Neurophysiology and Muscle Physiology

Every reflex you test with a hammer, every muscle you grade on a five point scale and every patient who describes burning, tingling or weakness is telling you something about excitable membranes. This chapter builds neuromuscular physiology from the ion up, through synapses, receptors and reflex arcs, to the contraction of skeletal, smooth and cardiac muscle. Physiology carries a large share of Part I, and these mechanisms return again in pathology, pharmacology and every neurological exam you will ever perform.

Section 1Membranes, resting potential and the action potential

The membrane as a capacitor with gates

The plasma membrane is a phospholipid bilayer that ions cannot cross on their own. Charged particles move only through proteins: channels (passive, fast, down an electrochemical gradient), carriers (facilitated diffusion), and pumps (primary active transport using ATP). Because the bilayer separates two conducting solutions, it behaves as a capacitor. Only a tiny number of ions need to cross to change the voltage across it, which is why the bulk concentrations inside and outside a neuron barely change during a single action potential.

The key concentration gradients in a typical mammalian neuron are shown below. Sodium, chloride and calcium are high outside; potassium and organic anions are high inside.

Equilibrium potentials: the Nernst idea

If a membrane were permeable to only one ion, that ion would diffuse down its concentration gradient until the electrical force pulling it back exactly balanced the chemical force pushing it out. The voltage at which this balance occurs is the ion's equilibrium (Nernst) potential. At body temperature the Nernst equation simplifies to E = (61.5 / z) log ([ion]outside / [ion]inside), where z is the ion's valence. For potassium, a roughly 30 fold higher concentration inside gives about -90 mV. For sodium, about 10 fold higher outside gives about +60 mV. Note the sign convention: the inside is always reported relative to the outside.

Two consequences matter clinically. First, the potassium equilibrium potential depends heavily on extracellular potassium, because extracellular potassium is low and small absolute changes alter the ratio greatly. Hyperkalemia makes E(K) less negative and depolarizes cells; hypokalemia hyperpolarizes them. Second, the driving force on any ion is the difference between the membrane potential and that ion's equilibrium potential (Vm minus E ion). The bigger the difference, the stronger the push when a channel opens.

The resting membrane potential: the Goldman idea

Real membranes are permeable to several ions at once. The Goldman Hodgkin Katz equation weights each ion's concentration gradient by its permeability. At rest, neurons have many open potassium leak channels (two pore domain and inward rectifier types), so potassium permeability dominates and the resting potential (about -70 mV in neurons, about -90 mV in skeletal and ventricular muscle) sits close to E(K). It is a little less negative than E(K) because a small sodium leak pulls it toward E(Na). Whichever ion has the highest permeability at a moment drags the membrane toward its own equilibrium potential: that single principle explains the resting potential, the upstroke and repolarization.

The Na+/K+ ATPase moves 3 Na+ out and 2 K+ in per ATP. It maintains the gradients that leak channels use, and because it moves net positive charge out it is electrogenic, contributing only a few millivolts directly. Its main job is long term: block it with digitalis (ouabain, digoxin) or starve it of ATP in ischemia, and the gradients slowly run down, cells depolarize and swell.

Graded potentials

Graded potentials are local changes in membrane potential at dendrites, cell bodies and sensory endings: excitatory and inhibitory postsynaptic potentials, receptor (generator) potentials and the end plate potential. They vary in size with stimulus strength, can be depolarizing or hyperpolarizing, decay with distance (decremental conduction) and can summate. How far they spread is set by the length constant (lambda), which increases with higher membrane resistance and lower internal (axial) resistance. The time constant (resistance times capacitance) sets how quickly they rise and fall.

The action potential

The action potential is an all or none, regenerative, non decremental depolarization that travels along axons and muscle fibers. It begins when graded depolarization at the axon initial segment (or axon hillock), which has the highest density of voltage gated sodium channels, reaches threshold (about -55 mV in neurons).

1. Upstroke (depolarization). Voltage gated Na+ channels open their activation (m) gates. Sodium entry depolarizes the membrane, which opens more sodium channels: a positive feedback loop (the Hodgkin cycle). The membrane overshoots zero and approaches, but never reaches, E(Na), peaking near +30 to +40 mV.
2. Peak and inactivation. Within about a millisecond the slower inactivation (h) gates close the sodium channels. They cannot reopen until the membrane repolarizes and the h gate resets.
3. Repolarization. Voltage gated (delayed rectifier) K+ channels open more slowly than sodium channels. Potassium leaves, returning the membrane toward rest.
4. Afterhyperpolarization (undershoot). Potassium channels close slowly, so for a few milliseconds total potassium conductance exceeds that at rest and the membrane dips toward E(K).

Threshold is the voltage at which inward sodium current just exceeds outward potassium and leak current. A stimulus that depolarizes slowly can fail to fire because sodium channels inactivate as they open, a phenomenon called accommodation. The pump plays no direct role in a single spike: an action potential still fires for thousands of cycles after the pump is poisoned.

Refractory periods

During the absolute refractory period no stimulus, however strong, can trigger a second action potential, because most sodium channels are inactivated. It lasts from threshold until repolarization is largely complete, and it limits maximum firing frequency (roughly 1 kHz for large neurons). During the relative refractory period, which overlaps the afterhyperpolarization, a stronger than normal stimulus can fire the cell: some sodium channels have recovered, but potassium conductance is still elevated and the membrane is farther from threshold. Refractoriness also forces one way propagation in the normal axon, because the patch just behind the advancing spike cannot fire again.

Conduction and myelination

An action potential spreads because local currents flow from the active region to adjacent resting membrane and depolarize it to threshold. Conduction velocity rises with axon diameter (lower internal resistance) and with myelination (higher membrane resistance and lower capacitance, so current spreads farther and charges the membrane faster). Myelin is formed by oligodendrocytes in the CNS (one cell wraps many axons) and Schwann cells in the PNS (one cell per internode). Voltage gated sodium channels cluster at the nodes of Ranvier, so the impulse jumps from node to node: saltatory conduction. It is faster and also metabolically cheaper, because ion exchange occurs only at nodes.

Demyelination slows or blocks conduction. In multiple sclerosis (CNS) and Guillain Barré syndrome (PNS), current leaks through the bare internode and may fail to reach threshold at the next node. Warmth worsens conduction in demyelinated fibers (Uhthoff phenomenon) because sodium channels inactivate faster at higher temperature.

Nerve fiber classification

Two systems coexist. The Erlanger Gasser (letter) system applies to all fibers, sensory and motor, and sorts them by conduction velocity. The Lloyd Hunt (Roman numeral) system applies to sensory fibers from muscle and skin. Boards test the matches.

Susceptibility to block differs by mechanism. Local anesthetics block small fibers first (C and B, then A delta), so pain and autonomic function disappear before touch and motor power. Pressure and ischemia affect large myelinated fibers first, which is why a "dead leg" from sitting on the sciatic nerve loses proprioception and motor function before pain. Hypoxia affects B fibers most.

Section 2Synapses and neurotransmitters

Electrical and chemical synapses

An electrical synapse is a gap junction: connexin proteins (six form a connexon, two connexons span the gap) create a pore that lets ions and small molecules pass directly from cell to cell. Transmission is nearly instantaneous, usually bidirectional, and synchronizes groups of cells. Gap junctions couple cardiac myocytes at intercalated discs, single unit smooth muscle cells, some brainstem and hypothalamic neurons, and glia. A chemical synapse releases a transmitter across a cleft of about 20 to 40 nm. It is slower (a synaptic delay of about 0.5 ms), strictly one way, and can amplify, invert (inhibit) and modulate signals. Nearly all synapses tested on boards are chemical.

Release: from spike to vesicle fusion

1. The action potential invades the presynaptic terminal and opens voltage gated calcium channels (N and P/Q types at most neuronal and neuromuscular terminals).
2. Calcium enters and binds synaptotagmin, the calcium sensor on docked vesicles.
3. The SNARE complex zips together: v SNARE synaptobrevin on the vesicle pairs with t SNAREs syntaxin and SNAP 25 on the membrane, pulling the membranes into fusion.
4. Transmitter is released by exocytosis in discrete packets, or quanta, each the contents of one vesicle. A spontaneous single quantum at the neuromuscular junction produces a miniature end plate potential.
5. Vesicle membrane is retrieved by clathrin mediated endocytosis (dynamin pinches it off) and refilled by vesicular transporters driven by a proton gradient.

Release depends steeply on calcium: roughly the fourth power of calcium concentration in the terminal. Botulinum toxin and tetanus toxin are proteases that cleave SNARE proteins. Botulinum acts at peripheral cholinergic terminals and causes descending flaccid paralysis. Tetanus toxin travels retrograde to the spinal cord and blocks release of glycine and GABA from inhibitory Renshaw and interneurons, causing spastic paralysis, trismus and opisthotonus. Lambert Eaton syndrome produces antibodies against presynaptic voltage gated calcium channels; weakness improves with repeated effort as calcium accumulates.

Receptors: ionotropic and metabotropic

Ionotropic receptors are ligand gated ion channels. They act in milliseconds. Examples: nicotinic acetylcholine receptors (Na+ and K+, excitatory), AMPA and kainate glutamate receptors (Na+ and K+), NMDA glutamate receptors (Ca2+ plus Na+, blocked by Mg2+ at rest), GABA-A and glycine receptors (Cl-, inhibitory) and 5-HT3 serotonin receptors. Metabotropic receptors are G protein coupled receptors that act over hundreds of milliseconds to minutes through second messengers. Examples: muscarinic, adrenergic, dopamine, most serotonin, GABA-B, metabotropic glutamate and opioid receptors.

Postsynaptic potentials and summation

An EPSP depolarizes the postsynaptic membrane, usually by opening channels permeable to both Na+ and K+, whose combined reversal potential is near 0 mV. An IPSP usually opens Cl- or K+ channels, holding the membrane near or below rest. When E(Cl) equals the resting potential, opening chloride channels produces no voltage change but still inhibits by shunting: it clamps the membrane and short circuits excitatory current. A single EPSP at a central synapse is often only 0.5 to 1 mV, far below what is needed to reach threshold, so neurons integrate many inputs.

• Temporal summation: successive inputs from one synapse arrive before the previous potential decays (favored by a long time constant).
• Spatial summation: simultaneous inputs from different synapses add (favored by a long length constant).
• Presynaptic inhibition: an axoaxonic synapse reduces calcium entry into another terminal, selectively reducing its release; this is a key mechanism in spinal pain modulation.

The axon initial segment is the integration point because it has the lowest threshold. Synapses on the cell body and near the initial segment (often inhibitory) have more influence than distant dendritic synapses.

The neuromuscular junction

At the skeletal neuromuscular junction, one motor axon spike releases about 100 to 200 quanta of acetylcholine. ACh binds nicotinic receptors (N_M type, two ACh molecules per receptor) concentrated on the crests of junctional folds. The end plate potential is large, well above what is needed, so transmission normally has a generous safety factor: every motor nerve spike produces a muscle spike. Acetylcholinesterase in the basal lamina hydrolyzes ACh to acetate and choline within about a millisecond, and choline is taken back up by a sodium coupled transporter (the rate limiting step for ACh synthesis, blocked by hemicholinium). ACh itself is made from acetyl CoA and choline by choline acetyltransferase and loaded into vesicles by the vesicular ACh transporter.

The major transmitters and their pathways

Neurons often co release a small molecule transmitter with a neuropeptide; peptides, stored in large dense core vesicles, need higher firing frequencies, are cleared by diffusion and must be resynthesized in the cell body. Reuptake transporters clear most amines and amino acids (SSRIs block SERT; cocaine blocks NET, DAT and SERT), while MAO and COMT degrade catecholamines.

Section 3Sensory and motor physiology

Receptors and transduction

A sensory receptor converts a stimulus into a receptor (generator) potential, a graded depolarization that, if large enough, fires action potentials in the afferent axon. Each receptor has an adequate stimulus, the form of energy to which it is most sensitive (the law of specific nerve energies: activation of a pathway produces its own sensation however it is stimulated, which is why pressing on the eye makes you "see" light). Stimulus intensity is coded by frequency of firing and by the number of receptors recruited (population coding). Location is coded by which fibers fire, and spatial resolution depends on receptive field size and receptor density; the fingertips and lips have small fields and the best two point discrimination.

Adaptation is a fall in firing during a constant stimulus. Tonic (slowly adapting) receptors signal sustained stimuli: Merkel discs (pressure, texture), Ruffini endings (skin stretch), muscle spindles, Golgi tendon organs, nociceptors and baroreceptors. Phasic (rapidly adapting) receptors signal change: Meissner corpuscles (light touch, flutter), Pacinian corpuscles (vibration, especially near 250 Hz; the onion like capsule filters out sustained pressure) and hair follicle receptors.

Pain physiology

Nociceptors are free nerve endings that respond to tissue damaging mechanical, thermal or chemical stimuli. A delta fibers carry first (fast) pain, sharp and well localized, and synapse mainly in laminae I and V of the dorsal horn using glutamate. C fibers carry second (slow) pain, burning, aching and poorly localized, and synapse in laminae I and II (the substantia gelatinosa) using glutamate and substance P.

Injured tissue releases bradykinin, prostaglandins (especially PGE2), histamine, serotonin, potassium and H+. Prostaglandins do not cause pain much by themselves but lower nociceptor threshold: this is peripheral sensitization and the basis of NSAID analgesia. Antidromic release of substance P and CGRP from C fiber terminals causes vasodilation and plasma extravasation: neurogenic inflammation. Repetitive C fiber input to the dorsal horn produces wind up and central sensitization (NMDA receptor dependent), giving hyperalgesia (more pain from a painful stimulus) and allodynia (pain from a normally innocuous stimulus such as light touch).

Referred pain arises because visceral and somatic afferents converge on the same dorsal horn projection neurons (convergence projection theory): cardiac ischemia felt in the left arm and jaw (T1 to T4), diaphragmatic irritation in the shoulder (C3 to C5), appendicitis first around the umbilicus (T10).

Gate control and descending modulation

The gate control theory (Melzack and Wall) proposes that large diameter A beta touch fibers excite inhibitory interneurons in the substantia gelatinosa, which in turn inhibit transmission from nociceptive fibers to the projection (T) cells of the spinothalamic tract. Small fiber input inhibits those same interneurons, "opening the gate." Rubbing a bumped elbow, TENS, vibration and manual therapy all exploit this mechanism.

The brain also closes the gate from above. The periaqueductal gray (midbrain) excites the rostral ventromedial medulla, including the nucleus raphe magnus (serotonin), and the locus coeruleus sends noradrenergic fibers; both descend in the dorsolateral funiculus to the dorsal horn, where they activate enkephalin interneurons. Enkephalins act on mu and delta opioid receptors to cause presynaptic inhibition (less calcium entry, less substance P release) and postsynaptic hyperpolarization. Stress induced analgesia, placebo analgesia and part of the analgesic effect of spinal manipulation are thought to engage this descending system.

Muscle spindles and Golgi tendon organs

The muscle spindle lies in parallel with ordinary (extrafusal) muscle fibers and senses length and rate of change of length. Inside its capsule are intrafusal fibers: nuclear bag fibers (dynamic and static) and nuclear chain fibers. Group Ia primary (annulospiral) endings wrap all intrafusal fibers and respond to both length and velocity (dynamic response). Group II secondary (flower spray) endings lie mainly on chain and static bag fibers and signal static length. Gamma motor neurons innervate the contractile poles of intrafusal fibers. When the muscle shortens, the spindle would go slack and stop reporting; alpha gamma coactivation contracts the intrafusal poles in step with the extrafusal fibers, keeping the spindle taut and sensitive throughout movement. Increased gamma drive raises spindle sensitivity and reflex gain.

The Golgi tendon organ lies in series with extrafusal fibers at the musculotendinous junction. Its Ib afferent endings are squeezed between collagen strands when the tendon is loaded, so it signals tension, especially tension produced by active contraction. It is sensitive throughout the normal range, not only at dangerous loads.

Spinal reflexes

The deep tendon reflex tests the whole arc: receptor, afferent, root, cord segment, alpha motor neuron, neuromuscular junction and muscle. Standard levels: biceps C5 to C6, brachioradialis C6, triceps C7, patellar L4 (L2 to L4), Achilles S1. Renshaw cells are glycinergic interneurons excited by alpha motor neuron collaterals; they feed back to inhibit the same and synergistic motor neurons (recurrent inhibition), limiting firing rate. The Jendrassik maneuver (clenching teeth or pulling hooked fingers apart) increases gamma drive and facilitates a hard to elicit reflex.

The motor control hierarchy

Motor control is organized in levels. The spinal cord contains the final common pathway (alpha motor neurons), reflex circuits and central pattern generators for rhythmic stepping. The brainstem (vestibulospinal, reticulospinal and tectospinal tracts) controls posture, balance, head and eye orientation and axial and proximal muscles. The motor cortex (primary motor area 4, premotor and supplementary motor areas 6) plans and commands voluntary, especially fine distal, movement through the corticospinal and corticobulbar tracts. Two side loops refine commands without projecting directly to the cord. The cerebellum compares intended with actual movement and corrects errors in timing, coordination and smoothness; damage gives ipsilateral ataxia, dysmetria, intention tremor and dysdiadochokinesia. The basal ganglia select wanted movements and suppress unwanted ones through the direct (facilitates movement, D1) and indirect (inhibits movement, D2) pathways; damage gives hypokinetic (Parkinson disease) or hyperkinetic (Huntington chorea, hemiballismus) disorders with resting tremor and rigidity rather than weakness.

Upper versus lower motor neuron signs

The upper motor neuron (UMN) runs from cortex or brainstem to the anterior horn; the lower motor neuron (LMN) runs from the anterior horn (or cranial nerve nucleus) to the muscle. The pattern of signs locates a lesion and is among the most tested topics in neurophysiology.

In the acute phase of a UMN lesion (spinal shock or acute stroke) tone and reflexes are depressed for days to weeks before spasticity appears. Amyotrophic lateral sclerosis characteristically produces both UMN and LMN signs, sometimes in the same limb, with sensation spared.

Section 4Skeletal muscle physiology

Structure that explains function

A skeletal muscle is made of fascicles, each a bundle of multinucleated muscle fibers (cells), each packed with myofibrils, each a chain of sarcomeres, the contractile unit running from one Z line (Z disc) to the next. Thin filaments are actin with the regulatory proteins tropomyosin and troponin (troponin C binds calcium, troponin I inhibits actin myosin interaction, troponin T binds tropomyosin) and are anchored at the Z line. Thick filaments are myosin II, each molecule with two heads that carry an ATPase site and an actin binding site, anchored at the M line. The giant elastic protein titin runs from Z line to M line, centers the thick filament and provides passive tension when the muscle is stretched. Nebulin sets thin filament length. Dystrophin links the cytoskeleton through the sarcolemma to the extracellular matrix; its absence (Duchenne) or truncation (Becker) makes the membrane fragile.

Excitation contraction coupling

Excitation contraction coupling links the muscle action potential to the rise in cytosolic calcium that starts contraction.

1. ACh at the end plate triggers an end plate potential and then a muscle action potential that spreads along the sarcolemma.
2. The action potential dives into the fiber along transverse (T) tubules, which in skeletal muscle lie at the A band to I band junction, two per sarcomere, flanked by terminal cisternae of the sarcoplasmic reticulum (a triad).
3. T tubule depolarization changes the shape of the dihydropyridine receptor (DHPR), an L type calcium channel that in skeletal muscle acts mainly as a voltage sensor.
4. The DHPR is mechanically coupled to the ryanodine receptor (RyR1) in the SR membrane, which opens and releases stored calcium into the cytosol. Skeletal muscle does not need extracellular calcium entry to contract.
5. Calcium binds troponin C, shifting tropomyosin off the myosin binding sites on actin.
6. Cross bridge cycling proceeds until calcium is pumped back into the SR by SERCA (sarcoplasmic and endoplasmic reticulum Ca2+ ATPase), where it is held by calsequestrin. Relaxation therefore also requires ATP.

The cross bridge cycle

1. Rigor (attached) state: myosin head bound to actin, no nucleotide.
2. ATP binds to the myosin head, reducing its affinity for actin; the head detaches.
3. ATP is hydrolyzed to ADP and Pi, which stay bound; the head swings into the cocked, high energy position.
4. Weak binding to a new actin site (possible only if calcium has moved tropomyosin).
5. Pi release triggers the power stroke: the head pivots, pulling the thin filament about 10 nm toward the M line.
6. ADP release returns the head to the rigor state, ready for another ATP.

ATP has three roles in muscle: powering the cross bridge (hydrolysis), detaching myosin from actin (binding) and pumping calcium back into the SR (SERCA), plus running the Na+/K+ ATPase. Without ATP, heads remain locked to actin: rigor mortis, which begins a few hours after death and resolves as proteins degrade.

Twitch, summation and tetanus

A single action potential produces a twitch. Because the muscle action potential lasts only 2 to 4 ms while the twitch lasts tens of milliseconds, a second stimulus can arrive before relaxation is complete, adding force: summation (wave summation). At higher frequencies twitches fuse into a smooth sustained contraction, tetanus, with force several times that of a single twitch, because calcium stays high and the series elastic elements are fully stretched. Skeletal muscle can be tetanized because its refractory period is far shorter than its contraction; cardiac muscle cannot (Section 5).

Length tension and force velocity

Isometric force depends on sarcomere length. Active tension peaks at the optimal length (about 2.0 to 2.2 µm), where overlap between thin filaments and the cross bridge region of thick filaments is maximal. When the muscle is stretched beyond that, overlap falls and active force declines; when shortened, thin filaments overlap one another and thick filaments butt against Z lines, also reducing force. Passive tension, from titin and connective tissue, rises steeply as the muscle is stretched past resting length. Total tension is the sum. In the body, most muscles operate near the peak of the active curve.

The force velocity relationship states that the faster a muscle shortens, the less force it can produce, because fewer cross bridges are attached at any moment. Maximum velocity occurs with zero load; maximum force is isometric. During eccentric (lengthening) contraction, force can exceed isometric maximum, which is one reason eccentric exercise causes more delayed onset muscle soreness and is also used to load tendons in rehabilitation.

Motor units and recruitment

A motor unit is one alpha motor neuron plus all the muscle fibers it innervates; all fibers of a unit are the same type and contract together. Muscles for fine control (extraocular, intrinsic hand, laryngeal) have small units of a few to about 10 fibers; postural and power muscles (gastrocnemius, quadriceps) have units of hundreds to over a thousand fibers. Force is graded in two ways: recruitment of more motor units, and rate coding, increasing firing frequency toward tetanus. Recruitment follows Henneman size principle: small motor neurons (lower threshold, because a given synaptic current produces a larger voltage change across their higher input resistance) are recruited first and innervate slow, fatigue resistant fibers; large motor neurons innervating fast, fatigable fibers are recruited last, only for strong efforts. This order gives fine control at low force and keeps the fatigable units in reserve.

Fiber types

Fiber type is set mainly by the motor neuron: cross innervation experiments convert slow to fast fibers and vice versa. Endurance training shifts IIx toward IIa and increases mitochondria and capillaries; resistance training produces hypertrophy (more myofibrils per fiber) with little true hyperplasia. Disuse, bed rest and spaceflight cause atrophy, especially of type I postural fibers.

Fatigue

Muscle fatigue is a reversible fall in force or power during sustained or repeated activity. Central fatigue arises in the CNS: reduced motor drive, motivation, serotonergic effects. Peripheral fatigue arises in the muscle: accumulation of inorganic phosphate (which reduces cross bridge force and calcium release; now considered a major contributor), reduced SR calcium release, extracellular potassium accumulation in T tubules impairing action potentials, glycogen depletion in prolonged exercise, and, to a lesser degree than once believed, H+ accumulation. Lactate itself is not the cause of fatigue and is actually a fuel that heart and type I fibers oxidize. Neuromuscular junction failure is rare in healthy people but central to myasthenia gravis.

Section 5Smooth and cardiac muscle and muscle energetics

Smooth muscle structure

Smooth muscle cells are small, spindle shaped and uninucleate, with no sarcomeres and therefore no striations. Actin filaments attach to dense bodies in the cytoplasm (the functional equivalent of Z lines, containing alpha actinin) and to dense plaques on the membrane. Myosin is arranged in side polar filaments, which let filaments slide over a long distance: smooth muscle can generate force over a much wider range of lengths than skeletal muscle, an advantage for hollow organs such as bladder and uterus. Smooth muscle lacks troponin. The SR is sparse, there are no T tubules, and instead shallow membrane invaginations called caveolae sit near the SR.

Single unit and multiunit smooth muscle

Single unit smooth muscle shows slow waves, rhythmic oscillations of membrane potential generated by interstitial cells of Cajal in the gut. Spikes, and contraction, occur when slow wave peaks reach threshold. Single unit muscle also shows the myogenic response (stretch causes contraction, important in arteriolar autoregulation) and stress relaxation (plasticity): after a filled bladder stretches, tension slowly falls back toward its prior level, so the organ can hold more volume without a large rise in pressure. Autonomic nerves release transmitter from varicosities along the axon rather than from discrete end plates.

Calcium and the regulation of smooth muscle contraction

Smooth muscle uses myosin based (thick filament) regulation. Calcium comes from two sources: extracellular entry through L type voltage gated channels, receptor operated and stretch activated channels, and release from the SR through IP3 receptors (after Gq activation) and ryanodine receptors (calcium induced calcium release).

1. Calcium binds calmodulin.
2. Calcium calmodulin activates myosin light chain kinase (MLCK).
3. MLCK phosphorylates the regulatory light chain of myosin, allowing cross bridge cycling.
4. Myosin light chain phosphatase (MLCP) dephosphorylates the light chain, causing relaxation.

The ratio of MLCK to MLCP activity sets tone. Calcium sensitization occurs through the RhoA and Rho kinase pathway, which inhibits MLCP so that more force is generated at the same calcium. Relaxation is promoted by cAMP (protein kinase A phosphorylates and inhibits MLCK; beta 2 agonists relax bronchi this way) and by cGMP (protein kinase G activates MLCP and lowers calcium; this is how nitric oxide, nitroglycerin and sildenafil act).

The latch state

When calcium and MLCK activity fall after a contraction, some myosin heads become dephosphorylated while still attached to actin. These latch bridges detach very slowly, so force is maintained with minimal cross bridge cycling and very low ATP use (up to several hundred times more economical than skeletal muscle). The latch state explains how sphincters, vascular smooth muscle and the uterus hold tone for hours without fatigue.

Cardiac muscle properties

Cardiac myocytes are striated (they have sarcomeres, troponin and tropomyosin), branched, usually have one or two central nuclei, and are joined end to end by intercalated discs containing desmosomes (mechanical coupling) and gap junctions (electrical coupling), so the atria and ventricles each behave as a functional syncytium. T tubules lie at the Z lines and pair with SR cisternae to form dyads. Mitochondria occupy about a third of cell volume, reflecting almost purely aerobic metabolism (fatty acids are the preferred fuel, plus lactate, glucose and ketones).

The key difference from skeletal muscle is calcium induced calcium release. During the plateau of the ventricular action potential, L type calcium channels (DHPRs) admit a small trigger of extracellular calcium, which opens RyR2 and releases much larger amounts from the SR. The strength of contraction therefore depends on extracellular calcium entry and SR load: catecholamines (beta 1, cAMP, protein kinase A) increase L type current and SERCA activity via phosphorylation of phospholamban, raising both contractility and relaxation rate (lusitropy). Digoxin inhibits the Na+/K+ ATPase, raising intracellular sodium, slowing the sodium calcium exchanger (NCX) and leaving more calcium in the cell for the SR: a positive inotrope.

The ventricular action potential has a long plateau (phase 2) produced by L type calcium entry balanced by potassium exit, so it lasts about 250 to 300 ms, nearly as long as the contraction. The effective refractory period therefore extends until the muscle has begun to relax, which prevents summation and tetanus and guarantees that the ventricles fill between beats. Chapter 8 develops pacemaker potentials and the ECG.

Muscle energetics: where the ATP comes from

Resting muscle holds only enough ATP for a few seconds of maximal effort (about 5 to 8 mM). Three systems regenerate it, overlapping in time rather than switching on and off.

• Phosphagen system. Creatine phosphate (phosphocreatine) donates its phosphate to ADP via creatine kinase: PCr + ADP gives Cr + ATP. Stores are 3 to 5 times those of ATP and support maximal efforts of roughly 10 seconds (sprint, lift, jump). Creatine is made in liver and kidney from arginine, glycine and methionine; spontaneous breakdown of creatine phosphate produces creatinine, excreted at a rate proportional to muscle mass, which is why serum creatinine estimates kidney function. Elevated serum CK indicates muscle damage (rhabdomyolysis, muscular dystrophy, statin myopathy, intense exercise).
• Anaerobic glycolysis. Muscle glycogen and blood glucose are broken down to pyruvate in the cytosol, netting 2 ATP per glucose (3 from glycogen, because glycogen phosphorylase produces glucose 1 phosphate without spending ATP at hexokinase). When oxygen delivery or mitochondrial capacity is exceeded, lactate dehydrogenase converts pyruvate to lactate to regenerate NAD+ so glycolysis can continue. This system dominates efforts of about 10 seconds to 2 minutes. Lactate travels to the liver for gluconeogenesis (Cori cycle) or to heart and type I fibers for oxidation.
• Oxidative phosphorylation. Pyruvate, fatty acids (via beta oxidation) and amino acids feed acetyl CoA into the citric acid cycle; the electron transport chain yields about 30 to 32 ATP per glucose. It is slow to ramp up but has nearly unlimited capacity and dominates any effort longer than about 2 to 3 minutes. Fat dominates at low intensity; carbohydrate rises as intensity increases (the crossover concept).

After exercise, oxygen consumption stays elevated: excess post exercise oxygen consumption (once called oxygen debt) pays for resynthesis of creatine phosphate and ATP, reloading of myoglobin and hemoglobin, lactate clearance and the higher temperature and catecholamine levels. The lactate threshold (onset of blood lactate accumulation, often 50 to 60 percent of VO2max in untrained and 70 to 80 percent in trained people) marks the intensity above which glycolytic flux outpaces clearance.

Words worth knowing