WEEK3 - Muscular system (1) learning contents

Topic 2.1: The nature of the muscular system and terminology 

Topic 2.2: Muscles of the face and neck, thorax and abdomen 

Topic 2.3: Muscles of the upper limb 

Topic 2.4: Muscles of the lower limb 

 

 

- Characteristics and Functions

Types of Muscle Tissue:

A.   Skeletal muscle is associated with the bony skeleton and consists of large cells that bear striations and are under voluntary control.

B.   Cardiac muscle, found only in the heart, consists of small cells that are striated and under involuntary control.

C.   Smooth muscle is found in the walls of hollow organs, and consists of small, elongated, unstriated cells that are under involuntary control.

 

Characteristics of Muscle Tissue:

1.   Excitability, or responsiveness, is the ability to receive and respond to a stimulus.

2.   Contractility is the ability to contract forcibly when stimulated.

3.   Extensibility is the ability to be stretched.

4.   Elasticity is the ability to resume the cells’ original length once stretched.

 Functions of Muscle Tissue:

1.   Muscles produce movement by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems.

2.   Muscles aid in maintaining posture by adjusting the position of the body with respect to gravity.

3.   Muscles stabilize joints by exerting tension around the joint.

4.   Muscles generate heat as a function of their cellular metabolic processes.

 

 

- Differences between Muscle Types

Differences between Smooth and Skeletal Muscle Fibers

1.   Smooth muscle cells are small, spindle-shaped cells with one central nucleus, and lack the coarse connective tissue coverings of skeletal muscle.

2.   Smooth muscle cells are usually arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer.

3.   Contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called peristalsis연동, which propels substances through the organs.

4.   Smooth muscle lacks neuromuscular junctions, but has varicosities: numerous bulbous swellings that release neurotransmitters to a wide synaptic cleft.

5.   Smooth muscle cells have a less developed sarcoplasmic근형질의 reticulum, sequestering고립시키다 large amounts of calcium in extracellular fluid within caveolae in the cell membrane.

6.   Smooth muscle has no striations, no sarcomeres, a lower ratio of thick to thin filaments compared with skeletal muscle, and has tropomyosin but no troponin.

7.   Smooth muscle fibers contain longitudinal bundles of non contractile intermediate filaments anchored to the sarcolemma and surrounding tissues via dense bodies.

 

Mechanisms of Smooth Muscle Contraction

a.   Smooth muscle fibers exhibit slow, synchronized contractions due to electrical coupling by gap junctions.

b.   Like skeletal muscle, actin and myosin interact by the sliding filament mechanism; contraction is triggered by a rise in intracellular calcium level, and the process is energized by ATP.

c.   During excitation-contraction coupling, calcium ions enter the cell from the extracellular space, bind to calmodulin, and activate an enzyme, myosin light chain kinase, powering the cross-bridging cycle.

d.   Smooth muscle contracts more slowly and consumes less ATP than skeletal muscle.

 

Regulation of Smooth Muscle Contraction

a.   Autonomic nerve endings release either acetylcholine or norepinephrine, which may result in excitation of certain groups of smooth muscle cells, and inhibition of others.

b.   Hormones and local factors, such as lack of oxygen, histamine, excess carbon dioxide, or low pH, act as signals for contraction.

Special Features of Smooth Muscle Contraction

a.   Smooth muscle initially contracts when stretched, but contraction is brief, and then the cells relax to accommodate the stretch.

b.   Because the muscle filaments have an irregular overlapping pattern, smooth muscle stretches more and generates more tension when stretched than skeletal muscle.

c.   Hyperplasia, an increase in cell number through division, is possible in addition to hypertrophy, an increase in individual cell size.

Types of Smooth Muscle

1.   Unitary smooth muscle, called visceral muscle, is the most common type of smooth muscle. It contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials.

2.   Multi-unit smooth muscle is located in large airways to the lungs, large arteries, arrector pili muscles in hair follicles, and the iris of the eye. It consists of cells that are structurally independent of each other, has motor units, and is capable of graded contractions.

 

 

- Developmental aspects

A.   Nearly all muscle tissue develops from specialized mesodermal cells called myoblasts.

B.   Skeletal muscle fibers form through the fusion of several myoblasts, and are actively contracting by week 7 of fetal development.

C.   Myoblasts of cardiac and smooth muscle do not fuse but form gap junctions at a very early stage.

D.   Muscular development in infants is mostly reflexive at birth, and progresses in a head-to-toe and proximal-to-distal direction.

E.   Women have relatively less muscle mass than men due to the effects of the male sex hormone testosterone, which accounts for the difference in strength between the sexes.

F.    Rich blood supply enables skeletal muscle to be highly resistant to infection.

G.   With age, the amount of connective tissue in muscles increases, and the number of cells decreases, resulting in significant loss of strength.

 

 

 

- Gross Anatomy and Terminology

A.   Each skeletal muscle is a discrete organ, in which muscle fibers predominate, but also includes blood vessels, nerves, and connective tissue.

1.   Nerve and blood supply allows neural control and ensures adequate nutrient delivery and waste removal.

2.   Connective tissue sheaths are found at various structural levels of each muscle: endomysium surrounds each muscle fiber, perimysium surrounds groups of muscle fibers, and epimysium surrounds whole muscles.

3.   Skeletal muscles span joints and cause movement to occur from the movable attachment (the muscle’s insertion) toward the less movable attachment (the muscle’s origin).

4.   Muscle attachments may be direct, in which the epimysium fuses with the periosteum or perichondrium; or indirect, in which the connective tissue wrappings of the muscle extend into a rope-like or sheet-like structure that attaches to the bone, cartilage, or fascia.

a.   Indirect attachments, either rope-like tendons, or sheet-like aponeuroses, are the most common because they are durable and are small in size, conserving space across joints.

 

 

- Agonist, antagonist, synergist / fixator

A.   Muscles only pull; they never push, and as a muscle shortens, the insertion is pulled toward the origin.

B.   The muscle that provides the major force for the specific movement is called the prime mover or the agonist.

C.   Muscles that oppose or reverse a particular movement are called the antagonists, and are usually located on the opposite side of the joint from the agonist.

D.   Synergists help the prime movers by adding extra force to the same movement, or by reducing undesirable or unnecessary movements.

1.   When synergists immobilize a bone to provide stability for the action of a prime mover, they are acting as fixators.

 

 

- Naming of muscles

Criteria used to name skeletal muscles include location, shape, size, direction of muscle fibers, number of origins, location of attachments, or action. A muscle name often incorporates more than one of these criteria.

1.   An example of a muscle named for its location is the temporalis, which covers the temporal bone.

2.   An example of a muscle named for its shape is the deltoid, which has a triangular shape.

3.   Terms such as maximus, minimus, longus, and brevis indicate relative muscle size.

4.   Terms such as transversus and oblique are often relative to the body’s midline, and indicate the fiber direction of the muscle relative to that line.

5.   Biceps, triceps, and quadriceps indicate two, three, or four origins, respectively.

6.  An example of a muscle named for its attachment sites is the sternocleidomastoid, which attaches at the origin at the sternum and clavicle, and at the insertion, the mastoid process.

7.   Flexor, extensor, or adductor are examples of action terms that are part of many muscles’ names.

Fascicle Arrangement

In skeletal muscles, the common arrangement of the fascicles varies, resulting in muscles with different shapes and functional capabilities.

 

1.   The fascicular pattern is circular when the fascicles are arranged in concentric rings.

2.   A convergent muscle has a broad origin and its fascicles converge toward a single tendon of insertion.

3.   In a parallel arrangement, the long axis of the fascicles runs parallel to the long axis of the muscle.

a.   A spindle-shaped parallel arrangement of fascicles is sometimes classified as a fusiform muscle.

4.   In a pennate pattern of arrangement, the fascicles are short and attach obliquely to a central tendon that runs the length of the muscle.

a.   Unipennate muscles have fascicles that insert into only one side of the tendon.

b.   Bipennate muscles have fascicles that insert into opposite sides of the tendon, forming a feather-like pattern.

c.   Multipennate muscles resemble several “feathers” arranged side-by-side.

 

 

- Regional functional anatomy

We will cover the regional muscles in the online labs. For a detailed listing of the names, actions, origins, and insertions of muscles involved in swallowing, refer to the pages listed in the textbook, lab workbook and lab manual.

Additionally, access the MASTERING A&P online environment for learning, practising these muscles (virtual lab).