Muscular System:
Histology and Physiology
Muscular System Functions
• Body movement (Locomotion)
• Maintenance of posture
• Respiration
– Diaphragm and intercostal contractions
• Communication (Verbal and Facial)
• Constriction of organs and vessels
– Peristalsis of intestinal tract
– Vasoconstriction of b.v. and other structures
• Heart beat
• Production of body heat
Properties of Muscle
• Excitability: capacity of muscle to
respond to a stimulus
• Contractility: ability of a muscle to
shorten and generate pulling force
• Extensibility: muscle can be stretched
back to its original length
• Elasticity: ability of muscle to recoil to
original resting length after stretched
Types of Muscle
• Skeletal
– Attached to bones
– Makes up 40% of body weight
– Responsible for locomotion, facial expressions, posture, respiratory
movements, other types of body movement
– Voluntary in action; controlled by somatic motor neurons
• Smooth
– In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
– Some functions: propel urine, mix food in digestive tract,
dilating/constricting pupils, regulating blood flow,
– In some locations, autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
• Cardiac
– Heart: major source of movement of blood
– Autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
Connective Tissue Sheaths
• Connective Tissue of a Muscle
– Epimysium. Dense regular c.t. surrounding entire muscle
• Separates muscle from surrounding tissues and organs
• Connected to the deep fascia
– Perimysium. Collagen and elastic fibers surrounding a
group of muscle fibers called a fascicle
• Contains b.v and nerves
– Endomysium. Loose connective tissue that surrounds
individual muscle fibers
• Also contains b.v., nerves
• Collagen fibers of all 3 layers come together at each
end of muscle to form a tendon.
Nerve and Blood Vessel Supply
• Motor neurons
– stimulate muscle fibers to contract
– Neuron axons branch so that each muscle fiber (muscle cell)
is innervated
– Form a neuromuscular junction (= myoneural junction)
• Capillary beds surround muscle fibers
– Muscles require large amts of energy
– Extensive vascular network delivers necessary
oxygen and nutrients and carries away metabolic
waste produced by muscle fibers
Basic Features of a Skeletal Muscle
• Muscle attachments
– Most skeletal muscles
run from one bone to
another
– One bone will move –
other bone remains
fixed
• Origin – less
movable attachment
• Insertion – more
movable attachment
Basic Features of a Skeletal
Muscle
• Muscle attachments (continued)
– Muscles attach to origins and insertions by
connective tissue
• Fleshy attachments – connective tissue fibers
are short
• Indirect attachments – connective tissue forms
a tendon.
Skeletal Muscle Structure
• Composed of muscle cells (fibers),
connective tissue, blood vessels,
nerves
• Fibers are long, cylindrical, and
multinucleated
• Tend to be smaller diameter in small
muscles and larger in large muscles.
1 mm- 4 cm in length
• Develop from myoblasts
• Striated appearance
• Nuclei are peripherally located
Muscle Fiber Anatomy
•
•
Sarcolemma - cell membrane
– Surrounds the sarcoplasm (cytoplasm of fiber)
• Contains many of the same organelles seen in other cells
• An abundance of the oxygen-binding protein myoglobin
– Punctuated by openings called the transverse tubules (Ttubules)
• Narrow tubes that extend into the sarcoplasm at right angles
to the surface
• Filled with extracellular fluid
Myofibrils -cylindrical structures within muscle fiber
– Are bundles of protein filaments (=myofilaments)
• Two types of myofilaments
1. Actin filaments (thin filaments)
2. Myosin filaments (thick filaments)
– At each end of the fiber, myofibrils are anchored to the inner
surface of the sarcolemma
– When myofibril shortens, muscle shortens (contracts)
Sarcoplasmic Reticulum (SR)
• SR is an elaborate, smooth endoplasmic
reticulum
– runs longitudinally and surrounds each myofibril
– Form chambers called terminal cisternae on either
side of the T-tubules
• A single T-tubule and the 2 terminal cisternae
form a triad
• SR stores Ca++ when muscle not contracting
– When stimulated, calcium released into sarcoplasm
– SR membrane has Ca++ pumps that function to
pump Ca++ out of the sarcoplasm back into the SR
after contraction
Sarcoplasmic Reticulum (SR)
Figure 9.5
Parts of a Muscle
•
Sarcomeres: Z
Disk to Z Disk
Sarcomere - repeating functional units
of a myofibril
– About 10,000 sarcomeres per
myofibril, end to end
– Each is about 2 µm long
•
Differences in size, density, and
distribution of thick and thin filaments
gives the muscle fiber a banded or
striated appearance.
– A bands: a dark band; full length of thick
(myosin) filament
– M line - protein to which myosins attach
– H zone - thick but NO thin filaments
– I bands: a light band; from Z disks to ends
of thick filaments
• Thin but NO thick filaments
• Extends from A band of one sarcomere to
A band of the next sarcomere
– Z disk: filamentous network of protein.
Serves as attachment for actin
myofilaments
– Titin filaments: elastic chains of amino
acids; keep thick and thin filaments in
proper alignment
Structure of Actin and Myosin
Myosin
(Thick)
Myofilament
•
•
•
•
Many elongated myosin molecules
shaped like golf clubs.
Single filament contains roughly 300
myosin molecules
Molecule consists of two heavy myosin
molecules wound together to form a
rod portion lying parallel to the
myosin myofilament and two heads
that extend laterally.
Myosin heads
1. Can bind to active sites on the
actin molecules to form crossbridges. (Actin binding site)
2. Attached to the rod portion by a
hinge region that can bend and
straighten during contraction.
3. Have ATPase activity: activity that
breaks down adenosine
triphosphate (ATP), releasing
energy. Part of the energy is used
to bend the hinge region of the
myosin molecule during
contraction
•
•
•
•
Thin Filament: composed of 3 major
proteins
1. F (fibrous) actin
2. Tropomyosin
3. Troponin
Two strands of fibrous (F) actin
form a double helix extending the
length of the myofilament; attached
at either end at sarcomere.
– Composed of G actin monomers
each of which has a myosinbinding site (see yellow dot)
– Actin site can bind myosin
during muscle contraction.
Tropomyosin: an elongated protein
winds along the groove of the F
actin double helix.
Troponin is composed of three
subunits:
– Tn-A : binds to actin
– Tn-T :binds to tropomyosin,
– Tn-C :binds to calcium ions.
Actin (Thin)
Myofilaments
Sliding Filament Model of
Contraction
• Thin filaments slide past the thick ones
so that the actin and myosin filaments
overlap to a greater degree
• In the relaxed state, thin and thick
filaments overlap only slightly
• Upon stimulation, myosin heads bind to
actin and sliding begins
Sliding Filament Model of
Contraction
• Each myosin head binds and detaches
several times during contraction, acting
like a ratchet to generate tension and
propel the thin filaments to the center
of the sarcomere
• As this event occurs throughout the
sarcomeres, the muscle shortens
PLAY
InterActive Physiology®: Muscular System: Sliding Filament Theory
Neuromuscular Junction
• Region where the motor neuron stimulates the
muscle fiber
• The neuromuscular junction is formed by :
1. End of motor neuron axon (axon terminal)
• Terminals have small membranous sacs (synaptic
vesicles) that contain the neurotransmitter acetylcholine
(ACh)
2. The motor end plate of a muscle
• A specific part of the sarcolemma that contains ACh
receptors
• Though exceedingly close, axonal ends and muscle
fibers are always separated by a space called the
synaptic cleft
Neuromuscular Junction
Figure 9.7 (a-c)
Motor Unit: The NerveMuscle Functional Unit
• A motor unit is a motor neuron and all the
muscle fibers it supplies
• The number of muscle fibers per motor unit
can vary from a few (4-6) to hundreds
(1200-1500)
• Muscles that control fine movements
(fingers, eyes) have small motor units
• Large weight-bearing muscles (thighs, hips)
have large motor units
Motor Unit: The Nerve-Muscle Functional
Unit
Figure 9.12 (a)
Motor Unit: The NerveMuscle Functional Unit
• Muscle fibers from a motor unit are spread
throughout the muscle
– Not confined to one fascicle
• Therefore, contraction of a single motor unit
causes weak contraction of the entire muscle
• Stronger and stronger contractions of a muscle
require more and more motor units being
stimulated (recruited)
Smooth
Muscle
• Cells are not striated
• Fibers smaller than those in skeletal
muscle
• Spindle-shaped; single, central nucleus
• More actin than myosin
• No sarcomeres
– Not arranged as symmetrically as
in skeletal muscle, thus NO
striations.
• Caveolae: indentations in sarcolemma;
– May act like T tubules
• Dense bodies instead of Z disks
– Have noncontractile intermediate
filaments
Smooth Muscle
• Grouped into sheets in walls of hollow organs
• Longitudinal layer – muscle fibers run parallel to organ’s long axis
• Circular layer – muscle fibers run around circumference of the organ
• Both layers participate in peristalsis
Figure 9.24
Smooth Muscle
• Is innervated by autonomic nervous system (ANS)
• Visceral or unitary smooth muscle
– Only a few muscle fibers innervated in each group
– Impulse spreads through gap junctions
– Who sheet contracts as a unit
– Often autorhythmic
• Multiunit:
– Cells or groups of cells act as independent units
– Arrector pili of skin and iris of eye
Cardiac Muscle
• Found only in heart where it forms a thick layer called
the myocardium
• Striated fibers that branch
• Each cell usually has one centrally-located nucleus
• Fibers joined by intercalated disks
– IDs are composites of desmosomes and gap junctions
– Allow excitation in one fiber to spread quickly to adjoining fibers
• Under control of the ANS (involuntary) and endocrine
system (hormones)
• Some cells are autorhythmic
– Fibers spontaneously contract (aka Pacemaker cells)
Cardiac Muscle Tissue
Figure 10.10a
Disorders of Muscle Tissue
• Muscle tissues experience few disorders
– Heart muscle is the exception
– Skeletal muscle – remarkably resistant to
infection
– Smooth muscle – problems stem from
external irritants
Disorders of Muscle Tissue
• Muscular dystrophy – a group of
inherited muscle destroying disease
– Affected muscles enlarge with fat and
connective tissue
– Muscles degenerate
• Types of muscular dystrophy
– Duchenne muscular dystrophy
– Myotonic dystrophy
Disorders of Muscle Tissue
• Myofascial pain syndrome – pain is
caused by tightened bands of muscle
fibers
• Fibromyalgia – a mysterious chronicpain syndrome
– Affects mostly women
– Symptoms – fatigue, sleep abnormalities,
severe musculoskeletal pain, and headache
Developmental Aspects:
Regeneration
• Cardiac and skeletal muscle become amitotic, but can
lengthen and thicken
• Myoblast-like satellite cells show very limited
regenerative ability
• Cardiac cells lack satellite cells
• Smooth muscle has good regenerative ability
• There is a biological basis for greater strength in men
than in women
• Women’s skeletal muscle makes up 36% of their body
mass
• Men’s skeletal muscle makes up 42% of their body
mass
Developmental Aspects: Male
and Female
• These differences are due primarily to
the male sex hormone testosterone
• With more muscle mass, men are
generally stronger than women
• Body strength per unit muscle mass,
however, is the same in both sexes
Developmental Aspects: Age
Related
• With age, connective tissue increases and
muscle fibers decrease
• Muscles become stringier and more sinewy
• By age 80, 50% of muscle mass is lost
(sarcopenia)
• Decreased density of capillaries in muscle
• Reduced stamina
• Increased recovery time
• Regular exercise reverses sarcopenia