Muscular system

• basic properties of muscle tissues:

  • excitability
  • contractility
  • extensibility
  • elasticity
• properties of muscle tissue: excitability

  • ability to receive and respond to stimulus
• properties of muscle tissue: contractility 


  ability to shorten and exert a pull on other structures
• properties of muscle tissue: extensibility 


  ability to be stretched without damage
• property of muscle tissues: elasticity 


  ability to return to original shape after being stretched
• 3 types of muscle tissue:

  • skeletal muscle
  • cardiac muscle
  • smooth muscle
• skeletal muscle:

  • most important muscle
  • each cell is a single muscle fibre
  • cells are multinucleated
  • multinucleated skeletal muscle fibres form by the fusion of myoblasts - takes place during foetal development
  • some stay outside the cell - Myosatellite cell
• skeletal muscle:

  • Myofibrils are responsible for skeletal muscle contraction
  • main protein content is myosin and actin
• skeletal muscle:

  A) sarcomere

  B) myosin filament

  C) actin filament

  D) h zone

  E) z disc

  F) A band

  G) I band

  H) M line

  I) sarcoplasmic reticulum

  
• Functions of skeletal muscle:

  skeletal movement = contractions pull on tendons and move bones

  posture and body position = tension in muscles maintains posture

  support = layers of muscle in abdominal wall and floor of pelvic cavity support visceral organs and shield from injury

  entries and exits = muscle at openings of digestive and urinary tracts

  body temp = contractions produce heat

  nutrient reserve = contractile proteins broken down and amino acids released into circulation for energy
• skeletal muscle:

  under neural control - we control when they contract
• can skeletal muscles regenerate?

  YES

  • Myosatellite cells normally mitotically quiescent
  • but can initiate proliferation in response to stress induced by injury
  • to mediate regeneration of muscle
• Hypertrophy:

  increase in individual muscle fiber size
• hyperplasia:

  • increase in number of muscle fibers
  • usually have smaller organs and find it hard to give birth naturally
• atrophy:

  decrease in size of muscle
• cardiac muscle:

  • cells have one nucleus, are branched, connected by intercalated discs
  • cells are smaller and have less sarcomeres than skeletal muscle cells
• cardiac muscle cell

  
• control of cardiac muscle:

  • involuntary
  • dont rely on nerve contraction
  • pacemaker cells establish a regular rate of contraction
  • pacemaker cells respond to hormones that modulate heart rate to control blood pressure
• can cardiac muscles regenerate?

  • NO
  • if myocardial cells die, they are replaced by fibrous non - contractile scar tissue
  • this is not elastic so cant contract
• smooth muscle:

  • cells have one nucleus, are small and spindle shaped
  • have single unit muscle and mult unit muscle
• single unit (visceral) smooth muscle:

  • cells jointed by gap junctions so the muscle contracts as a single unit
• multi unit smooth muscle:

  • no gap junctions - contraction is confined to the stimulated cell
• smooth muscle:

  • dont have organised sarcomeres
  • actin and myosin arranged in dense body
  • autonomic nerve fibres branch on top of smooth muscle cells - multiple varicosities along nerve fiber with vesicles containing neurotransmitters
• functions of smooth muscle:

  • regulates movement of food, urine and reproductive tract secretions
  • controls diameter of respirator passageways
  • regulated diameter of blood vessels
• control mechanism of smooth muscle:

  • involuntary but can be controlled by nervous system
  • triggers for contraction include: hormones, neural stimulation and local factors
  • a pacesetter cell can trigger action potentials and contractions
• can smooth cells regenerate?

  YES

  they can divide after injury
• muscle characteristics:

  
• Skeletal muscle contraction: initiation

  • master component = calcium
  • AP arrives at neuromuscular junction - depolarises the sarcolemma and T-tubules
  • ACh is released and binds to receptors
  • this opens sodium ion channels, leading to action potential in sarcolemma
  • action potential travels along t - tubules
  • triggers release of calcium ions, initiates contraction
  • sustained by ATP
• skeletal muscle contraction:

  • calcium ion levels increase in cytosol
  • calcium ion binds to troponin
  • troponin pulls tropomyosin away from actin - myosin binding site
  • myosin binds to actin and completes power stroke
  • actin filament moves
• skeletal muscle contraction: contractile cycle

  • ATP binds to myosin
  • myosin releases actin
  • myosin hydrolyses ATP, the energy is used to rotate the myosin head to the cocked position so binds weakly to actin - muscle contraction occurs
  • power stroke occurs
  • myosin releases ADP
  • cycle start again
• skeletal muscle relaxation:

  • contraction stops when signalling from motor neuron ends
  • repolarises sarcolemma and t - tubules
  • closes voltage gated calcium channels in sarcoplasmic reticulum
  • calcium ions pumped back into sarcoplasmic reticulum
  • can also stop contracting if it runs out ATP
• Cardiac muscle contraction:

  • same as skeletal muscle
  • action potential triggering contraction differs
  • depolarizes - plateaus - then repolarizes
• smooth muscle contraction:

  • signal for contraction = cytosolic calcium
  • cross bridge formation regulated by the regulatory protein calmodulin
• smooth muscle contraction:

  • calcium ions released from sarcoplasmic reticulum and enter cell
  • calcium ions bind to calmodulin (CaM)
  • activates myosin light chain kinase (MLCK)
  • phosphorylates light chains in myosin heads & increase myosin ATPase activity
  • myosin cross bridges slide along actin and create muscle tension
• relaxation of smooth muscle:

  • calcium ions leave cytosol
  • calcium ion unbinds from calmodulin and myosin light chain kinase activity decreases
  • myosin phosphatase removes phosphate from myosin light chains
  • decreases ATPase activity
  • decreased muscle tension
• contraction in smooth muscle:

  • depolarisation is due to entry of calcium ions rather than sodium ions
  • can take place after an action potential, a subthreshold graded potential, or with no change in membrane potential
• smooth muscle cells:

  • some contain stretch activated calcium ion channels
  • open when pressure distorts cell membrane
  • resultant contraction is called myogenic contraction
• smooth muscle membrane potentials:

  • slow wave potentials = fire action potentials when they reach threshold
  • pacemaker potentials = always depolarize threshold