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Muscle tissue
Characteristics
Excitability
·When
a muscle is stimulated by a nerve impulse or electrical
stimulus it contracts: property of excitability.
Contractibility
·Stimulated
muscle responds by contracting.
Extensibility
·Pull
on the ends of muscle it stretches; extensibility
Elasticity
·Released
from stretching the muscle returns to its original shape.
Functions
Motion
·Muscles
provide the force for motion; skeletal, cardiac and smooth.
Posture
·Generates
a force to maintain posture
Heat
·Every
muscle action requires mechanical and chemical processes;
generates heat; used to maintain body temp.
Type
Skeletal
·Muscle
attached to a bone to provide for motion and posture.
Cardiac
·Associated
with the heart, to generate the force used to drive
blood through the vessels.
Smooth
·Associated with the many organs of the body;
·E.g., generates a squeezing force that pushes material
digestive tract.
·Also,
found in the bronchial tubes and blood vessels.
down the
Connective tissue
Fascia
·Sheets
of CT fibers (collagenous) found under the skin and between
muscles and organs.
·Helps to separate one organ or structure from another.
·Consists
of superficial, deep and subserous fascia
deep fascia
·Dense CT, arranged in layers.
·In each layer, the fibers are parallel
·each layer is oriented in a different direction.
·Deep fascia around each muscle blends into the periosteum
bone.
Epimysium
·Dense
CT layer which surrounds the
entire muscle; continuous with the
surrounding deep fascia.
·Separates
the muscles from
surrounding structures.
Perimysium
·An
extension of the epimysium within
the muscle; divides the muscle into
bundles of muscle cells; fascicles.
Endomysium
·An
extension of the perimysium
within each fascicle; separates each
muscle cell.
of the
tendons
·Dense regular CT bundles ; bind muscles to bones.
·Tendons are continuous with the epimysium of the muscle
·by implication, the peri- and endomysium as well.
·Forces generated within each muscle cell are transmitted through
the various CT layers; transferred through the tendon to the bone.
Skeletal muscle
striated voluntary
·Under
the microscope; each muscle has minute cross-banding
patterns; a “striated” appearance.
·The muscle is under conscious control; voluntary.
Histology
Myofibers or muscle fibers or muscle cells
·All
are equivalent terms.
Sarcolemma
·Sarco
means muscle. Lemma= plasma membrane (PM).
·Muscle
cell membrane.
Sarcoplasm
·The
cytoplasm within the muscle cell.
Multinucleate
·Muscle cells have many nuclei.
·Started out as multiple uninucleate
cells; fuse into the much larger
muscle fiber.
·Original
nuclei are retained.
transverse tubules
·Tube-like
extensions of the PM
into the sarcoplasm.
·They
carry the stimulus
( in this case, a type of
“electrical” signal) from
the sarcolemma deep
into the cell.
sarcoplasmic reticulum
·Similar to the ER, endoplasmic reticulum.
·Stores large amounts of calcium within the
·Is closely associated with the T-Tubules.
SR lumen.
Calcium
·Calcium
is released into the
sarcoplasm in response to a stimulus.
Calsequestrine
·An
ion pump used to move calcium
into the SR; uses ATP
calcium-ATPase
·A
ion pump used to move calcium into the SR; uses ATP
Triad
·One
T-tubule and the adjacent SR on either side.
myofibrils
·Are
bundles of protein filaments, called myofilaments,
within the muscle fiber (cell.)
·Made
of two types of myofilaments.
myofilaments
myofibril is made of alternating and interdigitating thin
and thick myofilaments aligned with the long axis of the cell.
·Each
pattern seen within each myofiber (muscle cell) is
caused by this arrangement of myofilaments.
·Striated
Thin
·Appears
·Made
lighter under the microscope.
of several proteins; actin,
troponin and tropomyosin.
Thick
·Appears darker under the microscope.
·Made of a protein called myosin.
sarcomeres
·Myofilaments
sarcomeres.
are organized into some 10,000 repeating units called
Z-lines
end of the sacromere. Thin filaments of adjacent sarcomeres are linked
together here.
·Each
A-band
·Consists
of overlapping thin and thick filaments.
I-band
·Only thin filaments.
·The I-band shortens
as the sarcomere contracts.
H-zone
·Only
thick filaments. This also shortens during contraction.
16
thin
Actin
·Each
thin filament is made of three different
proteins.
filaments are made of subunits called G-actin; these are
globular proteins which are linked together to form a filament.
·Actin
myosin binding site
·Each
G-actin contains a binding site for myosin head groups (part of
the thick filament)
Troponin ·This protein is associated with actin and it binds calcium.
Tropomyosin
·The
third protein of the
group
·when muscle fiber is relaxed
tropomyosin covers the
myosin binding site, blocks
the myosin head groups from
binding to actin.
thick
·Consists
of a bundle of proteins called myosin
Myosin
Tails
heads=cross bridges
·Each
tail has two head groups at the same end
and each has two binding sites.
actin binding site
·This
binds to the myosin binding site on actin, when it is
exposed as tropomyosin moves.
ATP-binding site
·Binds to ATP
·splits ATP
· released energy
drives the
movement of the myosin
head groups.
Motor end plate (neuromuscular junction)
·Signals passed down through nerves to stimulate muscle contraction
·need to be converted from an action potential (AP, nerve impulse) into a
chemical signal at the neuromuscular junction.
synaptic end bulbs
·Signals reach the end of the nerve;
enter the synaptic end bulb.
synaptic vesicles
·Signal triggers synaptic vesicles to release
signaling chemicals into the synaptic cleft.
acetlycholine- ACh (neurotransmitter)
·In muscles, this chemical is called
acetlycholine, a neurotransmitter.
acetylcholinesterase
·Ach needs to be broken down as soon as
it is used by the enzyme
acetlycholinesterase.
Q8
20
Molecular events in muscle contraction
·Best
reviewed by studying the animation.
1) Signals pass down the nerve and cause the release of NT at the
neuromuscular junction.
2) A signal is generated in the sarcolemma within the junction.
3) Signal passes down the T-tubules.
4) Triggers the release of calcium from the SR.
5) The calcium diffuses into the sarcoplasm and binds to troponin on the thin
filament.
6) Troponin causes tropomyosin to move and exposes the myosin binding
site.
7) Myosin head groups bind to the active site on actin and then undergo a
power stroke; this pulls the actin filament along
8) Head groups detach and are free to bind again only if ATP is present.
9) As nerve signals stop the calcium levels fall and tropomyosin moves back
to cover the myosin binding sites. Contraction stops.
Q9
23
Rigor mortis
·During
·ATP
death ATP levels fall rapidly, as cellular respiration stops.
is required to detach myosin head groups from the actin filaments.
·Myosin and actin filaments
· muscles become rigid and
are bound together
locked into the position they were in at death.
·Eventually the myosin and actin filaments
·enzymatic and chemical degradation
·muscle becomes flaccid.
detach
Energy for muscle contraction
ATP is needed for…
1) contraction
·Cocking
and detachment of the myosin head.
2) calsequestrin and Ca2+-ATPase
·Pumping
calcium into the SR of the sarcoplasm.
3) Na+/K+-ATPase
·Needed
for impulse conduction.
ATP lasts
·...only
5-6 seconds during active muscle contraction as ATP stores are used
up.
ATP is quickly reconstituted
·Several
mechanisms that replenish the ATP stores.
Sources of energy for ATP production
1) phosphocreatine (creatine phosphate, CP)
·ATP is produced by the
·another high energy
phosphogen system from phosphocreatine
molecule.
creatine kinase
·Breaks down phosphocreatine, releasing a phosphate and energy.
·Energy is used to make new ATP.
·The first system that gets activated during contraction; it is simple
produce ATP quickly.
·This store of energy lasts about 8-15 seconds.
and can
2) anaerobic respiration, glycolysis
·ATP is now
·None of the
made by anaerobic mechanisms.
previously listed processed are turned off; rather NEW
mechanisms are added.
Glycogen ·A polymer of glucose.
·Primary
energy source used in this process; lasts about 30-40 secs.
lactic acid/pyruvic acid
·Product
of anaerobic respiration is pyruvate; as no oxygen is available the
pyruvate is converted to lactic acid.
·Lactic acid builds up in the muscles; changes the pH of the muscle
·Causes a decrease in the efficiency of enzymes
·Leads to soreness and fatigue.
·Muscle recovery; lactic acid must be removed quickly
by a well vascularized
muscle.
no oxygen
·Anaerobic respiration; no oxygen is used
· most of the energy in glucose is not converted
·Only 2 ATPs per glucose are produced.
·Remaining energy is trapped in lactic acid.
to ATP.
fermentation
·Aka,
anaerobic respiration; usually not applied to humans.
oxygen debt
·If
oxygen is available; lactic acid can be converted back into pyruvate
·used
by mitochondria to generate ATP aerobically.
·During anaerobic respiration a need for oxygen is built up; oxygen debt.
3) Oxidative metabolism
Aka, aerobic respiration
Oxygen
·Oxygen used by mitochondria to produce 36 net ATPs
·2 from glycolysis and 34 from Krebs and ETC.
·Can produce ATP indefinitely as long as you have oxygen and
energy stores (fat, proteins or glucose.)
Muscle heat
·All mechanical and chemical reactions
·Used to maintain body temperature.
produce heat.
Initial ·Heat generated during muscle contraction.
Recovery ·Heat produced to recover from contraction;
oxygen debt, replenishing ATP stores.
1.
Phosphogen
system
2.
Glycogen-lactic acid
system
3.
Aerobic respiration
system
anaerobic respiration
Creatine phosphate
Glycogen
Lactic
acid
Glucose
fatty acids
amino acids
+
02
Glucose
Creatine + PO3
Pyruvic
acid
ADP
CO2 + H2O
ATP
Lasts for 10-15 secs
Lasts for 30-40 secs
100 m dash, weight lifting
200-400 m dash
Lasts indefinitely
30
All or none principal
·All the muscle fibers of a motor unit respond to a threshold
·contracting as fully as they are physiologically able to.
stimulus
Threshold stimuli
·A
stimulus that is sufficient to cause a response; the response is to
contract fully.
Sub-threshold
stimulus that is not sufficient to cause a response; none of the
motor units respond.
·A
Contractions
Twitch ·A
contraction-relaxation event in response to a single threshold
stimulus.
latent phase
·Period
immediately after a stimulus; muscle is preparing to contract; the
stimulus is moving over the sarcolemma, down the t-tubules, etc.
·No contraction yet.
contraction phase
·Filaments
are sliding past each other and generating the contractile force.
relaxation phase
·Ca2+
levels falls and the filaments can’t interact; forces decline.
refractory period
·Period
of time after the initial stimulus in which another stimulus has no
effect; during the time of action potential and Ca2+ diffusion.
tetanus
·Sustained
muscle contractions in normal muscle contractions.
Incomplete, unfused or wave summation
·If
a second stimulus arrives before the completion of the relaxation
phase, the second contraction is added to the first, causing a stronger
second contraction.
Complete
·Stimulation
rate is increased, the stimuli arrive during the contraction
phase and the increase in tension is continuous until a maximum level is
reached.
treppe
·A
step-wise increase in muscle contraction when the stimuli are applied
immediately after the relaxation phase.
Types of contractions
Isotonic
·Iso= same, tonic=tension. Same tension.
·Muscle tension stays the same while the muscle length shortens.
·Activities such as walking, lifting objects, etc. involve isotonic
contractions.
Isometric
·Iso= same, metric=length. Same length.
·Muscle length stays the same while the muscle
tension increases.
Fibers types
Slow-twitch red
Fast twitch red
Fast twitch white
Myoglobin
lots
lots
low
Mitochondria
lots
lots
low
Capillaries
lots
lots
low
ATP production
aerobic
Aerobic
Contraction
slow
Fast
Fatigue
Very resistant
Moderately resistant
Anaerobic
Fast
Low resistant to fatigue
Myoglobin A protein similar to hemoglobin that binds oxygen and found in
muscles.
Cardiac muscle
·Heart
muscle.
Striated involuntary
·Striations
present; unconscious control.
Intercalated discs
·Characteristic
of cardiac muscle; region of gap junctions between
cardiocytes.
Long refractory period
·Reduces
the chance for an
uncontrolled contractions of the
atria and ventricles.
Smooth muscle
Non-striated involuntary
·Cross banding pattern is not present and control is not conscious.
Arteries, airways & others
·Cross banding pattern is not present; control is not conscious.
Slower and longer lasting contractions