Skeletal The skeletal system serves many important functions; it provides the shape and form for the body, support and protection, allows bodily movement, produces
blood for the body, and stores minerals. The number of bones in the human skeletal system is a controversial topic. Humans are born with over 300 bones; however, many bones fuse together between birth and maturity. As a result, an average adult
skeleton consists of 206 bones. The number of bones varies according to the method used to derive the count. While some consider certain structures to be a single bone with multiple parts, others may see it as a single part with multiple bones. There are five general classifications of bones. These are
long bones,
short bones,
flat bones,
irregular bones, and
sesamoid bones. The human skeleton is composed of both fused and individual bones supported by
ligaments,
tendons,
muscles and
cartilage. It is a complex structure with two distinct divisions; the
axial skeleton, which includes the
vertebral column, and the
appendicular skeleton. The red marrow of some bones is an important site for
blood cell production, approximately 2.6 million
red blood cells per second in order to replace existing cells that have been destroyed by the
liver. Muscles are innervated, whereby nervous signals are communicated by
nerves, which conduct electrical currents from the
central nervous system and cause the muscles to contract.
Contraction initiation In
mammals, when a muscle contracts, a series of reactions occur.
Muscle contraction is stimulated by the
motor neuron sending a message to the muscles from the
somatic nervous system.
Depolarization of the
motor neuron results in
neurotransmitters being released from the
nerve terminal. The space between the nerve terminal and the muscle
cell is called the
neuromuscular junction. These neurotransmitters diffuse across the
synapse and bind to specific receptor sites on the
cell membrane of the muscle fiber. When enough receptors are stimulated, an
action potential is generated and the
permeability of the
sarcolemma is altered. This process is known as initiation.
Tendons A tendon is a tough, flexible band of
fibrous connective tissue that connects muscles to bones. The extra-cellular connective tissue between muscle fibers binds to tendons at the distal and proximal ends, and the tendon binds to the
periosteum of individual bones at the muscle's origin and insertion. As muscles contract, tendons transmit the forces to the relatively rigid bones, pulling on them and causing movement. Tendons can stretch substantially, allowing them to function as springs during locomotion, thereby saving energy.
Joints, ligaments and bursae Joints are structures that connect individual bones and may allow bones to move against each other to cause movement. There are three divisions of joints:
diarthroses, which allow extensive mobility between two or more articular heads;
amphiarthroses, which allow
some movement; and false joints or
synarthroses, which allow little or no movement and are predominantly
fibrous.
Synovial joints, joints that are not directly joined, are lubricated by a solution called
synovial fluid that is produced by the
synovial membranes. This fluid lowers the
friction between the articular surfaces and is kept within an
articular capsule, binding the joint with its taut tissue. Ligaments connect the ends of bones together in order to form a joint. Most ligaments limit
dislocation, or prevent certain movements that may cause breaks. Since they are only
elastic they increasingly lengthen when under pressure. When this occurs the ligament may be susceptible to break resulting in an unstable joint. Ligaments may also restrict some actions: movements such as
hyper extension and
hyper flexion are restricted by ligaments to an extent. Also ligaments prevent certain directional movement.
Bursae A bursa is a small fluid-filled sac made of white
fibrous tissue and lined with
synovial membrane. Bursa may also be formed by a synovial membrane that extends outside of the
joint capsule. It provides a cushion between bones and tendons or muscles around a joint; bursa are filled with
synovial fluid and are found around almost every major joint of the body. ==Clinical significance==