Muscular System
Muscles – organs composed of specialized
cells that use the chemical energy stored in
nutrients to contract.
Three Types of Muscles
 Skeletal
 Smooth
 Cardiac
Structure of Skeletal Muscle
 Connective Tissue Coverings
o Fascia – layer of dense connective
tissue that surrounds and separates
each muscle
o Tendons – attach muscle to bone
o Epimysium – surrounds each whole
muscle
o Perimysium – separate muscle cells
into small compartments called
fascicles
o Endomysium – layer of connective
tissue in which each muscle fiber in
a fascicle lies
 Skeletal Muscle Fibers
o Single, long, cylindrical muscle cells
o Contract in response to stimulation
and relaxes when the stimulation
ends
o Cytoplasm contains myofibrils that
play a role in muscle contraction
(made of actin and myosin) –
produce striations
o Sarcoplasmic and transverse tubules
(membranous channels) activate
muscle contraction when the fiber is
stimulated
 Neuromuscular junctions
o Site where a motor neuron (axon
from nerve cell) and muscle fiber
meet
o Nerve impulse sent from brain –
reaches motor neuron end – releases
neurotransmitters (chemicals) – this
action stimulates the muscle fiber to
contract
 Motor Unit – a motor neuron and all of
the muscle fibers it controls
Energy Sources for Contraction
 ATP supplies the energy for muscle fiber
contraction – limited supply – must be
regenerated
 Creatine phosphate – molecule that
stores excess energy – regenerates ATP
from ADP and phosphate
Oxygen supply
 Pigment called myoglobin stores oxygen
in muscle tissue
 Oxygen deficiency – lactic acid build up
Muscle Fatigue
 When a muscle loses its ability to
contract during strenuous activity
 Causes – accumulation of lactic acid –
lowers pH which prevents muscles from
contracting
Muscle Responses
 Threshold stimulus – minimal strength
required to cause a contraction
 All or none response – muscles do not
contract partially
Recording a Muscle Contraction
 Myogram – recording of an
electronically stimulated muscle
contraction
 Twitch – single contraction that lasts
only a fraction of a second
 Latent period – delay between the time
the stimulus was applied and the time
the muscle responded
Sustained Contractions
 Muscle tone – response to nerve
impulses that originate repeatedly from
the spinal cord and stimulate a few
muscle fibers (ex. Posture)
Smooth Muscles
 Elongated with tapering ends – lack
striations
 Two types
1.Multiunit Smooth muscle – fibers
occur separately rather than in sheets
(ex. Blood vessels and iris of the eye)
2.Visceral Smooth muscle – occurs in
sheets – these fibers can stimulate one
another – display rhythmicity (ex.
Peristalsis of intestines)
Smooth Muscle Contraction
 Slower to contract and relax than
skeletal muscle
 Can maintain a forceful contraction
longer with a given amount of ATP
 Two neurotransmitters and hormones
responsible for contractions
Cardiac Muscle
 Found only in the heart
 Self exciting and rhythmic
 Extra calcium is supplied to allow for
longer contractions
Skeletal Muscle Actions
 Origin – immovable end of muscle
 Insertion – movable ends
 Prime mover (agonist) – muscle doing
the majority of the work
 Synergists – helper muscles
 Antagonists – opposing muscles
Major Skeletal Muscles
 Named according to size, shape,
location, action, number of attachments,
or direction of fibers.
 See Table