Musculoskeletal System Lecture 31

Question 1

What is the function of muscles?

Answer 1

To convert chemical energy (ATP) to mechanical energy

Question 2

What functions can contraction of muscle tissue give?

Answer 2

Movement - the most obvious function of muscle. Movement is not confined to the movement of the bones in the skeletal system. Other examples of movement include: moving gut contents & lymph transportation (smooth muscle) and circulating blood (cardiac muscle)

Stability - muscle plays a very important role in stabilising joints and maintaining posture. Muscle is especially important in stabilising those joints that have a wide range of movement. In these joints, stability (normally provided with the ligaments and/or articular capsule) has been replaced with active contraction of the surrounding muscles.

Communication - muscles are used for facial expression, body language, writing and speech.

Control of body openings and passages - some ring-like muscles (sphincters) help control the admission of light (eyelids and pupils) and food and drink (muscles around the mouth) that enter our bodies. The elimination of waste is also controlled by the urethral and anal sphincters (smooth and skeletal muscle.)

Heat production - skeletal muscle can produce as much as 85% of our body heat. This heat is used to maintain the body at 37 degrees for normal function.

Question 3

General atonomy of skeletal muscle

Answer 3

Question 4

Insertion

Answer 4

The point where the muscle attaches to the bone that moves the most during muscle contraction.

Question 5

Tendon

Answer 5

Tendons are connective tissues that attach muscles to bones. They transmit the force generated by the muscle to move the bone.

Question 6

OTJ (Osteotendinous junction)

Answer 6

This refers to the area where the tendon attaches to the bone.

Question 7

MTJ (Myotendinous junction)

Answer 7

This refers to the area where the muscle transitions into a tendon.

Question 8

Origin

Answer 8

This refers to the point where the muscle attaches to a bone, usually the point that moves the least during muscle contraction.

Question 9

Overview of a myocyte (muscle cell)

Answer 9

• A myocyte, also called a muscle cell, forms the basic unit of a muscle.
• These cells can span the length of a muscle and may reach several centimeters in length, although they vary.
• Myocytes are sometimes referred to as “myofibres” due to their elongated shape, but they are still contractile cells.
• The thickness of a myocyte ranges from 10 microns to 100 microns, with 100 microns being about the same as a human hair. This highlights the size variability of muscle cells.
• A myocyte has multiple nuclei due to its formation by the fusion of smaller cells, making it a syncytium. (large cell-like structure that contains multiple nuclei and forms by the fusion of smaller individual cells). This allows myocytes to be long but also have multiple control points along their length.

Question 10

What is the structure of myocyte?

Answer 10

• The cell membrane of a myocyte is called the sarcolemma, which is specialised for conducting action potentials electrical impulses that are required for muscle contraction.
• The neurosmuscular junction is where motor neurons interact with the sarcolemma to initiate muscle contraction.
• Inside the cell, the sarcoplasm (cytoplasm) contains several organelles that support the muscle’s contractile function.

Question 11

Energy and Oxygen Storage of myocytes

Answer 11

Myocytes contain many mitochondria to support high ATP production necessary for muscle contraction.
They store fuel, such as glucagon and lipids, which are used for energy.
Myoglobin is present in myocytes and functions to store oxygen, although it is less efficient than hemoglobin. This allows the muscle to store oxygen for immediate use during fight-or-flight responses before the cardiovascular system ramps up oxygen supply.

Question 12

What are myofibrils?

Answer 12

Contractile organelles packed inside the myocyte, pushing other organelles to the edges to the cells,

Question 13

What are myofibrils composed of?

Answer 13

Repeating contractile units called sacromeres, which are the smallest functional units of muscle contraction.

Question 14

Structure of a sacromere

Answer 14

A sarcomere is defined by Z-discs (or Z-lines) at either end. The area between two Z-discs forms a single sarcomere.
A-bands are dark bands within the sarcomere where thick filaments (myosin) are found. The I-bands are lighter and contain the Z-disc. The I-bands shorten during muscle contraction, bringing Z-discs closer together to generate force.

Question 15

Muscle Organization

Answer 15

A bundle of myocytes forms a fascicle, and each myocyte within a fascicle is supported by endomysium, a loose irregular connective tissue.
The endomysium contains capillaries and nerves, providing blood supply and nervous signals to the myocytes.
Groups of fascicles form the whole skeletal muscle, which is supported by layers of connective tissue. The perimysium (dense irregular connective tissue) surrounds each fascicle, and the epimysium (dense irregular connective tissue) surrounds the entire muscle, providing strength and structural integrity.

Question 16

Connective tissue layers

Answer 16

Endomysium: Surrounds each myocyte.
Perimysium: Surrounds each fascicle (bundle of myocytes).
Epimysium: Encloses the entire muscle organ.

Question 17

Capillaries and Blood Supply

Answer 17

Muscles are highly vascular, receiving an extensive blood supply to meet their metabolic demands.
Capillaries run through the endomysium to ensure efficient oxygen and nutrient delivery to the muscle cells.
A good blood supply is essential for aerobic respiration in muscle cells, although they can function anaerobically if needed.

Question 18

Microscopic Observations

Answer 18

Under a microscope, myocytes in cross-section appear polygonal, tightly packed within fascicles.
Myocyte nuclei are usually located near the edges of the cell due to the presence of packed myofibrils.
The sarcolemma (cell membrane) and endomysium are visible as supporting structures around the myocytes.

Question 19

Why is the nuclei of the myocyte pressed to the outer edge and never in the center?

Answer 19

To leave space for the myofibrils

Question 20

Why are skeletal and cardiac muscle striated?

Answer 20

Due to perfect alignment of A bands and I bands across all myofibrils

Question 21

What is deep fascia?

Answer 21

Deep fascia is a fibrous, stocking-like layer that covers muscles.
To reach the deep fascia, you dissect through the skin and subcutaneous fatty layer.
Regional names are given to the deep fascia based on the muscle it covers (e.g., pectoral fascia over pectoralis major, brachial fascia over the arm muscles).
This defines the outer boundary of the musculoskeletal system and aids in muscle identification.

Question 22

What are the dissection layers?

Answer 22

If dissecting down to the myofibrils, the order of layers from the surface would be:
- Skin
- Subcutaneous fatty layer
- Deep fascia (which marks the outer boundary of the muscle).

Question 23

Deep Fascia and Muscle Compartments

Answer 23

The deep structures of the body are usually covered in a wrapping of dense connective tissue (regular and irregular) called deep fascia. The deep fascia underlies skin and the subcutaneous tissue (also known as superficial fascia).
Muscles that are supplied by the same nerves or have a similar action can sometimes be found
grouped together in a muscle compartment. The outer walls of the compartments are made up of deep fascia.
The deeper walls or septa are
often referred to as investing
fascia (eg. intermuscular septa,
interosseous membranes).
Where investing fascia comes
into contact with bone it fuses
with the periosteum.
In most areas the outer layer
of a muscle (epimysium) can
move and glide under the
deep fascia. In other areas
the deep fascia is part of the
muscle tendon and can act as
an attachment point for the
muscle.

Question 24

Muscle Compartment Functions

Answer 24

Compartmentalization helps organize muscles with similar functions.
For example, the dorsi flexor compartment in the front of the leg lifts the toes and foot.
Opposite compartments often perform opposite actions (e.g., plantar flexor compartment in the back of the leg points the foot).

Question 25

Blood Supply and Nerve Supply

Answer 25

Muscles in the same compartment often share a common artery, nerve, and veins.
Venous return: As muscles contract, their bellies expand and press against veins, helping push blood back to the heart.
This mechanism is important for proper venous return in the lower limbs.

Question 26

What is Compartment Syndrome?

Answer 26

When muscles swell (e.g., from overuse or rapid muscle growth), the deep fascia can limit expansion, leading to compartment syndrome.
Compartment syndrome occurs when pressure builds in a muscle compartment, compressing veins, then potentially arteries and nerves, leading to ischemia.
A notable case involved All Blacks rugby players, whose calf muscles grew too quickly during pre-season, necessitating surgery (fasciectomy) to release the fascia and allow expansion.

Question 27

What is hyperplasia?

Answer 27

When a tissue/organ increases in size due to an increase in cell number. Skeletal muscles do not typically undergo hyperplasia. This is due to the large size and length of muscle cells (myocytes), which makes mitotic division difficult.
The number of myocytes is mostly determined by birth, with minimal changes throughout adulthood.

Question 28

What is hypertrophy?

Answer 28

The increase in muscle size due to increases in the size of the individual myocytes (myofibres). The myocytes will increase in diameter as more myofibrils are packed into each muscle cell.
Typically, the effect will be an increase in overall muscle size
and strength but the same number of cells will still contribute to the contraction.

Example: A normal myocyte contains a certain number of myofibrils, and hypertrophy increases that number, making the cell stronger.
Resistance training can induce hypertrophy by damaging the muscles, which prompts the body to repair them, making them larger and stronger.

Question 29

What factors can stimulate skeletal muscle hypertrophy?

Answer 29

Repetitive contraction of muscles to near maximal
tension (heavy resistance training) and the use of anabolic steroids.

Question 30

What are anabolic steroids?

Answer 30

• Variants of the male sex hormone testosterone that have been synthesised by pharmaceutical companies.
• Increase protein synthesis through their interactions with
  specific target tissues that include skeletal muscle and bone.
• Can also effect other tissues which can have side effects such as; acne, hair loss, excess hair gain in the wrong places, liver failure, shrivelled testes, infertility, increased susceptibility to coronary artery disease and mood swings ‘roid rage’.

Question 31

What is atrophy?

Answer 31

Atrophy is when the muscle decreases in size due to the reduction of myofibrils in the myocytes.
Muscular atrophy occurs when muscles are not used or stimulated by motor neurons. For example when a limb is immobilized in a cast for a period of time or if a muscle is paralysed.
Muscle atrophy also occurs as part of the complex pathology in diseases such as heart failure, diabetes, cancer and AIDS.
The normal loss of muscle mass starts at the age of 20 years. The rate of loss is accelerated after the age of 50. By the time we reach
80 years approximately 40% of our muscle mass will be lost.
If atrophy is not permitted to proceed too far, it can normally be reversed. Muscle is replaced by fat and connective tissue.
Muscle loss can also occur due to the loss of myocyte = hypoplasia. This is difficult to reverse.

Question 32

Satellite cells (Myoblasts)

Answer 32

Myocytes are created by the
fusion of many myoblasts during
the embryonic stage of life =
Syncytium).
Because myocytes contain many nuclei and are very large cells they are unable to divide by mitosis.
During the formation of myocytes not all of the myoblasts fuse, some remain as individual cells and
become Satellite cells. These cells lie beside the muscle fibres, outside the sarcolemma but within
the same basement membrane.
Satellite cells are the only cells in muscle that can divide and fuse with each other and the myocytes to repair any damage that may have occurred.
While the number of muscle fibres is more or less set at the time of birth, satellite cells do have a limited ability to replace muscle fibres that die from old age or injury.

Question 33

What is myostatin?

Answer 33

A protein produced by myocytes that regulates muscle growth by turning off satellite cells.

Question 34

Irregulation of myostatin

Answer 34

In animals like Wendy the Whippet (a dog) and Belgian Blue cattle, genetic mutations prevent myostatin from functioning, leading to excessive muscle growth (referred to as “double muscling”).
This condition leads to rapid muscle growth but can put stress on bones, joints, and internal organs. In cattle, it often necessitates C-section births due to the large size of calves.

Question 35

Function of the Skeletal Muscle Connective Tissue: (epi, peri and endomysium)

Answer 35

1. To provide the organisation and scaffolding upon which
   the muscle is constructed.
2. To provide a medium for blood vessels and nerves to
   gain access to the myocytes.
3. To prevent excessive stretching and therefore damage to
   the myocytes.
4. To distribute the forces generated by muscle fibre
   contraction.

Question 36

Background knowledge

Answer 36

Sequential sarcomeres in a myofibril share a Z-line.
- When all the sarcomeres in a myofibril are stimulated to
contract, all the Z-lines are pulled closer together by the
filaments that make up the A and I bands (sliding filament
theory). Therefore the whole myofibril will shorten.
If myofibrils acted independently, damage to just one sarcomere in the chain could render the
entire myofibril useless. However this is not the case. Myocytes that are cut in ‘vivo’ can still exert a pulling force on the muscle tendons

Question 37

Structural proteins

Answer 37

• The Z-lines of adjacent sarcomeres within a myocyte are held together
  by a number of structural proteins. One such protein is called Desmin.
• These proteins help align sarcomeres between the myofibrils. The result is that sarcomeres
  shorten together and pull in unison. It is believed this is why skeletal muscle fibres (myocytes)
  have a uniform striated appearance under the microscope.
  At the surface of a myocyte the Z-lines of the outermost myofibrils
  are attached to the sarcolemma, and to the surrounding basement
  membrane and endomysium.
• A group of proteins form a ‘protein complex’ and are responsible for this bridge between the
  myocyte and surrounding connective tissue.
• The protein complex is also thought to contribute to the strengthening of the sarcolemma while
  transmitting contractile forces generated by the sarcomeres to the surrounding endomysium.

Question 38

Muscular Dystrophy

Answer 38

Dystrophin is an example of a protein that contributes to this Protein complex.
- Muscular Dystrophy is a disease where the protein Dystrophin is not transcribed correctly, or is
missing. This results in myocytes that have a weaker sarcolemma that can tear easily, eventually
causing the death of the cell.