Peristalsis

smooth muscle

found throughout body, forms sheets, bundles, or sheaths around tissues

smooth muscle around blood vessels

regulates blood flow

smooth muscle along passageways in digestive/urinary systems

forms ring shaped sphincters that regulate movement around passageways

smooth muscle in digestive tract

organized into inner circular layer and outer longitudinal layer; within each layer, cells are aligned parallel to each other

smooth muscle cells

relatively long and slender; contain actin and myosin filaments; no striations

thin filaments of smooth muscle cells

are attached to dense bodies; distributed thru out sarcoplasm; are attachment points for adjacent smooth muscle cells

thick filaments of smooth muscle cells

are scattered throughout the sarcoplasm; more myosin heads per thick filament than in skeletal or cardiac muscle

sarcoplasm

cytoplasm of muscle cells

sarcoplasmic reticulum

responsible for storing calcium ions that regulate muscle contraction/relaxation; found in muscle cells

smooth muscle cell contraction

involves interaction of thin and thick filaments; cell twists like a corkscrew as it contracts

multi unit smooth muscle cells

innervated in motor units like skeletal muscle, but each cell may by connected to more than one motor neuron

locations of multi unit smooth muscle cells

iris of eye, portions of male reproductive system, walls of large arteries, arrector pili muscles of skin

visceral smooth muscle cells

lack any direct connection w/ motor neuron; arranged in sheets

visceral smooth muscle cells are connected

by gap junctions and dense bodies; cells contract in a wave as a single unit

visceral smooth muscle cells stimulation

can be neural, hormonal, or chemical; some rhythmically stimulated by pacesetter cells

visceral smooth muscle cells location

digestive tract walls, gallbladder, urinary bladder, and other internal organs

function characteristics of smooth muscle

plasticity and smooth muscle tone

plasticity of smooth muscle

ability to function over a wide range of lengths due to scattered arrangement of filaments; important for digestive tract to change shape

smooth muscle tone

normal background activity and tension due to various stimulations

peristalsis

wave of muscle contraction

bolus

term for food enters the digestive contract; moist compact mass of material

process of peristalsis

bolus is propelled along tract by contractions of muscularis externa

circular muscles

contract behind bolus during peristalsis

longitudinal muscles

contract ahead of bolus during peristalsis

segmentation

cycles of contraction; churn and fragment bolus while mixing w/ intestinal secretions

segmentation occurs

in most areas of small intestine and some portions of large intestine

segmentation pattern

no set pattern of contractions, no particular direction of movement

digestive regulation mechanisms

local factors, neural control mechanisms, and hormonal control mechanisms

1: local factors

primary stimulus for digestive activities

examples of local factors

change in pH, distortion of digestive tract wall, presence of chemicals

2: neural control mechanisms

short reflexes and long reflexes

short reflexes (myenteric reflexes)

triggered by chemoreceptors or stretch receptors in digestive tract walls; controlling neurons located in myenteric plexus

long reflexes

higher level of control involving interneurons and motor neurons of cns; generally control large scale peristalsis; may involve parasympathetic motor fibers that synapse in myenteric plexus

large scale peristalsis

moving material from one region of tract to another

hormonal control mechanisms

involve 18 hormones that affect digestive system

hormones

peptides produced by enteroendocrine cells

enteroendocrine cells

endocrine cells in epithelium of digestive tract

congenital megacolon

characterized by absences or marked reduction in number of ganglion cells in myenteric plexus of rectum

congenital megacolon results in

paralysis of smooth muscle (absence of peristaltic movement)

congenital megacolon symptoms

abnormal dilation, hypertrophy of colon, chronic constipation, bloating, abdominal pain