Lecture 8: Muscles I: Introduction and Overview
Muscle Types: Classification
1) Skeletal muscle
● Attached to bones
● Attachment to bones makes locomotion possible
● 40% of body mass is made up of muscle
● Muscles generate heat as a byproduct of their contraction and thus
participate in thermal homeostasis
● The muscle cell, or myocyte, develops from myoblasts derived from the
mesoderm
● skeletal muscle is arranged in bundles surrounded by connective tissue
● skeletal muscle is striated due to the regular alternation of contractile
proteins actin and myosin
● The cells are mutinucleated as a result of the many myoblasts that fuse
to form each long muscle fibre
2) Cardiac muscle forms the contractile walls of the heart
● The cells of cardiac muscle, cardiomyocytes also appear striated under
the microscope
● Unlike skeletal fibres, cardiomyocytes are single cells typically with a
single centrally located nucleus
● A principal characteristic of cardiomyoctes is that they contract on their
own intrinsic rhythms without any external stimulation
● Cardiomyocytes attach to each other via specialized cell junctions
called intercalated discs - these hold adjacent cells together across the
dynamic pressure changes of the cardiac cycle
● The cardiac muscle pumps blood through the body and is under
involuntary control
3) Smooth muscle tissue contraction is responsible for the involuntary
movements in internal organs
● It forms the contractile component of the digestive, urinary and
reproductive systems, as well as the airways and arteries
● Each cell is spindle-shaped with a single nucleus and no visible
striations
, Muscle Types: Muscle Action
● Flexion - a movement that decreases the angle between two body parts e.g.
flexion at the elbow is decreasing the angle between the ulna and the
humerus
○ Reduced angle
● Extension
○ Increased angle
● Abduction - movement away from the midline
● Adduction - movement towards the midline
● Medial rotation - movement towards the midline
○ Rotating your straight leg to point the toes inwards
● Lateral rotation - movement away from the midline
● Circumduction - full rotation around the midline
Pronation and Supination
● Pronation = facing down
● Supination = facing up
Protraction and Retraction
● Protraction = movement in a forward direction
● Retraction = movement in a backward direction
Skeletal Muscle
Two major groupings of muscles
● Forelimb
○ Including thorax and neck
● Hindlimb
○ Including abdomen
Skeletal Muscle: Organisation
● Each skeletal muscle has three layers of
connective tissue called mysia that enclose it and
provide structure to the muscle as a whole
○ They compartmentalize the muscle fibers
within the muscle
Muscle Types: Classification
1) Skeletal muscle
● Attached to bones
● Attachment to bones makes locomotion possible
● 40% of body mass is made up of muscle
● Muscles generate heat as a byproduct of their contraction and thus
participate in thermal homeostasis
● The muscle cell, or myocyte, develops from myoblasts derived from the
mesoderm
● skeletal muscle is arranged in bundles surrounded by connective tissue
● skeletal muscle is striated due to the regular alternation of contractile
proteins actin and myosin
● The cells are mutinucleated as a result of the many myoblasts that fuse
to form each long muscle fibre
2) Cardiac muscle forms the contractile walls of the heart
● The cells of cardiac muscle, cardiomyocytes also appear striated under
the microscope
● Unlike skeletal fibres, cardiomyocytes are single cells typically with a
single centrally located nucleus
● A principal characteristic of cardiomyoctes is that they contract on their
own intrinsic rhythms without any external stimulation
● Cardiomyocytes attach to each other via specialized cell junctions
called intercalated discs - these hold adjacent cells together across the
dynamic pressure changes of the cardiac cycle
● The cardiac muscle pumps blood through the body and is under
involuntary control
3) Smooth muscle tissue contraction is responsible for the involuntary
movements in internal organs
● It forms the contractile component of the digestive, urinary and
reproductive systems, as well as the airways and arteries
● Each cell is spindle-shaped with a single nucleus and no visible
striations
, Muscle Types: Muscle Action
● Flexion - a movement that decreases the angle between two body parts e.g.
flexion at the elbow is decreasing the angle between the ulna and the
humerus
○ Reduced angle
● Extension
○ Increased angle
● Abduction - movement away from the midline
● Adduction - movement towards the midline
● Medial rotation - movement towards the midline
○ Rotating your straight leg to point the toes inwards
● Lateral rotation - movement away from the midline
● Circumduction - full rotation around the midline
Pronation and Supination
● Pronation = facing down
● Supination = facing up
Protraction and Retraction
● Protraction = movement in a forward direction
● Retraction = movement in a backward direction
Skeletal Muscle
Two major groupings of muscles
● Forelimb
○ Including thorax and neck
● Hindlimb
○ Including abdomen
Skeletal Muscle: Organisation
● Each skeletal muscle has three layers of
connective tissue called mysia that enclose it and
provide structure to the muscle as a whole
○ They compartmentalize the muscle fibers
within the muscle
