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.
• Sodium exists in higher concentrations outside the neuron, while potassium is more concentrated inside.
• The sodium-potassium pump actively transports sodium out of the neuron and potassium in to the neuron, maintaining the concentration gradients.
• 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.
• Dopamine: Involved in reward, motivation, and motor control.
• Norepinephrine: Plays a role in attention and responding actions in the brain.
• Serotonin: Regulates mood, appetite, and sleep.
• 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.
• Stratum corneum: The outermost layer, consisting of dead keratinized cells.
• Stratum lucidum: A thin, clear layer found only in thick skin.
• Stratum granulosum: Contains granules that contribute to keratin formation.
• Stratum spinosum: Provides strength and flexibility to the skin.
• Stratum basale: The deepest layer, where new skin cells are generated.
• The dermis is the middle layer, containing connective tissue, blood vessels, and nerves.
• Collagen and elastin fibers provide strength and elasticity.
• This layer houses sweat glands, sebaceous glands, hair follicles, and sensory receptors.
• 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.
• Meissner's corpuscles detect light touch.
• Pacinian corpuscles detect deep pressure and vibration.
• Merkel cells detect light touch and pressure.
• Ruffini endings detect skin stretch and sustained pressure.
• Temperature receptors (thermoreceptors) detect changes in temperature.
• Pain receptors (nociceptors) respond to harmful stimuli.
• Transpiration is the process of water vapor loss from the skin. This is regulated by sweat glands.

Musculoskeletal System

Muscle Types

• Skeletal muscle is responsible for voluntary movements.
• Cardiac muscle makes up the heart and is involuntary.
• Smooth muscle is found in hollow organs and is involuntary.

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.
• Synovial joints are freely movable joints.
• Cartilaginous joints allow limited movement.
• Fibrous joints are immovable.
• Syndesmoses are a type of fibrous joint connected by ligaments.
• Gomphoses are another type of fibrous joint, connecting teeth to their sockets.
• Synchondroses are a type of cartilaginous joint connected by hyaline cartilage.

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.
• Insulin regulates blood sugar levels by promoting glucose uptake into cells.
• Glucagon raises blood sugar levels by stimulating the liver to release glucose.
• Protein hormones are larger molecules made up of chains of amino acids and include growth hormone and prolactin.
• Growth hormone stimulates growth and cell reproduction.
• Prolactin stimulates milk production in the mammary glands.
• Steroid hormones are derived from cholesterol.
• Cortisol is involved in stress response and metabolism regulation.
• Testosterone is responsible for the development of male secondary sexual characteristics.
• Amine hormones are derived from amino acids.
• Thyroid hormones regulate metabolism and growth.
• Adrenaline (epinephrine) is involved in the fight-or-flight response.

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.
• Helper T cells (CD4+ cells) assist in activating B cells and other immune cells.
• Cytotoxic T cells (CD8+ cells) directly kill infected or cancerous cells.
• Regulatory T cells (Tregs) help maintain immune tolerance and prevent autoimmune responses.
• 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.
• Macrophages are large phagocytic cells that play a key role in the immune response.
• Neutrophils are the most abundant type of white blood cell and are the first responders to infection.
• Dendritic cells are antigen-presenting cells that activate T cells by presenting antigens.

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 corpuscle is the initial filtering component of the nephron.
• The renal tubule is responsible for reabsorbing water and solutes from the filtrate.
• The loop of Henle is involved in concentrating urine by reabsorbing water and salts.
• The glomerulus is a network of capillaries that filters blood in the renal corpuscle.
• The Bowman’s capsule surrounds the glomerulus and collects the filtrate.
• Fenestrated capillaries in the glomerulus allow for the passage of small molecules while retaining larger ones.
• Podocytes are specialized cells in the Bowman’s capsule that wrap around the glomerulus and help filter blood.
• 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.