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 (1525 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
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Action performed
Direction of fibers
Location
Number of divisions
Shape
Point of attachement
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Pronator teres
Transverse abdominis
External oblique
Bicep
Deltoid
Sternocleidomastoid