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Smooth Muscle
Smooth muscle surrounds the major
hollow organs - including: blood vessels,
bronchi, gut, uterus, bladder.
Diverse Functions
Responsive to a variety of stimuli:
neural input (autonomic and enteric
nervous systems)
broad array of hormones and drugs
metabolites
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
Slow Waves
Rhythmic Changes in Smooth Muscle
Membrane Potential
Interstitial Cells of Cajal (ICC):
Pacemaker Cells with Ion Channel
Characteristics Similar to SA node cells
Inhibitory Input from Myenteric Interneurons Limits the Level of Depolarization
Slow Waves Exhibited in Duodenal Visceral Smooth
Muscle Cells
tetrodotoxin
membrane slow waves
potential
mV
Force
(gms)
spikes
Patterns of Motor Activity
• Digestive
Inter-digestive
• Patterns of Contractility
Patterns of Motor Activity
• Digestive
Inter-digestive
• Patterns of Contractility
Tonic
Force
(gms)
Phasic
Patterns of Motor Activity
Peristalsis: Propulsive MovementsCentral Coordination
Rhythmic Segmentation:
Mixing and Slow PropulsionLocal Control
Paced by Slow Wave Frequency
3 Functions of Motility
• Reservoir: Colon, Proximal Stomach
• Mixing: Distal Stomach, Small and
Large Intestine
• Propulsion: Distal Stomach through
Colon, Esophagus
• Proximal Stomach
- Reservoir: Receptive Relaxation
• Distal Stomach
- Mixing and Grinding
- Regulation of Emptying
Intraluminal
pressure
Distal
Esophagus
Swallow
mm
Hg
0
LES
mm
Hg
0
Proximal
Stomach
Basal
Tone
mm
Hg
Relaxation
0
Seconds
Basal
Tone
Distal Stomach
Fraction in Stomach
1.0
0.8
Solid
0.6
0.4
Liquid
0.2
0
0
30
60
90
120
Minutes
150
180
Gastric Emptying
100
pH 2.0
Volume
Remaining
in Stomach
%
50
pH 4.0
pH 7.0
0
0
10
Minutes
20
Patterns of Motor Activity
Peristalsis: Propulsive MovementsCentral Coordination
Rhythmic Segmentation:
Mixing and Slow PropulsionLocal Control
Paced by Slow Wave Frequency
SLOW WAVE FREQUENCY
: Mixing and Movement
12
11
cycles 10
/min 9
8
Duodenum
Pylorus
Jejunum
Ileum
Ileocecal
Junction
Motility within the Large Intestine
Haustrations: Proximal Colon- Mixing
Mass Movements: Propulsive Contractions
:: Rectosphincteric Reflex
Primary Role of Individual Sphincters
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
Transit Times During Digestion/Absorption of a
Meal
Esophagus: Seconds
Stomach: 1-3 Hours
Small Intestine: 3-5 Hours
Colon: 1-3 Days
Inter-Digestive Period: Temporal Patterns of
Motility
MMC Phase
Duration (min)
% Slow Waves
with Spikes
I
45 - 60
0
II
30 - 45
50
Myoelectric
mV
Activity
Contraction mm Hg
Amplitude
Time
III
5 - 10
100
Inter-Digestive Period: Migrating Motor
Complex.
Phase III
Phase III
MINUTES
Inter-Digestive Period: Migrating Motor Complex
Intravenous Motilin
Endogenous
Motilin
MMCs
Duodenum
Prox. Jejunum
Mid Jejunum
hours
Phase I
• Exogenous
Phase II
Phase III
motilin stimulates both MMCs and endogenous motilin release