Chapter 6: Muscular System

Popeye

Functions

• Essential function of muscle is contraction or shortening.

• Movement – muscle contractions lead to movement of skeleton

• Posture – partial contractions• Stabilizing Joints • Heat Production – catabolism produces heat,

leading to homeostasis of body temperature

Types of Muscles

• Smooth muscle – no striations, involuntary; abdominal organs

• Cardiac muscle – intercalated disks, striations, involuntary; heart

• Skeletal muscle – striations, voluntary; biceps, deltoid

Muscle Cells (Fibers)

• Skeletal and smooth muscle cells are elongated

• These are called fibers.• New terms for cells of muscle

fibers:• Sarcolemma – cell membrane• Sarcoplasm – cytoplasm• Sarcoplasmic reticulum –

same as ER but contains high levels of calcium

Muscle Fibers• Each skeletal muscle fiber (cell) is

enclosed in a connective tissue layer called endomysium.

• Several sheathed muscle fibers are then wrapped by a fibrous membrane called a perimysium in bundles called fascicles.

• Groups of fascicles are bound together and wrapped in connective tissue called epimysium.

• These muscles are then attached to bones by tendons or aponeuroses.

The Man Whose Arms Exploded

• Connective tissues gave way because of extreme size

• Nothing to keep muscle fibers bundled

Make-up of Fibers• Myofibrils – fine fibers packed

close together• Alternating light (I) and dark (A)

bands along the length of myofibril give striated appearance.

• Tiny contractile units of muscle fiber called sarcomeres.

• Myofilaments – structural units within sarcomere

• 2 types of myofilaments• Actin – thin protein

• Myosin – thick protein

The Sarcomere

Parts of the Sarcomere

• Z-Disc: separates each sarcomere

• H-zone - space between each actin molecule

• A-band – region of myosin (thick)

• I-band – only actin (thin)• M-line – holds thick

filaments together• Cross-bridges – “arms” on

myosin that attach to binding sites on actin

Skeletal Muscle Activity• To contract, skeletal

muscle cells are stimulated by nerve impulses sent by brain.

• One neuron + muscle cells it stimulates = motor unit.

• Impulse travels along nerve on long extensions of neuron are called axons.

• When they reach muscle they branch into axon terminals which join with sarcolemma.

Skeletal Muscle Activity• These junctions are called neuromuscular junctions.• The nerve endings and muscle never touch – this gap is

called the synaptic cleft.• When nerve impulse reaches axon terminal, a

neurotransmitter is released from neuron.• This neurotransmitter is called acetylcholine, or ACh.

Skeletal Muscle Activity• ACh diffuses across synaptic cleft and attaches to receptors

on muscle cell.• Sacrolemma becomes temporarily more permeable to Na+

ions than to K+ ions (Na+ in / K+ out).• This gives cell interior an excess of positive ions and

changes electrical conditions of cell.• Creates electrical current called an action potential (AP).• Electrical impulse travels across muscle cell, leading to

contraction of the cell.• Cell returned to resting state by sodium-potassium pump• Acetylcholinesterase (AChE), an enzyme, breaks down

Acetylcholine to prevent repeated stimulation (one impulse = one contraction)

Neuromuscular Junction

Neuromuscular Junction• Action potential travels down Transverse

Tubules (T Tubules)• AP reaches SR, Ca2+ ions are released into

sarcoplasm• In a relaxed cell, a protein, tropomyosin,

covers the binding site of myosin.• Ca2+ binds to troponin, causing tropomyosin to

shift, allowing myosin to bind to actin• Ca2+ ions trigger binding of myosin to actin to

initiate filament sliding.

Mechanism of Muscle Contraction• When activated, myosin heads

attach to binding sites on thin filaments and sliding begins.

• Each cross-bridge attaches and detaches creating tension that pulls the thin filaments towards the center of sarcomere

• This leads to cell shortening, or contractions.

• A) Relaxed sarcomere• B) Fully contracted

• H Zone disappeared• Z discs closer • I bands nearly gone

• Sodium Potassium Pump - Frogs

Sliding Filament Theory

Sliding Filament Theory

Contractions

• “All or None” Muscle Law – a muscle cell will contract to its fullest extent; never partially

• However, entire muscle has graded response– Frequency of nerve impulses– Number of muscle cells stimulated at once

• Rigor mortis – Muscle fibers run out of ATP and SR cannot pump Ca2+ ions out of sarcoplasm. – Cross bridges cannot detach and skeletal muscles

become locked in contraction– Lasts until lysosomes can break down muscle fibers (15-

25 hours later)

Types of Contractions• Muscle twitches – single, brief, jerky contractions caused by single

impulse• Complete tetanus – smooth, sustained contraction• Isotonic contraction – muscle shortens, tension remains same; leads to

movement• Isometric contraction – muscle remains the same, and tension

increases; no movement

Fast and Slow Muscles

• Speed of contraction is related to muscle’s specific function

• Red (Type I)muscle is slow twitch, can generate ATP quickly and contract for long periods of time

• Red muscles have many mitochondria for ATP

• Endurance

Fast and Slow Muscles

• White (Type IIb) muscles are fast twitch, can not make ATP quickly, and fatigue relatively rapidly

• White muscles have few mitochondria

• Strength

Naming Muscles

• Action performed• Direction of fibers• Location• Number of divisions• Shape• Point of attachement

• Pronator teres• Transverse abdominis• External oblique• Bicep• Deltoid• Sternocleidomastoid