Topic 1-neurons

• Neuron: A nerve cell that carries electrical impulses around the body.
• Structure of a neuron:

  1. Dendrite (input)
  2. Axon hillock (integrate)
  3. Cell body
  4. Axon (output)
  5. Synapse
• Synapse: the gap between neurones that allows the transmission of nerve impulses
• Membrane potential: The difference in electrical potential between the inside and outside of a cell.
• Ion channel: passive transport, selective permeability

  Ion pumps: active transport, require energy (ATP)
• Nernst equation: equilibrium potential for ion

  R = universal gas constant

  F = Faraday constant

  T = temperature

  z = valence

  ionO = outside concentration of ion

  ionI = inside concentration of ion
• Sodium/potassium pump:

  • for 1 molecule of ATP, 2 K+ in, 3 Na+ out
  • results in a concentration gradient of greater Na+ outside and K+ inside
  • higher electrical potential outside
• Neuronal signaling steps:

  1. Initiation of action potential (axon hillock)
  2. Propagation of action potential (axon)
  3. Synaptic transmission (synapse)
  4. Synaptic integration (axon hillock)
• Hyperpolarization: the resting membrane potential of a neuron is more negative than the threshold potential
• Inhibitory postsynaptic potentials: increase membrane potential
• Depolarization: The process of the membrane becoming more positive, causing the sodium channels to open and the potassium channels to close
• Excitatory postsynaptic potentials: A type of postsynaptic potential that is generated by the action potentials of an excitatory neuron
• Action potential: rapid depolarization and repolarization of cell

  • occurs at threshold potential
  • all or none action
  • refractory period
• Voltage-gated ion channels: open and close in response to transmembrane voltage changes
• Action potential events:

  1. at the threshold, voltage-gated Na+ channels open and sodium ions flow into the cell
  2. the opening of Na+ channels increases depolarization
  3. voltage-gated K+ channels open and potassium ions flow out of the cell
  4. Na+ channels close while K+ channels are still open during hyperpolarization
  5. K+ channels close when the membrane is hyperpolarized and the membrane potential returns to a steady state
• Absolute refractory period: The time interval between the initiation of an action potential and the next action potential.
• Relative refractory period: The time taken for the membrane potential to recover after a stimulus
• Hodgkin-Huxley model: A model of the action potential that shows the relationship between the voltage across the membrane and the rate of ion flow across the membrane
• Electrotonic conduction: the passive flow of a change in electric potential along a nerve or muscle membrane

  • passive, relatively fast, exponentially attenuating
• Self-regenerating propagation: active propagation of an action potential, travels long distances, relatively slow
• Nodes of Ranvier: Gaps between myelinated axons in the CNS; helps to propagate an action potential down an axon
• Myelin: A fatty substance that insulates axons and speeds up the transmission of nerve impulses
• Speed of propagation depends on:

  • axon diameter
  • myelination
  • temperatue
• Synaptic transmission: passing the signal from one cell to the next

  • neurotransmitter released from presynaptic cell
  • received by receptors on postsynaptic cell
  • ion channels open on postsynaptic cell
• Postsynaptic potentials: the electrical signal that is generated at the end of a neuron after a synaptic connection has been made

  • depends on neurotransmitters from presynaptic cell
  • receptors of the postsynaptic cell
  • EPSP or IPSP
  • Fast or slow
• Inhibitory postsynaptic potential: A type of EPSP that is caused by a decrease in the probability of firing
• Neurotransmitters: chemicals that are released from neurons and diffuse across the synapse

  • major CNS neurotransmitters:
  • Glutamate (primary excitatory)
  • GABA (primary inhibitory)
• Neurotransmitter characteristics:

  • synthesized by and localized within presynaptic neurons, stored in terminal bouton
  • released by presynaptic neurons when action potentials occur
  • postsynaptic neurons contain receptors specific to it
  • a mechanism exists to remove it after release
• Neurotransmitter reuptake: The process by which neurotransmitters are removed from the synaptic cleft. Can be:

  • active reuptake
  • enzymatic breakdown
  • diffusion
• Ionotropic receptors: Receptors that respond to changes in the ionic composition of the extracellular fluid (fast EPSP)
• Metabotropic receptors: embedded in cell membrane; when neurotransmitter is detected, initiates an extended cascade of molecular processes that eventually leads to ion channels opening
• Nicotinic acetylcholine receptor-channel: unlocked by two ACh molecules, can also be unlocked by nicotine
• Electrical transmission: not always chemical synapses

  • Gap junction:
  • cell membrane of presynaptic neuron and postsynaptic neuron are flush against each other
  • open ion channels form a tunnel
  • change in membrane potential conducts straight through open channels into postsynaptic cell
• Strength of PSP at the trigger zone at a certain time depends on:

  1. strength of the PSP
  2. timecourse of PSP
  3. distance from the synapse
  4. time since action potential
• Summation of PSPs:

  • trigger zone starts at the resting membrane potential
  • EPSPs add to membrane potential
  • IPSPs subtract from membrane potential
  • an action potential fires if the sum exceeds the threshold potential
• Types of PSP summation:

  1. Spatial summation (different locations)
  2. Temporal summation (different times)
• Artificial neural network: A network of interconnected nodes that mimics the structure of the brain.

  • Applications:
  • understanding mind and brain function
  • artificial intelligence
• Brainbow: A technique that uses fluorescent dyes to label different parts of the brain.
• Green fluorescent protein:

  • originally isolated from a jellyfish
  • gene for GFP inserted into DNA near the gene for a target protein
  • when the target protein is expressed, so is GFP
• Brainbow basics:

  • a single brainbow transgene expresses red, cyan, or yellow
  • with multiple brainbow transgenes more colours can be expressed