Wiktor Wise
Neurons are the key players in the nervous system, conveying information through electrical and chemical signals. Neurons are essentially electrical devices, with channels in their membranes that allow ions to flow in and out of the cell, creating a voltage difference. This movement of electrical charge within the neuron is what allows it to transmit information. While neurons can communicate within themselves, they also connect and communicate with other neurons through chemical signals, or neurotransmitters, which are released at the synapse, the junction between two neurons.
| Characteristics | Values |
|---|---|
| How neurons transmit information | Through electrical signals and chemical signals |
| How electrical signals are conveyed within neurons | Along the cell membrane |
| How chemical signals are conveyed between neurons | Through small messenger molecules called neurotransmitters |
| How neurotransmitters are released | Through an action potential or spike |
| What is an action potential | A brief (~1 ms) electrical event that signals the neuron as 'active' |
| How is an action potential generated | Through a change in membrane potential or membrane electrical charge |
| What is membrane potential | The electrical potential across the neuron's cell membrane, which arises due to different distributions of positively and negatively charged ions within and outside of the cell |
| What is the resting membrane potential | The membrane potential (electrical charge) in a neuron that is not currently transmitting a signal; maintained by the sodium potassium pump and potassium leak channels |
| What is an action potential | A brief depolarization (reduction in magnitude of the charge) along the neuron’s axon |
| How does depolarization occur | Due to the uneven distribution of electrically charged particles, or ions, such as sodium (Na+), potassium (K+), chloride (Cl−), and calcium (Ca2+) |
| What are gap junctions | Intercellular structures that serve as pathways of low resistance for the spread of electrical currents between the interior of two cells |
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What You'll Learn
The role of ions in electrical signals
Neurons are not good conductors of electricity, yet they have evolved mechanisms to generate electrical signals based on the flow of ions across their plasma membranes. Ions are atoms or groups of atoms that gain an electrical charge by losing or acquiring electrons. The electrical events that constitute signalling in the nervous system depend on the distribution of ions on either side of the nerve membrane.
The nerve membrane is selectively permeable to some ions and not others. This selective permeability is due to ion channels, which are proteins that allow only certain ions to cross the membrane in the direction of their concentration gradients. Ion channels work against the ion transporters, which actively move ions into or out of cells against their concentration gradients. Together, they generate the resting membrane potential, action potentials, and the synaptic and receptor potentials that trigger action potentials.
The movement of cations toward the less-concentrated solution creates a separation of electrical charge across the membrane. This separation of charge, or difference in electrical potential, is the starting point of all electrical events in nervous systems. When present in the plasma membrane of the neuron, the potential difference transforms the neuron into an electrolytic cell capable of generating and transmitting electrical impulses.
The sodium-potassium pump and potassium leak channels maintain the resting potential, or the membrane potential of a neuron that is not currently transmitting a signal. The action potential is a brief depolarization along the neuron's axon, regulated by voltage-gated sodium and potassium channels, and the sodium-potassium pump. Action potentials are propagated along the length of axons and are the fundamental signal that carries information from one place to another in the nervous system.
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Action potentials
Neurons are electrically excitable cells that react to input by producing electrical impulses, which are propagated as action potentials throughout the cell and its axon. The axon is a tube-like structure that propagates the integrated signal to specialised endings called axon terminals.
The resting membrane potential is ordinarily negative, at around -60mV to -70mV, and can be measured by recording the voltage between the inside and outside of nerve cells. When the resting membrane potential becomes less negative, it depolarises, and an action potential is generated. This change in membrane potential will open voltage-gated cationic channels (sodium channels), resulting in the process of depolarisation and the generation of the action potential. The action potential abolishes the negative resting potential and makes the transmembrane potential transiently positive.
There are three stages in the generation of the action potential: depolarisation, repolarisation, and after-hyperpolarisation. Firstly, depolarisation changes the membrane potential from -60mV to +40mV, primarily due to the sodium influx. Secondly, repolarisation returns the membrane to its resting potential, mainly due to the potassium efflux. Finally, after-hyperpolarisation is a recovery from a slight overshoot of the repolarisation. Immediately after an action potential is generated, the neuron cannot immediately generate another; this is the absolute refractory period.
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Neurotransmitters
Neuromodulators are unique as they can affect multiple neurons simultaneously and regulate neuronal populations. They include dopamine, which is involved in motor control, reward and reinforcement, and motivation. Another neuromodulator is noradrenaline (norepinephrine), the primary neurotransmitter in the sympathetic nervous system, which controls vital functions such as blood pressure and heart rate.
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Membrane potential
The membrane potential, or resting membrane potential, is the electrical charge in a neuron that is not currently transmitting a signal. It is the result of the movement of several different ion species through various ion channels and transporters in the plasma membrane. These movements result in different electrostatic charges across the cell membrane.
The resting membrane potential of a cell is defined as the electrical potential difference across the plasma membrane when the cell is in a non-excited state. The potential difference is expressed by its value inside the cell relative to the extracellular environment. The inside of a nerve cell is negatively charged (around -70 mV) compared to the outside. This electrical gradient is maintained by the sodium-potassium pump, which moves three sodium ions out of the cell and two potassium ions into the cell for each ATP molecule used.
The sodium-potassium pump is crucial to the functioning of nerve cells, accounting for the majority of their ATP usage. Neurons also have a large number of potassium leak channels, which allow potassium to diffuse down its concentration gradient and leak out of the cell. The positively charged potassium ions leaving the cell contribute to the negative charge inside the cell.
The resting membrane potential is important for the proper functioning of the nervous and muscular systems. When a neuron is excited, it deviates from its resting membrane potential and undergoes a rapid action potential before returning to rest. The action potential is a brief depolarization (reduction in the negative charge) along the neuron's axon, which is propagated along the length of the axon. It is the fundamental signal that carries information from one place to another in the nervous system.
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Gap junctions
Neurons are not naturally good electricity conductors, but they have evolved mechanisms to generate electrical signals. Gap junctions, also called electrical synapses, are channel-forming structures in contacting plasma membranes that allow direct metabolic and electrical communication between almost all cell types in the mammalian brain.
The coupling of neurons by gap junctions and the expression of the neuronal gap junction protein Cx36 increase during early postnatal development in the mammalian central nervous system. The levels of both subsequently decline and remain low in adults, though they are still expressed in certain areas of the adult brain, including the retina. Following neuronal injury, such as ischemia, traumatic brain injury, or epilepsy, the coupling and expression of Cx36 rise again.
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Frequently asked questions
Neurons transmit information both electrically and chemically. Within the neuron itself, information is passed along through the movement of an electrical charge.
An action potential is a brief (~1 ms) electrical event that signals the neuron as 'active'. It is generated in the axon and travels its length, causing the release of neurotransmitters into the synapse.
Neurotransmitters are small messenger molecules that are released from a neuron following an action potential. They travel across the synapse to excite or inhibit the target neuron.
Ions play a crucial role in generating electrical signals in neurons. The movement of ions, such as sodium (Na+), potassium (K+), chloride (Cl-), and calcium (Ca2+), across the cell membrane creates a voltage difference, known as the membrane potential.
Electrical signals can move between neurons through "gap junctions," which are low-resistance pathways that allow the spread of electrical currents between the interiors of two cells.
