Systems Biology
Neurotransmitters facilitate the communication between neurons at synapses. A synapse consists of a presynaptic neuron, a postsynaptic neuron or other receiving cell, and the synaptic cleft between them. Neurotransmitters are stored inside vesicles in the nerve terminal of the presynaptic cell and are released into the synaptic cleft when an action potential arrives. Receptors on the postsynaptic cell are either ligand-gated or G protein-coupled receptors. Depending on the receptor, a change in membrane polarization occurs, inducing an excitatory or inhibitory response in the postsynaptic cell.
Some important neurotransmitters are norepinephrine, epinephrine, acetylcholine, dopamine, serotonin, GABA, and glutamate, which play crucial roles in various brain functions and behaviors, including the fight-or-flight response, alertness, muscle activation, pleasure/reward pathway, mood regulation, sleep/wake cycle, and inhibition or excitation. To cease transmission at a synapse, there are three methods: enzymatic breakdown, reuptake, or diffusion, all of which remove neurotransmitters from the synapse and end their ability to cause a response in the postsynaptic cell.
Lesson Outline
<ul> <li>Introduction to synapse and neurotransmitters</li> <ul> <li>Synapse: junction between two cells (typically neurons) that use neurotransmitters to send chemical signals</li> <li>Common mistake: calling just the gap between neurons the synapse</li> <li>Components of a synapse: presynaptic neuron, postsynaptic neuron (or other receiving cell), and synaptic cleft</li> </ul> <li>Presynaptic neuron</li> <ul> <li>Releases neurotransmitters from the nerve terminal at the end of its axon</li> </ul> <li>Postsynaptic neuron</li> <ul> <li>Contains receptors that neurotransmitters bind to</li> </ul> <li>Synaptic cleft: the gap between the presynaptic and postsynaptic neurons</li> <li>Types of synapses</li> <ul> <li>Between two neurons (most common)</li> <li>Neuromuscular junction: between a neuron and a muscle cell</li> <li>Neuroglandular junction: between a neuron and a gland</li> </ul> <li>Role of calcium ions</li> <ul> <li>Enter the presynaptic nerve terminal upon depolarization</li> <li>Trigger release of neurotransmitters from vesicles</li> </ul> <li>Receptors on the postsynaptic neuron</li> <ul> <li>Ionotropic or ligand-gated: open ion channels, directly impacting membrane polarization</li> <li>Metabotropic or G protein-coupled: initiate secondary messenger cascades, leading to various downstream effects</li> </ul> <li>Excitatory and inhibitory responses</li> <ul> <li>Excitatory: influx of positive ions, resulting in depolarization (less negative)</li> <li>Inhibitory: influx of negative ions, resulting in hyperpolarization (more negative)</li> </ul> <li>Common neurotransmitters and their roles</li> <ul> <li>Norepinephrine: mostly excitatory, responsible for alertness</li> <li>Epinephrine: mostly excitatory, involved in the fight-or-flight response</li> <li>Acetylcholine: usually excitatory, activates skeletal muscle fibers</li> <li>Dopamine: can be excitatory or inhibitory, associated with pleasure/reward pathways and motor movement regulation</li> <li>Serotonin: generally inhibitory, regulates mood and sleep/wake cycle</li> <li>GABA: key inhibitory neurotransmitter in the brain</li> <li>Glutamate: key excitatory neurotransmitter in the central nervous system</li> </ul> <li>Stopping neurotransmitter signals</li> <ul> <li>Enzymatic breakdown: enzymes break down neurotransmitters, rendering them useless</li> <li>Reuptake: neurotransmitters are taken back up by the presynaptic neuron to be repackaged into vesicles</li> <li>Diffusion: neurotransmitters disperse and dilute, reducing their likelihood of binding to a receptor</li> </ul> </ul>
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FAQs
Neurotransmitters are chemical messengers that transmit signals across the synaptic cleft between the presynaptic and postsynaptic neurons. When an action potential reaches the synapse, neurotransmitter molecules are released into the synaptic cleft. These molecules bind to specific receptor sites on the postsynaptic neuron, triggering membrane polarization and modulating the activity of the postsynaptic neuron. This process enables the propagation of action potentials and the integration of neural information.
When neurotransmitters bind to receptors on the membrane of the postsynaptic neuron, they can either cause depolarization or hyperpolarization. Excitatory neurotransmitters such as glutamate typically promote depolarization, while inhibitory neurotransmitters like GABA often lead to hyperpolarization. The change in membrane potential depends on the type of neurotransmitter and the receptor it binds to. Membrane polarization influences the likelihood of an action potential being generated in the postsynaptic neuron, allowing for signal transmission and synaptic integration.
Some common neurotransmitters include norepinephrine, dopamine, and serotonin. Norepinephrine is involved in the flight-or-fight response and plays a role in attention, alertness, and arousal. Dopamine is associated with reward and motivation and is crucial for motor function. Serotonin is involved in the regulation of mood, appetite, and sleep. Each neurotransmitter has a specific function within the nervous system and can be excitatory, inhibitory, or have modulatory effects depending on the receptor types and neuronal circuits involved.
The synaptic cleft is the extracellular space between the presynaptic and postsynaptic neurons where neurotransmission takes place. When an action potential arrives at the presynaptic neuron's axon terminal, voltage-gated calcium channels open, allowing calcium to enter the cell. This influx of calcium triggers vesicles containing neurotransmitters to merge with the presynaptic membrane, releasing the neurotransmitters into the synaptic cleft. The neurotransmitters then diffuse across the cleft and bind to specific receptors on the postsynaptic neuron, which can result in membrane polarization and the generation or inhibition of an action potential in the postsynaptic neuron.
After neurotransmitters are released into the synaptic cleft, they can either bind to receptors, be taken back into the presynaptic neuron via transporter proteins through a process called reuptake, or be broken down by enzymes. The balance between release, reuptake, and degradation ensures the proper functioning of neurotransmission and helps regulate neuronal activity. Some drugs, such as selective serotonin reuptake inhibitors (SSRIs), specifically target the reuptake of neurotransmitters to increase their availability and modulate neural communication.
