The
Skeleton and the Muscles
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The
human skeleton is an endoskeleton (internal skeleton) made of bone and
cartilage.
Major Functions
- Support for the soft tissues
and largely responsible for the shape of the human body.
- Movement: the strong rigid
bones function as levers moved by the contraction of muscles.
- Protection: the tough bones
protect delicate vital organs
- The brain is
protected by the skull.
- The heart and lungs
are protected by the rib cage.
- The spinal cord is
protected by the backbone.
Structure
The skeleton is divided into:
1. Axial Skeleton:
skull, backbone, ribs and sternum.
2. Appendicular Skeleton: pectoral
girdle, pelvic girdle and limbs.
Axial Skeleton
1. Skull: composed of
22 fused bones except for the mandible (lower jaw).
2. Spine or
Backbone: Also known as the vertebral column. It is composed of 33 small
bones in a line - cervical (7), thoracic (12), lumbar (5), sacrum (5), coccyx (4).
The
vertebrae of the sacrum and coccyx are fused together. The vertebrae of the
other regions can move slightly giving flexibility to the backbone. There are
cartilage discs between these that act as shock absorbers. They help to protect
the vertebrae. The vertebrae are held in position by ligaments. Muscles
attached to their surfaces also support the vertebrae. Spinal nerves emerge in
pairs from the spinal cord between the vertebrae.
Rib Cage
The rib cage consists of the sternum and 12
pairs of ribs. All 12 pairs all attached to the backbone. The first seven
are true ribs joining also to the sternum. The next three are the false ribs,
which also join to the seventh rib. The last two are the floating ribs. They
are only joined at one end to the backbone. The sternum or breastbone is a flat
thin bone at the centre of the chest wall.
The Appendicular Skeleton
The Pectoral Girdle is composed of four bones — two clavicles (collar
bones) and two scapulae (shoulder blades). The arms connect with the scapulae at a
ball and socket joint.
The Pelvic
Girdle
The Pelvic Girdle appears to be one large cylindrical bone but
is actually six fused bones. The legs articulate with the pelvis at a ball and
socket joint. The pelvis is securely attached to the backbone at the sacrum.
Limbs
Human Forelimb (arm)
Human Hindlimb (leg)
The arms and legs have similar design.
There are long upper bones. In the arm it is the humerus
and in the leg it is the femur. Also, there are two long bones in the
medial region: The ulna and radius in the arm and the tibia and
fibula in the leg.
The foot and hand have similar bones. The carpals are in
the wrist of the arm and the tarsals in the ankle of the leg. The metacarpals
are in the palm of the hand and metatarsals in the hind foot. The phalanges
are in the fingers and toes.
Long Bone
Structure
A
membrane called the periosteum encloses long bones. It contains blood
vessels and nerves.
The diaphysis
is the shaft or long main portion of the bone. The epiphysis is at each
end of the long bone and is formed separately of the diaphysis.
Cartilage is found at each end of the long
bones. It is made of protein called collagen. Collagen fibres are wound
within the surrounding material of calcium and phosphorous salts. The cartilage covers the epiphyses
protecting them from friction and shock at freely moveable joints. Cartilage does
not have blood vessels or nerves. Useful materials enter the cartilage by
diffusion.
Types of Bones
There are 2 types of bones: compact bone and
spongy bone.
1.
Compact Bone: Is a solid bone that at microscopic
level has a concentric ring structure. It is made of bone cells called osteoblasts.
These cells are growing within a material that is called a bone matrix.
The bone matrix is made of 70% inorganic
salts such as calcium phosphate and 30% protein called collagen.
The calcium salts give the bone strength while the protein gives it
flexibility. Blood vessels as well as nerve fibres are within the bone.
2. Spongy
Bone: irregular openwork of thin
plates of bone. It is also known as trabecular or cancellous bone. The mineral
deposits are arranged as a system of struts. Bone marrow fills the spaces
between the plates.
The marrow cavity is the space within the diaphysis that
also contains marrow.
There are 2
types of bone marrow:
1. Red Bone Marrow: Is responsible for the formation of
red blood cells, white blood cells and platelets.
2. Yellow Bone Marrow: Is responsible for energy storage in
the form of lipid (fat). This can be converted into red marrow if the body
needs increased blood cell formation. It is only found in adults’ medullary
cavity.
Inside view of cavities within spongy bone. They are filled with red bone marrow.
Bone Growth
The embryo is made of cartilage until about the 8th week of development. It then is replaced with bone. Osteoblasts produce the protein collagen. A hard covering of mainly calcium phosphate forms around the collagen. The osteoblasts become trapped within the calcium phosphate and become dormant bone cells.
The bone becomes longer as a result of
growth plates found between the diaphysis and
the epiphysis. At this plate, cartilage is continuously formed and then turned
into bone. This process is called ossification. This stops when a person reaches the adult age.
Bone Development
Even though
bones stop growing in length in early adulthood, they can continue to increase
in thickness or diameter throughout life in response to stress from increased
muscle activity or to weight. The increase in diameter is called appositional
growth. Osteoblasts in the periosteum form compact bone around the external
bone surface. At the same time, osteoclasts in medullary cavity break down bone
on the internal bone surface, around the medullary cavity. These two processes
together increase the diameter of the bone and, at the same time, keep the bone
from becoming excessively heavy and bulky.
Factors
that affect bone growth
1.
Stress on the bones by physical activity. Osteoblasts are
stimulated. More bone grows.
2.
Lack of Stress causes bones to become thin.
3.
Growth hormone and sex hormones increase bone size.
Joints
A
joint is the junction between two or more bones. There are three major types of
joints:
1. Fused Joints: These joints include the skull,
sacrum, pelvis, and coccyx. As the name
suggests, these joints are points where joints fuse or grow together. The place where they grow together is called
the suture. These joints provide
strength, support, and protection.
2. Slightly Moveable Joints: These joints are located between the vertebrae of the upper
spine. There is cartilage within the joints. They help pad and protect the
bones. The bones are held together by ligaments.
The ligaments are tightly bound and limit the movement of the bones. This
protects the spinal cord.
3. Freely Moveable or Synovial Joints: At
these joints the ends of the bones are covered with cartilage and there is a cavity that separates the bones. The
bones are held in place by ligaments which stop the bones from moving too much.
In addition to the ligaments the two bones are joined together by sleeve-like
capsule. The capsule encloses the synovial cavity. The outer layer of the
capsule is composed of ligaments. As
stated previously, the ligaments keep bones together preventing dislocation and
control the range of movement. The
inner layer of the capsule is the synovial
membrane. The synovial membrane
secretes the lubricating synovial fluid. Lubrication is essential to prevent
frictional wear and tear. The cartilage
at the contact ends of the bones also reduces friction. The cartilage pads also
acts as shock absorbers against mechanical damage.
Classes of Synovial Joints
1. Gliding: The bones of these joints move across
each other, back-and-forth and side-to-side. Examples are between the carpals
of the wrist and tarsals of the ankle.
2. Pivot:
These joints allow a turning movement. Examples are between the first and
second vertebras when turning the head, between the ulna and the radius of the
lower arm when turning the palm of the hand up or down.
3. Hinge: These
joints allow movement in one plane during flexion and extension. They act, as
the name implies, like the hinge of a door. Examples are bending the elbow or
knee.
4. Ball and Socket: This type of joint permits movement
in three planes, i.e., in all directions. Examples are the shoulder and hip
joints.
Ligaments
Ligaments
are strong, slightly elastic tissues that connect bone to bone at joints. These
tissues are more flexible when warm. That is why you should gently warm up
before exercising. Ligaments prevent dislocation of the joint and control the
range of movement of the bones at the joint.
Tendons
Tendons are strong inelastic cords or
bands of connective tissue that connect muscle to bone. They are composed of
collagen and contain blood vessels. The inelastic tendon will not stretch when
the muscle contracts. Therefore the full pull is transmitted to the bone and
the full range of motion is accomplished.
Arthritis
There are 2 types of arthritis, Osteoarthritis and Rheumatoid Arthritis. Both of these conditions involve the swelling
and inflammation of the joint. See your text book page 352 for more information
on this topic.
Muscles
There are 3 types of muscle: skeletal,
smooth, and cardiac.
Skeletal Muscle
Skeletal muscle, as its name implies, is the muscle
attached to the skeleton. It is also called striated muscle. The
contraction of skeletal muscle is under voluntary control. These muscles are
mainly responsible for movement of the body. Other purposes are posture
maintenance, support of the joints, and heat production. While its contraction
is fast and strong, skeletal muscle tires easily.
Smooth Muscle
Smooth muscle is found in the walls of all the hollow organs of the
body (except the heart). Its contraction reduces the size of these structures.
Thus it regulates the flow of blood in the arteries, moves your breakfast along
through your gastrointestinal tract, expels urine from your
urinary bladder, sends babies out into the world from the uterus, and regulates
the flow of air through the lungs. The contraction of smooth muscle is not
under voluntary control. It is called involuntary
muscle. It contracts slowly and is slow to tire.
Cardiac Muscle
Your heart is made of cardiac muscle. This type of
muscle only exists in your heart. Unlike other types of muscle, cardiac muscle
never gets tired. It works automatically and constantly without ever pausing to
rest. Cardiac muscle contracts to squeeze blood out of your heart, and relaxes
to fill your heart with blood.
Antagonistic muscles are pairs of muscles. The
action of one member is opposite to that of the other member. Muscles can contract but they do not have the ability to
lengthen (stretch) themselves. They are arranged in pairs such that after one
muscle or muscle group contracts, a skeleton transfers
the movement to stretch another muscle or muscle group. The pairs of muscles
that stretch each other are said to be antagonistic.
Example of an
antagonistic muscle pair
The biceps and triceps muscles of the arm are an example of an
antagonistic pair. Contraction of the biceps moves the arm toward the body and
stretches the triceps. Contraction of the triceps extends the arm and stretches
the biceps. In this example the bicep is said to be the flexor while the tricep is the extensor.
Extensors are not as strong as flexors.
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