Movements and Locomotion Study Notes
Components of Movement and Locomotion
Movement is defined as a change in the position of any part of the body with respect to a definite point. When an organism as a whole moves from one place to another, these collective movements are referred to as locomotory movements or locomotion. While all locomotion is movement, not all movements constitute locomotion, such as the curvature movements seen in plants. Locomotion is essential for organisms to find food, seek shelter, escape from enemies, and find mates. These movements contribute to the enjoyment of life and are critical for maintaining it by allowing the body to respond to changes in both the external and internal environment. In vertebrates, movements are brought about by the coordinated action of three main components: the skeleton or skeletal system, the muscles, and the joints.
Classification and Origin of the Skeletal System
The skeleton or skeletal system consists of all the hard parts of the body that form the supporting framework. In vertebrates, this internal framework is composed of bones and cartilages. The skeletal system is divided into two main types based on its location. The exoskeleton includes all hard parts present on the outer surface of the body, such as the shells of molluscs, the chitinous plates or sclerites of arthropods, the scales of fishes and reptiles, the feathers and claws of birds, and the hair, nails, and hoofs of mammals. The exoskeleton is always dead and typically ectodermal in origin, with the notable exception of fish scales. In contrast, the endoskeleton consists of the hard parts, specifically bones and cartilages, located inside the body. The endoskeleton is always living and mesodermal in origin, providing support and permitting movement. While the exoskeleton offers superior protection against external injury, the endoskeleton provides better structural support and facilitates more complex locomotion. Most exoskeleton structures are formed primarily by keratin protein, whereas the endoskeleton is composed of both organic and inorganic compounds.
Functions of the Human Skeleton
The skeleton performs multiple vital functions beyond structural support. It serves as the supporting framework of the body and provides and maintains the body's shape. Hard bony structures protect delicate vital organs; for instance, the skull protects the brain and sense organs, while the ribs protect the heart and lungs. Bones and joints together constitute levers, and the skeleton provides surfaces for muscle attachment to facilitate movement. Bones also act as a major reservoir for calcium, which is deposited or withdrawn according to physiological needs to maintain constant blood calcium levels. The production of blood corpuscles, known as haemopoiesis, occurs within the red bone marrow. Furthermore, the marrow cavity of long bones is filled with bone marrow made of adipose tissue for fat storage. Three tiny bones in the middle ear, the ear ossicles, are essential for hearing. The sternum and ribs play a critical role in the breathing process. Finally, the hardness of bones gives them palaeontological importance because they are usually preserved as fossils, providing authentic evidence to trace the evolution of various animals.
The Human Axial Skeleton
The human endoskeleton is divided into the axial skeleton and the appendicular skeleton. The axial skeleton consists of 80 bones present along the median longitudinal axis of the body. In adults, the human skeleton total is 206 bones, while a newly born baby possesses approximately 270 bones. The axial skeleton includes 74 bones forming the upright axis—comprising the skull, vertebral column, sternum, and ribs—along with six tiny middle ear bones. The skull itself is made of 28 irregularly shaped bones and is divided into the cranium, the face, and the ear ossicles. The cranium, or brain-box, is a large, hollow structure made of 8 bones connected by immovable joints called sutures. At the back of the cranium is the foramen magnum, a large hole through which the spinal cord continues from the brain into the vertebral canal. The face contains 14 bones, all of which form immovable joints except for the mandible or lower jaw bone, which articulates movably. Each middle ear contains three ear ossicles—the malleus (hammer), incus (anvil), and stapes (stirrup)—totaling six bones that convey sound waves from the external to the internal ear. Additionally, a single U-shaped bone called the hyoid bone is located in the throat; it is unique because it does not form a joint with any other bone.
Characteristics of the Vertebral Column and Vertebrae
The vertebral column is a curved, vertical rod about long, located in the mid-posterior line of the neck and trunk. While it is almost straight in newborns, the adult vertebral column develops four curvatures—cervical, thoracic, lumbar, and sacral—to increase carrying strength and maintain balance in an upright position. The column is formed by 33 ring-like bones called vertebrae, which enclose the vertebral or neural canal housing the spinal cord. These are grouped into 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the thorax, 5 lumbar vertebrae in the abdomen, 5 fused sacral vertebrae forming the sacrum, and 4 fused coccygeal vertebrae forming the coccyx. The vertebral formula for humans is expressed as . A typical vertebra consists of a thick body or centrum on the ventral side and a neural arch forming the neural canal. Seven processes arise from the neural arch: one spinous process, a pair of superior articular processes, a pair of inferior articular processes, and a pair of transverse processes. Intervertebral discs between the centra provide flexibility, and intervertebral foramina allow the exit of spinal nerves. Specific vertebrae include the atlas (), the axis (), and typical cervical vertebrae. Thoracic vertebrae are marked by long downward-directed spinous processes, and lumbar vertebrae are characterized by their large, strong bodies.
The Thoracic Cage and Appendicular Skeleton
The thoracic cage is formed by the sternum and 12 pairs of ribs. The sternum, a dagger-shaped bone in the front chest wall, consists of the manubrium, the body, and the xiphoid process. The 12 pairs of ribs articulate with the thoracic vertebrae at the back. The first seven pairs are true ribs, directly attached to the sternum. The next three pairs (8th, 9th, and 10th) are false ribs, attached to the 7th rib. The last two pairs (11th and 12th) are floating ribs as they are free in front. Some individuals may possess a 13th pair known as a gorilla rib. The appendicular skeleton consists of 126 bones, including the girdles and limb bones. The pectoral or shoulder girdle consists of the clavicle (collar bone) and the scapula (shoulder blade). The scapula features a glenoid cavity where the head of the humerus articulates. The pelvic or hip girdle is a bowl-like structure made of two innominate halves, each comprising the ilium, pubis, and ischium. Each innominate bone has a depression called the acetabulum for the femur's head. The upper limb consists of 30 bones: 1 humerus, 1 radius, 1 ulna, 8 carpals (wrist), 5 metacarpals (palm), and 14 phalanges (fingers). The lower limb also has 30 bones: 1 femur (thigh bone, the longest bone), 1 patella (knee cap), 1 tibia, 1 fibula, 7 tarsals (ankle), 5 metatarsals (sole), and 14 phalanges (toes).
Classification and Structure of Joints
Arthrology is the study of joints, which are the points of articulation between bones. Joints are classified into three types. Fibrous or immovable joints (synarthroses) occur where bones are tightly bound by white fibrous collagen tissue, such as the sutures of the skull or the peg-and-socket joints of teeth. Cartilaginous or slightly movable joints (amphiarthroses) involve cartilage joining two bones, allowing limited movement. Examples include the pubic symphysis, joints between vertebrae, and the synchondroses between ribs and the sternum via costal cartilage. Synovial or freely movable joints (diarthroses) are the most mobile. In these joints, the articular surfaces are covered by hyaline articular cartilage and enclosed in a synovial cavity filled with synovial fluid secreted by the synovial membrane. This fluid lubricates the joint and reduces friction. In old age, a decrease in synovial fluid can lead to joint stiffness and difficult movement.
Types of Synovial Joints
Synovial joints are subdivided into six types based on their structure and the movements they permit. Ball and socket joints allow movement in nearly all directions and occur where a ball-like end fits into a cup-like socket, such as the shoulder and hip joints. Hinge joints allow movement in only one plane, exemplified by the elbow and knee. Pivot joints involve a bone rotating on a projection of another bone that acts as an axis, such as the joint between the atlas and axis vertebrae. Angular or ellipsoidal joints permit movement in two directions (side-to-side and back-and-forth), found in the wrist and metacarpophalangeal joints. Gliding joints allow two bones to slide over each other, occurring between the carpals of the wrist, tarsals of the ankle, and zygapophyses of the vertebrae. Saddle joints resemble poorly developed ball and socket joints; only one pair exists in the human body, specifically between the trapezium (a carpal bone) and the metacarpal of the thumb.
Muscles and the Mechanism of Movement
Movement is produced by the contraction and relaxation of muscular tissue. A muscle is a bundle of fibers covered by connective tissue and has two ends: the origin, attached to an immovable or less movable bone, and the insertion, attached to the more movable part. There are 639 named muscles in the human body, of which approximately 600 are voluntary. The largest muscle is the gluteus maximus (buttock to thigh), while the smallest is the stapedius, which controls the stapes bone in the middle ear. Muscles can only contract (shorten and thicken) and relax; they cannot expand or elongate on their own. They can pull parts together but cannot push them away. Therefore, muscles work in antagonistic pairs to move body parts in opposite directions. For example, the biceps muscle contracts to bend the arm at the elbow while the triceps relaxes; to straighten the arm, the triceps contracts while the biceps relaxes. Muscular contraction requires energy provided by ATP molecules, and all movements are coordinated by the nervous system.