An otherwise healthy 58-year-old man comes to the physician because of a 1-year history of episodic coughing whenever he cleans his left ear. There is no history of hearing loss, tinnitus, or vertigo. Stimulating his left ear canal with a cotton swab triggers a bout of coughing. The physician informs him that these symptoms are caused by hypersensitivity of a cranial nerve. A peripheral lesion of this nerve is most likely to manifest with which of the following findings on physical examination?

Ipsilateral sensorineural hearing loss

Ipsilateral deviation of the tongue

Inability to raise ipsilateral eyebrow

Decreased secretion from ipsilateral sublingual gland

Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

Inhaling the entire vital capacity (VC)

Inhaling the inspiratory reserve volume (IRV)

Exhaling the entire vital capacity (VC)

Exhaling the expiratory reserve volume (ERV)

Breath holding maneuver at functional residual capacity (FRC)

A 35-year-old woman volunteers for a study on respiratory physiology. Pressure probes A and B are placed as follows: Probe A: between the parietal and visceral pleura Probe B: within the cavity of an alveolus The probes provide a pressure reading relative to atmospheric pressure. To obtain a baseline reading, she is asked to sit comfortably and breathe normally. Which of the following sets of values will most likely be seen at the end of inspiration?

Probe A: -6 mm Hg; Probe B: 0 mm Hg

Probe A: 0 mm Hg; Probe B: -1 mm Hg

Probe A: -4 mm Hg; Probe B: 0 mm Hg

Probe A: -4 mm Hg; Probe B: -1 mm Hg

Probe A: -6 mm Hg; Probe B: -1 mm Hg

A researcher is studying receptors that respond to epinephrine in the body and discovers a particular subset that is expressed in presynaptic adrenergic nerve terminals. She discovers that upon activation, these receptors will lead to decreased sympathetic nervous system activity. She then studies the intracellular second messenger changes that occur when this receptor is activated. She records these changes and begins searching for analogous receptor pathways. Which of the following receptors would cause the most similar set of intracellular second messenger changes?

Dopamine receptors in the brain

Muscarinic cholinoreceptors in the gastrointestinal tract

Growth hormone receptors in the musculoskeletal system

Vasopressin receptors in the kidney

Aldosterone receptors in the kidney

A 22-year-old man presents with a history of lightheadedness, weakness, and palpitations when he assumes an upright position from a supine position. He is otherwise a healthy man without a history of alcohol or other substance abuse. His supine and standing blood pressures (measured at 3-minute intervals) were 124/82 mm Hg and 102/72 mm Hg, respectively. He was advised to perform a Valsalva maneuver while monitoring blood pressure and heart rate to assess the integrity of his baroreflex control. Which of the following statements is correct?

During late phase II, there is an increase in both blood pressure and heart rate

During early phase II, there is an increase in blood pressure and a decrease in heart rate

Phases III and IV are mediated by baroreceptor reflexes that require intact efferent parasympathetic responses

During phase I, the blood pressure decreases due to increased intrathoracic pressure

The Valsalva ratio is defined as the maximum phase II tachycardia divided by the minimum phase IV bradycardia

A 28-year-old woman is brought to the emergency department 1 hour after being involved in a motor vehicle collision. She was riding a bike when she lost control and hit a car on the opposite side of the road. On arrival, she is unconscious. She has a history of intravenous heroin use. Her pulse is 56/min, respirations are 8/min and irregular, and blood pressure is 196/102 mm Hg. Examination shows a 2-cm laceration over the left cheek and a 3-cm laceration over the left chest. There are multiple abrasions over her face and chest. She opens her eyes and flexes her extremities to painful stimuli. The pupils are dilated and react sluggishly to light. There are decreased breath sounds over the left lung. The trachea is central. There is no jugular venous distention. Cardiac examination shows no abnormalities. The abdomen is soft and nontender. The left knee and right ankle are swollen; range of motion is limited. Two large-bore peripheral intravenous catheters are inserted. She is intubated and mechanical ventilation is initiated. A focused assessment with sonography in trauma is negative. An occlusive dressing is applied over the left chest wound. She is scheduled for a noncontrast CT scan of the brain. Which of the following is the underlying cause of this patient's hypertension?

Increased intrathoracic pressure

Reduced parasympathetic response

A 57-year-old man was brought into the emergency department unconscious 2 days ago. His friends who were with him at that time say he collapsed on the street. Upon arrival to the ED, he had a generalized tonic seizure. At that time, he was intubated and is being treated with diazepam and phenytoin. A noncontrast head CT revealed hemorrhages within the pons and cerebellum with a mass effect and tonsillar herniation. Today, his blood pressure is 110/65 mm Hg, heart rate is 65/min, respiratory rate is 12/min (intubated, ventilator settings: tidal volume (TV) 600 ml, positive end-expiratory pressure (PEEP) 5 cm H2O, and FiO2 40%), and temperature is 37.0°C (98.6°F). On physical examination, the patient is in a comatose state. Pupils are 4 mm bilaterally and unresponsive to light. Cornea reflexes are absent. Gag reflex and cough reflex are also absent. Which of the following is the next best step in the management of this patient?

Second opinion from a neurologist

Withdraw ventilation support and mark time of death

Repeat examination in several hours

Vital centers in brainstem for USMLE Step 3: High-Yield Anatomy Lessons

Vital Centers - The Brain's Life Support

Midsagittal section of brainstem and cerebellum

Located primarily within the reticular formation of the medulla oblongata, these centers regulate autonomic functions essential for life.

Cardiac Center:
- Controls heart rate and force of contraction.
- Input from baroreceptors & chemoreceptors.
Vasomotor Center:
- Regulates blood pressure via vessel diameter.
Respiratory Rhythmicity Centers:
- Dorsal group (DRG) for inspiration.
- Ventral group (VRG) for forced expiration.
- Influenced by pontine centers (apneustic & pneumotaxic).

⭐ Cushing Reflex: A late sign of ↑ intracranial pressure, presenting as hypertension, bradycardia, and irregular respiration, indicating brainstem compression.

Respiratory Rhythms - Just Breathe

Medullary Centers (Primary Control)
- Dorsal Respiratory Group (DRG): The integrator. Receives sensory input from peripheral chemoreceptors (via CN IX) and mechanoreceptors (via CN X). Modulates the VRG.
- Ventral Respiratory Group (VRG): The rhythm generator. Contains inspiratory & expiratory neurons. Sets basic eupnea pace; drives forced breathing.
Pontine Centers (Fine-Tuning)
- Pneumotaxic Center: Limits inspiration, thereby controlling respiratory rate and tidal volume. Prevents over-inflation.
- Apneustic Center: Stimulates DRG to prolong inspiration. Overridden by pneumotaxic signals.

Brainstem Visceral Nuclei and Respiratory Centers

⭐ Ondine's Curse: Congenital Central Hypoventilation Syndrome (CCHS) is a rare disorder caused by a PHOX2B gene mutation. It impairs autonomic respiratory control, leading to severe hypoventilation, particularly during sleep.

Cardiac Control - The Heart's Conductor

Located in the medulla oblongata, these centers fine-tune cardiac output by modulating heart rate (chronotropy) and contractility (inotropy).

Cardioacceleratory Center (Sympathetic):
- Originates in the reticular formation.
- Projects via T1-T5 spinal segments to the sympathetic chain.
- Innervates SA/AV nodes and ventricular muscle.
- Action: ↑ Heart rate & ↑ contractility.
- Neurotransmitter: Norepinephrine.
Cardioinhibitory Center (Parasympathetic):
- Originates in the Nucleus Ambiguus.
- Fibers travel within the Vagus nerve (CN X).
- Innervates SA/AV nodes.
- Action: ↓ Heart rate.
- Neurotransmitter: Acetylcholine.

Cardiac Autonomic Nervous System

⭐ The Bezold-Jarisch reflex, often seen in inferior wall MI, involves activation of the cardioinhibitory center, leading to a paradoxical triad of bradycardia, hypotension, and coronary artery dilation.

Reflex Central - Coughs, Sneezes, & Swallows

Location: Primarily medulla oblongata, coordinating complex motor responses.
Cough Reflex
- Afferent (sensory): CN X (Vagus n.) from larynx/trachea.
- Efferent (motor): CN X, phrenic & spinal nerves to glottis, diaphragm, abdominal muscles.
Sneeze Reflex
- Afferent: CN V (Trigeminal n.) from nasal mucosa.
- Efferent: CN V, VII, IX, X, & spinal motor nerves.
Swallowing (Deglutition)
- Afferents: CN IX, X.
- Efferents: CN V, VII, IX, X, XII.

⭐ The nucleus ambiguus in the medulla is the primary motor nucleus for swallowing, supplying muscles of the soft palate, pharynx, and larynx via CN IX and X.

Brainstem nuclei for swallowing and cough reflex

The medulla oblongata is the primary control center for vital reflexes.

It houses the cardiac center (regulates heart rate), vasomotor center (controls blood pressure), and centers for vomiting, coughing, and swallowing.

The medullary respiratory center sets the basic rhythm of breathing via the dorsal (DRG) and ventral (VRG) respiratory groups.

The pons fine-tunes respiration using the pneumotaxic and apneustic centers.

The pneumotaxic center limits inspiration, thereby controlling respiratory rate and depth.

Lesions in the medulla can cause rapid cardiopulmonary collapse.

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