Muscles

• The three types of muscles are: skeletal, cardiac, and smooth
• Skeletal muscles are the most abundant muscle type
• Skeletal muscles are voluntary
• Mycoytes are muscle cells
• The myocytes of skeletal muscles are long and cylindrical
• Skeletal myocytes are multinucleated
• Sarcolemma are the plasma membrane of myocytes
• Sarcoplasma fills the myocyte like cytoplasm
• Myocytes are filled with myofibrils
• Myofibrils have alternating light I bands and dark A bands running through the skeletal myocyte that create the striated appearance seen under the microscope
• Z disks are found in the middle of light I bands
• Sarcomeres are the contractile unit of the myocyte; they run from one z band to the next
• Myofibrils are made of myofilaments, mainly composed of the proteins: actin and myosin

  • actin and myosin slide past each other during muscle activity to contract the muscle cells
• There are thick and thin areas of sarcomeres depending on the concentration of actin and myosin: thin areas contain only actin, thick have myosin and actin (in all areas but the M line)
• At the junctions of each A and I bands, the sarcolemma indents into the muscle cell, forming a transverse (T) tubule which run deep into the skeletal muscle fibre between cross channels/terminal cisterns of the sarcoplasmic reticulum
• Regions where the sarcoplasmic reticulum terminal cisterns border a T tubule on each side are called triads
• The sarcomoplasmic reticulum is the smooth endoplasmic reticulum of muscle cells
• Endomysium is made of areolar connective tissue (particularly, reticular fibres) that enclose each myocyte
• Perimysium is a loose connective tissue that wraps several myocytes together into a fasicle
• Epimysium is a dense irregular connective tissue that sheaths many fasicles into a muscle
• All three sheaths join to form tendons or aponeuroses, which either attach muscles to each other or indirectly to bones
• Aponeuroses are flat sheet like tendons
• The insertion is a muscles movable attachment point
• The origin is the muscles fixed attachment point
• Tendons perform several functions including:

  • providing durability (can span rough bony projections that would damage from delicate muscle tissue)
  • conserve space (smaller size allows more tendons to pass over joints that fleshy muscles can)
  • provide strength to muscles
  • provide a route of entry and exit for nerves and blood vessels that serve muscle fibres
• Larger more powerful muscles have more connective tissue than muscles involved in fine or delicate movements
• Voluntary skeletal muscles cells must be stimulated by motor neurons via nerve impulses
• The junction between an axon of a motor neuron, and a muscle fibre (motor plate) is called a neuromuscular junction
• Each motor neuron typically divides into many terminal branches as it approaches the muscle

  • each branch ends in an axon terminal that forms the neuromuscular junction with one muscle fibre
  • this means that one neuron can stimulate many myocytes
• Motor unit: a neuron and all the muscle fibre it stimulates

  • the neuron and muscle fibre don't touch, but are separated by a fluid filled synaptic cleft
• Axon terminals contain many mitochondria and vesicles containing acetylcholine (ACh)

  • when an action potential reaches the axon terminal, voltage gated calcium ion channels open, and calcium ions enter the axon terminal, causing ACh to be released through exocytosis, where is diffuses across the synaptic cleft and combines with receptors on the sarcolemma
  • when receptors bind to ACh, a change in sarcolemma permeability occurs, ion channels open briefly, depolarizing the sarcolemma, causing contraction of the muscle fibres
• H zones of myocytes have only thick filaments/myofibrils
• Myocyte Labels:

  A) Dark A band

  B) Light I band

  C) Muscle fiber

  D) nuclei

  E) sarcolemma

  F) mitochondria

  G) myofibril

  H) Z disc

  I) Z disc

  J) H zone

  K) Light I band

  L) Light I band

  M) Dark A band

  N) Z disc

  O) Z disc

  P) Thin (actin) filament

  Q) Thick (myosin) filament

  R) I band

  S) H zone

  T) A band

  U) Thin (actin) filament

  V) Thick (myosin) filament

  W) Dark A band

  X) Light I band

  Y) nucleus

  
• Skeletal muscle diagram:

  A) Light I band

  B) Dark A band

  C) Light I band

  D) Z disc

  E) Z disc

  F) H zone

  G) sarcolemma

  H) Triad

  I) T tubule

  J) Terminal cisterns of the SR

  K) Tubules of the SR

  L) myofibrils

  M) mitochondria

  N) sarcolemma

  O) myofibril

  
• Smooth muscle myocytes lack striations and are spindle shaped
• Each smooth myocyte has one nucleus
• Smooth muscles myocytes aren't arranged into sarcomeres which is why they lack striations
• Smooth muscles have a distinct endomysium and a distinctive but slim epimysium and perimysium
• Smooth muscles are involuntary and found primarily in the walls of hollow organs
• Calveolae are found in the walls of smooth muscle myocytes and help modulate contractions