HSCI 101 Chapter 11 Drexel

organization of skeletal muscle

muscle; fascicles; muscle fibers; myofibrils; myofilaments (actin and myosin)

epimysium

dense regular CT surrounds entire muscle

perimysium

dense regular CT surrounds groups of fascicles, fuses together to form a tendon

endomysium

areolar CT surrounds each muscle fiber

myocyte

muscle cell

sarcoplasmic reticulum

modified ER, stores calcium

Terminal cristae

enlarged part of the sarcoplasmic reticulum

sarcolemma

plasma membrane of muscle

sarcoplasm

cytoplasm of a muscle cell

triad

2 terminal cristae + t-tubule

sarcolemma goes inside to make t-tubule

myofibrils

composed of thick and thin filaments

Thick filaments

made up of multiple myosin

thin filaments

actin, troponin, tropomyosin

myosin heads

bind with actin

troponin

locks tropomyosin in place

tropomyosin

covers actin active sites

actin active site

where myosin head binds

sarcomere

smallest contractile unit of a muscle fiber

z-disc to z-disc

I band

thin filaments only

light appearance

z disc

middle of I band

anchors thin filaments

"Endline"

A band

overlap of thin and thick filaments

dark appearance

H zone

middle of A band

thick filaments

M line

"midline"

bisects H zone of A band

anchors thick filaments

skeletal muscles are stimulated by

somatic motor neurons

neuromuscular junction (NMJ)

where the motor neuron contacts the skeletal muscle

transmission of AP from neuron to sarcolemma

NMJ components

synaptic knob, synaptic cleft, motor end plate

Skeletal muscle contraction phases

excitation

excitation-contraction coupling

crossbridge cycling

end-plate potential

initial depolarization of motor end plate, excitation phase

Excitation-Contraction coupling

generation of an AP

AP propagates down sarcolemma and into t-tubules

Steps of generation of an AP

generation of an end plate potential

depolarization

repolarization

generation of an end plate potential

ACh released from motor neuron

Chemically-gated Na+ channels open

result: local depolarization

depolarization

if end plate potential reaches threshold

voltage-gated Na+ channels open

AP spreads across sarcolemma

repolarization

voltage-gated Na+ channels close

voltage-gated K+ channels open

Na+/K+ pump maintains RMP

crossbridge cycling

muscle shortening caused by the movement of the contractile protein (muscle contraction)

rigor mortis

stiffening/contraction of skeletal muscles after death, no more ATP

crossbridge formation

myosin head binds to actin active site

crossbridge detachment

addition of ATP to myosin head

crossbridge cycling step 1

ATP hydrolysis "cocks" the myosin head

crossbridge cycling step 2

myosin head binds to actin

crossbridge cycling step 3

phosphate detaches from the myosin head and myosin pulls actin toward the center of the sarcomere

crossbridge cycling step 4

ATP breaks the attachment of myosin to actin

ATP is needed for

release myosin heads from actin

muscle relaxation

ATP is generated by

immediate cytosolic reactions

glycolysis

aerobic cellular respiration

immediate cytosystolic reactions

ATP rapidly consumed during contraction

ATP regenerated by creatine phosphate

energy lasts 10-15 seconds

glycolysis

glucose is broken down

provides ATP once immediate sources are depleted

DOESNOT require oxygen

energy lasts 30-60 seconds

aerobic cellular respiration (oxidative catabolism)

provides unlimited supply of ATP as long as oxygen and fuels are available

Diameter influences

strength of contraction

Type I / slow oxidative fibers

slow-twitch fibers

fatigue-resistant

produce less forces for longer periods of time

relies on oxidative catabolism

Type IIa fast fibers

relies on oxidative catabolism and glycolysis

ex) walking, jogging

Type IIx fast fibers

relies on glycolysis

ex) short burst

smaller motor units

fine motor control

ex) fingers, eyes

larger motor units

less control, but more power

ex) large leg muscles motor

motor unit

a single motor neuron and all the muscle fibers it innervates

Frequency of Stimulation

repeated stimulation of a muscle fiber by a motor neuron

results in summation of the contractions (wave summation)

Length Tension Relationship

Tension is dependent upon sarcomere length

at "optimal" length, sarcomere can generate greatest tension

Isometric contractions

consistent muscle length but tension changes

isotonic contractions

constant tension but muscle length changes

Concentric isotonic contractions

shortening of muscle

generate enough force to move an object

eccentric isotonic contractions

lengthening

deceleration of a joint/brake