Excitatory Signals and Inhibitory Signals

Membrane Potential and Action Potentials

1. Ion Distribution in the Cell: The inside of a cell and the outside environment have different concentrations of ions, which are critical for setting up the membrane potential.

   • Cations:

     • Outside the Cell: High amounts of Calcium (Ca²⁺) and Sodium (Na⁺).

     • Inside the Cell: High amounts of Potassium (K⁺).

   • Anions:

     • Outside the Cell: The main anion is Chloride ions (Cl⁻).

     • Inside the Cell: Negatively charged proteins serve as the main anion, affecting the internal charge.

2. Resting Membrane Potential: The difference in ion concentration creates a resting membrane potential typically around -60 to -70 millivolts. This means that the inside of the cell is slightly negative compared to the outside, which is essential for starting nerve impulses and muscle contractions. This resting potential can change based on the cell's condition.

3. Action Potentials: An action potential is the rapid change in membrane potential when a cell is stimulated. It involves the movement of ions across the cell membrane.

   • The entry of Sodium (Na⁺) into the cell is crucial for depolarization. As Na⁺ moves in, it decreases the negativity inside, making the membrane potential approach zero.

   • Calcium (Ca²⁺) can also enter the cell through special channels, enhancing the depolarization needed to generate an action potential.

4. Facilitated Diffusion: This process allows ions to move through the cell membrane. Special protein channels help certain ions enter the cell, causing shifts in the membrane charge critical for action potentials.

5. Understanding Depolarization and Hyperpolarization:

   • Depolarization: This occurs when positive ions (like Na⁺ and Ca²⁺) enter the cell, making the inside less negative, which is necessary for generating an action potential.

   • Hyperpolarization: This happens when negative ions (like Cl⁻) enter or when positive ions (like K⁺) leave the cell, making the inside more negative. This process makes it harder for the cell to trigger an action potential.

6. Neurotransmission: Neurons communicate at synapses, where action potentials in one neuron trigger the release of neurotransmitters into the synaptic cleft. These neurotransmitters determine whether the next neuron will become excited or inhibited.

   • Excitatory Neurotransmitters like Acetylcholine help depolarize the next neuron, making it more likely to fire an action potential.

   • Inhibitory Neurotransmitters such as Gamma-Aminobutyric Acid (GABA) cause hyperpolarization in the next neuron, making it less likely to fire an action potential.

7. Role of Ion Channels: The effect of neurotransmitters relies on which ions can flow through their channels. Sodium and calcium channels support excitatory signals, while chloride channels (activated by GABA) lead to inhibitory signals.