Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation
Monitor the pathway, not just the waveform
Every modality interrogates a specific circuit, and questions are usually built on pathway detail. The dorsal column–medial lemniscus system (fasciculus gracilis/cuneatus → nucleus cuneatus of the caudal medulla → decussation → medial lemniscus → VPL thalamus → postcentral gyrus) carries vibration and proprioception and is the substrate for SSEPs: mixed nerve → plexus → dorsal root ganglion → ipsilateral dorsal column → contralateral cortex. The corticospinal tract (motor cortex → corona radiata → internal capsule → cerebral peduncle → pyramidal decussation → lateral corticospinal tract → anterior horn cell → motor unit) underlies MEPs; because the volley crosses a synapse at the anterior horn cell, it is abolished by neuromuscular blockade and is far more anesthetic-sensitive than SSEP. The auditory pathway (VIII nerve → cochlear nuclei → superior olive → lateral lemniscus → inferior colliculus → medial geniculate → auditory cortex) is the most resistant to anesthesia.
Instrumentation essentials
A differential amplifier amplifies the difference between two inputs and rejects what is common to both; that property, CMRR, is what lets microvolt responses be seen in a millivolt environment. High input impedance limits signal loss at the electrode–tissue interface. Sampling rate sets bandwidth: by Nyquist, the highest resolvable frequency is half the sampling rate. Low-frequency (high-pass) and high-frequency (low-pass) filters trade noise rejection against distortion of latency and amplitude.
Traps: assuming SSEPs cross within the spinal cord; confusing the decussation level of the dorsal columns with that of the pyramids; believing NMB suppresses SSEPs; setting filters that clip the peak of interest.
Rule of thumb: 60 Hz contamination is a common-mode/grounding problem first - check impedance and reference placement before touching filter settings.