Properties of Smooth Muscle

1. Structural / Histological Properties

• Cell length ranges from ~20 µm (small blood vessels) to ~500 µm (pregnant uterus); diameter at the widest point is 5-10 µm.
• Adjacent cells are packed in a "tail-to-head" arrangement so that cross-sections show profiles of varying diameter, with only the largest profiles containing nuclei.
• The cell surface shows numerous small plasmalemmal invaginations called caveolae, which are enriched in ion channels and act as functional substitutes for the T-tubule system of striated muscle.
• There are no T-tubules; this is unnecessary because the smaller cell diameter allows Ca²⁺ diffusion across the whole cell without them.
• The sarcoplasmic reticulum (SR) is rudimentary - a peripheral SR close to the sarcolemma handles local Ca²⁺ release for membrane excitability, and a deeper central SR delivers Ca²⁺ to the contractile apparatus.
• Junqueira's Basic Histology 17e, pp. 527-529

2. Types of Smooth Muscle

3. Excitation-Contraction (E-C) Coupling

1. A stimulus (depolarization, hormone, neurotransmitter) raises intracellular [Ca²⁺] through three routes:
   • Voltage-gated Ca²⁺ channels (open with membrane depolarization - unlike skeletal muscle, even sub-threshold depolarization can open these)
   • Ligand-gated Ca²⁺ channels (activated by hormones/neurotransmitters via G-proteins)
   • IP₃-gated SR channels (hormones → PLC → IP₃ → Ca²⁺ release from SR)
   • Additional amplification via Ca²⁺-induced Ca²⁺ release (CICR) through ryanodine receptors (RYR2 isoform) on the SR
2. Ca²⁺ binds calmodulin (4 Ca²⁺ ions cooperatively). The Ca²⁺-calmodulin complex activates MLCK.
3. MLCK phosphorylates the myosin light chain at serine-19 → increases myosin ATPase activity → myosin head binds actin → cross-bridge cycling → tension.
4. Calponin and caldesmon (thin filament proteins) also participate: at low [Ca²⁺], they inhibit myosin ATPase; rising Ca²⁺-calmodulin phosphorylates them, releasing their inhibition.
5. Relaxation occurs when [Ca²⁺] falls via: closing of voltage-gated channels (hyperpolarization), cAMP/cGMP-mediated channel inhibition, decreased IP₃ production, increased SR-Ca²⁺-ATPase activity, or activation of myosin light chain phosphatase (dephosphorylates myosin).

• Costanzo Physiology 7th Edition, pp. 49-50; Ganong's Review of Medical Physiology 26e, p. 127-128

4. Electrical Properties

• Resting membrane potential is typically -50 to -60 mV (less negative than skeletal muscle's ~-90 mV).
• Action potentials in smooth muscle have a slower time course, with a plateau phase mediated primarily by L-type Ca²⁺ channels (rather than Na⁺ channels as in skeletal muscle).
• Slow waves (pacemaker potentials) arise from spontaneous oscillations in membrane potential in unitary smooth muscle. When slow waves reach threshold, bursts of action potentials are superimposed, each followed by a contraction.
• Unlike skeletal and cardiac muscle, smooth muscle can be activated to contract by subthreshold depolarization alone - full action potentials are not required.

5. Autonomic Innervation and Pharmacology

• Acetylcholine (muscarinic receptors, via PLC/IP₃): depolarizes unitary smooth muscle, increases spike frequency, increases tone and contractility. - Ganong's 26e, p. 128
• Norepinephrine/epinephrine: typically hyperpolarizes intestinal smooth muscle (relaxation) by opening K⁺ channels (β₂ receptors), but contracts vascular smooth muscle (α₁ receptors).
• Unitary smooth muscle also shows 2 key autonomous properties: (1) spontaneous activity without nervous stimulation, and (2) contraction in response to stretch (myogenic response). Stretch → depolarization → increased spike frequency → increased tone. This is the physiological basis of Bayliss' myogenic response in blood vessels. - Ganong's 26e, p. 128

• NO (nitric oxide) from endothelium → activates soluble guanylyl cyclase → ↑cGMP → protein kinase G activation → Ca²⁺ channel inhibition/enhanced Ca²⁺ removal → relaxation.
• Phosphodiesterase V inhibitors (sildenafil, tadalafil) block cGMP breakdown → prolonged smooth muscle relaxation in corpus cavernosum and pulmonary vasculature.
• β₂-agonists (albuterol) → ↑cAMP → relaxation of airway smooth muscle (used in asthma).
• Ganong's 26e, p. 129

6. Mechanical Properties

• Smooth muscle contracts and relaxes slowly (cross-bridge cycling rate is ~100× slower than fast skeletal muscle).
• The latch-bridge mechanism allows sustained contraction with very little ATP expenditure: after MLCK activity declines and [Ca²⁺] falls, myosin cross-bridges remain attached to actin for prolonged periods in a high-force, low-energy state. This is especially important in vascular smooth muscle maintaining resting vessel tone. - Ganong's 26e, p. 127
• Smooth muscle can sustain prolonged contraction without fatigue.
• It can shorten to a much greater fraction of its resting length than skeletal muscle (up to 70-80%), allowing hollow organs like the bladder and uterus to change volume enormously.
• Plasticity: smooth muscle can adjust its resting length over a wide range and still generate near-maximal force - this is critical for organs like the bladder that fill and empty repeatedly.

7. Location and Functions

• GI tract - peristalsis and segmentation (two layers: inner circular, outer longitudinal)
• Vasculature - control of vessel caliber and blood pressure
• Bladder, ureter, uterus - propulsion and voiding
• Bronchioles - airway resistance regulation
• Iris and ciliary body - pupil diameter and lens accommodation
• Vas deferens, seminal vesicles

The Gut-Brain Axis

1. Structural Anatomy of the Axis

The Enteric Nervous System - "The Second Brain"

• Myenteric (Auerbach's) plexus - between the longitudinal and circular muscle layers; primarily controls motility
• Submucosal (Meissner's) plexus - in the submucosa; controls secretion and local blood flow

The Vagus Nerve - The Highway

• Afferent (gut → brain): ~80% of vagal fibers are sensory, carrying information from chemoreceptors, osmoreceptors, and mechanoreceptors in the gut mucosa to autonomic centers in the medulla. These form vagovagal reflexes - loops that integrate gut sensory input at the brainstem and send efferent motor output back to the ENS.
• Efferent (brain → gut): Preganglionic parasympathetic fibers from the dorsal motor nucleus of the vagus synapse on ENS neurons. Parasympathetic stimulation (overall) increases secretion and motility.
• The vagus nerve innervates the gut from the pharynx to the distal colon; the remaining distal colon and rectum receive parasympathetic innervation from pelvic nerves (S2-S4).
• Medical Physiology (Boron & Boulpaep), p. 1279-1280

Sympathetic Innervation

Hierarchical Reflex Loops

1. ENS (lowest level) - independent reflexes
2. Autonomic ganglia - control ENS and other organs
3. Spinal cord - integrates across levels
4. Brainstem - coordinates all visceral control
5. Forebrain (hypothalamus, limbic system, cortex) - top-down modulation of all lower centers

2. Communication Pathways

A. Neural Pathways

B. Neuroendocrine / HPA Axis Pathway

1. Stress → hypothalamus releases corticotropin-releasing factor (CRF)
2. CRF → anterior pituitary releases ACTH
3. ACTH → adrenal cortex releases cortisol
4. Cortisol feeds back to hypothalamus/pituitary (negative feedback) AND acts directly on gut:
   • Increases colonic motility
   • Increases visceral sensitivity
   • Promotes mast cell degranulation
   • Increases intestinal permeability
   • Modulates the local gut immune response

C. Immune Pathway

• Activated mast cells and mucosal immune cells release cytokines (IL-1β, IL-6, TNF-α) that activate afferent sensory neurons
• The vagus nerve has anti-inflammatory properties: vagal efferent activity suppresses peripheral cytokine release via the cholinergic anti-inflammatory pathway (ACh acting on α7-nicotinic receptors on macrophages)
• Post-infectious IBS is associated with persistently increased gut lymphocytes, enterochromaffin cells, and IL-1β expression, linking prior immune activation to chronic gut-brain dysregulation - Yamada's 7e, p. 1461

D. Enteroendocrine / Humoral Signaling

3. The Microbiota-Gut-Brain Axis

Top-down (Brain → Microbiota)

• Stress, psychological states, and experiences alter intestinal microbiota composition in animal models
• Mechanism: ANS-mediated changes in gut motility, secretion, mucus, and permeability change the gut environment, which selects for different microbial populations
• Norepinephrine released under stress can directly stimulate bacterial growth

Bottom-up (Microbiota → Brain)

1. ENS activation - Microbes alter ENS function; ENS signals are relayed to CNS via the vagus nerve. Ingestion of Lactobacillus rhamnosus changed GABA receptor expression in limbic brain regions and reduced anxiety/depression - an effect blocked by vagotomy, proving vagal dependence. - Harrison's Principles of Internal Medicine 22e, p. 3442
2. HPA axis programming - Microbial metabolites (especially SCFAs) target areas not protected by the blood-brain barrier (BBB), including the hypothalamus. Neonatal microbiota program HPA axis stress reactivity across the lifespan - germ-free animals show exaggerated HPA responses that are normalized by microbial colonization. - Sleisenger & Fordtran's
3. Bloodborne microbial metabolites crossing the BBB
   • Short-chain fatty acids (SCFAs) - butyrate, propionate, acetate; produced by bacterial fermentation of dietary fiber; regulate BDNF production, microglial development, and epigenetic changes in brain cells
   • Tryptophan metabolites - serotonin precursors and kynurenine pathway products; influence serotonergic signaling
   • Secondary bile acids - affect neuroinflammation via TGR5 and FXR receptors
   • Bacterial neurotransmitters - gut bacteria produce GABA, serotonin, dopamine precursors, and neuroactive peptides
4. Immune modulation - Microbiota are required for normal development and function of brain microglia (CNS-resident immune cells); dysbiosis → immune dysregulation → neuroinflammation

• Harrison's Principles 22e; Sleisenger & Fordtran's GI and Liver Disease

4. Stress and the Gut-Brain Axis

• Cannon (1915): Observed cessation of bowel activity in cats exposed to a growling dog
• Pavlov: Demonstrated that psychic stimuli alter gastric acid secretion via the vagus
• Beaumont & Wolf: Observed direct mucosal changes (hyperemia, altered motility/secretion) in response to emotions (anger, anxiety)

• Increased colonic motility (CRF-mediated)
• Increased visceral sensitivity (central and peripheral sensitization)
• Increased intestinal permeability ("leaky gut") - allowing translocation of bacterial products and antigens
• Mast cell degranulation → histamine and cytokine release → sensitization of afferent neurons
• Altered microbiota composition
• Sleisenger & Fordtran's, pp. 350-351; Yamada's 7e, pp. 1459-1461

5. Clinical Relevance - Disorders of Gut-Brain Interaction (DGBIs)

Irritable Bowel Syndrome (IBS)

• Visceral hypersensitivity - lowered pain thresholds in the gut, partly from central sensitization (altered pain processing in brain) and peripheral sensitization (sensitized mucosal afferents)
• Altered motility - exaggerated motility response to stress
• HPA dysregulation - chronic stress is the strongest predictor of IBS symptom intensity over time
• Post-infectious IBS - acute gut infection → persistent immune activation → altered ENS → IBS; predicted by psychological stress and anxiety at time of infection
• Dysbiosis - altered microbiome composition contributes to both gut and central symptoms

Neurodegenerative Diseases

• Autism Spectrum Disorder (ASD) - children with ASD have documented dysbiosis; severity of dysbiosis correlates with autism severity; murine models show microbiome manipulation can ameliorate ASD behaviors (IL-17 implicated)
• Multiple sclerosis - A. muciniphila peptides may drive cross-reactive immune responses via molecular mimicry (HLA-related)
• Alzheimer's disease - gut dysbiosis → neuroinflammation → amyloid processing (active research area)
• Guillain-Barré syndrome - Campylobacter jejuni molecular mimicry of GM1 ganglioside
• Hepatic encephalopathy - gut bacterial metabolites (ammonia, etc.) directly worsen brain function; treated with rifaximin/lactulose
• Harrison's Principles of Internal Medicine 22e

Psychiatric Disorders

• Depression and anxiety - gut dysbiosis and altered 5-HT production are implicated; gut microbiota influence BDNF levels, GABA receptor expression, and HPA reactivity
• Anorexia nervosa / eating disorders - gut hormones (ghrelin, CCK, GLP-1) modulate satiety circuits
• Germ-free mice display reduced anxiety, less stress reactivity, more exploratory behavior, and impaired memory - all reversible with microbial colonization

6. Microbiome as a Therapeutic Target

Summary

BRAIN (affect, cognition, visceral perception)
        ↕  (vagal afferents/efferents)
HPA AXIS (CRF → ACTH → cortisol)
        ↕
AUTONOMIC NERVOUS SYSTEM (sympathetic + parasympathetic)
        ↕
ENTERIC NERVOUS SYSTEM (myenteric + submucosal plexus)
        ↕  (immune system)
GUT EPITHELIUM & MICROBIOTA
(serotonin, SCFAs, bile acids, neurotransmitters, cytokines)

Mechanism of Intestinal Secretion

Overview: The Fluid Budget of the GI Tract

1. Anatomical Basis: The Crypt-Villus Unit

• Villi (finger-like projections): covered by enterocytes that absorb fluid, electrolytes, and nutrients
• Crypts of Lieberkühn (pits between villi): lined by epithelial cells (enterocytes + goblet cells + Paneth cells) that secrete fluid and electrolytes

2. Core Cellular Mechanism: Cl⁻-Driven Secretion

Step-by-Step (Crypt Cell)

• The Na⁺-K⁺-ATPase pump on the basolateral membrane continuously pumps 3 Na⁺ out and 2 K⁺ in, using ATP.
• This maintains a low intracellular Na⁺ concentration and a negative intracellular electrical potential - the driving force for the next step.

• The low intracellular Na⁺ drives the Na⁺-K⁺-2Cl⁻ (NKCC1) cotransporter on the basolateral membrane.
• This brings Na⁺, K⁺, and 2Cl⁻ into the cell together from the interstitial fluid.
• Result: intracellular [Cl⁻] rises above its electrochemical equilibrium.

• The K⁺ brought in by NKCC1 exits back across the basolateral membrane through basolateral K⁺ channels (KCNQ1, etc.).
• This keeps intracellular [K⁺] in check and maintains the negative membrane potential.

• Cl⁻, which has accumulated inside the cell, now moves down its electrochemical gradient out through CFTR (Cystic Fibrosis Transmembrane conductance Regulator) Cl⁻ channels in the apical membrane into the intestinal lumen.
• CFTR channels are normally closed at rest - they must be opened by intracellular signals (see below).
• Other apical Cl⁻ channels (Ca²⁺-activated CaCC channels) also contribute.

• Cl⁻ secretion into the lumen creates a negative lumenal charge.
• Na⁺ is dragged through the paracellular pathway (tight junctions) by this electrochemical gradient.

• The accumulation of NaCl in the lumen creates an osmotic gradient.
• Water moves osmotically from the cell and interstitium into the lumen.
• Net result: secretion of isotonic NaCl solution into the lumen.
• Costanzo Physiology 7th Edition, p. 391-392; Guyton & Hall, p. 811

3. Regulation: What Opens the Cl⁻ Channels?

A. cAMP Pathway (most important)

Secretagogue (VIP, ACh, PGE₂, cholera toxin)
         ↓
Gs protein → Adenylyl cyclase activated
         ↓
ATP → cAMP ↑
         ↓
Protein Kinase A (PKA) activated
         ↓
PKA phosphorylates CFTR → CFTR Cl⁻ channel OPENS
         ↓
Cl⁻ secretion → Na⁺ + H₂O follow → Net fluid secretion

• VIP (Vasoactive Intestinal Peptide) - the most potent endogenous secretagogue
• Prostaglandins (PGE₂, PGI₂) - released during inflammation; explain secretory diarrhea in enteritis
• Secretin - from S cells of duodenum; stimulates pancreatic and intestinal secretion
• Cholera toxin (see below)

B. Ca²⁺ Pathway

Secretagogue (ACh via muscarinic M3, 5-HT, substance P)
         ↓
Gq protein → Phospholipase C activated
         ↓
PIP₂ → IP₃ + DAG
         ↓
IP₃ → Ca²⁺ release from ER; DAG → PKC
         ↓
↑ intracellular Ca²⁺ → Ca²⁺-activated CaCC channels open
         ↓
Cl⁻ secretion → fluid secretion

• Acetylcholine (from ENS neurons): major neural secretagogue - acts via M3 receptors → ↑Ca²⁺
• Serotonin (5-HT): released by enterochromaffin cells in response to luminal stimuli → activates ENS neurons → Cl⁻ secretion
• Substance P, neurotensin: neuropeptides released by ENS

C. cGMP Pathway

• Heat-stable E. coli enterotoxin (STa): binds guanylyl cyclase C (GCC) receptor on apical membrane → ↑cGMP → activates PKG → phosphorylates CFTR
• Guanylin and uroguanylin: endogenous peptides that physiologically activate GCC → regulate fluid balance
• Linaclotide, plecanatide: drugs for constipation that mimic guanylin → activate GCC → ↑ intestinal secretion/motility
• Medical Physiology (Boron & Boulpaep), p. 2084-2087

Summary Table: Secretagogues and Their Mechanisms

4. Neural Regulation

• Tactile/chemical stimulation of mucosal mechanoreceptors and chemoreceptors (e.g., by chyme) → activate submucosal plexus neurons
• These release VIP and ACh onto crypt cells → Cl⁻ secretion
• Parasympathetic stimulation (vagus nerve, pelvic nerves) → increases secretion via ENS
• Sympathetic stimulation → generally inhibits secretion (α₂ adrenergic receptors → ↓cAMP; also α-adrenergic stimulation increases NaCl absorption)

5. Composition of Intestinal Secretion

6. Large Intestine Secretion

• Mucus only (from abundant goblet cells) - the dominant secretion
• Small amount of NaHCO₃ - neutralizes acid from bacterial fermentation
• Volume: much smaller than small intestine (~few hundred mL/day at most)

• Direct tactile stimulation of epithelium → reflex mucus secretion
• Parasympathetic stimulation (pelvic nerves, S2-S4) → marked increase in mucus secretion + peristalsis
• Extreme parasympathetic activation (emotional disturbance) → copious ropy mucus without feces

1. Protects wall from mechanical excoriation
2. Holds fecal matter together
3. Shields epithelium from bacterial activity and their metabolic acids
4. The alkaline (pH 8.0) HCO₃⁻-rich mucus neutralizes acidic fermentation products

• Guyton & Hall Medical Physiology, p. 812

7. Clinical Application: Secretory Diarrhea

Cholera (prototype)

1. Vibrio cholerae colonizes small intestine → releases cholera toxin (CT)
2. CT B subunit binds GM1 ganglioside on apical membrane → A subunit enters cell
3. A subunit catalyzes ADP-ribosylation of Gsα → Gsα locked in active (GTP-bound) state
4. Adenylyl cyclase permanently activated → cAMP remains maximally elevated
5. CFTR channels stay permanently open → massive Cl⁻ secretion
6. Na⁺ and H₂O follow → up to 20 L/day of "rice-water stool"
7. Electrolyte losses: HCO₃⁻ loss → metabolic acidosis; K⁺ loss → hypokalemia

Oral Rehydration Solution (ORS) - Exploiting Normal Absorption

• ORS (glucose + Na⁺ + Cl⁻ + HCO₃⁻ + K⁺) → SGLT1 absorbs Na⁺ with glucose → water follows
• This absorption mechanism works normally even in cholera
• This is the basis of WHO Oral Rehydration Therapy - arguably one of the greatest public health achievements

Types of Diarrhea (MBBS)

• Costanzo Physiology 7th Edition, p. 392-393

Quick Revision Summary (MBBS)

CRYPT CELL SECRETION (active Cl⁻ secretion)

Basolateral:
  Na⁺-K⁺-ATPase → lowers intracellular Na⁺ (uses ATP)
  NKCC1 cotransporter → Na⁺ + K⁺ + 2Cl⁻ enter cell (driven by low intracellular Na⁺)
  K⁺ channels → K⁺ recycles back out

Apical:
  CFTR Cl⁻ channels (normally CLOSED)
  Opened by: cAMP (via VIP, cholera toxin) or Ca²⁺ (via ACh, 5-HT) or cGMP (via STa, guanylin)
  Cl⁻ exits into lumen

Na⁺ follows paracellularly (electrochemical drag)
H₂O follows osmotically

→ Isotonic NaCl fluid secreted into lumen
→ Absorbed by villus cells further up (net absorption normally)
→ If secretion overwhelms absorption → DIARRHEA

• Triggered by VIP, cholera toxin, PGE₂, secretin
• Gs → adenylyl cyclase → cAMP → PKA → CFTR phosphorylation and opening
• Cholera toxin irreversibly ADP-ribosylates Gsα → permanently elevated cAMP → massive fluid loss

• Triggered by ACh (M3), serotonin (5-HT), substance P
• Gq → PLC → IP₃ + DAG → Ca²⁺ release from ER → CaCC channels open
• Also activates PKC, which can synergize with cAMP