Lecture 5-6: The Muscular System

Three types of muscle

Skeletal, Smooth, Cardiac

Skeletal Muscle Fiber

A single 30cm long cell which contains multiple nuclei, and cannot undergo mitosis

Myoblasts

Single nucleus cells which join to form a skeletal muscle fiber

Myofibril

Subcellular units which bundle up longitudinally to form muscle fibers

Z-lines

Protein disks which are pulled together by actin and myosin to shorten a sarcomere

Sarcomeres

Subsections of myofibrils that contain staggered actin and myosin

Actin

Thin protein filaments that attach to Z-lines, and gap at M-lines

Myosin

Thick, headed, protein filaments that bind to actin sites to pull M-line closed

Low-energy configuration (myosin)

Head is folded and binds ATP

High-energy configuration (myosin)

Head becomes upright and able to bind to actin by cleaving ATP

Cross-Bridging

High-energy myosin binds to actin molecule, releasing ADP and a free Phosphate

Sarcomere contraction

Myosin returns to low energy state while still bound to actin

Twitch of a muscle fiber (energy)

290 ATP cross bridging cycles

Troponin

Complexes on actin filaments which have Ca2+ binding sites, which signals Troponin to release Tropomyosin

Tropomyosin

Strand-like protein structures which block the active sites of Actin molecules in the absence of Ca2+ signals

What ion signals muscle contraction

Ca2+ (calcium)

Resting state of Myosin

High energy, awaiting the availability of actin binding site, to contract immediately upon signal

Effects of Ca2+ concentration

The more Ca2+ is released, the more Tropomyosin is removed, contributing to a stronger contraction

Neuromuscular junction

Synapse that joins a single muscle cell to the nervous system 

Sarcoplasmic Reticulum

Series of internal sacs that surround a myofibril, ready to release Ca2+ ions upon signal

Neuromuscular junction pathway

Acetylcholine release triggers Na+ channels to open, creating an action potential that allow the opening of voltage gated calcium channels, which then binds to Troponin

Difference between pumps and channels

Pumps use ATP to move molecules against their concentration gradient, while channels allow the bidirectional flow of molecules with their concentration gradient

motor unit

one neuron and the group of muscle fibers it controls

recruiting

the activation of motor units in order to facilitate an action

small vs large motor units

small: down to 10 fibers, used for fine motor control

large: up to 1000 fibers, used for strong muscles

recruitment of differently sized motor units

smaller motor units will “fill” up faster at a lower frequency of action potentials in a process similar to temporal summation. Larger units will take higher frequencies to be recruited

muscle twitch

activation of a single motor neuron, caused by a small amount of Ca2+ released resulting in a small amount of cross bridging 

temporal summation

the buildup of Ca2+ within a myofibril due to the continued release of it before it can be absorbed back in

Ca2+ cycle

released through voltage gated channels into the tissue fluid, binding to troponin, pumped back into the Sarcoplasmic Reticulum by Ca2+ pumps

tissue fluid

interstitial fluid but for muscles

3 systems for obtaining ATP

immediate system, glycolytic system, oxidative system

immediate system

a phosphate from the stored molecule phosphocreatine is transferred to adp creating atp and creatine (

glycolytic system

through glycolysis, 2ATP are created in the absence of O2 (anaerobically), and dispersed into tissue fluid

oxidative system

through chemiosmosis, 26-28 ATP are generated and deposited in tissue fluid to be taken up by muscle fibers

fatigue

oxygen deficit due to the limits of the circulatory system

myoglobin

molecule that draws oxygen from tissue fluid into muscle fibers

slow oxidative fibers

contract slowly, contain many mitochondria, high myoglobin content (red)

fast oxidative fibers

contract quickly, contain many mitochondria, high myoglobin content (red)

fast glycolytic fibers

contract quickly, source ATP from glycolysis, contain few mitochondria, low myoglobin content (white)

fast-twitch fibers

enable quick, reactive movements, robust SRs to activate strong cross-bridging fast

slow-twitch fibers

enable slower, sustained movements, less robust SR, and are always oxidative

typical fiber twitch distribution

45% fast twitch, 55% slow twitch