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Anatomy (Subject) / Anatomy III (Lesson)

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  • Characteristics of muscle tissue 1. Contractibility  2. Extensibility  3. Excitability  4. Elasticity 
  • Components of a muscle fibre (Myocyte) - Nuclei  - Sarcolemma - Sarcoplasm - Myofibrils - Striations
  • Components of a myofibril - Thin (actin) filaments - Thick (myosin) filaments  - Sarcomeres (basic mechanical unit that makes muscles work)
  • How do sarcomeres work? 1. Relaxation: Actin loosely interlinks myosin in the middle 2. Contraction: actin fiercely interlinks myosin in the middle
  • Sorts of Connective tissue between/around muscles 1. Micro - interspersed between/around mucle fibers (cells) --> hold them together (in large muscles) 2. Macro - tendons/apaneuroses between muscle and bone - Fascia/septa between/aroud individual muscle --> compartimentation - Retinaculum - thick ending of fascia --Y retain structures in place 
  • Muscle compartments of the limbs - muscles in the limbs are contained in compartments seperated by connective tissue septa --> muscles in the same compartment = same function, in opposite departments = opposite functions
  • Muscle shapes (6) 1. Flat - parallel fibres --> abdominal wall 2. Pennate - feather like 3. Fusiforn - spindle shaped --> biceps 4. Convergent - attached to a broad area and converge to a single lendons --> pectoralis major 5. Quadrate - four equal sides --> retus abdominis  6. Circular (sphincter) - surround body openings (e.g. anal)
  • Muscle actions (4) 1. Prime mover (Agonist) - effects a particular movement 2. Antagonist - opposes the action of another muscle 3. Synergist - aids the movement of the agonist 4. Fixator - Immobilises bones, allowing other muscles to act from a stable base 
  • Classification of muscle tissue (3) 1. Smooth - non striated --> lines the tissue of walld of blood vessels, tubes, hallow organs (GI tract, urinary tract,...) 2. Cardiac - striated --> e.g. myocardium  3. Skeletal - striated --> e.g. biceps
  • Characteristics of skeletal muscle tissue - under conscious control --> innervated by Somatic Nervous System (SNS) - comprises muscles of the locomotor system and "visceral" muscles (e.g. tongue, diaphragm) - limited regenerating capacity --> differentiation of satellite cells into myoblasts
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  • Skeletal muscle and connective tissue 1. Endomysium - loose CT, surrounds individual myocytes  2. Perimysium -  dense CT, segregates muscle into bundles of myocytes (fascicles) 3. Epimysium - dense CT, sheath of the deep fascia
  • Skeletal muscle attachment to bone 1. Direct - collagen of the epi,peri- and endomysium merge with that of the bone matrix (Sharpy's fibres) 2. Indirect - attachment at "myotendinous junctions (tendons and muscle are interdigitated and tendons insert into bone matrix)
  • Neuromuscular junction and motor unit 1. Neuromuscular junction - contact between a motor neuron and a myofibre  2. Motor unit - myofibres innervated by a single motor neuron
  • Types of skeletal muscle fibres (3) 1. Type I fibres (red) - fatigue-resistant, postural muscles, long-lasting activity (marathon runners) 2. Type II fibres (intermediate) - fatigue-resistant, high-force output (marathon runners) 3. Type III fibres (white) - fatigable and short max. contractions (power lifters, sprinters)
  • Characteristics Cardiac muscle - only present in the heart - Involuntary, intrinsic spontaneous contraction --> ANS + endocrine system - some cells are conducting cells (Purkinje fibres) - no regenerating capacity - mononucleate  - intercalated discs --> connection between muscle cells to allow heart to act as a single organ (synactium)
  • Characteristics Smooth muscle - Involuntary --> ANS + endocrine system  - easy cell regeneration  - spindle shaped, often in groups/layers with similar orientation - single nucleus (centrally located, wrinkled) - cytoplasm rich in mitochondria, actin, myosin 
  • Cellular structure in muscle cells 1. Sarcoplasmic reticulum  - network of tubules and cisternae, store intracellular Ca2+ ions - aligned with sarcomere with 2 flattened sacs at regular intervals (terminal cisternae) 2. Transverse tubular system - Invaginations of sarcolemma at the junctions of A- and I bonds  - convey the AP from the sarcoplasmic reticulum by initiating the release of Ca2+ --> myofibril contraction 3. Sarcotubular system - sarcoplasmic reticulum + transverse tubular system - Muscle triad = transverse tubule and 2 terminal cisternae 
  • Microanatomy of striation (skeletal/cardiac muscle) 1. Z line - marks at the beginning and end of a sarcomere (passing through I band) 2. I band - actin/thin filaments only  3. A band - myosin/thick and actin/thin filaments 4. H zone - only thick/myosin filaments 5. M line - divides H zone and connects thick/myosin filaments together 
  • Cross bridge formation/cycling (Process) - sarcomeres shorten when myosin heads form crossbridges with actin molecules  1. Activation of Actin Sarcoplasmic reticulum releases Ca2+ which binds to troposin --> conformational change of troposin, moving tropomyosin away from the myosin binding site --> myosin heads can now bind actin by forming a crossbridge  2. The power stroke a) ATP binds to myosin head --> dissociation from actin b) ATP is hydrolysed into ADP + P and angle of mysin altered to "cocked" position --> activation of myosin head! c) Myosin head binds to actin, forming a crossbidge  d) Phosphate is released and myosin reverts to its original structure, pulling actin with it (Power stroke!) --> Cycle repeats!
  • Smooth muscle types (2) 1. Visceral/Single unit smooth muscle - widely distributed  - many cells act as one unit due to electrical coupling (gap junctions= --> e.g. circular smooth muscle GI tract (peristalsis) 2. Multi unit smooth muscle  - not common - each monocyte has its own nerve supply, behaving independently  --> e.g. piloerector muscles of the skin and iris muscles of the eye 
  • Smooth muscle contraction (mechanism) - not striated --> indicates different arrangement of myosin and actin - contractile apparatus contains myosin, actin, tropomyosin but no troponin --> replaced by calmodulin  1. Calcium signalling  - Ca2+ enters the cell from extracellular fluid --> release of Ca2+ from SR 2. MLCK activation - Intracellular Ca2+ binds to calmodulin --> activation of myosin light chain kinase (MLCK) 3. Myosin activation - MLCK phosphorylates the light chain of the myosin using ATP --> activation 4. Actin binding  - Activated myosin binds to actin --> Increase of muscle tension
  • Smooth muscle relaxation (process) 1. Dephosphorylation of myosin by the enzyme myosin light chain phosphotase (MLCP) 2. Removal of Ca2+ by... a)Pumped across the plasma membrane --> Ca2+ ATPase; Na+/Ca2+ ion exchanger  b) Actively pumped into intracellular stores 
  • Contractile characteristics (Smooth muscle) 1. Slow contraction and relaxation rate 2. Equivalent in tension/force --> number of crossbridges  3. Few mitochondria --> anaerobic ATP generation 4. Do not fatigue --> low energy expenditure because of low rate of cross bridge cycling --> don't recycle their cross bridges at the same rate as cardiac/skeletal muscle but leave myosin and actin together (tension hold and not relieved)

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