Neuromuscular Physiology Foundations

The anterior horn cell: the trophic centre of the motor unit

The lower motor neuron soma resides in the ventral grey matter of the spinal cord (Rexed laminae IX) or in the motor nuclei of the brainstem. It is the metabolic and trophic command centre for an entire motor unit — one neuron plus every muscle fibre it innervates — and its destruction (as in motor neuron disease, poliomyelitis, or West Nile virus) denervates that whole territory at once. Two functional classes must be distinguished:

• Alpha (α) motor neurons — large somata (~50–70 µm) giving rise to large, fast, heavily myelinated Aα efferents that innervate extrafusal muscle fibres and generate force. These are the neurons whose axons we stimulate and whose health we interrogate in routine motor conduction studies and needle EMG.
• Gamma (γ) motor neurons — smaller somata projecting in thin Aγ fibres to the intrafusal fibres of the muscle spindle. They set spindle sensitivity (fusimotor drive) and do not contribute to recordable force; their activity is electrically silent on surface and standard concentric recordings, a point that matters when reconciling clinical tone with electrophysiology.

The motor neuron is dominated by an enormous dendritic arbor that may receive on the order of 10,000 synaptic contacts; this arbor performs the spatial and temporal integration of descending corticospinal, propriospinal, and segmental reflex input that determines whether the cell reaches threshold at its axon initial segment. Crucially, the soma and dendrites are arranged with strict somatotopy and a myotopic columnar organization: motor neurons supplying a single muscle form a longitudinal column spanning two to four spinal segments, with flexors and proximal muscles represented differently from extensors and distal muscles. This anatomical clustering is the substrate of the myotome, and it is why a single root lesion produces a reproducible, segmentally distributed pattern of denervation rather than a random scatter.

Peripheral nerve architecture: a layered, protected cable

Beyond the cord, the axon travels inside a connective-tissue scaffold engineered for both insulation and protection. Understanding the three sheaths explains both normal conduction and the localization logic of focal lesions:

• Endoneurium — the loose collagenous matrix surrounding each individual myelinated axon and its Schwann cell. Within it run the endoneurial capillaries that form the blood–nerve barrier, an analogue of the blood–brain barrier created by tight junctions between endothelial cells and reinforced by the perineurium. This barrier maintains a precisely regulated endoneurial fluid composition; its breakdown (vasculitis, inflammatory demyelination, diabetic microangiopathy) admits inflammatory cells and oedema and is central to immune-mediated neuropathy.
• Perineurium — concentric lamellae of flattened perineurial cells joined by tight junctions that ensheath each fascicle. It is the principal diffusion barrier and the chief mechanical load-bearing layer, generating a slight positive endoneurial pressure. Its segregation of axons into fascicles is why a partial nerve injury can spare some fascicles entirely, producing patchy, fascicle-specific deficits.
• Epineurium — the outer areolar sheath binding fascicles into the named nerve trunk and cushioning them; it is proportionally thickest where nerves cross joints and is the tissue that absorbs stretch and compression.

This laminar design has a direct electrodiagnostic corollary: because fascicles are functionally insulated, a focal compressive or demyelinating lesion can affect conduction in some fibres while neighbours are normal, generating the conduction block and temporal dispersion that we exploit to localize lesions to a segment of nerve.

Myelin: the biophysics of insulation

In the peripheral nervous system, each Schwann cell myelinates exactly one internode of one axon by spiralling its plasma membrane into a multilamellar sheath whose protein and lipid composition (P0, PMP22, myelin basic protein) excludes water and maximizes resistance. The functional geometry that results is the basis of all fast conduction:

• Internodes — the myelinated segments, typically 0.5–1.5 mm long and roughly proportional to axon diameter. Across the internode the high-resistance, low-capacitance sheath forces current to flow longitudinally inside the axon rather than leaking radially.
• Compact myelin — the bulk insulating spiral, where membrane lamellae are tightly apposed (major dense line and intraperiod line) and ion channels are excluded.
• Paranodal and juxtaparanodal regions — specialized non-compact zones flanking each node where the myelin loops attach to the axolemma through septate-like junctions (Caspr/contactin/neurofascin complexes). These junctions are the molecular fence that segregates nodal Na⁺ channels from juxtaparanodal K⁺ channels; autoantibodies against these proteins (the nodo-paranodopathies) produce a distinct conduction-failure phenotype.

Electrically, myelin does two things at once: it raises transverse membrane resistance (so less current is wasted) and it lowers membrane capacitance (so less charge is needed to swing the voltage). Both effects lengthen the cable's space constant and shorten its time constant, allowing the depolarizing wavefront to leap from node to node — the geometric prerequisite for the saltatory conduction examined in the conduction chapter.

Axonal conduction: cable properties and the molecular node

The axon behaves as a leaky electrical cable described by two passive parameters. The length (space) constant λ — the distance over which a subthreshold voltage decays to 1/e of its value — equals √(r_m / r_i), so it grows with high membrane resistance (good insulation) and low axoplasmic resistance (large diameter). The time constant τ = r_m · c_m governs how fast the membrane charges. Myelination optimizes both, but the regenerative spike itself depends on discrete molecular machinery:

• Nav1.6 at the nodes of Ranvier — voltage-gated sodium channels are clustered at the node at extraordinarily high density (on the order of 1,000–2,000 channels/µm², versus a sparse internodal axolemma). This concentration produces a large, brief inward current sufficient to charge the next internode to threshold despite the intervening cable losses.
• Kv channels (Kv1.1/1.2) — delayed-rectifier potassium channels sequestered in the juxtaparanode beneath the myelin. Hidden in health, they are unmasked by demyelination, where their exposure produces aberrant repolarization, reduced excitability, and part of the basis for fatigable, temperature-sensitive symptoms.

The interplay is the entire story of the safety factor: nodal Na⁺ current must exceed the threshold demand of the downstream node by a comfortable margin. Demyelination increases capacitative load and current leak across the denuded internode, eroding that margin until conduction slows, blocks, or fails — the mechanism we will quantify later.

The neuromuscular junction and the safety factor

Where the motor axon meets muscle, the all-or-none nerve impulse is converted to a graded chemical signal and then back to a regenerative muscle spike. The arriving action potential opens presynaptic voltage-gated Ca²⁺ channels (P/Q-type, Cav2.1); calcium influx triggers the synchronous exocytosis of roughly 50–300 quanta of acetylcholine, each quantum being the ~5,000–10,000 ACh molecules packaged in one synaptic vesicle. ACh diffuses across the cleft and binds nicotinic acetylcholine receptors (AChR) clustered at the crests of the postsynaptic junctional folds, opening cation channels that generate the endplate potential (EPP). The folds concentrate voltage-gated Na⁺ channels in their depths, so a suprathreshold EPP ignites a propagating muscle fibre action potential. Acetylcholinesterase (AChE) in the basal lamina then hydrolyzes ACh within milliseconds, terminating the signal and enforcing one-to-one transmission.

The decisive concept is the safety factor: the EPP (~30 mV depolarization) normally overshoots the threshold for the muscle action potential by a wide margin (a factor of roughly 3–5×). This redundancy guarantees faithful transmission across the physiological range of firing. Disease erodes it from opposite ends — presynaptically (reduced quantal release in Lambert–Eaton syndrome or botulism) or postsynaptically (reduced functional AChR in myasthenia gravis) — and once the EPP intermittently fails to reach threshold, intermittent transmission block appears. This is the electrophysiological substrate of the decrement on repetitive nerve stimulation and of jitter and blocking on single-fibre EMG.

Muscle fibre types and the motor neuron's authority

The muscle fibre is the final effector, and its phenotype is dictated by its motor neuron, not by the muscle. Cross- innervation experiments demonstrate that the firing pattern of the neuron — tonic low-frequency versus phasic high-frequency — instructs the fibre's myosin heavy-chain isoform, oxidative capacity, and contractile speed:

• Type I (slow-oxidative) — innervated by small, tonically active α neurons; rich in mitochondria and myoglobin, fatigue-resistant, recruited first for posture and sustained effort.
• Type IIa (fast oxidative-glycolytic) — fast, relatively fatigue-resistant, intermediate metabolism.
• Type IIx (fast glycolytic) — fastest and most powerful, rapidly fatigable, recruited last for high-force bursts.

Because each motor unit comprises a single fibre type, healthy muscle shows a normal mosaic — adjacent fibres of differing type intermixed like a checkerboard. After denervation and collateral reinnervation, surviving axons adopt orphaned neighbours and convert them to their own type, replacing the mosaic with enlarged, uniform clusters: fibre-type grouping, the histological fingerprint of a chronic neurogenic process and the structural correlate of the large, polyphasic, long-duration motor unit potentials we will later read as evidence of reinnervation.