3.3

• Motor neuron


  Class of neurons that control muscle

  
• Alpha motor neurons


  • Allow for fast control of skeletal muscle
  • Innervate skeletal muscle fibres with large myelinated (Aa) nerves

  
• Motor unit


  A single motor neuron and the group of muscle fibres it innervates

  
• Neuromuscular junction


  Synapse between motor neuron and skeletal muscle

  
• Neuromuscular junction


  • Designed to maximise the chances of producing a post-synaptic (muscle) response
  • Motor axon terminal is enlarged and lies in a groove on the surface of the muscle fibre
  • Separated by a synaptic cleft, with no direct contact between nerve terminal and motor end plate
  • Motor end plate on post-synaptic side is highly folded (junctional folds)
  • Axon terminal contains thousands of vesicles that store the neurotransmitter Acetylcholine (ACh)

  
• Criteria for chemical transmission

  • Synthesis of neurotransmitter in pre-synaptic nerve terminals
  • Neurotransmitter stored in secretory vesicles
  • Regulated release of neurotransmitter into synaptic cleft
  • Specific receptors on post-synaptic membrane
  • Specific post-synaptic means for termination of neurotransmitter action

  
• Acetylcholine (ACh)


  The neurotransmitter used at the neuromuscular junction

  
• Life cycle of Acetylcholine (ACh)

  1. Precursor Acetyl-CoA produced in mitochondria and released into cytoplasm

  2. Combines with Choline to produce ACh, which is then stored in vesicles

  3. Fusion of vesicles with terminal membrane releases ACh into synaptic cleft

  4. ACh in synaptic cleft is broken down by enzyme to release Acetate and Choline

  5. Choline is re-absorbed by pre-synaptic terminal

  
• Depolarization of the motor neuron terminal triggers Ca++ entry
• Action potential traveling down the axon flows electrotonically into the terminal
• Depolarisation of the motor neuron terminal

  Depolarization triggers Ca++ entry
• The whole process of chemical signalling at the NMJ via transmitter release is to allow transmission of the command from the motor neuron to muscle fibre, to cause the muscle fibre to contract

  
• There has to be an Action Potential (or more normally, a wave of Action Potentials) traveling down the axon to the terminal
• The current from the Action Potential flows electrotonically into the terminal
• The terminal can't produce an Action Potential because it doesn't contain the voltage-gated Na++ channels needed to cause the depolarisation phase of the Action Potential
• The depolarisation caused by the electrotonic flow of current into the terminal does not result in an Action Potential
• The depolarisation of the terminal by the flow of current opens voltage-gated Ca++ channels
• Calcium


  • Plays a critical role in vesicle docking
• Intracellular Ca++ (0.1 mM) is very low compared to extracellular Ca++ (2 mM) and the amount required intracellularly for neurotransmitter release is 50-100 mM
• Intracellular [Ca++] must increase about 500-1000 fold for neurotransmitter release to occur

  
• Origin of Calcium for vesicle fusion

  1. Extracellular sources: Depolarization opens voltage-gated Ca++ channels allowing influx

  2. Intracellular sources: Depolarization activates G-protein coupled receptor, activating Phosphoinositol-Phospholipase C, releasing IP3 to release Ca++ from ER
• Active zones


  • Maximise Calcium interaction with docked vesicles
• The entry of Ca++ into the terminal is an explosive but brief event (a couple of milliseconds)
• Ca++ can diffuse only 850Å at most and so only vesicles in this proximity can be influenced by Ca++
• The terminal has to be organised to optimise the use of the Ca++ that floods the terminal
• Organisation of the terminal

  1. Vesicles docked at Active Zones

  2. Voltage-gated Ca++ channels concentrated at Active Zones
• The time between Ca++ influx and exocytosis of transmitter in nerve terminals can be very short (0.5 -1.0 msec at the Neuro Muscular Junction; 200 msec in the squid axon; 60 msec in neurons in the Central Nervous System)
• The efficient organisation results in fusion of many vesicles with the terminal membrane to release large amounts of transmitter
• Proteins involved in vesicle fusion

  • Synaptogamin and Synaptobrevin on the vesicle membrane

  SNARE proteins (SNAP-25 and Syntaxin) in the terminal membrane

  Process involves ATP
• Botulinum toxin (BoTox) degrades the SNAP-25 protein while tetanus toxin attacks Synaptobrevin, preventing synaptic vesicles from fusing with the cell membrane and releasing their ACh to activate the underlying muscle
• Vesicle membrane is recycled after fusion with the terminal membrane
• Recycling of vesicle membrane

  Bulk Endocytosis: large volumes of external material endocytosed (dominant during prolonged/intense neuronal activity)

  Clathrin-mediated Endocytosis: selective recycling (dominant during low/mild neuronal activity)
• Clathrin-mediated Endocytosis


  Vesicle membrane extruded of proteins, leaving lipid bilayer

  Membrane coated with Clathrin triskelia

  Membrane pinched off from terminal membrane using GTP

  Clathrin lattice assembles around vesicle membrane
• Unlike exocytosis, endocytosis of the vesicle membrane takes place away from the active zone
• Life cycle of a vesicle

  Import of neurotransmitter into vesicle using H+ pump and secondary active transport

  2. Movement of filled vesicle to active zone

  3. Docking at active zone plasma membrane

  4. Fusion and exocytosis triggered by Ca++ increase

  5. Clathrin-coated vesicle pinched off from plasma membrane

  6. Endocytosis of clathrin-coated vesicles for reuse

  
• When AcetylCholine is released from the motor neuron terminal into the synaptic cleft, it has four potential fates: binding to receptors, binding to enzymes that destroy it, diffusing away, or being re-uptaken by the nerve terminal

  
• The structure of the Neuromuscular Junction ensures that option 3 (diffusing away) is the least likely
• Nicotinic AcetylCholine Receptors


  Ionotropic receptors, chemically-gated

  
• Muscarinic AcetylCholine Receptors


  Metabotropic receptors, G-protein coupled

  
• Nicotinic AcetylCholine receptors


  One of the two major classes of AcetylCholine receptors in the body