C2.2 Neural Signalling

• electron transport chain

  1. NADH carries hydrogen/e- & 1P from the reactions in the matrix to the inner membrane space. 2. Hydrogen protons are pumped from the inner membrane to inter membrane space 3. Protons flow back into the mitochondrial matrix through the ATP synthase producing ATP
• neurons

  • Transmits the message
  • Synapses modulate the message
• Neurons


  Specially adapted cells that can carry electrical impulses (nerve impulses)
• Nerve impulse


  Electrical signals passed between two cells
• Nerve impulses can go from

  • Neuron to neuron
  • Neuron to muscle
  • Neuron to gland
• Transmission along neurons


  Transmission between neurons
• Functions of neuron parts

  • Dendrites: Receive signals from other cells
  • Cell body: Organizes and keeps the cell functional
  • Cell membrane: Protects the cell
  • Nucleus: Contains the genetic material and controls the entire neuron
  • Axon hillock: Generates impulse in the neuron
  • Axon: Transfers signals to other cells and organs
  • Node of Ranvier: Allows diffusion of ions
  • Myelin sheath: Increases the speed of the signal
  • Schwann cell: Produces the myelin sheath
  • Axon terminal: Forms junctions with other cells
• The cell body contains the nucleus and other organelles, coordinating the neuron's metabolic activities
• Transmission across a synapse

  1. Pump: Active transport

  2. Facilitated diffusion

  3. Exocytosis

  4. Diffusion

  5. Acetylcholine also helped to open it insulin or hormones

  6. Facilitated door - gated, - facilitated diffusion

  7. Action potential flip-flop
• In typical sensory neurons, dendrites receive sensory information, the cell body integrates signals, and the axon transmits signals to the central nervous system
• In typical motor neurons, dendrites receive signals from other neurons, the cell body integrates these signals, and the axon transmits signals away from the central nervous system to muscles or glands
• Membrane potential


  The difference in electric charge between the interior and exterior of a cell membrane
• Resting potential


  The membrane potential of a neuron when it is not being stimulated, with the inside of the neuron being relatively negative (-70mV)
• Resting potential gained by the sodium potassium pump

  1. The Na+/K+ pump actively pumps out 3 Na+ ions and pumps in 2K+ ions, transporting against the natural concentration gradient and requiring ATP

  2. More positive ions are pumped out than in, so the inside of the cell is negative compared to the outside
• Mechanisms for resting potential

  • Sodium-potassium pump: actively transports sodium ions out of the cell and potassium ions into the cell
  • Potassium leak channels: Allow some sodium ions to leak into the cell
• Voltage of resting potential is around -70 millivolts (mV)
• Steps of sodium-potassium pump action

  1. Binding of cytoplasmic sodium ions

  2. Phosphorylation by ATP

  3. Release of sodium ions outside the cell

  4. Binding of extracellular potassium ions

  5. Dephosphorylation

  6. Release of potassium ions inside the cell
• Electrical potential across a membrane

  • Volt - measure from one point to another
  • Potential energy
  • 'Charged battery'
  • Battery = ion
• Nerve impulses


  The electrical signal that travels along the length of a neuron
• Action potential


  A rapid change in the membrane potential that allows for the transmission of nerve impulses, arising from the transport of positively charged ions
• Neurons transmit the message, synapses modulate
• The sodium potassium pump needs active transport
• Active translocation of the sodium potassium pump

  Pumps 3 sodium ions out of the cell and 2 potassium ions into the cell, requiring ATP
• Depolarization


  Membrane potential goes from negative to positive
• Repolarization


  Membrane potential goes from positive back to negative
• Steps of action potential

  1. Voltage-gated sodium ion channels open

  2. Sodium ions diffuse into the cell (facilitated diffusion)

  3. Depolarization - to +

  4. Voltage gated sodium ion channels close, and voltage gated potassium ions channels open

  5. Potassium ions diffuse out of the cell (facilitated diffusion)

  6. Repolarization + to -

  7. Sodium-potassium pump re-establishes resting potential by actively pumping sodium ions out and potassium ions in
• Action potential is self-propagating, with depolarization in one part triggering depolarization in the next part due to the opening of voltage gated channels
• Ways to speed up nerve impulses

  • Increase diameter: Decreases resistance to flow
  • Myelination: Results in myelin sheath, allowing saltatory conduction
• Saltatory conduction


  Myelinated axon, with the nodes of Ranvier between Schwann cells, allows much faster conduction by the impulse jumping from node to node
• Effects of myelination


  • Increases speed from 1m/s to 100m/s
  • Decreases energy requirements by needing less ATP to power the sodium potassium pump
• Axon diameter


  Larger diameter correlates with faster action potential
• Myelinated fibres conduct faster than unmyelinated fibres
• Larger animals generally have faster conduction speeds
• Synapse


  A junction between two neurons or between a neuron and an effector cell, where signals are passed by neurotransmitters across a 20nm gap
• An electrical current cannot cross the space between two nerve cells (the synapse), so it must be converted into a chemical signal
• Synaptic cleft


  The gap between the presynaptic and postsynaptic membranes
• Effector


  A muscle or gland that responds to stimulation
• Signal can only pass from the presynaptic neuron to the postsynaptic neuron or effector cell
• Role of neurotransmitters


  Transmit signals across synapses, and can be either excitatory or inhibitory
• Release of neurotransmitters from a presynaptic membrane

  1. Depolarization of the presynaptic membrane leads to the opening of calcium channels

  2. Calcium ions influx triggers exocytosis of neurotransmitter vesicles

  3. Neurotransmitters diffuse across the synaptic cleft