histo 12 muscle tissue

8/3/2019 Histo 12 Muscle Tissue

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Histology Script # 12

Topic : Muscle TissueDr Qasim al - Dwairi

Motion and movement or motility is an essential need for all organisms. We

have seen in the previous classes when we spoke about the cells that there are

extracellular movements which are achieved by a certain areas like the

microtubules, the actin and so on. But in the multicellular organism like the

human being, you need motion. And this motion could be external that

involves the skeletal system, or it could be internal; moving the certain content

within the different visceral organs like the gastrointestinal system, like the

renal system, etc. So motion is essential; a very important process and of

course when we talk about motion there's always involvement of energy in the

form ofATP.

And of course when we talk about the movement, in another word, there are

certain contractile component which are essential and important mainly the

main ones which are the myosin and actins which are present in thesecontractile cells. Now, the muscles: they are defined as excitable tissues. I think

you have already come across another excitable tissue which is the nervous

tissue.

Now, the muscles we have according to the location, we have three types of

muscles :

1) Skeletal muscle - associated with the skeletal system

2) Cardiac muscle - present within the wall of heart

3) Smooth muscle - present within walls of visceral organs

Now, there are certain terminology which we use when we talk about the

muscles.


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For example, when we talk about the cell cytoplasm in the muscle, we say

sarcoplasm. So the suffix 'sarc' is said before the name of structure.. We say

sarcoplasm for cytoplasm, sarcoplasmic reticulum, sarcolemma, etc. So the

'sarc' suffix describe the different organelles which we normally refer to when

we study the cells in the previous lectures.

Alright, so regarding the muscles, basically the muscle in particular the skeletal

muscle, the cardiac muscle, they have terminally differentiated themselves,

they originated themselves from the mesoderm layer. They provide, of course,

contraction, that results in movement for motility and they are vascular and

they have nerve supply. Of course these skeletal muscles are important in themovement of the levels and different parts of the skeletal system. And the

cardiac muscle; contraction of the heart. The smooth muscle is responsible for

the movement contents that is present in different system; as in peristalsis in

the visceral organs, we also have other things which are vasodilation and

vasoconstriction which are essentially the function of the smooth muscle

within the blood vessels' walls; and that is how the blood pressure is regulated

as you studied in physiology

The skeletal muscle form almost 50% of the body weight. And of course, the

tissues responsible for the contraction, you know that there is glycogen

deposits within the muscles as well as in the?.... where else? ----the liver!

Alright, now we describe the skeletal muscle as striated. Now, let's have a lookat the description of the skeletal muscle and let's start from how they begin

their development. In the beginning, the mother cells that gives rise to the

skeletal muscle - we call them myoblast. Just like we said osteocytes,

chondroblast...etc. Which are derived from the mesoderm. So what happens to

this myoblast ; they fuse end to end and form the myotube. And after they

form the myotube. After that we have the lay down ofcontractile proteins.


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Myoblast (fused end to end), each myoblast has a nucleus. When all the

myoblast fuse forming the tube, we have several nuclei. After that we have the

lay down of the contractile tissue; and there will be lots of contractile tissue,

we're going to stuff it with contractile tissue. So these contractile tissues which

are protein, they are going to displace nuclei peripherally outwards, and

squeeze them. That is why they will look flat. So, base on this, if I go back to

describe the structure of the skeletal muscle, we will say they are, unbranched,

long tubular fibres and these they have many peripheral flat nuclei and they

show striations.. So now, we understand why the fibres are elongated -

because many myoblasts fuse together and form the tube. We understand why

they have many nuclei because each myoblast has a nucleus. And we

understand why the nuclei are peripheral and flat - because lots of contractile

protein are being stuffed within these tubes so the nuclei are displaced outside

the periphery.

Now let's go on to what they call the striation. We have the arrangement -

individual fibres that they refer as myofibril. And the myofibrils are arranged

within fascicle. And the fascicles are arranged into what we call the musclefibre. So we have myofibril; and the myofibril are surrounded by a CT

(connective tissue) layer and that is called the endomysium. And these

myofibrils are arranged into fascicles and again they are surrounded by CT

layer which isperimysium. Then, all the fascicles are surrounded by another CT

layer and that is the epimysium. And that is why I told you earlier we spoke

about the dense CT, and I gave you an example; the tendon. I said because this

tendon is the connection of all ; the endomysium, perimysium, and epimysium.


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Now let's go more to the ultrastructure.

So as you can see each one muscle fibres and the endomysium

(shown in picture, appear as white, adjacent to muscle fibres). We

see many flat peripheral nuclei.. And we

see these cross striations. What are these

cross striations?? So here you can see in

another section, individual muscle fibres,

elongated, non-branched, flat peripheral

nuclei, the adjacent white color is the

endomysium and we see these striations.

These striations are usually due to two main reasons:-

1) The arrangement of the contractile protein

2) The arrangement of myofibrils with each other

Contractile protein - that is the actin and myosin. Within individual units, we

call them sarcomeres. The skeletal muscle and cardiac muscle, their contractile

proteins are arranged into units called sarcomeres. What is a sarcomere? --It isthe distance between one Z-line and another Z-line.

A-band - myosin.

H-band - the band that is not overlapped by actin no matter how much is the

contraction .

Contractile Process(just like in physiology) :P

What is created on these myosin? They are globular heads of myosins - that

have active site underneath the actin. Those active sites usually are covered by

Tropomyosin. The Tropomyosin is removed when the Calcium bind to Troponin

C. In a while we will see where is this Calcium. Action potential is from a nerve

ending that reaches a muscle with what you call the neuromuscular junction.

Let's say the example for the neurotransmitter is acetylcholine, alright?

Acetylcholine is present within the vesicles. It is exocytosed into the cavity


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between 2 membranes; the nerve and the muscle. What happens to the

acetylcholine after it is released? It binds to surface. After that, there will be

conformational change and that will lead to opening of channels.

Now, the ions which are present more on the muscle are the Potassium (K+);Outside are the Sodium (Na+). Then the process of the passage of ions; they

attempt to reach the equilibrium - this is Action Potential. Now, this Action

Potential will be transmitted by the muscle membrane via the T-tubule

(discussed later in this lecture). That will reach to the storage calcium in the

muscle - the sarcoplasmic reticulum (also further discussed later). Calcium is

released, binds to troponin C. The troponin C will remove the tropomyosin that

covers the active site of actin. There will be binding of globular head of myosin

with the active site of actin.

And what's going to happen? we have actin sites, acting from the site, they will

pull the actin towards the inside. (actin pulled towards center)

Now we come back to the actual structure of sarcomere.

We have between muscle fibres, hundreds and thousands of sarcomeres. Thatis one point. The other point, you can see that each myofibril, the arrangement

of these dark lines are also in register with the other myofibril. So the cross

striation, it is not due ONLY to the arrangement of contractile protein but the

arrangement of the myofibrils with each other.

______________________

Now, we see another arrangement and that is the T-tubule system. What is T-

tubule system? It is invagination from sarcolemma (plasma membrane of

muscle cells). And these T-tubules will go around each fibre(myofibril) and

move to the second fibre and go around the second fibre, and so on...until they

finally reach this cisternae - that is what they call the sarcoplasmic reticulum

(the storage of calcium)


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Action Potential transmitted from the membrane of muscle via T-tubulesystem till it reaches the storage of calcium within the sarcoplasmic

reticulum. So that is where calcium is released.

_________

Another thing we want to talk about the skeletal muscle and this you have

come across in physiology(really?? :P) and that is muscle tone. We do know

that the skeletal muscle is a voluntary muscle. That means, we decide how

much force we need. According to the force that we need, we have

recruitment of different motor units to the contractile. In other words, not all

the motor units in the skeletal muscle contracts. They are recruited according

to the force I need; because it is a voluntary muscle. In certain places (and most

of it), if you touch the muscle, you feel certain firmness of the muscle, different

than when you touch fatty part of your body. Now this firmness of the muscle

is what we call the tone of the muscle. The muscle eventhough it is at rest,

some fibres or some sarcomeres are recruited to contract. So these few

members of sarcomeres that they are required to contract at different times,

give the muscle the firmness that you feel - so this is what you call the tone ofthe muscle.

Important Question that involves Physiology & Histology :During contraction of skeletal muscle, does the myosin length change??

No, only the actin changes. Regarding the band, the H-band changes.

(The doctor showed and described several pictures on the slides to visualize what he just mentioned earlier)

Axons have branches that penetrates the muscle. Usually the nerve supply for

the muscle is somewhere in the middle.


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Where do you find the sarcoplasmic reticulum? Between the junction of the A-band and the I-band.

*The doctor said that they will ask these questions* He said that later, you will find the location is

different in the cardiac muscle.

In the skeletal muscle, we have an arrangement of sarcoplasmic reticulum and

the T-tubules ; we call this triads. We explain that in relation to cardiac muscle,

where we have diads ; one opening of sarcoplasmic reticulum and one of T-

tubule.

Skeletal Muscle Types : -

1) Type I - (Slow) Red Muscle

2) Type II - (Fast) White Muscle

Type I

Thinner, smaller in size, more amount of myoglobulin and mitochondria,more blood supply

Low ATPase activity (slow utilization of energy), rely on ready-madeenergy

Postural function (function in maintaining posture) Contract slowly but for long period of time

(eg:Dajaj Baladi / Country Chicken)

Type II

Thicker, less myoglobulin High ATPase activity Function in fight-or-flight situation Contract fast and powerful but for short period (fatigue easily)


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How athletes are trained :Marathon runners - More red fibres

Question: What determines the contraction of muscle fibres ; whether itis slow or fast?

The nerve supply (Either fast twich or slow twich muscle fibres)

The doctor gave a few analogies for the muscle types to help us understand and

remember better:

1)Deer - Has more White fibres

Tiger - Has more Red fibres

At the beginning, tiger can't keep up with deer because it is running too fast

(fight or flight; white muscles), but soon, deer will tire easily and tiger can

catch it.

2) Shopping using Visa Card (White Muscle) or Cash (Red Muscle)

Type of neuron/innervation determines type of musclesNow, not all the muscles in the body are Fast or Slow muscle.

- Limbs : red fibres like soleus muscle (under gastrocnemius muscle)- Rectus abdominis : white fibres

Most of the muscles are actually Mixed Muscle. WHY? because of their

functional need.


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Examples of mixed type of muscle :

Diaphragm.Responsible for main movement of respiration. Sometimes, while at rest, we

are at ease, and muscle contracts at ease. When we run and so on, respiratory

rate increase (15 - 52), so we're required to recruit more red muscle fibres.

Gastrocnemius

Blinking your eyeWhen come in front of your eyes, your eyes twich fastly. (more white fibres)

Done by : Farah Nadzirah

Special Thanks to :

- Nabihah comel (x ikhlas nih..haha)- Aina Amalia and Haniz Amirah (nak jugak kan :P)

Success doesnt mean achieving high grades or being on the top,

Its about not giving up everytime you feel like youre at the bottom

Believe in yourself, believe in the power of perseverance. Believe that He can

help you through anything.

Good Luck!

histo 12 muscle tissue

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