5.6 bio notes

Electrophysiology Introduction

• Muscle and nerve tissues are excitable tissues, meaning they can send and receive electrical signals.
• Muscle and nerve cells send electrical signals by manipulating the diffusion of sodium (Na+) and potassium (K+) ions down their chemical gradients.
• Diffusion of these ions provides an electrical charge to the plasma membrane.
• This electrical charge can be transmitted along the membrane of a cell by controlling the permeability of the membrane to those ions.

How Membranes Become Charged

• The hydrophobic barrier of the cell membrane allows the creation of concentration gradients of electrolytes (ions).
• A concentration gradient of ions is also a gradient of electrical charges.
• By allowing electrolytes to diffuse through the membrane, a flow of electrical charges can be created.
• This flow of electrical charges can be sensed by cells as a change in the overall charge of the membrane.

Ion Gradients

• Five gradients to know:
  • Three are in highest concentration in the extracellular fluid (ECF):
    1. Sodium (Na+)
    2. Calcium (Ca2+)
    3. Chloride (Cl-)
  • When the membrane is permeable to these three, they will diffuse down their concentration gradient into the intracellular fluid.
    • Two are in highest concentration in the intracellular fluid (ICF):
      1. Potassium (K+)
      2. The cell’s proteins (overall negative charge)
    • Of these two, the membrane can only become permeable to potassium.
    • When it does, potassium will diffuse down its concentration gradient into the extracellular fluid.
    • Proteins are too large to diffuse through the membrane.

How Membranes Become Charged by Diffusion

• The charge on a membrane is discussed in terms of what the electrical charge is on the inside of the membrane.
  • Example 1: Sodium diffusion
    • The membrane becomes permeable to sodium when there are active sodium transporters in the membrane.
    • When sodium diffuses through these transporters, positive charges diffuse into the intracellular fluid (ICF).
    • This means that the inside of the membrane gains an overall more positive charge.
    • And the outside has a more overall negative charge
  • Example 2: Chloride diffusion
    • The membrane becomes permeable to chloride when there are active chloride transporters in the membrane.
    • When chloride diffuses through these transporters, negative charges diffuse into the intracellular fluid (ICF).
    • This means that the inside of the membrane gains an overall more negative charge.
    • And the outside has an overall more positive charge
  • Example 3: Potassium diffusion
    • The membrane becomes permeable to potassium when there are active potassium transporters in the membrane.
    • When potassium diffuses through these transporters, positive charges diffuse into the extracellular fluid (ECF).
    • In other words, positive charges are leaving the inside of the cell
    • This means that the inside of the membrane gains an overall more negative charge.
    • And the outside has an overall more positive charge
    • The proteins of the cell stay inside so the inside of the cell while potassium leaves

Electrophysiology Terms

• When diffusion charges a membrane, that membrane becomes polarized.
• The strength of the charge on the membrane is called its membrane potential.
• Membrane potential is expressed in a number of millivolts (mV) of charge.
  • Example: The membrane potential of a nerve cell is -70 mV.
    • The sign tells you what the charge is on the inside of its membrane.
    • The number tells you the strength of the charge in that direction.
    • In the example, a nerve cell membrane has a negative charge on the inside with a strength of 70 millivolts.
• Every single cell in the body (not just nerves and muscle) is polarized, and this charge is called the resting membrane potential.

Resting Membrane Potential

• Every cell in the body has a potassium leak channel in its plasma membrane.
  • A channel is a transport protein that makes a water-filled opening through which electrolytes can diffuse.
  • This channel is built so that only potassium can diffuse through it.
  • The opening in a leak channel opens and closes randomly, which makes it so that the membrane is always permeable to potassium.
• The leak channel causes potassium to diffuse out of the cell and into the ECF.
  • The inside of the cell membrane becomes negatively charged.
• In addition to the potassium leak channels, each cell also has sodium leak channels.
  • Sodium leak channels open and close randomly, which makes it so that the membrane is always permeable to sodium.
  • Sodium diffuses into the cell.
  • This makes the charge in the cell more positive.
• The reason they don’t just cancel out is that there are usually a lot more potassium leak channels then sodium leak channels
  • So there is a lot of potassium diffusion happening
  • And only a little sodium diffusion
• Adding these two effects together is what creates the resting membrane potential
• Because there is more potassium diffusion than sodium diffusion, cells usually have a negative resting membrane potential
• The ratio of potassium leak to sodium leak is different in most cells of the body.
• This means that most types of cells have their own specific resting membrane potential.
• Nerve and muscle cells can adjust their membrane potential to send electrical signals.

Adjusting Membrane Potential

• Nerve and muscle cells have two kinds of membrane channels:
  1. Leak channels
     • These channels open and close randomly to set the resting membrane potential
  2. Gated channels
     • These channels are normally closed and only open in response to some kind of signal
• For example, an open sodium channel will allow sodium to diffuse into the cell.
  • This makes the inside of the cell move to a more positive voltage.
• When a potassium channel opens, it will allow potassium to diffuse out of the cell
  • This makes the inside of the cell move to a More negative voltage
• We track changes in membrane potential on a chart called a trace.
  • Traces put a membrane with no charge (0 mV) in the middle of the y-axis.
  • Traces start at the resting membrane potential.
    • In this case, the resting potential is -70 mV
  • The trace line moving up indicates moving to a more positive voltage.
  • The trace line moving down indicates moving to a more negative voltage
• Adjustments to membrane potential are almost always done by gated ion channels

Gated Ion Channel Types

1. Chemically-gated ion channels
   • These channels open when a chemical signal binds to them
   • The chemical signal will bind to a receptor on the ion channel
   • Binding of the signal changes the shape of the channel, which opens it
   • Opening the channel allows ions to diffuse through
2. Mechanically-gates ion channels
   • These channels open when a physical pressure is applied to their cell membrane
   • Converts a physical signal to an electrical signal
   • Examples: mechanoreceptors in the skin
     • Touching the skin opens mechanically gated ion channels
     • Electrical signal made by these channels is sent to the brain for processing
3. Voltage-gated ion channels
   • These channels open in response to a change in membrane potential
   • There are dozens of these in the body and each is tuned to open a specific voltage
   • These channels are special because they can be activated in chains to produce complex electrical signals called action potentials