Skeletal muscle Flashcards
What is muscle?
soft tissue composed of myocytes (muscle cells/fibres) which contracts to produce force and motions.
-By transforming chemical energy in the form of ATP into to mechanical energy
What else can a muscle be used to refer to ?
to an organ , containing muscle tissue and other tissue.
What is muscle responsible for ?
- Many tasks such as:
1. Locomotion
2. Posture (muscle tone)
3. Generating heat (shivering)
4. Retaining heat (hair follicles)
5. Nutrition – propelling substances (peristalsis)
6. Respiration (breathing)
7. Circulation (pumping blood)
8. Regulating sight and hearing
9. Facial expression
What are the 3 types of muscle tissue?
- Skeletal
- Cardiac
- Smooth
Where is the location of the 3 muscle tissues ?
- Skeletal- inside skeletal muscles- attached to bones or skin-
- Cardiac- within the heart-
- Smooth-walls of hollow organs-
What is the morphology of the 3 muscle tissues?
skeletal and cardiac tissue is striated
smooth muscle is non striated
What is the physiology of the 3 muscle tissues?
Voluntary(somatic)- Skeletal
Involuntary(autonomic)- cardiac and smooth
What does a generalised cell contain ?- cellular anatomy
- Plasma membrane: a selectively permeable outer boundary
- Cytoskeleton: series of filaments and tubules for cell structure and motion
- Cytoplasm: intracellular fluid packed with specialised organelles –
1. Nucleus: controls cellular activity. Typically located near the centre.
2. Mitochondria: the powerhouse of the cell (source of ATP)
3. Rough endoplasmic reticulum: catalytic enzymes for protein synthesis
4. Smooth endoplasmic reticulum: catalytic enzymes for other processes (incl. calcium storage and release)
What is the cytoskeleton like in the myocyte ?
-Cytoskeleton: densely packed with myofilaments of actin, myosin, & titin–packed full of contractile proteins and those are the proteins that a re specialised for causing motion
What is the plasma membrane like in a myocyte ?
it is -called Sarcolemma
What is the cytoplasm like in a myocyte?
-called Sarcoplasm, particularly rich in mitochondria- convert chemical energy into motion (mehcnical energy)
What is the nucleus like in the myocyte?
-Multinucleated (up to 100).
As densely packed with my-filaments the Nuclei pushed out to the periphery
called Sarcosome, unusually large
What is the smooth endoplasmic reticulum in the generalised cell called in a myocyte?
-called Sarcoplasmic reticulum, unusually large & full of calcium
What is the size of the myocte ?
Diameter : 10-40um
Length: 1-40mm
What is the skeletal tissue composed of?
4 tissues
- Muscle tissue
- connective tissue
- blood vessels
- nerve tissue
What are each skeletal muscle ?
a discrete organ, attached to bone
What is the muscle tissue of the skeletal muscle?
-each skeletal muscle is composed of multiple fascicles
What is each fascicle composed of?
bundle of multiple fibres
what do fascicles look like?
bundle of sticks
what are MYOcytes known as?
fibres because they are elongated in one direction ion
What does a muscle contain ?
1000s of densely packed fibres
What does each myocyte contain?
multiple myofibrils
What is the connective tissue of skeletal muscle?
What is holding everything together is:
-Tendons- which attach muscle to bone
What are the 3 types of sheath that compartmentalise the muscle and transmit blood(separate the muscle in different compartments and transmit blood around the muscle) in the connective tissue of skeletal muscle?
- Epimysium- wraps the muscle as a whole
- Perimysium- wraps each fascicle
- Endomysium - extends between individual myocytes
What is the blood vessel tissue of a skeletal muscle?
- Muscle tissue is well vascularised with abundant blood vessels - artery and vein supplying muscle
- A lot of blood is required to as job of ,muscle is to convert chemical energy via ATP to mechanical energy and that requires oxygen and takes a ,lot of nutrients to metabolise hence good blood flow for this and need to take away metabolic waste .
What is the nerve tissue of a skeletal muscle?
each skeletal myocyte is innervated by a single motor axon
- The same axon may alsoinnervateother fibres in the same motor unit (one-to-many).
- Many nerve fibres target muscle. Muscles are the effectors of most of our behaviours.
What is the most prominent feature of skeletal muscle ?
their striations
What are striations ?
are a property of individual myofibrils. (They are visible in skeletal myocytes as a whole since each fibril is precisely aligned
When muscle tissue is viewed longistudinally- can see the striations
What is muscle made of?
Many fascicles which contain myocytes which contain myofibrils which contain myofilaments
What are the 3 types of myofilametns the myofibrils contain?
- thick (myosin
- thin (Actin)
- elastic (titin)
What is the structure of myosin?
- Myosin is an elongated protein molecule composed of a heavy ‘tail’ and 2 light ‘heads’
- The junction of the heads and tail can be regarded as a ‘hinge’
- The heads form part of the ‘cross bridges’ that can attach to actin
- thick (myosin) filament is made up of many individual myosin molecules
What is the size of myosin?
15nm in diameter
What is the structure of actin?
Made up of 3 components:
A helix of two strands of globular actin (a multi-functional protein containing ‘active sites’)
Thin rod like tropomyosin molecules in a helix which lie in the groove formed by twisted actin (cover ‘active’ sites in resting state)
Troponin at regular intervals along tropomyosin – complex of three proteins one of which has a high affinity for calcium ions
What is the size of actin?
7n in diameter
What is then structure of titin?
- are composed of titin (aka ‘connectin’): a large springy protein
- They run through the core of each thick filament, anchoring it in place. It aids in preventing overstretching of the thick filament, allow recoiling like a spring whenever a muscle is stretched
What is a sarcomere ?
Groups of thick (myosin), thin (actin), and elastic (titin) myofilaments are arranged together
What is the structure of a sarcomere?
- At the end of each sarcomere is connective tissue which is the Z disc
- Thin actin filaments are attached to the Z disc
- nterdigitating between the actin filaments are the thicker myosin filaments, anchored to the Z disc by titin
What are striations ?
- property of individual myofibrils. They are visible in skeletal myocytes as a whole since each fibril is precisely aligned.
- precise alignment and the way myofibrils and myofilaments are arranged in the sarcomere
What is each myofibril made up of?
a repeating series of sarcomeres composed of the myofilaments actin, myosin, and titin.
define sarcomere?
A sarcomere is the distance between two Z discs (connective tissue)
What is the I band ?
The region containing thin actin filaments alone appears light
What is the A band?
The area of overlapping actin and thicker myosin is dark.
What is the H zone?
The region of myosin only
What is the structure of sarcoplasmic reticulum?
Within a fibre, small bundle of myofibrils are surrounded by
What is the sarcoplasmic reticulum ?
modified smooth ER - network of tubes enclosed in a membrane
- stores calcium and releases and demand when muscle fibre stimulates it.
- Run londitiudinally
What is the Trasnverse tubular system?
- Another system of tubes running at right angles to the surface of the fibre,
- ), is a continuation of the sarcolemma (plasma membrane ). This system ends, along with the terminal cisternae of the SR, on each sarcomere (forming a ‘triad’).
What are the ER tubules joined by?
the transverse tubular system
What is the job of the transverse tubular system ?
to conduct electrical impulses right down into the deepest regions of muscle cell
Summary of structure of skeletal muscles ?
Muscle (organ)
Muscular, vascular, connective & neural tissues
Fascicles
Bundles of myocytes
Myocytes (aka myofibers aka muscle cells)
Elongated cells specialized to contract and cause movement
Myofibrils
Repeated sarcomere units
Sarcomere
Multiple precisely aligned myofilaments
Myofilaments
Thick (myosin), thin (actin), & elastic (titin)
Protein molecules
myosin & actin molecules (contractile proteins) & titin (aka ‘connectin’) for elasticity, + regulatory proteins
What is the sliding filament theory ?
During contraction, it can be observed with EM that:
Width of A (dark/myosin) band unchanged
Width of I (light/actin) band decreases
This implies that shortening is due to actin and myosin sliding over each other
What are the 4 main stages of muscle contractions?
- Initiation
The propagation of an ‘onset’ signal from a motor neuron- contracts on demand skeletal muscle is voluntary - Excitation–contraction coupling
The process by which amuscular action potential(at the sarcolemma) causes themyofibrilsto begin contraction - The cross bridge cycle
The contraction itself - Cessation
Stopping contraction and return to resting state
What is stage 1 of muscle contraction?
Initiation
- Starts with a motor neuron Action potential in motor neuron
- Voltage-gated Ca2+ channels open; passive influx of Ca2+
- When enters in axon terminal in motor neuron Ca2+ causes ACH to be released by exocytosis( form of active transport-requires energy-molecules physically
- ACh travels cross synaptic cleft & binds to post synaptic receptors
- Depolarisation of muscle fibre – end plate potential (EPP)- spark
What does initiation cause?
causes EPP (end plate potential)
What is the stage 2?
Excitation-Contraction coupling
- Depolarisation of muscle fibre – end plate potential (EPP)- generated at neuromuscular junction causes a wave of depolarissation- opens up Na channels, na enters along ec gradient then that causes a.p to eb generated which spreads to the next voltage gated sodium channel
- Hence have propagation of electrical signals of action potentials spreading out on the membrane of a muscle cell - Surface propagation of action potential (AP)
What is phase 2 of stage 2 ?
The contraction
- The AP propagates along the sarcolemma and down the T tubule- carry electrical impulse deep into the heart of the myocytes
- causes Voltage-gated Ca2+ channels in the two neighbouring terminal cisterns of the SR open (re: the triad)
- causes Ca2+ stored in SR to floods into the cytosol down its concentration gradient
- Ca2+ binds to troponin(on actin), causing it to change shape & remove the blocking action of tropomyosin that cover binding sites. The myosin-binding sites on the thin (actin) filaments are now exposed.
- A ‘cross bridge’ is formed between thin (actin) & thick (myosin) filaments. Cross bridge cycling can begin
What is the 3rd stage?
Cross bridge cycling
1.The myosin head is
‘Hinged’- can move up and down and back and forth
‘Lazy’- low energy configuration
- Adenosinetriphosphate(ATP) gives it the energy it needs to ‘stand up straight’- through the transmission of energy through ATP
When in this high energy state, is said to be ‘cocked’- NOW REAADY TO bind onto actin
What are the 4 steps of stage 3 ?
1.Cross bridge formation
EC-coupling exposes myosin binding site. High-energy myosin head binds, forming a cross bridge
2. Power stroke by myosin head
ADP & P released & myosin head returns to bent, low-energy state. This pivot pulls the thin (actin) filament towards the centre of the sarcomere. (still attached)
3.Cross bridge detachment
ATP molecule binds to myosin head, causing it to lose its actin affinity and detach. (still in low-energy state)
4.Reactivation of myosin head
Energy released by hydrolysis of ATP reactivates the myosin head, returning it to its original high-energy state
What does the cross bridge cycle do?
-. This cycle will continue to repeat, progressively dragging the actin filament along and attaching to the next myosin site and dragging the actin along.
Why are myosin heads not synchronised?
-so can be in different steps of the cycle.
-This allows some to hold the actin filament in place stopping the actin form slipping back between strokes (like a ratchet).
Remember, there are 1000s of sarcomeres in a myofibril, and 1000s of myofibrils in a myocyte, and 1000s of myocytes in muscle tissue
What is the 4th stage of muscle contraction?
Cessation
- Removal of Ca2+ by active transport back into the SR
- Tropomyosin blockage restored, blocking myosin binding sites in the actin.
- No new cross bridges can now be formed- as everything is detached
- Contraction ends and myocyte relaxes
How are we able to perform different gradations of movement?
- Spatial summation (aka motor unit recruitment)
- Temporal summation
What is rigor mortis/in dead people?
-Rigor Mortis:
Without respiration, ATP synthesis ceases, and detachment becomes impossible.- in cessation
How can the size of muscle contraction is controlled in two ways?
- Spatial summation (aka motor unit recruitment)
- Temporal summation
What’s spatial summation?
- Each muscle is served by at least one motor nerve with a variable number of axons
- Each axon forms a neuromuscular junction with a single muscle fibre
- A motor unit consists of one motor neuron and all the muscle fibres it innervates
- When a motor neuron fires (transmits an action potential), all the muscle fibres it innervates contract.
How can the size of muscle contraction can be varied in spatial summation”?
by altering the number of motor units active
What do you need for the finest possible control ?
the size of each motor unit would then be 1 (the smallest possible)
What is an example of really fine control in spatial summation?
so if you consider one muscle that is made up of 10 motor units as N=1
-1 active motor unit would result in a small contraction, 2 active units in a bigger contraction etc
-So therefore the muscle would have 10 different degrees of contraction
which is extremely precise (fine control) and very expensive.
-Less muscle fibres in a each motor unit - more fine control
What is an example of less fine control in spatial summation?
If, instead, the muscle, had 2 motor units, each consisting of N = 5 muscle fibres, the muscle would only have 2 degrees of contraction etc
-More muscle fibres in each motor unit- less Fine control
So what do small delicate muscles with fine degree of control consist of?
motor units containing only a few muscle fibres e.g. motor units of larynx only 2-3 muscle fibres/motor unit – eye muscles: 3-10/unit
What do larger muscles (e.g. those involved in locomotion) consist of?
units of about 150-500 muscle fibres
What happens with postural muscles ?
- No fine control
- 800+/units of muscle fibres controlled by 1 motor neuron
What is the size principle?
- For further Fine tuning
- The motor units with the smallest muscle fibres are activated first because they are controlled by the smallest, most highly excitable motor neurons
- As motor units with larger and larger muscle fibres begin to be excited, contractile strength increases.
- The largest motor units, containing large, coarse muscle fibres, are controlled by the largest, least excitable (highest threshold) neurons and are activated only when the most powerful contraction is necessary .
How can the size of muscle contraction can be varied in temporal summation? And what does it mean?
This is varying the frequency of stimulation of a muscle (rather than altering the number of active fibers)
The higher the frequency, the greater the strength of contraction of a given motor unit
How does temporal summation work ?
It has 3 phases:
- Latent period
- Contraction phase
- Relaxation phase
What is the latent period?
: interval between stimulus and the beginning of contraction
During this period, cross bridges begin to cycle but muscle tension is not yet measurable so the myogram does not show a response.
What is the contraction phase?
when the muscle is shortening
–Myocytes pulling together and muscle shorten -Cross bridges active. 10-100 ms.
What is the relaxation phase?
during which tension declines
Ca2+ actively pumped back into SR. Cross bridges declining (& contractile force with it)- takes a while to do it
What is a muscle twitch?
the response of muscle to a single stimulus.
How can temporal dynamics vary between muscles ?
via Slow muscles and fast muscles
-These differences reflect variations in enzymes and metabolic properties of the myofibrils.
What are slow muscles ?
Usually active for prolonged periods of time(e.g. posture)
Uses a lot of energy
Thus, have many mitochondria & myoglobin (for oxygen storage)
Myoglobin gives muscle a red colour
What are fast muscles?
Usually only briefly active
Large sarcoplasmic reticulum (for rapid release and uptake of calcium)
Little myoglobin, resulting in white colouration
If the the electrical event (nerve impulse) leading to muscle contraction is over before muscle twitch begins, What does this mean?
It is therefore possible to stimulate the muscle again, before the contraction caused by the first stimulus is over.
What is temporal summation?
When the time between stimuli is decreased, the 2nd contraction starts before the previous one is over – the amplitude of contraction is bigger
What is a fused tetanus ?
A large smooth contraction in response to a series of stimuli is known as fused tetanus (no relaxation between stimuli)
When will the largest contraction be observed?
The largest contraction will be observed when the second contraction begins before the muscle has started to relax from the previous stimulus
What happens when when the time between successive stimuli is far?
apart one observes separate twitches
How are cardiac muscle similar to skeletal muscle tissue?
Cardiac muscle is striated (i.e., arranged into sarcomeres). It can therefore generate strong force
How is cardiac muscle tissue different to skeletal muscle tissue?
Involuntary
Cardiac fibres have only a single, central nucleus
Cardiac fibres branch
Cardiac fibres are joined by gap junctions (intercalated discs) forming a syncytium (cell-to-cell ion passage ensures rapid & synchronised propagation of APs throughout the heart)
Cardiac muscle is myogenic (activation arising from muscle tissue, rather than nerve impulses)
How is smooth muscle similar to skeletal muscle?
- Smooth muscle part of the musculoskeletal system
- Has actin and myosin fiolamnents and the way they produce force is the same
How is smooth muscle different. to skeletal muscle?
Involuntary
One, central nucleus
Small, spindle shaped cells
No striations (no sarcomeres)
The sarcomere arrangement of skeletal muscle ensures rapid and powerful contractions.
But sarcomeres will not contract if the muscle is too stretched
The more irregular arrangement of smooth muscle actin/myosin of smooth muscle means there is always enough overlap to generate tension
Less actin & myosin (10%)
Poorly developed SR – slow calcium release
Modified E-C mechanism
Contraction can be chemically induced
Innervated from multiple sources, both intrinsic (within organ) and extrinsic (CNS and both the parasympathetic and sympathetic pathways of the ANS)
What do smooth muscles also do?
important role in vision.
Why is it good that smooth muscle contraction is relatively slow and feeble?
It uses less energy, which is important as smooth muscle is continuously active (e.g. in stomach, eye, blood vessels etc.)
ones does not always want to generate a lot of force (imagine violent contractions of the stomach wall – pain & food ejection from mouth!
