module 6 - muscular system

types of muscle tissue

skeletal, cardiac, smooth

skeletal muscle

attaches to bone = movement

voluntary control

muscle fibers → striated and multiple nuclei

cardiac muscles

found on wall of heart

involuntary control

striations 1-2 nuclei

smooth muscle

found on walls of hollow organs

involuntary

one nucleus no striations

spindle shaped

characteristics of muscle tissue

excitable, contractable, extensible, elastic

function of skeletal muscle

movement, stabilize joints, maintain posture and position, heat generation

skeletal muscle connective tissue

epimysium, perimysium, endomysium

epimysium

dense irregular CT surrounds WHOLE muscle

perimysium

fibrous CT surrounds fascicles

endomysium

fine areolar connective tissue; surrounds individual muscle fiber

where does skeletal muscle attach to bone

extension of epimysium that is continuous with tendons

skeletal muscle type of attachements

direct and indirect

direct attachment

epimysium fuses with periosteum of bone or pericardium of cartilage

indirect attachement

connective tissue wrappings gather and extend beyond muscle as ropelike (tendon) or sheetlike (aponeurosis)

aponeurosis

sarcolemma

plasma membrane of muscle fibers

sarcoplasm

cytoplasm of muscle fibers

myofibrils

for contraction; modified organelles gives muscle striated appearance

sarcoplasmic reticulum

smooth ER for Ca2+ storage

smooth ER surrounding myofibrils

T-tubules

extension of sarcolemma

allows for transmission of electrical nerve impulses to travel to interior

myofilaments

allows for contraction to occur; consists of thick and thin filaments

make up sarcomeres

thick filaments

composed of myosin

thin filaments

composed of actin and regulatory proteins tropomyosin and troponin

myosin

contains 2 heavy and 4 light polypeptide chains

bind and unbind to form contraction

tropomyosin and troponin

regulatory proteins controlling thin filament binding sites

sarcomere

smallest contractile unit in muscle fiber

made of thick and thin filaments

sarcomere regions

A bands → dark regions (thick and thin)

H zone → light region in middle of A band

M line → middle of H zone

I bands → lighter region (thin)

z disc → coin shaped

filaments and proteins within sacromere

elastic filament, dystrophin protein, nebulin, myosin, C proteins

dystrophin

links thin filament to proteins on sarcolemma

nebulin, myosin and c proteins

bind filaments/sacromere together and maintain alignment of sacromere

elastic filament

titin protein → holds thick filament in place; helps recoil after stretch

Duchenne muscular dystrophy (DMD)

caused by defective dystrophin gene = sarcolemma of patients tear easily = entry of excess Ca = contractile fibre dmg

triads

formed by t tubule surrounded by terminal cistern of SR on either side

important in transmission of electrical signals and release of Ca

neuromuscular junction

end of axon → splits into many branches that interact with fiber

axon terminal

where axon meets sarcolemma

synaptic cleft

gap between axon terminal and sarcolemma

steps needed for muscle contraction

nerve stimulation

action potential

propagation across sarcolemma

Ca release

steps occurring at neuromuscular junction

nerve stimulation and action potential generation

ACh

neurotransmitter that carries signal along axon to innervate skeletal muscle

ligand gated channels

chemical messengers bind to receptors to open the channels

voltage gated channels

changes in voltage open or close channels

propagation of signal at NMJ

AP arrives at terminal

VGCC on axon terminal opens = Ca flows into axon terminal

Ca influx = ACh released

ACh diffuses across and binds to ligand gated receptors on sarcolemma

channel opens = Na flow into muscle fiber

acetocholinesterase degrades ACh and channel closes

end plate potential

when Na flows into muscle fiber and membrane potential becomes positive

how does end plate potential lead to action potential

membrane change causes wave of depolarization to spread away from motor end plate to rest of sarcolemma where VGC exist

causes Na channels to open and K channels to close

travels down to t tubules

RMP of skeletal muscle

90 mV

excitation contraction coupling

at rest → myosin binding sites on actin are closed = covered by tropomyosin

Ca levels rise → bind to troponin

troponin pulls tropomyosin off binding sites → myosin binds to form cross bridge

sliding filament model

thick and thin myofilaments don’t change in length

thin slides against thick = more overlap (shortens)

cross bridge cycling

ATP hydrolyzed = energy allows myosin head to bind to actin

ADP and Pi released = creates energy → myosin head bends and pulls actin filament to M line

ATP binds to myosin head → cross bridge breaks

ATP is hydrolyzed → myosin head returns to ready position

motor nerve stimulation summation

successive stimuli added together

produces stronger muscle contraction each time

tetanus

fusion of contractions produces continuous contraction

factors increasing force generation

increased frequency of stimuli

recruitment of more muscle fibers

large muscle fibers and type

length of overlap of myofilaments

isotonic contraction

change in muscle length in response to given load

types of isotonic contractions

eccentric and concentric

concentric contraction

muscle contract and shorten

eccentric contraction

muscle contract and lengthens

isometric contractions

muscle length remains constant as it contracts

type 1 muscle fibers

slow contraction

low force generated

high mitochondria and capillaries

high fatigue resistance

type II A fibers

intermediate contraction speed

medium force generated

some mitochondria medium capillary density

30 mins of use before fatigue

type II B

fast speed of contraction

high force generated

few mitochondria and capillaries

fast fatigue

myoglobin

hemoglobin but in muscle

aids with oxygen delivery to skeletal muscle tissue

metabolic pathways

direct, anaerobic, aerobic pathways

direct pathways

creatine phosphate quickly turned into ATP via creatine kinase; limited

anaerobic pathways

glycolysis → no oxygen; inefficient and produces lactic acid

for high intensity short duration

aerobic pathway

for long duration exercise (day to day)

large gain of ATP

produces lots of CO2

smooth muscle structure

connective tissue covering similar to skeletal muscle

only covers groups of cells (not whole muscle)

smooth vs skeletal

less sarcoplasmic reticulum

have gap junctions

involuntary control

excited or inhibited

slow contraction

mainly aerobic

shorter myosin no sarcomeres

thick and thin arranged diagonally

calmodulin

replaces troponin in smooth muscle

controls binding

smooth muscle contraction

caveolae have VGCC → opens during depolarization

Ca entering cell leads to release of Ca from SR

phases of muscle healing

destruction, repair, remodeling

muscle injury - destruction

myofiber rupture (necrosis)

hematoma → capillaries rupture, bruising

inflammatory response

muscle injury repair

macrophage cleans debris

myofibrils begin to regenerate

angiogenesis

muscle injury remodeling

myofibers mature

scar tissue reorganized to regain functional capacity

rhabdomyolysis

rapid breakdown of muscle fibers → leads to buildup in kidney (harmful)