Lesson 23 - 24 Flashcards
three properties of smooth and cardiac muscle
- their cells are myocytes
- they are involuntary
- they receive innervation from the autonomic nervous system
myocytes
smooth muscle cells, not as long and fibrous as skeletal muscles
properties of cardiac muscle (5)
- contracts with regular rhythm
- muscle cells of a given chamber contract in unison
- contractions must last long enough to expel blood
- highly resistant to fatigue
- works in sleep or wakefulness, without fail, and without conscious attention
structure of cardiac muscle cells (3)
- shorter and thicker than skeletal muscle cells
- surrounded by endomysium
- no peri or epimysium
cardiomyocytes branch and join to other cells at connections called _____ _____
intercalated discs
how do intercalated discs appear?
thick, dak lines in stained tissue sections
intercalated discs
mechanical junctions that keep cardiomyocytes from pulling apart
_____ _____ in the intercalated discs allow each cardiomyocyte to directly stimulate its neighbors
gap junctions
cardiac muscle doesn’t… (2)
- have satellite cells
- very limited mitosis
muscles can easily be replaced by scar tissue
pacemaker
rhythmically sets off a wave of electrical excitation
the heart is autorhythmic, meaning…
it is able to contract rhythmically and independently
places smooth muscle provide fine control (2)
- smooth muscles of the iris controls pupil size
- piloerector muscles raise hairs on skin
smooth muscle myocyte structure (5)
- fusiform shape
- single nucleus near middle of cell
- thick and thin filaments present, but not aligned
- minimal SR and no T tubules
- Z discs replaced by dense bodies
dense bodies
protein masses that form plaques on the inner face of the plasma membrane and scattered throughout sarcoplasm
- dense bodies of neighboring cells are linked together
types of smooth muscle: unitary (single unit) (3)
myocytes coupled by gap junctions, so it contracts as a single unit
- most common
- aka visceral muscle
types of smooth muscle: multiunit
individual myocytes respond independently of all other
smooth muscle can contract or relax in response to a variety of stimuli: (5)
- autonomic nerve fibers and neurotransmitters
- chemicals
- temperature
- stretch
- autorhythmicity
contraction is always triggered by _____, energized by ___, and achieved by the sliding of myosin and actin
Ca2+, ATP
where does smooth muscle get most of its ATP
the extracellular fluid through gates Ca2+ channels
- other Ca2+ comes from the limited SR
caveolae
pockets on the sarcolemma
where are calcium gated channels concentrated?
caveolae
in smooth muscle, what does calcium bind to?
calmodulin
what does calmodulin activate?
myosin light-chain kinase
what does myosin light-chain kinase do?
adds a phosphate group to a regulatory protein on myosin head which then activates ATPase
why do smooth muscles contract so much slower? (2)
- slow myosin ATPase, slow calcium pumps, and the presence of Ca2+ in the cytosol
- latent period in smooth muscle is 50-100 ms (2ms in skeletal)
latch-bridge mechanism
myosin doesn’t immediately detach from act when Ca2+ levels fall
- resistant to fatigue, doesn’t consume any more ATP
what does the latch-bridge mechanism maintain? (2)
- smooth muscle tone
- vasomotor tone in blood vessels
peristalsis
waves of contraction brought about by food distending the esophagus or feces distending the colon; propels contents along the organ
what can stretch do to smooth muscles?
can open mechanically gated calcium channels in the sarcolemma causing contraction
stress-relaxation response
helps hollow organs gradually fill
- when stretched, tissue briefly contracts then relaxes, helping prevent emptying while something is filling
unlike skeletal muscle, smooth muscle contracts _______ when greatly stretched
forcefully
plasticity
the ability to adjust its tension to the degree of stretch
why are smooth muscles able to contract forcefully even when stretched? (3)
- no Z discs, thick filaments cannot butt against them
- bc not arranged in orderly sarcomeres, there is no situation of too little overlap
- thick filaments have myosin heads along their entire length so cross bridge can form anywher
muscular dystrophy (MD)
a group of hereditary disease in which skeletal muscles degenerate and weaken and are replaced with fat and fibrous scar tissue
Duchenne muscular dystrophy (DMD)
caused by a sex-linked recessive trait on the X chromosome
- most common form
when is DMD diagnosed
around the age of 5
what causes DMD?
a mutation in the gene for muscular protein dystrophin, needed to link thin filaments in the sarcomere
- w/o actin isn’t linked to the sarcolemma and cell membranes are damaged during contraction
myasthenia gravis
autoimmune disease in which antibodies attack neuromuscular junctions and bind ACh receptors together in clusters, then muscle fibers removed the clusters via endocytosis causing muscles to be less and less sensitive
