The Skeletal System – Coordination of the Bones and Muscle (part 2 of 2)

Muscle tissue has 4 unique properties that allow it to function.
1. ELECTRICAL EXCITABILITY is the ability to receive and respond to stimuli (something that causes a response).
2. CONTRACTILITY is the ability to shorten and thicken in response to a stimulus.
3. EXTENSIBILITY is the ability to stretch(extend).
4. ELASTICITY is the ability to return to its original shape after contracting or extending.

Contractions allow the muscles to perform 3 important functions.
1. When muscles are integrated with bones and joints, they contribute to body movements like walking, running, grasping etc. Contraction of muscles helps the body stay in a stationary position and maintain posture.
2. Other muscles control movement without any attachment to bones or joints, like the heartbeat or contractions of the digestive tract.
3. A BYPRODUCT of muscle contractions is HEAT. This heat helps to maintain the body’s temperature. Involuntary contractions of skeletal muscles(SHIVERING) can increase the rate of heat production.

Humans have 3 major types of muscle: SKELETAL, SMOOTH and CARDIAC.
– Skeletal muscle accounts for between 40% and 50% of the body’s weight.
– Skeletal muscles are usually attached to the bones of the body. Some however are not attached to bone, like the muscles that allow you to open and close your eyelids.
– The muscle tapers at the end, forming a dense connective cord (TENDON) which attaches the muscle to the bone.
– There are about 600 skeletal muscles that mobilize the skeletal system.
– They control the voluntary actions of the body, like walking. They move the arms, legs, back, face, jaw etc.
– Skeletal muscles are categorized as either FLEXOR or EXTENDOR.
– When a FLEXOR muscle (like the biceps) contracts, it causes a joint to bend.
– When an EXTENDOR muscle (like the triceps) contracts, the joint straightens.
– Muscles shorten and pull.
– The biceps/triceps relationship is called an ANTAGONISTIC MUSCLE PAIR, because one muscle reverses the effects of the other.
– Skeletal muscles are known for the speed of their contractions.
– The muscle cell is cylinder shaped and can be long in length. It is often referred to as a MUSCLE FIBER because it runs the entire length of the muscle.
– Because of its length there are thousand of nuclei per cell.
– Skeletal muscle consists of bundles of MUSCLE FIBERS.
– Skeletal muscle has STRIPES or STRIATIONS (many dark lines) going across the cell surface when viewed under the microscope.
– Together, many skeletal muscle fibers form BUNDLES, which are wrapped in connective tissue to form muscles.

– Smooth muscle are found in the organs of the body.
– The most common function the smooth muscle is to squeeze (exert pressure) on the space inside the tube or organ it surrounds. Food is moved down the esophagus by the squeezing action of smooth muscle. Blood is pumped through the arteries with the aid of smooth muscle.
– They are found in the digestive tract, blood vessels and other internal organs.
– These muscles are usually not under voluntary control, the contract INVOLUNTARYLY. They are stimulated by the AUTOMATIC NERVOUS SYSTEM.
– Smooth muscle consist of spindle-shaped cells that contain their own nuclei.
– The fibers of smooth muscle are smaller that skeletal muscle.
– The cells form thin broad sheets of tissue.
– Smooth muscle cells do not have any striped or striations.
– Smooth muscle allow electrical impulses to travel from one smooth muscle to another, without the aid of nervous stimulation.
– Because of their structure, smooth muscle takes longer than skeletal muscle to trigger a contraction and longer for the contraction to subside.

– Cardiac muscle is only found in the heart. It controls the beating of the heart.
– The muscle has striations, like skeleton muscle.
– Like smooth muscle it is under involuntary control and has a single, centrally located nucleus.
– The cells form rows of fibers.
– Cardiac muscle tissue needs a constant supply of oxygen, therefore the cytoplasm is more abundant and the cell’s mitochondria are larger and more plentiful.
– Unlike other muscle tissue, cardiac muscle cells are lined up end to end and are joined to each other by a dense band (INTERCALATED DISK).
– The intercalated dicks strengthen cardiac muscle tissue and help conduct the electrical impulse from one muscle fiber to another.
– When a single fiber is stimulated, all are stimulated, and they contract in a synchronous manner.
– Cardiac tissue contract independently of a nerve supply. Each heartbeat is started by self-activating electrical activity of the heart’s PACEMAKER, called the SINOATRAIL NODE (S-A NODE), located in the wall of the right atrium.
– Cardiac muscle remains contracted 10 to 15 times longer than skeletal muscle.

Muscle cells contract either entirely or not at all.

When a muscle receives a stimulus, the response or how strong the muscle contracts depends on the number of muscle cells stimulated. And this all depends on the strength of the initial stimulus.

By increasing the stimulus, more muscle fibers will contract, which is the MAXIMAL STIMULUS. After reaching a maximal stimulus, any increase will not cause a stronger muscle contraction.

When a muscle is stimulated, a certain base level of electricity is necessary to produce a contraction, after this contraction, it takes a brief break for the muscle to contract and then relax before it can contract again.

The time taken to contract is the CONTRACTION PERIOD and the time taken to relax before the muscle can contract again is the RELAXATION PERIOD.

The amount of time from when the initial stimulus is administered until the contraction begins is called the LATENT PERIOD.

The latent, contraction and relaxation periods together make up a muscle twitch.

When a muscle is not allowed to relax completely before being stimulated again, the next contraction stimulate by the same electrical input will have a stronger response.

The energy required for muscle contraction is ATP (adenosine triphosphate), which is stored in the muscles until needed.

The storage of ATP in muscles can be depleted in seconds.

A high-energy compound (Phosphocreatine) in muscle fibers converts ADP to ADT and provides additional energy for muscle contractions.

Once that is used up, GLYCOGEN in the skeletal muscle and liver is broken down to GLUCOSE, which will provide energy for several minutes of activity.

Once the glycogen supply has been used up, muscles break down fat to provide the energy to resynthesize ATP.

The fine structure of a muscle fiber controls contractions.

A muscle fiber is made up of a bundle of finer fibers called MYOFIBRILS.

A single myofibril is composed of smaller units called SARCOMERES, which are the pieces that contract in muscle tissue. Sarcomeres are arranged in a single file along the length of the myofibrils.

In the sarcomeres are alternating rows of thin (ACTIN PROTEIN) and thick (MYOSIN PROTEIN) FILAMENTS.

The thin filaments are attached to two vertical bands of thick protein called the Z LINES. A Z line marks the boundary between sarcomeres.

Muscles contract due to a SLIDING FILAMENT MECHANISM. When the thick and thin filaments slide past each other, the Z lines of the sarcomeres are pulled together.

When the sarcomeres contract (pull closer together), the myofibrils also contract, causing the contraction of the muscle finer.


The Skeletal System (part 1 of 2)

The human skeleton is made up of 2 types of connective tissue: bone and cartilage.

The primary purpose of the skeleton is to carry the weight and maintain the shape of the body, protect and support the internal organs, and provide an area of support where muscles can attach for movement to occur.

The skeleton is strong enough to absorb reasonable amounts of shock without fracturing, and at the same time it is flexible and light enough to allow movement.

Skeletal bones move in response to muscles that work like levers. Bones act as levers to which muscles are attached. When the muscles contract the bones produce movement.

At birth, the human body is made up of 270 bones, due to the fusion of separate bones; the mature skeleton has only 206 bones.

There is considerable difference between the male and female skeletons. The bones of the male are generally larger and heavier than those of the female. The ends of the bones are thicker in relation to the shafts in males. The points of attachment of muscles are larger in males due to the fact that males have larger muscles. The pelvic girdle of females is designed to allow the birth of children.

Bone replaces itself throughout adult life. Worn or injured bone is removed and replaced by new tissue.

Bone replaces itself at different intervals. The distal end (furthest from the center) of the femur is replaced about every 4 months; however the bone in parts of the shaft of the femur may not be completely replaced during a lifetime. This process allows the bone to serve as the body’s storage are for calcium.

Bones are the major site of mineral storage, particularly calcium and phosphorous, these minerals can be distributed to other parts of the body on demand.

– Long bones are longer than they are wide and they are slightly curved for strength.
– A bone that is curved is structurally able to absorb the stress of the body at several points so the stress will be evenly distributed.
– The straight bone would not be able to evenly distribute the weight of the body and the bone would break more easily.
– Long bones are shaped like tubes with rounded ends that are designed to fit into other bones to form JOINTS.
– The ends of long bones are filled with spongy bone; this makes them light but strong.
– Long bones can be found in the thigh, lower leg, toes, arms, forearms and fingers.
– Short bones are cubed-shaped and nearly equal in length and width.
– They are a spongy bone except at the surface, where there is a thin layer of compact bone.
– Short bones can be found in the wrist and ankle.
– Flat bones are usually thin and are composed of several plates of compact bone over a layer of spongy bone.
– They provide considerable protection and provide ample area for muscle attachment.
– cranial bones, the sternum, ribs and the scapula are flat bones.
– Irregular bones have complex shapes and cannot be grouped into any of the other 3 categories.
– The vertebrae and some of the facial bones are irregular.

Most of the human skeleton is living tissue that is growing constantly.

The bones are made up of cells embedded in a matrix.

About 20% of living bone is water. The remaining 80% is a MATRIX that consists of minerals (tri-calcium phosphate, magnesium and other elements) and protein (collagen fibers).

When the minerals are deposited, the bone becomes OSSIFIED (hardened).

The protein fibers and the minerals make up the non-living matrix of the bone.

Bone is not a completely solid substance; bone has some spaces between the hard components.

The spaces provide channels for blood vessels that supply bone cells with nutrients and make bones lighter.

The bone matrix has a network of canals called HAVERSAIN CANALS in it. These canals allow blood vessels and nerve fibers to extend into interior bone.

Bone also contains many living cells and blood vessels that provide the movement of nourishment into the cell and the removal of waste from the cells.

The surface of the bone is covered by a tough membrane called the PERIOSTEUM.

The periosteum has many microscopic blood vessels that provide nourishment to the bone.

Living bone cells are found in small spaces in the mineral matrix of the bone. There are 3 types of living bone cells and each has a special function.

– Osteoblasts repair broken bones and produce new bone material.
– They also secrete the mineral and protein compounds that form the matrix.
– Osteoclasts are the “bone breakers”. They are able to dissolve pieces of bone that are in the way of “efficient” skeletal design.
– The destructive work is often followed by the constructive work of the osteoblast in the rebuilding of the bone.
– Osteocytes function as the “caretaker” of the bone tissue. They provide coverall maintenance of the bone.

Depending on the size and distribution of the spaces in the bone, the regions of the bone can be categorized as COMPACT or SPONGY.
– Compact bone is dense tissue (thicker) with few spaces.
– It is deposited in a layer over the spongy bone tissue.
– Compact bone tissue provides protection and support and helps long bones resist the stress of weight placed on them. It can resist considerable weight and stress.
– Spongy bone is less dense and contains many large spaces. It is filled with soft tissue called RED MARROW, which makes red blood cells and helps the body store fat.
– Red bone marrow consists of immature blood cells, called STEM CELLS, fat cells, and macrophages.
– Red marrow produces red blood cells, some white blood cells and platelets.
– Spongy bone make up most of the short, flat and irregularly shaped bones. It also makes up most of the ends of long bones.

– Cartilage is both firm and flexible, unlike bone that is harder and more brittle.
– Cartilage is usually located where firmness and flexibility are needed, like joints, the nose, the ears, trachea, and larynx and in between the vertebrate.
– Cartilage is made up of circular cells embedded in a rubbery matrix that has supporting fibers.
– During the development of the skeletal system of humans, embryos begin with a cartilaginous skeleton. Gradually most of the cartilage is replaced by bone.

The axial skeleton includes the bones that lie along the longitudinal axis. It provides the protective functions of the skeleton.

– All of the bones of the head make up the skull. There are 2 regions of the skull; the CRANIUM and the FACE.
– The skull is made up of 22 flat and irregular shaped bones.
– There are 8 bones in the cranium; their function is to protect the brain.
– The facial region is made up of 14 bones and it protects the eyes, nose, mouth and ears.
– The SINUSES are air space in the facial bones. They aid in reducing the weight of the skull.
– The 3 bones in the middle ear (the HAMMER, the ANVIL, and the STIRRUP) function in
transmitting sound to the inner ear and are the smallest bones in the body.

– The vertebral column has 26 bones that are called VERTEBREA.
– At birth the vertebral column has 33 bones.
– 5 SACRAL VERTEBREA. The 5 sacral bones fuse into one large triangular bone, called the SACRUM, located at the back of the pelvis.
– 4 CAUDAL/COCCYGEAL (tail) VERTEBREA. The 4 coccygeal bones fuse into a single COCCYX.
– The vertebrae column is called the BACKBONE; it is made flexible by the cartilage and ligaments that join the individual vertebrae.
– This flexibility allows movement of the head and the trunk of the body.
– DISC of CARTILAGE separate the individual vertebrae. The disc prevent friction due to the rubbing of the bones and act as shock absorbers.

– Just below the neck is the THORACIC BASKET (12 pair of ribs attached to the vertebrae column).
– The 1st 10 pair of ribs are attached to the STERNUM (BREASTBONE) by cartilage strips.
– The 11th and 12th pair of ribs are called the FLOATING RIBS, because they are attached to the vertebrae column, but not to the sternum.
– The loose connections of the ribs to the vertebrae and the flexible cartilage connections at the sternum allow the ribs to move when the lungs are inflated.

– Appendicular is the adjective of the word appendage.
– An appendage is an attachment to a main body or structure. Arms and legs are attached to the axial skeleton.
– The moveable limbs attached to the axial skeleton make up the appendicular skeleton.
– The appendicular skeleton forms a system of levers, providing movement and dexterity.
– The arms and hands, the legs and feet, and the bones of the shoulders and the pelvis make up the appendicular skeleton.
– The sites where the arms and legs are attached to the axial skeleton are bones referred to a GIRDLES.
– The PECTORAL(shoulder)GIRDLE, 2 SCAPULA(shoulder blades) and 2 CLAVICAL(collar bones) hold the arm to the axial skeleton.
– The legs are attached to the PELVIC GIRDLE, which is formed by the fusion of 3 bones. The ILIUM, the ISCHIUM, and the PUBIS on each side of the midline of the body.
– The PELVIC GIRDLE receives the weight of the upper body from the vertebral column and transfers it to the leg bones or the surface when seated.
– The arched shape of the foot allow it to withstand tremendous force and weight.

– The fused bones creating the CRANUIM make up the SKULL.
– The lower TEETH are located in the MANDIBLE(jaw).
– The collar bone is the CLAVICAL.
– The wings in the upper back are called SCAPULAS.
– The bone connecting all of the RIBS in the middle of the chest is the STERNUM.
– The ribs are connected in the back to the VERTEBRAL COLUMN(backbone), which is composed of VERTEBREA.
– The vertebrae in the neck are called CERVICAL VERTEBREA; THORACIC VERTEBREA join with the ribs; LUMBAR VERTEBREA descends from the thoracic vertebrae to the pelvis; and together the fused bones in the PELVIS make up the SACRUM.
– The COCCYX(tail) is made up of CAUDAL VERTEBREA.
– The bone in the upper arm is the HUMERUS and the two bones in the lower arm are the RADIUS and ULNA.
– The wrist bones are the CARPALS.
– At the base of the fingers(the palm of the hand) are the METACARPALS.
– The smaller bones extending out to the fingertips are the PHALANGES.
– The largest bones in the body are the FEMURS, the thigh bone connects the upper legs with the pelvis.
– The upper and lower legs meet at the KNEE covering that is the kneecap, or PATELLA.
– Each lower leg has two long bones, the TIBIA and FIBULA.
– The little bones in the ankles are the TARSALS, that connect to the METATARSALS
– The bones extending into the toe tips are called PHALANGES.

The site where 2 bones come together is called a JOINT.

Most joints allow the bones to move at the connection point.

Some joints are immovable. They are usually bones that are fused together, like the SUTURES located between the skull bones.

Moveable joints are held together with LIGAMENTS, a strong, flexible tissue that connects bone to cartilage.

The more mobile the joint is, the weaker it is.

– No movement is allowed by this type of joint.
– Suture joints in the skull are an example.
– Bones meeting at these joints have some ability to move.
– The spaces between the vertebrae in the back are an example.
– Bones meeting at these joints have the possibility of great movement.
– BALL and SOCKET JOINTS (found in the shoulder), HINGE JOINT (a back and forth movement joint, like the knees and elbows), and PIVOT JOINTS (a rotating joint, like the forearm at the elbow) are all examples of freely moving joints.

Circulatory System – The Lymphatic System (part 4 of 4)

The human body has a second circulatory system, a system of vessels that transport a fluid called LYMPH throughout the body.

The capillaries are responsible for most of the diffusion of materials into the surrounding tissue; the small size of the capillaries can cause the loss of plasma into the body.

The lymphatic system drains the fluids that weren’t reabsorbed from the blood capillaries.

The lymphatic system is a series of vessels located throughout the body that help collect the plasma fluid/tissue fluid or LYMPH (a protein-containing fluid) that has leaked out of the blood vessels into the surrounding tissue. The Lymphatic system brings this fluid back into the bloodstream.

Lymph differs from plasma in that it has 50% fewer proteins and it does not contain red blood cells.

The LYMPHATIC CAPILLARIES are located in the spaces between cells where the fluid accumulates.

The lymph capillaries are slightly larger and more permeable then the blood capillaries.

The lymph capillaries merge into larger lymph capillaries called LYMPHATICS, which resemble veins but have thinner walls and more valves.

Lymph fluids are propelled through the body by capillary pressure, muscle action, intestinal movements and respiratory movements. These movements squeeze the lymph vessels and push the fluid along.

Lymph moves in only one direction, TOWARD THE HEART.

In addition to returning fluids back to the bloodstream, the lymphatic system is also the way that fat enters the bloodstream from the small intestine. On the lining of the small intestine are hair-like extensions called VILLI that contain vessels called LACTEALS. The lacteals are part of the lymphatic system and they absorb fat from food.

EDEMA is the swelling of a body area (like the feet or legs) that results from inadequate draining of lymph from the body.

Edema may be caused by heart or kidney disorders, malnutrition or injury.

The lymphatic system contains many oval-shaped structures called LYMPH NODES or GLANDS that are made up of connective tissue that contain PHAGOCYTOTIC CELLS (cells that can ingest other particles).

Lymph nodes are filtering organs that clear the tissue fluids of bacteria and other foreign particles. They filter and process the lymph.

The lymph nodes return the waste they filter back to the blood, where they are carried to the lungs, kidneys and sweat glands that eliminate them from the body. These wastes are detoxified by the liver.

Lymph nodes act as a defense and are located in the head, face, neck, chest area, groin, pelvic and abdominal regions of the body.

Lymph nodes can become swollen when the body is fighting an infection (swollen glands in the neck are usually a sign of an illness).

The lymph nodes and other lymphoid tissue like the spleen, thymus and tonsils are sites where LYMPHOCYTES (white blood cells) are formed.

Circulatory System – The Heart (part 3 of 4)

Cells must be constantly supplied with the materials necessary for life. The blood in humans is the transport system of these materials and this system must be constantly moving materials to and from the cells.

The heart is a muscular pump and the major organ of the circulatory system. It keeps the blood moving in its’ constant motion.

The human heart is about the size of a clenched fist.

The heart lies in the chest cavity, behind the breast bone and slightly to the left.

The heart is a bundle of cardiac muscles specialized for rhythmic contractions and relaxations, also know as the HEARTBEAT.

The rate of the average heartbeat is 72 beats per minute. Over an average lifetime of about 70 years, the heart beats about 2.5 billion times and pumps 200 million liters of blood.

The heart re-circulates the entire volume of blood, about 5 liters, every minute.

The heart is a strong, muscular organ that has two sides and contains four chambers.

Both sides of the heart beat simultaneously.

The walls are thicker on the one side than on the other.

The upper chambers are called the ATRIA/ARTIUM. The atrium are thin-walled and receive the blood from the veins. They are the receiving chambers of the heart.

The lower chambers are called the VENTRICLES. The ventricles are larger and receive the blood from the atrium above it. They are the pumping chambers of the heart.

Each atrium is separated from the ventricles by a VALVE.

The atrium and ventricles on the right are separated from the left atrium and ventricles by a thick wall of muscle called a SEPTUM.

Blood leaves the ventricles and flows through arteries.

Blood flows one way through the heart with the help of valves. ARTERIES CARRY BLOOD AWAY FROM THE HEART, VEINS CARRY BLOOD TO THE HEART.

The heart is a double pump.

– The right ventricle represents the 1st pump. It is the pump for the PULMONARY CIRCULATION.
– Blood that is low in oxygen returns from the upper and lower parts of the body and enters the right atrium of the heart and then the right ventricle.
– The right ventricle pumps the DEOXYGENATED blood into the PULMONARY ARTERIES that bring the blood to the lungs where it receives oxygen and gives up waste products, including carbon dioxide, which is exhaled.
– The blood returns from the lungs by way of the PULMONARY VEINS to the left atrium and the left ventricle of the heart. This begins the SYSTEMIC CIRCULATION, which carries blood from the heart to the rest of the body.

– The left side of the heart is the pump for the systemic circulation.
– Oxygenated blood enters the left atrium and then the left ventricle. The left ventricle contracts and pumps the blood through the largest artery in the body, the AORTA, to all parts of the body.
– The oxygenated blood passes through arteries that lead to smaller arteries, them to arterioles, then to capillaries, where oxygen, nutrients, hormones and other substances move from the blood to the cells.
– It is through the capillary walls that waste products, like carbon dioxide are picked up.
– Other substances such as nutrients and hormones enter the blood through the capillary walls.
– Due to the pumping action of the heart, blood is continuously circulating throughout the body, maintain the constant exchange of materials necessary for life to continue.
– The blood eventually returns to the right atrium of the heart to complete the cycle.

– The surface of the heart is covered with a number of small arteries and veins.
– Even though the heart has blood flowing through it internally all the time, it is unable to obtain the materials it needs from this blood.
– These small vessels are the CORONARY ARTERIES AND VEINS that carry blood full of oxygen and nutrients to the muscle fibers of the heart and remove waste from the heart tissue.
– These coronary vessels are essential for the health of the heart.
– A heart attack occurs when these vessels become blocked and do not allow the heart tissue to receive oxygen and nutrients.
– This causes death of the heart tissue and the chest pain associated with a heart attack.