Tissue Type 3- Muscle

Question 1

Learning Objectives

Answer 1

• List the three types of muscle tissue and where each may be found.
• State that the contractile proteins, actin and myosin, are found in large amounts in all three types.
• Describe the histological and ultrastructural features of each type of muscle
• Describe why the ultrastructural arrangement of actin and myosin gives smooth and striated muscle their typical appearance.
• Relate the histology and ultrastructure of striated muscle to the sliding-filament hypothesis of contraction.
• List the connective tissue elements which are associated with skeletal muscle.

Question 2

What are the 3 types of muscle?

Answer 2

Skeletal, Cardiac, Smooth
Important points to remember:
-ALL muscle types contain the contractile proteins, actin and myosin in large amounts
-All cells contain some actin and myosin

Question 3

What are the features of skeletal muscle?

Answer 3

• Also known as striated/voluntary muscle
• Found attached to bone, usually
• Has limited powers of regeneration but capable of hypertrophy (enlargement) of existing myofibre
• Long peripheral multinucleated cell as the cells are formed in the embryo by fusion of progenitor cells (myoblasts)

Question 4

What are the main histological features of skeletal muscle?

Answer 4

Main Histological Features:

• Myofibres have cross striations (dark and light) bands -Myofibres are long, unbranched and multinucleated
• Nuclei occur at periphery of fibre

Question 5

What is the hierarchy of skeletal muscle structure?

Answer 5

• Muscle(bundle of fascicles)
• Muscle fasciculus(bundle of fibres)
• Muscle fibre (bundle of myofibrils)
• Myofibrils (bundle of myofilaments)
• Myofilaments (actin & myosin)

Question 6

What are Satellite cells?

Answer 6

Satellite cells occur in close association w Sk. M cells- significant in maintenance and limited regeneration. Resting cells, then induced, through stress/ trauma, to become myogenic precursor cells, undergoing several rounds of mitosis before fusing w existing muscle cells.

Question 7

What is the loose CT surrounding a bunch of muscle fibres?

Answer 7

Perimysium

Question 8

What is the CT surrounding an individual myocyte?

Answer 8

Endomysium

Question 9

What is the CT outside the whole muscle?

Answer 9

Epimysium- all these 3 types contain vessels, nerves and collagen, and are continuous w the dense CT of tendon attaching muscle to bone

Question 10

What are the Ultrastructural Features of Skeletal Muscle?

Answer 10

Ultrastructural Features of Skeletal Muscle
Sarcolemma–plasma membrane of fibre
Transverse Tubules
-tubular invaginations of sarcolemma, carry wave of depolarisation into fibre
-form network with sarcomere
Sarcoplasm
-cytoplasm of fibre containing: myofibrils
numerous mitochondria
sarcoplasmic reticulum (rER)
myoglobin (oxygen-storing protein)

Question 11

How does a nerve impulse reach and deliver contractile signals to myofibrils?

Answer 11

Tranverse tubules- invaginations of sarcolemma into sarcoplasm of cell, which carry wave of depolarisation into cell- these T- tubules come into contact w membraneous sacks of SR called terminal cysternae
Triad = 2 TCs and a t tubule
Ach released into neuromuscular junction due to an AP. Channels of SR contain Ca2+ to activate muscle contractions when AP reaches T tubule

Question 12

What is the ultrastructural basis of cross striations?

Answer 12

-Dark (A) and Light (I) bands due to alternating arrangement of actin and myosin
Myofilaments\I band straddled by Z-line
-composed of actin myofilaments that are anchored to the Z-line ( distance between 2 z lines is sarcomere, functional unit of striated muscle)
A band- straddled by M line
-represents the overlap between actin and myosin (except at the H band)
-Myosin anchored to M-line

Question 13

What is cardiac muscle?

Answer 13

a.k.a. myocardium, involuntary muscle
Found in the heart
-Cells align in chain like arrangement
-Myocardium consists of tightly knit, interwoven cells

Cardiac Muscle Histological features

• Striated: dark (A) & light (I) bands
• Branched cells joined together by intercalated disc
• Nucleus positioned centrally in fibre (cell)

Question 14

What is the ultrastructure of cardiac muscle?

Answer 14

-Actin & myosin myofilaments organised as in skeletal muscle
-Numerous mitochondria: more than skel, as well as more capillaries (oxygen and nutrients as C cells constantly active)
-T-tubules and sarcoplasmic reticulum
-Intercalated Discs
Adhesion specialisations comprising fascia adherens (expanded desmosome) and gap junctions (nexus)
Do not contain triads- only 1 T. Cysternae associated w T tubule, DIAD

Question 15

What is the importance of intercalated discs?

Answer 15

2 functions- joins cells together strongly, ensuring tight bond between adjacent myocytes: rapid communication between cells
Cells connected together by gap junction/nexus, that are present as they allow rapid communication.
Desmosomal junction- mechanical junctions intermediate filaments
Fascia/ Adhering junctions- communicating junctions, use actin filaments

Question 16

What is smooth muscle?

Answer 16

• Also referred to as visceral or involuntary muscle
• Found in tubular organs (gut, blood vessels, bile duct, uterus, urinary bladder, respiratory tract)
• Can regenerate and undergo hypertrophy
• two types of unit- single and multi unit

Main histological features:

• NON- STRIATED
• Made of individual cells that are spindle shaped (long and tapered)
• Central nucleus, often corkscrew shaped

Question 17

Describe the contraction of skeletal muscle.

Answer 17

During contraction:The A band stays the same length. while other regions of the sarcomere (the I band and H zone) shorten. The sliding filament theory, states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to Zlines any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle.

Question 18

Describe the contraction of smooth muscle.

Answer 18

• No myofibrils -Actin and Myosin present, but not organised in regular way (i.e. no striations).
• Myofilaments attach to dense bodies (anchoring points -equivalent of Z-lines in striated muscle). Dense bodies connect to each other via intermediate filaments.
• Membrane invaginations called caveolae(equivalent of T-tubules) are important in Ca2+ transport.
• Most organelles stored at the poles of the nucleus, to avoid interference with contractile process.