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Lecture 7
Muscular System
Muscle Types and Functions
 Types of Muscle
o Skeletal muscle – voluntary striated muscle that is usually attached to one
or more bones
 Long, thin, cylindrical fibers
 Packed with protein microfilaments that overlap each other
producing light and dark bands (striations)
 More than one nucleus per cell
o Cardiac muscle – heart muscle, involuntarily controlled
 Striated, but involuntary
 One nucleus per cell
o Smooth muscle – involuntarily controlled
 No striations
 One nucleus per cell
 Functions of Muscle
o Movement – Allow body to move from place to place and movement of
individual body parts
 Also moves body contents in course of respiration, circulation,
digestion, urination, and childbirth
o Stability – Muscles maintain posture for resisting the pull of gravity
o Communication – Muscles are used for facial expression, body language,
writing and speech
o Control of body openings and passages – Sphincter muscles around
eyelids, pupils, and mouth control admission of light, food, and drink into
the body; other sphincter muscles control movements out of the body
o Heat production- The skeletal muscles produce 85% of body heat
 Properties of Muscle
o Excitability – When stimulated, muscle cells respond with electrical
changes across the plasma membrane
o Conductivity – When the muscle is stimulated, electrical excitation is
conducted throughout the entire plasma membrane
o Contractility- Muscle fibers shorten lengthwise when stimulated, which
allows them to create movement
o Extensibility – Muscle cells can be stretched as much as three times their
contracted length.
o Elasticity – If a muscle is stretched, and then the tension is released, it can
recoil to its original resting length
General Anatomy of Muscles
 Connective Tissues and Fascicles
o Endomysium – a thin sleeve of connective tissue that surrounds each
muscle fiber (muscle cell)
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o Fascicles – bundles of muscle fibers, visible to the naked eye as parallel
strands
o Perimysium – connective tissue that surrounds fascicles
o Epimysium – a fibrous sheath that surrounds and entire skeletal muscle
o Fascia – connective tissue that separates muscles from one another (deep
fascia) or that separates muscles from the overlying skin
Fascicles and Muscle Shapes
o Fusiform muscles – muscles that are thick in the middle and tapered at
each end
 Moderately strong contractions
o Parallel muscles – muscles that are of uniform width and that have parallel
fascicles
 Span long distances, weaker than fusiform
o Convergent muscles – muscles that are fan shaped, with fibers meeting at
a single point
 Relatively strong because of the relatively small insertion
o Pennate muscles- feather shaped, with fascicles inserting obliquely on a
tendon that runs the length of the muscle (fibers meet at a line, rather than
a single point)
 Strong muscles such as rectus femoris
o Sphincteral muscles (Circular muscles) – form rings around body
openings
Muscle Attachments
o Origin – relatively stationary (and proximal) point of muscle attachment
o Insertion – more mobile (and distal) point of muscle attachment
o Belly – middle region between the origin and the insertion
Functional Groups of Muscles
o Prime mover (agonist) – muscle that produces most of the force during a
particular contraction
 Example: biceps brachii for flexion of the elbow
o Synergist – Muscle that aids the prime mover to produce more power than
a single muscle could
 Example: brachialis muscle assists the biceps brachii
o Antogonist – Muscle that opposes the prime mover
 It relaxes to give the prime mover almost complete control over an
action
 It may maintain some tension on a joint, limiting the speed or
range of the prime mover, preventing excessive movement and
joint injury
 Example: triceps brachii opposes the biceps brachii
o Fixator – Muscle that prevents a bone from moving
 Example: The fixator muscles of the scapula hold the scapula
firmly in place during flexion of the elbow
Intrinsic and Extrinsic Muscles
o Intrinsic muscles are entirely contained within a particular region, having
both its origin and insertion there
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o Extrinsic muscles act on a particular region, but has its origin elsewhere
 Some movements of the fingers are produced by extrinsic muscles
in the forearm
Muscles, Bones, and Levers
o Lever- any elongated, rigid object that rotates around a fixed point
o Fulcrum – fixed point around which a lever rotates
o Effort- force applied against inertia; something done through exertion
o Resistance- force that tends to oppose motion
o Effort arm- part from fulcrum to the point of effort
o Resistance arm- part from fulcrum to point of resistance
o Functions of a lever?
 Exert more force against a resisting object than the force applied to
the lever
 OR move the resisting object farther or faster than the effort arm is
moved
 There is a trade-off between force and speed or distance
o Mechanical advantage-Ratio of its output force to its input force
o Lever classes
 First-class lever – Fulcrum is between the effort and the resistance
 Second-class lever – Resistance is between the effort and the
fulcrum
 Third-class lever – Effort is between the fulcrum and the resistance
Microscopic Anatomy
 Ultrastructure of Muscle Fibers
o Sarcolemma – plasma membrane of a muscle cell
o Transverse tubules – tunnel-like infoldings of the sarcolemma that
penetrate through the cell membrane and carry an electrical current from
the surface of the cell to the interior when the cell is stimulated
o Sarcoplam- cell membrane of a muscle cell
o Myofibrils – long protein bundles about 1 micrometer in diameter
o Glycogen – provides stored energy for the muscle during exercise
o Myoglobin – binds oxygen until it is needed for muscular activity
o Sarcoplasmic reticulum – reservoir for calcium ions which are needed to
activate the muscle contraction process
o Myofilaments – parallel protein microfilaments that make up myofibrils
 2 main kinds
 Myosin myofilaments – thick – about 15 nm in diameter
 Actin myofilaments – thin – about 7 nm in diameter
o Striations and Sarcomeres
 Actin myofilaments run parallel to each other and are anchored at
one end to a Z line (or Z disc)
 Mysosin myofilaments run parallel to each other and fit between
actin myofilaments
 A sarcomere is the term for the segment that spans from one Z line
to the next Z line
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Because the sarcomere contains areas where there are only thin
filaments, areas where there are only thick filaments, and areas
with overlapping thick and thin filaments, there are dark areas and
light areas
 I bands are light (lIght) and contain only thin filaments
 A bands are dark (dArk) and contain thick filaments
(overlapping with thin filaments at the ends)
 H bands are found in the middle of the A band
o They are found where actin and myosin filaments
do no overlap (containing only thick filaments)
 Shortening
 The sarcomere shortens during muscle contraction, but the
filaments do not
o As the Z lines get closer together, actin and myosin
filaments overlap more and more.
o Because I bands are the areas consisting of areas where
thin filaments are not overlapped by thick filaments, the I
bands shorten during muscle contraction
o Because the H bands are the areas consisting of areas
where thick filaments are not overlapped by thin
filaments, the H bands shorten during muscle contraction
o Because the A bands extend from one end of the thick
filaments to the other end (overlapping with thin, or not),
and because the filaments themselves do not shorten, A
bands remain the same length
The Nerve-Muscle Relationship
o Skeletal muscles only contract when stimulated by neurons
o Motor neurons are nerve cells that lead from the central nervous system to
muscles (or glands)
 Each motor nerve fiber branches about 200 times at its distal end,
leading to different muscle fibers
 Each muscle fiber is innervated by only one motor neuron
o Synapse – a junction at the end of an axon where it stimulates another cell
o Neuromuscular junction – a synapse between a nerve fiber and a muscle
cell
o Synaptic knob – The swollen tip at the distal end of an axon; the site of
synaptic vesicles and neurotransmitter release
o Synaptic vesicles – A spheroid organelle in a synaptic knob; contains
neurotransmitter
o Neurotransmitter – A chemical released at the distal end of an axon that
stimulates an adjacent cell
o Motor end plate – A depression in a muscle fiber where it has synaptic
contact with a nerve fiber and has a high density of neurotransmitter
receptors
o Synaptic cleft – a narrow space between the synaptic knob of an axon and
the adjacent cell
 A neurotransmitter has to diffuse across this space
 Motor Unit – a motor neuron and all the skeletal muscles innervated by it
o Small motor units – where fine control is needed
 Very few muscle fibers per nerve fiber
 Provide fine degree of control needed for subtle movements
 Small neurons that are easily stimulatetd
o Large motor units
 Many muscle fibers per nerve fiber
 Much stronger, but lacking fine control
 Large neurons that are harder to stimulate
Functional Perspectives
 Muscle Growth and Atrophy
o Growth - Exercise stimulates muscle fibers to produce more protein
myofilaments
 Myofibrils, as a result, grow thicker
 At a certain point, a large myofibril splits longitudinally, so a wellconditioned muscle cell has more myofibrils
 Some scientists think that entire muscle cells may split, leading to
increased numbers of muscle cells (not just myofibrils)
o Atrophy – muscle shrinks if it is not used
 Disuse atrophy – typically seen when a limb is kept in a cast for
weeks
 Denervation atrophy – seen in cases where the spinal cord or nerve
connections to a muscle are damaged
 Senescence atrophy – occurs due to aging, regardless of exercise
 Physiological Classes of Muscle Fibers
o Slow-twitch
 Well adapted to aerobic respiration
 Fibers do not generate lactic acid or fatigue easily
o Fast-twitch
 Rich in enzymes for anaerobic fermentation
 Fibers do produce lactic acid and fatigue more easily
 Well adapted for quick responses but not for endurance
Cardiac and Smooth Muscle
 Cardiac Muscle
o Makes up most of the heart
o Striated like skeletal muscle
o Short, stumpy, slightly branched cells
o Have intercalated discs where the cells meet
o Each cell has one centrally-placed nucleus
 Smooth Muscle
o Found in blood vessels, air passages, the iris of the eye, and digestive,
respiratory, urinary and reproductive tracts
o Fusiform in shape
o Each cell has a single nucleus