muscle physiology 1&2 Flashcards
outer membrane of muscle fiber is the
sarcolemma
each muscle fiber is innervated by
one motor neuron, making up the motor unit
muscle fiber is made up of smaller subunits called
myofibrils
myofibrils made up of repeating
sarcomeres, basic contractile unit of muscle fiber
sarcomere, the basic contracile unit of myofibrils has a disk at each end called the
z disk
lots if small thin filaments, called _____, are attached to the z disks, extending towards the middle of the sarcomere
actin
A band
one region of the repeated sarcomere remains relatively constant in length during contraction
I band
rich in thinner filaments, made of actin , changed its length along with the sarcomere during contraction
sliding filament theory
states that the sliding of actin past myosin generates muscle tension
because actin is tethered to the Z disks or Z bands, any shortening of the actin filament would result in shortening or sarcomere and therefore the muscle
sarcomeres have proteins
actin (thin filaments) and
myosin (thick fillaments)
actin; thin or thick
roped together with ____
thin filaments
tropomyosin
myosin; thin or thick
heads interact with
thick filaments
heads interact with actin
myosin heads and actin filaments are arranged
360 degrees all around these filaments
tropomyosin molecules (that link the actin togtehr) also hold on to a
troponin complex
troponin complex 3 subunits
- troponin T (bound to tropomyosin)
- troponin I
- troponin C (binds calcium)
when troponin C binds calcium, what occurs
- change in shape
- moves tropomyosin off the active site on actin –> enables interactin between actin and myosin
how actin and myosin filaments interact is called the process of
cross-bridge cycling
when the myosin heads bind to the active site on actin (when troponin C binds calcium) it forms a
at this point we’ve got ___ bound to myosin head
crossbridge
ADP
when myosin head binds to active site on actin we get a conformational change; moves the actin, this is called a
power stroke= force
ADP released from myosin head during the power stroke
during the power stroke (when myosin head binds to active site on actin we get a conformational change; moves the actin) the myosin cross bridges are rotating
at this point the ADP that was bound to myosin head is
towards the centre of the sarcomere; shortening the length of the sarcomere
released
one the ADP is released from myosin head during the power stroke, ____ binds to the myosin head
ATP
ATP binding to the myosin head causes a
- change in shape of myosin; decreases affinity of myosin heads for the actin filaments
- causes a release (myosin heads detach from actin)
after the myosin heads are detached from the actin, what do the myosin heads do with the ATP
cleave or hydrolyze it
myosin heads cleaving the ATP provides the energy for the myosin head
to undergo another conformational change, which recocks the myosin head: myosin head is displaced towards the pos end of the actin and then once in this position, myosin head has energy it needs to undergo another power stroke; repeats cycle
cross bridge cycling keeps on repeating as long as you have _____ and _____
ATP and calcium
muscle fiber contraction; how does it begin
- action potential arrives along motor neuron innervating that fiber
- acetylcholine (the neurotransmitter) is released from the presynaptic terminal of the motor neuron, opens sodium ion channels leading to action potenial in sarcolemma
- action potential travels along the T tubules
describe what occurs during transduction of the electrical signal across the T-tubule membrane
- action potential spreads down T-tubule
activates dihydropyridine (DHP) receptors which line the T-tubule - DHP receptors tethered to calcium release channels
- calcium diffuses out of sarcoplasmic reticulum into sarcoplasm
- calcium initiates contraction
muscle relaxation
- signal from motor neuron stops
- acetylcholinesterase breaks down acetylcholine
- allows sarcolemma and T-tubules to repolarise (resting membrane potenital)
- calcium concentration in sarcoplasm decreases
- troponin shields the binding sites on actin
- blocks the myosin/actin interaction
how is calcium pumped out during muscle relaxation
- voltage-gated calcium channels in sarcoplasmic reticulum close
- calcium ions pumped from cytosol (sarcoplasm) into sarcoplasmic reticulum
- sarcoendoplasmic reticulum calcium ATPase (SIRCA)
- requires energy
- pumping calcium against concentration gradient
- results in tropomyosin covering up binding sites on actin strands
- muscles will also stop contracting if they run out of ATP
motor unit includes the
alpha motor neuron and all the skeletal muscle fibers it innervates
activity of motor neurons produce
contraction: all or nothing fashion
____ motor units recruited first
smallest
larger ones recruited later for larger/stronger motions
Which of the following molecules is bound to myosin, when the myosin head binds to the actin site?
ADP
Which of the following is NOT a step in the muscle contraction process?
A.
Calcium binds to troponin C, which moves tropomyosin from the active site on the actin filament
B.
ATP binds to myosin causing the dissociation of the myosin head from the actin filament
C.
ADP molecule released during the power stroke
D.
Hydrolysis of ATP allows for the cocking of the myosin head
E.
Tropomyosin covering the active site on actin
E.
Tropomyosin covering the active site on actin
Which of the following is activated by membrane depolarisation within the T-tubule of a skeletal myocyte?
Dihydropyridine receptor (DHP)
Which neurotransmitter is responsible for the initiation of skeletal muscle contraction?
Acetylcholine
Which of the following allows for Ca2+ to re-enter the sarcoplasmic reticulum and terminate the muscle contraction?
Sarco/endoplasmic reticulum calcium-ATPase (SERCA)
Which of the following statements is TRUE regarding muscle developing maximum tension during active contraction?
A.
The muscle tension is inversely proportional to the length of the muscle
B.
The muscle develops maximum tension at the optimal length
C.
The muscle tension is directly proportional to the length of the muscle
D.
The muscle develops maximum tension when it is stretched the most
B.
The muscle develops maximum tension at the optimal length
Which of the following is the first component of the motor unit?
Alpha-motor neuron
smooth muscle in blood vessels function
control diameter, regulate blood flow
smooth muscle in lung airways function
control diameter, regulate air flow
smooth muscle in urinary system function
push urine through ureter, tone of bladder, internal spinchter
smooth muscle in male reproductive tract function
secretion, propels semen
smooth muscle in females reproductive tract function
propels eye (fallopian tube), parturition (myometrium of uterus)
smooth muscle in eye function
control of pupil diameter (iris muscle) and shape of lens
smooth muscle in kidney function
regulate blood flow
smooth muscle in skin function
hair erection (piloerection)
single unit smooth muscle
act as a single unit, in unison
electrically coupled by gap junctions
multi unit smooth muscle
muscle cells act independently allows finer control, more graded response
sphincters contraction/relaxation
normally contracted, when relaxes something passes then returns back to contracted
blood vessels and airways contraction/relaxation
normally partially contracted (tone)
esophagus and urinary bladder contraction/relaxations
opposite of sphincters; normally relaxed, contracts when something passes
stomach intestines contraction/relaxation
when physically activate lots of oscillations between contraction and relaxation
smooth muscle is not _____ unlike skeletal muscle
striated
dense bodies in smooth muscle
dense areas of membrane also help to maintain that
structural relationship of thick and thin filaments
which muscle can shorten to greater degree; smooth or skeletal muscle
smooth
is smooth muscle organized into sarcomeres
no
Smooth muscle still has actin and myosin, thin filaments and
thick filaments. However, it’s not organised into sarcomeres. Instead, it has a large number of intermediate filaments that help maintain its structural integrity
what do smooth muscles have for electrical and chemical communication
gap junctions
smooth muscle is _____ coupled
mechanically
smooth muscle has tropomyosin but no toponin, what does it use instead
calmodulin
smooth muscle has bundles of _____ anchored to the dense bodies, what do they do
non contractile intermediate filaments
serve to anchor the thin actin filaments
smooth muscle sequesters large amounts of _____ ions in the extracellular fluid within _____
calcium
caveolae
caveolae
small invagination in smooth muscle cell membrane (sarcolemma)
2 main ways to get smooth muscle to contract
CICR (calcium induced calcium release)
ligand mediated
ligand mediated way of smooth muscle contraction
- a hormone or a neurotransmitter
to bind to a receptor - This binding of to the G‐protein coupled receptor causes an enzymatic second messenger signal pathway to release calcium into the cell
- May have no direct nerve
supply - Respond to stimuli in the ECF
- Stimulates IP3 formation
- IP3 binds to IP3-gated Ca2+
channel in sarcoplasmic reticulum - Ca2+ released into cytosol
CICR way of smooth muscle contraction
-The first way is via an electrical potential spreading across the membrane
- which opens up calcium channels
- This influx of
calcium, causes calcium‐induced calcium release from the sarcoplasmic reticulum
- This calcium‐induced calcium release is also known as CICR. - - calcium enters into
the smooth muscle cell from the extracellular fluid, which engages a calcium channel
to release more calcium
- so calcium influx stimulates calcium release from sarcoplasmic reticulum
in smooth muscle calcium channels are located in the
caveolus
in CICR the calcium that comes into the cell from the caveolus isn’t really involved with the contraction of the smooth muscle to a great degree; most of the contraction is
induced from what is released from the
sarcoplasmic reticulum
smooth vs skeletal muscle contraction
SKELETAL
- intracellular Ca2+ concentration is sensed by troponin
- moves tropomyosin (thin filament regulation)
- exposes binding sites on actin
- which allows actin-myosin interaction
- leads to cross bridge cycling
- causes contraction
SMOOTH
- intracellular Ca2+ is sensed by calmodulin
- which activates MLCK (thick filament regullation)
- which phosphorylates myosin
- which allows actin-myosin interaction
- leads to cross bridge cycling
- causes contraction
smooth vs skeletal muscle which is thin filament regulation and which is thick
In skeletal muscle, the troponin moves tropomyosin off the active site. This movement of troponin is known as thin filament regulation. thin
filament: actin. So it’s regulating the thin filament.
In smooth muscle, calmodulin
activates myosin light chain kinase. This activation of myosin light chain kinase is
known as thick filament regulation. thick filament: Myosin. So calmodulin
is regulating the thick filament.
in smooth muscle ___ is being regulated
in skeletal muscle ____ is being regulated
myosin
actin
when Ca2+ binds to calmodulin in smooth muscle contraction, the calmodulin-calcium complex activates an enzyme:
myosin light chain kinase (MLCK) which phosphorylates the myosin head
2 mechanisms to relax smooth muscle
- pump calcium out of cell using calcium ATPase or via a sodium-calcium exchanger
- sequester calcium back into sarcoplasmic reticulum
is cardiac muscle striated
yes
does cardiac muscle have sarcomeres with actin and myosin like skeletal muscle
yes
what are some differences between cardiac muscle and skeletal muscle
- cells branch
- gap junctions present at intercalated discs where cells bind together
- contraction is synchronized
- energy from aerobic respiration
one alpha motor unit innervates
many skeletal muscle fibers
each muscle consists of a number of mixed ____ of different fiber types
motor units
for a weak contraction of the whole muscle only one or a few of its motor units are activated, which motor units are recruited first
smallest motor units recruited first
larger ones recruited later for larger/stronger motions
what determines the number of muscle fibers per motor unit and the number of motor units per muscle
the specific function of the muscle
the smaller the muscle unit, the ____ the control of movement
finer
what is a twitch
When a motor neuron innervating the muscle fibre has an action potential, when it
“fires”, it causes a brief increase in the intracellular concentration of calcium ions, and actin‐myosin binding. The result is a brief contraction called a ‘twitch’
what is recruitment
a whole muscle is controlled by the firing of lots of motor axons. The motor
nerves control movement by adjusting the number of motor axons firing, therefore
controlling the number of twitching muscle fibres. This is a process known as
‘recruitment‘.
aka the process of activating more motor units to increase the strength of contraction
in recruitment, why are smaller, fine motor units recruited first
because their neurons are
smaller and reach the excitation threshold more quickly than larger neurons
summation
adding together individual twitch contractions to increase the intensity of the whole muscle contraction
the force of
contraction increases by increasing
the frequency of contractions
tetany
sustained contraction of a muscle; result of a rapid succession of nerve impulses delivered to the muscle
refractory period
brief period of time in which muscle cells will not respond to a stimulus
if muscle is stimulated anytime during refractory period it will
not respond
Stimulating after refractory period but before complete recovery will lead to
second contraction on top of
first one, resulting in summation
what has a longer refractory period; cardiac or skeletal muscle
cardiac; to prevent heart from going into tetany
tetanus and cross bridge cycling
- Sustained elevation in Ca2+ permits greater cross-bridge cycling
- Frequency of action potentials increases
- Duration of elevated cytosolic Ca2+ concentration increases
- Contractile activity increases
- Maximum tetanic contraction reached
With tetanus, the maximum number of cross‐bridge
binding sites remain uncovered, so that
cross‐bridge cycling and tension development is at its peak
active force regulation
Types and number of motor units recruited
More motor units = greater force
passive force regulation
related to initial muscle length; what is ideal length for force generation
to determine total muscle tension have to add together
active and passive tension
Which of the following structures allows “single-unit” smooth muscle to contract in a coordinated manner?
Gap junctions
Which of the following functions is attributed to the smooth muscle located in the skin?
Hair erection
Which of the following is NOT a feature of smooth muscle?
Striations
Which of the following is usually contracted in its normal state?
A sphincter
Where in the myocyte are the smooth muscle calcium-induced release channels located?
Sarcoplasmic reticulum
Which of the following initiates the activation of the IP3 pathway in the sarcoplasmic reticulum?
Hormonal activation of a GPCR
Which protein in smooth muscle has a similar function as troponin does in skeletal muscle?
Calmodulin
Which type of stimulation generates the highest tension in the muscle over time?
Fused tetanus
Which of the following is the first component of the motor unit?
Alpha-motor neuron
Which of the following statements is TRUE regarding muscle developing maximum tension during active contraction?
A.
The muscle develops maximum tension when it is stretched the most
B.
The muscle tension is inversely proportional to the length of the muscle
C.
The muscle tension is directly proportional to the length of the muscle
D.
The muscle develops maximum tension at the optimal length
D.
The muscle develops maximum tension at the optimal length
