Animal Locomotion
What are the advantages of locomotion?
Lots of ways to get around…
Lots of ways to get around…
Lots of ways to get around…
Muscle 
Organization of Skeletal muscle
Human 
endoskeleton
Muscles movement 
•	Muscles do work by contracting
•	skeletal muscles come in 
antagonistic pairs
•	flexor vs. extensor
•	contracting = shortening
•	move skeletal parts
•	tendons
•	connect bone to muscle
•	ligaments
•	connect bone to bone

Structure of striated skeletal muscle 
•	Muscle Fiber
•	muscle cell
•	divided into sections = sarcomeres
•	Sarcomere
•	functional unit of muscle contraction 
•	alternating bands of 
thin (actin) & thick (myosin) protein filaments
Muscle filaments & Sarcomere 
•	Interacting proteins
•	thin filaments
•	braided strands 
•	actin
•	tropomyosin
•	troponin
•	thick filaments
•	myosin
Thin filaments: actin
•	Proteins
•	braid of actin molecules & tropomyosin fibers
•	tropomyosin fibers dotted with troponin molecules
Thick filaments: myosin
•	Protein
•	myosin molecule
•	long protein with globular head
Thick & thin filaments
•	Myosin tails aligned together & heads pointed away from center of sarcomere
Interaction of thick & thin filaments
•	Cross bridges 
•	connections formed between myosin heads (thick filaments) & actin (thin filaments) 
•	cause the muscle to shorten (contract) 
Where is ATP needed?
Cleaving ATP  ADP allows myosin head to bind to actin filament
Shortening sarcomere
•	Myosin pulls actin chain along toward center of sarcomere
•	Sarcomere shortens (Z lines move closer together)
•	Muscle contracts
•	energy from:
•	ATP
•	glycogen 

Closer look at muscle cell
Muscle cell organelles
•	Sarcoplasm 
•	muscle cell cytoplasm
•	contains many mitochondria
•	Sarcoplasmic reticulum (SR)
•	organelle similar to ER
•	network of tubes
•	stores Ca+2
•	Ca+2 released from SR through channels
•	Ca+2 pumps then restore Ca+2 to SR
•	remove Ca+2 from cytosol
•	pumps use ATP
Muscle at rest
•	Interacting proteins
•	at rest, troponin molecules hold tropomyosin fibers so that they cover the myosin-binding sites on actin
The Trigger: motor neurons 
•	Motor neuron triggers muscle contraction
Nerve trigger of muscle action
•	Nerve signal stimulates muscle cell’s sarcoplasmic reticulum (SR) to 
release stored Ca+2
Ca+2 triggers muscle action
•	At rest, tropomyosin blocks myosin-binding sites on actin
•	Ca+2 binds to 
troponin complex 
•	shape change 
causes movement 
of tropomyosin-
troponin complex
•	exposes myosin-binding sites 
on actin
How Ca+2 controls muscle
•	Sliding filament model
•	exposed actin binds to myosin
•	fibers slide past each other
•	ratchet system
•	shorten muscle cell
•	muscle contraction
•	muscle doesn’t relax until Ca+2 is pumped back into SR 
•	requires ATP

How it all works…
•	Action potential causes Ca+2 release from SR
•	Ca+2 binds to troponin
•	Troponin moves tropomyosin uncovering myosin binding site on actin
•	Myosin binds actin
•	uses ATP to "ratchet" each time
•	releases, "unratchets" & binds to next actin
•	Myosin pulls actin chain along
•	Sarcomere shortens 
•	Z discs move closer together
•	Whole fiber shortens  contraction!
•	Ca+2 pumps restore Ca+2 to SR  relaxation!
•	pumps use ATP
Fast twitch & slow twitch muscles
•	Slow twitch muscle fibers
•	contract slowly, but keep going for a long time
•	more mitochondria for aerobic respiration 
•	less SR  Ca+2 remains in cytosol longer
•	long distance runner
•	“dark” meat = more blood vessels 
•	Fast twitch muscle fibers
•	contract quickly, but get tired rapidly
•	store more glycogen for anaerobic respiration 
•	sprinter
•	“white” meat
Muscle limits
•	Muscle fatigue
•	lack of sugar
•	lack of ATP to restore Ca+2 gradient
•	low O2
•	lactic acid drops pH which 
interferes with protein function
•	synaptic fatigue
•	loss of acetylcholine
•	Muscle cramps
•	build up of lactic acid 
•	ATP depletion
•	ion imbalance
•	massage or stretching 
increases circulation
Diseases of Muscle tissue
•	ALS
•	amyotrophic lateral sclerosis
•	Lou Gehrig’s disease
•	motor neurons degenerate
•	Myasthenia gravis
•	auto-immune
•	antibodies to 
acetylcholine 
receptors
Botox
•	Bacteria Clostridium botulinum toxin
•	blocks release of acetylcholine
•	botulism can be fatal
Rigor mortis