Phys Unit 3.1

Skeletal and smooth muscle

Fascicles

Bundles of muscle fibers

Muscle Fiber

Muscle cell

Myofibrils

Overlapping thick and thin filaments

Sarcolemma

Muscle cell plasma membrane

Sarcoplasmic Reticulum

Modified Endoplasmic Reticulum in muscle cells

Motor Unit

Motor neuron and all the muscle fibers it innervatesMu

Muscle contractile proteins

Actin (thin filament) and Myosin (thick filament)

Muscle regulatory proteins

Troponin and tropomyosin (on actin)

Thick filament

• Myosin

• Contains myosin head (for cross bridge)

  • Head has a binding site for actin and ATPase (breaks down ATP to disconnect with actin)

Thin filament

Troponin - 2 bead complex

• Binds actin/tropomyosin

• Binds Ca2+

Tropomyosin - long strands

• Binds to myosin

  • Provides support and stability

A band

• Contains both actin and myosin

  • Spans entire length of myosin and overlaps with actin

H zone

Contains only myosin

I band (light band)

Contains only actin

Z disc

Anchors thin filaments

Sarcomere

Z disk to Z disk 

Sliding filament theory of contraction

During contraction, the thin filaments slide over the thick filaments toward the center of the sarcomere

• H band and I band shorten

Excitation-Contraction Sequence

1. ACh is released from axon terminal of motor neuron and diffuses across synaptic cleft to bind with nicotinic receptor on motor end plate

2. ACh opens many monovalent cation channels (Na+ influx>K+ efflux) - (EPP always triggers an action potential in muscle cells)

3. The AP is propagated into the T-tubules which contains DHP receptors

4. DHP receptors are physically attached to ryanodine receptors (Ca2+) - SR gates open and Ca2+ is released into the sarcoplasm

5. Ca2+ binds to troponin pulling the tropomyosin off the actin binding site and allowing cross bridges to form

6. Myosin head executes the power stroke

7. Actin filaments slide toward the cent of the sarcomere

Twitch

A single contraction-relaxation cycle

Relaxation of the muscle

• Ca2+ is actively pumped back into the SR using Ca2+-ATPase

• Leads to Ca2+ to unbind to troponin and tropomyosin slides back over to cover actin binding site

Rigor mortis

• No oxygen is present to drive oxidative phosphorylation

• Without ATP, myosin and actin are unable to separate

• Ca2+ is continually being pumped out of SR and binding with troponin allowing myosin and actin to bind

Sarin Gas (acetylcholinesterase inhibitor)

• Unable to break down acetylcholine

• Unable to breath because muscles can not contract and relax

• Without oxygen, no ATP can be produced so you go into a state of rigor

Curare

• Binds to nicotinic receptor making acetylcholine unable to bind

• Leads to no muscle contraction

Botox

• Makes acetylcholine never release from lower motor neuron

• Inhibits contraction (facial expressions)

Creatine phosphate

Quick energy storage (no oxygen)

Creatine phosphate + ADP → ← Creatine + ATP

Anaerobic metabolism

Glycolysis (lasts about 60 secs)

• 2 ATP/Glucose and no oxygen required

Aerobic metabolism

Oxidative phosphorylation (hours of use)

• 32 ATP/Glucose and requires oxygen

Tension

Force exerted on an object by a contracting muscle (pull)

Load

Force exerted on muscle by an object

Isotonic contraction

Contraction where the muscles shorten while the load remains (change of length)

Isometric contraction

What a muscle develops tension but does not change length (standing)

What influences muscle fiber force?

a. frequency of stimulation

b. fiber diameter

c. fiber length

Fiber diameter

More actin/myosin present and able to bind → more cross bridges able to form 

Frequency of stimulation

Ca2+ reuptake into the ST is not fast enough → Ca2+ remains bound to troponin → formation of more cross bridges (more Ca2+ released than absorbed)

Changes of fiber lengths

Optimal length occurs when the maximal amount of cross bridges can be formed

Tetanus

Maximum amount of tension a muscle can have

Summation of unfused tetanus

Stimuli are apart to allow muscle to relax slightly between stimuli

Fused summation

No relaxation of the muscle

Recruitment

Starting with small motor units and moving to large motor units as needed

Myoglobin

Allows oxygen to concentrate in muscle

• Red molecule that binds oxygen to be stored in the muscle

  • Gives a darker color to the muscle

Glycolytic fibers (white muscle)

• Fast APTase activity

• Glycolytic enzymes in the cytosol

• Used for rapid bursts of activity (sprinting/power lifting)

• Large diameter

• 2 ATP/Glucose without oxygen

• Few mitochondria

• Recruited last and fatigues quickly

• Few capillaries

• No myoglobin

Oxidative fibers

• Slower myosin ATPase

• Many mitochondria (required for oxidative phosphorylation)

• Small diameter

• Generates about 38 ATP/glucose with oxygen

• Many capillaries

• Recruited first and slower to fatigue

• Myoglobin

Slow (twitch) oxidative fibers (type 1)

Smallest motor units; most used - posture

Fast (twitch) oxidative fibers (type IIa)

Intermediate motor units; standing, walking

• very trainable to use more oxidative or glycolytic pathways based on physical activity

Fast (twitch) glycolytic fibers (type IIb)

Largest motor units; least used - jumping

• Bursts of activity

Muscle hypertrophy

Increase in muscle size

Muscle atrophy

Loss of muscle

Smooth muscle

• Lacks striations, no sarcomeres

• Found in internal organs and blood vessels

• Not under voluntary control

• No T-tubule system

• No troponin - uses calmodulin instead

• 2 sources of calcium (intracellular and extracellular)

• Activated by autonomic nervous system

• Less defined sarcoplasmic reticulum

Myosin light chains

Small regulatory protein chains in the myosin head

Smooth muscle activity

ATPase activity of myosin is much slower and the contraction phase of the twitch is longer

Single unit (visceral)

Most common

• All the cells are coupled by gap junctions

• Entire tissue behaves like a single unit

Multi-unit

Places where fine control is needed (iris/ciliary body of eye)

Smooth muscle contraction

1. Increase in intracellular Ca2+ (from ECF and ICF)

2. Ca2+ binds to calmodulin

3. Ca2+-calmodulin activates MLCK

4. MLCK phosphorylates light chains - increases myosin ATPase activity

5. Active myosin cross bridges - increased muscle tension

Smooth muscle relaxation

1. Ca2+ pumped out of cell and back into SR (pumped against concentration gradient - requires ATP)

2. Ca2+ unbinds from calmodulin

3. Myosin phosphatase removes phosphate (removes phosphate from light chains to create ATP - decreases ATPase activity)

4. Decreased muscle tension