skeletal muscle, smooth muscle, cardiac muscle

skeletal muscle structure

• striated

• multinucleated (many nuclei per cell)

• long, cylindrical fibers

• under voluntary control

skeletal muscle function

• produces body movements

• maintains posture

• generates head

skeletal muscle location

attached to bones by tendons

skeletal muscle special properties

• fast contraction

• fatigues relatively quickly

• required neural stimulation to contract

Skeletal muscle anatomy

CT sheaths

Epimysium

Perimysium

Endomysium

CT sheaths - Skeletal Muscle

support cells and reinforce whole muscle

epimysium

dense irregular CT surrounding entire muscle; may blend with fascia

perimysium

dense irregular CT surrounding fascicles (groups of muscle fibers)

endomysium

fine reticular fibers surrounding each muscle cell (= muscle fiber)

Skeletal muscle anatomy - attachments

muscles span joints and attach to bones in at least two places

• insertion

• origin

insertion

attachment to movable bone

origin

attachment to immovable/less movable bone

attachments can be…

direct or indirect

direct (fleshy)

epimysium fused to periosteum (bone) or perichondrium (cartilage)

ex: intercostals

indirect

connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis

Skeletal Muscle Microanatomy - Specialized features

• sarcolemma

• Sarcoplasm

• Glycosomes

• Myoglobin

• Modified organelles

  • myofibrils

  • sarcoplasmic reticulum

  • T-tubules

sarcolemma

muscle fiber plasma membrane

sarcoplasm

muscle fiber cytoplasm

glycosomes

glycogen storage

myoglobin

O2 storage

myofibrils

rodlike

densely packed, rodlike elements - one muscle fiber can contain 1000s (~80% of muscle cell volume)

myofibril features: striations, sarcomeres, myofilaments (thick - myosin; thin - actin)

sarcoplasmic reticulum

calcium storage

T tubules

transport

Sarcomere

smallest contractile unit (functional unit) of muscle fiber

Individual sarcomere formation

individual sarcomeres align end to end along a myofibril, like boxcars of a train

• striations

• myofilaments

striations

stripes formed from repeating series of dark and light bands along length of each myofibril

myofilaments

orderly arrangement of actin (thin) and myosin (thick) myofilaments within sarcomere

Components of a sarcomere

• Z discs: narrow, plate-shaped regions of dense material that separate one sarcomere from the next

• A band: dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments

• I band: lighter, less dense area of sarcomere that contains remainder of thin filaments but no thick filaments. A Z disc passes through center of each I band

• H zone: narrow region in center of each A band that contains thick filaments but no thin filaments

• M line: region in center of H zone that contains proteins that hold thick filaments together at center of sarcomere

contractile proteins

proteins that generate force during muscle contractions

myosin

contractile protein that makes up thick filament; molecule consists of a tail and two myosin heads, which bind to myosin-binding sites on actin molecules of thin filament during muscle contraction

actin

contractile protein that is the main component of thin filament; each actin molecule has a myosin-binding site where myosin head of thick filament binds during muscle contraction

what happens during contraction

heads link thick and thin filaments together, forming cross bridges

regulatory proteins

proteins that help switch muscle contraction process on and off

tropomyosin

regulatory protein that is a component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin

troponin

regulatory protein that is a component of thin filament; when calcium ions (Ca2+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin

what happens when calcium ions bind to troponin

it causes troponin to change shape - troponin’s shape change pulls on tropomyosin, which exposes binding sites on actin

sarcoplasmic reticulum (SR)

network of smooth endoplasmic reticulum tubules surrounding each myofibril

stores and releases Ca2+

transverse (T) tubules

tubes formed by protrusion of sarcolemma into cell interior - increases muscle fiber’s surface area

T tubules allow

electrical nerve transmissions to reach interior of cell

skeletal muscle fiber type is based on

speed of contraction

metabolic pathways used for ATP synthesis

• oxidative - use aerobic pathways (red)

• glycolytic - use anaerobic glycolysis (white)

skeletal muscle fibers can be classified into three types

1. slow oxidative fibers (red)

2. fast oxidative fibers (red-pink)

3. fast glycolytic fibers (white)

slow oxidative fibers (red)

low intensity, endurance activities (ex: maintaining posture)

fast oxidative-glycolytic fibers (red pink)

medium-intensity activities (ex: sprinting or walking)

fast glycolytic fibers (white)

short term intense or powerful movements (ex: hitting a baseball)

human muscles are

typically a mix (pink)

genes can determine percentage of fast and slow fibers

can also change with physical conditioning

cardiac muscle cells are usually

relatively small, usually 1 nucleus, can be branched

are there sarcomeres present in cardiac muscle

yes - causes striations

T tubules in cardiac muscle

wider and less numerous; SR simpler (no triads)

what is the release of Ca2+ triggered by in muscle cells

extracellular calcium

what type of respiration does cardiac muscle use

AEROBIC respiration only

large energy reserves

large numbers of mitochondria

abundant myoglobin

cardiac muscle cells contain

intercalated discs

gap junctions - electrically connect cells

desmosomes - keep cells from pulling apart

cardiac muscle can contract without

neural stimulation (automaticity)

pacemaker cells within the heart

cardiac muscle - system can make alterations to

rate of pacemaker cells

amount of tension in each contraction

cardiac muscle - contractions last

10 times as long as skeletal muscle

longer refractory periods

no fatigue

cardiac muscle - properties of the sarcolemma differ

twitches cannot undergo summation - no tetanic contractions

smooth muscle is found

in walls of most hollow organs

smooth muscle forms

sheets, bundles, or sheaths around other tissues

smooth muscle can

shorten and stretch to a greater extent

smooth muscle is arranged as

either single-unit (visceral) or multi-unit fibers

smooth muscle fiber shape

spindle shaped - thin and short

smooth muscle fiber - nucleus and striations

only 1 nucleus, no striations

smooth muscle lacks

connective tissue sheaths - endomysium only

SR and T tubules of smooth muscle

less elaborate SR and no T tubules

SR does store intracellular Ca2+, but most calcium used for contraction has extracellular origins

Troponin and tropomyosin in smooth muscle

no troponin complex, but does contain tropomyosin

what protein binds Ca2+ in smooth muscle

calmodulin

thin and thick filaments - smooth muscle characteristics

thin and thick filaments arranged diagonally

fewer thick filaments

spirally arranged, causing smooth muscle to contract in a corkscrew manner

smooth muscle contains a lattic-like arrangement of

non contractile intermediate filaments - resists tension

dense bodies in smooth muscle

anchor filaments to sarcolemma - like Z discs of skeletal muscle

what happens during contraction of smooth muscle

areas of sarcolemma between dense bodies bulge outward - cell looks puffy

functional characteristics of smooth muscle - sarcolemma

contains pouchlike infoldings (caveolae) that contain numerous Ca2+ channels that open to allow rapid influx of extracellular Ca2+

functional characteristics of smooth muscle - sarcolemma organization

organized into 2 layers of tightly packed fibers sheets oriented at right angles to each other

functional characteristics of smooth muscle - sarcolemma alternating contractions and relaxations of layers

alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs

smooth muscle contractions

slow, synchronized contractions and slower to relax - but maintains contraction for long periods with less fatigue

gap junctions in smooth muscle

transmit action potentials from fiber to fiber

some smooth muscle cells are

self-excitatory (depolarize without external stimuli) - act as pacemakers for muscle sheets

rate and intensity of contraction in smooth muscle cells

rate and intensity of contraction is modified by neural and chemical stimuli

how does smooth muscle respond to stretch

responds to stretch only briefly, then adapts to new length

this enables organ to temporarily store contents

smooth muscle contraction length

can contract when between ½ and 2x resting length

allows organ to have huge volume changes without becoming flabby when relaxed