Beatrice Lang
Neurons are the fundamental units of the nervous system and are responsible for sending messages from the brain throughout the body. Neurons communicate with each other through electrical and chemical signals. The electrical signal, or action potential, runs from the cell body area to the axon terminals, through a thin fibre called an axon. The speed of the signal transmission is influenced by an insulating layer called myelin. Myelin is a protective covering that insulates the axon and increases the speed of electrical communication along the length of the neuron. The chemical component of neuronal communication involves the release of neurotransmitters, which can either excite or inhibit the target neuron.
| Characteristics | Values |
|---|---|
| How neurons communicate | Electrical and chemical signals |
| Type of electrical signal | Action potential |
| What is an action potential | A brief (~1 ms) electrical event |
| What does an action potential do | It signals the neuron as 'active' and travels the length of the axon |
| What happens after an action potential | It causes the release of a neurotransmitter into the synapse |
| What is a neurotransmitter | A chemical released from a neuron following an action potential |
| What does a neurotransmitter do | It travels across the synapse to excite or inhibit the target neuron |
| What is a synapse | The junction between the axon of one neuron and the dendrite of another, through which the two neurons communicate |
| What is myelin | A protective covering that insulates the axon and increases the speed of electrical communication along the length of the neuron |
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What You'll Learn
Electrical signals and chemical messengers
Neurons communicate with each other through electrical and chemical signals. The electrical signal, or action potential, runs from the cell body area to the axon terminals through a thin fibre called an axon. The axon is insulated by a protective covering called myelin, which increases the speed of electrical communication along the length of the neuron. Myelin is a fatty layer formed by the concentric wrapping of oligodendrocyte cell processes around axons. The speed of signal transmission is influenced by the insulating layer of myelin. Myelin promotes fast transmission of electrical signals by increasing electrical resistance and reducing the leakage of electrical signals and ions along the axon, trapping them inside.
The electrical signal moves down the axon like a wave, with sodium ions entering the cell and diffusing to the next section of the axon, raising the charge past the threshold of excitation and triggering a new influx of sodium ions. This movement of the action potential down the length of the axon is an electrical event, and the movement of the neurotransmitter across the synaptic space represents the chemical portion of the process.
Within cells, electrical signals are conveyed along the cell membrane. For communication between cells, these electrical signals are converted into chemical signals conveyed by small messenger molecules called neurotransmitters. The neurotransmitters are released from presynaptic terminals, which may branch to communicate with several postsynaptic neurons. The neurotransmitters bind to receptors on the surface of the receiving (postsynaptic) neuron. There are approximately 100 different neurotransmitters, and each neuron produces and releases only one or a few types of neurotransmitters.
Second messengers, such as G proteins, are molecules that help relay signals from the cell's surface to its interior. Neurotransmitters that bind to second messenger-linked receptors, such as dopamine, initiate a complex cascade of chemical events that can either excite or inhibit further electrical signals. The neurotransmitters may also attach to receptors on the transmitting cell's own presynaptic sites, beginning a feedback process that can affect future communication through that synaptic cleft.
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Myelin's role in insulation
Myelin is a lipid-rich substance that coats nerve cell axons, providing a protective layer of insulation. This insulation increases the speed of electrical communication along the neuron. Myelin is made up of fat (lipids) and protein, and it is produced by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system.
The myelin sheath is not a single, solid covering but is made up of multiple long sheaths separated by small gaps known as nodes of Ranvier. These nodes are around one micrometre long and enable a much faster rate of conduction, known as saltatory conduction, where the action potential jumps over each gap to the next node. The speed of signal transmission is influenced by the distance between nodes, the thickness of the myelin wrapping, and the length of the exposed axon in the node.
Myelin's insulating function is essential for efficient motor function, such as walking, and sensory function, such as sight, hearing, and touch. It also plays a role in cognition. Myelin helps to regulate the timing of information flow through individual circuits in the nervous system and facilitates modes of nervous system function beyond the neuron doctrine.
Damage to the myelin sheath can slow or stop electrical signals from being transmitted. This damage can be caused by the body's immune system, which may recognise myelin as a foreign substance and produce inflammatory substances that attack it. Demyelination, or the loss of the myelin sheath, is associated with neurodegenerative autoimmune diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica.
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Neurotransmitters and receptors
Neurons communicate with each other through electrical and chemical signals. The electrical signal, known as an action potential, runs from the cell body area to the axon terminals through a thin fibre called an axon. The axon is insulated by a protective covering called myelin, which increases the speed of electrical communication along the length of the neuron.
At the junction between two neurons, called a synapse, the action potential causes the release of a chemical neurotransmitter. The neurotransmitter travels across the synapse to either excite or inhibit the target neuron. This process involves the conversion of the cell's electrical message into a chemical one.
Neurotransmitters are small messenger molecules that are released from presynaptic terminals, which may branch to communicate with several postsynaptic neurons. There are approximately 100 different types of neurotransmitters, and each neuron produces and releases only one or a few types. However, a neuron can carry receptors for several types of neurotransmitters.
Neurotransmitters, such as dopamine, can bind to receptors on the surface of the receiving neuron, initiating a complex cascade of chemical events. These chemical events can either excite or inhibit further electrical signals. The signals that the neuron receives can have excitatory or inhibitory effects, leading to either an increase or decrease in neuronal communication.
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Action potentials
An action potential is a rapid sequence of changes in the voltage across a cell membrane. It is also known as a nerve impulse or a "spike" when it occurs in a neuron. Action potentials occur in several types of excitable cells, including neurons, muscle cells, and some plant and endocrine cells. In neurons, action potentials play a central role in cell-to-cell communication by facilitating the propagation of signals along the neuron's axon toward synaptic boutons situated at the ends of an axon. These signals can then connect with other neurons at synapses or to motor cells or glands.
The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported back out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to its resting state.
The speed of action potential propagation along myelinated axons is influenced by the thickness of the myelin sheath. Myelin is a protective covering that insulates the axon and increases the speed of electrical communication along the length of the neuron. Abnormal myelination can lead to a delay or reduction in the transmission of electrical and chemical signals, affecting the synchronicity of brain region activity and potentially leading to improper actions and behaviours.
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Electrical and chemical synapses
Neurons communicate with each other through electrical and chemical signals. The electrical signal, or action potential, runs from the cell body area to the axon terminals through a thin fibre called an axon. Axons can be very long, and the electrical signal that runs along the axon is based on ion movement. The speed of the signal transmission is influenced by an insulating layer called myelin, a fatty layer formed in the vertebrate central nervous system. Myelin insulates the axon and increases the speed of electrical communication along the length of the neuron.
At the junction between two neurons (synapse), an action potential causes neuron A to release a chemical neurotransmitter. The neurotransmitter can either help (excite) or hinder (inhibit) neuron B from firing its own action potential. The neurotransmitter travels across the synaptic cleft to the postsynaptic neuron, where the chemical signal is converted back into an electrical signal as charged ions flow into or out of the postsynaptic neuron.
At an electrical synapse, neurons pass signals directly via electrical current, moving between cells through channels. Electrical synapses can form between many different parts of neurons, and messages can flow through them in both directions. Ultimately, neural circuits are created by the interactions between both electrical and chemical synapses.
Neurotransmitters are small messenger molecules that can be excitatory or inhibitory. Examples include dopamine and serotonin. Each neuron produces and releases only one or a few types of neurotransmitters, but they can carry receptors on their surface for several types of neurotransmitters.
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Frequently asked questions
Neurons communicate with each other through electrical and chemical signals.
Electrical signals, also known as action potentials, are electrical events that run from the cell body area to the axon terminals through a thin fibre called an axon. The electrical signal that runs along the axon is based on ion movement.
Chemical signals are created by neurotransmitters, which are released from neurons following an action potential. These neurotransmitters can either excite or inhibit the target neuron.
Myelin is a protective covering that insulates the axon of a neuron. It increases the speed of electrical communication along the length of the neuron by reducing the leakage of electrical signals and ions along the axon.
