BIOMED SCIE Lecture 16 neuroscenze 1

• Neuron Structure, Resting Membrane Potential and Action Potential
• Also involved in cell communication and neurodegeneration
• Action Potential Initiation zone


  The cone-shaped base of an axon called the axon hillock, also known as the initial segment where Action potentials are initiated
• Neuron Structure and Function


  • Most of a neuron's organelles are in the cell body
  • Most neurons have dendrites, highly branched extensions that receive signals from other neurons
  • The axon is typically a much longer extension that transmits signals to other cells at synapses
• Myelin sheaths


  • Improve the efficiency of the axon by increasing the conduction velocity of the action potential
• Other Glial cells: Astrocytes and Microglia
• Action Potentials


  All or none: Electrical potential travels along the axon to the synaptic terminal
• Synaptic transmission


  Caused by the Action potential reaching the synaptic terminal
• Neurons form Networks


  • Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)
  • Most neurons are nourished or insulated by cells called glia
• Resting membrane potential (RMP)


  Required for normal function like muscle contraction, synaptic transmission, pacemaker activity, and cell to cell signalling
• Membrane Potential


  The voltage (difference in electrical charge) across the plasma membrane of a cell
• Resting potential


  The membrane potential of a neuron not sending signals
• Changes in membrane potential


  Act as signals, transmitting and processing information
• Neuronal Membrane


  • Under resting conditions the neuronal membrane is highly permeable to K+ ions with only a small permeability to Na+
  • The Resting membrane potential of the neurone is approx. -65 to -70mV
• Distribution of ions


  • Extracellular: Mainly Sodium and Chloride ions
  • Intracellular: Potassium and non-diffusible molecules e.g. proteins with negatively charged side chains and phosphate compounds
• Formation of the Resting Potential


  1. Sodium-potassium ATPase pumps use the energy of ATP to maintain K+ and Na+ gradients across the plasma membrane (pumps 3 Na+ out for 2 K+ in) producing approx -5mV of the membrane potential

  2. Open potassium channels in the neuronal membrane allow K+ ions to freely move out of the cell (Very little permeability to sodium ions)

  3. K+ can diffuse out of the cell down its concentration gradient
• Equilibrium potential (Eion)


  The membrane voltage for a particular ion at equilibrium, calculated using the Nernst equation
• The equilibrium potential of K+ (EK) is negative (Approx.-90mV), while the equilibrium potential of Na+ (ENa) is positive (+62mV)
• The Resting membrane potential (-65 to -70mV) is closer to the equilibrium potential for K+ (-90mV) as the membrane is highly permeable to K+ with some permeability to Na+ and Cl-
• Electrochemical Equilibrium


  The membrane potential is due to the balance of electrical driving force and chemical driving force
• Action potential


  All or None. It is NOT a graded potential (like a synaptic potential). Once it is initiated it will spread along the Axon and will NOT decrease in amplitude
• Action potential initiation


  1. Strong depolarizing stimulus

  2. Threshold (Approx -55mV) at which voltage-gated sodium channels open, allowing Na+ to enter the neuron

  3. Depolarizing event (e.g. synaptic response) causes Na+ to enter the cell down its chemical and electrical gradient, causing the cell to depolarise

  4. Explosive depolarization, potential reaches 0mV

  5. Na+ inactivation gate begins to close (to stop Na+ entering the neuron)

  6. K+ channels open, K+ begins to exit the neuron

  7. Peak of action potential, potential reversed (Approx +30mV)

  8. Repolarization begins as K+ leaves the cell

  9. Na+ inactivation gate opens, Na+ activation gate closes

  10. Action potential complete, after hyperpolarization begins due to K+ channels remaining open

  11. After hyperpolarization complete, return to resting potential
• two major cell types in the brain

  • neurons: Process information, sense environmental changes, communicate changes to other neurons, command body response.
  • Glial cells: insulate, support and nourish neurons
• For parts of neurons

  Dendrites, cell body, axon, axon terminal
• "soma " or cell body of a neuron

  • Nissl bodies: stacks of rough endoplasmic reticulum( protein synthesis).
  • prominent Golgi apparatus: packages material into vesicles for transport.
  • many mitochondria.
  • Cytoskeletal elements : Neurofilaments, thin rod like. Microtubules, larger cylindric like structure.
• The axon


  conducts information from cell body to the synapse
• The Axon

  • Arise from the cell body
  • May be ensheathed or bare
• The axon


  Axon hillock: highly specialised region where action potential is generated
• Myelinating Glial Cells

  Role:

  • physical support/separation
  • Produce myelin
  • Oligodendrocytes
  • Schwann Cells
• In a mammalian neuron at resting potential, the concentration ofK+ is highest inside the cell, while the concentration of Na+ ishighest outside the cell• Large negatively charged proteins are also trapped inside thecell.
• Resting potential can be modeled by an artificialmembrane that separates two chambers– The concentration of KCl is higher in the innerchamber and lower in the outer chamber– K+ diffuses down its gradient to the outerchamber– Negative charge (Cl–) builds up in the innerchamber
• At equilibrium, both the electrical and chemical gradients are balanced
• The membrane potential is due to the balance of• electrical driving force and• chemical driving force.This is the Electrochemical Equilibrium
• Na+ enters the cell down its chemical and electricalgradient causing the cell to depolarise
• Sodium enters the cell down its concentration and electrical gradient
• Repolarisation is caused as K+ leaves the cell
• Na+ inactivation gate opens;Na+ activation gate closes
• After hyperpolarization is complete; return to resting potential