Muscle Physiology (Final)

Types of Muscle

- Skeletal

- Cardiac

- Smooth

Sarcomere

- Contractile Unit of Muscle Fiber

The generation of action potentials in the sarcolemma triggers what?

- A sequence of events inside the cell that resuts in force development

Sarcolemma

- Plasma membrane of a muscle fiber (cell)

Muscle Structure: Largest → Smallest

- Muscle → Fascicle → Muscle Fiber (cell) → Myofibrils → Sarcomeres (repeating units inside myofibrils)

How do muscle cells generate force?

- Specialized motor proteins within the cell

Myofibrils consist of what?

- Sarcomeres., repeating contractual units

Components of a Sarcomere

- Z discs, A band, I band, H zone, M line

Z Disc

- Separates sarcomeres from each other

I Band

- Thin filaments only, attached to Z Disc

A Band

- Dark area; extends length of the thick filaments

H Zone

- The region at the center of an A band of a sarcomere that is made up of myosin only. The H zone gets shorter (and may disappear) during muscle contraction.

M Line

- Center of sarcomere, attaches to the thick filaments found in the H Zone

Light vs Dark Striations

- Light: I Bands → Thin

- Dark: A Bands → Thick

Components of Thin Filaments

- Backbone: Double helix of actin filaments

- Helical Grooves: Occupied by Tropomyosin + Troponin

Tropomyosin does what at rest?

- Obscures the binding sites between actin and myosin

What regulates skeletal muscle contraction?

- Tropomyosin + Troponin (T, C, I) Trimer

Troponin T

- Anchors trimer to tropomyosin

Troponin C

- Binds calcium

Troponin I

- Inhibits interactions between actin and myosin when intracellular calcium is low

How is skeletal muscle contraction regulated?

1. Motor neurons release Acetylcholine at a synapse

2. Action potentials sweep across the muscle fiber and into it along the transverse tubules

3. Ca2+ released from SR → myofibrils contract

Thick Filament Composition

- Head: Actin binding site + elements to bind and hydrolyze ATP for force development; cross bridges that bind to actin during muscle contraction

- Tails: Bundled together to form core of the filament

- Hinge: Connects the heads + tails

Titin

- Important for maintaining sarcomere structure, runs from Z disc → M Line

- Responsible for passive tension that is measured when a relaxed muscle becomes stretched

Elastic String of Muscle

Dystrophin

- Scaffolding protein that links thin filaments to proteins of sarcolemma

Sliding Filament Theory

- Theory that actin filaments slide towards eachother during muscle contraction, while the myosin filaments are still

What happens if intracellular [Ca2+] is low?

- Actin binding sites are unavailable → no muscle contraction

What happens if sufficient tension is generated across muscle fibers to overcome a resistance

- Tension is transmitted via tendons to insertion points on skeleton → movement at joints

Somatic Motor Neurons: What are they? What do they release? Its result?

- Nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles

- Releases acetylcholine → Ca2+ released from SR → Voluntary control

What is total tension proportional to?

- The number of cross-bridges formed

Steps of Skeletal Muscle Contraction Phase

1. Motor neuron excites muscle fibers → Ca2+ released from SR

2. Ca2+ from SR binds to Troponin C → Troponin shifts position

3. Shifting of troponin exposes myosin binding sites on thin filaments

4. Myosin head forms tension-producing cross-bridge with the actin filament (with ADP + Pi bound to Myosin Head)

Steps of Skeletal Muscle Cross Bridge Cycle

1. Myosin is bound to ADP + Pi as well as actin filament in a cross-bridge, ready to contract

2. ADP + Pi are released, causes myosin head to pivot forward → pulling actin filaments and Z discs towards center of sarcomere (POWERSTROKE)

3. New ATP molecule binds to myosin head → head detaches from actin filament

4. ATP is hydrolyzed → ADP + Pi → Allows myosin head to reform cross-bridge with actin filament

5. Cycles of ATP hydrolysis/removal of ADP + Pi allows continuous power strokes when ATP/Ca2+ are present

Steps of Skeletal Muscle Relaxation Phase

1. Motor neuron activation ceases

2. Ca2+ dissociates from troponin C → pumped back into SR

3. Troponin-Tropomyosin complex moves back into position covering myosin binding sites → cross-bridging is unable to form and muscle tension production decreases

Motor Unit

- A motor neuron and all of the muscle fibers it innervates

Fast vs Slow MU

- Fast: Generates tension and movement quickly

- Slow: Tension generation is slower and sustained

Muscle Fiber Types (Skeletal)

- Slow Oxidative

- Fast Oxidative

- Fast Glycolytic

Slow oxidative, Fast oxidative, Fast glycolytic: Fiber Diameter

- Slow: Smal

l- Oxidative: Intermediate

- Glycolytic: Large

Slow oxidative, Fast oxidative, Fast glycolytic: Tension Generation

- Slow: Low

- Oxidative: Intermediate

- Glycolytic: High

Slow oxidative, Fast oxidative, Fast glycolytic: Movement Velocity

- Slow: Slow

- Oxidative: Fast

- Glycolytic: Fast

Slow oxidative, Fast oxidative, Fast glycolytic: Fatiguability

- Slow: Low

- Oxidative: Intermediate

- Glycolytic: High

What do slow oxidative fibers do?

- Sustained, low intensity contractions

- Posturla muscles

- Walking and other types of "aerobic exercise"

Fast Oxidative Fibers: What do they do?

- Best for activités that requires both speed and moderate endurance

- Mid distance events, power walking, resistance training

Fast Glycolytic Fibers: What do they do?

- Powerful, maximal power, speed, and explosiveness

- Sprints

Myocardium

- Muscular, middle layer of the heart

Heart: Atria and Ventricles

- Four chambered (2 atria, 2 ventricles)

Atria and Ventricles: Steps of Contraction

1. Atria contracts first → moves blood into ventricles

2. Ventricular contractions move blood into pulmonary trunk and aorta

Intercalated Discs

- Specialized connections between myocardial cells containing gap junctions and desmosomes → excitation can spread rapidly across adjacent muscle fibers

How does cardiac muscle contract?

- Similar mechanism to skeletal

Where does the Ca2+ in cardiac muscle come from?

- Interstitium, SR

Smooth Muscle

- Involuntary muscle found inside many internal organs of the body

Smooth Muscle: Locations

- Blood vessels, airways

- Digestive/Urinary/Reproductive Tract

Smooth Muscle: Contraction leads to what?

- Constriction → Vasoconstriction → ↑ BP, ↓ Blood flow

Smooth Muscle: Relaxation leads to what?

- Dilation → Vasodilation → ↓ BP, ↑ Blood Flow

Morphology of Smooth Muscle Cells

- Short, spindle-shaped (fusiform) and feature a single, centrally located nucleus

What happens to a smooth muscle cell during contraction?

- There is a network of "dense bodies:" in the cytoplasm that serve as attachment points for actin filaments, thick filaments overlapping thin in an irregular way

- Cell shortens and twists during contraction

Visceral Smooth Muscle

- Composed of sheets of several hundred spindle-shaped cells in close contact with one another in large bundles

Why does each visceral (single unit) smooth muscle bundle behave as a functional syncytium?

- Gap junction connect adjacent smooth muscle cells, allows ions/electrical signals to pass directly between them → cells contract together as one unit

Multiunit Smooth Muscle vs Visceral

- Visceral = Team → Gap Junctions → Contract together

- Multi Unit = Solo → Little/no gap junctions → Contract Independently

When does contraction of smooth muscle begin?

- When intracellular [Ca2+] ↑↑

Source of Ca2+ in smooth muscle

- Ca2+ from Extracellular fluid

What happens when Ca2+ is introduced to smooth muscle?

1. Membrane becomes depolarized → voltage-gated Ca2+ channels open

2. Ca2+ enters from ECF → Binds Calmodulin

3. Ca2+ binds Calmodulin → Activates Myosin Light Chain Kinase → Phosphorylates myosin → muscle contraction

What can trigger depolarization of smooth muscle cells?

- A host of different hormones or neurotransmitters

What is another source of calcium for smooth muscle cells?

- Sarcoplasmic reticulum

How is smooth muscle contraction regulated?

- Different from striated, as there is no troponin in smooth muscle

- Instead, relies on phosphorylation of myosin by MLCK

Roles of Myosin Light Chain Kinase and Myosin Light Chain Phosphatase

- MLCK: Phosphorylates myosin → smooth muscle contracts

- MLCP → Dephosphorylates myosin → not able to contract

What forms when smooth muscle is stimulated and intracellular Ca2+ increases?

- Formation of Ca2+ : Calmodulin complex → Activates MLCK → Myosin phosphorylated → smooth muscle contracts

What is an isometric contraction?

- Muscle creates tension, but does not change length → no joint movement

Ex: Just holding a dumbbell

What is Isotonic Contraction?

- Muscle changes length → movement at joints

What is Concentric Contraction?

- Muscle shortens because muscle force > resistance

Ex: Lifting a dumbbell during bicep curl

What is Eccentric Contraction?

- Muscle lengthens while contracting because resistance > muscle force

Ex: Lowering the dumbbell during bicep curl

NOT RELAXATION

How are muscles generally arranged?

- As muscle pairs to balance opposing joint movements

Agonist vs Antagonist Muscles

- Agonist: Recruited to contract and cause joint movements

- Antagonist: Muscles that oppose agonists motion

Bicep Curls: Agonist vs Antagonist

- Agonist: Biceps

- Antagonists: Triceps

Tricep Extension: Agonist vs Antagonist

- Agonist: Triceps

- Antagonist: Biceps

What happens if muscle pairs contract simultaneously and equally?

- Isometric contraction → no joint movement

Muscle Stabilizer

- Helps keep your joint stable so that your agonist muscles can carry out the movement in the desired path of motion

Muscle Stabilizer: Examples

- Rotator cuff muscles

- Core abdominal muscles

- Gluteus medium

Muscle Fixator

A muscle that serves as a stabilizer of one part of the body during movement of another part.

Proprioception

- The ability to tell where one's body is in space.

Muscle Spindles: What does it detect? What does it do?

- Detects: How much and how fast a muscle is stretched

- Function: Makes the muscle contract to prevent over stretching

Stretch Sensor

Golgi Tendon Organs: What does it detect? What does it do?

- Detect: How much force/tension muscle is producing

- Function: Makes the muscle relax if the tension is too high to prevent injury.

What is a lever system?

- A lever system is a way your muscles use your bones and joints to create movement.

Like a seesaw

Lever Terminology: Fulcrum, Lever, Effort, Load

- Fulcrum: Joint (Pivot)

- Lever: Bone

- Effort: Muscle (pulling force)

- Load: Body part or object to be moved

First-Class Lever

- The fulcrum is positioned between the effort and resistance

- Atlanto-Occipital

Second-Class Lever

- The load is between the fulcrum and the effort

- Standing on tiptoes

Third-Class Lever

- The fulcrum is at one end of the bar and the effort is between the fulcrum and the resistance

- Bicep curls

Muscle Force Forumla

Fm = F(resistiance) x d(resistance)/ d(muscle)

Fresistance

- Resistance/external load to be moved

dmuscle

- The point of muscle insertion and distance from the joint center

ddistance

- Length of the lever arm

Sarcopenia

- The loss of muscle mass, strength, and function that comes with aging