Chapter 48: Neurons

### 1. Structure of a Neuron and Its Function

- Dendrites: Branch-like extensions that receive signals from other neurons. They have a high surface area for collecting input signals.

- Cell Body (Soma): Contains the nucleus and organelles. It integrates signals received by the dendrites and generates responses.

- Axon: A long, slender projection that transmits electrical impulses away from the cell body. Axons can be very long, enabling them to carry signals over distances.

- Axon Terminals: The end of the axon where neurotransmitters are released to communicate with other neurons or target cells.

- The structure of the neuron allows it to efficiently receive, process, and transmit information as electrical and chemical signals.

### 2. Role of Glial Cells

- Schwann Cells: Found in the peripheral nervous system, they wrap around axons to form the myelin sheath, which insulates the axon and speeds up signal transmission.

- Oligodendrocytes: Located in the central nervous system, they perform a similar function to Schwann cells by forming myelin sheaths around axons.

- Astrocytes: Star-shaped cells that provide structural support, regulate the chemical environment around neurons, and help form the blood-brain barrier.

### 3. Cell Membrane Potential

- Definition: The membrane potential is the electrical potential difference across the cell membrane, resulting from the distribution of ions.

- Relation to Voltage: Voltage is the measure of this potential difference. For neurons, it is typically around -70 mV at rest.

- Measurement: Membrane potential is measured using microelectrodes that detect voltage differences across the cell membrane.

### 4. Sodium/Potassium Pump and Resting Membrane Potential

- The sodium/potassium pump actively transports three sodium ions out of the cell and two potassium ions into the cell using ATP.

- This pump helps maintain the resting membrane potential by establishing a concentration gradient and keeping the inside of the neuron more negative relative to the outside.

### 5. Leak Channels and Resting Potential

- Leak Channels: Ion channels that are always open, allowing ions (mainly potassium) to move down their concentration gradients.

- Role in Resting Potential: Potassium leak channels allow K⁺ to exit the cell, contributing to the negative charge inside the neuron and maintaining the resting membrane potential.

### 6. Role of Ligand-Gated Ion Channels in Signal Conduction

- Ligand-Gated Ion Channels: These channels open in response to specific neurotransmitters (ligands), allowing ions to flow in or out of the neuron.

- Relation to Graded Potentials: When ligand-gated channels open, they cause small, local changes in membrane potential called graded potentials. The size of these potentials depends on the strength and duration of the stimulus.

### 7. Excitatory (Depolarizing) vs. Inhibitory (Hyperpolarizing) Stimuli

- Excitatory Stimuli: Cause depolarization by allowing positive ions (e.g., Na⁺) to enter the cell, bringing the membrane potential closer to the threshold.

- Inhibitory Stimuli: Cause hyperpolarization by allowing negative ions (e.g., Cl⁻) to enter or positive ions (e.g., K⁺) to leave the cell, moving the membrane potential farther from the threshold.

- These graded potentials sum at the axon hillock, and if they reach the threshold potential, an action potential will be triggered.

### 8. Integration of Stimuli at the Axon Hillock

- The axon hillock serves as the decision-making center of the neuron. Here, excitatory and inhibitory graded potentials are summed. If the net effect is enough to depolarize the membrane to the threshold, an action potential is generated and propagated down the axon.

### 9. Role of Voltage-Gated Ion Channels in Signal Conduction

- Voltage-Gated Ion Channels: These channels open in response to changes in membrane potential.

- They are essential for the propagation of action potentials. For example, voltage-gated Na⁺ channels open during depolarization, and voltage-gated K⁺ channels open during repolarization.

### 10. Sequence of Action Potential Generation

- Resting State: The membrane is at -70 mV, and voltage-gated channels are closed.

- Depolarization: If the threshold is reached, voltage-gated Na⁺ channels open, causing Na⁺ to rush into the cell and the membrane potential to become more positive.

- Repolarization: At the peak of the action potential, Na⁺ channels close and voltage-gated K⁺ channels open, allowing K⁺ to exit the cell, bringing the membrane potential back down.

- Hyperpolarization: The membrane briefly becomes more negative than the resting potential due to continued K⁺ efflux.

- Return to Resting Potential: K⁺ channels close, and the sodium/potassium pump restores the resting state.

### 11. Interpreting a Graph of Membrane Potential

- Resting Potential: The flat, baseline phase of the graph, typically around -70 mV.

- Graded Potentials: Small fluctuations from the resting potential that can be either depolarizing or hyperpolarizing.

- Action Potential: A sharp increase (depolarization) followed by a decrease (repolarization) and often a brief dip below resting potential (hyperpolarization).

- Recognizing these phases on a graph is essential for understanding how neurons transmit electrical signals.