What are the different muscle types?
skeletal, smooth and cardiac
What does skeletal muscles look like? (list the prominent features)
- multinucleated
- arranged in sarcomeres
- striated (they are associated to the arrangement and mechanism of contraction of thick and thin filaments)
- usually attached to bones and tendons
What does flexor mean?
brings bone together
What does extensor mean?
moves bone away
What is an example of an antagonistic muscle group?
-tricep and bicep
-back and chest
-lower back and abs
-quadriceps and hamstrings
(flexor-extensor pair) they have opposing actions
Describe the structure of skeletal muscle.
The core sheath: epimysium (connective tissue), perimysium (touch connective tissue) that binds together fascicles and within there is myofibrils which are arranged in sarcomeres. (this is from Z line to Z line).
In there the fascicle, there are thick (myosin) and thin (actin) filaments.
Many sarcomeres in a myofibril, the sarcolemma (membrane which is at the muscular junction in which the action potential travels through) overlies the myofibril.
Sarcoplasmic reticulum (type of smooth ER) unique to muscle and serves the function of releasing Calcium (which initiates contraction). It is important for Ca regulation during contraction.
T tubule is an inward invagination of the sarcoplasmic reticulum (an extension of the sarcolemma) so you can carry the action potentials to travel deep within the muscle cell to carry Ca there in order to trigger change in electrochemical gradient.
Triad arrangement: allows electrical signal to travel deep in muscle to stimulate contraction. (t tubule, SR, sarcolemma)
Muscles cells contain many mitochondria for ATP in order for contraction to occur.
Thick and thin filaments in the myofibril connect within Z line (sarcomere) gives the striations in skeletal muscle.
Explain what happens in the sarcomere during contraction.
Sarcomere is the contractile unit.
It is separated in segments of Z line to Z line.
A band: where thick and thin filaments overlaps. It is the densest region due to the overlapping. The Z line moves closer together during contraction.
What are the 4 protein molecules that make up the myofilaments?
- Myosin: thick filament, do not attach to the Z line.
- Actin: only they attach to the Z line, globular protein that form fibrous strands
- Tropomyosin: blocks the actin binding sites on actin so that the thick myosin head can only thin filaments when there is calcium present. so when calcium is not present.
- Troponin binds tropomyosin to block those active binding sites so there is no contraction without the presence of calcium.
What is the mechanism of contraction?
It requires excitation (the action potential) that arises in a motor neuron and travels down the axon of the motor neuron.
What kind of transmission would occur during contraction?
Celltatory conduction: requires axon with myelin sheaths and conduction of signal jumping from node of ranvier to node of ranvier. It needs to be fast.
What kind of signal is released at the neuromuscular junction?
ACh - chemical?
Sequence the events of contraction.
Presynaptic motor neuron release ACh, binds its receptor on the muscle fibre which causes depolarization of the membrane which is sarcolemma (due to increased sodium entry inside the cell). The action potential ravels deep into the membrane via the t tubules. Once it reaches the DHP receptor in the sarcolemma and since this receptor is connected to the SR surface RXR receptor, muscle contraction begins. The voltage change triggers the opening of calcium voltage gated channels on the SR which causes calcium to move out of the SR calcium stores. Calcium binds troponin which allows tropomyosin to shift in order to reveal active binding site so the myosin can bind to actin. This triggers hydrolysis of ATP to ADP (aka the power stroke), releases energy which causes a bending of the myosin head shifting the actin, pulling the Z lines together.
What are cross bridges?
When myosin binds onto actin it forms cross bridges.
What happens after contraction?
Calcium must be removed from troponin molecules which shuts down the contraction. Calcium must be moved back into the SR.
How does cardiac muscle contraction differ from skeletal?
Cardiac muscles are only in the heart. It forms the bulk of the walls of the four chambers.
- striated (looks similar to skeletal muscle)
- involuntary
- contracts rhythmically due to the ARC at SA nodes (it generates its own action potentials)
- muscle fibres are branched (intercalated disks important for electrical signal transmission around the heart)
- ARC located specifically at the SA node (set the beating rate)
- contractile, the generated action potential must reach the contractile cells which travels via the intercalated disks
- have SR (calcium storage unit) but not highly arranged as skeletal muscle
- diad of t tubule & SR (not triad like skeletal muscle)
What mechanisms allow the cardiac muscle to continuously pump blood?
- coupled gap junctions (intercalated disks which exhibit branching
- not tapered (tapered=the spindle shaped cells)
- continual contractile band that goes around chambers (allows coupled filling and emptying)
- longer retention of calcium in SR for longer contractions which allows complete filling and emptying of heart chambers
- cannot run low on ATP by having more mitochondria than other muscle types
- self stimulating due ARC: does not require any other input to contract but can receive input to be modulated by the nervous system in face of challenges by the body
How does smooth muscle differ from skeletal muscle?
- single cells with single nuclei
-no t tubules
-loosely organized
-calcium comes from outside and inside cell (where skeletal muscle calcium comes from only within the cell SR) - does not bind to troponin but calmodulin instead
-not striated due to different arrangement
the myofilaments are not organized in sarcomeres (which are the units of contraction in skeletal)
-smaller muscle fibres
-longer myosin and actin
-myosin ATPase activity is much slower
What plays a regulatory role in the mechanism of skeletal muscle contraction?
Troponin: there needs binding of calcium to troponin in order to expose actin binding sites
Where do we find smooth muscle?
Around any hollow organ.
bladder, stomach, uterus, etc.
Describe the anatomy of smooth muscle contraction.
contraction: balling up causes fibres to shorten, exhibits “dimples” since they are anchored to the plasma membrane - called dense bodies (anchored to cell membrane causing ball like formation)
What are the two types of smooth muscle?
- Single unit (visceral)
- continuous contraction spreading throughout the tissue due to gap junction
- most common - Multiunit
- each cell must be stimulated on its own for nervous input
Describe the single unit smooth muscle.
Release of neurotransmitter, it binds its receptor causing contraction where it spread throughout smooth muscle to cause contraction. It contracts as one via the spreading of gap junction. It is more similar to cardiac muscle where there is transmission of signal via coupled junctions = intercalated disks.
Describe the multiunit smooth muscle.
Must be stimulated individual by a nerve or neurotransmitter. No electrical coupled gap junctions.
Molecular mechanism of smooth muscle contraction?
Begins with rise or spike of intracellular calcium. Calcium enters cells triggered by neurotransmitter binding to receptor. Calcium moves into the cell and also released from the SR. It then binds to calmodulin (regulatory step) and binds myosin light chain kinase MLCK (which becomes activated). Now it can phosphorylate the light chain on the myosin heads and then allows ATPase activity and allows cross bridge cycling where the myosin heads can bind the actin filaments and contraction can occur.
