The Muscular System: Tissues and Functions

The Muscular System

• Muscular system functions primarily involve movement of bones, food, urine or blood
• Posture is supported by the muscular system sustaining the body's position
• Joint strength is increased through joint stabilization
• Body temperature is maintained, with muscle movement serving to generate heat

Muscle Tissue Types

• Skeletal muscle tissue builds skeletal muscles, enabling bone movement
  • Striated, due to striped appearance
  • Voluntary
• Smooth muscle tissue forms walls of hollow organs and resides within blood vessel walls
  • Unstriated with a smooth appearance
  • Involuntary
• Cardiac muscle tissue comprises the walls of the heart
  • Striated
  • Involuntary

Functional Characteristics of Muscle Tissue

• Excitability is the capacity to receive and respond to stimuli, implying any change in the environment
• Contractility allows the muscle to shorten and generate force
• Extensibility allows the muscle to lengthen, or to stretch
• Elasticity endows muscle fibers with the ability to recoil after being stretched

Organization of Skeletal Muscle

• Muscle is the whole organ broken down into fascicles
• Fascicle refers to a bundle of muscle fibers, or cells
• Muscle fiber/cell is the cell or cells of the tissue which contain myofibrils
• Myofibril pertains to contractile organelles that are divided into sarcomeres
• Sarcomere defines a myofibril segment that shrinks during contraction and is composed of myofilaments
• Myofilaments consist of thick and thin protein fibers conducive to muscle contraction

Anatomy of Skeletal Muscle

• Endomysium surrounds each individual muscle fiber (cell)
• Perimysium envelops bundles of muscle fibers, known as fascicles
• Epimysium forms a dense connective tissue, sheathing the entire muscle

Muscle Attachments

• Muscles must have at least two attachments to bone
• Attachments must span a joint to generate movement at that joint

Classifying Attachments

• Origin refers to the more immobile attachment site of a muscle
• Insertion represents the mobile attachment site of a muscle
• During contraction, the insertion point is pulled toward the origin

Types of Muscle Attachments

• Direct attachment happens when a muscle's epimysium fuses to a bone's periosteum
• Indirect attachment exists when the epimysium extends as a piece off the muscle
  • Tendons are rope-like connections
  • Aponeurosis provides a sheet-like connection

Microscopic Anatomy of Skeletal Muscle

• Skeletal muscle fibers (cells) are long and multinucleated with many contractile proteins and mitochondria, and have a 10-100 µm diameter
• Sarcolemma is the plasma membrane of a muscle fiber
• Sarcoplasm represents the cytoplasm of a muscle fiber
• Myofibrils consist of rod-like collections of contractile proteins filling the muscle fibers
• Glycosomes are compartments which store glycogen and ensure glucose provision
• Myoglobin functions as a red pigment that binds/stores oxygen
• Myofilaments consist of protein-based contractile elements accounting for a striated appearance
  • Thick filaments bundle together with myosin proteins
  • Thin filaments consist of actin and other proteins

Structure of Myofilaments

• Thick filaments include approximately 300 myosin proteins with a diameter of 16µm
• Myosin is comprised of a long tail and globular, flexible heads, which protrude out orienting themselves in opposite directions, possessing ATP and actin binding sites
• Thin filaments have an 8μm diameter
• Actin is the structural protein, with two strands forming a helix and containing myosin binding sites
• Tropomyosin refers to long protein strands spiraling the filament to block myosin binding sites
• Troponin describes proteins with subunits as follows
  • Tnl – binds to actin
  • TnT – binds to tropomyosin
  • TnC – allows sites for calcium binding

Microscopic Anatomy of Skeletal Muscle

• A band is the darker region that contains both thick and thin regions
• I band refers to the lighter region composed of thin filaments
• H zone signifies the central region of the A band with myosin only
• M line denotes the center of the H zone where proteins link adjacent thick filaments
• Z disc represents the area where proteins link adjacent thin filaments
• Sarcomere is the contractile unit, spanning from one z disc to another

Sliding Filament Theory

• During muscle contraction, thick and thin filaments slide to shorten sarcomeres
• Note a loss of I bands and H zones during concentration

Additional Structures

• SR (sarcoplasmic reticulum) is a series of tubules which enclose stored calcium ions
• Terminal cisternae are enlarged SR areas close to the A-I borders
• T tubules are sarcolemma tubes penetrating deep into muscle fibers
• Triad represents a T tubule accompanied by terminal cisternae on either side

Muscle Contraction

• It requires:
  • Excitation from a neurological source at the neuromuscular junction
  • Excitation-contraction coupling events within the muscle fiber
  • Cross bridge cycling during sliding of filaments

Neuromuscular Junction

• Motor neuron axons meet a skeletal muscle fiber here
• Action potential represents the nerve signal sent to the sarcolemma to trigger muscle contraction

Action Potential

• Separation of charges across the membrane creates potential energy
• Stimuli release energy when the ions flow, creating an electrical current
• The ion flow triggers a cascade along the membrane, working like a wave
• The membrane returns to its original state, enabling subsequent action potentials

Graph of Action Potential

• Resting membrane is polarized, creating separation between (+) sodium and (+) potassium
• Threshold signifies the minimum depolarization which triggers action potential
• Depolarization loses membrane polarity as (+) sodium ions surge into the cell
• Repolarization restores membrane polarity as (+) potassium ions surge out of the cell

Structures at Neuromuscular Junction

• Synaptic cleft designates the small gap between a neuron and muscle fiber
• Synaptic vesicles consist of cellular compartments storing neurotransmitters like calcium
• Motor end plate is the highly folded sarcolemma region containing neurotransmitters

Chemicals at the Neuromuscular Junction

• Acetylcholine is the neurotransmitter used by NMJ's to trigger contractions, released from neurons and crossing the synaptic cleft
• ACh receptors are proteins on muscle fibers that bind to ACh to start action potential
• Acetylcholinesterase refers to an enzyme breaking down excess ACh

Events at the Nueromuscular Junction:

• Firstly the potential energy flows to reach the axon terminal
• Secondly (+) calcium ions then flow within said terminal
• Thirdly Acetylcholine (Ach) then releases from the vesicles into the synaptic
• Fourthly the potential energy flows across the vessels of the Synaptic to bind to the Receptors.
• Fifthly the Acetyl receptors which bind to the Acetyl- Potential release depolarizations to the muscle fibres of the Sarcolemma.
• Sixthly, After that all the processes spread action to the Potential cells spread along the Sarcolemma and down the t Tubules.

EC Coupling

• EC Coupling consists of actions that have the muscle fiber connecting to the neurotransmitters sliding.
• First: Action Potential has to travel to the sarcolemma and down along the T Tubules
• There after, that causes the AP for action cells to trigger more calcium cells to be released from SR- -After that release causes the troponin cells to pull on the tropomyosin, exposing the sites binding.

More Excitation Coupling:

• Fourth, Myosin must head bind on actin to bind and pull on actin, to allow the sarcomere to shorten.
• There after, calcium, needs to be pumped back to the SR and A.p begins and is the A.p ends.
• Sixth, The Thick filament Release occurs between the filament to allow a release.

Calcium and Cross Bridge Formation

• The calcium and cross bridge for action is when the binding is connecting the thick and thin fibres
• The fibres begin to start shape change of the troponin where the troponin is pulled to expose sites.
  • There after the action is binding on site for actin, to allow the crossing fibres to bind to more.

The Cross Bridge Cycle

• The cross bridge release is when the fibre connects when the more fibres is formed on active site.
• The power of what the bridges use help create and build tension.

Rigor Mortis

• The stiffing of the muscles occur.
  • after dying for 3-4 hours. Then Peak builds around 12 hours than drops and dissolves.
• The reason for why the cells start to receive more calcium causing the cross bridge start and build. Than there is no available ATP to allow to release and drop.

Muscle Contraction – Motor Units

• Motor unit refers to a single motor neuron and all muscle fibers controlled
• Muscles with fine control feature smaller units versus those needing less precision
  • Weak muscle contractions recruit few motor units
  • Strong muscle contractions require a stronger stimulus, recruiting motor units

Motor Unit Recruitment

• Motor units recruited in a contraction hinge on stimulu strength
• Threshold stimulus represents a level where contraction is observed first
• Maximal srimulus indicates the leverl to when all muscles are recruitted.

Muscle Contraction – Muscle Twitches

• Muscle twitch indicates response of muscle fibers to a single action potential displayed on myogram
• Latent period identifies a time lag between stimulus and muscle contraction
• Contraction spans the duration in which muscle fibers are forcibly shortening
• Relaxation indicates time when muscle fibers elongate back to resting length

Isotonic Contractions

• Contractions that involve change to the muscle length
• Example is during load where a maximum is present
  • The contraction generates more force of with the contraction

Muscle Metabolism

• In the source of metabolism the source, it includes the source.
• 1 is stored 4.ATP as a muscle fibres, 4-6 seconds.
• 2 Create more phosphate which donates the phosphates to the adenosine T.P. (15 secs)
• 3 anerobic, very very few atp’s but its also very fast
• 4 Aerobic respirations is much more slower process because of slower energy and need oxygen.

Muscles, Anerobic,and Aerobic respiration

• In which where Glycolysis splits glucose molecules 2 to create to pyrufate of which can occur on both the aerobic and the anerobic. And in the case where oxygen is required The chemical formulation is to be required and and create water and produce CO2.
• There for, 32 AP are used to the mitochnodria and the process requires 32 AP.
• The Depleting oxygen is used to the Depleted oxygen where the muscle is depleting and needed and or is being replenish.

Factors Affecting Force of Contraction

• Number of fibres, more motor is required
• size more fibres mean more to get more four.
• A efficient tenus for more contraction produces more power.
• Degrees which are used with the best for forces to give and give to overlap to allow both to give power and be able to thin and filament to work.

Factors Affecting Velocity/Duration of Contraction

• The slow oxidative process which is oxygen dependent allow so the so more well Vascularized
• The fast glycosis is quick process which is independent allowing it quickly and white
• So in the power and high amount is dependent to the motor neurons. Which will generate more

Aerobic process: Which are to include by swimming like activitys.

• With more power in the ATP for production. The muscles gain an endurance
• The resistance process, is to do with lift exercise which leads to strength increase
• Resistance which provides more support and tissue, to the more fibre to allow for extra strength.

Smooth Muscle

• Not the same it is Non straided and not as a skeleton muscle and not a voluntary.
• Mostly found on the walls of areas. Which are blood. Mostly found on the walls of areas.
• There with circular and lingual muscle

Smooth Muscle Fibres

• The smooth muscles are a lot smaller then they scale muscles (17-500), and far shorter then normal, also less than 10 um.
• Only had 1 small fibre per area also which is centre focus.
• Muscle fibres are surround by more endomusium not as much, only the the sarcomere that are needed for the muscle fibres to rely. The use of gap junctions also create and help the fibre cells.

Disorders of Muscular System:

• Auto immune disease where the muscle is attacked
• the muscle becomes very lose and loses power.

DMD:

• Muscle tissues decreases, shorten lifespan and happens to boys more with the x factor of genetics
• Mutation is connected with non working tissues with proteins which limits and does not work due to sarcolemma issues.