Smooth muscle surrounds the majorhollow organs - including: blood vessels,bronchi, gut, uterus, bladder.

Responsive to a variety of stimuli:neural input (autonomic and enteric

nervous systems)broad array of hormones and drugsmetabolites

Diverse Functions

Smooth Muscle

Smooth Muscle – Non-Striated

Arteries – blood vessels

Lines Hollow Organs

- Uterus

- Alimentary Canal

Lines Ducts of Secretory Glands – e.g. Salivary

- Maintain Tension for long periods

- Rhythmically Contract to Propel Contents

- Speed of Contraction 10X less than striated muscle

* Coupled into a syncitium by gap junctions

Rhythmic Changes in Smooth MuscleMembrane Potential

Inhibitory Input from Myenteric Inter-neurons Limits the Level of Depolarization

Interstitial Cells of Cajal (ICC): Pacemaker Cells with Ion Channel Characteristics Similar to SA node cells

Slow Waves

membranepotential

mV

Force (gms)

tetrodotoxinslow waves spikes

Slow Waves Exhibited in Duodenal Visceral Smooth Muscle Cells

• Digestive Inter-digestive

• Patterns of Contractility

Patterns of Motor Activity

• Digestive Inter-digestive

• Patterns of Contractility

Tonic PhasicForce (gms)

Patterns of Motor Activity

Peristalsis: Propulsive Movements- Central Coordination

Rhythmic Segmentation:

Mixing and Slow Propulsion- Local Control

Paced by Slow Wave Frequency

Patterns of Motor Activity

• Reservoir: Colon, Proximal Stomach

• Mixing: Distal Stomach, Small and Large Intestine

• Propulsion: Distal Stomach through Colon, Esophagus

3 Functions of Motility

• Proximal Stomach- Reservoir: Receptive Relaxation

• Distal Stomach- Mixing and Grinding- Regulation of Emptying

DistalEsophagus

LES

ProximalStomach

mmHg

Intraluminalpressure

mmHg

Swallow

BasalTone Relaxation

BasalTone

Seconds

0

0

0

mmHg

Minutes

Fra

ctio

n in

Sto

mac

h

Solid

Liquid

1.0

0.8

0.6

0.4

0

0.2

0 30 60 90 120 150 180

Distal Stomach

VolumeRemaining in Stomach

%

Minutes

pH 2.0

pH 4.0

pH 7.0

100

50

0

0 10 20

Gastric Emptying

Peristalsis: Propulsive Movements- Central Coordination

Rhythmic Segmentation:

Mixing and Slow Propulsion- Local Control

Paced by Slow Wave Frequency

Patterns of Motor Activity

cycles/min

Duodenum

Pylorus IleocecalJunction

Jejunum Ileum

12

11

10

9

8

: Mixing and Movement

SLOW WAVE FREQUENCY

Haustrations: Proximal Colon- Mixing

Mass Movements: Propulsive Contractions

:: Rectosphincteric Reflex

Motility within the Large Intestine

Lower Esophageal: Retard Gastric (LES) Retropulsion

Pyloric: Retard Gastric Emptying

Ileocecal: Retard Colonic Retropulsion

Internal Anal: Regulate Emptying Into the Rectum

External Anal: Regulate Defecation

Primary Role of Individual Sphincters

Esophagus: Seconds

Stomach: 1-3 Hours

Small Intestine: 3-5 Hours

Colon: 1-3 Days

Transit Times During Digestion/Absorption of a Meal

I45 - 60

0

II30 - 45

50

III5 - 10100

MMC PhaseDuration (min)% Slow Waves

with Spikes

MyoelectricActivity

ContractionAmplitude

mm Hg

mV

Time

Inter-Digestive Period: Temporal Patterns of Motility

Phase III

MINUTES

Phase III

Inter-Digestive Period: Migrating Motor Complex.

EndogenousMotilin

MMCsDuodenum

Prox. Jejunum

Mid Jejunum

Intravenous Motilin

Phase I Phase II Phase III

hours

• Exogenous motilin stimulates both MMCs and endogenous motilin release

Inter-Digestive Period: Migrating Motor Complex