Neurotransmitters and Receptors — MCQs
Mechanism of action of d-tubocurarine is:
Which of the following neurotransmitters is primarily released from the sympathetic nervous system to increase heart rate in response to a DECREASE in blood pressure?
Purkinje cells of the cerebellar cortex project to:
What neurotransmitter is found in increased quantities in mania?
Which of the following is true regarding Lambert-Eaton myasthenic syndrome?
What is the neurotransmitter primarily involved in muscle contraction?
Which neurotransmitter primarily mediates slow synaptic transmission in the enteric nervous system?
What is the primary site of action of tetanospasmin in the nervous system?
Which of the following actions is NOT associated with tricyclic antidepressants?
Which neurotransmitter is primarily responsible for parasympathetic effects on heart rate?
Neurotransmitters and Receptors Indian Medical PG Practice Questions and MCQs
Question 1: Mechanism of action of d-tubocurarine is:
- A. Competitive, nondepolarizing block at the Nm cholinergic receptor (Correct Answer)
- B. Noncompetitive, depolarizing block at the Nm cholinergic receptor
- C. Non-competitive, nondepolarizing block at the Nm cholinergic receptor
- D. Competitive, depolarizing block at the Nm cholinergic receptor
Explanation: ***Competitive, nondepolarizing block at the Nm cholinergic receptor*** - **d-tubocurarine** acts as a **competitive antagonist** at the **nicotinic muscle (Nm) cholinergic receptors** on the motor endplate. - It competes with **acetylcholine (ACh)** for binding sites, preventing ACh from activating the receptor and causing **muscle paralysis** without depolarization. *Noncompetitive, depolarizing block at the Nm cholinergic receptor* - This describes the mechanism of action of **depolarizing neuromuscular blockers** like **succinylcholine**, which first *depolarize* the motor endplate before causing paralysis. - d-tubocurarine does not cause initial depolarization; it directly blocks the receptor. *Non-competitive, nondepolarizing block at the Nm cholinergic receptor* - While d-tubocurarine is **nondepolarizing**, it is a **competitive antagonist**, not a non-competitive one. - A non-competitive block would involve binding to a different site on the receptor or an associated ion channel, altering receptor function indirectly. *Competitive, depolarizing block at the Nm cholinergic receptor* - This option incorrectly combines the concepts, as **depolarizing blockers** like succinylcholine act initially by **depolarizing** the endplate, whereas d-tubocurarine is purely a **nondepolarizing** agent. - The "competitive" aspect would be true for the binding of ACh to its site on a depolarizing agent, but the effect of d-tubocurarine is simply to block, not depolarize.
Question 2: Which of the following neurotransmitters is primarily released from the sympathetic nervous system to increase heart rate in response to a DECREASE in blood pressure?
- A. Norepinephrine (Correct Answer)
- B. Dopamine
- C. Acetylcholine
- D. Epinephrine
Explanation: ***Norepinephrine*** - **Norepinephrine** is the primary neurotransmitter released by **postganglionic sympathetic neurons** directly onto the heart to increase heart rate and contractility in response to a drop in blood pressure. - It acts on **beta-1 adrenergic receptors** in the sinoatrial (SA) node, atria, and ventricles, leading to increased chronotropy (heart rate) and inotropy (contractility). *Dopamine* - While **dopamine** can have cardiovascular effects, particularly at high doses, it is not the primary neurotransmitter released by the sympathetic nervous system for direct heart rate regulation. - Dopamine is a precursor to norepinephrine and epinephrine, but its main physiological roles involve **renal blood flow regulation** and central nervous system functions. *Acetylcholine* - **Acetylcholine** is the primary neurotransmitter of the **parasympathetic nervous system**, which generally acts to **decrease heart rate** (bradycardia) through muscarinic receptors. - It is also released by **preganglionic sympathetic fibers**, but these do not directly innervate the heart to produce the desired effect of increasing heart rate. *Epinephrine* - **Epinephrine** (adrenaline) is primarily a **hormone** released from the **adrenal medulla** into the bloodstream, not directly from postganglionic sympathetic nerve terminals to the heart. - Although it has strong effects on beta-1 receptors in the heart, its release is more generalized and slower than the direct neuronal release of norepinephrine.
Question 3: Purkinje cells of the cerebellar cortex project to:
- A. Extrapyramidal system
- B. Cranial nerve nuclei
- C. Cerebellar nuclei (Correct Answer)
- D. Cerebral cortex
Explanation: ***Cerebellar nuclei*** - **Purkinje cells** are the sole output neurons of the cerebellar cortex and project inhibitory GABAergic signals to the **deep cerebellar nuclei** [1]. - These **deep cerebellar nuclei** then serve as the main output of the cerebellum, relaying modulated signals to various brainstem and thalamic targets [1]. *Extrapyramidal system* - The **extrapyramidal system** primarily involves motor pathways originating in the brainstem, such as the rubrospinal and reticulospinal tracts, and influences motor control. - While the cerebellum indirectly influences the extrapyramidal system via its connections to the brainstem and thalamus, Purkinje cells do not directly synapse on its components. *Cranial nerve nuclei* - **Cranial nerve nuclei** are collections of neuronal cell bodies in the brainstem that control the functions of the cranial nerves. - Purkinje cells do not directly project to these nuclei; rather, cerebellar outputs from the deep cerebellar nuclei modulate activity in regions that then influence cranial nerve functions. *Cerebral cortex* - The **cerebral cortex** is the outer layer of the cerebrum, responsible for higher-level functions, and receives input from the thalamus, not direct Purkinje cell projections [2]. - While the cerebellum and cerebral cortex interact extensively, this interaction is an indirect loop involving the **thalamus** and pons, not direct synapses from Purkinje cells [2].
Question 4: What neurotransmitter is found in increased quantities in mania?
- A. Noradrenaline
- B. Serotonin
- C. GABA
- D. Dopamine (Correct Answer)
Explanation: ***Dopamine*** - **Dopamine levels are consistently elevated during manic episodes**, representing one of the most robust neurobiological findings in mania. - Increased dopaminergic activity contributes to **psychomotor agitation**, **goal-directed behavior**, **reward-seeking**, **decreased need for sleep**, and **psychotic features** (delusions, hallucinations). - **Dopamine antagonists** (antipsychotics like haloperidol, olanzapine) are **first-line treatments** for acute mania, supporting the dopamine hypothesis. - The dopamine hypothesis of mania is well-established in psychiatric literature and supported by neuroimaging studies. *Noradrenaline* - Noradrenergic hyperactivity is also implicated in mania, contributing to increased arousal, energy, and reduced sleep. - However, while elevated, the evidence is less consistent than for dopamine, and noradrenaline's role appears more modulatory. - The question asks for the neurotransmitter "found in increased quantities" - both are elevated, but dopamine has stronger evidence. *Serotonin* - Serotonin dysfunction is implicated in mood disorders, but manic episodes are generally associated with **reduced or dysregulated serotonin activity**, not an increase. - Low serotonin may contribute to impulsivity and mood instability in bipolar disorder. *GABA* - **GABA** (gamma-aminobutyric acid) is the primary **inhibitory neurotransmitter** in the CNS. - **Reduced GABAergic activity** has been reported in mania (not increased), which may contribute to disinhibition and hyperexcitability. - GABAergic drugs (e.g., benzodiazepines, valproate) are used as adjuncts in mania, supporting reduced GABA activity.
Question 5: Which of the following is true regarding Lambert-Eaton myasthenic syndrome?
- A. Deep tendon reflexes are preserved
- B. Pyridostigmine can be used as treatment
- C. Associated with autoantibodies against P/Q-type calcium channels (Correct Answer)
- D. It is a presynaptic disorder of the neuromuscular junction that can cause weakness
Explanation: ***Associated with autoantibodies against P/Q-type calcium channels*** - **Lambert-Eaton Myasthenic Syndrome (LEMS)** is an **autoimmune disorder** where antibodies attack **presynaptic P/Q-type voltage-gated calcium channels** at the neuromuscular junction [1]. - This attack impairs the release of **acetylcholine**, leading to **muscle weakness**, particularly in the proximal limbs [2]. *Deep tendon reflexes are preserved* - In LEMS, **deep tendon reflexes are typically diminished or absent** at rest due to insufficient neurotransmitter release [2]. - Reflexes may transiently improve with **post-tetanic potentiation** after sustained muscle contraction [2]. *Pyridostigmine can be used as treatment* - **Pyridostigmine**, an **acetylcholinesterase inhibitor**, has limited efficacy in LEMS because the primary defect is in **acetylcholine release**, not its breakdown [1]. - While it may provide some mild benefit, it is **less effective** compared to its use in **myasthenia gravis** [1]. *It is a presynaptic disorder of the neuromuscular junction that can cause weakness* - While LEMS is indeed a **presynaptic disorder of the neuromuscular junction** that causes weakness, this statement is **less specific** than the correct option. - The most defining characteristic, which is specific to its pathophysiology, is the presence of **autoantibodies against P/Q-type calcium channels** [2].
Question 6: What is the neurotransmitter primarily involved in muscle contraction?
- A. Glutamate
- B. Acetylcholine (Correct Answer)
- C. Dopamine
- D. Serotonin
Explanation: ***Acetylcholine*** - **Acetylcholine (ACh)** acts at the **neuromuscular junction** to initiate muscle contraction by binding to nicotinic receptors on the muscle fiber membrane. - This binding causes depolarization and triggers the release of **calcium** from the sarcoplasmic reticulum, essential for the interaction of actin and myosin filaments. *Glutamate* - **Glutamate** is the primary **excitatory neurotransmitter** in the central nervous system, mainly involved in synaptic transmission, learning, and memory. - It does not mediate signal transmission at the **neuromuscular junction** for skeletal muscle contraction. *Dopamine* - **Dopamine** is a neurotransmitter involved in reward, motivation, and motor control pathways within the **central nervous system** (basal ganglia). - It does not play a direct role in the peripheral process of **skeletal muscle contraction** at the neuromuscular junction. *Serotonin* - **Serotonin** primarily regulates mood, sleep, appetite, and gastrointestinal function in the **central nervous system**. - It is not involved in directly signaling **skeletal muscle fibers** for contraction at the neuromuscular junction.
Question 7: Which neurotransmitter primarily mediates slow synaptic transmission in the enteric nervous system?
- A. Substance P
- B. Serotonin
- C. Acetylcholine
- D. Nitric oxide (Correct Answer)
Explanation: **Nitric oxide** - **Nitric oxide (NO)** is a key **non-classical neurotransmitter** in the **enteric nervous system (ENS)**, mediating **slow synaptic transmission** due to its gaseous nature allowing for diffusion and longer-lasting effects. - It is involved in **smooth muscle relaxation**, **vasodilation**, and diverse gastrointestinal functions, including **peristalsis** and **sphincter relaxation**. *Substance P* - **Substance P** is a **neuropeptide** that acts as an **excitatory neurotransmitter** in the ENS, primarily mediating **fast synaptic transmission** and smooth muscle contraction. - It is involved in pain perception, inflammation, and is released by sensory neurons and some enteric neurons. *Serotonin* - **Serotonin (5-HT)** is a major neurotransmitter in the ENS, largely mediating **fast excitatory or inhibitory synaptic transmission** depending on the receptor subtype. - It plays a crucial role in regulating gut motility, secretion, and visceral sensation, and is involved in both rapid signaling and neuromodulation. *Acetylcholine* - **Acetylcholine (ACh)** is the primary **excitatory neurotransmitter** of the **parasympathetic nervous system** within the ENS, mediating **fast synaptic transmission** by binding to nicotinic and muscarinic receptors. - It is crucial for stimulating **smooth muscle contraction** (promoting peristalsis), increasing glandular secretions, and generally enhancing gut motility.
Question 8: What is the primary site of action of tetanospasmin in the nervous system?
- A. Presynaptic terminals of the spinal cord (Correct Answer)
- B. Postsynaptic terminals of the spinal cord
- C. Neuromuscular junction
- D. Muscle fibers
Explanation: ***Presynaptic terminals of the spinal cord*** - **Tetanospasmin** is transported via **retrograde axonal transport** to the central nervous system, specifically targeting the **presynaptic terminals** of inhibitory interneurons in the spinal cord. - It interferes with the release of **inhibitory neurotransmitters** like **GABA** and **glycine**, leading to uncontrolled muscle spasms. *Postsynaptic terminals of the spinal cord* - This is incorrect because tetanospasmin acts by preventing the release of inhibitory neurotransmitters from the **presynaptic terminal**, rather than directly affecting the postsynaptic receptor. - While the absence of inhibition is perceived at the postsynaptic terminal, the direct mechanism of action is presynaptic. *Neuromuscular junction* - This is incorrect because **tetanospasmin** does not primarily act at the neuromuscular junction; that is the site of action for toxins like **botulinum toxin**. - Tetanospasmin is transported to the central nervous system to exert its effects. *Muscle fibers* - This is incorrect as **tetanospasmin** does not act directly on muscle fibers. - Its action is on the **nervous system**, leading to altered neuronal signaling that indirectly affects muscle contraction.
Question 9: Which of the following actions is NOT associated with tricyclic antidepressants?
- A. Block 5-HT or NE reuptake
- B. Anticholinergic action
- C. MAO inhibition (Correct Answer)
- D. Causes sedation
Explanation: ***MAO inhibition*** - Tricyclic antidepressants (TCAs) primarily exert their effects by inhibiting the reuptake of **norepinephrine** and **serotonin**, not by inhibiting monoamine oxidase (MAO). - **MAO inhibitors** are a distinct class of antidepressants with a different mechanism of action and side effect profile. *Anticholinergic action* - Many TCAs have significant **anticholinergic effects**, blocking muscarinic receptors and leading to side effects like dry mouth, constipation, and blurred vision. - These effects contribute to the **adverse event profile** of TCAs, especially in elderly patients. *Block 5-HT or NE reuptake* - The primary mechanism of action of TCAs involves the **inhibition of serotonin (5-HT)** and **norepinephrine (NE) reuptake** into presynaptic neurons. - This action increases the concentration of these neurotransmitters in the **synaptic cleft**, thereby potentiating their effects. *Causes sedation* - TCAs frequently cause **sedation**, particularly the more histaminergic ones (e.g., amitriptyline, doxepin), due to their **histamine H1 receptor antagonism**. - This side effect can be beneficial for patients with insomnia but can be problematic for daytime functioning.
Question 10: Which neurotransmitter is primarily responsible for parasympathetic effects on heart rate?
- A. Norepinephrine
- B. Dopamine
- C. Acetylcholine (Correct Answer)
- D. Epinephrine
Explanation: ***Acetylcholine*** - **Acetylcholine** is the primary neurotransmitter released by postganglionic parasympathetic neurons. - It acts on **muscarinic receptors** (M2 receptors) in the heart to decrease heart rate. *Norepinephrine* - **Norepinephrine** is primarily associated with the **sympathetic nervous system**, increasing heart rate and contractility. - It acts on **beta-1 adrenergic receptors** in the heart. *Dopamine* - **Dopamine** is a precursor to norepinephrine and epinephrine, and primarily functions as a neurotransmitter in the **central nervous system** and in regulating renal blood flow. - While it can have cardiac effects, it is not the primary neurotransmitter for parasympathetic actions on heart rate. *Epinephrine* - **Epinephrine** (adrenaline) is a hormone released by the adrenal medulla and a neurotransmitter in the sympathetic nervous system, causing an **increase in heart rate** and contractility. - It works through **beta-1 adrenergic receptors**, antagonistic to parasympathetic effects.
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