Neuroscience Action Potentials Quiz

Action Potentials

• Action potentials (APs) are generated and conducted by voltage-activated ion channels, primarily sodium channels (Na+).
• At rest, Na+ channels are closed, but when the membrane potential reaches the threshold of -55mV, they open due to a conformational change.
• Small depolarizations do not trigger APs because Na+ channels remain closed, but they are voltage-gated and open when the membrane potential reaches the threshold.
• The electrochemical gradient drives Na+ ions into the cell, causing depolarization.
• During rapid depolarization, Na+ channels open, allowing Na+ influx and flipping the membrane potential from negative to positive.
• Na+ channels have an inactivation mechanism that automatically shuts them off after approximately 1 millisecond.
• Inactivated Na+ channels cannot open again until the membrane potential returns to the resting state, creating an absolute refractory period.
• Repolarization is driven by potassium (K+) ions moving out of the cell.
• K+ leak channels are always open, but during AP, voltage-gated K+ channels open, allowing more K+ out of the cell.
• The slow closing of these voltage-gated K+ channels leads to a hyperpolarization phase and a relative refractory period.
• The Na+/K+ pump restores ion balance over time but does not contribute directly to AP generation.
• Subthreshold stimulation produces an excitatory potential that is not strong enough to elicit an AP.
• Supra-threshold stimulation produces an excitatory potential that exceeds the threshold, generating an AP that propagates down the axon.
• Conduction in unmyelinated axons is slow because Na+ channels are present along the entire axon, requiring more Na+ influx for signal propagation.
• Myelin insulates axons, reducing ion leakage and speeding up conduction.
• In myelinated axons, Na+ channels are concentrated at the nodes of Ranvier, the gaps between myelin sheaths.
• Myelinated axons conduct APs faster because fewer Na+ channels are required for signal propagation.
• APs travel in one direction because Na+ channels in the wake of the AP are inactivated, preventing retrograde propagation.

Dopamine and Schizophrenia

• Schizophrenia medications are dopamine D2 receptor antagonists, blocking D2 receptors to counteract the effects of excessive dopamine in the brain.
• The dopamine theory of schizophrenia suggests that high levels of dopamine are associated with the disorder's positive symptoms.
• Individuals with schizophrenia do not exhibit higher baseline levels of pleasure, but their positive symptoms (delusions, hallucinations, disorganized thoughts and speech) are reduced by D2 receptor antagonists.

Psychostimulants and Dopamine

• Psychostimulants act as agonists on the monoamine systems, particularly dopamine, norepinephrine, and serotonin.
• These drugs induce various effects, including euphoria, but high doses can cause temporary psychosis.
• The effects of psychostimulants on dopamine are similar to those observed with schizophrenia.

Separating Pleasure from Motivation

• Studies using the T-maze task have shown that dopamine antagonists reduce motivation but not pleasure.
• In the task, rats were presented with options involving different levels of effort and reward.
• Dopamine antagonists caused rats to shift their preference towards lower effort, higher reward options despite not affecting their ability to enjoy rewards.

Dopamine and Reward Prediction Error

• Dopamine neurons in the ventral tegmental area (VTA) exhibit specific firing patterns related to reward prediction.
• At first, they fire in response to unexpected rewards, but after learning, they fire in anticipation of expected rewards.
• When a predicted reward is not delivered, dopamine neuron firing decreases.
• Dopamine appears to play a role in encoding reward prediction error, the difference between expected and actual rewards.

Hallucinogens

• Psychedelic drugs act as serotonin receptor agonists, altering conscious perception and thoughts with minimal impact on mood.
• Recent research is exploring the therapeutic potential of hallucinogens for conditions like end-of-life care, PTSD, addiction, and more.
• Serotonin plays a crucial role in organizing cortical networks, contributing to our perception of sensory patterns.

Acetylcholine

• Acetylcholine was the first discovered neurotransmitter, found at the neuromuscular junction.
• Motor neurons release acetylcholine into muscle tissue, causing muscle contractions in both skeletal and smooth muscles.
• Acetylcholine is also present in the basal forebrain, implicated in functions like wakefulness and attention.
• Nicotine is an acetylcholine receptor agonist, activating acetylcholine receptors and causing various effects, including mild psychostimulant effects.

Endocannabinoids

• Endocannabinoids are neurotransmitters that bind to cannabinoid receptors, both of which are G protein-coupled receptors (GPCRs).
• Unlike conventional neurotransmitters, endocannabinoids are produced on demand in dendrites and not stored in vesicles.
• Endocannabinoids can travel from dendrites to axons, demonstrating retrograde transmission.
• They have an inhibitory effect, weakening the connection between two cells at the synapse by reducing neurotransmitter release.
• THC in cannabis is a cannabinoid receptor agonist, potentially explaining the memory impairments associated with heavy cannabis use.

Adenosine

• Adenosine is a byproduct of ATP metabolism.
• Adenosine receptors build up throughout the day, increasing inhibition and contributing to sleepiness.

The Hypothalamus and Neuroendocrine Cells

• The hypothalamus (HTh) is the link between the nervous system and the endocrine system.
• It contains neuroendocrine cells (neurosecretory cells) that release hormones into the bloodstream rather than onto neurons.
• Some hormones have dual functions as neurotransmitters, such as epinephrine and norepinephrine.

Hormone Types

• Peptide hormones are large chains of amino acids and typically act outside of the cell.
• Amine hormones are small, single-segment molecules, with some acting inside the cell.
• Steroid hormones are unique in their ability to cross cell membranes, allowing them to act inside the cell.

Hormone Receptor Types

• Membrane receptors, such as GPCRs, are located on the cell surface and generally mediate fast hormonal effects.
• Intracellular receptors, located near the nucleus, primarily function as transcription factors and typically mediate slow hormonal effects.
• Steroid hormones can act through both membrane and intracellular receptors.

Methods for Measuring Hormones and Receptors

• Radioimmunoassay measures hormone levels in blood, providing information about hormone prevalence.
• Autoradiography identifies brain areas affected by hormones by using radioactive hormones to label their binding sites.

Test your understanding of action potentials and how voltage-activated ion channels, particularly sodium channels, function in generating and conducting electric signals in neurons. This quiz covers the mechanisms of depolarization, inactivation, and repolarization associated with action potentials.