8/14/2019 histology #09

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8/14/2019 histology #09

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Here are some points Dr. faraj said in his class, I hope you will benefit

from them.

Skeletal Muscle:

The plasma membrane of skeletal muscle is called sarcolemma. The cytoplasm of skeletal muscle is called sarcoplasm. In nervous system, we mean by nerve fiberaxon.

But in the case of skeletal muscle we mean by musclefibermuscle cell.

But what are the characteristics of muscle cells?1. Cylindrical2. Shows cross striations under light microscope ( which means

alternating between light and dark bands)

3. Multinucleated cells and these nuclei are found at theperiphery of the cell (under the sarcolemma).

4. It is length 1-40 mm which is long5. The diameter 10-100 um. Muscle cell fiber consists of huge number of myofibrils

myofilaments (thin and thick filaments).

The fibers can be seen by light microscope but the thefilaments can be only seen by electron microscope.

As the fiber shows cross striations the myofilament alsoshow cross sriations which is a repeating pattern of lighand dark patter

tns.

Dark band (A- band) anisotropic thick filamentsmainly myosin.

Light band (I- band) isotropic thin filaments mainlyactin but there are also troponin and tropmyosin.

At the middle of the A- band there is a white area called theH-zone or h- band and in the center of it there is the M-line

(which is a protein connecting the thick filaments).

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At the middle of I band there a is a dark line called Z- line.Sarcomere: is the structuralandfunctional unit of

muscle tissue. So sarcomere is part of both myofibril

and myofilament.

The sarcomere is formed of thin and tick filaments. Sowhen these filaments are organized as a sarcomere, this

means that they show cross striations and the filaments areorganized in A and I band.

The sarcomere is between two Z-lines. And we have twohalves of I bands on both sides of the sarcomere.

So the I-band is belongs to 2 sarcomeres.

Whenever there is an overlap between thin and thick filamentsthere is a chance to interact contraction.

So if we excessively stretch the sarcomere, we will nothave overlap and the will be no contraction.

In a contraction the sarcomeres get shortened, the fibrils getshortened but the filaments do not shorten.

The cytoplasm of muscle fiber is full of muscle fibrils. And these muscle fibrils are surrounded by a network

called sarcoplasmic reticulum (Smooth endoplasmic

reticulum) which is the site of storage and release of calcium.

Each reticulum ends by 2 bags (at each end) calledterminal cisternae.

Each sarcomere is flanked by 4 terminal cisternae (2 pairs).Each pair is found between the junction of A and I band.

T-tubules transfer action potential to inside which releasesthe calcium from its storages places.

At the t-tubule wall there is a receptor called DHP, and it isnot away from the protein that closes the channels (calcium

release channels).

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The calcium channels are also called ryanodine calciumrelease channel.

During relaxation there is active calcium pump to reuptake thecalcium back to its storage areas.

When there is an action potential Electrical change

Conformational change in the DHP

The is will pull the foot process away from the calcium

release channels

Release of calcium which will bind to its receptor on

troponin C and this will cause sliding of the filaments which

will cause contraction.

Muscle tone = partial contraction in the muscle. We consider the diaphragm a skeletal muscle.

Cardiac muscle:

It has many characteristics:1. Cardiac fiber means many cells joined by special junction

(intercalated disc) which has gap junction.

2. Shows branching.3. Has gap junctions = low electrical resistance areas.4. There is functional syncitium but notanatomical

syncitium.

5. It has atypical sarcomeres.6. Cross striations are not as obvious as in skeletal muscle.7. The T-tubules are found on the Z-band.8. The sarcoplasmic reticulum is less developed.

Specialized cardiac muscle fibersPurkinje fibers larger than

cardiac muscle cells has more glycogen and less filaments to

produce and conduct action potential

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Smooth muscles:

Have the following characteristics:1.No cross striation2.No sarcomere3. There is thin and thick filaments4. Cytoplasm pink5.No z-line6.No t-tubules7. Myosin filaments quite larger and longer than those present

in skeletal and cardiac muscles

8. Contraction slow but sustained

GOOD LUCK for u all in the FINALS

Special Thanks for Wael Toukan & Hamzeh Jassar fortheir great efforts.