Muscles
3 Types of Muscle
Skeletal Muscle  Reacts to nervous system stimulation
Cardiac Muscle  Can respond to nervous system
stimulation or independent
Smooth Muscle  Responds to a variety of mechanisms
No Striations
Striations 
Intercalated Disk 
Skeletal Muscles & Bones
More than 600 skeletal muscles
in the body
Usually arranged in pairs
Some work together to promote the
same movement (synergistically)
Others work in opposition
(anatagonistically) or reverse
the movement of each other
Example of Antagonism
Biceps and Triceps
When biceps contract, triceps relax
When biceps relax, triceps contract
Muscle Contraction & Bones
Origin  Bone of attachment that stays motionless
during movement
Insertion  Bone of attachment that moves during
contraction
Example
Biceps Brachii (contracted)
Insertion is the radius
Origin is the scapula
Muscle Fiber Structure & Function
•Sarcomere  functional unit of a muscle fiber
•Each end of the sarcomere is defined by a Z-line
•Thick Filaments are called myosin
•Thin filaments are called Actin
Muscle Contraction
•Skeletal muscles always pull on bones, never push
•Nerve impulse stimulates the muscle fiber to contract
•Thick myosin filaments grab and pull the thin actin filaments closer
together, shortening the muscle
•The point where myosin “grabs” actin filament is called a cross bridge
•ATP is split by an enzyme to form ADP
•This reaction creates a “high energy” myosin that binds (and pulls) actin
•A single contraction consists of multiple pulls of myosin
•After death, no ATP, myosin stays “stuck” to actin, rigor mortis sets in
Muscle Contraction
Muscle Contraction Animation
Skeletal Muscle Types & Exercise
2 Types of Skeletal Muscle
Slow Muscle Fibers (Red)  Cells in this muscle are packed with iron-rich
proteins that bind to oxygen. Lots of blood vessels attach to slow muscle
fibers. These fibers contract slowly, but can hold the contraction for long
periods of time
Examples: Muscles of the legs and back
Fast Muscle Fibers (White)  Fewer blood vessels; relatively small
amount of mitochondria. These muscles contract very quickly; but
cannot contract for long periods of time
Example  Muscles of the hands
Exercise
Benefits of Exercise for Muscles
Increases the size and number of mitochondria
Increases the diameter and number of blood
vessels supplying the muscle
Increases the amount of myoglobin (which binds to
hemoglobin)
Muscle Contraction & Activation
Muscle contraction can be studied by removing the calf
leg from a frog and attaching it to an electrical
physiograph which initiates electrical currents through
the muscle
Single muscle fibers have an all or none approach to
electrical stimulation
A weak electrical stimulus will likely have no effect on
the fiber
When the stimulus reaches a threshold, the single fiber
contracts completely
The intensity of a muscle contraction is a result of the
number of muscle fibers shortening
Muscle Twitch
A muscle fiber can be given an instant burst of
electrical stimulation
This causes the muscle fiber to contract and
immediately relax
This action is called a muscle twitch
Muscle Twitch
Components of a Muscle Twitch
Contraction Period  period when the
muscle fiber is actively shortening
Relaxation Period  Period when the
muscle fiber is lengthening
Latent Period  Period when the muscle
fiber is recovering (refractory period)
If a muscle fiber is exposed to 2 threshold
stimuli within a very short period of time, it
will respond to the first but not the second
stimulus
Summation & Tetanus
If a muscle fiber is given a rapid series of threshold
stimuli before the muscle relaxes, muscle maintains
constant tension, called summation, or tetanus
Most muscles rarely undergo tetanus because some
fibers are contracting while others are relaxing
Muscle Activation
Muscles are highly innervated by nerve fibers
Brain initiates electrical impulse
Electrical impulses are transmitted through axons
Axon carries the impulse away from the CNS to the muscle fiber
At the end of each axon is a bulb, which contains little packages called
synaptic vesicles
Inside the vesicles is a neurotransmitter called acetylcholine
Acetylcholine empties into the muscle fiber, and stimulates the release of
calcium from storage sacs in the muscle itself
The calcium attaches to actin filaments and allows for the binding of
myosin
Myosin then pulls on the actin filaments, causing contraction
Muscle Activation
Muscle
Muscle Activation
Neuron Animation
Muscle Movements
Flexion: moving a part so that the angle at the joint is decreasing
Example: Bending elbow up
Extension: moving a part so that the angle at the joint is increasing
Example: straightening the arm from a bent elbow
Abduction: moving a bone away from the midline of the body
Example: moving the arms out from the body (from the supine position)
Adduction: moving a bone toward the midline of the body
Example: moving the arm back down
Rotation: Movement of a bone along the axis
Examples: Lateral, Medial, Axial
Muscle Movements
Circumduction: A combination movement that involves flexion/
extension and adduction/abduction when a bone moves in a circle
around an axis
Example: moving the whole arm in a circle; moving the whole leg in a
circle
Elevation: Movement of bone upward
Example: Movement of the shoulder up
Depression: Movement of bone downward
Example: movement of shoulder down
Muscle Movements
Protraction: Pushing out from the torso (shoulder and jaw)
Retraction: pulling back towards the torso (shoulder jaw)
Dorsiflexion: bending the foot upward
Example: walking on the heels
Plantarflexion: bending the foor down toward the surface
Example: pointing toes
Eversion: Movement of the soles of the feet outward so they are facing
away from each other
Inversion: movement of the soles of the feet so they are facing toward
each other
Opposition: movement of the thumb toward the pinkie
Pronation: movement of the palm so that the palm is facing down
Supination: movement of the palm so that the palm is facing up