All of the following statements apply to first order neurons that transmit discriminative tactile sensations from the arm EXCEPT:

Correct Answer: C

Rationale: They form internal arcuate fibers as they decussate , which is false for first-order neurons. In the dorsal column pathway, first-order neurons from the arm (pseudounipolar, D) ascend in the fasciculus cuneatus and synapse in the nucleus cuneatus . Second-order neurons from the nucleus cuneatus form internal arcuate fibers and decussate. Choice C attributthis to first-order neurons, which dont decussate they end in the medulla. Thus, C is the exception, making it the correct answer.

Correct Answer: B

Rationale: The anterior spinothalamic tract is found in the anterior white column . The anterior spinothalamic tract, carrying crude touch, ascends in the anterior (ventral) white column of the spinal cord after crossing. Choice A is wrong (posterior spinocerebellar is in the lateral column). Choice C is incorrect (lateral spinothalamic is in the lateral column). Choice D is false (fasciculus gracilis is in the posterior column). Thus, B correctly locatthe anterior spinothalamic tract, making it the right answer.

Correct Answer: C

Rationale: serotonin . The descending pain inhibitory pathway, originating in the brainstem (e.g., periaqueductal gray, raphe nuclei), usserotonin to activate inhibitory interneurons in the spinal cord dorsal horn, reducing pain transmission. Choice A (endorphin) and B (enkephalin) are opioids involved in pain modulation but not the primary neurotransmitter here. Choice D (substance P) enhancpain. Thus, C correctly identifiserotonin, making it the right answer.

Correct Answer: D

Rationale: Their function is subconscious regulation of balance and muscle tone . The lateral corticospinal tract, a pyramidal tract , controls voluntary fine movements, not subconscious balance/tone (extrapyramidal role). Choice B and C are correct (passthrough midbrain pedunculi and pons basilar part). Thus, D misattributfunction, making it the wrong statement and the correct answer.

Correct Answer: A

Rationale: Located in the anterior white column . The rubrospinal tract, from the red nucleus , cross, facilitatflexors , and runs in the lateral white column, not anterior (ventral corticospinal is anterior). Thus, A misplace the tract, making it the wrong statement and the correct answer.