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Organ Systems Overview


The Nervous System

Neural Cells

  • Astrocytes: Provide structural support, regulate blood flow, and maintain the blood-brain barrier.
  • Microglia: Act as immune cells in the CNS. Consists of small, mobile cells that can phagocytize debris and pathogens.
  • Oligodendrocytes: Myelinate CNS axons.
  • Schwann Cells: Myelinate PNS axons and support neuronal regeneration.

    • Potentials and Neurotransmitters

  • An action potential is a rapid change in membrane potential that propagates along the axon of a neuron.
  • A graded potential is a localized change in membrane potential that varies in magnitude and can lead to an action potential if it reaches the threshold.
  • At resting potential, the inside of the neuron is more negative than the outside, primarily due to the distribution of ions across the membrane.
  • Depolarization occurs when sodium channels open, allowing sodium to enter the neuron, making the inside more positive.
  • Repolarization occurs when potassium channels open, allowing potassium to exit the neuron, restoring the negative internal environment.
  • Hyperpolarization occurs when potassium continues to exit the neuron, making the inside more negative than at rest.
  • Neurotransmitters are chemical messengers that transmit signals across a synapse from one neuron to another.
  • Amino acid neurotransmitters are involved in signal communication. They include glutamate (excitatory) and gamma-aminobutyric acid (GABA, inhibitory).
  • Peptide neurotransmitters are chains of amino acids that can modulate neuronal activity. Examples include endorphins and opioids.
  • Biogenic amines are small molecules derived from amino acids that function as neurotransmitters.
  • Acetylcholine is a neurotransmitter that plays a key role in muscle contraction and autonomic nervous system functions.

    • Neurons

  • Dendrites receive signals from other neurons.
  • The cell body (soma) contains the nucleus and organelles.
  • The axon transmits signals away from the cell body.
  • Axon terminals release neurotransmitters into the synapse.
  • Gray matter consists of neuronal cell bodies, dendrites, and unmyelinated axons.
  • White matter consists of myelinated axons and is responsible for transmitting signals between different brain regions.
  • Myelination increases the speed of signal transmission.
  • Nodes of Ranvier are gaps in the myelin sheath that facilitate rapid signal conduction.
  • Saltatory conduction is the process by which action potentials jump from one node of Ranvier to the next, increasing conduction velocity.

    • Brain Regions

  • The cerebrum is the largest part of the brain, responsible for higher cognitive functions, sensory processing, and voluntary motor control.
  • The cerebellum coordinates voluntary movements, balance, and posture.
  • The brainstem controls basic life functions such as heart rate, breathing, and blood pressure.
  • The thalamus acts as a relay station for sensory information, directing it to the appropriate areas of the brain.
  • The hypothalamus regulates homeostasis, including temperature, hunger, and thirst.
  • The limbic system is involved in emotion, memory, and motivation.
  • The hippocampus is crucial for memory formation and spatial navigation.
  • The amygdala processes emotions, particularly fear and pleasure.
  • The basal ganglia are involved in motor control and learning.
  • The corpus callosum connects the left and right hemispheres of the brain, allowing communication between them.
  • The nucleus accumbens is involved in reward processing and motivation.
  • The midbrain is involved in consciousness and motor control.
  • The cerebellum is responsible for autonomic functions and coordination.
  • The pons connects the cerebellum to the brainstem and is involved in regulating sleep and arousal.
  • The medulla oblongata controls vital autonomic functions such as heart rate and breathing.
  • The spinal cord is a long, tubular structure that extends from the brainstem down the vertebral column.

    • Integumentary System

    Skin Layers

  • The epidermis is the outermost layer of skin, providing a barrier against the environment.
  • The dermis is the middle layer, containing connective tissue, blood vessels, and nerves.
  • The hypodermis is the innermost layer, consisting of fat and connective tissue.

    • Skin Functions

  • The skin acts as a barrier, protecting against pathogens, chemicals, and physical damage.
  • The skin contains sensory receptors that detect touch, temperature, and pain.
  • Transpiration is the process of water vapor loss from the skin. This is regulated by sweat glands.

    • Musculoskeletal System

    Muscle Types

    Muscle cells

  • Myocytes are the muscle cells responsible for contraction.
  • Myofibrils are long, thread-like structures within myocytes that contain the contractile proteins actin and myosin.
  • The sarcomere is the basic contractile unit of muscle fibers.
  • Actin is a thin filament that interacts with myosin during muscle contraction.
  • Myosin is a thick filament that interacts with actin to produce muscle contraction.
  • The sarcoplasmic reticulum stores calcium ions and releases them during muscle contraction.
  • The sarcolemma is the plasma membrane of a muscle cell.

    • Muscle Contraction

  • Troponin is a protein that regulates muscle contraction by binding to calcium ions.
  • Tropomyosin is a protein that blocks myosin-binding sites on actin filaments.
  • Calcium ions bind to troponin, causing a conformational change.
  • This change moves tropomyosin away from myosin-binding sites, allowing contraction to occur.

    • Power Stroke

  • Myosin heads bind to actin, forming cross-bridges and pulling the filaments past each other.
  • Then ATP binds to myosin, causing it to release actin and re-cock for another contraction.
  • Hydrolysis of this molecule provides the energy for the power stroke.
  • A force is generated as myosin pulls actin filaments toward the center of the sarcomere.
  • A new ATP molecule binds to myosin, allowing it to release actin.
  • Myosin is now ready to bind to a new actin site and repeat the cycle.

    • Ligaments and Joints

  • Tendons connect muscles to bones.
  • Ligaments connect bones to other bones at joints.

    • Skeleton

  • Sesamoid bones are small, round bones that form within tendons.
  • The axial skeleton includes the skull, vertebral column, and rib cage.
  • The appendicular skeleton includes the limbs and pelvic girdle.
  • Hyaline cartilage provides support and flexibility, found in the nose, trachea, and joints.
  • Elastic cartilage provides strength and elasticity, found in the ear and epiglottis.
  • Fibrocartilage provides tensile strength, found in intervertebral discs and pubic symphysis.

    • Cardiovascular System

    Blood Vessels

  • Arteries carry blood away from the heart.
  • Veins carry blood toward the heart.
  • Capillaries are small vessels where gas exchange occurs.
  • Venules are small veins that collect blood from capillaries.<
  • Arterioles are small arteries that regulate blood flow into capillaries.
  • Blood is composed of red blood cells, white blood cells, platelets, and plasma.
  • Plasma is the liquid component of blood.
  • Red blood cells transport oxygen and carbon dioxide.
  • White blood cells are involved in the immune response.
  • Platelets are involved in blood clotting.
  • Hemoglobin is a protein in red blood cells that binds to oxygen.
  • Hematopoiesis is the process of blood cell formation, occurring in the bone marrow.

    • Heart Structure

  • The right atrium receives deoxygenated blood from the body.
  • The right ventricle pumps deoxygenated blood to the lungs.
  • The left atrium receives oxygenated blood from the lungs.
  • The left ventricle pumps oxygenated blood to the body.
  • The mitral and tricuspid valves control blood flow between the atria and ventricles.
  • The aortic and pulmonary valves control blood flow out of the heart.

    • Respiratory System

    Lungs

  • The left lung has two lobes, while the right lung has three lobes.
  • The alveoli are tiny air sacs where gas exchange occurs between the air and blood.
  • The respiratory tract includes the trachea, bronchi, and bronchioles.
  • The diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity and plays a key role in breathing.

    • Gas Exchange

  • Gas exchange occurs in the alveoli, where oxygen is absorbed into the blood and carbon dioxide is released.
  • Henry's law states that the amount of gas dissolved in a liquid is proportional to its partial pressure.
  • Carbon dioxide exists in the body in three forms: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin.

    • Digestive System

    Digestive Tract

  • Parotid glands produce an enzyme called amylase, which breaks down carbohydrates.
  • Submandibular glands are located beneath the jaw and produce the most saliva.
  • Von Ebner's glands are located at the back of the tongue and produce a watery secretion that helps with taste.
  • Food is converted into bolus in the mouth before swallowing.
  • Mastication is the process of chewing food to break it down into smaller pieces.
  • When food enters the stomach, it is mixed with gastric juices to form chyme.
  • Chief cells secrete pepsinogen, which is activated to pepsin in the acidic environment of the stomach.
  • Parietal cells secrete hydrochloric acid, which helps to denature proteins and activate pepsin.
  • G cells secrete gastrin, a hormone that stimulates the secretion of gastric acid.
  • The duodenum is the first part of the small intestine and is responsible for the majority of chemical digestion.
  • The jejunum is the second part of the small intestine, where most nutrient absorption occurs.
  • The ileum is the final part of the small intestine, where vitamins and bile salts are absorbed.
  • When the food enters the small intestine, enteropeptidase is secreted, which activates pancreatic enzymes.
  • Most bacteria that aid digestion are located in the large intestine.

    • Accessory Organs

  • The liver produces bile, which helps emulsify fats.
  • The gallbladder stores bile until it is needed in the small intestine.
  • The pancreas produces digestive enzymes and bicarbonate to neutralize stomach acid.

    • Endocrine System

    Hormones

  • Hormones are chemical messengers that regulate various functions in the body.
  • Peptide hormones are made up of amino acids and include insulin and glucagon.
  • Protein hormones are larger molecules made up of chains of amino acids and include growth hormone and prolactin.
  • Steroid hormones are derived from cholesterol.
  • Amine hormones are derived from amino acids.

    • Hormone Action

  • Hormones bind to specific receptors on target cells to initiate a response.
  • Primary messengers are hormones that bind to receptors on target cells to initiate a response.
  • Secondary messengers are molecules that relay signals from primary messengers within the cell.
  • Hormones can be classified as either hydrophilic (water-soluble) or hydrophobic (lipid-soluble).
  • Hydrophilic hormones bind to receptors on the cell surface, while hydrophobic hormones pass through the cell membrane and bind to intracellular receptors.

    • Endocrine Glands

  • The hypothalamus is the master regulator of the endocrine system, controlling the release of hormones from the pituitary gland.
  • The pituitary gland is divided into the anterior and posterior lobes, each secreting different hormones.
  • The anterior pituitary produces hormones under the regulation of the hypothalamus.
  • The posterior pituitary stores and releases hormones produced by the hypothalamus, including oxytocin and vasopressin.
  • The thyroid gland produces hormones that regulate metabolism, including thyroxine (T4) and triiodothyronine (T3).
  • The parathyroid glands regulate calcium levels in the blood by secreting parathyroid hormone (PTH).
  • The adrenal glands produce hormones involved in stress response, including cortisol and adrenaline.

    • Immune System

    Immune Cells

  • B cells produce antibodies that target specific antigens.
  • T cells are involved in cell-mediated immunity.
  • Natural Killer (NK) cells are part of the innate immune system and can kill infected or tumor cells without prior sensitization.
  • Phagocytes are immune cells that engulf and digest pathogens and debris.

    • Immune Response

  • The innate immune response is the body's first line of defense against pathogens.
  • The adaptive immune response is a specific response that develops over time and involves memory cells.
  • The humoral immune response involves B cells and the production of antibodies.
  • MHC (Major Histocompatibility Complex) molecules are essential for the recognition of antigens by T cells.

    • Reproductive System

    Male Reproductive System

  • The testes produce sperm and hormones such as testosterone.
  • The epididymis stores and matures sperm.
  • The vas deferens transports sperm from the epididymis to the ejaculatory duct.
  • The seminal vesicles produce a fluid that nourishes and helps transport sperm.
  • The prostate gland produces a fluid that helps protect and activate sperm.
  • The seminiferous tubules are the site of spermatogenesis within the testes.

    • Female Reproductive System

  • The ovaries produce eggs and hormones such as estrogen and progesterone.
  • The fallopian tubes transport eggs from the ovaries to the uterus.
  • The uterus is where a fertilized egg implants and develops during pregnancy.
  • The cervix is the lower part of the uterus that opens into the vagina.
  • The follicular phase is characterized by the maturation of ovarian follicles and increased estrogen production.
  • The ovulation phase is the release of a mature egg from the ovary, triggered by a surge in luteinizing hormone.
  • The luteal phase follows ovulation and involves the formation of the corpus luteum, which secretes progesterone.

    • Fertilization

  • Fertilization occurs when a sperm cell successfully penetrates an egg cell.
  • Cleavage is the series of rapid cell divisions that occur after fertilization. At its final stage, it forms a blastocyst.
  • Implantation is the process by which the blastocyst attaches to the uterine wall.
  • The blastocyst contains an inner cell mass that will develop into the embryo and an outer layer called the trophoblast that will form the placenta.
  • The primitive streak marks the site of gastrulation, where the three germ layers begin to form.
  • The mesoderm is the middle germ layer that gives rise to muscles, bones, and the circulatory system.
  • The endoderm is the innermost germ layer that forms the lining of the digestive and respiratory systems.
  • The ectoderm is the outermost germ layer that develops into the skin and nervous system.
  • During the embryo stage, the notochord forms and provides support for the developing nervous system.
  • Neurulation is the process by which the neural tube forms, giving rise to the central nervous system.

    • Renal System

    Kidneys

  • First, unfiltered blood enters the kidneys through the renal arteries.
  • Within the kidneys, blood is filtered in the nephrons, which are the functional units of the kidney.
  • The renal pelvis collects urine from the nephrons and transports it to the ureters.
  • The renal cortex is the outer layer of the kidney, containing the renal corpuscles and renal tubules.
  • The renal medulla is the inner layer of the kidney, containing the loops of Henle and the collecting ducts.
  • The ureters are tubes that transport urine from the kidneys to the bladder.
  • The bladder is a muscular sac that stores urine until it is ready to be excreted.
  • The urethra is the tube that carries urine from the bladder to the outside of the body.

    • Nephron Function

  • Filtration occurs in the glomerulus, where blood is filtered to form filtrate.
  • Reabsorption occurs in the renal tubule, where water, ions, and nutrients are reabsorbed back into the bloodstream.
  • Secretion occurs in the renal tubule, where waste products and excess ions are secreted into the filtrate.
  • Excretion is the final step, where urine is formed and transported out of the body.