Skeletal Muscles: Anatomy & Excitation

• Skeletal Muscle - Anatomy and Excitation

  
• Muscle is one of the four basic types of tissue. Muscle tissue is able to contract and produce mechanical force.

  
• Subtypes of muscle tissue


  • Striated muscle
  • Smooth muscle

  
• Each skeletal muscle is connected to the skeletal system via tendons (or aponeuroses). Skeletal muscles are often organized within layers of fasciae.

  
• Skeletal muscles


  • Organized as bundles of fascicles, each of which contains several muscle fibres (cells)
  • Each individual muscle is an organ that consists of several tissue types

  
• Each myofiber (aka muscle fibre, muscle cell) contains myofibrils and is associated with supporting cells and tissues.

  
• Myofibre


  A single cell that contains many nuclei and stretches from the muscle origin to its insertion (tendon to tendon)

  
• Myofibres are produced by the fusion of many myoblasts during development, and can be repaired in adulthood by myosatellite cells.
• Motor unit


  The combination of motor neuron and muscle cell(s)

  
• Each mature myofiber has one neuromuscular junction and is part of a motor unit.
• Neuromuscular junction (NMJ)


  Made up of the axon terminal, the synaptic cleft and the motor end plate
• Organelles in myofibres


  • Sarcoplasmic reticulum
  • Myofibrils
  • Cell membrane (Plasmalemma)
  • Sarcoplasm
  • Endoplasmic reticulum

  
• Excitation


  The sarcolemma generating an electrical signal that precedes contraction
• Contraction


  Myofibres/myofibrils becoming shorter or producing tension
• Excitation-contraction (E-C) coupling


  The mechanism(s) by which excitation triggers contraction
• Excitation of a muscle cell at the NMJ

  1. Electrical signal

  2. Chemical signal from the motor neuron

  3. Electrical excitation of the NMJ

  4. Propagation of the electrical signal
• Contraction of a skeletal muscle fibre

  Only triggered when a command is received from the nervous system, activating the motor neuron, which excites the myofibre
• Transmembrane potential


  All cells have a transmembrane potential. 'Excitable cells' can rapidly vary their transmembrane potential to send and receive signals
• At rest, sodium CANNOT move through ion channels in the sarcolemma (though K+ can).
• Excitation at the NMJ

  Motor neurons release a chemical neurotransmitter, which causes the opening of ion channels (receptors) that allow sodium into the myofibre
• Acetylcholine (ACh)


  Neurotransmitter released at the NMJ
• Acetylcholine receptors (AChRs)


  Provide a path for sodium ions to cross the cell membrane
• If sodium (Na+) is able to cross the cell membrane of a cell

  It will tend to move inward, because it will diffuse down its concentration gradient (high outside, low inside)
• Excitation of a muscle cell

  1. Electrical signal

  2. Release of neurotransmitter

  3. Neurotransmitter binding and NMJ excitation

  4. Spread of excitation

  5. Enzymatic removal of neurotransmitter
• Curare (drug)


  Prevents ACh binding to ACh receptors, leading to lower excitation and contraction
• Novichok (nerve agent)


  Blocks the enzyme that breaks down ACh in the synaptic cleft, leading to higher excitation and contraction
• Excitation propagation in the myofibre

  1. Depolarization (excitation) spreads near the NMJ

  2. Depolarization can cause voltage-gated ion channels permeable to sodium to open, creating an 'action potential'

  3. Action potential propagates through the entire length of the myofibre's plasma membrane
• The spread of an action potential is very rapid (many m/s).
• 1. tubules


  Bring the plasma membrane's action potential into the vicinity of the sarcoplasmic reticulum, triggering calcium release
• Two membrane proteins physically and physiologically connect electrical signals in the sarcolemma to calcium channels in the SR.
• Electrical muscle stimulation (EMS)


  Used to help enhance muscle strength and increase muscle mass, and to maintain or build muscle mass even when physical mobility is limited
• Skeletal muscle cells, or myofibres, are multinucleate cells that span the length of a muscle and which contain specialized organelles that support contraction.

  
• Skeletal muscles are triggered to contract only through electrical and chemical signals that begin in nervous tissue.

  
• A muscle action potential spreads throughout the length of the myofibre, and triggers the release of calcium ions from the sarcoplasmic reticulum.