Muscle Types Flashcards
describe skeletal muscle
large multinucleate cells that appear striped or striated under the microscope
Skeletal muscle is attached to bone (mostly), allowing the body to move
- Cells are large, elongated, cylindricalshape – also called fibres
- Fibres extend full length of muscle
- Many muscle fibres lie parallel to one another, bundled together by connective tissue (epimysium)
- Highly organised internal structure -striated or striped appearance
- Cells are multinucleated (fusion of myoblasts)
- Contain many mitochondria (high energy demand) = supply this high energy process
describe smooth muscle
small and lack sarcomeres NOT striated
contracts differently
Smooth muscle is found throughout the body…
eg: - vascular walls e.g. gastrointestinal tract/respiratory walls
- Much more actin than in skeletal or cardiac muscle – each myosin molecule is surrounded by 12-15 actin molecules
- Myosin chains(filaments) are also longer than in skeletal muscle
- Contractile units run more-or-less parallel to the long axis of the cells
in skeletal/cardiac : myosin heads point in the same direction on both sides of myosin filament
in smooth : myosin heads point in opposite directions on the two sides
describe cardiac muscle
appear striped or striated under the microscope
smaller, branched and uninucleate
cells joined in series by junctions called intercalated discs (specialised cells junctions that link the cardiac muscle)
=> Allow firing together and transmit electrical potentials from one fibre to the adjacent fibres
* Intercalated discs are made up of desmosomes and gap junctions – join the cells and propagate the action potentials and contractions
=> by allowing the flow of ions for the passing on of AP within cells to contract silmutaneously
the spiral arrangement of ventricular muscle in the heart (like wringing a wet towel out) => more effieically pumps blood up to the apex of heart to the rest of the body
- 99% of cardiac muscle cells is contractile (contains sarcomeres)
- 1% are autorhythmic (pacemaker cells) – smaller than contractile, less fibres and sarcomeres
=> initiate electrical singal within the heart spontaneously for heart contraction
aren’t innervated by nerves
==> have pacemaker cells within cells to initiate AP
–> frequency of pacemaker contractions can be controlled by para and sympathetic NS
tendons
attaches bone to muscle vice versa
comprised of:
- type I and III collagen
- Elastin
- Proteoglycans and
- tendon fibroblasts
muscle structure
muscle > fascicles > muscle fibres > myofibrils > protein filaments
within muscle, many fascicles are separated by the perimysium
within individual fascicles, muscle fibres are separated by the endomysium
muscle fibres are single cells made from myoblasts fused together and made of many myofibrils
myofibrils contain protein filmanets
protein filmanets required for muscle contraction
intrafusal fibres have no myofibrils, hence no contraction
striated muscle
Striation : alternating dark and light bands are created by protein structures in the muscle.
- Myosin (thick filaments = dark bands)
- Actin (thin filaments = light band)
The protein ‘myosin’ creates the thick, dark bands and chains of ‘actin’ protein create the light band.
–> overlap of these bands acheive muscle contraction
repeat unit of myofibrils = sarcomere
AP at msucle
long nerve axons attach to the muscle fibre
at NMJ, axon terminal, synapse with the motor end plate
–> folding of membrane on motor end plate and it contains high conc of nACh receptors
T-tubules allow ECF to enter into the muscle
- influx of Na+/Ca2+ = depolarisation along length of T-tubule
- VGCC (voltge gated Ca channels) down T-tubule
- T-tubule close to SR (abundant in Ca2+) == AP activates release of Ca2+ out of SR
sarcomere structures
- Sarcomere is the basic contractile unit of the muscle fiber
- Contains two main protein filaments\
Has distinct regions/bands/zones:
- One sarcomere is the region between two Z-lines
- I-band contains only thin filaments (actin) that do not overlap with thick filaments (myosin)
- A-band contains entire length of a single thick filament (can include thin filament also)
- H-zone contains thick filaments with no actin
- M-line is the middle of the sarcomere and contains cross-connecting elements of the cytoskeleton
strucutre of the thin and thick filaments
thin filaments :
nebulin runs along in the middle of the filament : anchors G-acting molecules
tropomysoin hides binding sites for myosin heads
G-acting molecules are what the myosin heads bind to
thick filaments :
M -line, myosin heads, and hinge region attaches heads to tails
excitation-contraction coupling
excite muscle with neurotransmitter for muscle contraction
- Skeletal muscle is stimulated to contract by release of acetylcholine (ACh) at the neuromuscular junction
- ACh binds to receptor-channels on the motor end plates of the muscle and sets off an action potential along the surface of the muscle fibre and transverse tubules (ttubules).
–> influx of Ca2+ activates Ca2+ dependent channels (DHP and also RyR) to cause the release of Ca2+ from SR - The t-tubules dip into the muscle fibres and activate the release of Ca2+ from intracellular stores inside the sarcoplasmic reticulum into the sarcomeres = muscle contraction.
structures
* t-tubules
allows ions e.g. Na+/Ca2+ from ECF into muscle cells to initiate AP along T-tubule
–> interact with Ca2+ dependent channels on SR to induce release of Ca2+
ryanodine = Ca2+ dependent channels ; also known as RyR
- Sarcoplasmic reticulum
intracellular store of Ca2+ - ATP pumps Na+ and K+ out and in to maintain membrane potentail and electrical graidents for your ions
- sodium calcium exchange to pump out 1 Ca2+ molecules = 3 Na+ molecule
- high ATP required to mantain the high [Ca2+] in SR
–> drive Ca2+ back into SR
pacemaker cells
- 99% of cardiac muscle cells is contractile (contains sarcomeres)
- 1% are autorhythmic (pacemaker cells) – smaller than contractile, less fibres and sarcomeres
=> initiate electrical singal within the heart spontaneously for heart contraction
aren’t innervated by nerves
==> have pacemaker cells within cells to initiate AP
Frequency of pacemaker contractions can be controlled by para and sympathetic NS
–> ligands produced by para/sympathetic NS can bind to pacemaker cells to increase/decrease muscle contraction
= regulate heart rate
Cardiac muscle - contraction
- Gap junctions in the intercalated discs allow the electrical waves of depolarization to travel from cell to cell – leading to almost simultaneous contraction of entire myocardium
ryanodine receptors aren’t directly coupled
=> directly activated by binding of Ca2+
Smooth muscle
contraction initiation
Note: there are no t-tubules
Triggered by:
* Signal ligand (e.g. adrenaline, NO or histamine)
* Autorhythmic (i.e. from myogenic mechanisms)
* Stretch
instead of Ca2+ directly binding to myosin filaments => Ca2+ binds to CaM to activate MLCK to create this movement
These will all increase Ca2+ into the cytosol and trigger cascade for initiation of contraction
multi / single- unitsmooth muscle
Some smooth muscle has cells connected by electrical transmitting gap junctions (somewhat like myocardial cells) – these cells therefore contract together as a unit
= single- unitsmooth muscle (eg small intestine)
all connected by gap junctions : contract as a single unit
- Some smooth muscle is comprised of cells that function independently of each other
= multi-unitsmooth muscle (eg iris and ciliary eye muscle)
no gap junctions ==> no electrically linked and each cell must be stimulated independently ==> allow finer control
Smooth muscle – types of contractions
- phasic : usually relaxed (esophagus) => relaxed at rest
- tonic : usually contracted (sphinctor) => force at rest => only relaxes when material needs to pass
