Skeletal muscle functions and structure

Sliding filament theory

(when actin filaments slide towards the M line=when muscle contracts): 1. the I band gets smaller. Also H band gets smaller (narrower) 2. overlapping zone gets bigger 3. the A band stays the same **contraction weakens if I bands disappear (too much overlapping)

blood vessels in muscles

--extensive network needed to supply nutrients and remove waste -enter (w nerves) through the perimysium -small arterioles enter the endomysium to supply indivifual muscle cells

F-actin strand

-F for filamentous -strand composed of two rows of G-actin molecules (300-400 per row) -long strand of nebulin holds both rows together

Thick filaments

-about 300 myosin molecules -10-12 nm diameter, 1.6 microm length -Parts: 1. myosin molecules (each with a pair subunits, i.e. 2 heads) 2. titin

Z line

-also called Z disc bc in sectional view looks like a disk with a meshwork -mark boundary between sarcomeres -consist of proteins ACTININS which interconnect actin

H band

-are of only myosin (thick filaments) -in the middle of a\n A band, around M line

striations

-are seen in the microscope slides of skeletal muscle -they are formed by the alternations of A (dark) and I bands

nerves in muscles

-axon penetrate the epimysium, branch through the perimysium, and enter endomysium to innervate individual muscle fibers -skeletal msucles are voluntary muscle bc we have voluntary control over them (even at subconscious level like the diaphragm)

Muscle contraction

-begins when stored Ca++ is released into the cytosol and then it diffuses into the individual sarcomeres -done by interactions of thin and thick filaments

aponeurosis

-broad sheet made of collagen fibers from epimysium, perimysium, and endomysium that fuse together at the end of each muscle -attach skel muscle to bone (Sharpey's fibers)

tendon

-bundle made of collagen fibers from epimysium, perimysium, and endomysium that fuse together at the end of each muscle -attach skel muscle to bone (Sharpey's fibers)

Myofibrils

-cylindrical structures inside the muscle fiber/cell -1-2 micrometers diameter. very long -responsible for skeletl muscle contraction -tbey are bundles of: 1. actin myofilaments (thin) 2. myosin myofilaments (thick)

sarcoplasm

-cytoplasm of a muscle cell -more negative potential

A bands

-dark bands (anisotropic) -regions of overlapping myosin and actic

titin

-elastic protein -attaches myosin to the Z line; which ensures cross bridge formation by ensuring the presence of an overlapping zone. -goes from Z line to M line: extends through the whole length of the sarcomere. One portion is inside the myosin filament on the A band; the other one (the elastic portion) extends from the end of myosin (overlapping zone) until the Z line -helps muscle resist extreme stretching without disrupting the contraction -aids in restoring resting sarcomere length after contraction

Myoblasts

-embryonic cells that differentiate into nuclei in muscle cell (if they fuse) or into myosatellite cells (involved in regeneration) -several fused myoblasts form a muscle fiber/cell (multinucleated; one myoblast per nuclei)

G-actin molecule

-globular molecules that conform each F-actin strand on an actin myofibril or thin filament -they contain an active site where myosin (thick fil) can bind (*in resting state, troponin-tropomyosin complex prevents myosin from binding)

zone of overlap

-has both actin and myosin molecules -is where the triad attaches

Sarcolema

-is the plasma membrane of the muscle cell -it encloses the cytoplasm (sarcoplasm) -it is deeper to the endomysium -has a characteristic membrane potential (>negative inside) -is able to conduct electrical impulse

sarcomere

-is the smaller functional unit of a muscle cell/fiber -about 10.000 per myofibril -resting length about 2micrometers

tropomyosin

-it extends along the F-actin strand covering the active sites of actin molecules -when it is lifted by troponin, it exposes the binding site of G-actin molecules, so contraction can happen

I bands

-light bands (isotropic) -have no thick filaments (myosin)

M line

-middle line of a sarcomere -is in the middle of the A band (and h band) -holds myosin filaments together

troponin

-molecule consisting of three globular subunits: 1. one subunit bind to tropomyosin and holds both together 2. another binds to a G-actin molecule, attaching the troponin-tropomyosin complex to the F-actin strand 3. the last binding site is empty during resting state; it binds to Ca++ to initiate muscle contraction. When Ca++ binds, troponin lifts the tropomyosin molecule exposing the binding site of G-actin, so myosin heads can bind

Myosin

-molecules arranged in spiral that conform the thick filaments -each has a pair subunits, i.e. 2 heads -heads project towards the thin filaments and they can pivot towards/away M-line (=cause of muscle contraction) -no myosin heads on the H-band?? -make cross-bridges

types of muscle tissue

-skeletal muscle tissue (pulls bones) -cardiac muscle tissue (pumps blood) -smooth muscle tissue (pushes fluids and solids along digestive tract

Muscle contaction

-sliding of I band towards the M line in every sarcomere. Result: myofibril shortens, thus entire muscle fiber shortens generating a pull (TENSION) on the tendon

Epimyseum

-surrounds entire muscle -dense layer -connected to the deep fascia

Perimyseum

-surrounds fascicles -collagen and elastic fibers -has blood vessels and nerves that supply each fascicle

Endomyseum

-surrounds individual skel muscle fibers/cells and interconnects adjacent fibers -delicate and flexible connective tissue -contains capillaries, myosatellite cells (stem), and nerves that are all in direct contact with each muscle fiber -

terminal cisternae

-t tubules -they wrap around myofibrils -muscle remove Ca++ ions from the cytosol and actively transport them to the terminal cisternae -has protein calsequestrin which binds to Ca++ -in cisternae, very high [Ca++] -located in zones of overlap

Transverse tubules

-they conduct the action potential throughout interior of cell -their surface is continuos w the sarcolemma (see "pores" on sarcolemma) nd they extend deep into sarcoplasm -able to conduct electrical impulse -filled w extracellular fluid

Layers of connective tissue in skeletal muscle

1. Epimyseum: surrounds entire muscle 2. Perimyseum: surrounds fascicles 3. Endomyseum: surrounds muscle fibers

Functions of skeletal muscle

1. Produce movement: pull on tendons to move bones 2. Maintain posture and body position: tension maintain posture, constant muscular activity 3. Support soft tissues: like the abdominal wall and floor of pelvic cavity 4. Guard body entrances and exits: like the openings of the digestive, urinary tract; these muscles give us voluntary control 5. Maintain body temperature: whenever energy is used in body, heat is released 6. Store nutrients: contractile proteins in skeletal muscles can be broken down into AA when our diet is not sufficient

thin filaments

actin -5-6 nm diameter and 1 microm in length -Parts: 1. F-actin (F=filamentous) strand with G-actin molecules 2. Nebulin 3. Troponin 4. Tropomyosin

cross-bridge

is the interaction between a myosin head and a thin filament that results in muscle contraction

Nebulin

long strand that holds the two rows of G-actin molecules together in an F-actin strand (thin filament)

actinins

proteins that interconnect actin filaments and conform the Z line

sarcoplasmic reticulum

similar to the smooth endoplasmic reticulum -tubular network around the myofibrils -firs a triad with the t tubule

triad

t tubule+2 terminal cisternae -membranes are tied togerther but fluid contained is different -triads are located in zones of overlap (myosin + actin) for better interaction

elastic myofilaments

titin