A 52-year-old woman presents to her primary care physician for her annual checkup. She lost her job 6 months ago and since then she has been feeling worthless because nobody wants to hire her. She also says that she is finding it difficult to concentrate, which is exacerbated by the fact that she has lost interest in activities that she used to love such as doing puzzles and working in the garden. She says that she is sleeping over 10 hours every day because she says it is difficult to find the energy to get up in the morning. She denies having any thoughts about suicide. Which of the following neurotransmitter profiles would most likely be seen in this patient?

Decreased serotonin and norepinephrine

Decreased gamma-aminobutyric acid

A 52-year-man is brought to the physician because of a 2-week history of memory loss. Three weeks ago, he had a cardiac arrest that required cardiopulmonary resuscitation and intravenous epinephrine. On mental status examination, he cannot recall objects shown to him 20 minutes earlier but vividly recalls memories from before the incident. The remainder of the examination shows no abnormalities. Which of the following structures of the brain is most likely affected?

Pyramidal cells of the hippocampus

Internal pyramidal layer of the amygdala

Microglial cells of dorsal midbrain

Purkinje cells of the cerebellum

Astroglial cells of the putamen

A 43-year-old male with a history of thyroid cancer status post total thyroidectomy presents to his primary care physician after repeated bouts of headaches. His headaches are preceded by periods of anxiety, palpitations, and sweating. The patient says he is unable to pinpoint any precipitating factors and instead says the events occur without warning. Of note, the patient's father and uncle also have a history of thyroid cancer. On exam his vitals are: T 36.8 HR 87, BP 135/93, RR 14, and O2 Sat 100% on room air. The patient's TSH is within normal limits, and he reports taking his levothyroxine as prescribed. What is the next best step in diagnosing this patient's chief complaint?

Plasma fractionated metanephrines

Abdominal CT scan with and without IV contrast

High dose dexamethasone suppression test

Every time your neighbor in the adjacent apartment flushes the toilet, the water in your shower becomes very hot, causing you to jump out of the way of the water stream. After this has occurred for several months, you note that you reflexively jump back from the water directly after the sound of the flushing toilet but before the water temperature changes. Which of the following situations is the most similar to the conditioning process detailed above?

White coat syndrome (patient anxiety evoked at the sight of a white lab coat)

You consistently check the slots of pay telephones as you have previously found change left there

A young child elects to not throw a temper tantrum to avoid being grounded by his parents

A mouse repeatedly presses a red button to avoid receiving an electric shock

You now sleep through the noise of the train running past your apartment that kept you up 1 year ago

Neurobiological basis of anxiety for USMLE Step 3: High-Yield Psychiatry Lessons

Fear Circuitry - The Brain's Alarm System

Brain regions and their functions related to anxiety

Amygdala: Central processing hub; rapidly detects threats from sensory input (thalamus) and assigns fear value.
Prefrontal Cortex (PFC): The "brake." Exerts top-down inhibitory control over the amygdala, assessing and dampening fear responses.
Hippocampus: Provides context to fear-provoking stimuli; links fear to specific situations.
Hypothalamus & Brainstem: Execute the physiological fear output (fight-or-flight).

⭐ In anxiety disorders, ↓ PFC activity fails to inhibit an overactive amygdala, resulting in an exaggerated and poorly controlled fear response.

Chemical Imbalance - Neurotransmitter Chaos

Serotonin (5-HT): ↓ levels are central to anxiety pathophysiology.
- SSRIs & SNRIs are first-line treatments; they ↑ synaptic 5-HT.
- Affects mood, sleep, and appetite regulation, all disturbed in anxiety.
Norepinephrine (NE): ↑ activity, especially from the Locus Coeruleus (LC).
- Drives autonomic symptoms: tachycardia, tremors, sweating.
- SNRIs also target NE reuptake.
GABA (γ-aminobutyric acid): ↓ function of this primary inhibitory neurotransmitter.
- Leads to neuronal hyperexcitability.
- Benzodiazepines enhance GABA-A receptor function.
Glutamate: ↑ levels of the primary excitatory neurotransmitter can be anxiogenic.

⭐ The Locus Coeruleus (LC) is the principal site for norepinephrine synthesis in the brain. Overactivation of the LC is strongly linked to the somatic and autonomic arousal seen in panic attacks.

Neurotransmitter Pathways and Associated Drugs

Stress Response - The HPA Axis Overdrive

Core Feature: Anxiety disorders are marked by a hyperactive Hypothalamic-Pituitary-Adrenal (HPA) axis.
Initiation: The amygdala (fear center) over-stimulates the hypothalamus, initiating the stress cascade.
Dysregulation: Chronic cortisol exposure impairs the negative feedback function of the hippocampus and prefrontal cortex (PFC), preventing the system from shutting down.
Result: Persistently ↑ Corticotropin-Releasing Hormone (CRH), ↑ Adrenocorticotropic hormone (ACTH), and ↑ Cortisol.

⭐ In anxiety, failure of cortisol's negative feedback on the hypothalamus is a key mechanism. Dexamethasone suppression tests can reveal this non-suppression, indicating HPA axis hyperactivity.

HPA axis pathway in anxiety with amygdala and hippocampus

Genetic Factors - Nature's Predisposition

Family & Twin Studies: First-degree relatives show a 4-6x increased risk for anxiety disorders. Heritability is estimated at ~30-50%, with higher concordance in monozygotic than dizygotic twins.
Key Gene Associations:
- SLC6A4: Gene for the serotonin transporter (SERT). The short (S) allele of the 5-HTTLPR promoter region is linked to reduced transporter expression and increased anxiety traits.
- BDNF: The Val66Met polymorphism can impair fear extinction circuitry.

Twin heritability and DEP-PGS prediction for anxiety

⭐ The short allele of the serotonin transporter gene (5-HTTLPR) is linked to heightened amygdala reactivity to negative stimuli, forming a biological basis for vulnerability to stress.

The amygdala is hyperactive, central to processing fear and anxiety signals.

The prefrontal cortex (PFC) provides top-down inhibition of the amygdala, which is often deficient.

Key neurotransmitters involved include ↑ norepinephrine, ↓ serotonin, and ↓ GABA.

Benzodiazepines enhance GABAergic inhibition, while SSRIs modulate serotonin levels.

The locus coeruleus, the principal site for norepinephrine synthesis, shows increased activity.

The HPA axis can be dysregulated, leading to altered cortisol levels.

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