What are cholinergic neurons, and where are three major places they are found?
Neurons that use acetylcholine (ACh) as their primary neurotransmitter.
Key Locations:
Motor neurons at neuromuscular junctions. (used to excite muscle).
Neurons throughout the autonomic nervous system.
Neurons in the brainstem regulating arousal, sleep, and attention. to make sure we are awake during the day.
Local interneurons in other brain regions (e.g., the striatum).
Describe the specific and different usage of acetylcholine in the sympathetic vs. parasympathetic nervous systems.
Autonomic Nervous System Context: Controls involuntary bodily functions (e.g., heart rate, digestion). It uses a two-neuron chain: preganglionic and postganglionic.
Parasympathetic Nervous System (“Rest and Digest”): ACh is used as the neurotransmitter for BOTH neurons in the chain.
Preganglionic neurons → ACh → Postganglionic neurons → ACh → Target Organs
Sympathetic Nervous System (“Fight or Flight”): ACh is used ONLY by the preganglionic neurons.
Preganglionic neurons → ACh → Postganglionic neurons → Norepinephrine → Target Organs
What are the specific roles of cholinergic neurons within the central nervous system?
Arousal, Sleep, and Attention: Specific cholinergic neurons in the brainstem and basal forebrain are crucial for maintaining the sleep-wake cycle, promoting wakefulness, and ensuring we are attentive to sensory stimuli.
Local Circuit Computation: Interneurons in regions like the striatum use ACh for specialized local processing, distinct from the long-range arousal systems.
What are the two precursors for acetylcholine, and what enzyme catalyzes their reaction in the presynaptic terminal?
Precursors:
Acetyl-CoA: A central metabolic compound from the citric acid cycle.
Choline: An amino alcohol used in lipid synthesis.
Enzyme: Choline acetyltransferase (ChAT).
Reaction: This enzyme transfers the acetyl group from acetyl-CoA to choline, forming an ester bond to create acetylcholine.
How is acetylcholine concentrated inside synaptic vesicles?
Transporter: The Vesicular Acetylcholine Transporter (VAChT).
Mechanism: VAChT is a solute carrier that performs secondary active transport.
Energy Source: It functions as an antiporter, using the energy from protons flowing out of the vesicle (down their gradient) to power the transport of acetylcholine into the vesicle against its concentration gradient.
How is the acetylcholine signal in the synapse terminated, and how is the choline precursor conserved?
Degradation: The enzyme Acetylcholinesterase (AChE) in the synaptic cleft rapidly hydrolyzes acetylcholine, breaking the ester bond.
Products: This reaction yields acetate and choline.
Recycling: The choline is taken back up into the presynaptic terminal by specific transporters to be reused for synthesizing new acetylcholine.How is the acetylcholine signal in the synapse terminated, and how is the choline precursor conserved?
What is the underlying cause of the autoimmune disease Myasthenia Gravis?
Mechanism: The body produces autoantibodies that attack and bind to nicotinic acetylcholine receptors on the postsynaptic membrane of muscle cells at the neuromuscular junction.
Result: This reduces the number of functional receptors, impeding synaptic transmission and causing muscle weakness and fatigue, especially during sustained activity.
How do acetylcholinesterase inhibitors help treat the symptoms of Myasthenia Gravis?
Drug Action: They inhibit the enzyme acetylcholinesterase (AChE) in the synaptic cleft.
Effect: This inhibition slows the breakdown of acetylcholine, leading to a longer-lasting presence of the neurotransmitter in the cleft.
Therapeutic Benefit: The prolonged ACh activity partially compensates for the reduced number of receptors, allowing for more effective synaptic transmission and helping to prevent muscle weakness.
What type of receptor is a nicotinic acetylcholine receptor (nAChR), what ions does it conduct, and what is the resulting postsynaptic potential?
Type: Ionotropic receptor (ligand-gated ion channel).
Ion Permeability: Permeable to sodium (Na⁺) and potassium (K⁺), and some subtypes are also permeable to calcium (Ca²⁺).
Result: Its reversal potential is far more positive than the action potential threshold, so its activation always generates a fast Excitatory Postsynaptic Potential (EPSP).
Describe the structure of a nicotinic receptor and explain the source of its functional diversity.c
Structure: A pentameric complex composed of five protein subunits that form a central ion pore.
Source of Diversity: The subunits are encoded by several different genes. The specific combination of subunits used to assemble the receptor determines its functional properties, such as calcium permeability, conductance, and activation kinetics.
Where in the nervous system are nicotinic acetylcholine receptors found?
Neuromuscular Junctions: On muscle cells.
Autonomic Nervous System: At synapses between preganglionic and postganglionic neurons in both the sympathetic and parasympathetic divisions.
Central Nervous System: At many cholinergic synapses throughout the brain.
What are the key agonist and antagonist for nicotinic receptors, and what are their effects?
Agonist: Nicotine
Source: Tobacco.
Effect: Stimulates the receptor, acting as an agonist.
Antagonist: α-Bungarotoxin
Source: Venom of the banded krait snake.
Effect: Binds to the receptor and prevents it from opening, causing paralysis by blocking neurotransmission at neuromuscular junctions.
What type of receptors are muscarinic acetylcholine receptors (mAChRs), and how can they produce opposite effects?
Type: A family of metabotropic receptors (GPCRs), with five subtypes (M1-M5).
Divergent Signaling: They couple to different G proteins:
Gq protein: Activation decreases potassium (K⁺) channel conductance, leading to slow Excitatory Postsynaptic Potentials (EPSPs).
Gi protein: Activation increases potassium (K⁺) channel conductance, leading to slow Inhibitory Postsynaptic Potentials (IPSPs).
Where are muscarinic acetylcholine receptors primarily located in the body?
Peripheral Nervous System: On the target cells of parasympathetic innervation, such as smooth muscle cells (e.g., in airways, iris) and secretory cells.
Central Nervous System: In neurons of the forebrain, especially in regions like the striatum.
Name three clinical antagonists of muscarinic receptors and their medical applications.
Atropine:
Source: Plant alkaloid from deadly nightshade (Atropa belladonna).
Use: Causes pupillary dilation (mydriasis) for eye examinations.
Scopolamine:
Use: Prevents motion sickness.
Ipratropium:
Use: Treats asthma by inhibiting parasympathetic input, leading to bronchodilation.
What is the dual role of glutamate in the body?
Primary Role: An abundant non-essential amino acid used for protein synthesis.
Neurotransmitter Role: It is the major excitatory neurotransmitter in the central nervous system (CNS).
How prevalent is glutamate as a neurotransmitter in the brain?
Prevalence: It is used by more than 50% of all synapses in the brain.
Function: It is the primary neurotransmitter for most excitatory synapses throughout the central nervous system (CNS).
Where is glutamate used as a neurotransmitter outside of the brain (in the peripheral nervous system)?
Location: It is the neurotransmitter for many primary sensory neurons and sensory receptor cells.
Examples:
Photoreceptors in the retina.
Olfactory receptor neurons in the nose.
How is the neurotransmitter glutamate synthesized and concentrated into synaptic vesicles?
Synthesis: In the presynaptic terminal, the enzyme glutaminase converts the precursor glutamine into glutamate by hydrolyzing its amide group.
Packaging: The vesicular glutamate transporter (VGLUT) loads glutamate into synaptic vesicles via secondary active transport.
Energy Source: VGLUT is an antiporter that uses the energy from protons flowing out of the vesicle down their gradient to power glutamate uptake against its concentration gradient.
Describe the process that clears glutamate from the synapse and recycles it for re-use.
Step 1 - Clearance: After release, glutamate is rapidly cleared from the synaptic cleft by excitatory amino acid transporters (EAATs) on nearby glia and neurons.
Step 2 - Conversion in Glia: Inside glial cells, glutamate is converted back into glutamine.
Step 3 - Shuttling: Glutamine is transported out of the glia and taken up by the presynaptic neuron.
Step 4 - Re-synthesis: In the neuron, glutamine is converted back to glutamate by glutaminase, completing the cycle.
Why is it critical to rapidly clear glutamate from the synaptic cleft?
Reason: Glutamate is the main excitatory neurotransmitter.
Consequence: If it persists in the extracellular space, it can cause overexcitation of surrounding neurons, leading to excitotoxicity and potential cell damage. The glutamate-glutamine cycle ensures precise, transient signaling.
What are the three families of ionotropic glutamate receptors, and how did they get their names?
Families:
AMPA receptors
NMDA receptors
Kainate receptors
Naming: They are named after synthetic agonists that selectively activate them: AMPA, NMDA, and kainic acid.
What ions do ionotropic glutamate receptors conduct, and what type of postsynaptic potential do they always produce?
Ion Permeability: All conduct sodium (Na⁺) and potassium (K⁺) ions. Some subtypes are also permeable to calcium (Ca²⁺).
Result: Their reversal potential is always more positive than the action potential threshold. Therefore, their activation always generates a fast Excitatory Postsynaptic Potential (EPSP).
Describe the structure of ionotropic glutamate receptors and their distribution in the nervous system.
Structure: Composed of four subunits. Multiple genes encode these subunits, leading to great diversity in properties like conductance and kinetics.
Distribution:
AMPA & NMDA receptors: Found together at most glutamatergic synapses in the brain.
Kainate receptors: Present at some synapses, and can be located on both postsynaptic and presynaptic membranes ( will change the membrane potenial,, have feedback system with what regulates glutamate).
