Muscle (2)

Muscle (2)

1. Muscle - bundle of fascicles

   • Epimysium- dense irregular conenctive tissue surrounding the perimysum + entire muscle
     • Perimysium- dense irregular connective tissue surrounding the fascicles
       • Fasicles

2. Fascicle- a bundle of myocytes

   • Endomysium- loose irregular connective tissue surrounding myocyte. contains the nerves and capillaries that supply the myocytes
     • thin basement membrane- between the myocyte and endomysium. this is thin, specialised sheet of connective tissue that surrounds the muscle fibres and blends with the endomysium
       • myocytes ( highly variable numbers depending on muscle type)

3. Myocyte/ myofibre- bindle of myofibrils

   • Sarcolemma - cell membrane that is highly conductive
     • sarcoplasm- cell cytoplasm, area between myofibrils ( contains lipids + glucagon + myoglobin
       • many myocyte nucleus
         • myofibrils - contractile organelles

4. Myofibrils - many sarcomeres

5. Deep Fascia and muscle compartments

   • From the outer surface of skin, right under is superficial fascia.
     • right below the superficial fascia is deep fascia - dense, connective tissue ( can be regular or irregular)
       • there are muscle compartments- compartment- group of muscle that are all innervated by the same nerve or have similar action roles
         • outer sleeve + walls of compartments made up of deep fascia
           • walls/ septa are referred to investing fascia ( inter muscular septa- wall of deep fascia near the muscle, interosseous membrane- fascia near bone)
             • where investing fascia comes into contact with bone, it fuses with the periosteum
               • usually, epimysium can move and glide under the deep fascia
                 • otherwise, deep fascia is part of muscle tendon and acts as an attachment point for the muscle

6. size and repair of a muscle fibre

   • Hypertrophy- increase in muscle size is due to increases in size of fibres (hypertrophy) rather than any increase in fibre numbers ( hyperplasia )
     • myocytes increase in diameter with more myofibrils packed into each cell
       • effect- increase in overall muscle size and strength but the same number of cells still contribute to contraction
         • factors leading to hypertrophy - heavy resistance training + use of anabolic steroids
           • anabolic steroids- variants of testosterone, they increase protein synthesis through their interactions with specific target tissues including skeletal muscle + bone
             • however anabolic steroids don't just effect those target tissues, they also lead to acne, hair loss, excess hair growth in inappropriate places, liver failure, shrivelled testes, infertility, increased susceptibility to coronary heart disease + extreme mood swings ( roid rage )
   • atrophy - decrease in the size of the myocytes
     • occurs when muscles are not used or stimulated by motor neurons for e.g. limb immobilised in a cast for a long time
       • can also occur as part of complex pathology in diseases such as AIDS, cancer, heart failure + diabetes
         • decrease in size of muscle is due to the myocytes decreasing in size ( less myofibrils) , as we age myocytes number can also reduce- hypoplasia
           • normal muscle loss begins around 20, accelerated at around age 50, by the time we are 80, 40% muscle mass has been lost
             • muscle loss is replaced by fat + connective tissue , some loss is due to individual fibre shrinkage- atrophy, some is due to fibre loss- hypoplasia
               • if atropy has not proceeded too far, it can be reversed

7. Satellite cells (myoblasts)

   • lie beside muscle fibres outside the sarcolemma but within the same basement membrane
     • because myocytes are large cells created from fusion of many myoblasts - syncytium- they are unable to divide through mitosis
       • satellite cells are the only ones that are able to divide through mitosis, so can divide and fuse with other satellite cells and myocytes to repair any damage
         • though muscle fibres are pretty much set in number form birth, satellite cells do have a limited ability to replace muscle fibres that die from old age or injury