GVSU - BMS 250 - Lanier - Final

Excitability

ability to respond to stimulus by changing electrical membrane potential

Conductivity

involves sending an electrical change down the length of the cell membrane

Contractility

exhibited when filaments slide past each other.

Enables muscle to cause movement

Extensibility

ability to be stretched

Elasticity

ability to return to original shape after being stretched

function of skeletal muscle tissue

body movement

maintain posture

temperature regulation

storage/movement of materials

support soft tissue

true or false = skeletal muscles' fibers are striated and usually attached to bone

true

skeletal muscle organ composed of;

muscle fibers

connective tissue

blood vessels

nerves

muscle fiber

muscle cell

where are muscle fibers located?

bundled within a fascicle

fasicle

consists of many muscle fibers

3 concentric layers of muscle

epimysium (superficial)

perimysium

endomysium (deepest)

epimysium

dense irregular connective tissue wrapping the whole muscle

perimysium

Dense irregular connective tissue wrapping fascicle.

Houses many blood vessels & nerves

endomysium

Areolar connective tissue wrapping individual fiber.

Electrical insulation, capillary support, binding of neighboring cells.

tendon

Attaches muscle to bone.

Cord-like structure of dense regular connective tissue

Aponeurosis

Thin, flattened sheet of sense irregular CT.

Takes the place of a tendon in flat muscles having a wide area of attachment.

Deep Fascia

Dense irregular CT superficial to epimysium.

Separates individual muscles; binds muscles with similar functions

Superficial Fascia

Areolar and adipose CT superficial to deep fascia.

Separates muscles from skin

Origin

FIXED end of skeletal muscle (does not move)

Insertion

MOVABLE end of skeletal muscle

Sarcoplasm

(Cytoplasm)

Has typical organelles plus contractile proteins and other specializations

Multiple nuclei

(individual cells are multinucleated)

- Cell is formed in embryo when multiple myoblasts fuse

- Some nearby myoblasts become undifferentiated satellite cells for support and repair of muscle fibers

Sarcolemma

Plasma membrane of a muscle fiber.

Sarcolemma and its T-tubules have voltage-gated ion channels that allow for conduction of electrical signals

Sarcoplasmic reticulum

- Specialized endoplasmic reticulum of muscle cells.

- Contain; terminal cisternae, calcium pumps, calcium release channels

Terminal cisternae of sarcoplasmic reticulum

blind sacs that serve as reservoirs of calcium ions. (2 cisternae with T-tubule between = triad)

Myofibrils

- Contractile proteins within myofibrils.

- Bundles of myofilaments (contractile proteins within myofibrils).

- Enclosed in sarcoplasmic reticulum

think filaments

Bundles of many myosin protein molecules.

(myosin heads point toward ends of the filament)

thin filament

Bundles of many myosin protein molecules

I band

•Light-appearing regions that contain only thin filaments

•Bisected by Z disc

•Get smaller when muscle contracts (can disappear with maximal contraction)

A band

•Dark-appearing region that contains thick filaments and overlapping thin filaments

•Contains H zone and M line

•Makes up central region of sarcomere

H zone

•Disappears with maximal muscle contraction

•Only thick filaments present

M line

•Protein meshwork structure

•Attachment site for thick filaments

Myoglobin within muscle fibers allows for __________.

storage of oxygen used for aerobic ATP production in the mitochondria

_____________ will give up its phosphate group to help replenish ATP supply

Creatinine phosphate

glycogen

stored for when fuel is needed quickly

Motor unit

A motor neuron and all of the muscle fibers it controls

Axons of motor neurons from spinal cord (or brain) innervate ________.

muscle fibers

small motor units

- Have less than 5 muscle fibers

- Precise control of force output

large motor unit

- Have thousands of muscle fibers

- Production of large amount of force (not precise)

Neuromuscular Junction

- Location: where motor neuron innervates muscle.

- Made up of; synaptic knob, synaptic cleft, motor end plate.

Synaptic knob of motor neuron

- Expanded tip of the motor neuron axon.

- Houses synaptic vesicles.

- Has Ca2+ pumps in plasma membrane.

- Has voltage-gated Ca2+ channels in membrane.

Motor end plate

Specialized region of the sarcolemma.

Has many ACh receptors.

•Plasma membrane protein channels

•Opened by binding of ACh

•Allow Na+ entry and K+ exit

Synaptic cleft

- Narrow fluid-filled space.

- Separates synaptic knob from motor end plate.

- Acetylcholinesterase resides here (enzyme that breaks down ACh molecules).

Resting Membrane Potential (RMP)

•Fluid inside cell is negative compared to fluid outside cell

•RMP of muscle cell is about -90 mV

•RMP established by leak channels and Na+/K+ pumps (voltage-gated channels are closed)

Neuromuscular Junction: Excitation of a Skeletal Muscle Fiber

1. Calcium entry at synaptic knob

2. Release of ACh from synaptic knob

3. Binding of ACh at motor end plate

Calcium entry at synaptic knob

Nerve signal propagated down motor axon

Triggers opening of voltage-gated Ca2+ channels

Movement of calcium down concentration gradient

from interstitial fluid into synaptic knob

Binding of calcium with proteins on synaptic vesicles

Release of ACh from synaptic knob

•Vesicles merge with cell membrane at synaptic knob: exocytosis

•Thousands of ACh molecules released from about 300 vesicles

Binding of ACh at motor end plate

Diffusion of ACh across synaptic cleft

Binds with ACh receptors within motor end plate

Causes excitation of muscle fiber

Excitation-Contraction Coupling

- Stimulation of the fiber is coupled with the sliding of filaments

- Coupling includes the end-plate potential (EPP), muscle action potential, and release of Ca2+ from the sarcoplasmic reticulum

myasthenia gravis (MG)

- Autoimmune disease (primarily women)

- Receptors removed from muscle = decreased muscle stimulation/muscle weakness.

End-plate potential (EPP)

- Depolarization of a membrane region by a sodium influx.

- Cell membrane briefly becomes less negative at the end plate region.

- EPP is local but it does lead to the opening of voltage-gated ion channels in the adjacent region of the sarcolemma

action potential =

= rapid rise (depolarization) and fall (repolarization) in the charge of the membrane

Action Potential Propagation (T-tubules)

- AP travels down T-tubules

- Causes voltage-sensitive calcium channels in T-tubule membrane to trigger the opening of calcium release channels in SR terminal cisternae

Action Potential Propagation (SR)

- Release of Ca2+ from the sarcoplasmic reticulum

- Ca2+ interacts with myofilaments triggering contraction

crossbridge cycling steps

1. Calcium binds troponin; myosin binding site uncovered

2. Crossbridge formation

3. Power stroke

4. Release of myosin head

5. Reset myosin head

what triggers crossbridge cycling

Ca2+ binds to troponin,

(Troponin and tropomyosin move so actin is exposed)

1)Crossbridge formation

Myosin head attaches to exposed

binding site on actin

Power stroke

•Myosin head pulls thin filament

toward center of sarcomere

•ADP and Pi released

Relesae of myosin head

ATP binds to myosin head causing its release from actin

Reset myosin head

•ATP split into ADP and Pi by myosin ATPase

•Provides energy to "cock" the myosin head

Sarcomere Shortening

- Cycling continues as long as Ca2+ and ATP are present

- Z discs move closer together

- Narrowing of H zone and I band

- Thick and thin filaments remain the same length but slide past each other

Stored ATP in the muscles

Muscle cells have little ATP in storage

•Stored ATP is spent after about 5 seconds of intense exertion

•Additional ATP rapidly produced via myokinase

Ways to generate additional ATP in skeletal muscle fiber

1. Creatine phosphate

2. Glycolysis

3. Aerobic cellular respiration

creatine phosphate

• High-energy bond (creatine and phosphate)

• Phosphate can be transferred to ADP to form ATP

• Catalyzed by creatine kinase

• Provides 10-15 seconds of additional energy

Glycolysis

• Does NOT require oxygen

• Glucose converted to 2 pyruvate molecules

• 2 ATP released per glucose molecule

• Occurs in cytosol

aerobic cellular respiration

• Occurs within mitochondria (requires oxygen)

• Pyruvate oxidized to carbon dioxide

• Triglycerides can also be used as fuel to produce ATP

Lactate formation

from pyruvate occurs under conditions of low oxygen availability

lactate dehydrogenase

pyruvate converted to lactate

Lactic Acid Cycle

cycling of lactate to liver where it's converted to glucose, and transport of glucose back to muscle

Oxygen Debt

Amount of additional oxygen needed after exercise to restore pre-exercise conditions

Classification of Muscle Fiber Types

1. Type of contraction generated

2. Means for supplying ATP

Muscle Fiber Types: Differences in power, speed, and duration

• Power related to diameter of muscle fiber

• Speed and duration related to type of myosin ATPase, of action potential propagation, and quickness of Ca2+ release and reuptake by sarcoplasmic reticulum

•Oxidative fibers

- (fatigue-resistant) use aerobic cellular respiration.

- Extensive capillaries, many mitochondria, large supply of myoglobin

- Red fibers

•Glycolytic fibers

- (fatigable) use anaerobic cellular respiration

- Fewer capillaries, fewer mitochondria, small supply of myoglobin, large glycogen reserves

- White fibers

Three types of skeletal muscle fibers:

1.Slow oxidative fibers

2.Fast oxidative fibers

3.Fast glycolytic fibers

slow oxidative fibers (type I)

- Contractions are slower and less powerful

- High endurance since ATP supplied aerobically

- About half the diameter of other fibers, red in color due to myoglobin

Fast oxidative fibers (type IIa, intermediate)

• Contractions are fast and powerful

• Primarily aerobic respiration, but delivery of oxygen lower

• Intermediate size, light red in color

Fast glycolytic fibers (type IIb, fast anaerobic)

•Contractions are fast and powerful

•Contractions are brief, as ATP production is primarily anaerobic

Largest size, white in color due to lack of myoglobin

muscle tension

force generated when a muscle is stimulated to contract

muscle twitch

brief contraction in response to a single stimulus (minimum voltage that triggers a twitch is the threshold)

3 phases of muscle twitch

1. latent period

2. contraction period

3. relaxation period

Latend period of muscle twitch

•Time after stimulus but before contraction begins

•No change in tension

contraction period of muscle twitch

•Time when tension is increasing

•Begins as power strokes pull thin filaments

relaxation period of muscle twitch

•Time when tension is decreasing to baseline

•Begins with release of crossbridges

•Generally lasts a little longer than contraction period

True or False: as voltage increases, less units are recruited to contract

false; as voltage increases, more units are recruited to contract

maximum contraction

all units are recruited (above certain voltage)

recruitment order of muscle unit

small first

large last

Isometric contraction

• Although tension is increased, it is insufficient to overcome resistance

• Muscle length stays the same

• E.x. holding a weight while arm doesn't move

Isotonic contraction

•Muscle tension overcomes resistance resulting in movement

•Tone doesn't change , but length changes

Isotonic Contraction: Eccentric

muscle lengthens as it contracts

E.x. biceps brachii when lowering

Isotonic Contraction: Concentric

muscle shortens as it contracts

E.x. biceps brachii when lifting

Fiber at resting length generates __________

= maximum contractile force

•Optimal overlap of thick and thin filaments

Fiber at a shortened length generates ___________

= weaker force

•Filament movement is limited (already close to Z disc)

Fiber at an extended length generates __________

= weaker force

•Minimal thick and thin filament overlap for crossbridge formation

Muscle fatigue

- Reduced ability to produce muscle tension

- Primarily caused by a decrease in glycogen stores during prolonged exercise

Other possible causes of fatigue: Excitation at neuromuscular junction

• Insufficient Ca2+ to enter synaptic knob

• Decreased number of synaptic vesicles

Other possible causes of fatigue: Crossbridge cycling

• Excessive Pi slows release of Pi from myosin head

• Less Ca2+ available for troponin (some Ca2+ is bound to Pi)

Other possible causes of fatigue: Excitation-contraction coupling

Altered ion concentrations impair action potential conduction and Ca2+ release from sarcoplasmic reticulum

Cardiac muscle cells characteristics

• Short, branching fibers

• One or two nuclei

• Striated

•Many mitochondria

•Intercalated discs join ends of neighboring fibers

•Contractions started by heart's autorhythmic pacemaker cells

•Heart rate and contraction force influenced by autonomic nervous system