Muscular Skeletal System

• Properties of Muscle:

  • Contractability
  • The ability of the muscle to shorten
• Properties of Muscle

  • Extensibility
  • The ability of the muscle to be stretched, muscles can't increase their own length but can be stretched
• Properties of Muscle

  • Elasticity
  • The ability of muscle to return to its original length after being stretched
• Properties of Muscle:

  • Excitability
  • Responds to stimulus including, electrical, hormonal and mechanical
• Structure of Skeletal Muscle

  • Each skeletal muscle is an organ that consists of various integrated tissues (skeletal muscle fibres, blood vessels, nerve fibres, and connective tissue). 
  • Each skeletal muscle has three layers of connective tissue that enclose it, provide structure to the muscle, and compartmentalize the muscle fibres within the muscle.
• Structure of Skeletal Muscle:

  • In skeletal muscles that work with tendons to pull on bones, the collagen in the three connective tissue layers intertwines with the collagen of a tendon. 
  • At the other end of the tendon, it fuses with the periosteum coating the bone.
  • The tension created by contraction of the muscle fibres is then transferred though the connective tissue layers, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton
• Epimysium:

  • Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. 
• Fascicles:

  • Inside each skeletal muscle, muscle fibres are organized into bundles, called fascicles, surrounded by a middle layer of connective tissue called the perimysium. This allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle.
  • The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.
  • Inside each fascicle, each muscle fibre is encased in a thin connective tissue layer of collagen and reticular fibres called the endomysium.
• Skeletal Muscle Fibers:

  • Skeletal muscle fibres have many nuclei to allow for production of the large amounts of proteins and enzymes needed for maintaining normal function of these large protein dense cells.  
  • Skeletal muscle fibres also contain cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum. However, some of these structures are specialized in muscle fibres.
• The specialized smooth endoplasmic reticulum, called the sarcoplasmic reticulum (SR), stores, releases, and retrieves calcium ions (Ca++).
• • The plasma membrane is called the sarcolemma and the cytoplasm is referred to as sarcoplasm.
  • Each muscle fiber/cell is surrounded by a sarcolemma
  • Within this is the sarcoplasm (cytoplasm)
• • Within a muscle fibre, proteins are organized into organelles called myofibrils that run the length of the cell and contain sarcomeres connected in series.
  • The sarcomere is the smallest functional unit of a skeletal muscle fibre. 
  • It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibres (and ultimately the whole muscle).
• Skeletal muscles are also:

  • richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. 
  • supplied by the axon branch of a somatic motor neuron, which signals the fibre to contract.
• Myofibrils

  • Are made of sarcomeres
  • A combination of thick and thin filaments known as myofilaments give a banded effect
  • Bands are known as striations 
  • These are the contracting units of a muscle
• Types of Myofilaments

  • Thick myofilaments made of myosin
  • Thin myofilaments made of actin
• Sliding filament model

  • This model is the representation of how muscles contract
  • This contraction occurs because of the acting and myosin filaments
  • Actin slides over myosin shortening the sarcomere
• Resting state

  • In a relaxed muscle, the thick and thin filaments overlap slightly
• Key parts of sarcomere

  • Z line - separates one sarcomere from the next
  • H zone - space between the thin actin myofilaments of one sarcomere
  • A Band - the length of the thick myosin filaments within one sarcomere
  • I band - distance between the thick myosin myofilaments of one sarcomere to the next
• Actin

  • The thin filament known as actin consists of two chains of actin subunits twisted around each other.
  • Would around the actin molecule is a protein called tropomyosin and this has three molecules of troponin attached.
• Myosin

  • The thick filament known as myosin has two protruding heads which stick out at the end of each molecule and are mobile
  • These heads enable this protein to bind to the exposed binding sites which are found on the thin filament
• Muscle contraction works via sliding filament theory:

  • Thin actin filaments slide between the thick myosin filaments
  • They do not contract themselves so they stay the same length
  • This causes the sarcomeres to shorten in length in turn shortening the muscle fibres and causing contraction
• 1. Initiation of muscle contraction

  • When a nerve impulse signals a muscle to contract stored calcium ions are released from vesicles into the sarcoplasm
  • These calcium ions bind to troponin proteins on the thin filaments causing them to change shape and move the tropomyosin
  • This results in the exposure of binding sites
• 2. Cross bridge formation

  • Myosin heads (part of the thick filaments) bind to the exposed sites of the action (thin filaments) forming cross bridges.
• 3. Power stroke

  • Energy from ATP is used to move the myosin heads, causing them to pull the actin filaments towards the centre of the sarcomere
  • This shortens the sarcomere leading to muscle contraction
• 4. Actin-Myosin cross bridge breaks

  • An ATP molecule binds to the myosin head, providing enough energy to break the bond between the myosin head and the actin filament
• 5. Myosin head reattaches to a different actin-myosin binding site

  • Hydrolysis of the ATP makes energy available, allowing the myosin to bind further along the length of the actin fibre
  • Results in shortening of sarcomere
• 6. The cycle is repeated

  • The myosin head can make and break the cross-bridges up to 100 times a second, so the process is very rapid
• Relaxation

  • When the nerve impulse ceases, calcium ions are actively transported back into storage in the muscle cell
  • Without calcium the binding sites on the actin filaments are covered again, preventing further cross-bridge formation
  • As a result the muscle relaxes
• Steps in Muscle relaxation

  1. Ach broken down by AChE
  2. Sarcoplasmic reticulum recaptures Calcium
  3. Active sites covered, no cross bridge interaction
  4. Contraction ends
  5. Relaxation occurs, passive return to resting length
• Interactions of skeletal muscles

  • To move the skeleton the tension created by the contraction of the fibres in most skeletal muscles is transferred to the tendons
• • Tendons - strong bands of dense regular connective tissue that connects muscles to bones
• Origin and insertion

  • To pull on a bone (move the skeleton) a skeletal muscle must also be attached to a fixed part of the skeleton
  • The moveable end of the muscle that attaches to the bone being pulled is called the muscles insertion
  • The end of the muscle attached to a fixed (stabilized) bone is called the origin
• Identify the: of the gastrocnemius (calf) muscle in the lower leg

  • Origin - near the knee
  • Insertion - by the heal
• Prime movers:

  • Although a number of muscles may be involved in an action the principle muscle involved is called the prime mover or agonist
• Synergists - muscles that act to help the prime mover, this can be by producing the same movement as the mover or by stabilizing a joint
• Fixator - when a synergistic muscle contracts to immobilize/stabilise a joint
• Antagonists

  • A muscle with the opposite action of the prime mover (agonist) is called an antagonist
  • They play two important roles in muscle function
  • They maintain body or limb position, such as holding the arm out or standing erect
  • They control rapid movement as in shadow boxing without landing a punch or the ability to check the motion of a limb
• Antagonistic pairs

  • Antagonist and agonist
• Skeletal muscles - movement at joints

  • Belly - is the thick, fleshy mid portion of the muscle
  • Flexor - a muscle in an antagonistic pair which causes flexion bending at joint, decrease angle between bones at joint
  • Extensor - a muscle in an antagonistic pair which causes extension (straightening at joint increases angle between bones at joint.)
• • Abduction - pulls a structure away from the middle of the body e.g. moving the leg to the side