The Muscular System Overview

Introduction to the Muscular System

• Key Functions:

  • Physiological processes

  • Dynamic interactions with the environment

• Skeletal Muscles:

  • Composed of elongated cells known as muscle fibers

  • Fibers contract along the longitudinal axis

Types of Muscle Tissue

• Skeletal Muscle:

  • Voluntary contractions

  • Responsible for pulling on bones

• Cardiac Muscle:

  • Involuntary contractions

  • Pumps blood through the circulatory system

• Smooth Muscle:

  • Involuntary contractions

  • Moves fluids and solids in organs (e.g., digestive tract)

Properties of Muscle Tissues

• Excitability:

  • Responds to stimuli

• Contractility:

  • Can shorten and exert pull or tension

• Extensibility:

  • Can contract over a range of resting lengths

• Elasticity:

  • Can rebound to original length after contraction

Functions of Skeletal Muscles

• Produce Skeletal Movement:

  • Contracts and pulls on tendons connected to bones

• Maintain Posture and Body Position:

  • Stabilizes joints to support posture

• Support Soft Tissue:

  • Supports visceral organs' weight

• Regulate Material Entry/Exit:

  • Voluntary control in digestive and urinary systems

• Maintain Body Temperature:

  • Energy from contraction converted to heat

Anatomy of Skeletal Muscles

Gross Anatomy

• Overview:

  • Connective tissues, nerves, and blood vessels associated with muscles

• Connective Tissues:

  • Epimysium: Surrounds the entire muscle

  • Perimysium: Divides muscle into fascicles

  • Endomysium: Surrounds individual muscle fibers

  • Tendons: Connect muscle to bone; aponeuroses connect muscle to muscle

• Nerve & Blood Vessels:

  • Nerves penetrate epimysium for chemical communication at neuromuscular junctions

  • Blood vessels supply muscles and accommodate movement

Microanatomy of Skeletal Muscle Fibers

• Key Components:

  • Sarcolemma: Membrane around muscle cells

  • Sarcoplasm: Cytosol of the muscle cell

  • Myofibrils: Structures responsible for contraction, consists of thick (myosin) and thin (actin) filament organize into sarcomeres

• Sarcomeres:

  • Basic functional unit of muscle fibers

  • Each has Z lines, A bands (actin & myosin), I bands (only actin), and H band (only myosin)

Muscle Contraction

Sliding Filament Theory

• Mechanism of Contraction:

  • Thick and thin filaments slide over each other to shorten the muscle

  • Requires ATP and calcium ions

  • Cross-bridge cycling: binding, pivoting, detachment of myosin heads on actin

• Process Overview:

  • Calcium binds to troponin, altering tropomyosin's position to expose actin's active sites

  • Myosin heads attach, pulling actin, causing muscle contraction

Neural Control of Muscle Contraction

• Neuromuscular Junction:

  • Axon terminal releases acetylcholine (ACh) to activate muscle contraction

  • ACh fragments are cleared by acetylcholinesterase to end contraction

Muscle Units and Control

Motor Units

• Defined as a motor neuron and the muscle fibers it controls

• Types of Control:

  • Precise control: Few fibers (e.g., eye muscles)

  • Less precise: Many fibers (e.g., leg muscles)

Muscle Tone and Adaptation

• Muscle Tone:

  • Constant tension in muscles at rest, providing stability

• Hypertrophy and Atrophy:

  • Hypertrophy: increase in muscle size due to exercise

  • Atrophy: decrease in muscle mass from disuse

Types of Skeletal Muscle Fibers

• Fast Fibers:

  • White fibers, rapid contraction, fatigue quickly

• Slow Fibers:

  • Red fibers, slower contractions, fatigue-resistant, high myoglobin

• Intermediate Fibers:

  • Characteristics between fast and slow fibers

Organization of Skeletal Muscle Fibers

• By Fiber Arrangement:

  • Parallel, convergent, pennate (unipennate, bipennate, multipennate), circular

  • Fiber arrangement influences the force and range of motion

Muscle Terminology

Origins and Insertions

• Origin: Non-movable attachment

• Insertion: Movable attachment

Muscle Actions

• Prime Movers: Primary muscle for a movement

• Antagonists: Opposes action of prime mover

• Synergists: Assist prime movers

• Fixators: Stabilize joints during movement

Muscle Names

• Named based on:

  • Location, shape, orientation, size, function, or origin/insertion points

Levers and Pulleys in Muscles

• Structure of Levers:

  • Fulcrum, load, applied force determines movement made by muscles

• Classes of Levers:

  • First (fulcrum in middle), Second (load in middle), Third (force in middle)

Aging and the Muscular System

• Age-Related Changes:

  • Muscle fiber size diminution, reduction in elasticity, increase in fatigue, and rehabilitation challenges

Axial Musculature

Muscles of the Head and Neck

• Major Groups Include:

  • Muscles of facial expression, extra-ocular muscles, mastication, tongue, and pharynx

  • Each group has specific innervations (e.g., CN VII for facial expression)

Muscles of the Vertebral Column

• Layers:

  • Superficial: move neck

  • Intermediate: move vertebral column

  • Deep: interconnect vertebrae

Muscles of the Pelvic Diaphragm

• Components:

  • Urogenital and anal triangles with associated muscles (e.g., levator ani, coccygeus)

Appendicular Musculature

Groups of Appendicular Muscles

• Main Functions:

  • Stabilize and move upper/lower limbs

• Upper Limbs:

  • Muscles that position the pectoral girdle, and move arms and hands.

• Lower Limbs:

  • Muscles that move the thighs, legs, feet, and toes.