Skeletal System
The adult human skeletal system consists of 206 bones, as well as a network of
tendons, ligaments and cartilage that connects them. The skeletal system performs
vital functions support, movement, protection, blood cell production, calcium
storage and endocrine regulation.
The skeletal system has two distinctive parts: the axial skeleton and the
appendicular skeleton.
The axial skeleton, with a total of 80 bones, consists of the vertebral column, the
rib cage and the skull. The axial skeleton transmits the weight from the head, the
trunk and the upper extremities down to the lower extremities at the hip joints,
which help humans maintain our upright posture.
The appendicular skeleton has a total of 126 bones, and is formed by the pectoral
girdles, the upper limbs, the pelvic girdle and the lower limbs. Their functions are to
make walking, running and other movement possible and to protect the major
organs responsible for digestion, excretion and reproduction.
Skeleton Components
The skeleton is composed of fibrous and mineralized connective tissues that give it
firmness and flexibility. It consists of bone, cartilage, tendons, joints, and ligaments.
Bone - a type of mineralized connective tissue that contains collagen and
calcium phosphate, a mineral crystal. Calcium phosphate gives bone its
firmness. Bone tissue may be compact or spongy. Bones provide support
and protection for body organs. Cartilage - a form of fibrous connective tissue that is composed of closely packed
collagenous fibers in a rubbery gelatinous substance called chondrin.
Cartilage provides flexible support for certain structures in adult humans
including the nose, trachea, and ears. Tendon - a fibrous band of connective tissue that is bonded to bone and
connects bone to bone.
Ligament - a fibrous band of connective tissue that joins bones and other
connective tissues together at joints.
Joint - a site where two or more bones or other skeletal components are joined
together.
Skeleton Divisions
Bones are a major component of the skeletal system. Bones that comprise the
human skeleton are divided into two groups. They are the axial skeletal bones and
appendicular skeletal bones. An adult human skeleton contains 206 bones, 80 of
which are from the axial skeleton and 126 from the appendicular skeleton.
The axial skeleton includes bones that run along the medial sagittal plane of the
body. Imagine a vertical plane that runs through your body from front to back and
divides the body into equal right and left regions. This is the medial sagittal plane.
The axial skeleton forms a central axis that includes bones of the skull, hyoid,
vertebral column, and thoracic cage. The axial skeleton protects numerous vital
organs and soft tissues of the body. The skull provides protection for the brain, the
vertebral column protects the spinal cord, and the thoracic cage protects the heart
and lungs.
Axial Skeleton
Skull - includes bones of the cranium, face, and ears (auditory ossicles).
Hyoid - U-shaped bone or complex of bones located in the neck between the
chin and larynx.
Vertebral Column - includes spinal vertebrae.
Thoracic Cage - includes ribs and sternum (breast bone).
The appendicular skeleton is comprised of body limbs and structures that attach
limbs to the axial skeleton. Bones of the upper and lower limbs, pectoral girdles,
and pelvic girdle are components of this skeleton. Although the primary function of
the appendicular skeleton is for bodily movement, it also provides protection for
organs of the digestive system, excretory system, and reproductive system.
Appendicular Skeleton
Pectoral Girdle - includes shoulder bones (clavicle and scapula).
Upper Limbs - includes bones of the arms and hands.
Pelvic Girdle - includes hip bones.
Lower Limbs - includes bones of the legs and feet.
Bone Classification
Bones of the skeletal system can be classified into four major types. They are
categorized by shape and size. The four main bone classifications are long, short,
flat and irregular bones. Long bones are bones that have greater length than width.
Examples include arm, leg, finger, and thigh bones. Short bones are almost the
same in length and width and are close to being cube shaped. Examples of short
bones are wrist and ankle bones. Flat bones are thin, flat, and typically curved.
Examples include cranial bones, ribs, and the sternum. Irregular bones are atypical
in shape and can not be classified as long, short, or flat. Examples include hip
bones, cranial bones, and vertebrae.
The Muscular System
Muscle tissue has four main properties: Excitability (ability to respond to stimuli),
Contractibility (ability to contract), Extensibility (ability of a muscle to be stretched
without tearing) and Elasticity (ability to return to its normal shape).
Through contraction, the muscular system performs three important functions:
Motion - walking, running etc.
Heat production - maintain normal body temperature
Maintenance of posture - standing, sitting etc.
Motion
To understand how the muscles combine with the skeleton in providing motion we
must look at the basic mechanics of movement. The main framework of the body is
covered by muscle, whose function is to permit movement. We know that to move
or lift a load against another force, it is easier to use levers, and it is this principle
which the musculoskeletal system adopts and which we must examine.
The component parts that are used in a lever are as follows:
Lever - nearly always the bone
Fulcrum - pivot point of the lever, which is usually the joint
Muscle Force - force that draws the opposite ends of the muscles together
Resistive Force - force generated by a factor external to the body that acts
against muscle force
Torque - the degree to which a force tends to rotate an object about a specified
fulcrum
There are different types of levers dependent upon the position of fulcrum, effort
and resistive force.
First Class lever: Muscle force and resistive force is on different sides of the
fulcrum e.g. the head resting on the vertebral column. As the head is raised, the
facial portion of the skull is the resistance, the fulcrum is between the atlas and
occipital bone, and the effort is the contraction of the muscles of the back.
Second Class lever: Muscle force and resistive force act on the same side of the
fulcrum, with the muscle force acting through the level longer than that through
which the resistive force acts - e.g. raising the body up onto the toes. The body is
the resistance, the ball of the foot is the fulcrum, and the effort is the contraction of
the calf muscle.
Third Class lever: Muscle force and resistive force act on the same side of the
fulcrum, with the muscle force acting through the lever shorter than that through
which the resistive force acts - e.g. adduction of the thigh. The weight of the thigh
is the resistance, the hip joint is the fulcrum, and the contraction of the adductor
muscle is the effort.
Most of the limbs of the human body are articulated by third class levers.
Agonist, Antagonist, Fixator & Synergist Muscles
Muscles can only exert a pulling force so work in pairs. When we move a limb one
muscle, the agonist muscle also known as the prime mover, causes the movement
and an antagonist muscle works in opposition to the agonist muscle.
Example: Biceps curl – the biceps is the agonist muscle causing the movement
and the triceps are the antagonist muscle working in opposition to the biceps.
The function of a fixator muscle is to stabilize the origin of the agonist muscle so
that it can move efficiently.
Other muscles, known as synergist muscles, stabilize muscle movements to keep
them even, and control the movement so that it falls within a range of motion which
is safe and desired.
Maintenance of posture
As well as enabling movement, muscles also maintain posture and body position.
Sensory receptors in the muscles monitor the tension and length of the muscles
and provide the nervous system with crucial information about the position of the
body parts, therefore enabling posture to be maintained. Muscles are never
completely at rest, nor do they actually have to shorten in length when they
contract. The tension or tone produced as a result of these contractions between
various opposing groups of muscle helps us remain in a static position, even when
we are asleep.
Muscle Origin & Insertion
Each end of a muscle is attached to a bone and these connections are known as
the origin and insertion. The muscle's origin is attached to the immovable bone and
the muscle's insertion is attached to the movable bone.
Effect of exercise on the muscular system
The effects of regular exercise on the muscular system:
Strengthens muscles and the connective tissues
Improves muscle control and balance
An increase in muscle size and efficiency
The amount of myoglobin within skeletal muscle increases
Muscles are capable of storing a larger amount of glycogen
Muscle became more efficient at disposing of waste products through the
bloodstream