Muscle Tissue and the Muscular System: Structure, Function, and Mechanics

Muscle Tissue

Types and Functions of Muscle Tissue

Muscle tissue is a specialized tissue that contracts to produce movement. There are three major types of muscle tissue, each with distinct structure and function:

• Skeletal Muscle – voluntary, striated, attached to bones; responsible for body movement.

• Cardiac Muscle – involuntary, striated, only in heart; pumps blood throughout the body.

• Smooth Muscle – involuntary, non-striated, walls of hollow organs; propels substances such as food, urine, and blood.

Shared Muscle Characteristics

All muscle tissues share certain properties that enable their function:

Property	Meaning	Example
Excitability	Responds to stimuli	Nerves triggering contraction
Contractility	Can shorten forcefully	Lifting weight
Extensibility	Can stretch	Stretching hamstring
Elasticity	Returns to original length	Recoil after stretch

Primary Muscle Functions

• Movement of body and substances

• Maintaining posture

• Joint stabilization

• Heat production (shivering increases body temperature)

Skeletal Muscle Anatomy

• Connective Tissue Layers:

  • Epimysium – surrounds whole muscle

  • Perimysium – surrounds fascicles (bundles of fibers)

  • Endomysium – surrounds individual muscle fibers

Muscle Fiber Microanatomy

• Sarcolemma – muscle cell membrane

• Sarcoplasm – cytoplasm with glycogen and myoglobin

• Myofibrils – contractile organelles (composed of actin and myosin)

• T-tubules – carry electrical impulses deep into fiber

• Sarcoplasmic Reticulum (SR) – stores and releases Ca2+ for contraction

Muscle Contraction

Muscle Fiber Structure: Sarcomere Bands

Band/Line	Contents	Notes
Z-disc	Anchors actin	End of sarcomere
A band	Thick (myosin) ± thin overlap	Dark
I band	Thin only	Light
H zone	Thick only	Disappears with contraction
M line	Anchors myosin	Middle of sarcomere

Sliding Filament Theory

• Actin slides toward the M-line, shortening the sarcomere.

• Myosin heads form cross-bridges and pull actin filaments inward.

Muscle Contraction Steps

1. Neural Activation at Neuromuscular Junction

   • Action potential (AP) arrives at terminal.

   • Voltage-gated calcium channels open; Ca2+ enters motor neuron.

   • Ca2+ triggers release of acetylcholine (ACh) into synaptic cleft.

   • ACh binds to ACh receptors on sarcolemma, opening Na+ channels and generating AP in muscle fiber.

   • ACh is degraded by acetylcholinesterase.

2. Muscle Fiber Excitation

   • AP travels along sarcolemma and T-tubules.

   • Voltage-gated Na+ and K+ channels propagate AP.

3. Excitation-Contraction (EC) Coupling

   • AP triggers Ca2+ release from SR.

   • Ca2+ binds to troponin, exposing binding sites on actin.

4. Cross-Bridge Cycle (Requires ATP and Ca2+)

   1. Myosin binds to actin.

   2. Power stroke pulls thin filaments.

   3. ATP attaches, myosin detaches.

   4. ATP hydrolysis "re-cocks" myosin head.

ATP Regeneration Pathways

Muscles require ATP for contraction. Stored ATP lasts only 4–6 seconds, so regeneration is essential:

Pathway	Oxygen?	Speed	Duration	Notes
Creatine Phosphate	No	Fast	10–15 sec	CP donates phosphate to ADP; instant ATP
Anaerobic Glycolysis	No	Very fast	~1 min	Glucose → lactate; used when O2 limited
Aerobic Respiration	Yes	Slow	Hours	Uses glucose, then fats; main long-duration source

Muscle Fatigue

• Physiological inability to contract.

• Caused by ionic imbalances or SR damage—not usually lack of ATP.

• EPOC (Excess Post-Exercise Oxygen Consumption): Needed to restore oxygen stores, ATP, CP, and glycogen.

Muscle Fiber Types

Type	Contraction	Metabolism	Fatigue?	Best For
Slow Oxidative	Slow	Aerobic	Highly fatigue-resistant	Posture, distance running
Fast Oxidative	Fast	Aerobic	Moderate	Sprinting
Fast Glycolytic	Fast	Anaerobic	Fatigues quickly	Power lifting, jumping

Whole Muscle Mechanics

Motor Units

• One motor neuron and all muscle fibers it controls.

• Small units = fine control; large units = strength.

• Graded contractions:

  1. Frequency of stimulation (twitch → summation → tetanus)

  2. Recruitment (more motor units = more force)

Types of Contraction

Type	Length Change?	Example
Isometric	No change	Holding a weight still
Isotonic Concentric	Shortens	Curling dumbbell up
Isotonic Eccentric	Lengthens	Lowering dumbbell slowly

Smooth Muscle

Smooth Muscle Characteristics

Feature	Smooth Muscle Characteristics
Control	Involuntary (ANS)
Appearance	No striations, spindle-shaped cells
Contraction	Slow, sustained, very energy-efficient
Regulator Protein	Uses Calmodulin (not troponin)
Behavior	Can stretch greatly without losing function (e.g., bladder)

The Muscular System

Naming and Grouping Skeletal Muscles

• Muscles can be named by:

  • Location (e.g., temporalis)

  • Shape (e.g., deltoid = triangle)

  • Size (maximus, minimus, longus)

  • Fiber direction (rectus = straight, oblique = angled)

  • Origin count (biceps = 2)

  • Attachments

  • Action (flexor, extensor, etc.)

• Four main functional groups:

  • Prime movers (agonists): Provide major force

  • Antagonists: Oppose/reverse a movement

  • Synergists: Add force to movement (assist prime mover)

  • Fixators: Immobilize/stabilize origin

Muscle Mechanics – Lever Systems

• Skeletal muscles move bones by using leverage.

• Levers have:

  • Lever – the bone

  • Fulcrum – the joint (pivot point)

  • Effort – force from muscle contraction

  • Load – the weight/resistance being moved

• Mechanical Advantage: Effort farther from fulcrum than load ("power lever").

• Mechanical Disadvantage: Effort closer to fulcrum than load ("speed lever").

Class	Arrangement	Example	Function
1st Class	Fulcrum between load & effort	Seesaw, nodding head	Can be strength or speed
2nd Class	Load between fulcrum & effort	Wheelbarrow, standing on tiptoe	Mechanical advantage (strength)
3rd Class	Effort between fulcrum & load	Tweezers, most skeletal muscles	Mechanical disadvantage (speed & range); most common in body

The body usually prioritizes speed and movement range (3rd-class levers), allowing quick and precise limb movement rather than maximum strength.