BI-151 (MUSCULAR SYSTEM)

Muscular System Study Topics

4 Main Functions of the Muscular System

1. Producing body movement (locomotion)

2. Stabilizing body positions

3. Storing and moving substance within the body

4. Generating heat

3 Types of Muscle Tissue (functions, location, characteristics)

1. Skeletal Muscle

   • Striated, multi-nucleated (cylindrical & parallel, long)

   • Moves along skeleton, responsible for locomotion & heat production (voluntary muscle)

   • Attaches to bone & sometimes skin, found in facial muscles and voluntary sphincters as well

2. Cardiac Muscle

   • Striated, uni-nucleated (short, bifurcated, intercalated discs between cells)

   • Involuntary contraction & relaxation, pumps blood into the heart

   • Found in the myocardium/heart wall

3. Smooth Muscle

   • Non-striated, uni-nucleated (fusiform, short)

   • Involuntary muscle movements, moving materials through organs

   • Found in walls of hollow internal organs (vessels, airways, stomach, bladder, uterus)

3 Layers of Skeletal Muscle

1. Epimysium

   • Outer-most layer wrapping the entire muscle organ

   • Allows the muscle to contract & move without losing structural integrity

2. Perimysium

   • Connective tissue that separates into fascicles

   • Peri = allows nervous system to trigger specific movements via activating fascicles

3. Endomysium

   • Inner-most layer, muscle fibers encased within collagen & reticular fibers

   • Endo = contains extracellular fluid & nutrients to support the muscle fibers

Sarcolemma vs. Sarcoplasm vs. Sarcomere

1. Sarcolemma

   • The plasma membrane of a myocyte

2. Sarcoplasm

   • The cytoplasm of a myocyte

3. Sarcomere

   • The functional contractile unit of a myofibril (bundles of protein filaments, specialized contractile organelles)

Contractile vs. Regulatory vs. Structural Proteins

CONTRACTILE

1. Myosin (thick filaments)

   • Motor proteins that convert chemical energy into mechanical energy

   • Found in A-Bands within the sarcomere

2. Actin (thin filaments)

   • Globular protein involved in cell division, motility, and cell signaling

   • Found in A and I-Bands within the sarcomere

REGULATORY

1. Troponin

   • Calcium-regulatory protein that promotes muscle contraction; has three types: C, I, T

   • C: binds to calcium; I: inhibits myosin and actin interaction; T: binds to tropomyosin

2. Tropomyosin

   • Protein that prevents muscle contraction via blocking myosin-binding sites

STRUCTURAL

• Link myofibrils to the sarcolemma

Motor Unit vs. Motor Neuron

A motor neuron is a nerve cell that controls muscle fibers, while a motor unit is a term used to describe a motor neuron and all of the muscle fibers it innervates.

• Fine Control / Movement: few fibers, one motor neuron (more accuracy, less power)

• Gross Control / Movement: ~hundreds of fibers (more force & movement, less precision)

EXAMPLES

• Writing involves fine motor skills (precision, less force)

• Waving your arm involves gross motor skills (large, sweeping movements)

Neuromuscular Junction

Where the neuron meets skeletal muscle fiber

Motor Endplate

A specific site on skeletal muscle fiber that contributes to the NMJ

• Embedded in the motor endplate sarcolemma are ligand-activated Na+/K+ gates

Steps Involved in Muscular Contraction

1. Motor neuron sends signal via motor endplate on NMJ (action potential), triggers the release of ACh

2. ACh triggers the release of another action potential along the sarcolemma, resulting in the release of Sodium ions

3. Depolarization causes the release of Calcium ions in the SR

4. Calcium ions bind to troponin, pulling tropomyosin out from the myosin-binding sites on actin

5. ATP Hydrolysis: ATP binds to myosin heads, acting as an ATP-ase

6. Myosin binds to actin, forming a cross-bridge formation

7. Power Stroke: myosin heads pull actin towards the center of the sarcomere, momentarily detaching then reattaching, resulting in muscle shortening and contraction

8. As Calcium ions get pumped back into the SR, troponin pushes tropomyosin back onto the myosin-binding sites

9. Muscle relaxes

Types of Muscle Contractions

• Isotonic: length of muscle changes as tension is maintained to move load

  • Eccentric: fibers lengthen to move load

  • Concentric: fibers shorten to move load

• Isometric: length of muscle does not change as tension is produced

How is ATP produced for Muscle Contraction?

ATP is produced through cellular respiration within the mitochondria

What affects contractile muscle force?

• Amount of muscle fibers

• Length of muscle

• Amount of sodium within the muscle

Origin/Insertion vs. Agonist/Antagonist

• Origin/Insertion: the fixed / stabilized end of muscle

• Agonist/Antagonist: the movable end of a muscle

The origin / agonist point of a muscle is always susceptible to change depending on what moves and where

Ways muscles can be named

• Location (attachment on body)

• Shape or size

• Direction of muscle fibers

• # of origins

• # of muscles involved / function

• Origin