Muscle Unit

Characteristics of muscle

Responsiveness (excitability)Conductivity ContractilityExtensibility (up to 3x)ElasticityExcitability (irritability)

Functions of muscle:

Produces movementMaintains postureStabilizes jointsGenerates body heat

Types of muscle tissue:

Skeletal muscleSmooth muscleCardiac muscle

Skeletal muscle characteristics Defined as voluntary striated muscle

attached to one or more bones Have contractile proteins Striations are light/dark bands of muscle Striations occur because of overlapping

contractile proteins Typical muscle cells are 100 microns in

diameter and 3 cm long Some as big as 500 microns and 30 cm long Composed of muscle tissue AND fibrous

connective tissue Makes up most of our “stereotypical” muscle

Smooth muscle characteristics

Capable of mitosisNo visible striationsNo Z-discsFound in walls of visceral organs,

blood vessels, iris of eye, hair follicles

Cardiac muscle characteristics

Rich in myoglobinRich in glycogenLarge mitochondria (fills 25% of cell

instead of 2% in skeletal muscle cells)Little use of anaerobic fermentation, so

highly resistant to fatigue

Skeletal muscle anatomy Glycogen - starch-like carbohydrate

provides energy to cells Myoglobin - pigment that stores oxygen Satellite cells - unspecialized stem cells that

can multiply to replace injured muscle cells Sarcolemma - plasma membrane of muscle

fibers Sarcoplasm - cytoplasm of muscle fibers Sarcoplasmic reticulum - smooth

endoplasmic reticulum that serves as a calicum ion reservoir

Skeletal muscle fiber anatomy Muscle fibers surrounded by endomysium Muscle fiber bundles form a fasicle Fascicles are surrounded by perimysium Fascicle bundles surrounded by epimysium Epimysium surrounded by deep fascia Built together like a rope Thick filaments called myosin Thin filaments called actin

Sliding filament theory of muscle contractionHugh Huxley (1954)Thin fibers (actin) overlap thick fibers

(myosin)Associated vocab words include…. A

band, H band, I band, Z disc, sarcomere

Contraction mechanisms:

Troponin complex attached to actin and blocks binding site

Myosin head cannot attachTroponin complex must be moved to

the side to allow myosin head to connect to actin

Ionic calcium pushes it to the sideMyosin head must attach to actin

binding site, forming a cross bridge

Continued …. At rest, myosin head has a high amount of

ATP Myosin head attaches Power stroke acts as lever, and uses ATP ATP must detach myosin head from binding

site after power stroke or rigor mortis can occur.

Myosin head re-cocks itself into a high-energy state once again

Muscle bioelectricity Skeletal muscle cannot contract unless stimulated Fibers stimulated by somatic motor neurons “motor unit” = a single nerve fiber and all the muscle fibers it

supplies One nerve fiber supplies stimulus to approx. 200 fibers The junction between nerve cells and muscle tissue is called

a “neuromuscular junction” The most common neurotransmitter with the NMJ is called

acetylcholine (Ach) Ach stimulates Ach receptors in the muscle fibers

Polarization mechanisms Polarized state = resting state (unstimulated) In the polarized state, there are more sodium ions outside of

the cell and more potassium ions inside the cell Resting membrane potential = small amount of voltage

difference across the membrane (approx. 90 mV) Depolarization = sodium ion channels opening allowing

sodium to flood into the nerve cells Repolarization = potassium channels open allowing

potassium to flood out of the nerve cells Repolarization re-establishes the negative charge inside the

cell Action potential = quick up & down voltage shift along the

membrane

Associated vocab:Threshold = minimum voltage

necessary to generate an action potential

Twitch = quick cycle of contraction/relaxation (takes between 7 and 100 milliseconds)

Latent period = takes 2 milliseconds between stimulus and the twitch

Recruitment = process of causing more motor units to contract

Muscle Tone Defined as a continual state of contraction Even relaxed muscles are slightly

contracted Caused by spinal reflexes that try to balance

activation of motor units vs. activation of stretch receptors

In other words, things are continually pulling in two different directions

Does not produce active movements Keeps muscles firm, healthy, responsive

Contraction Types:

Isotonic contractions cause muscle to change in length.

Tension remains constant through most of the contractile period

Isometric contractions neither shorten nor lengthen

Examples include muscles that help posture and to stabilize joints

Importance of ATP:

ATP is the ONLY source of energy that can be used for contractile activities

ATP provides energy for contractionATP is used and turns into ADPATP is quickly depletedThat means ATP must be regenerated

again (ADP turns into ATP)

Regeneration of ATP Method #1: Creatine phosphate is coupled with ADP Creatine phosphate (CP) is a high-energy molecule stored in

muscles Reaction: Creatine phosphate + ADP = creatine + ATP Muscle cells store about 5x as much CP as ATP ATP and CP provide for 15 seconds of max. muscle power

Regeneration of ATP Method #2: Anaerobic glycolysis Glucose breaks down to 2 pyruvic acids and +2 ATP The process doesn’t use oxygen so it’s anaerobic Caused from muscles acting vigorously, such as running

600m with maximal effort Bulging muscles compress blood vessels, reducing oxygen

flow to them The pyruvic acids are converted to lactic acid Washed out of muscle tissue within 30 minutes and into blood

stream Picked up by liver, heart, kidney cells to be used as energy Anaerobic pathway harvests 5% as much ATP as aerobic

pathway BUT … it produces ATP about 2 1/2 times faster

Regeneration of ATP Method #3: Aerobic respiration 95% of ATP used for muscle activity comes from this Occurs in the mitochondria Requires oxygen Glucose is broken down into water, carbon dioxide, and ATP Glucose + oxygen = carbon dioxide + water + ATP

Major Muscles and Muscle Facts Extensors are used to extend body parts, such as stretching

out the forearm or legs (increase the angle) Flexors are used to bring the angle of two body parts closer

together such as flexing your biceps in your arm(decrease the angle)

“Longus” muscles are long Look for hints that go along with the words…. Extensor carpi ulnaris = involves extending finger and lays

over the ulna Flexor carpi ulnaris = involves flexing fingers and lays over the

ulna

Biceps brachiDeltoidRectus abdominis