11.7 - Cardiac & Smooth Muscle

myocytes

muscle cells

cardiomyocytes

cardiac muscle cells

cardiac muscle (properties)

contracts with a regular rhythm, functions unconsciously in sleep and wakefulness, is highly resistant to fatigue, cardiomyocytes of a given heart chamber contract in unison so the chamber can effectively expel blood, and each contraction lasts long enough to expel blood

intercalated discs

end-to-end intercellular connections between cardiomyocytes that appear as thick dark lines in stained tissue sections

gap junctions

electrical junctions of intercalated discs that allow each cardiomyocytes to directly stimulate its neighbors

mechanical junctions

junctions of intercalated discs that keep the cardiomyocytes from pulling apart when the heart contracts

pacemaker

built-in component of the heart that rhythmically sets off a wave of electrical excitation, which travels through the muscle and triggers the contraction of the heart chambers

autorhythmic

possessing the ability to contract rhythmically and independently

varicosities

swellings along the length of nerve fibers that contain synaptic vesicles from which it releases neurotransmitters

smooth muscle (location)

forms layers in the walls of larger organs like the stomach, intestines, uterus, and urinary bladder

smooth muscle myocytes

fusiform myocytes enclosed in endomysium only, with only thick and thin filaments, one nucleus, and scant sarcoplasmic reticulum

dense bodies

protein plaques that replace Z discs in smooth muscle

two types of smooth muscle

multiunit and unitary smooth muscle

multiunit smooth muscle (location)

occurs in some of the largest arteries and pulmonary air passages, arrector muscles of the hair follicles, and eye muscles that control the iris and lens

multiunit smooth muscle (innervation)

the terminal branches of a nerve fiber synapse with individual myocytes and form a motor unit, which contracts in response to variable electrical changes in the sarcolemma or even in the absence of electrical excitation

unitary smooth muscle (location)

occurs in most blood vessels, as well as in the digestive, respiratory, and reproductive tracts

unitary smooth muscle (innervation)

no nerve varicosity is associated with a specific myocyte; instead, the varicosities simply release a flood of neurotransmitter into the tissue

five modes of smooth muscle stimulation

autonomic nerve fibers and neurotransmitters, chemicals, temperature, stretch, and autorhythmicity

autonomic nerve fibers and neurotransmitters

parasympathetic nerves secrete acetylcholine and sympathetic nerves secrete norepinephrine

chemicals

smooth muscle responds to hormones, carbon dioxide, oxygen, nitric oxide, low pH, and other chemical stimuli

temperature

cold induces contraction of smooth muscle, while warmth relaxes smooth muscle

stretch

smooth muscle contracts when stretched

autorhythmicity

some myocytes spontaneously depolarize at regular time intervals and set off waves of contraction throughout entire layers of muscle

immediate trigger for muscle contraction

calcium ions

caveolae

pockets in the sarcolemma where its calcium chanells are concentrated

calmodulin

protein associated with myosin that is similar to troponin and activates myosin light-chain kinase

myosin light-chain kinase

enzyme that adds a phosphate group to a small regulatory protein on the myosin head

latch-bridge mechanism

mechanism that enables myosin to remain attached to actin for a prolonged time without consuming more ATP

smooth muscle tone

state of continual tonic contraction assisted by smooth muscle’s fatigue resistance and latch-bridge mechanism

vasomotor tone

state of partial constriction maintained in arteries

peristalsis

wave of contraction that propels the contents along the organ, caused by distention

stress-relaxation response (receptive-relaxation response)

when stretched, smooth muscle briefly contracts and resists, but then relaxes

plasticity

ability to adjust its tension to the degree of stretch