ANAT 260 Midterm Study Guide - Comprehensive Review

Foundations of Human Anatomy and Body Organization

The study of human anatomy is based on a hierarchical structure of biological organization. This hierarchy begins at the chemical and cellular levels and progresses through tissues and organs. The organ system level of organization is specifically defined as the level composed of multiple individual organs that work together in a coordinated fashion to achieve a shared physiological function. To analyze these structures at a microscopic level, various sub-disciplines exist; the branch of microscopic anatomy that focuses specifically on the study and identification of tissues is known as histology.

The human body is categorized into $4$ primary tissue types, which include muscle tissue, connective tissue, epithelial tissue, and nervous tissue. Other specialized systems, such as the endocrine system, are classified as organ systems rather than primary tissue types. Within these tissues, specific functional properties are present. For example, muscle tissue is characterized by several classic properties: contractility, which allows for the shortening of cells; elasticity, which allows tissue to return to its original shape; and conductivity, which refers to the ability to transmit electrical impulses.

Anatomical Terminology and Position

To ensure clarity in medical communication, the anatomical position serves as a standard reference point where the body is standing upright and the palms face anteriorly (forward). Directional terms are relative to this position. The term proximal is used to describe a structure that is "closer to the trunk" of the body or a point of attachment. In contrast, the term distal refers to a structure farther from the trunk. Relative to the midline of the body, the term lateral indicates a position "farther from the midline," while the term medial signifies a position closer to it. These terms are used to describe the relationship between bones in the forearm; in the anatomical position, the radius is located lateral to the ulna.

Specific regional terms are also utilized to describe surface landmarks. The cheek region of the face is medically referred to as the buccal region. For clinical and anatomical description, the abdomen is divided into regions; the epigastric region is the area that sits directly superior to (above) the umbilical region. Deep within the body, various cavities house vital organs. The cranial cavity is the space that contains and protects the brain, while the thoracic cavity is separated from the abdominopelvic cavity by the thin, muscular sheet known as the diaphragm.

Cell Biology and Organelles

The cell is the basic functional unit of life, and its organelles are specialized for specific tasks. The plasma membrane serves as the outer boundary of the cell, strictly controlling the passage of substances entering and leaving the intracellular environment. Many cells possess surface specializations to enhance their function. Microvilli are finger-like extensions of the plasma membrane that serve to increase the surface area of the cell for more efficient absorption. In contrast, cilia are hair-like structures that function to move substances, such as mucus, along the surface of an epithelial layer.

Metabolic needs dictate the prevalence of certain organelles within a cell. For instance, cells that require unusually high amounts of ATP (adenosine triphosphate) to fuel their activities, such as muscle cells or active transport cells, typically contain a higher density of mitochondria. These organelles are the main site of cellular respiration and energy production.

Cell Division and Genetics

Cells undergo a regulated process of division known as mitosis to ensure growth and repair. The correct sequence of mitotic phases is prophase, followed by metaphase, then anaphase, and concluding with telophase. During anaphase, a critical transition occurs where the sister chromatids separate from one another and move toward opposite poles of the cell.

In human reproduction, specialized cells called gametes, or sex cells, are used. Unlike most cells in the body, which are somatic, gametes are involved in the transfer of genetic material. A standard diploid human cell contains a full complement of genetic information, which consists of $23$ pairs of chromosomes, totaling $46$ individual chromosomes. These pairs are known as homologous pairs, ensuring that the cell has two copies of each gene.

Integumentary System

The skin and its associated structures comprise the integumentary system. Another formal term for the skin is the cutaneuous membrane. The most superficial layer of the skin, the epidermis, is composed of keratinized stratified squamous epithelium, which provides a durable, waterproof barrier. The epidermis is organized into layers known as strata. Moving from the deepest layer to the most superficial layer, the correct sequence is the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum (found only in thick skin), and finally the stratum corneum.

The skin contains specialized structures for protection and regulation. Melanin is a pigment produced by melanocytes primarily to protect the underlying tissues against damage from ultraviolet (UV) radiation. Within the dermis, small smooth muscles known as arrector pili are attached to hair follicles; when these muscles contract, they pull the hair upright, resulting in the phenomenon commonly known as "goose bumps." Furthermore, the ear canal contains modified sweat glands that produce cerumen, which is the clinical term for earwax.

The Skeletal System and Bone Histology

Bones serve several vital roles in the body, including providing structural support, protecting internal organs, assisting in movement, and performing mineral storage (specifically calcium and phosphate). The process of blood cell production, known as hemopoiesis, occurs mainly within the red bone marrow found in the spongy sections of various bones. Bones are classified by their shape; for example, a metatarsal bone in the foot is classified as a long bone, despite its relatively small size, because its length exceeds its width.

At the microscopic level, compact bone is organized into structural units called osteons. These units consist of concentric rings of bone matrix, known as lamellae, arranged around a central canal. The skeletal system is maintained by specialized cells: osteoblasts are the cells responsible for building bone by secreting the organic components of the bone matrix, while osteoclasts break it down. During growth, the lengthening of long bones occurs at the epiphyseal plate, which is located in the metaphysis—the region between the diaphysis (shaft) and the epiphysis (end) of the bone.

Axial and Appendicular Skeleton

The human skeleton features specific landmarks and articulated joints. On the skull, the sella turcica is a saddle-shaped depression located on the sphenoid bone that houses the pituitary gland. Another critical structure in the skull is the cribriform plate of the ethmoid bone, which is perforated with small holes to allow the olfactory nerves to pass from the nasal cavity to the brain for the sense of smell.

In the appendicular skeleton, a sesamoid bone is a bone that develops within a tendon; the patella (kneecap) is the most prominent example in the human body. In the lower limb, the talus is the specific ankle bone that articulates directly with the distal end of the tibia to form the ankle joint. In terms of connective structures, a tendon is a tough band of fibrous connective tissue that connects muscle to bone, whereas a ligament connects bone to bone.

Skeletal Muscle Physiology

Skeletal muscle contraction is a complex process involving molecular interactions. Muscle fibers contain myofilaments, categorised as thick and thin. Thick myofilaments are primarily composed of the protein myosin, while thin filaments are composed mainly of actin, troponin, and tropomyosin. The functional unit of contraction within a muscle fiber is the sarcomere, which extends from one Z disc to the next.

Contraction is triggered by the nervous system at the neuromuscular junction. The synaptic cleft is the physical space between the terminal of a motor neuron and the sarcolemma of a muscle fiber. When a nerve impulse arrives, the neurotransmitter acetylcholine ($ACh$) is released into this cleft. This leads to the release of calcium ions, which are stored within the terminal cisternae of the sarcoplasmic reticulum. The presence of calcium in the sarcoplasm allows the myosin heads to bind to actin, initiating the sliding filament mechanism. In the heart, cardiac muscle tissue is distinguished by the presence of intercalated discs, which allow for rapid electrical communication between adjacent cells.

The Nervous System

The nervous system is divided functionally into afferent and efferent pathways. The afferent division is synonymous with the sensory division, as it carries information toward the central nervous system. Nervous tissue itself is composed primarily of two types of cells: neurons, which transmit electrical signals, and glial cells (or neuroglia), which provide support and protection. Among the glial cells in the central nervous system, ependymal cells play a vital role in the production and circulation of cerebrospinal fluid ($CSF$).

The anatomy of a neuron includes the soma, also known as the perikaryon or cell body, which contains the nucleus. Peripheral nerves are organized by layers of connective tissue: the epineurium is the outermost covering, the perineurium surrounds bundles of axons (fascicles), and the endoneurium is the innermost layer surrounding individual axons.

Neuroanatomy and the Brain

The brain is protected by three layers of membranes called meninges. The middle layer, characterized by its "spider web" appearance, is the arachnoid mater. The brain is divided into two cerebral hemispheres, which are primarily connected and communicate through a large white matter tract called the corpus callosum. The brain is further divided into lobes with specialized functions; the occipital lobe is the region most heavily associated with the processing of visual information.

Other brain structures include the cerebellum, which contains a branching pattern of white matter known as the arbor vitae (or "tree of life"). The brainstem is a critical region responsible for regulating vital autonomic functions, including breathing and the regulation of blood pressure. Extending from the central nervous system are 31 pairs of spinal nerves that distribute throughout the body. The largest and longest individual nerve in the human body is the sciatic nerve, which originates in the lower spine and runs down the back of each leg.

Muscular Anatomy of the Torso

The abdominal wall consists of several layers of muscle that protect internal organs and provide core stability. The transversus abdominis is identified as the deepest of the abdominal wall muscles. This is distinct from the superficial rectus abdominis and the internal and external obliques. As previously noted, the diaphragm serves as the primary muscle of respiration and acts as the anatomical boundary between the thoracic and abdominopelvic cavities.