Last lecture

Electrical properties of neurons

• Charges come from ions
  • Excess of positive charge outside, negative inside
  • $V_m$ membrane potential is -65 mV.
• Can open up a channel that allows ions to flow—only Na+
  • Influx of sodium in the cell, increasing $V_m$.
  • Flows into due to the diffusion current due to concentration
  • Drift current: electrical driving force
• Charges align and create a capacitor

Action potentials

• Neurons are excitable: they can change potential
• Sodium channels exist among all dendrites and will increase voltage
• All these tiny potentials propagate towards the axon hillock and combine
  • Has a voltage gated ion channel
  • This creates a positive feedback loop, having a spike, so tons of sodium will enter
  • The voltage goes back down since potassium has an ion channel
• Long range, but also receptor and synaptic potentials which decay in millimeters
  1. Receptor potential is graded and local (analog)—when stretching a muscle, it will stretch open an ion channel
     1. Analog since more stretch means more potential
  2. Action potentials are all or nothing and long range (digital)—from receptor potentials
     1. Has a threshold at a certain value due to voltage gated ion channels
     2. Neurotransmitter release is next to other neurons, probabilistic like
        1. Is probability since not all dopamine is guaranteed to get to the other neurotransmitter
• Both types come from change in membrane potential

Ion Channels

Neuron signaling depends on rapid changes in membrane potential.

• Require 500 V/s slew rate: time to peak is 0.2 ms and peak is difference in potential of 100mV, so 500 V/s
• TTX in pufferfish makes sodium ion channels not work
• Ion channels can be heterogenous

Three properties of ion channels:

• Ionic current $10^8$ ions/channel
• Recognize and select specific ions
  • K+ channels are permeable to K+ 100x more
  • Na+ 10x more