chapter 10

primary tissue consists of

skeletal muscle

cardiac muscle

smooth muscle

skeletal muscles

move the body by pulling on bones

cardiac and smooth muscles

control movements inside the body

common properties include

excitability, contractility, extensibility, elasticity

skeletal muscles contain

skeletal muscle tissue, connective tissues, blood vessels, nerves

skeletal muscle fiber

a skeletal muscle cell

muscle fasicile

a bundle of muscle fibers

skeletal muscle formed

by several muscle fascicles

skeletal muscle layers of connective tissue

endomysium, perimysium, epimysium

endomysium

surrounds individueal muscle cells (muscle fibers)

endomysium contains

capillary networks, myosatellite cells (stem cells) that repair damage, nerve fibers

perimysium

surrounds muscle fiber bundles (fasicles)

perimysium contains

collagen fibers, elastic fibers, blood vessels, nerves

epimysium

layer of collagen fibers that surrounds the muscle, connected to deep fascia, seperates muscle from surrounding tissues

collagen fibers of epimysium, perimysium, and endomysium come together

at ends of muscle to form tendon (bundle) or aponeurosis (sheet)

extensive vascular networks that

deliver oxygen and nutrients

remove metabolic wastes

skeletal muscle fibers

enormous compared to other cells; contain hundreds of nuclei (multinucleate); develop by fusion of embryonic cells (myoblasts); also known as striated muscle cells due to striations

sarcolemma

plasma membrane of a muscle fiber; surrounds the sarcoplasm (cytoplasm of a muscle fiber); sudden change in membrane potential initiates a contraction

transverse tubules (T tubules)

tubes that extend from surface of muscle fiber deep into sarcoplasm; transmit action potentials from sarcolemma into cell interior

sarcoplasmic reticulum (SR)

tubular network surrounding each myofibril; similar to smooth endoplasmic reticulum

sarcoplasmic reticulum forms

chambers (terminal cisternae) that attach to T tubules

triad

two terminal cisternae plus a T tubule forms a triad

ions

are actively transported from cytosol into terminal cisternae by calcium pump

SR releases

calcium ions into sarcomeres to begin muscle contraction

myofibrils

lengthwise subdivisions within a muscle fiber; responsible for muscle contraction; made of bundles of protein filaments (myofilaments)

two types of myofilaments

thin and thick filaments

sarcomeres

-Smallest functional units of a muscle fiber

-Interactions between filaments produce contraction

-Arrangement of filaments accounts for striated pattern of myofibrils

dark bands

A bands

light bands

I bands

the A band

M band, H band, and zone of overlap

M line

in center of A band

H band

on either side of M line; has thick filaments but no thin filaments

zone of overlap

dark region; where thick and thin filaments overlap

I band

contains thin but no thick filaments

Z lines

bisect I band and mark boundaries between adjacent sarcomeres

Titin

Elastic protein

Extends from tips of thick filaments to the Z line

Keeps filaments in proper alignment

Aids in restoring resting sarcomere length

thin filament

composed primarily of actin, F-actin, nebulin, tropomyosin, and troponin proteins

filamentous actin (F-actin)

-Twisted strand composed of two rows of globular G-actin molecules

-Active sites on G-actin bind to myosin

nebulin

Holds F-actin strands together

tropomyosin

-covers active sites on G-actin

-prevents actin-myosin interaction

troponin

globular protein; binds G-actin and Ca2+

thick filaments

composed primarily of myosin; contains about 300 myosin molecules

myosin molecule consists of

tail and head

tail

binds to other myosin molecules

head

made of two globular protein subunits; projects toward nearest thin filament

initiating contraction

Ca2+ binds to receptor on troponin molecule

Troponin-tropomyosin complex changes

Exposes active site of F-actin

myosin actin

interact with actin filaments forming cross-bridges; pivot producing motion

during a contraction

H bands and I bands narrow

during contraction 2

zones of overlap widen

during contraction3

Z lines move closer together

during contraction4

width of A band remains constant

exciatable membranes

found in skeletal muscle fibers and neurons

depolarization and repolarization

events produce action potentials (electrical impulses)

skeletal muscle fibers 1

contract due to stimulation by motor neruons

process of contraction

neural stimulation of sarcolemma; causes excitation- contraction coupling

muscle fiber contraction

interaction of thick and thin filaments

neuromuscular junction (NMJ)

synapse between a neuron and a skeletal muscle fiber; axon terminal of the motor neuron releases a neurotransmitter into the synaptic cleft

neurotransmitter

acetylcholine (ACh)

ACh

binds to and opens a chemically gated Na+ channel on the muscle fiber

Na+ enters cell

depolarizes motor en plate and action potential is generated

action potential travels

down T tubules to triads

Ca2+

released from terminal cisternae of SR; binds to troponin and changes its shape

generation of muscle tension

muscle cells contract they produce tension (pull); produce movement tension must overcome the load (resistance)

entire muscle shortens at the same rate

all sarcomeres contract together; speed shortening depends on cycling rate

duration of a contraction

duration of neural stimulus at NMJ; presence of free calcium ion in cytosol; availability of ATP

During relaxation

neural stimulus is terminated; no more action potentials and calcium channels close

as Ca2+ is pumped back into SR

Ca2+ concentration in cytosol falls

rigor mortis

fixed muscular contraction after death; ATP runs out and ion pumps cease to function; calcium ions build up in cytosol

amount of tension produced by a muscle fiber depends on

number of power stroke performed

tension produced by a muscle fiber

relates to the length of the sarcomeres

amount of tension produced depends on

number of power strokes performed by cross-bridges in each of the myofibrils

maximum tension

produced when the maximum number of cross-bridges is formed

frequency of stiumulation

single neural stimulation produces a single contraction (twitch)

myogram

a graph showing tension development in muscle fibers

muscle twitches vary in duration depending on

muscle type, its location, internal and external environmental conditions

latent period

action potential moves across sacolemma; SR releases Ca2+

contraction phase

Calcium ions bind to troponin and cross-bridges form

Tension builds to a peak

relaxation phase

Ca2+ levels in cytosol fall; cross bridges detach and tension decreases

treppe

-A stair-step increase in tension

-Caused by repeated stimulations immediately after relaxation phase

•Stimulus frequency

wave stimulation

increasing tension due to summation of twitches; caused by repeated stiumulations before the end of relaxation phase

tetanus

max tension

incomplete tetanus

muscle produces near-maximum tension; caused by rapid cycles of contraction and relaxation

complete tetanus

Higher stimulation frequency eliminates relaxation phase

Muscle is in continuous contraction

All potential cross-bridges form

tension production depends

internal tension and external tension

internal tension

produced by muscle fibers

external tension

exerted by muscle fibers on elastic extracellular fibers

motor unit

a motor neuron and all of the muscle fibers it controls

small motor units

contains few muscle fibers; fine or precise movements

large motor units

contains many muscle fibers; weight-bearing muscles less precise

fasciculation

involuntary “muscle twitch”

unlike a true twitch it involves more than one muscle fiber

synchronous contraction of a motor unit

maximum tension

achieved when all motor units reach complete tetanus

sustained contractions

motor units are activated on a rotating basis

muscle tone

normal tension and firmness of a muscle at rest

types of muscle contractions

isotonic and isometric contraction

isotonic contraction

skeletal muscle changes length (motion)

isotonic concentric contraction

muscle tension >load; muscle shortens (lifting groceries)

isotonic eccentric contraction

muscle tension< load; muscle elongates (putting groceries down)

isometric contractions

skeletal muscle develops tension that never exceeds the load; muscle does not change length (holding groceries in position)

elastic forces

tendons recoil after a contraction; helps return muscle fibers to resting length

opposing (antagonist) muscle contractions

opposing muscle to resting length quickly