Slide 5a

Question 1

What are the different muscle types?

Answer 1

skeletal, smooth and cardiac

Question 2

What does skeletal muscles look like? (list the prominent features)

Answer 2

• 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

Question 3

What does flexor mean?

Answer 3

brings bone together

Question 4

What does extensor mean?

Answer 4

moves bone away

Question 5

What is an example of an antagonistic muscle group?

Answer 5

-tricep and bicep
-back and chest
-lower back and abs
-quadriceps and hamstrings
(flexor-extensor pair) they have opposing actions

Question 6

Describe the structure of skeletal muscle.

Answer 6

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.

Question 7

Explain what happens in the sarcomere during contraction.

Answer 7

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.

Question 8

What are the 4 protein molecules that make up the myofilaments?

Answer 8

1. Myosin: thick filament, do not attach to the Z line.
2. Actin: only they attach to the Z line, globular protein that form fibrous strands
3. 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.
4. Troponin binds tropomyosin to block those active binding sites so there is no contraction without the presence of calcium.

Question 9

What is the mechanism of contraction?

Answer 9

It requires excitation (the action potential) that arises in a motor neuron and travels down the axon of the motor neuron.

Question 10

What kind of transmission would occur during contraction?

Answer 10

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.

Question 11

What kind of signal is released at the neuromuscular junction?

Answer 11

ACh - chemical?

Question 12

Sequence the events of contraction.

Answer 12

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.

Question 13

What are cross bridges?

Answer 13

When myosin binds onto actin it forms cross bridges.

Question 14

What happens after contraction?

Answer 14

Calcium must be removed from troponin molecules which shuts down the contraction. Calcium must be moved back into the SR.

Question 15

How does cardiac muscle contraction differ from skeletal?

Answer 15

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)

Question 16

What mechanisms allow the cardiac muscle to continuously pump blood?

Answer 16

• 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

Question 17

How does smooth muscle differ from skeletal muscle?

Answer 17

• 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

Question 18

What plays a regulatory role in the mechanism of skeletal muscle contraction?

Answer 18

Troponin: there needs binding of calcium to troponin in order to expose actin binding sites

Question 19

Where do we find smooth muscle?

Answer 19

Around any hollow organ.

bladder, stomach, uterus, etc.

Question 20

Describe the anatomy of smooth muscle contraction.

Answer 20

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)

Question 21

What are the two types of smooth muscle?

Answer 21

1. Single unit (visceral)
   - continuous contraction spreading throughout the tissue due to gap junction
   - most common
2. Multiunit
   - each cell must be stimulated on its own for nervous input

Question 22

Describe the single unit smooth muscle.

Answer 22

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.

Question 23

Describe the multiunit smooth muscle.

Answer 23

Must be stimulated individual by a nerve or neurotransmitter. No electrical coupled gap junctions.

Question 24

Molecular mechanism of smooth muscle contraction?

Answer 24

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.

Question 25

Molecular smooth muscle relaxation?

Answer 25

It is much slower because contraction is much longer for smooth muscle. Calcium must be removed out of cell and gets shuttled back into the SR which both require energy. Calcium unbinds from calmodulin so MLCK is no longer active and there’s no more ATPase hydrolysis related to the movement of myosin heads which stops contraction.

Question 26

Compare the length of contraction for each muscle type.

Answer 26

Smooth muscle has the longest contraction which is very important in maintaining homeostasis. It is involved in the movement of material through the lumen (expelling things in the uterus, excretion, etc. within hollow structures.)
Skeletal has the shortest period of contraction and cardiac has longer period of contraction. This contraction is related to allowing for complete filling and emptying of the 4 blood chambers.

Question 27

What is the mechanism related of to the long period of contraction in the smooth muscle?

Answer 27

It is due to the removal of calcium. It time to remove it which causes a longer contraction. Calcium biding the calmodulin is the major regulatory step, calcium needs to get out of cell and back to SR in order to stop activating the MLCK.

Question 28

What are the 6 divisions of smooth muscle types?

Answer 28

gastrointestinal, vascular, urinary, respiratory, reproductive and eye. They are controlled by hormones
(travel long distance in body to find receptor and cause contraction in that tissue type)
eg. estrogen and progesterone produced by ovary but released in blood vessels to travel around body until they find receptors on the uterus to cause menstruation, labour and delivery, ovulation etc.
eg. eye, smooth muscle control contraction of the eye

Question 29

Differentiate the smooth muscle from skeletal muscle.

Answer 29

• smaller
• longer actin/myosin filaments
• not arranged in sarcomeres and has less SR

Question 30

What are the two ways for smooth muscle to contract?

Answer 30

1. MLCK pathway
2. Stretch activated calcium channels: pressure or other force that disturbs the cell membrane
   eg. full stomach to activated stretch receptors to cause this contraction

Question 31

Describe the mechanism of the stretch activated calcium channel in smooth muscle.

Answer 31

Myogenic contraction (pressure stretched the receptors) triggers calcium entry and release of calcium stores.

Question 32

What is the difference between neuromuscular junction vs. the axon terminal

Answer 32

Neuromuscular junction: bigger picture- when a neuron touches the muscle

Axon terminal: smaller picture- the end of a neuron (axon) about to release ACh or other things to the next neuron/muscle (there is the synaptic cleft between).

Question 33

What happens on membrane potential if there is an inhibition of the Achase?

Answer 33

It remains depolarized since ACh causes depolarization, if there is not degradation of it, it continues to depolarize.