Communication in the Nervous System: Electrochemical Signaling and Neurotransmission

Communication in the Nervous System

Electrochemical Communication

The nervous system relies on both electrical and chemical signals to transmit information between neurons. This dual mode of communication is essential for rapid and coordinated responses throughout the body.

• Electrical communication occurs within neurons via action potentials.

• Chemical communication occurs between neurons at synapses via neurotransmitters.

• Neurons communicate through electrical impulses and chemical signals.

• Action potentials travel along axons; neurotransmitters cross synapses.

Electrical Communication: Action Potentials

Action Potential Basics

An action potential is a rapid change in voltage across a neuron's membrane, allowing for neural signaling. It is initiated when the neuron's membrane potential reaches a specific threshold.

• Ions are particles that carry an electrical charge (e.g., sodium Na+, potassium K+).

• Neurons have ion channels that allow ions to flow in and out of the cell.

• Ion channels are voltage-gated, meaning they open or close depending on the membrane voltage.

• The electrical gradient across the membrane is called the membrane potential.

Example: Sodium ions are attracted toward areas of the same charge. Potassium ions are repelled by areas of opposite charge.

Action Potential Sequence

Neurons start at a resting potential (about -70 mV). When the internal voltage reaches about -55 mV (threshold), an action potential is triggered.

• Depolarization: Sodium ions rush in, making the inside of the neuron more positive.

• Repolarization: Potassium ions flow out, restoring the negative charge inside the neuron.

• Refractory period: The neuron is temporarily unable to fire another action potential.

Action potential is all-or-nothing: Once the threshold is reached, the action potential will occur fully.

Equation:

$V_{m} = V_{inside} - V_{outside}$

where $V_{m}$ is the membrane potential.

Action Potential Graph

The action potential can be visualized as a graph showing changes in membrane potential over time:

• a: Depolarization

• b: Repolarization

• c: Return to resting potential

• d: Refractory period

Example: A neuron’s resting potential is -65 mV. If the threshold is reached, the action potential will fire. The refractory period means there is a time when the neuron cannot fire again.

Chemical Communication: The Synapse

The Synapse

When an action potential reaches the terminal button, it is converted into a chemical message at the synapse—the gap between the presynaptic and postsynaptic neurons.

• Presynaptic neuron: Releases neurotransmitters into the synaptic gap.

• Postsynaptic neuron: Has specialized receptors for different neurotransmitters.

• Neurotransmitters bind to receptors, opening ion channels and changing the voltage of the postsynaptic neuron.

Example: The synapse is the gap between two neurons.

Pathway of Signal Transmission

• Action potential moves down the axon.

• Terminal vesicles open in response to the incoming action potential.

• Neurotransmitters cross the synapse.

• Specialized receptors on the dendrite receive the neurotransmitter.

Chemical Communication: Neurotransmitters

Types of Neurotransmitters

Neurotransmitters can be classified as excitatory (increase the probability of firing an action potential) or inhibitory (decrease the probability).

Neurotransmitter	Known Effects
Glutamate	Major excitatory neurotransmitter in the brain
Gamma-aminobutyric acid (GABA)	Major inhibitory neurotransmitter in the brain
Serotonin	Mood, sleep, and appetite
Dopamine	Processing movement, attention, and cognitive functions
Norepinephrine	Involved in fight or flight response

Example: Benzodiazepines are drugs that enhance the activity of GABA, producing inhibitory effects in the brain.

Neurotransmitter Removal Mechanisms

Leftover neurotransmitters are removed from the synapse by several mechanisms:

Mechanism	Description
Diffusion	Neurotransmitters drift away from the synapse into the extracellular space
Degradation	Enzymes break down neurotransmitters in the synapse
Reuptake	Neurotransmitters are taken back into the presynaptic terminal

Example: Selective Serotonin Reuptake Inhibitors (SSRIs) are drugs that prevent the reuptake of serotonin, increasing its amount in the synapse.

Defining Neural Transfer

Integration of Signals

Neurons receive signals from hundreds or thousands of other neurons simultaneously. These signals can be excitatory, inhibitory, or both. If the excitatory messages outweigh the inhibitory ones, neural communication occurs.

• Neural communication is the result of the integration of multiple signals.

• Excitatory signals increase the likelihood of action potential; inhibitory signals decrease it.

Additional info: This process is known as summation, where the total input determines whether the neuron will fire.