membrane potential

• structure of a neuron

  • Dendrites receive information
  • axon conducts outgoing information
  • Terminal knobs are where impulses are transmitted to the target cell
  • Myelin sheath help conduct the electrical information quicker

  
• Resting potential

  Membrane potential/voltage = -70 V

  What contributes to the resting membrane potential?

  1. Na+/K+ ATPase pump maintains the electrical difference by keeping the inside of the cell more negative than the outside (pumps 3 Na+ out and 2 K+ in)
  2. K+ ions are the charged substance with the most permeability in a resting nerve cell
  3. Flow out through potassium leak channels (although they are not constantly flowing out and are based on an equilibrium) which maintains the inside being more negative
• Stages of an action potential

  1. Resting potential
  2. Depolarization: stimulus causes Na+ channels to open and allow Na+ to diffuse into the cell. Threshold value of -50mV must be reached for Na+ diffuse in large amounts, which allows the membrane potential to increase to +40mV. Sodium channels spontaneously close after 1ms.
  3. Repolarization: depolarization triggers the opening of voltage-gated K+ channels and K+ can leave the cell. Na+ gated channels close. Membrane potential goes back to negative (-80mV) but more negative than the membrane potential since K+ channels are slow to close.
• Although the voltage changes during the action potential, the Na+ and K+ concentration gradients are barely affected since very few Na+ and K+ have to move to make this diference
• Propagation of an action potential

  Nerve impulse: action potential propagated along a neuron by triggering action potentials in adjacent portions of the membrane.
• • Continuous conduction (unmyelinated axons): flow of current causes the membrane region just ahead to become depolarized.
  • Action potential is propagated without loss of intensity
  • portion of membrane that just experienced an action potential will be in a brief refractory period
  • Saltatory conduction (myelinated axons): action doesn’t have depolarize every part of the region and can just skip to the next node of ranvier. Myelin prevents ions from moving across the membrane.
  • Nodes of Ranvier: unmyelinated region where most Na+ and K+ channels are found
• Synaptic transmission

  • Synapse: specialized junction of neuron with its target cell
  • Presynaptic cell: conducts the impulse towards a synapse
  • Synaptic vesicles: storage for neurotransmitters in the terminal knob of axons
  • Neurotransmitters: chemical that binds to the postsynaptic cell
  • Synaptic cleft: space that separates the two cells
  • Post-synaptic cell: receives the impulse
• Step 1 in synaptic transmission

  • Nerve impulse causes depolarization and triggers the voltage-gated Ca2+ channels to open in the presynaptic cell. Ca2+ diffuses into the presynaptic cell.
• Step 2 in synaptic transmission

  • Increased Ca2+ triggers synaptic vesicles to fuse with the plasma membrane, thus allowing neurotransmitters to be transported out of presynaptic cell and released into cleft to bind to receptors.
  • Excitatory neurotransmitter: Binding triggers Na+ channels to open and if enough Na+ enters
  • Inhibitory: binding triggers influx of Cl- ions which decreases voltage and makes it harder to reach threshold.
• Step 3 in synaptic transmission

  After being released from vesicles, neurotransmitters have a very short half-life.

  • Enzymes destroy the neurotransmitter in the synaptic cleft
  • Reuptake of neurotransmitter into the presynaptic cell