The Functioning of Neurons: University of Auckland Module 5

The Neuron as an Information Processor
- Neurons are specialized cells that process information by receiving signals from multiple sources.
- They integrate these signals to determine whether or not to generate an output.
- The resulting signal is sent to other neurons or to muscles.
Key Components of a Neuron
- Dendrites: These serve as the receptive area of the neuron. Signals from other neurons are received here.
- Cell Body: This is the site where all incoming signals are added together (integrated) to generate a final output.
- Axon: This acts as the output pathway of the neuron, conducting the signal away from the cell body.
- Axon Terminal: This is located at the furthest end of the axon.
- Synapses: These are the junctions where the neuron connects with other cells.
Functional Stages
- Inputs: Received via dendrites.
- Processing: Occurs in the cell body.
- Transmission: Conducted via the axon to the synapses.

Nature of the Signal
- Signals within neurons are electrical in nature and are known as action potentials.
- An action potential is defined as a pulse of electrical activity that travels down the neuron in a wave-like motion.
The Role of the Cell Membrane
- The cell membrane acts as a physical barrier to ions.
- Ions are atoms that carry an electrical charge.
- The membrane allows the cell to create and maintain different environments inside versus outside the cell.
Ion Channels
- These are special proteins embedded in the membrane that can open and close.
- They allow specific ions, such as Sodium ( $Na^+$ ) and Potassium ( $K^+$ ), to cross the membrane only when an action potential is occurring.

Two Primary Forces
- Two physical principles determine how ions move across the membrane when a channel opens: concentration and charge.
Concentration (Chemical Gradient)
- Definition: Concentration refers to the amount of a substance present in a liquid.
- Movement Principle: Substances tend to move from areas of high concentration to areas of low concentration until the concentration becomes even.
- Analogies provided:
  - Salt: Moving from a "saltier" area to a "less salty" area.
  - Dye: Moving from a "darker" area to a "clearer" area.
Charge (Electrical Gradient)
- Electrical charges can be either positive ( $+$ ) or negative ( $-$ ).
- Principles of Attraction and Repulsion:
  - Opposite charges ( $+$ and $-$ ) attract each other.
  - Like charges (e.g., $+$ and $+$ ) repel each other.
- Examples provided:
  - Balloon and Water: Rubbing a balloon on a cloth makes it negatively charged, allowing it to attract water (which contains positive charges).
  - Trampoline Hair: Bouncing on a trampoline can create an uneven (usually positive) charge in a person's hair. Because the hairs then have the same charge, they repel each other and stand up.

Understanding Voltage
- Voltage, or electrical potential, is related to the amount of energy a charge has as it travels between two terminals.
- Battery Comparisons:
  - A standard heavy-duty battery shown in the example has a voltage of $9.00\,V$ .
  - An AA battery has a voltage of $1.5\,V$ .
Neuron Membrane Potentials
- Neurons maintain a similar, though much smaller, electrical potential between the inside and outside of the membrane.
- Resting Potential: At rest, the potential is typically $-70\,mV$ .
- Action Potential Peak: During an action potential, the potential reaches approximately $+40\,mV$ .

Voltage-Sensing Channels
- Ion channels in the membrane sense changes in voltage. At rest, Sodium ( $Na^+$ ) and Potassium ( $K^+$ ) channels are closed.
Current Flow and Membrane Reversal
- The process begins when the membrane potential becomes less negative.
- Sodium ( $Na^+$ ) Influx: The $Na^+$ channels open. $Na^+$ ions flow into the cell because they are moving from a high to low concentration and are attracted to the negative charge inside the neuron.
- The Spike: The inward flow of positive $Na^+$ ions causes the membrane potential to become more positive, resulting in the "spike" of the action potential.
- Potassium ( $K^+$ ) Efflux: Shortly after, $K^+$ ions flow through Potassium channels to move out of the cell. This causes the membrane potential to return to its resting state of $-70\,mV$ .

Travel Down the Axon
- Action potentials originate at the cell body.
- When $Na^+$ ions flow in, they spread to neighboring sections of the axon.
- This makes the adjacent sections more positive, triggering an action potential there.
- This cycle repeats sequentially down the entire length of the axon until it reaches the terminals.
Biological Variation
- In giraffes, axons can be several meters long.
- Despite the length, the mechanism of action potential propagation works exactly the same way as in shorter axons.

The Synaptic Junction
- Synapses are the connections between neurons located at the end of the axon.
- Transmission is chemical rather than purely electrical.
Neurotransmitters and Receptors
- Information is sent to the next neuron via chemicals called neurotransmitters.
- These chemicals are released into the small space (synaptic cleft) between neurons.
- Special proteins called receptors on the membrane of the receiving neuron capture these neurotransmitters.
Signal Integration and Summation
- The activation of receptors causes very small changes in the membrane potential of the receiving neuron.
- Summation: When enough of these small changes occur close together in time and space on the neuron, they add up.
- If the sum is sufficient, it triggers a new action potential in the next neuron, restarting the process.
Complexity of Neural Networks
- Neurons do not exist in isolation but form complicated networks.
- Each individual neuron may receive tens of thousands of different inputs.