Neuro AM 03-24-25

Synapse StructureTwo synapses depicted:

• Synapse A: Between a neuron releasing an excitatory neurotransmitter and neuron A. Excitatory synapses facilitate action potentials by depolarizing the postsynaptic neuron.

• Synapse B: Between a neuron releasing an inhibitory neurotransmitter and neuron B. Inhibitory synapses prevent action potentials by hyperpolarizing the postsynaptic neuron, stabilizing neural circuits.

Membrane PotentialsMeasuring membrane potentials when neurotransmitters are released:

• Excitatory neurotransmitter: Increases membrane potential (depolarization), allowing Na+ ions to flow into the neuron, making it more likely to fire an action potential.

• Inhibitory neurotransmitter: Decreases membrane potential (hyperpolarization), often allowing Cl- ions to enter or K+ ions to exit, making it less likely for the neuron to reach the threshold for firing.

Key Definitions

• Depolarization: A process that makes a neuron’s membrane potential more positive, leading to activation (excitation) of the neuron and potential action generation.

• Hyperpolarization: A process causing the membrane potential to become more negative, inhibiting neuron activity and making it harder to stimulate action potential.

Neurotransmitter ActionsNeurotransmitters can be classified as excitatory or inhibitory, significantly affecting cellular responses:

• Excitatory (e.g., glutamate) = depolarization, which increases the likelihood of action potential firing.

• Inhibitory (e.g., GABA) = hyperpolarization, reducing the likelihood of action potential generation.

• Some neurotransmitters can exert both excitatory and inhibitory effects depending on the type of receptors they bind to, showcasing their versatility in neurotransmission.

Neurotransmitter LevelsImbalances in neurotransmitter levels can significantly contribute to various neurological and psychiatric disorders:

• Too low: May require agonist medications to stimulate receptors, potentially alleviating symptoms of disorders such as depression or anxiety.

• Too high: May require antagonist medications to block receptors, such as in conditions where hyperactivity of neurotransmitters like dopamine leads to psychosis.

Agonists and Antagonists

• Agonist: Mimics neurotransmitter action, effectively stimulating a receptor when natural neurotransmitter levels are too low (e.g., certain medications for depression).

• Antagonist: Blocks neurotransmitter action, inhibiting a receptor when levels are too high (e.g., antipsychotic drugs).

• Reuptake Inhibitors: Slow down the reabsorption of neurotransmitters, maintaining higher levels in the synaptic cleft, allowing for prolonged signaling and are commonly used in treating conditions like depression (e.g., SSRIs).

Acetylcholine Overview

• Acetylcholine can be both excitatory and inhibitory based on its target cell interactions and receptor types, influencing various physiological processes.

  • Excites skeletal muscle by binding to nicotinic receptors, leading to muscle contractions essential for movement.

  • Inhibits heart muscle activity through muscarinic receptors, slowing heart rate and contributing to parasympathetic control of the heart.

Functions of Acetylcholine

• Memory: A critical neurotransmitter for learning and the formation of memories, primarily through its action in the hippocampus.

• Attention & Sleep: Plays significant roles in maintaining attention, modulating arousal, and regulating sleep patterns through cholinergic pathways.

• Arousal: Involved in enhancing alertness and facilitating cognitive processes essential for responsiveness to the environment.

Acetylcholine Abnormalities

• Alzheimer's Disease: Characterized by decreased levels of acetylcholine, leading to symptoms such as memory loss and cognitive decline. Treatments may involve acetylcholine reuptake inhibitors to improve cholinergic signaling.

Monoamine Neurotransmitters

• Serotonin: Linked to mood regulation, temperature control, and nausea/vomiting feedback mechanisms; crucial in mood disorders.

• Norepinephrine: Influences attention, decision-making, and overall mood; imbalances can impact anxiety and depression severity.

• Dopamine: Associated with pleasure, reward, and movement regulation; dysregulation can lead to movement disorders, such as Parkinson’s disease, and psychiatric conditions like schizophrenia.

Treatment for Imbalances

• Low levels of serotonin and dopamine can lead to conditions like depression and anxiety, commonly treated with reuptake inhibitors and psychotherapy targeting neurotransmitter regulation.

Amino Acid Neurotransmitters

• Glutamate: Exclusively excitatory; vital for synaptic plasticity involved in learning and memory processes, but excess can lead to neuronal toxicity and seizures.

• Seizure treatment involves administering antagonists to the glutamate receptors to reduce excitotoxicity.

• GABA: The sole inhibitory neurotransmitter, crucial for regulating neuronal excitability and overall balance within the nervous system, preventing overactive neuronal firing.