Download Muscular System

Muscular System
Dr. Anderson
GCIT
Function
• Muscle is a collective term for tissue that
causes movement of the body, or materials
through the body
• Muscle can also serve other functions such as
– Stabilizing the body
– Producing heat
Muscle Tissue
• Three main types of
muscle
– Skeletal – attaches to
skeleton and enables
body movement striated, voluntary
– Smooth – lines body
cavities to move fluids
throughout the body –
smooth, involuntary
– Cardiac – makes up the
heart – striated,
involuntary
Physiology
• Muscles function by
converting the
energy stored in ATP
into mechanical
energy that the
muscle can use to
contract – muscles
only expend energy
to get shorter
Muscle Characteristics
• Excitability – the muscle’s ability to react to a
stimulus
• Contractility – the ability to shorten forcibly
• Extensibility – the ability to be stretched
• Elasticity – the ability for a muscle to return to its
pre-stretched length
Skeletal Muscle Anatomy
• There are roughly 640 skeletal muscles in the
human body
• Each one is a discrete unit (organ) of
contraction, separated by membranes and
served by
– One major nerve
– One artery
– One to a few veins
Muscle Connections/Separations
Individual muscles are separated by sheaths of
connective tissue
• Epimysium – surrounds entire muscle
• Perimysium and fascicles – perimysium surrounds
bundles of muscle fibers (fascicles)
• Endomysium – very thin sheath of tissue that
surrounds each muscle fiber (cell)
Muscle Sheaths
Skeletal Origins and Insertions
• Skeletal muscles are connected to bone in at least
two places
• Origin – end connected to the immovable bone
• Insertion – connected to moveable side
• Direct – Epimysium of muscle is fused directly to
the periosteum of bone (less common)
• Indirect – The connective tissue connects to bone
via a tendon (more common)
Direct vs. Indirect Muscle Attachment
Direct Attachment
(to skull)
Indirect Attachments
Muscle Fiber Anatomy
•
•
•
•
Sarcolemma – cell membrane of muscle cell
Sarcoplasm – cytoplasm of muscle cell
Glycosomes – granules of stored glycogen
Myoglobin – red pigment that can store O2 in
muscles
Myofibrils
• Complex organelle composed of bundles of
myofilaments in a muscle fiber (appear
banded in skeletal muscle)
Sarcomere
• The smallest contractile unit of the muscle
• Composed of protein filaments
(myofilaments) that shorten the muscle upon
stimulation
• Here’s where it gets complicated…..
Thick and Thin Filaments
• Thick filaments (myosin) – contain globular
myosin “heads” that will bond to thin filaments
during muscle contraction
• Thin filaments – composed of:
– Actin – the structural protein that provides tensile
strength
– Troponin and Tropomyosin -proteins that regulate
muscle contraction
• Titin – allows for extensibility and elasticity
during and after contraction
Ultrastucture of Muscle Fibrils
Thick
Filament
(Myosin)
Titin
Thin Filament
(Actin)
Sarcomere Components
Muscle Contraction
• Thick filament heads and thin filaments link
and form cross-bridges
– The heads swivel and pull the opposing thin
filaments of the sarcomere together
– Helpful video!
– http://www.youtube.com/watch?v=ELyoJZom5N0
Regulatory Proteins
• Tropomyosin – bound within the actin
filaments and block myosin-binding sites
(when muscle is relaxed)
• Troponin – helps stabilize actin and binds Ca+
ions
Sarcoplasmic Reticulum
• Specialized smooth endoplasmic reticulum (sER)
that surrounds each myofibril
– Most run longitudinally along the length of the cell
– However, between the thick filaments and the Z disc,
they form pairs of terminal cisternae
• This organelle is critical in that it stores and
releases Ca+ ions, which are the triggers for
muscle contraction
T- Tubules
• An invagination of the sarcolemma – located
between the terminal cisternae in each
sarcomere
• The structure of 2 Terminal cisternae divided
by t-tubules is called a TRIAD
• Why is all of the surface area in triads
necessary?
Triads
Triad
The Sliding Filament Model (Sum-up)
• Thin filaments are “captured” by the myosin
heads of the thick filaments by forming crossbridges
• As the myosin heads swivel, they ratchet the
thin (actin) filaments towards the H-line
• This shortens the sarcomeres, and in turn the
entire muscle, causing movement
Muscle Flexing
• In order for muscles to flex:
– They must be stimulated by nerves (or chemicals)
– An action potential must propagate (travel) along
the sarcolemma
– Ca+ ions must increase in concentration, causing
the filaments to slide past one another
Muscle Innervation (Motor Nerves)
• Motor nerves from the spinal cord innervate
the muscle to communicate with the muscle
fibers
• The fibers do not touch but come exceedingly
close (1-2 nm apart) forming the synaptic cleft
Synaptic Cleft (Neuromuscular
Junction)
• From the nerve terminal, a neurotransmitter
called acetylcholine (ACh) is released
• ACh is then picked up by receptors on the
sarcolemma
Electrical Gradient
• Normally, the sarcolemma is polarized (more
negative on the inside than the outside)
• When ACh binds to it receptors, channel
proteins let ions (Ca+ and K+) pass
• More Ca+ ions stream into the cell than K+ ions
stream out, therefore the inside of the cell
becomes less negative – this is called
depolarization
Synaptic Cleft Arrangement
Helpful Video
• http://www.youtube.com/watch?v=kvMFdNw
35L0
Synaptic Cleft
• From the nerve terminal, a neurotransmitter
called acetylcholine (ACh) is released
• ACh is then picked up by receptors on the
sarcolemma
Electrical Gradient
• Normally, the sarcolemma is polarized (more
negative on the inside than the outside)
• When ACh binds to receptors, it opens
channel proteins which lets ions (Ca+ and K+)
pass
• More Ca+ ions stream into the cell than K+ ions
stream out, therefore the inside of the cell
becomes less negative – this is called
depolarization
Depolarization- Successive Steps
1. Action potential (nerve impulse from motor neuron)
reaches the muscle fiber(s)
2. ACh is released, opening Ca channels (ACh is quickly
degraded by acetylcholinesterase
3. The “local” depolarization from the neuron (end plate
depolarization) depolarizes adjacent membrane areas
3. Then voltage-gated ion channels allow Ca+ ions to
enter
4. This action potential is the propagated through the
sarcolemma, opening up more channels
Depolarization- Successive Steps
• 5. Voltage-dependent gates open, and more
Ca+ floods through the sarcolemma
• 6. The influx of Ca+ leads to muscle
contraction by interacting with tropomyosin,
freeing the myosin binding sites
Repolarization
• K+ channels open, which allows K+ to flood out
of the cell, restoring the negative conditions
inside
• How do we restore the electrochemical
balance of the cell?
Sodium-Potassium Pump
• ATP-dependent protein “pump” that restores
the ionic concentration that was present
before depolarization
Excitation-Contraction Coupling
• Calcium ion influx (through T-tubules and into
terminal cisternae)
• Calcium binds to troponin and removes the
blocking action of tropomyosin, allowing
myosin to bind to the thin (actin) filaments
• Myosin cross-bridges with actin, leading to
contraction
Let’s Sum Up
• http://www.youtube.com/watch?v=70DyJww
FnkU
Muscle Innervation
• The single motor nerve that innervates a
muscle can contain hundreds of motor
neurons
• A motor unit is a motor neuron and all of the
muscle fibers that it serves – the more motor
units, the larger the force (more fibers
contract)
– Which muscles might have small motor units?
Large motor units?
Motor Unit
Strength of Contraction
• Increasing strength can be due to
– Increased frequency of stimuli
• The less time between nerve stimulation,
the greater the force of flexion due to Ca
being left over in the muscle
– Increased number of stimulated motor
units
• The more motor units stimulated, the
greater the force of flexion
(Frequency of Stimuli)
Twitch
Latent – Ca floods in
Contraction – Myosin heads pull and make sarcomeres shorter
Relaxation – muscle activity stops as ions are replaced to normal
concentrations
Contraction Strength
• How is the strength of a contraction
controlled?
• The size/number of motor neurons stimulated
– Small neurons (and therefore small muscles) are
most excitable and fire first
– Large neurons are the least excitable and fire last
(only used when strongest contractions are
needed)
Isotonic vs Isometric Contraction
• Isotonic – muscle moves when flexed
– Concentric – muscle moves and shortens
– Eccentric – muscle moves while lengthening
• Isometric – muscle flexes, but does not
shorten or lengthen
Muscle Physiology
• Where do muscles get the energy to flex, and
keep flexing?
Muscle Metabolism
• ATP!!
– Provides the energy needed for cross bridge
movement and detachment (of myosin heads),
but short supply in most muscles
• ATP-ADP-ATP cycles rapidly by:
– Creatine phosphate phosphorylation
– Anaerobic respiration (burning sugar without O2)
– Aerobic respiration (burning sugar with O2)
Creatine Phosphate
• Another high-energy molecule that converts
ADP to ATP by direct phosphorylation via an
enzyme called creatine kinase
ADP + creatine phosphate
creatine kinase
ATP + creatine
Rigor Mortis
• Lack of ATP after
death causes
contractures, where
myosin heads cannot
detach from the actin
filaments, locking up
the muscle until
muscle proteins start
to break down
Length vs. Force
• The strength of a
muscle
contraction varies
drastically over
its length
– Strongest from
80-120% of its
resting
sarcomere length
Velocity of Contraction
• “Fast twitch” fibers – contract rapidly
– Large fibers that depend on stored glycogen reserves
(does not depend on delivery of nutrients)
– Low myoglobin
– Tires quickly
• “Slow fibers” – contract slowly
– Uses nutrients delivered from blood, therefore rich
capillary supply
– High myoglobin
– High endurance
Smooth Muscle
• Different microstructure, but similar function
as skeletal muscle
• Flexing is under control of the autonomic
nervous system, but also under the control of
local chemistry
Smooth Muscle
• Lines the:
– Stomach and
intestines
– Blood vessels
– Lungs
– Urinary system
– Reproductive organs
Muscle Sheets
• In Digestive organs, smooth muscle sheets are
arranged at 90 degree angles to each other
• What will this lead to during flexing?
http://www.youtube.com/watch?v=pfNYbzGEgO8
Smooth Muscles
• Smooth Muscle Cells
are:
– Uninucleate
– Spindle-shaped
– Much narrower and
shorter than skeletal
muscle cells
– Calveoli instead of Ttubules
– No sarcomeres!
Smooth Muscle Sheaths
• Only a small amount of endomysium runs
between smooth muscle cells
• This network is also perfused and innervated
Longitudinal section
Transverse Section
Distribution
• Most smooth muscle occurs in walls of organs
that transport fluids through the body (liquids
and gases)
• Many organs have muscle cells arranged in 2
layers that run perpendicular to each other
– Why?
Peristalsis
• Rhythmic, alternate contractions between
muscle sheets in digestive system
• By alternating contractions, the bolus of food
(or waste) can be moved through the
intestines
• http://www.youtube.com/watch?v=o18UycW
RsaA
Stimulation
• Smooth muscles lack synaptic clefts, but
neurotransmitter is released in the general area
of muscle cells by nerve bundles called
varicosities
Contraction – Smooth vs. Skeletal
• Thick and thin filaments arranged diagonally
• Calmodulin instead of troponin complex
• Thick filaments have myosin heads across
their entire length
Anchoring of Smooth Muscle
• Intermediate Filaments – non-contractile
filaments that anchor to dense bodies which
are attached to adjacent sarcolemmas
– Why is this important?
Smooth Muscle Cell
Contraction vs. Extension
• Ca++ binds to calmodulin, which activates
myosin kinase which phosphorylates the
myosin heads, activating them
• Myosin is then able to form cross-bridges with
actin, and contract the muscle
Response to Stimuli
• Smooth muscle takes up to 30 times longer to
react to a stimulus than skeletal muscle
• Smooth muscles may stay in a contracted
(flexed) state for long periods, as myosin
heads remain attached to actin filaments for
longer periods
Nerve- Based Contraction
• Contraction of smooth muscle is still due to
the release of neurotransmitters
– Highly dependent on receptors for
neurotransmitters
– E.g. norepinephrine relaxes smooth muscle in
bronchioles
Response to Stretch
• Stress-relaxation
response – abrupt
stretches may
cause muscle to
contract, but long
slow stretches
may allow the
muscles to extend
– Examples?
Stretch and Tension
• Different arrangement of
filaments allows smooth
muscle to be stretched quite
and still contract strongly
• Example?