Neurons: Electrical Signals and Resting Potential

• Thoughts, feelings, and actions rely on neural communication via electrical and chemical signals.

Electric Signaling

• Electric signals are conducted inside the neuron from dendrites to the cell body, then down the axon.
• Chemical signals transmit from one neuron to another across the synapse.
• The process of neuronal communication is electrochemical.
• A neuron's cell membrane contains pores that act as channels for ions to flow in and out.
• Ions are atoms or molecules carrying a positive (+) or negative (−) electric charge.
• Ion flow across the cell membrane creates electric current within the neuron.

Resting Potential

• Resting potential refers to the difference in electric charge between the inside and outside of a neuron’s cell membrane.
• When a neuron is at rest, potassium ions (K+) and protein ions (A−) are more abundant inside the neuron.
• Sodium ions (Na+) are more abundant outside the neuron.
• The inside of the neuron has a slight negative electric charge relative to the outside.
• The resting potential is typically about -70 millivolts.
• The resting potential was discovered in the 1930s by biologists working with the squid giant axon.
• Special channels in the cell membrane restrict ion movement in and out of the cell.

Action Potential

• An action potential is an electric signal conducted along the length of a neuron’s axon to a synapse.
• Biologists discovered that stimulating the squid giant axon with an electric shock triggers a larger electrical impulse traveling down the axon.
• The action potential occurs only when the electric shock reaches a threshold.
• Above the threshold, increases in the electric shock do not increase the strength of the action potential.
• The action potential is all or none, meaning electric stimulation below the threshold fails to produce an action potential.
• Electric stimulation at or above the threshold always produces the action potential at the same strength.
• A neuron "fires" when it produces an action potential.

Action Potential Movement

• The action potential occurs due to changes in the axon’s membrane channels.
• During the resting potential, sodium ion membrane channels are closed, but when the electrical charge reaches the threshold, the channels open.
• Na+ ions rush in, causing the local electric charge to surge from -70 millivolts to +40 millivolts in less than one millisecond.
• The inrush of Na+ ions spreads inside the cell, increasing the electric charge in neighboring areas.
• This triggers channels in the adjacent cell membrane to open, letting in more Na+ ions, spreading the charge even farther.
• The process repeats down the entire axon.

Myelin Sheath

• The myelin sheath increases the conduction of the action potential in many neurons.
• It prevents electric current from leaking out of the axon.
• Myelin clumps around the axon with breakpoints called nodes of Ranvier.
• Nodes of Ranvier were discovered by French pathologist Louis-Antoine Ranvier.
• Current "jumps" from node to node in a process called saltatory conduction, speeding the flow of information down the axon.

Refractory Period

• The action potential always spreads onward, never backward, because Na+ channels are temporarily inactivated after the action potential passes.
• This brief period of inactivation is called a refractory period, during which a new action potential cannot be initiated.
• During the refractory period, electrical and chemical balance is restored.
• Na+ channels inactivate for milliseconds, stopping the inrush of Na+ ions, and K+ channels open, allowing excess K+ ions to escape.
• Ion pumps redistribute the ions until concentrations are rebalanced and the resting potential is restored.

Chemical Signaling

• Electric action potentials cross the synaptic gap to other neurons.
• Axons have hundreds or thousands of branches reaching out to other neurons and organs.
• Axons end in terminal buttons containing vesicles filled with neurotransmitters.
• Neurotransmitters transmit information across the synapse to a receiving neuron’s dendrites.
• Dendrites contain receptors that receive the neurotransmitter and either initiate or prevent a new electric signal.
• An action potential in the presynaptic neuron stimulates the release of neurotransmitters from vesicles into the synapses.
• Neurotransmitters float across the synapse and bind to receptor sites on the postsynaptic neuron.
• Synaptic transmission underlies thoughts, emotions, and behavior.
• When the postsynaptic neuron receives a neurotransmitter, it activates ion channels, raising or lowering the voltage across the membrane.
• Neurotransmitters’ chemical messages create an electrical signal.
• Neurons form pathways in the brain characterized by specific types of neurotransmitters.
• Neurotransmitters and receptor sites act like a lock-and-key system, where only some neurotransmitters bind to specific receptor sites.

Neurotransmitter Processes

• Neurotransmitters leave the synapse through three processes:
• Reuptake: Neurotransmitters are absorbed by the terminal buttons of the presynaptic neuron’s axon or by neighboring glial cells.
• Enzyme Deactivation: Enzymes in the synapse break down specific neurotransmitters.
• Diffusion: Neurotransmitters drift out of the synapse and can no longer reach receptors.
• Neurotransmitters can also bind to autoreceptors on the presynaptic neuron, which detect how much neurotransmitter has been released and may stop further release.