Which neurotransmitter is responsible for stimulating muscle contraction?

Correct Answer: B

Rationale: Acetylcholine is the neurotransmitter responsible for stimulating muscle contraction at the neuromuscular junction. When an action potential reaches the end of a motor neuron, acetylcholine is released into the synaptic cleft and binds to receptors on the muscle cell membrane, leading to muscle contraction. Dopamine, serotonin, and norepinephrine are neurotransmitters that have different functions in the nervous system and are not directly involved in stimulating muscle contraction. Dopamine is involved in reward and pleasure pathways, serotonin regulates mood and behavior, and norepinephrine is involved in the 'fight or flight' response.

Correct Answer: C

Rationale: The involuntary reflex arc that withdraws your hand from a hot object involves both the central nervous system (CNS) and the peripheral nervous system (PNS). When your hand touches a hot object, sensory neurons in the PNS detect the heat and send signals to the spinal cord in the CNS. The spinal cord processes this information and sends a signal back through motor neurons in the PNS to move your hand away from the hot object. This coordinated response requires the collaboration of both the CNS and PNS to protect the body from harm. Choice A is incorrect because the reflex arc involves more than just the central nervous system. Choice B is incorrect because the reflex arc involves more than just the peripheral nervous system. Choice D is incorrect because the reflex arc also involves motor neurons, not just sensory neurons.

Correct Answer: B

Rationale: The pituitary gland is correctly known as the 'master gland' of the endocrine system. It is situated at the base of the brain and plays a crucial role in secreting a variety of hormones that regulate the functions of other endocrine glands in the body. These hormones control growth, metabolism, reproduction, and various other essential bodily functions. The other choices, thyroid gland, adrenal gland, and pancreas, are incorrect as they do not serve as the 'master gland' of the endocrine system nor are located at the base of the brain like the pituitary gland.

Correct Answer: B

Rationale: The 'fight-or-flight' response is triggered by the release of adrenaline (epinephrine) and norepinephrine from the adrenal glands. These hormones prepare the body to either confront or flee from a perceived threat or stressor. The adrenal glands are crucial in initiating this rapid physiological response. Choices A, C, and D are incorrect because the thyroid gland primarily regulates metabolism and energy levels, the pituitary gland controls other endocrine glands but does not directly trigger the 'fight-or-flight' response, and the pancreas is responsible for regulating blood sugar levels through insulin and glucagon, not for triggering the 'fight-or-flight' response.

Correct Answer: B

Rationale: The correct answer is the Mitral valve. The mitral valve is located between the left atrium and the left ventricle of the heart. Its function is to prevent the backflow of blood from the left ventricle into the left atrium during ventricular contraction. Choice A, the Tricuspid valve, is incorrect as it is located between the right atrium and right ventricle. Choice C, the Aortic valve, and Choice D, the Pulmonic valve, are also incorrect as they are involved in the circulation of blood leaving the heart rather than preventing backflow within the atria and ventricles.