unit 3.1, 3.2, 3.5 muscles intro and specifics on skeletal + smooth

what is a muscle?

a tissue specialized to convert biochem reactions into mech work

two main functions of a muscle

to generate motion and force
- can also generate heat and contribute to body temp homeostasis

restrictions on movement of muscle

- can only contract and relax
- cannot expand unless when externally pulled by another muscle group (stretching for example)

3 types of muscles (intro to each)

skeletal
- attach to bones of the skeleton
- control body movement
- can only contract when there is a somatic motor neuron signal (meaning it CANNOT be influenced by hormones or contract on its own)
- very striated and organized
cardiac
- found only in the heart
- controls blood movement
- striated, but not as organized as skeletal
smooth
- primary muscle of internal organs and tubes (stomach, vessels, bladder, etc)
- controls material movement throughout body
- no striations or order

skeletal muscle characteristics

- responsible for positioning and movement of the skeleton
- attached to bones via TENDONS
- makes up ~half of body weight

tendons

- connects muscle to bone
- composed of dense regular connective tissue called collagen (a protein arranged in parallel alignment)

5 components of a skeletal muscle

epimysium, perimysium, fascicle, endomysium, myofibrils

epimysium

the outermost connective tissue layer of a muscle
- contains bundles of muscle tissue (so outer layer of groups of groups)
- contains nerves and blood vessels within
- epi = above, myo = muscle

perimysium

connective tissue that covers a single bundle of muscle fibres
- peri = around, myo = muscle
- specifically covers fascicles

fascicle

a bundle of muscle cells/fibres
- covered by perimysium

functional unit of skeletal muscle

myofibril, which is in every muscle fibre
- they are contractile and elastic protein bundles

endomysium

innermost connective tissue layer
- sheath around exactly one muscle fibre
- endo = inner, myo = muscle

why do muscle cells not have many organelles?

there are so many myofibrils that there's very little room for organelles

what's in the cytosol of a muscle cell?

- glycogen granules (energy storage)
- mitochondria (ATP synthesis)

general structure of muscle fibre

long and cylindrical cell with hundreds of nuclei on the surface

what is the cell membrane of a muscle cell called?

sarcolemma
- sarco = flesh, lemma = shell

what is the endoplasmic reticulum of a muscle cell called?

sarcoplasmic reticulum
- associated with this SR are a series of branching tubes: t-tubules
- stores a lot of Ca ions

t-tubules

aka transverse tubules
- indentations of membrane with lumen (aka space within a tube) that is full of ECF (extracellular fluid)
- closely associated with terminal cisternae (aka "large vessel"), which are the ends of an SR
- allow for rapid AP diffusion into the muscle fibre (necessary for proper movement)

what is the cytoplasm of a muscle cell called?

sarcoplasm

triad in muscle cell

one t-tubule with one terminal cisternae on each side

4 general terms that have different names in muscle context

muscle cell = muscle fibre
cell membrane = sarcolemma
cytoplasm = sarcoplasm
endoplasmic reticulum = sarcoplasmic reticulum

3 types of proteins in a myofibril

contractile, regulatory, accessory

contractile proteins in myofibrils

CAM
- specifically myosin and actin
- "can the cell contract?": responsible for the physical component of a contraction

regulatory proteins in myofibrils

RTT
- specifically troponin and tropomyosin
- "will the cell contract?": determines whether a muscle will actually contract or not

accessory proteins

ATN
- specifically titin and nebulin

sarcomere

one repeated pattern of myofibrils

sarcomere structure (has 5 parts)

ZIAHM:
z line
i band
a band
h zone
m line

what causes striations in muscle cells?

the organization of protein components (thick and thin filaments; myosin and actin respectively) of the myofibrils

myosin

- motor protein that consists of two coiled protein molecules/chains, each consisting of two parts: a head and tail
- head and tail are stiff and connected by a flexible hinge, which allows heads to pivot (this is important for generating contractions)
- are thick filaments due to how many molecules coil together
- heads are always toward the ends of the SR/toward Z

actin

- subunits of G-actin (G for globular)
- polymerize to form a chain, called F-actin (F for filamentous)
- two F-actins coil to form the basis of thin filament: associates with regulatory proteins (RTT) and cause contractions when interacting
- completed thin filament is F-actins + troponin + tropomyosin
- myosin heads directly interact with F-actin (interactions are called CROSSBRIDGES)

z-line

- think z for zigzag
- discs that act as site of attachment for thin filaments
- signify the ends of a sarcomere (one full sarcomere is two z-discs and the filaments between them)

i band

- contains only thin filaments
- one z disc will run through the middle of an i band so that an i band is split in half: one half to one sarcomere, the other to a separate sarcomere

a band

- contains thick and thin filaments: very dense (hence the darker color of it)!
- thick and thin overlap at outer edges of a band
- center consists of thick ONLY...h band

h band

- center of a band, has only thick filaments
- lighter than outer edges of a band

m line

- m line = midline; the center of the sarcomere
- site of attachment for thick filaments

cross-sections of sarcomeres and the thin/thick ratio

- every thin filament is surrounded by 3 thick filaments
- every thick filament is surrounded by 6 thin filaments

titin

- largest known protein
- elastic protein that stretches from one z disc to an m line in the next sarcomere
- stabilizes myosin, which are contractile filaments
- returns stretched muscles to their resting length (think titin tightens back)

nebulin

- non-elastic and attaches to z disc
- guides actin filaments in the sarcomere to properly align them

if you took a cross-section of a sarcomere through the outer edge of the a band, what would you see?

both myosin thick filaments and actin thin filaments

smooth muscle arrangement

has two distinct arrangements
1) single unit
- muscle cells are coupled by gap junctions
- don't need electrical stimulation for each individual cell
- think lining of organs: network of cells
2) multi unit
- no gap junctions
- every cell works on its own, innervated separately
- think of iris of eyes

general differences between smooth and skeletal

- contraction of smooth changes shape and length, skeletal only changes length
- smooth develops tension/force slowly
- smooth muscle doesn't fatigue as quickly due to contraction (think bladder: it's constantly adjusting, as that's its function)

cellular differences between smooth and skeletal

- smooth muscle fibres are much smaller
- no sarcomere arrangement in smooth, so no striations
- actin is anchored at dense bodies of cell membrane, unlike skeletal which is attached to z lines
- no t-tubules for smooth and not much SR (instead, smooth has caveolae vesicles within the sarcolemma to store calcium)
- force of contraction in smooth is related to amount of calcium released

what is the effect of not having t-tubules for smooth muscles?

no direct coupling of AP to release calcium from SR through dhp-ryr receptor coupling (skeletal). instead, Ca entering sarcolemma results in Ca release from SR.