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Answer: 4) It activates myosin kinase (MLCK) in the presence of calcium
Explanation: Calcium binds to calmodulin, forming a calcium-calmodulin complex. This complex then activates myosin kinase, which phosphorylates the myosin light chain, leading to the formation of cross-bridges between actin and myosin and initiating muscle contraction in smooth muscles.
What is the role of calmodulin in the contraction of smooth muscle cells?
1) It binds to calcium and initiates muscle contraction directly
2) It activates the sarcoplasmic reticulum to release calcium
3) It inhibits the release of calcium from the sarcoplasmic reticulum
4) It activates myosin kinase (MLCK) in the presence of calcium
Answer: 4) Stimulation of mitochondrial oxidative phosphorylation
Explanation: Thyroid hormones, particularly T3, increase metabolic rate by enhancing the activity of mitochondrial oxidative phosphorylation. This leads to increased ATP production and overall cellular energy metabolism.
What is the primary mechanism by which thyroid hormones increase metabolic rate?
1) Stimulation of the sympathetic nervous system
2) Increased expression of uncoupling proteins
3) Increased activity of the sodium-potassium ATPase pump
4) Stimulation of mitochondrial oxidative phosphorylation
Answer: 2) Dilatation (bronchodilatation)
Explanation: Epinephrine acts on beta-2 adrenergic receptors in the bronchial smooth muscles, leading to their relaxation and dilatation of the bronchioles. This allows for increased airflow and oxygenation of the lungs, which is essential during the fight-or-flight response.
What is the effect of epinephrine on the bronchial smooth muscles during the fight-or-flight response?
1) Constriction (bronchoconstriction)
2) Dilatation (bronchodilatation)
3) No effect
4) Spasm
In the presence of which substance does vitamin B12 combine in the stomach to prevent its degradation by gastric acid?
1) Intrinsic factor
2) Pepsin
3) Gastric acid
4) Pancreatic amylase
Answer: 1) Decreased heart rate
Explanation: In the heart, acetylcholine acts on muscarinic receptors, which are present on the sinoatrial (SA) and atrioventricular (AV) nodes. This leads to a decrease in heart rate by slowing the sinoatrial (SA) node's automaticity and increasing the time it takes for an electrical impulse to pass through the AV node.
What is the primary effect of acetylcholine on the cardiac muscles?
1) Decreased heart rate
2) Increased heart rate
3) No effect on heart rate
4) Increased force of contraction
What is the osmolarity of the tubular fluid leaving the early distal tubule?
1) Isotonic to plasma
2) Hypertonic
3) Hypotonic
4) Variable
What is the role of the acrosome in sperm penetration of the egg during fertilization?
1) It produces energy for the sperm's movement
2) It contains enzymes that help dissolve the egg's outer layers
3) It transfers genetic material to the egg
4) It has no role in the penetration process
The first sensation of bladder filling, known as the first sensation of urgency, typically occurs when the bladder is about one-quarter full, which is around 150 ml. This is the point where an individual begins to become aware of the need to urinate.
The first sensation of bladder filling occurs at approximately:
1) 150 ml
2) 250 ml
3) 450 ml
4) 750 ml
Answer: 2) Epinephrine
Explanation: During the fight-or-flight response, epinephrine is released from the adrenal medulla and acts on the pupillary dilator muscles to cause mydriasis (dilatation of pupils). This effect is mediated by beta-adrenergic receptors on the smooth muscle of the iris, leading to increased pupil size to allow more light in for better vision.
What is the primary neurotransmitter involved in the pupil dilatation during the fight-or-flight response?
1) Acetylcholine
2) Epinephrine
3) Norepinephrine
4) Dopamine
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