Electrical Activity, Cations, Anions & Ion Distribution

• Electrical activity in living cells relates to ions
• Atoms/molecules carry charges based on the balance of electrons and protons

Cations vs Anions

• Cations carry positive charges because they have fewer electrons than protons
• Anions carry negative charges because they have more electrons than protons
• Key ions: sodium (Na+), potassium (K+), hydrogen (H+), calcium (Ca2+), and chloride (Cl-)
• Opposite charges attract, while like charges repel

Ion Distribution

• Ions are not evenly distributed inside and outside cells
• Overall, bodies are electrically neutral
• A neuron at rest has excess negatively charged ions (anions) inside
• Excess ions line up along the plasma membrane
• There is an excess of positive charges outside the neurons

Membrane Potential Energy

• Separation of charges creates potential energy, measured in volts, which is the ability to do work
• A neuron at rest has potential energy stored across its plasma membrane, known as resting membrane potential
• Convention sets the outside voltage
• The inside membrane voltage represents the difference

Electrochemical Gradient

• The electrochemical gradient combines concentration and electrical (charge) differences across a membrane
• This gradient determines where an ion will move

Electrochemical Gradient Example

• In a resting membrane potential model, sodium "leaks" out of the cell
• Electrically, sodium is attracted to the negatively charged cell interior

Membrane Polarity

• Polarized means distinct separation of structures/charges; divides areas based on properties/jobs
• By default, a neuron's intracellular surface is more negative than the extracellular surface
• Neurons at rest have no electrical signal passing through
• The plasma membrane in neurons is electrochemically polarized due to the ATP pump and leak channels
• Most animal cells, not just neurons, are electrically polarized

Sodium-Potassium Pump

• The Sodium-Potassium ATP pump is a membrane transporter that actively forces ions against a gradient

Sodium-Potassium Pump Functions

• Forces 3 sodium ions out of the cell
• Forces 2 potassium ions into the cell

Sodium Potassium Uneven Distribution

• Creates an uneven distribution of sodium and potassium ions intracellularly vs. extracellularly
• There is more sodium and potassium in extracellular fluid
• ECF concentration of Sodium = 150mM
• ICF concentration of Sodium = 15mM
• ECF concentration of Potassium = 5mM
• ICF concentration of Potassium = 150mM

Leak Channels

• Leak channels: Sodium has leaky channels with 25 times more potassium leaky channels
• Functions:
  • Slow leak of ions back across the membrane, electrochemical gradient
  • Shifts the resting membrane potential more negative vs what the pump would do alone
• The K+ leak channels allow potassium to leak out of the cell

Membrane Transport Protein

• Na-K+ ATPase/Pump: Separates sodium and potassium across the plasma membrane; sodium is high outside, potassium is high inside which is -10mV
• K+ Leak Channel: Lets potassium leak out which causes loss of + charges from inside, so inside becomes more negative to -80mV
• Na+ Leak Channel: Lets sodium leak in which causes gain of + charges from outside to -70mV

Topic 10 Summary:

• Ions are unevenly distributed
• Separated ions have potential energy
• The membrane has potential energy at rest including the potential energy to communicate
• the potential uses Neurons, muscle cells communicating with neighboring or distant cells

Parts of a Neuron:

• Dendrite: Gathers electrical signals
• Cell Body: Contains the Soma and main support machinery
• Axon hillock: Electrical signals start here
• Axon Terminal: Sends Electrical signals
• Axon: Location of action potentials
• Electrical signals must pass through the Axon hillock before beginning the action potential