Download Muscle Physiology

Skeletal Muscle 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 (pupils)
• Heart beat
• Production of body heat (Thermogenesis)
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 to its
original length (and beyond to a certain
degree)
• 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
Muscle Tissue Types
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, and satellite cells (embryonic stem cells
function in repair of muscle tissue
• Collagen fibers of all 3 layers come together at each end
of muscle to form a tendon or aponeurosis.
Parts of a Muscle
Z disc
Z disc
Microanatomy of Skeletal
Muscle
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 (T-tubules)
• 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)
•
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
•
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 disc: 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
Sarcoplasmic Reticulum (SR)
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
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 cross-bridges.
(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 myosin-binding
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
Now, putting it all together to perform the function
of muscle: Contraction
Neuromuscular Junction
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
Motor Unit
All the muscle cells controlled by one
nerve cell
Muscle Contraction Summary
1. Nerve impulse reaches myoneural junction
2. Acetylcholine (Ach) is released from
motor neuron (axon terminals) and crosses
synaptic cleft.
3. Ach binds with receptors in the muscle
membrane to allow sodium to enter
synaptic cleft
4. Sodium influx will generate an action
potential in the sarcolemma
Muscle Contraction (Cont’d)
5. Action potential travels down T tubule and
to sarcoplamic reticulum (SR)
6. SR releases calcium ions
7. Calcium binds with troponin to move the
troponin/tropomyosin complex on the actin
8. Binding sites on the actin filament are
exposed
Muscle Contraction (cont’d)
9. ATP helps myosin head attach to binding
sites (forming a crossbridge) and create a
power stroke which moves actin, causing
contraction
10. ATP detaches myosin heads and energizes
them for another contraction
11. When action potentials cease the muscle
stop contracting
Sarcomere Relaxed
Sarcomere Partially Contracted
Sarcomere Completely
Contracted
Binding Site
Troponin
Ca2+
Tropomyosin
Myosin
Excitation-Contraction Coupling
Muscle contraction
•Alpha motor neurons release Ach
•ACh produces large EPSP in muscle fibers (via
nicotinic Ach receptors
•EPSP evokes action potential
•Action potential (excitation) triggers Ca2+
release, leads to fiber contraction
•Relaxation, Ca2+ levels lowered by organelle
reuptake
Excitation-Contraction Coupling
Excitation-Contraction Coupling
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
How striated muscle works: The Sliding Filament Model
The lever movement drives displacement of the actin filament relative to the myosin
head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).
Thick and thin filaments interdigitate and “slide” relative to each other.
Three Potential Actions During Muscle Contraction:
• isotonic
Biceps muscle shortens
during contraction
*shortening against fixed load =
muscle building
*No movement
(but as tension
increases muscles
grow
larger/stronger)
• isometric
• lengthening
Biceps muscle lengthens
during contraction
*Most likely to cause
muscle injury
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
– Women’s skeletal muscle makes up 36% of
their body mass
– Men’s skeletal muscle makes up 42% of their
body mass
• 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 lean
• 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