muscle Flashcards
primary function of all muscle
and the other functions
The primary function of all muscle is to generate force and/or movement in response to stimuli.
body movement
posture maintenance
respiration
production of body heat
communication
constriction of organs and vessels
heartbeat
skeletal muscle
attached to
tendons
myofilaments
think/thick filaments
F-Actin: backbone of thin filaments, contains the binding site for thick filaments
tropomyosin: two identical alpha helices that coil around each other, regulating the binding of myosin to actin
troponin complex: consists
troponin C: ca binding to site
troponin T: binds to tropomyosin
troponin I: bound to actin
thick filament is myosin.
myosin head contains a region for binding actin as well as a site for binding and hydrolyzing ATP
light chains regulate Myosin ATPase
sarcomere
z-discs: zigzag protein structure that is the attachment site for thin filaments
I bands: lightest band of the sarcomere, only actin
a band: darked band, contains both thick and thin filaments
h zone: central region of A band, only myosin
line: proteins from the attachment site for thick filaments, same thing z disk for think filaments
neuromuscular junction
the area where the lower motor neuron makes synaptic contact with the muscle fiber
three components:
presynaptic motor neuron filled with synaptic vesicles
the synaptic cleft
postsynaptic membrane of the skeletal muscle fiber
initiation of the AP
AP travels down motor neuron into axon terminal.
Ca channels activated, and Ca enters
vesicles containing ACh bind to the presynaptic membrane
ACh binds and opens ion channels
Na entry
the entry of Na generates an excitatory end plate potential. (EPP) that spreads to adjacent voltage-gated Na channels and initates an action potential
ceasing neural transmission
must get rid of
once AP stops firing in the alpha motor neuron, ACh must be removed and will diffuse away or be broken down into acetate and choline by acetylcholinesterase.
excitation-contraction coupling
what is it
how can we get Ca in the sacroplasm?
triggers contraction in all muscle types is a rise in intracellular calcium.
depending on the muscle type, Ca can enter the sarcoplasm from the extracellular space via voltage-gated Ca channels or
can be released into the sarcoplasm from the intracellular Ca storage reservoir of the SR
electrical excitation of the membrane triggers an increase of Ca is EC coupling
what are the channels? Ca can enter the sacroplasm with
DHP receptor: L-type Ca channel
RyR: ryanodine receptor
Ca release channel on SR
besides mechanically, they can open by Ca (Ca induced Ca release)
how does an increase in Ca allow contraction?
it removes the inhibition of cross-bridge cycling.
Ca2+ binds low-affinity sites of troponin C, which changes the troponin complex.
causes the troponin complex as well as tropomyosin to move revealing the myosin binding site of actin
cross bridge cycling
before it starts, tropomyosin must be shifted
1) ATP binding: ATP binds to the head of myosin heavy chain, which breaks the bond of myosin and actin
2) ATP hydrolysis: ATP is broken down to ADP and Pi inorganic phosphate, resulting in the movement of the myosin head into the cocking position. allows for binding of new actin
3) the power stroke: PI is dropped and strengthens bond between myosin and actin, triggering the power stroke, myosin head returns to un cocked state well doing so pulls the actin.
4) ADP release: disassociation of ADP from myosin casuses myosin to remain bound to actin until until ATP initiates the cycle again.
termination of contraction requires removal of CA
Ca must be removed so that the myosin binding site on actin can be covered by tropomyosin
Ca can be removed to the extracellular space by the Na-Ca exchanger or by the Ca pump which uses ATP.
SERCA type Ca pump
calquestrin and calreticulin maximize CA uptake by the SR
rigor mortis
development of rigid muscle several hours after death
what is ATP needed for and whre can you get it from
needed for:
myosin ATPase (contraction)
Ca ATPase: SERCA (relaxation)
Na/K ATPase (after AP in muscle fibre)
sources:
free intracellular ATP
ATP formed from phosphocreatine
anaerobic metabolism
aerobic metabolism
anaerobic metabolism vs
aerobic metabolism
anaerobic: ATP is needed so glycogen is converted back into glucose (glucogenesis).
glucose is broken down into pyruvate by glycolysis resulting in 2 ATP molecules.
occurs in the sacroplasm
aerobic: O2 is present, pyruvate enters the citric acid cycle, producing 2 more molecules of ATP as well electrons H
the electrons and H combine with O2 in the ETC to produce 26-28 ATP
occurs in mitochondria
muscle fatigue
central vs peripheral
decreased in muscle tension as a result of previous contractile activity that is reversible with rest
central fatigue: feeling of tiredness; low pH from acid production during ATP hydrolysis may influence the fatigue of the brain
peripheral fatigue: EC coupling problems at the t tubule
with repeated AP firing, K builds up in the t-tubules, changing the threshold for APs in the muscle fiber.
accumulation of phosphate, acid, ADP
decrease in Ca release, reuptake, and storage by the SR
direct inhibition of the binding and power stroke of myosin cross bridges
how is summation formed
another contraction occurs before the muscle can relax
single aP does not cause release of the entire Ca store from the SR.
the Ca released from one AP may not lead to all troponin complexes being activated for a sufficient amount of time, some regions may be recovered with tropomyosin
a second AP causes a second wave of Ca that may keep additional troponin complexes activated allowing more cross bridges to be formed
tetanus
a maintained contractile response to repeated stimuli
unfused stimuli: allows for relxation in between b.c stimuli are far enough apart
fused stimuli: fast enough stimuli that fiber does not relax; instead, it reaches max tension and remains there
how do isometric contractions generate force?
sacromere shortens by elastic elements in tendons, elastic and connective tissue in and around muscle fibers
motor unit vs motor neuron pool
motor unit: a single motor neuron and all the muscle fibers it innervates
motor neuron pool: the group of all motor neurons innervating a single muscle
muscle hypertrophy
increase in size of sarcomeres and number of contractile proteins (myosin and actin)
increased number of sarcomeres within a muscle length, increased myofibrils
increased sacroplamsic storage (glycogen)
skeletal muscle reflexes
what is it
how does it work
2 responses
what are the 4 components
involved in almost all movements
receptors sense change in joint movements, muscle tension and muscle length and feed info to the CNS which responds in 2 ways:
if muscle contraction is needed, the CNS activates motor neurons to the muscle fibers.
if relaxation is needed sensory input activates inhibitory interneurons in CNS which inhibit activity in motor neurons, leading to relaxation
4 components: 1) sensory receptor 2) integrating center 3) efferent neurons 4) effector
proprioceptors
provide info into the CNS about the position of our limbs in space, movements, and the effort exerted by muscle
there are muscle spindles, golgi tendon organs, and joint receptors.
muscle spindles
parallel to the muscle fibres
send info to the CNS about muscle length adn changes in lenght
made up of sensory neurons wrapped around intrafusal muscle fibers
extrafusal muscle fibers are regular muscle fibers innervated by alpha motor neurons
contracts the agonist and relaxes the antagonist
if load is added to the arm arm will stay up
golgi tendon organs
respond to tension
control inhibitory reflex to prevent muscle damage
if load on the arm, the muscle will relax due to GTO and the load will be dropped
(recirprocal inhibiton) contraction of the antagonist and relaxation of agonist
flexion reflexes
pull limbs away from painful stimuli
1) Painful stimuli activate nociceptors.
2 types of cells in the cardiac muscle
and their AP’s
autorhythmic andcontractile myocardial cells
autorhymic cells
starts at -60mv
HCN channels open -60mv- to -40mv
just before -40mV thershold HCN channels close and t-type Ca channels open, very small
once the threshold is reached, L-type Ca channels open, causing big depolarizing
they close, and then K channels open, causing K to flow out of the cell, causing repolarize to -60 mV
contractile cell
starts at -90mv
Na channel opens until +20mV
na channels close and fast K channels open before Ca channels open
Ca channels open and fast K channels close (the plateau)
once Ca channels close, k channels open repolarizing the cell
EC coupling in the cardiac muscle
Ca-induced Ca release
in cardiac muscle, L-type Ca channels (DHP receptors) are not mechanically coupled to ryanodine receptors.
Ca entry is necessary for contraction.
Ca-induced Ca release
1) Ap enters from adjacent cell
2) Ca channels open, Ca enters the cell
3) Ca induces Ca release through RyR
4) local release causes Ca spark
5) summed Ca sparks create a Ca signal
6) Ca ions bind to troponin to initiate contraction
how is the SERCA pump differnet in cardiac muscle
it is regulated by phospholamban.
when phosphorylated, Ca pump inhibition is removed, enhancing relaxation rates and contractability
rate and amount of Ca uptake is increased, causing quicker relaxation and larger store of Ca
enhancing contractile force in cardiac muscle
in cardiac muscle, an inc in intracellular Ca in the cardiac myocytes enhances contractile force. allows more troponin complexes
length-tension relationship
cardiac muscle generates a greater force when slightly stretched
needs to be the right amount of stretch not too much not too little.
smooth is categorized by
location
contraction pattern
communication with neighboring cells
location: in humans, smooth muscle can be divided into 6 major groups:
vascular
gastrointestinal
urinary
respiratory
reproductive
ocular
contraction pattern:
phasic relaxed
phasic contraction and relaxed
tonic contracting
tonic whose contraction varied
communication with neighboring cells:
unitary (single unit): contains gap junctions
multunit: not electrically coupled
iris and ciliary body of the eye
electrical activity in smooth muscle
Single unit
autonomic AP initiation
spikes of plateaus
spontaneous AP: slow wave, pacemaker
graded potential –> multunit
Ca entry through smooth muscle
intracellular and extracellular
1) Ca entry through voltage-gated channels or ligand-gated channels
2) Ca release from the SR
Ca-induced Ca release from RYr
3) Ca entry through voltage-independent channels
smooth muscle anatomy
spindle-shaped, uninucleate cells
troponin and t tubules absent
intermediate filaments
