Trystan Casey
Neurons are essentially electrical devices that convey information both electrically and chemically. They are not intrinsically good conductors of electricity, but they have evolved mechanisms for generating electrical signals based on the flow of ions across their plasma membranes. These electrical signals are conveyed along the cell membrane and are converted into chemical signals conveyed by small messenger molecules called neurotransmitters. The mechanism underlying signal transmission within neurons is based on voltage differences (or potentials) that exist between the inside and outside of the cell.
| Characteristics | Values |
|---|---|
| Neurons' Electrical Signaling | Action potentials, or nerve impulses, are the basis of neurons' electrical signaling. |
| Action Potential | A rapid change in the membrane potential of a neuron, typically involving a brief spike of positive voltage followed by a return to the resting state. |
| Resting Potential | Neurons maintain a resting potential of approximately -70 millivolts (mV) across their cell membranes. |
| Membrane Potential | The difference in electric charge across the neuron's cell membrane results from differences in ion concentrations inside and outside the cell and the permeability of the membrane to these ions. |
| Ion Channels | Neurons have specific ion channels that allow the selective passage of ions (such as sodium, potassium, calcium, and chloride ions) into and out of the cell. |
| Action Potential Threshold | When enough excitatory stimuli or neurotransmitters cause a local change in membrane potential, it can reach a threshold of around +50 mV, triggering an action potential. |
| All-or-None Principle | Once the threshold is reached, the action potential occurs in an all-or-none manner, meaning it always reaches the same level of voltage and duration. |
| Action Potential Propagation | Action potentials typically propagate down axons in one direction, from the cell body to the axon terminals, with the help of voltage-gated ion channels. |
| Neurotransmitters | Neurotransmitters are released at the axon terminals and carry the signal across the synapse to the next neuron or target cell. |
| Synaptic Transmission | The electrical signal of the action potential triggers the release of neurotransmitters, which then bind to receptors on the postsynaptic neuron, influencing its membrane potential and potentially generating a new action potential. |
| Refractory Period | After an action potential, there is a brief refractory period when the neuron is less responsive to stimuli, ensuring that the action potential has time to propagate fully before another can be generated. |
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What You'll Learn
Neurons communicate via electrical events called 'action potentials'
Neurons are specialized cells that transmit information throughout the body through electrical signals. This transmission of information is crucial for our nervous system to function properly, allowing us to interact with and respond to our environment. The process begins with a stimulus, which can be a sensory input or a signal from other neurons. This stimulus triggers a response in the neuron, leading to the generation of an electrical signal.
At the core of neuronal communication are electrical events known as action potentials. These action potentials are rapid, transient changes in the electrical potential across the cell membrane of a neuron. They occur when there is a sudden reversal of the membrane potential, resulting in a spike of electrical activity. This spike then propagates down the neuron's axon, transmitting the signal to the next neuron in the circuit.
The generation of an action potential involves the movement of ions across the neuron's membrane. Specifically, there is a flow of positively charged ions, such as sodium (Na+) and potassium (K+), into and out of the cell. In its resting state, a neuron has a higher concentration of negative ions inside the cell compared to the outside, resulting in a negative membrane potential. When a stimulus is strong enough, it triggers the opening of ion channels, allowing the flow of positive ions into the cell, thereby changing the membrane potential.
During an action potential, there are distinct phases. It begins with the stimulus, which causes the membrane potential to become less negative, a process known as depolarization. If the stimulus is strong enough, it reaches a threshold, leading to the rapid opening of ion channels and a surge of positive ions into the cell. This results in a spike in the membrane potential, with a brief period of positive voltage inside the cell. This spike is the action potential, and it quickly propagates down the axon.
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Neurotransmitters and receptors
Neurons communicate with each other via synapses, where neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic cell. Neurotransmitters are chemical substances released from nerve endings into the synaptic cleft. They can either excite or inhibit the target neuron. Once the neurotransmitter binds to its receptor, the ligand-gated channels of the postsynaptic membrane either open or close, altering the permeability of the postsynaptic membrane to ions such as calcium, sodium, potassium, and chloride. This leads to a stimulatory or inhibitory response.
Neurotransmitters can be classified according to their function as either excitatory or inhibitory. Excitatory neurotransmitters activate receptors on the postsynaptic membrane and enhance the effects of the action potential, while inhibitory neurotransmitters prevent an action potential. Acetylcholine (ACh), for example, is an excitatory neurotransmitter secreted by motor neurons that innervate muscle cells. Its main function is to stimulate muscle contraction. However, acetylcholine is an inhibitory neurotransmitter at the parasympathetic endings of the vagus nerve. The best-known neurotransmitters responsible for fast, but short-lived excitatory action are acetylcholine, norepinephrine, and epinephrine, while GABA is the major inhibitory neurotransmitter.
The neurotransmitter released into the synaptic cleft acts for a very short duration, only minutes or even seconds. It is then either destroyed by enzymes or reabsorbed into the terminal button of the presynaptic neuron by reuptake mechanisms and then recycled. Repeated synaptic activities can have long-lasting effects on the receptor neuron, including structural changes such as the formation of new synapses and alterations in the dendritic tree.
The existence of neurotransmitter receptor heteromers is becoming broadly accepted, and they are thought to function as processors of computations that modulate cell signaling. Neurotransmitter receptor heteromers can influence both pre-and postsynaptic neurotransmission.
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The role of synapses
Our brain consists of billions of neurons, and these neurons communicate with each other through specialized junctions called synapses. Synapses play a critical role in the transmission of electrical signals between neurons, facilitating the complex processes of thought, memory, and action. At the most basic level, a synapse allows one neuron to form a connection with another neuron, a muscle cell, or a gland cell. This connection enables the transmitting neuron to send a signal to the target cell, influencing its activity.
The process of synaptic transmission begins with the arrival of an action potential at the presynaptic terminal of the sending neuron. This electrical impulse triggers the opening of voltage-gated calcium channels, resulting in an influx of calcium ions into the terminal. The increased calcium concentration stimulates the fusion of synaptic vesicles with the presynaptic membrane, leading to the release of neurotransmitters into the synaptic cleft. Neurotransmitters are small, signaling molecules that carry the message across the synaptic gap.
On the other side of the synapse, the neurotransmitters bind to specific receptors located on the postsynaptic membrane of the receiving neuron. These receptors are typically ion channels or G-protein-coupled receptors. When the neurotransmitters bind to these receptors, it can either excite or inhibit the postsynaptic neuron. Excitatory neurotransmitters typically act by allowing positive ions to flow into the neuron, while inhibitory neurotransmitters work by allowing negative ions to flow in or positive ions to flow out, making it more difficult for an action potential to occur.
The strength and efficiency of synaptic transmission can be modified through a process known as synaptic plasticity. This is a fundamental mechanism that underlies learning and memory. One well-studied form of synaptic plasticity is long-term potentiation (LTP), which involves the strengthening of synaptic connections following repeated stimulation. Conversely, long-term depression (LTD) weakens synaptic connections. These processes of synaptic plasticity enable the brain to adapt and change in response to experiences, forming the basis of memory formation and cognitive function.
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Electrical transmission between neurons
Neurons are the basic units of the nervous system and are essentially electrical devices. They transmit electrical and chemical signals through their three main parts: the cell body, dendrites, and axon. The cell body contains the nucleus, dendrites receive signals, and the long axon conducts signals away from the cell body.
Neurons communicate with each other via synapses, where neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic cell. This allows signals to be transmitted electrically along neurons and chemically between neurons. The neurotransmitters released by the presynaptic neuron can be chemicals like acetylcholine or glutamate. These neurotransmitters bind to and open ion channels in the postsynaptic neuron, generating an electrical signal. This transmission allows neurons to form complex communication networks and coordinate the functions of the nervous system.
At rest, ion concentrations create a resting membrane potential. Stimuli cause graded potentials by opening or closing channels, making the inside of the neuron slightly more or less negative. An action potential occurs when enough channels open to rapidly reverse the potential, signalling along the neuron. The action potential is a brief (~1 ms) electrical event that signals the neuron as 'active'. It travels the length of the axon and causes the release of neurotransmitters into the synapse.
The gap junction is an intercellular structure that serves as a pathway of low resistance for the spread of electrical currents between neurons. It is formed by the docking of two individual channels, named hemichannels or connexons, one from each of the coupled cells. Gap junctions are not exclusive to neurons and are present in virtually every tissue.
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How neurons send and receive signals
Neurons are the basic structural and functional units of the nervous system. They transmit electrical and chemical signals. The cell has three main parts: the cell body, dendrites, and axon. The cell body contains the nucleus. Dendrites receive signals, and the long axon conducts signals away from the cell body.
Neurons communicate with each other via synapses, where neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic cell. This allows signals to be transmitted electrically along neurons and chemically between neurons. At rest, ion concentrations create a resting membrane potential. Stimuli cause graded potentials by opening or closing channels, making the inside slightly more or less negative. An action potential occurs when enough channels open to rapidly reverse the potential, signalling along the neuron.
The presynaptic neuron releases neurotransmitters like acetylcholine or glutamate. These chemicals bind to and open ion channels in the postsynaptic neuron, generating an electrical signal. This transmission allows neurons to form complex communication networks and coordinate the functions of the nervous system.
The action potential and consequent transmitter release allow the neuron to communicate with other neurons. Neurotransmitters are chemicals released from a neuron following an action potential. They travel across the synapse to excite or inhibit the target neuron.
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Frequently asked questions
Neurons are the basic structural and functional units of the nervous system. They transmit electrical and chemical signals and have three main parts: the cell body, dendrites, and axon.
Neurons communicate with each other via electrical events called "action potentials" and chemical neurotransmitters. At the junction between two neurons (synapse), an action potential causes neuron A to release a chemical neurotransmitter. The neurotransmitter can either excite or inhibit neuron B from firing its own action potential.
An action potential is a brief (~1 ms) electrical event generated in the axon that signals the neuron as 'active'. It travels the length of the axon and causes the release of neurotransmitters into the synapse.
A neurotransmitter is a chemical released from a neuron following an action potential. The neurotransmitter travels across the synapse to excite or inhibit the target neuron.
Neurons have electrically charged membranes and generate electrical signals through ion channels in their membranes. At rest, ion concentrations create a resting membrane potential. Stimuli cause graded potentials by opening or closing channels, making the inside slightly more or less negative. An action potential occurs when enough channels open to rapidly reverse the potential, signalling along the neuron.
