Multiple Choice
Which of the following substances is a hormone and a neurotransmitter?
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14. The Autonomic Nervous System
Neurotransmitters of the ANS
2:40 minutesProblem 26
Textbook Question
In the Netherlands a young man named Jan was admitted to the emergency room. He and his friends had been to a rave. His friends say he started twitching and having muscle spasms which progressed until he was 'stiff as a board.' On examination, staff found a marked increase in muscle tone and hyperreflexia involving facial and limb muscles. In his pocket, he had unmarked dark yellow tablets with dark flecks. Analysis of the tablets showed them to contain a mixture of ecstasy and strychnine. Ecstasy would not cause this clinical picture, but strychnine, which blocks glycine receptors, could. Explain how.
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Understand the role of glycine in the central nervous system: Glycine is an inhibitory neurotransmitter primarily found in the spinal cord and brainstem. It binds to glycine receptors on postsynaptic neurons, causing chloride ions (Cl⁻) to flow into the neuron, hyperpolarizing it and reducing its ability to fire action potentials. This inhibitory effect helps regulate muscle tone and prevent excessive muscle contractions.
Recognize the effect of strychnine on glycine receptors: Strychnine is a neurotoxin that acts as a competitive antagonist of glycine receptors. By blocking glycine from binding to its receptors, strychnine prevents the inhibitory effect of glycine, leading to unchecked excitatory signals in the nervous system.
Connect the physiological consequences of glycine receptor blockade: Without glycine's inhibitory action, motor neurons in the spinal cord become overactive, leading to excessive firing of action potentials. This results in hyperreflexia (exaggerated reflexes) and increased muscle tone, causing muscle spasms and stiffness.
Relate the clinical symptoms to the mechanism: The symptoms described in the case, such as twitching, muscle spasms, and 'stiff as a board' rigidity, are consistent with the effects of strychnine poisoning. The hyperreflexia and increased muscle tone are direct consequences of the loss of glycine-mediated inhibition in the spinal cord and brainstem.
Summarize the role of strychnine in the clinical picture: Strychnine's blockade of glycine receptors disrupts the balance between excitatory and inhibitory signals in the nervous system, leading to the observed symptoms. Ecstasy, while a stimulant, does not interfere with glycine receptors and thus would not cause this specific clinical presentation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Strychnine Mechanism of Action
Strychnine is a potent neurotoxin that acts as a competitive antagonist at glycine receptors in the central nervous system. By blocking these inhibitory receptors, strychnine disrupts the normal inhibitory signaling, leading to increased neuronal excitability. This results in symptoms such as muscle spasms and rigidity, as seen in Jan's case, where the lack of glycine-mediated inhibition causes excessive muscle contraction.
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Action Potential
Glycine Receptors
Glycine receptors are ligand-gated ion channels that mediate inhibitory neurotransmission in the spinal cord and brainstem. When glycine binds to these receptors, it typically allows chloride ions to enter the neuron, leading to hyperpolarization and reduced neuronal excitability. Inhibition of these receptors by substances like strychnine can lead to a loss of this inhibitory effect, resulting in hyperreflexia and increased muscle tone.
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Hyperreflexia and Muscle Tone
Hyperreflexia refers to an exaggerated response to stimuli, often resulting in involuntary muscle contractions or spasms. This condition can occur when inhibitory pathways in the nervous system are disrupted, as seen with strychnine's action on glycine receptors. The increased muscle tone and spasms in Jan's case illustrate how the loss of inhibitory control can lead to severe motor symptoms, highlighting the importance of balanced excitatory and inhibitory signaling in muscle function.
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