2.2 Nervous System, Neuromuscular Junction & Electrophysiology

Nervous System, Neuromuscular Junction & Electrophysiology

The nervous system coordinates bodily functions through rapid, highly organized electrical and chemical signaling. For Biomedical Equipment Technicians (CBETs), mastering electrophysiology is the foundation for servicing and validating diagnostic biopotential recording systems—such as Electroencephalographs (EEG), Electromyographs (EMG), Evoked Potential (EP) systems, Intraoperative Neurophysiological Monitoring (IONM) consoles, and Train-of-Four (TOF) peripheral nerve stimulators.

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1. Structural Organization of the Nervous System

The nervous system is partitioned into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

+-----------------------------------------------------------------------------+
|                   ORGANIZATION OF THE NERVOUS SYSTEM                        |
|                                                                             |
|   [CENTRAL NERVOUS SYSTEM (CNS)]                                            |
|   ├── Brain (Cerebrum, Diencephalon, Brainstem, Cerebellum)                 |
|   └── Spinal Cord (Cervical, Thoracic, Lumbar, Sacral, Coccygeal)           |
|                                                                             |
|   [PERIPHERAL NERVOUS SYSTEM (PNS)]                                         |
|   ├── Sensory (Afferent) Division (Transmits signals to CNS)                |
|   └── Motor (Efferent) Division (Transmits signals from CNS to effectors)   |
|         ├── Somatic Nervous System (Voluntary skeletal muscle control)      |
|         └── Autonomic Nervous System (ANS) (Involuntary visceral control)   |
|               ├── Sympathetic Division ("Fight or Flight" - Thoracolumbar)  |
|               └── Parasympathetic Division ("Rest & Digest" - Craniosacral)|
+-----------------------------------------------------------------------------+

Brainstem Vital Centers in the Medulla Oblongata

The brainstem (midbrain, pons, medulla oblongata) connects the cerebrum to the spinal cord. The medulla oblongata houses the autonomous reflex centers essential for patient survival:

1. Respiratory Rhythmicity Center: Contains the Dorsal Respiratory Group (DRG) (governing inspiratory pacing) and the Ventral Respiratory Group (VRG) (activating during forced inspiration/expiration). It receives chemoreceptor feedback regarding arterial $\text{PaCO}_2$ and $\text{pH}$.
2. Cardiac Control Center: Divides into the cardioacceleratory center (sympathetic fibers via cardiac nerves increasing heart rate and stroke volume) and the cardioinhibitory center (parasympathetic fibers via the Vagus Nerve [Cranial Nerve X] decreasing heart rate via acetylcholine release at the SA node).
3. Vasomotor Center: Controls peripheral vascular resistance and systemic blood pressure by regulating smooth muscle tone in the walls of arterioles through sympathetic vasoconstrictor tone.

Autonomic Division: Sympathetic vs. Parasympathetic

• Sympathetic Nervous System (Thoracolumbar Outflow, T1–L2): Prepares the body for stress ("fight or flight"). Preganglionic fibers release acetylcholine (ACh) at nicotinic receptors; postganglionic fibers release norepinephrine (NE) onto target adrenergic receptors ($\alpha_1, \beta_1, \beta_2$), increasing heart rate, dilating bronchioles, and shunting blood to skeletal muscle.
• Parasympathetic Nervous System (Craniosacral Outflow, CN III, VII, IX, X and S2–S4): Mediates vegetative functions ("rest and digest"). Both preganglionic and postganglionic fibers release acetylcholine (ACh), acting on muscarinic receptors ($M_1–M_5$) to slow heart rate, stimulate digestive peristalsis, and constrict pupils.

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2. Cellular Neurophysiology & The Action Potential

Neurons communicate via transient electrical impulses termed action potentials, generated by selective ionic permeability across the phospholipid bilayer.

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|                      THE NEURONAL ACTION POTENTIAL                          |
|                                                                             |
|   Voltage (mV)                                                              |
|     +40 + - - - - - - - - - - - - - - - [PEAK DEPOLARIZATION: +30 mV]       |
|         |                             / \                                   |
|     +20 |                            /   \                                  |
|         |                           /     \  [REPOLARIZATION]               |
|       0 |                          /       \ (Voltage-gated K+ out)         |
|         |   [DEPOLARIZATION]      /         \                               |
|     -20 |   (Voltage-gated       /           \                              |
|         |    Na+ rushes in)     /             \                             |
|     -40 |                      /               \                            |
|     -55 | - - - - - - - - - - * [THRESHOLD: -55 mV]                         |
|     -70 | -------------------*                 \   [HYPERPOLARIZATION]      |
|         |  [RESTING POTENTIAL: -70 mV]          \____  (-80 to -90 mV)      |
|     -90 +--------------------------------------------\-----------+--------> |
|         0                    1                 2     3           4 Time(ms) |
|                                                                             |
|   * Absolute Refractory Period (0-2 ms): Inactivation gate (h) closed       |
|   * Relative Refractory Period (2-4 ms): Stronger stimulus required         |
+-----------------------------------------------------------------------------+

The Resting Membrane Potential (-70 mV)

In the resting state, the intracellular fluid is electrically negative relative to the extracellular fluid, exhibiting a potential difference of approximately $-70\text{ mV}$ in neurons (and $-90\text{ mV}$ in skeletal muscle fibers). This equilibrium is maintained by:

1. The Sodium-Potassium ATPase Pump ($ ext{Na}^+/\text{K}^+$ Pump): An electrogenic transport protein that hydrolyzes one ATP molecule to actively pump $3\text{ Na}^+$ ions OUT of the cell while transporting $2\text{ K}^+$ ions IN. This maintains high extracellular $\text{Na}^+$ ($\approx 145\text{ mEq/L}$) and high intracellular $\text{K}^+$ ($\approx 140\text{ mEq/L}$).
2. High Resting Potassium Permeability: Neuronal resting membranes possess non-gated "leak" potassium channels that are 50 to 100 times more permeable to $\text{K}^+$ than to $\text{Na}^+$. Potassium diffuses out down its concentration gradient until electrical attraction pulls it back, establishing a resting potential near the potassium equilibrium potential (calculated via the Nernst Equation):

$E_K = \frac{RT}{zF} \ln \left(\frac{[\text{K}^+]_{\text{out}}}{[\text{K}^+]_{\text{in}}}\right) = 61.5 \log_{10}\left(\frac{4.0}{140}\right) \approx -95\text{ mV}$

Action Potential Phases

1. Resting State: $-70\text{ mV}$. Voltage-gated $\text{Na}^+$ channels have activation ($m$) gates closed and inactivation ($h$) gates open.
2. Depolarization to Threshold: A local graded potential depolarizes the axon hillock to $-55\text{ mV}$ (threshold).
3. Rapid Depolarization: Threshold triggers the rapid opening of activation gates in voltage-gated $\text{Na}^+$ channels. Sodium rushes inward down its electrical and chemical gradient, depolarizing the membrane to $+30\text{ to }+40\text{ mV}$.
4. Repolarization: At peak voltage, the $\text{Na}^+$ channel inactivation gates ($h$) snap shut, halting sodium influx. Simultaneously, slower voltage-gated $\text{K}^+$ channels open, allowing $\text{K}^+$ to rush out of the cell, restoring negative intracellular polarity.
5. Hyperpolarization (Undershoot): Voltage-gated $\text{K}^+$ channels remain open briefly after reaching resting potential, driving the membrane to $-80\text{ to }-90\text{ mV}$ before closing.
6. Refractory Periods:
   • Absolute Refractory Period: From threshold until $\text{Na}^+$ channel inactivation gates reset. No stimulus, regardless of strength, can initiate a second action potential.
   • Relative Refractory Period: Corresponds to hyperpolarization. A suprathreshold stimulus can fire an action potential because $\text{Na}^+$ channels have reset, though $\text{K}^+$ channels remain open.

Saltatory Conduction along Myelinated Axons

Axons are insulated by myelin sheaths formed by Schwann cells in the PNS and Oligodendrocytes in the CNS. The sheath is interrupted at regular $1–2\text{ mm}$ intervals by uninsulated Nodes of Ranvier, which contain dense clusters of voltage-gated $\text{Na}^+$ channels.

• Instead of continuous propagation along the entire membrane ($0.5–2\text{ m/s}$ in unmyelinated C-fibers), the action potential electrotonically "jumps" from node to node in a process known as saltatory conduction, increasing conduction velocity up to $100–120\text{ m/s}$ ($>250\text{ mph}$) while conserving cellular ATP.

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3. The Neuromuscular Junction (NMJ) & Synaptic Transmission

The neuromuscular junction (NMJ) is the specialized biochemical synapse between a lower alpha motor neuron terminal and a skeletal muscle fiber motor endplate.

+-----------------------------------------------------------------------------+
|                      NEUROMUSCULAR JUNCTION SYNAPSE                         |
|                                                                             |
|   [MOTOR AXON TERMINAL]                                                     |
|   ├── Action potential arrives at presynaptic terminal                      |
|   ├── Voltage-Gated Ca2+ channels open ---> Ca2+ rushes IN                  |
|   └── Ca2+ triggers vesicle exocytosis ---> Acetylcholine (ACh) released    |
|                                    |                                        |
|                                    v (20-50 nm Synaptic Cleft)              |
|   [POSTSYNAPTIC MOTOR ENDPLATE]                                             |
|   ├── ACh binds Nicotinic Receptors (nAChR - ligand-gated ion channels)     |
|   ├── Na+ influx > K+ efflux ---> Endplate Potential (EPP) generated        |
|   ├── EPP fires Muscle Action Potential along Sarcolemma & T-tubules        |
|   ├── Sarcoplasmic Reticulum releases Ca2+ ---> Muscle contraction          |
|   └── Acetylcholinesterase (AChE) hydrolyzes ACh into Choline + Acetate     |
+-----------------------------------------------------------------------------+

Sequence of Synaptic Events:

1. Presynaptic $\text{Ca}^{2+}$ Influx: Depolarization of the presynaptic terminal opens voltage-gated $\text{Ca}^{2+}$ channels, allowing extracellular calcium to enter.
2. Exocytosis of Acetylcholine: Intracellular calcium activates synaptotagmin and SNARE proteins, causing synaptic vesicles containing Acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the $20–50\text{ nm}$ synaptic cleft.
3. Postsynaptic Receptor Activation: ACh diffuses across the cleft and binds to pentameric nicotinic acetylcholine receptors (nAChR) on the folded motor endplate. Receptor opening allows massive $\text{Na}^+$ influx (and minor $\text{K}^+$ efflux), generating a localized Endplate Potential (EPP).
4. Muscle Action Potential & Contraction: The EPP exceeds threshold, opening adjacent voltage-gated $\text{Na}^+$ channels on the sarcolemma. The action potential propagates down Transverse (T) tubules, activating dihydropyridine (DHP) receptors and ryanodine receptors (RyR1) on the sarcoplasmic reticulum to release stored $\text{Ca}^{2+}$, initiating actin-myosin cross-bridge cycling.
5. Signal Termination: Acetylcholinesterase (AChE) anchored in the synaptic basement membrane hydrolyzes ACh into choline and acetate within milliseconds ($>25,000\text{ molecules/sec/enzyme}$), allowing the endplate to repolarize.

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4. Neuromuscular Blockade & Train-of-Four (TOF) Monitoring

In surgical anesthesia, Neuromuscular Blocking Agents (NMBAs) induce temporary skeletal muscle paralysis to facilitate endotracheal intubation and surgical exposure.

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|                 TRAIN-OF-FOUR (TOF) NERVE STIMULATION PATTERN               |
|                                                                             |
|   Pulse 1 (T1)       Pulse 2 (T2)       Pulse 3 (T3)       Pulse 4 (T4)     |
|      |                  |                  |                  |             |
|      | (0.5 sec / 2 Hz) | (0.5 sec / 2 Hz) | (0.5 sec / 2 Hz) |             |
|      v                  v                  v                  v             |
|   +-----+            +-----+            +-----+            +-----+          |
|   |     |            |     |            |     |            |     |          |
|   |     |            |     |            |     |            |     |          |
|   +-----+            +-----+            +-----+            +-----+          |
|   <------------------ TOTAL DURATION: 1.5 SECONDS ----------------->        |
|                                                                             |
|   * TOF Ratio = Amplitude of T4 / Amplitude of T1                           |
|   * Non-depolarizing block exhibits FADE (T4 < T3 < T2 < T1)                 |
|   * Depolarizing block (Phase I) exhibits equal reduction without fade       |
+-----------------------------------------------------------------------------+

Pharmacological Classes of NMBAs:

1. Depolarizing Blockers (e.g., Succinylcholine): Structurally mimics ACh, binding to nAChR to cause persistent endplate depolarization. This causes visible muscle twitching (fasciculations) followed by flaccid paralysis because voltage-gated $\text{Na}^+$ channels remain inactivated. It is not hydrolyzed by AChE (metabolized by plasma pseudocholinesterase). Phase I block produces no fade on TOF stimulation.
2. Non-Depolarizing Blockers (e.g., Rocuronium, Vecuronium, Cisatracurium): Act as competitive antagonists, binding nAChRs without opening the channel. They block endogenous ACh binding and exhibit characteristic fade during repetitive stimulation. Reversible with anticholinesterases (neostigmine with glycopyrrolate) or the cyclodextrin encapsulating agent sugammadex.

Train-of-Four (TOF) Stimulator Quantification

A peripheral nerve stimulator (e.g., stimulating the ulnar nerve at the wrist to assess adductor pollicis thumb adduction, or the facial nerve to assess orbicularis oculi) delivers 4 supramaximal monophasic square-wave electrical pulses ($0.2\text{ ms}$ duration) at a frequency of $2\text{ Hz}$ ($0.5\text{ s}$ intervals) over a $1.5\text{ s}$ window.

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5. Electroencephalography (EEG) Frequency Bands

Electroencephalography (EEG) records the synchronized postsynaptic potentials of millions of pyramidal cortical neurons oriented perpendicular to the scalp surface using non-invasive surface electrodes placed according to the International 10–20 System.

+-----------------------------------------------------------------------------+
|                        EEG FREQUENCY BAND SPECTRUM                          |
|                                                                             |
|   DELTA (0.5 - 4 Hz)       | Deep sleep (NREM 3/4), coma, infant baseline   |
|   /\x5c__/\x5c__/\x5c__/\x5c__/\x5c__/\x5c       | High amplitude (>75 uV), slowest rhythm        |
|                            |                                                |
|   THETA (4 - 8 Hz)         | Drowsiness, light sleep (NREM 1), meditation   |
|   /\x5c/\x5c/\x5c/\x5c/\x5c/\x5c/\x5c/\x5c/\x5c/       | Moderate amplitude (20-50 uV), parietal/temp   |
|                            |                                                |
|   ALPHA (8 - 13 Hz)        | Relaxed, awake, EYES CLOSED, Berger rhythm     |
|   MMMMMMMMMMMMMMMM         | 20-60 uV, prominent occipital; blocked by eyes |
|                            |                                                |
|   BETA (13 - 30 Hz)        | Alert, active concentration, mental workload   |
|   WWWWWWWWWWWWWWWW         | Low amplitude (5-20 uV), frontal/central       |
|                            |                                                |
|   GAMMA (>30 - 100 Hz)     | High cognitive binding, sensory integration    |
|   ~~~~~~~~~~~~~~~~         | Very low amplitude (<5 uV), cross-modal binding|
+-----------------------------------------------------------------------------+

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6. Electrophysiological Signal Comparison & Instrumentation Design

Biomedical engineers and technicians must interface with varied biopotential signals generated across the human body. The table below summarizes their core electrical properties and instrumentation requirements:

Biomedical Instrumentation Circuit Requirements:

1. Differential Instrumentation Bioamplifiers: Because microvolt-level signals (EEG/EMG) are recorded in electrically noisy hospital environments, amplifiers must possess a Common-Mode Rejection Ratio (CMRR) $\ge 100–120\text{ dB}$ at 60 Hz to reject common-mode line interference.
2. Ultra-High Input Impedance: Input impedance must be $\ge 10–100\text{ M}\Omega$ (typically achieved via FET-input operational amplifiers) to prevent signal attenuation caused by electrode-skin contact impedance ($1–10\text{ k}\Omega$).
3. Patient Isolation & Grounding: Must comply with IEC 60601-1 Type CF or Type BF floating isolation barriers ($>4000\text{ V}$ dielectric breakdown rating, optical or transformer coupled) with active Right Leg Drive (RLD) or active patient reference circuits to suppress common-mode displacement currents.
4. Anti-Aliasing & Nyquist Sampling: In digital processing, analog low-pass filtering must precede Analog-to-Digital Conversion (ADC) at a sampling rate ($f_s$) at least twice the highest frequency component ($f_s \ge 2 \cdot f_{\text{max}}$); for diagnostic EMG ($f_{\text{max}} = 5\text{ kHz}$), $f_s \ge 10\text{ kHz}$ is required.