• Body Plan

  • Asymmetrical animals are sponges.

    • They have no symmetry, therefore no specific form.

    • They are the simplest types of animals, and they lack tissues.

  • Symmetrical animals come in two forms: those that have radial symmetry and those that have bilateral symmetry.

    • Example: Humans have bilateral symmetry.

• Body Planes

  • A standing vertebrate animal can be divided by several plans:

    • Sagittal - divides the body into right and left portions

    • Midsagittal - divides the body exactly in the middle, making two equal right and left halves.

    • Frontal (Coronal) - separates the front from the back

    • Transverse (Horizontal) - divides the animal into upper and lower portions

      • This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane.

• Body Cavities

  • The human body is divided into two main cavities: the dorsal (back) and ventral (front) cavities.

  • These cavities contain several cavities within them, look at the figure below.

4.2 Tissues and Their Functions

• Tissues

  • A tissue is a group of cells with similar functions.

• Four Types of Tissues

  • Epithelial - covers the body and lining the cavities

    • Classified based on:

      • Shape

        • Squamous (flat)

        • Cuboidal

        • Columnar

      • Layers

        • Simple

        • Stratified (more than 1 layer)

  • Connective tissue - holds tissue together

    • Composed of cells and a matrix (made of protein and a ground substance)

    • Fibrous, supportive, and fluid

• Muscle tissue - the cells and fibers of this tissue can contract and shorten to provide movement.

  • Three types: Skeletal, smooth, and cardiac

  • These types differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntary or involuntary controlled, and their location within the body.

• Nervous tissue - excitable tissue

  • Found in two types of cells: neurons (functional cells) and neuroglia (supportive cells)

4.3 Homeostasis

• Homeostasis

  • Process by which an organism can maintain internal stability while adjusting to changing external conditions maintained in our body by negative feedback loops, limiting the stimulus.

    • Example: Sweating on a hot day

  • Positive feedback loops maintain the direction of the stimulus, pushing the organism further out of homeostasis.

    • Example: Giving birth

• Endotherms and Ectotherms

  • Endotherms are organisms that maintain a constant body temperature. 

    • These organisms are often referred to as warm blooded and have a higher metabolic rate.

    • Example: Humans are considered endotherms.

  • Ectotherms are organisms that change their temperature to suit their environment and are often called cold blooded. 

    • Example: Reptiles are ectotherms. 

4.4 The Skeletal Systems

• Endoskeleton

  • Humans have an endoskeleton, which means our skeletal system consists of hard, mineralized structures located within the soft tissue of our body.

    • Humans have 206 bones.

  • Our skeletal system is divided into two parts: the axial skeleton (skull, vertebral column, rib cage, and sternum) and the appendicular skeleton (shoulder, upper limb, pelvis, and lower limb).

    • The figure to the right shows the axial skeleton in gray, and the appendicular skeleton in pink.

• Skull

  • The bones of the skull support the face and protect the brain.

  • The skull consists of 22 bones, which are divided into two categories: cranial and facial bones

    • The 8 cranial bones (frontal, two parietal, two temporal, occipital, sphenoid, and the ethmoid bone) form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head and neck.

    • The 14 facial bones (nasal, maxillary, zygomatic, palatine, vomer, lacrimal, inferior nasal conchae, and the mandible) occur in pairs and form the face.

• Vertebral Column

  • The vertebral column is composed of 33 vertebrae, and is an S-shaped column that houses and protects the spinal cord.

    • The adult vertebrae is divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae.

  • Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass and a notch on each side through which the spinal nerves pass.

  • Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum.

    • Each disk is part of a joint that allows for some movement of the spine and acts as a cushion to absorb shocks from movement such as walking and running.

• Thoracic Cage 

  • The thoracic cage, also known as the ribcage, consists of the ribs, sternum, thoracic vertebrae, and costal cartilages.

  • It encloses and protects the heart and lungs. 

  • It provides support for the shoulder girdles and upper limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders.

  • The sternum, or breastbone, is a long flat bone located at the anterior of the chest.

    • It is formed from three bones that fuse in the adult.

  • 12 pairs of ribs attach to the thoracic vertebrae and curve toward the front of the body, forming the ribcage.

    • Costal cartilages connect the anterior ends of the ribs to the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs.

• Pectoral Girdle 

  • The pectoral girdle bones provide the points of attachment of the upper limbs to the axial skeleton.

    • It consists of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the posterior.

• Upper Limb

  • The upper limb contains 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius), the wrist (carpals), and hand (metacarpals and phalanges). 

• Pelvic Girdle

  • The pelvic girdle attaches to the lower limbs of the axial skeleton.

  • It is responsible for bearing the weight of the body and for locomotion.

  • It is attached to the axial skeleton by strong ligaments.

    • It has deep sockets with robust ligaments to securely attach the femur to the body.

  • The pelvic girdle is further strengthened by two large hip bones.

    • In adults, the hip or coxal bones are formed by the fusion of three pairs of bones: the ilium, ischium, and pubis.

  • The pelvis joins together in the anterior of the body at a joint called the pubis symphysis and with the bones of the sacrum at the posterior of the body.

  • Important: The female pelvis has a wider pubic angle and larger diameter of the pelvic canal which helps during childbirth.

• Lower Limb

  • The lower limb consists of the thigh, the leg, and the foot.

  • The bones of the lower limb are the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot). 

    • Important: The bones of the lower limbs are thicker and stronger than the bones of the upper limbs because of the need to support the entire weight of the body and the resulting forces from locomotion.

    • The femur, or thighbone, is the longest, heaviest, and strongest bone in the body.

  • The femur and pelvis form the hip joint at the proximal end. And at the distal end, the femur, tibia, and patella form the knee joint.

  • The patella, or kneecap, is a triangular bone that lies anterior to the knee joint.

    • It is embedded in the tendon of the femoral extensors (quadriceps).

    • It improves knee extension by reducing friction.

  • The tibia, or shinbone, is a large bone of the leg that is located directly below the knee.

    • It articulates with the femur at its proximal end, with the fibula and the tarsal bones at its distal end.

    • It is the second largest bone in the human body and is responsible for transmitting the weight of the body from the femur to the foot.

  • The fibula, or calf bone, parallels and articulates with the tibia.

    • It does not articulate with the femur and does not bear weight.

    • It acts as a site for muscle attachment and forms the lateral part of the ankle joint. 

  • The tarsals are the seven bones of the ankle.

    • The ankle transmits the weight of the body from the tibia and the fibula to the foot.

  • The metatarsals are the five bones of the foot.

  • The phalanges are the 14 bones of the toes.

    • Each toe consists of three phalanges, except for the big toe that has only two.

4.5 Bone

• Bone

  • Bone, or osseous tissue, is a solid connective tissue that makes up the endoskeleton. It contains specialized cells and a matrix of mineral salts and collagen fibers.

    • The mineral salts primarily include hydroxyapatite, a mineral formed from calcium phosphate.

    • Calcification is the process of deposition of mineral salts on the collagen fiber matrix that crystallizes and hardens tissue.

      • Only occurs in the presence of collagen fibers.

  • The bones of the human skeleton are classified by their shape: long bones, short bones, flat bones, sutural bones, sesamoid bones, and irregular bones.

  • Long bones are longer than they are wide and have a shaft and two ends.

    • Most limb bones are long bones.

    • Example: Femur tibia, ulna, and radius (exceptions are the patella and bones of the wrist and ankle)

  • The diaphysis, or central shaft, contains bone marrow in a medullary (marrow) cavity.

    • The rounded ends, the epiphyses, are covered with articular cartilage and are filled with red bone marrow, which produces blood cells.

  • Short bones, or cuboidal bones, are bones that are the same width and length, giving them a cube-like shape.

    • Example: The bones of the wrist (carpals) and ankle (tarsals) are short bones.

  • Flat bones are thin and relatively broad bones that are found where extensive protection of organs is required or where broad surfaces of muscle attachment are required.

    • Example: The sternum (breast bone), ribs, scapulae (shoulder blades), and the roof of the skull are flat bones

  • Irregular bones are bones with complex shapes

    • These bones may have short, flat, notched, or ridged surfaces.

    • Example: The vertebrae, hip bones, and several skull bones

  • Sesamoid bones are small, flat bones and are shaped similarly to a sesame seed.

    • They develop inside tendons and may be found near joints at the knees, hands, and feet.

    • Example: The patellae

  • Sutural bones are small, flat, irregularly shaped bones.

    • They may be found between the flat bones of the skull.

    • They vary in number, shape, size, and position.

• Bone Tissue

  • Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue.

    • Osteocytes are the living cells of bone tissue that form the mineral matrix of bones.

    • There are two types of bone tissue: compact and spongy.

  • Compact bone (or cortical bone) forms the hard external layers of all bones and surrounds the medullary cavity, or bone marrow.

    • It provides protection and strength to bones.

    • This tissue consists of units called osteons or Haversian systems; They are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi, which transport blood.

      • They are aligned parallel to the long axis of the bone.

      • Each osteon consists of lamellae, which are layers of compact matrix that surround a central canal called the Haversian canal (this contains the bone’s blood vessels and nerve fibers).

      • Osteons in compact bone tissue are aligned in the same direction along lines of stress and help the bone resist bending or fracturing.

    • Compact bone tissue is prominent in areas of bone at which stresses are applied in only a few directions. 

  • Spongy bone forms the inner layer of all bones.

    • It consists of trabeculae, which are lamellae that are arranged as rods or plates.

    • Red bone marrow is found between the trabeculae.

      • Red bone marrow of the femur and the interior of other large bones, such as the ilium, forms blood cells.

    • Blood vessels within this tissue deliver nutrients to osteocytes and remove waste.

    • Spongy bone reduces the density of bone and allows the ends of long bones to compress as the result of stresses applied to bone.

    • Prominent in areas of bones that are not heavily stressed or where stresses arrive from many directions

• Cell Types in Bones

  • Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells.

  • Osteoblasts are bone cells that are responsible for bone formation.

  • Osteoclasts are large bone cells with up to 50 nuclei.

    • They remove bone structure by releasing lysosomal enzymes and acids that dissolve the bony matrix.

    • They help regulate calcium concentrations in the blood.

    • Bones may also be resorbed for remodeling, if the applied stresses have changed.

  • Osteocytes are mature bone cells and are the main cells in bony connective tissue.

    • They cannot divide.

    • They maintain normal bone structure by recycling the mineral salts in the bony matrix. 

  • Osteoprogenitor cells are squamous stem cells that divide to produce daughter cells that differentiate into osteoblasts.

    • They are important in the repair of fractures.

• Development of Bone

  • Ossification (osteogenesis) is the process of bone formation.

  • It is different from calcification.

    • Calcification takes place during the ossification of bones, it can also occur in other tissues.

  • Ossification begins approximately six weeks after fertilization in an embryo.

    • Before this time, the embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage.

  • Bone growth continues until approximately age 25.

    • Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.

• Intramembranous Ossification

  • Intramembranous ossification is the process of bone development from fibrous membranes.

  • It is involved in the formation of the flat bones of the skull, mandible, and clavicles.

  • Ossification begins as mesenchymal cells form a template of the future bone.

  • They then differentiate into osteoblasts at the ossification center.

    • Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix.

  • The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone.

  • Connective tissue in the matrix differentiates into red bone marrow in the fetus.

    • Spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone. 

• Endochondral Ossification

  • Endochondral ossification is the process of bone development from hyaline cartilage.

    • All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification.

  • In long bones, chondrocytes form a template of the hyaline cartilage diaphysis. 

    • Responding to complex development signals, the matrix begins to calcify.

      • This calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage.

    • Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone.

      • Osteoclasts then break down some of the spongy bone to create a medullary, or marrow, cavity in the center of the diaphysis.

    • Dense, irregular connective tissue forms a sheath (periosteum) around the bones.

      • The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments.

    • The bone continues to grow and elongate as the cartilage cells at the epiphysis divide.

  • In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify.

    • Secondary ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert hyaline cartilage into spongy bone.

  • Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth plate), which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of long bones. 

• Growth of Bone

  • Long bones continue to lengthen, potentially until adolescence, through the addition of bone tissue at the epiphyseal plate.

  • They also increase in width through appositional growth.

• Lengthening of Long Bones

  • Chondrocytes on the epiphyseal side of the epiphyseal plate divide.

    • One cell remains undifferentiated near the epiphysis, and one cell moves toward the diaphysis.

    • The cells, which are pushed from the epiphysis, mature and are destroyed by calcification.

      • This process replaces cartilage with bone on the diaphyseal side of the plate, resulting in a lengthening of the bone. 

  • Long bones stop growing at around the age of 18 in females and the age of 21 in males in a process called epiphyseal plate closure.

    • Cartilage cells stop dividing and all cartilage is replaced by bone.

    • The epiphyseal plate fades, leaving a structure called the epiphyseal line or epiphyseal remnant, and the epiphysis and diaphysis fuse.

• Thickening of Long Bones

  • Appositional growth is the increase in the diameter of bones by the addition of bony tissue at the surface of bones. 

  • Osteoblasts at the bone surface secrete bone matrix, and osteoclasts on the inner surface break down bone.

    • Osteoblasts differentiate into osteocytes,

  • A balance between these two processes allows the bone to thicken without becoming too heavy.

• Bone Remodeling and Repair

  • Bone renewal continues after birth into adulthood.

  • Bone remodeling is the replacement of old bone tissue by new bone tissue

    • It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts.

  • Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorus, and magnesium.

    • Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for proper bone growth and maintenance.

  • Bone turnover rates are quite high, with 5 to 7% of bone mass being recycled every week.

    • Differences in turnover rates exist in different areas of the skeleton and in different areas of a bone.

    • Example: The bone in the head of the femur may be fully replaces every six months, whereas the bone along the shaft is altered much more slowly.

  • Bone remodeling allows bones to adapt to stresses by becoming thicker and stronger when subjected to stress.

  • Bones that are not subject to normal stress, for example when a limb is in a cast, will begin to lose mass.

4.6 Joints and Skeletal Movements

• Joints

  • The variety of motion that the bones are able to do in our body is made possible by joints.

  • These are found where bones meet.

  • They are either classified by: structure or function (extent of mobility provided by the joint)

    • Types of Joints by function: Synarthroses (immovable), amphiarthrosis (slightly movable), diarthroses (freely movable)

    • Types of Joints by structure: Fibrous (tend to be immovable), synovial (tend to be freely movable), and cartilaginous (exhibit a range of mobilities)

  • Fibrous joints

    • Contain lots of dense fibrous connective tissue 

    • No joint cavity

    • For connecting bones that don’t require a lot of movement

      • Three types: sutures (found only in skull), syndesmoses (found where bones are connected only by ligaments), and gomphoses (found only in mouth)

  • Cartilaginous joints

    • Bones are connected by cartilage

    • Lack a joint cavity 

    • Not particularly movable

      • Two types: synchondroses (contain hyaline cartilage), symphyses (contain fibrocartilage, compressible)

  • Synovial joints

    • Contain a cavity filled with fluid

    • Most joints are of this type, especially the ones in our limbs.

• Types of Motion

  • Muscles have an origin attached to an immovable bone, and an insertion attached to a movable bone. 

  • When muscles contract around joints, we get movement, and we can describe this motion by referencing certain lines or axes as well as certain planes of space. 

  • Synovial joint movement: nonaxial movement (slipping movement), uniaxial movement (movement in one plane), biaxial movement (movement in two planes), and multiaxial movement (movement in all three planes)

  • Other Synovial movements:

    • Gliding movement occurs when one flat bone surface slips over another.

      • Occurs at the ankles and wrists

    • Angular movement happens when the angle between two bones changes.

      • Flexion decreases the angle of the joint (i.e. bending the head forward).

      • Extension increases the angle of the joint (i.e. straightening your neck)

      • Hyperextension goes beyond extension (i.e. bending your head back).

    • Abduction is motion of a limb away from the midline plate of the body (i.e. moving arms up and away from your side).

    • Adduction is the opposite of abduction, moving a limb toward the midline plate of the body (i.e. bringing arms down to your side).

    • Circumduction involves making circles with a limb, such as arm circles.

    • Rotation involves the turning of a bone around its own axis

      • Two types: Lateral (external) rotation and medial (internal) rotation

  • Special Movements:

    • Supination and pronation (refers to the radius moving around the ulna)

    • Dorsiflexion and plantar flexion (refers to movements in the foot)

    • Protraction and retraction (refer to movements in the mandible)

4.7 Muscle Contraction

• Skeletal Muscle Structure

  • Skeletal muscle, also called striated muscle, is used to move the skeleton.

  • They are under the direct control of the nervous system and can produce contractions ranging from quick twitches to powerful sustained tension.

  • Individual muscle cells, muscle fibers, are among the largest cells of the human body ranging  from 10 to 100 micrometers in diameter.

    • Each muscle fiber runs the entire length of the muscle.

  • Each muscle fiber contains many individual contractile subunits known as myofibrils, extending from one end of the fiber to the other.

    • Each myofibril is surrounded by sarcoplasmic reticulum, a complex of membranes forming a network of interconnected hollow tubes.

      • It contains a fluid rich in calcium ions.

    • Deep indentations of the muscle cell membrane called transverse or t tubules extend down into the muscle fiber, passing very close to portions of the sarcoplasmic reticulum.

      • This arrangement is crucial in controlling muscle contraction.

    • Myofibrils contain subunits called sarcomeres, made up of precise arrangements of actin and myosin filaments.

      • Sarcomeres are attached end to end throughout the length of the myofibril and their junction points are called z-lines. 

    • Attached to the z-lines are strands of actin and two accessory proteins that form the thin filaments.

      • Suspended between the thin filaments are thick filaments composed of the protein myosin. 

    • Important: Alternating thin and thick filaments give myofibrils their striped appearance.

    • Small arms called cross bridges extend from the strands of biosin and contact the thin filaments. 

      • Each subunit of actin in thin filaments has a binding site for a myosin cross bridge.

      • In a relaxed muscle, these sites are covered by two accessory proteins which prevent the myosin cross bridges from attaching to the thin filaments.

        • When relaxed, these accessory proteins move aside so biosin cross bridges can attach to the binding sites on the actin subunits.

• How Signal Arrives at The Neuromuscular Junction

• Skeletal Muscle Contraction