Muscles

• There are 3 types of muscle fibre/cell

  • smooth muscle
  • cardiac muscle
  • skeletal/striated muscle
• smooth muscle fibres have tapered ends and are not striated. They are found in the trachea, blood vessels (not capillaries) and the small intestine. They are controlled by the autonomic nervous system.
• In the gut wall, smooth muscle is arranged in a circular layer and longitudinal layer. The circular layer contracts to allow segmentation to occur, which allows for greater mixing of food with digestive juices. The longitudinal layer allows for peristalsis.
• Cardiac muscle cells are branched and striated. They form cardiac tissue that makes up the heart. Cardiac muscle cells contract rhythmically without stimulation from nerves or hormones. This is called automaticity. It is also under control of the autonomic nervous system.
• Branches of cardiac muscles are called cross bridges and are present to ensure that electrical stimulation spread evenly across the walls of the heart chamber. Its arrangement also ensures that the contraction is a squeezing action rather than one-dimensional.
• Cardiac cells are joined by intercalated disks, which are specialised cell surface membranes fused to produce gap junctions that allow free diffusion of ions between cells so action potentials can easily pass through.
• Many myofibrils make up a muscle fibre/cell. Lots of muscle fibres surrounded by connective tissue make up a bundle of muscle fibres. Lots of bundles of muscle fibres make up a muscle.
• At the neuromuscular junction

  1. an action potential arrives at the end of the axon causing calcium ion channels to open, allowing them to flood in
  2. This triggers vesicles of ACh to fuse with membrane
  3. Ach molecules diffuse across gap and bind to receptors on sarcolemma
  4. Sodium ion channels open causing it to flood in and cause depolarisation
  5. wave of depolarisation passes across sarcolemma and down transverse tubules
  6. triggers sarcoplasmic reticulum to release calcium ions
• A-band contains thick filaments and some overlapping thin filaments and are dark (anisotropic)
• I-bands contain thin filaments only (isotropic)
• The H-zone contains thick filaments only and is where the M-line is found, which is the middle of the sarcomere
• the Z-line is found in the middle of the I-band and one sarcomere is marked from Z-line to Z-line
• Thick filaments are myosin filaments and thin filaments are actin filaments
• Each actin filaments consists of 2 actin subunits twisted around each other. A molecule of tropomyosin is wound around the two and troponin is bound to that in 3 subunits
• Function of each troponin:

  1. binds to actin
  2. binds to tropomyosin
  3. binds to calcium ions
• When calcium ions are present, they bind to troponin which causes shape to alter and tropomyosin to move and expose binding sited on actin
• 1. cross bridges form between myosin heads and binding sites of actin
  2. the myosin head tilts backwards and causes the thin filaments to slide past the myosin. This is the power stroke. During this ADP and Pi are released from the myosin head
  3. the cross bridge is broken and a new ATP attaches to myosin
  4. the head returns to its original position and swings forwards as ATP is hydrolysed (the head can now form a new cross bridge)
• ATP is needed to break the cross bridge after the power stroke. Myosin heads contain ATPase which allow the head to return to its original position when ATP is hydrolysed.
• When nervous stimulation stops, calcium ions are actively transported back into sarcoplasmic reticulum and the muscle can relax
• The sliding filament theory explains how muscles contract by shortening