will open.  

        • -55mV is called threshold potential. 

4th step of Muscle Contraction

The sarcolemma is stimulated. An impulse travels over the surface of the muscle fiber and deep into the fiber through the transverse tubules (t-tubules) 

• Action potential is spread into the muscles fiber by T-tubules 

  • T-Tubules are extensions of the sarcolemma

5th step of muscle contraction

the impulse reaches the sarcoplasmic reticulum and the calcium channels open

• The action potential passes to the SR and causes the terminal cisternae of the SR to release Ca²⁺ (this is different from the Ca²⁺ ions in the axon) into the muscle cell.

  • Note the triad includes the T-Tubules of the sarcolemma and the terminal cisternae of the sarcoplasmic reticulum

6th step of Muscle contraction

Calcium ions diffuse from the sarcoplasmic reticulum into the cytosol and bind to the troponin molecules

7th step of muscle contraction

Tropomyosin molecules move and expose specific sites where myosin heads can bind

8th step of muscle contraction

cross-bridges form, linking thin (actin) and thick (myosin) filaments

9th step of Muscle contraction

thin filaments are pulled toward the center of the sarcomere by the pulling of the cross-bridges

• Myosin heads pull the actin

10th step of muscle contraction

the muscle fiber exerts a pulling force on the attachments as a contraction occurs.

• This allows the muscle to do work

• When ATP is present AND the stimulus is still present (muscle is still depolarized) this cycle will repeat. 

  • The ATP is needed for the myosin head to detach and the energy of the ATP is used to return the head to its original shape (so it can grab the actin again)

  • When the stimulus is no longer present, Ca²⁺ ions will not be available to bind the troponin

1st step of muscle relaxation

Acetylcholinesterase decomposes ACh, and the muscle fiber membrane is no longer stimulated.

• The impulse is no longer present in the motor neuron. 

• Na⁺ no longer moves in and K⁺ moves out of voltage gated K⁺ channels to return the membrane potential to -70mV

• The Sodium/Potassium pump then reestablishes the sodium and potassium concentrations that existed before the contraction.

• The sodium potassium pump uses ATP to pump 3 Na⁺ out of the cell for every 2 K⁺ it moves in

2nd step of muscle relaxation

Calcium ions are actively transported into the sarcoplasmic reticulum

3rd step of muscle relaxation

ATP breaks cross-bridges linkages between actin and myosin filaments without breakdown of the ATP itself

4th step of muscle relaxation

ATP breaks down and turns into ADP and P (inorganic) and returns myosin head to its original position

5ht step of muscle relaxation

Troponin and tropomyosin molecules block the interaction between myosin and actin filaments

6th step of muscle relaxation

The muscle fiber reamlian relaxed, yet ready, until stimulated again

Rigor Mortis

• Several hours after death, skeletal muscles partially contract

• May continue for up to 72 hours

• Results from release of Ca²⁺ ions which allows for the formation of cross-bridges

  • With no ATP present, the cross-bridges cannot detach 

  • The condition will persist until the muscle proteins begin to breakdow

Myosin heads

• contain the enzyme ATPase, which catalyzes the breakdown of ATP to ADP and phosphate. 

•  can attach to a myosin binding site, forming a cross-bridge, and bend slightly, pulling the actin filament. (shortening the sarcomere)

Motor neurons

Neurons that control muscles (effectors)

Synapse

The functional connection between a neuron and another cell

Neuromuscular Junction

The functional connection between a neuron and a muscle

Motor end plate

• the sarcolemma has indentations where the ends of the neuron are located. 

Neurotransmitters

how Neurons communicate with other cells

Synaptic cleft

The space between the neuron and its effector (the muscle cell)

Axon

• end of the motor neuron and is rich in mitochondria and vesicles that store neurotransmitters. 

ADPase

• catalyzes the breakdown of ATP to ADP and phosphate. 

ADP + Pi (inorganic phosphate

the byproducts of ATP hydrolysis, which releases the energy needed for the muscle to contract.