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.

Invertebrates do not have bone.

Protists

Protists wave cilia or move pseudopods; animals contract muscles.

In animals that have a hydrostatic skeleton, the force of contraction is applied to a fluid-filled chamber.

Circular muscles surround the chamber and longitudinal muscles extend from one end to the other.   

Circular and longitudinal muscles form an antagonistic system.   When circular muscles  contract, the animal becomes thinner and longer and longitudinal muscles are stretched.  When longitudinal muscles contract, the chamber becomes shorter and thicker and circular muscles are stretched. 

Click here for diagrams of a hydrostatic skeleton

Hydras (cnidarians) and planarians (flatworms, platyhelminthes) have a fluid-filled gastrovascular cavity.

Roundworms (nematodes) have a fluid-filled pseudocoelom.

Annelids are segmented. Separate longitudinal and circular muscles control each segment independently.

Exoskeletons are hard structures found on the outside of the animal. They provide support, protection and facilitate movement by enabling muscles to move the parts.

Mollusks

Snails (gastropods) and clams (bivalves) have a shell composed of calcium carbonate.

The shell functions mainly for protection.

As the animal grows, new shell material is produced by the mantle, thus enlarging the shell.

Arthropods

The exoskeleton of arthropods is composed of chitin.

The skeleton provides an attachment site for muscles, allowing rapid movement. It also protects and prevents the animal from drying out.

The arthropod exoskeleton is composed of separate pieces that are hinged with the muscles contained inside.

The arthropod exoskeleton does not grow, so arthropods must periodically undergo molting, a process in which the old exoskeleton is shed and a new, soft one expands and then hardens.

Animals with an exoskeleton are limited in size because as the exoskeleton becomes larger to accommodate the heavier animal, there is less space for the internal organs.

Flexors are muscles that bend a joint; extensors straighten it.

A fly wing is elevated by the contraction of elevator muscles. A depressor muscle contracts to lower it.

The wings of some insects beat too fast for nerve impulses. In these insects, the contraction of the elevator muscle triggers the contraction of the depressor muscle without any nerve signals. This allows the wings to beat more rapidly than would be possible if nerves controlled their contraction.

Vertebrates have rigid, jointed  endoskeletons that are contained within the body of the animal. They are composed of living tissues (bone and cartilage) and grow as the animal grows. The skeletons of chondrichthyes (cartilaginous fish) are composed of cartilage only.

Animals with endoskeletons can grow to a larger size because the internal space is not limited by the size of the skeleton as it is in animals with exoskeletons.

It provides support and protection while enabling movement.

It functions to store calcium and phosphorus.

It is the site of blood cell production in adults.

It is the site of maturation of B-lymphocytes.

Bone Structure

Bone is composed of fibers of a protein called collagen that are embedded in hardened calcium phosphate and other minerals. The strength of bones is due to the combination of fibers and hardening minerals.

Ounce for ounce, bone is stronger than steel.

Haversian canals (center canals) contain blood vessels and nerve cells. Osteocytes (bone cells) are arranged in concentric circles around the Haversian canals.

Click on the photograph to view an enlargement.

Spongy bone contains marrow; compact bone is solid.

The shafts of the long bones are  hollow while the tips of the bones are strengthened by the presence of spongy bone.

The human femur shown below has been cut to show the internal structure. Click on the photograph to view an enlargement.

The spaces within the long bones contain marrow.  Red marrow produces red blood cells.  Yellow marrow is mostly fat. It can be converted to red marrow if needed.

Bone is formed in the embryo from cartilage or sheets of dense connective tissue.

Long bones

The long bones are cartilaginous (made of cartilage) during prenatal development.

Osteoblasts (bone-secreting cells) secrete calcium salts within the cartilage. The conversion of soft tissues to bone is called ossification.

Ossification first occurs in the middle of the bones and later at the ends.

A cartilaginous area called the epiphyseal plate remains near each end of the bone. Bones lengthen by depositing tissue on the inside surfaces of the epiphyseal plate. At maturity, these areas become ossified and disappear.

Bones become wider by the deposition of bone tissue underneath the surrounding connective tissue.

As a bone matures, the osteoblasts become surrounded by the bone that they have secreted. These bone cells are now called osteocytes.

Flat bones (ex: sternum)

Flat bones such as those found in the skull originate from sheets of dense connective tissue instead of cartilage.

Below: This flat bone (scapula) has been cut to reveal the spongy interior.

In order to regulate calcium levels in the blood, bone is constantly broken down and renewed by reabsorption and deposition. This breakdown and renewal process is called remodeling.

Osteoporosis is a decrease in bone mass. It is occurs most often in the vertebral column, legs, and feet of older women.

The axial skeleton includes the skull, vertebrae, ribs, and sternum.

The skull protects the brain. The bones of face protect sense organs and form the jaws.

Some skull bones contain sinuses, which are open spaces that function to make the skull lighter.

The lower portion of a skull has been cut to reveal sinuses.

The skull of an infant contains fontanels, regions of connective tissue where bone is not completely formed. This allows the bones of the head to move during childbirth, facilitating the fit through the birth canal. The fontanels become ossified (converted to bone) by the age of two.

Below: A region of softer connective tissue (fontanel) can be seen between the incompletely-formed bones of the skull of an infant.

The vertebral column serves directly or indirectly as the anchor for all other bones. Curves in the vertebral column give strength and resiliency.

The vertebrae are named according to location:

cervical vertebrae - neck region

thoracic vertebrae - middle and upper back

lumbar vertebrae - lower back

sacral vertebrae - in pelvic region

Vertebrae protect the spinal cord.

Disks between the vertebrae act as shock absorbers.  Excessive pressure, particularly on disks that have weakened with age, may cause them to slip or rupture.   The swelling might put pressure on spinal cord or spinal nerves, necessitating their removal and the fusion of vertebrae.

The ribs are connected directly to thoracic vertebrae in the back. All but 2 pairs of ribs are connected to the sternum in the front. The lower ribs are unattached.

Below: The bottom two pairs of ribs are not attached to the sternum.

The appendicular skeleton includes the pectoral girdle (shoulder), pelvic girdle (hips), and the bones attached to them (arms and hands, legs and feet).

The pelvic girdle and appendages (legs) are specialized for strength.

The pectoral girdle and appendages (arms) are specialized for maximum flexibility, not strength.

Clavicle

The clavicle (collarbone) connects to the sternum in front and the scapula in back but the scapula is attached to the body by muscles only. This arrangement allows free movement of the arm but is not very strong.

The clavicle is the most frequently broken bone because it has a small diameter and it is the only direct attachment between the arm and the rest of the body.

Scapula

The scapula is broad and flat, providing a large area for attachment to muscles.   This is necessary because its only connection to the rest of the body is the relatively small and weak clavicle (collarbone).  The scapula links the humerus (upper arm) with the clavicle (collarbone).

Humerus

The humerus fits into a shallow socket of the scapula. This allows maximum movement but it provides little stability, making the shoulder easy to dislocate.

Other Bones of the Pectoral Girdle

radius, ulna - lower arm

8 carpals - wrist

5 metacarpals, 14 phalanges - hand

The hipbones are anchored to the sacrum to form the pelvis.

The femur is the largest bone in the body.

The tibia forms shin and inner ankle. The end of the fibula forms the outer ankle.

Tarsals, 5 metatarsals, and 14 phalanges are located in the foot.

In humans, metatarsals form the arches, which improve stability and provide shock absorption.

Joints

Ligaments connect bones to each other. The connection between two bones is a joint.

Types of Joints

The bones of the cranium (the part of the skull that protects the brain) are joined together by immovable (fibrous) joints.

The vertebrae are joined by slightly movable (cartilaginous) joints.

Freely movable (synovial) joints allow the most movement. Three kinds of freely movable joints are discussed below.

The knee and elbow are hinge joints because, like a door hinge, they allow movement in only one plane.

Below: Knee joint seen from behind. Notice that the broad head of the femur (upper bone) prevents side-to-side movement. The tibia (lower bone) can move forward (into the page) or back (out of the page).

Ball and socket joints such as the shoulder and hip allow a wide range of movement.

Below: The hip contains a ball and socket joint.

Pivot joints allow the lower arm and leg to rotate while the upper part of the limb remains stationary.

Below: The elbow is a hinge joint. The lower arm can rotate because it also contains a pivot joint.

The bones in a joint are separated by a cavity and are surrounded ligaments that hold them together and form a capsule.

The joint is lined with a synovial membrane that produces synovial fluid.  The fluid lubricates and separates the bones.

The ends of the bones are covered by cartilage to reduce friction.

The bones within a joint are supported by pieces of cartilage called menisci (sing: meniscus).

In the knee, 13 fluid-filled sacs called bursae reduce friction between tendons and ligaments and tendons and bones.

Arthritis

Osteoarthritis is when the cartilage at the end of the bone has worn away with aging.

In rheumatoid arthritis, the synovial membrane becomes inflamed, the cartilage degenerates, and calcium is deposited in the joint. This could be due to a bacterial or viral infection. Some people may have a genetic predisposition to this disease. It may be caused by an autoimmune reaction.

Skeletal Muscle

Skeletal muscle is voluntary.

The cells are long (up to 30 cm or 12 in), striated, and multinucleate.

Groups of cells are surrounded by connective tissues to form bundles.

Cardiac Muscle

Cardiac muscle is found in the heart.

The cells are short, branched, and striated. The blue arrows in the photograph below point to branches.

Intercalated disks are regions where cells join together (see below).

Smooth Muscle

Smooth muscle is involuntary.

It lines the gut, blood vessels, and reproductive tract.

The cells are tapered on the ends.

Skeletal muscles are composed of many cells. Muscle cells are also called muscle fibers.

Myofibrils are strands found within muscle cells that are composed of the proteins actin and myosin. They extend from one end of the cell to the other and are capable of contraction, causing the cell to shorten.

The plasma membrane is referred to as the sarcolemma.

Extensions of the endoplasmic reticulum form a network of canals that surround the myofibrils and function to store Ca⁺⁺. It is called the sarcoplasmic reticulum.

T-tubules (transverse tubules) are membranous tubules formed from the sarcolemma (plasma membrane) that extend throughout the cell. They are next to but not fused with the sarcoplasmic reticulum. T-tubules spread action potentials throughout the cell.

During muscle contraction, actin and myosin filaments slide over each other as diagrammed below.

Cross-bridges (not shown) form between the heads of myosin molecules and binding sites on the actin filaments.

After attachment, the myosin heads bend, causing the two filaments to move with respect to each other.

The energy is derived from ATP. ATP binds the myosin heads, causing them to be released from the actin filaments. When ATP is hydrolyzed to ADP + P_i,  the myosin heads bend to their high-energy configuration. After bending to this configuration, they myosin heads bind to binding sites on the actin filaments. When ADP and P_i are released, the myosin heads return to their original low-energy configuration, causing the actin and myosin filaments slide past each other. ATP binds to the myosin heads, causing them to be released from the actin filaments and the process can begin again.

At death, there is no ATP to cause the heads to detach, and the body enters rigor mortis (it becomes stiff) because the myosin heads remain attached to the actin filaments.

An action potential depolarizes the sarcolemma (the plasma membrane of the muscle cell). T-tubules spread the action potential throughout the cell. When the depolarization reaches the sarcoplasmic reticulum (calcium storage sacs), they release Ca⁺⁺.

Calcium triggers the contraction.

Pumps in the sarcoplasmic reticulum return the Ca⁺⁺ to the sarcoplasmic reticulum for storage causing the muscle to relax.

The actin filaments are composed of two rows of actin subunits that are wound around with tropomyosin threads.

In a relaxed muscle, contraction does not occur because the myosin binding sites on the actin filament are covered by the tropomyosin threads.

Troponin occurs at intervals along the threads. When Ca⁺⁺ combines with troponin, the tropomyosin threads shift, exposing the myosin binding sites.

When the binding sites are exposed, the myosin heads attach and the filament contracts as previously described.

Animation

Click here to view a 3D animation of the processes described in this section.

Each motor axon branches to several muscle fibers and each branch has several terminal knobs. A single action potential can therefore stimulate several muscle fibers (cells).

Collectively, a motor neuron and all of the muscle fibers that it controls are called a motor unit.

The neurotransmitter is acetylcholine.

The region where a motor neuron forms a synapse with a muscle cell is called a neuromuscular junction.

An action potential reaches the terminal end of a motor neuron where it arrives at several muscle fibers (cells).

Acetylcholine is released into the synaptic cleft of the neuromuscular junction.

The muscle fiber depolarizes, and the action potential is spread to the interior of the cell by the T-tubules.

When the action potential reaches the sarcoplasmic reticulum, calcium is released into the cell.

Calcium binds with troponin, causing tropomyosin to uncover the myosin binding sites on the actin filaments.

When ATP bound to the myosin heads is hydrolyzed to ADP + P_i, causing the myosin heads bend to their high-energy configuration.

The myosin heads bind to the actin filaments, forming cross-bridges.

ATP and Pi are then released from the myosin heads, causing them to release their energy and return to their original configuration. 

As the myosin heads bend, the actin and myosin filaments move because the myosin has formed cross bridges with the actin.

ATP binds to the myosin heads, causing them to be released from the actin filaments.

If binding sites are available on the actin filaments, the process will repeat and the muscle will continue to contract.