Medical Terminology and Body Systems Comprehensive Review

Foundations of Medical Terminology

Kodie Warn defines medical terminology as the specialized language utilized within the medical profession. The structure of this language relies on specific components: root words, prefixing elements, suffixes, and combining vowels. Root words and their meanings include: Abin/o (white), Bi/o (life), Cry/o (cold), cutane/o (skin), Cyan/o (blue), Derm/o or dermat/o (skin), Diaphor/O (profuse sweating), Electr/o (electricity), ichthy/o (scaly), Kerat/o (hard), Lip/o (fat), Melan/O (black), Myc/o (fungus), Necr/o (death), Pil/o (hair), Py/o (pus), Rhytid/o (wrinkle), Seb/o (oil), and Ungu/o (nail).

A suffix is a component that attaches specifically at the end of a root word to modify its meaning. Medical suffixes include: -algia (pain, e.g., gastralgia/stomach pain), -cele (hernia or protrusion, e.g., cystocele/protrusion of the bladder), -cise (cut, e.g., excise/to cut out), -cyte (cell, e.g., erythrocyte/red cell), -dynia (pain, e.g., cardiodynia/heart pain), -ectasis (dilation, e.g., bronchiectasis/dilated bronchi), -gen (that which produces, e.g., mutagen/that which produces mutations), -genesis (produces or generates, e.g., osteogenesis), and -genic (produces or produced by, e.g., carcinogenic/producing cancer). Other suffixes include -ia (state or condition, e.g., hemiplegia/condition of being half paralyzed), -iasis (abnormal condition, e.g., lithiasis/abnormal condition of stones), -ism (state of, e.g., hypothyroidism/state of low thyroid), -itis (inflammation, e.g., cellulitis/inflammation of cells), -logist (one who studies, e.g., cardiologist/one who studies the heart), and -logy (study of, e.g., cardiology/study of the heart).

Further suffixes include -lysis (destruction, e.g., osteolysis/bone destruction), -malacia (abnormal softening, e.g., chondromalacia/abnormal cartilage softening), -megaly (enlargement or large, e.g., cardiomegaly/enlarged heart), -oma (tumor or mass, e.g., carcinoma/cancerous tumor), -osis (abnormal condition, e.g., cyanosis/abnormal condition of being blue), -pathy (disease, e.g., myopathy/muscle disease), -plasia (development or growth, e.g., dysplasia/abnormal development), -plasm (formation or development, e.g., neoplasm/new formation), -ptosis (drooping, e.g., proctoptosis/drooping rectum), -rrhage (excessive or abnormal flow, e.g., hemorrhage/excessive bleeding), -rrhea (discharge or flow, e.g., rhinorrhea/discharge from the nose), -rrhexis (rupture, e.g., hysterorrhexis/ruptured uterus), -sclerosis (hardening, e.g., arteriosclerosis/hardening of an artery), -stenosis (narrowing, e.g., angiostenosis/narrowing of a vessel), -therapy (treatment, e.g., chemotherapy/treatment with chemicals), and -trophy (nourishment or development, e.g., hypertrophy/excessive development).

Combining vowels are the "in between" sounds that help connect and pronounce medical terms. Prefixes define or make the root word more specific by expanding on location, frequency, number, color, or negation. Examples include: A- (without, away from, e.g., aphasia/without speech), an- (without, e.g., anoxia/without oxygen), ante- (before, in front of, e.g., antepartum/before birth), anti- (against, e.g., antibiotic/against life), auto- (self, e.g., autograft/a graft from one's own body), and Brady- (slow, e.g., bradycardia/slow heart beat). Additional prefixes are: dys- (painful, difficult, e.g., dyspnea/difficulty breathing), endo- (within, inner, e.g., endoscope/instrument to view within), epi- (upon, over, e.g., epigastric/upon or over the stomach), Eu- (normal, good, e.g., eupnea/normal breathing), hetero- (different, e.g., heterograft/a graft from another person's body), homo- (same, e.g., homozygous/having two identical genes), hyper- (over, above, e.g., hypertrophy/over development), hypo- (under, below, e.g., hypoglossal/under the tongue), infra- (under, beneath, below, e.g., infraorbital/below or under the eye socket).

Prefixes also include: inter- (among, between, e.g., intervertebral/between the vertebrae), intra- (within, inside, e.g., intravenous/inside or within a vein), macro- (large, e.g., macrocephalic/having a large head), Micro- (small, e.g., microcephalic/having a small head), Neo- (new, e.g., neonate/newborn), pan- (all, e.g., pancarditis/inflammation of all the heart), para- (beside, beyond, near, e.g., paranasal/near or alongside the nose), per- (through, e.g., percutaneous/through the skin), peri- (around, e.g., pericardial/around the heart), post- (after, e.g., postpartum/after birth), pre- (before, in front of, e.g., prefrontal/in front of the frontal bone), pseudo- (false, e.g., pseudocyesis/false pregnancy), retro- (backwards, behind, e.g., retrograde/movement in backward direction), sub- (below, under, e.g., subcutaneous/under, below the skin), super- (above, excess, e.g., supernumerary/above the normal number), Supra- (above, e.g., suprapubic/above the pubic bone), tachy- (rapid, fast, e.g., tachycardia/fast heartbeat), trans- (through, across, e.g., transurethral/across the urethra), and ultra- (beyond, excess, e.g., ultrasound/high-frequency sound waves).

Rules for Creating Plural Forms

Specific rules exist for converting singular medical terms to plural forms based on word endings. If a word ends in -a, keep the -a and add -e (e.g., singular vertebra becomes plural vertebrae). If the word ends in -ax, drop the -x and add -ces (e.g., singular thorax). For words ending in -ex, drop -ex and add -ices (e.g., singular apex becomes plural apices). If the word ends in -ix, drop -x and add -ces (e.g., singular appendix). Words ending in -ma require keeping the -ma and adding -ta (e.g., singular sarcoma becomes plural sarcomata). For -is endings, drop -is and add -es (e.g., singular metastasis becomes plural metastases). For -on endings, drop -on and add -a (e.g., singular spermatozoon becomes plural spermatozoa). For -us, drop -us and add -i (e.g., singular alveolus becomes plural alveoli). For -um, drop -um and add -a (e.g., singular ovum becomes plural ova). If the word ends in -y, drop -y and add -ies (e.g., singular biopsy becomes plural biopsies). Finally, if the word ends in -x, drop -x and add -ges (e.g., singular phalanx becomes plural phalanges).

Anatomical Position, Terminology, and Movements

Anatomical position is defined as standing erect, facing forward, with the palms of the hands facing forward and the thumbs pointed outward. Key directional terms include: anterior (front surface), posterior (back side), superior (above another structure), inferior (below another structure), medial (pertaining to the middle), and lateral (pertaining to the side). Specialized terms include ipsilateral (same side of the body), contralateral (opposite side of the body), proximal (near the point of attachment to the trunk), and distal (farther away from the attachment). Organ-specific terms include the apex (tip or summit) and base (bottom). Deep refers to positions away from the surface, while superficial indicates on the surface. Lying on the belly is prone, while lying on the back is supine.

Body movements are categorized based on the plane in which they occur. Flexion is bending the joint, occurring in the sagittal plane, while extension is straightening the joint, also in the sagittal plane. Abduction involves moving away from the median plane, occurring in the coronal plane, while adduction is moving toward the medial plane, also in the coronal plane. Lateral flexion is the movement of the spine in the coronal plane. Depression is a downward movement, while elevation is an upward movement. Forearm movements include supination (palm facing upwards) and pronation (palm facing backwards). Toward the front is protraction (anteriorly), and toward the back is retraction (posteriorly).

Body Planes, Regions, and Cavities

Body planes divide the body into sections. The frontal (or coronal) plane divides the body vertically into anterior and posterior portions. The sagittal (or lateral) plane divides the body into left and right sides. The transverse plane (or cross section) creates upper and lower sections. Specific regions include: abdominal (center anterior trunk), brachial (arm), cephalic (head), cervical (neck), crural (lower extremity), dorsum (the back), gluteal (buttocks), pelvic (lower anterior trunk), pubic (genital), thoracic (chest), trunk (torso), and vertebral (spinal column).

A cavity is an empty space. There are two major cavities: dorsal and ventral. The dorsal cavity is subdivided into the cranial cavity (formed by the skull to protect the brain) and the spinal cavity (vertebral canal protecting the spinal cord). The ventral cavity contains the thoracic and abdominopelvic cavities and is lined by a serous membrane to reduce friction, secreting serous fluid. In the thoracic cavity, the outer membrane layer is the parietal pleura, and the inner layer is the visceral pleura.

The thoracic cavity contains the pleural cavities (each containing a lung), the pericardial cavity (containing the heart), and the mediastinum (containing the esophagus, trachea, thymus gland, aorta, and inferior vena cava). The abdominopelvic cavity is divided into the abdominal and pelvic cavities, lined by the peritoneum (parietal peritoneum on the outer layer and visceral peritoneum on the inner layer). The abdominal cavity contains the stomach, spleen, liver, gallbladder, pancreas, and most of the small intestine and colon. The pelvic cavity holds the urinary bladder, ureters, urethra, lower large intestine, part of the small intestine, and reproductive organs.

Abdominal clinical divisions consist of four quadrants: Right Upper Quadrant (RUQ) containing the liver, gallbladder, pancreas portion, right kidney, and colon; Left Upper Quadrant (LUQ) with the liver portion, spleen, stomach, majority of the pancreas, left kidney, and colon; Right Lower Quadrant (RLQ) containing half of the small intestines/colon, right ovary/fallopian tube, appendix, and right ureter; and Left Lower Quadrant (LLQ) containing half of the small intestines/colon, left ovary/fallopian tube, and left ureter. Anatomical divisions divide the abdomen into nine regions: Right Hypochondriac, Epigastric, Left Hypochondriac, Right Lumbar, Umbilical, Left Lumbar, Hypogastric, Right Iliac, and Left Iliac.

The Integumentary System

The biological hierarchy starts with cells, which form tissues; tissues create organs; organs form organ systems; and organ systems perform all bodily functions. The integumentary system includes the skin, hair, nails, sweat glands, sebaceous glands, and nerve receptors. Dermatology is the study of skin. The skin has three layers: epidermis, dermis, and subcutaneous. The skin contains veins, arteries, nerves, touch receptors, and hair follicles. The subcutaneous layer contains a large fat layer. Functions of the skin include waterproofing, protection from pathogens, sensing touch, synthesizing Vitamin D, screening radiation, secreting oil, and regulating body temperature and water content. Skin color is determined by melanin and carotene, influenced by hemoglobin and emotions. Hair and nails grow through specific stages and processes.

Integumentary conditions include sebaceous gland issues like Acne vulgaris (chronic inflammation of skin pustules caused by sebum overproduction), Seborrhea (overactive sebaceous glands), and Seborrheic keratosis (benign waxy growths in the elderly). Nail conditions include onychia (inflammation of the nail bed where the nail falls off), onychocryptosis (ingrown nail), and onychomycosis (fungal infection). Skin infections include mycosis (fungal), tinea (ringworm), impetigo (contagious bacterial pustules), and gangrene (tissue death from loss of circulation). Surface lesions include macules ($< 1\,cm$), nodules ($> 0.5\,cm$), weals (smooth elevated swollen areas), and exudate (pus). Psoriasis presents as red papules with silvery scales. Pigmentation issues include albinism (absence of pigment), vitiligo (destruction of melanocytes), and chloasma (pregnancy brown spots). Hair conditions include alopecia (baldness), folliculitis (follicle inflammation), and hirsutism (excessive female facial hair). Bleeding under the skin involves contusions (discoloration without break), ecchymosis (bruising), hematomas (swelling of clotted blood), and petechia (hemorrhages $< 2\,mm$). Dermatitis types include contact and eczema (atopic dermatitis). Chronic pressure can cause decubitus (pressure sores).

The Muscular System

Muscles are formed by muscle fibers (myofibrils) bound by connective tissue called fascia. Fascia forms tendons to connect muscles to bones. The hierarchy is: myofibrils protected by endomysium -> fascicles protected by perimysium -> muscle protected by epimysium. Types of muscle include skeletal, cardiac (myocardial), and smooth. Muscle attachment points include the origin (immovable end closest to the body) and the insertion (movable end). Muscles are named by origin/insertion, action, location, fiber direction, division number, or size/shape. Functions include movement, posture, heat generation, and moving food/fluids. The system works with the nervous system for contraction and relaxation.

Muscle disorders include fasciitis, fibromyalgia (chronic fatigue and muscle/joint pain), and tendonitis. Muscle tone conditions include atonic (lack of tone), dystonia (abnormal tone), and hypotonia (diminished tone). Myasthenia gravis is an autoimmune disease affecting the neuromuscular junction, causing double vision, difficulty swallowing, and drooping eyelids with no cure. Carpal tunnel syndrome results from overused tendons compressing the nerve in the wrist. Sports injuries include sprains (ligament), strains (muscle/tendon), and shin splints (muscle tearing from the tibia). Movement disorders include ataxia (lack of coordination), contracture (permanent tightening of tissue), and muscle spasms/tics. Muscular dystrophy (MD) includes Duchenne (DMD), which affects boys aged $3$ to $5$ and progresses to inability to walk by age $12$. Paralysis includes myoparesis (slight), hemiplegia (one side total), paraplegia (lower body), and quadriplegia (all extremities). Compartment syndrome results from pressure within muscle groups.

The Skeletal System

The skeletal system comprises $206$ bones and is divided into the axial system ($80$ bones of the skull, spine, hyoid, ossicles, and rib cage) and the appendicular system ($126$ bones of the appendages). The spinal column protects nerves and has five divisions: Cervical ($C1-C7$), Thoracic ($T1-T12$), Lumbar ($L1-L5$), Sacrum ($5$ fused vertebrae), and Coccyx ($3-5$ fused vertebrae). There are four types of bones: long, flat, short, and irregular. Long bones consist of spongy bone, compact bone, and a medullary cavity. Physiology involves supporting organs, storing calcium, and bone marrow function. Red bone marrow forms blood cells, while yellow bone marrow is fat.

Joints (articulations) include fibrous (inflexible sutures of the skull), cartilaginous (slight movement, e.g., ribs to sternum), and synovial (wide range of motion with synovial fluid and capsule). Ligaments connect bone to bone, and a bursa acts as a cushion. Skeletal pathologies include ankylosis (loss of mobility), bursitis, chondromalacia (softening of cartilage), and dislocations (luxation). Arthritis types include osteoarthritis (degenerative), gout (uric acid crystals), and rheumatoid arthritis (autoimmune). Curvature disorders include scoliosis (lateral), lordosis (forward/swayback), and kyphosis (outward/humpback). Bone conditions include rickets (Vitamin D deficiency), osteomyelitis (infection), and osteoporosis (loss of density). Fractures are classified as closed (simple), open (compound), comminuted (crushed), greenstick (partial), or spiral. Bone cancers include osteosarcoma and myeloma. Treatment may involve allogenic bone marrow transplants (from a compatible donor) or autologous (using the patient's own cells).

The Endocrine System

The endocrine system uses glands to secrete hormones, which are chemical messengers. The pineal gland (brain center) secretes melatonin for circadian cycles. The hypothalamus controls the pituitary gland. The pituitary (pea-sized) has two lobes. The anterior lobe secretes $ACTH$, $FSH$ (estrogen/sperm growth), $GH$ (growth), $LTH$ (prolactin/milk), $LH$ (ovulation/testosterone), $MSH$ (melanin), and $TSH$. The posterior lobe stores and releases $ADH$ (blood pressure/water control) and oxytocin ($OXT$ for uterine contractions). The thyroid gland regulates metabolism via thyroxine and triiodothyronine and secretes calcitonin. The parathyroid glands (size of a grain of rice) secrete $PTH$ to balance calcium. The thymus gland secretes thymosin for the immune system before puberty.

The pancreas controls blood glucose through glucagon (raises sugar) and insulin (lets glucose enter cells/stores glycogen). Adrenal glands have a cortex (secreting androgens, corticosteroids, aldosterone, and cortisol) and a medulla (secreting epinephrine/adrenalin and norepinephrine for fight-or-flight). Gonads include ovaries (producing eggs/ovum and progesterone/estrogen) and testicles (producing sperm and testosterone). The placenta provides nutrients to the fetus and removes waste. Endocrine disorders include acromegaly (large hands/feet), giantism (pre-puberty growth), Graves' disease (hyperthyroidism/goiter), Hashimoto's (autoimmune hypothyroidism), and diabetes. Type $1$ diabetes is autoimmune insulin deficiency; Type $2$ is insulin resistance related to obesity. Addison's disease results from low cortisol, while Cushing's syndrome involves prolonged high cortisol exposure.

The Cardiovascular System

The vascular system consists of arteries (carrying blood away, largest being the aorta), veins (carrying blood toward the heart, including vena cavae), arterioles, venules, and capillaries. The heart is a hollow organ in the pericardium with three wall layers: epicardium, myocardium, and endocardium. It has four chambers (two atria, two ventricles) and four valves. The electrical system includes the $SA$ node, $AV$ node, bundle of His, and purkinje fibers. Blood is a fluid tissue: $55\%$ plasma ($91\%$ water, $9\%$ proteins/fibrinogen) and $45\%$ formed elements (erythrocytes/RBC, leukocytes/WBC, and thrombocytes/platelets). Hemoglobin in RBCs transports oxygen.

Circulation includes pulmonary (between heart and lungs) and systemic (between heart and the rest of the body). Blood pressure ($BP$) measures systolic (contraction/highest pressure) and diastolic (relaxation/lowest pressure). Normal $BP$ is $< 120/80\,mm\,Hg$. Disorders include atherosclerosis (plaque buildup), coronary artery disease ($CAD$), and myocardial infarction (heart attack). Heart failure can be left-sided (pulmonary edema) or right-sided (body edema). Arrhythmias include asystole (flat line), bradycardia (slow), and tachycardia (fast). Blood vessel conditions include aneurysms (ballooning due to weak spots), phlebitis (vein inflammation), and thrombosis (clots). Blood disorders include anemia (low RBC), leukemia (blood cancer), and hemochromatosis (too much iron). Life-threatening hypertension levels reach $180/120\,mm\,Hg$ or higher.

The Respiratory System

The respiratory system involves gas exchange (respiration) through inhalation and exhalation controlled by the diaphragm and intercostal muscles. Anatomy includes the nostrils, nasal cavities, pharynx, larynx, trachea, epiglottis, bronchi, bronchioles, and alveoli ($O_2/CO_2$ transfer point). The right lung has three lobes; the left has two. Chronic obstructive pulmonary disease ($COPD$) includes chronic bronchitis and emphysema (destruction of alveoli). Asthma involves bronchial spasms. Upper respiratory disorders include the common cold (often human rhinovirus), croup (barking cough), and diphtheria. Pertussis (whooping cough) is immunized against via $DPT$.

Breathing disorders include eupnea (normal), bradypnea ($\le 10\,breaths/min$), and tachypnea ($\ge 20\,breaths/min$). Serious conditions include anoxia (absence of oxygen causing brain damage in $4$ to $6$ minutes), asphyxia (suffocation), and cyanosis (bluish skin). Lung disorders include pleurisy, hemothorax (blood in pleural cavity), and pulmonary embolism. Pneumoconiosis and asbestosis are environmental lung diseases. Sleep apnea includes obstructive sleep apnea ($OSA$). Sudden Infant Death Syndrome ($SIDS$) is the unexplained death of a healthy infant aged $2$ to $6$ months; back-sleeping is recommended.

The Lymphatic and Immune Systems

The lymphatic system filters lymph fluid through nodes ($700$ in total, including cervical, axillary, and inguinal groups). Lymphatic circulation moves one-way toward the neck to return to the venous system. Lymph consists of interstitial fluid, proteins, and debris. Spleen functions include filtering microorganisms and destroying old RBCs. Lymphocytes (RBCs from bone marrow) include Natural Killer ($NK$) cells, B cells (produce antibodies), and T cells (cell-mediated immunity). The immune system's first lines of defense are the skin, respiratory tract (mucus/hairs), and digestive system (acid). Antigens are foreign substances; antibodies are disease-fighting proteins. Immunoglobulins ($IgG, IgA, IgM, IgD, IgE$) participate in the antigen-antibody reaction.

Disorders include splenomegaly, lymphedema, and autoimmune diseases (e.g., Crohn's, Graves'). Immunodeficiency includes $SCIDS$. Human Immunodeficiency Virus ($HIV$) kills T cells; Acquired Immunodeficiency Syndrome ($AIDS$) is its advanced fatal stage. Infections include bacterial (cellulitis, impetigo, STDs like syphilis/chlamydia), viral (hepatitis, Covid-19, influenza), and fungal (athlete's foot, ringworm). Tonsillitis is the inflammation of the tonsils (adenoids, palatine, and lingual).

The Gastrointestinal System

The gastrointestinal ($GI$) tract includes the upper $GI$ (oral cavity, pharynx, esophagus, stomach) and lower $GI$ (small intestine: duodenum, jejunum, ileum; large intestine: cecum, colon, rectum, anus). Accessory organs are the liver (removes toxins, secretes bile), gallbladder (stores bile), and pancreas (secretes juices). Digestion breaks food into nutrients; metabolism includes anabolism (building) and catabolism (breaking down/energy release). Peristalsis moves food forward. Food in the stomach becomes chyme. The large intestine reabsorbs water, forming feces processed by gut microflora. Defecation is the evacuation of waste.

GI disorders include aphthous ulcers (canker sores), oral thrush (fungal), and xerostomia (dry mouth). Dental issues include caries (cavities), plaque, and bruxism (grinding). Esophageal problems include $GERD$ (reflux) and dysphagia. Stomach issues include gastritis and peptic ulcers. Nutrition disorders include anorexia, bulimia, cachexia (physical wasting), and obesity (diagnosed when individual is $> 20\%$ over weight standards). Vomiting terms include emesis, hematemesis (blood), and hyperemesis. Intestinal conditions include Celiac disease (gluten reaction), diverticulosis, and Irritable Bowel Syndrome ($IBS$). Liver disorders include jaundice (high bilirubin), hepatitis, and cirrhosis. Gallbladder issues include cholelithiasis (gallstones).

The Urinary System

The urinary system consists of two kidneys, two ureters ($10$ to $12\,inches$ long), the urinary bladder, and the urethra (female: $1.5\,inches$; male: $8\,inches$). Kidneys filter approximately $200\,quarts$ of blood daily to produce $2\,quarts$ of urine. Each kidney has a cortex containing $1\,million$ nephrons and a medulla. Filtration occurs in the glomerulus within Bowman's capsule. The four urinary processes are Filtration (keeping proteins in blood), Reabsorption (nutrients/water back to blood), Secretion (waste ions to tubules), and Excretion. Urine is $95\%$ water.

Pathologies include nephropathy, renal failure (uremia), and acute renal failure (sudden blood volume drop). Chronic kidney disease is progressive; End-Stage Renal Disease ($ESRD$) requires dialysis or transplant. Nephrotic syndrome involves high protein loss in urine (hyperproteinuria) and low protein in blood (hypoproteinemia), often causing edema. Kidney stones (calculi) cause renal colic. Bladder issues include cystocele (prolapsed bladder) and interstitial cystitis. Neurogenic bladder results from nerve pathway interference. Urination disorders include anuria (absence), dysuria (painful), enuresis (involuntary/bedwetting), and nocturia (frequent night urination). Scanty urination is oliguria.

The Reproductive System

Male anatomy includes the scrotum, testicles (sperm produced in seminiferous tubules), epididymis (storage/maturation), vas deferens, seminal vesicles (nourish sperm), prostate gland (alkaline fluid for motility), and bulbourethral glands (pre-ejaculate). Female anatomy includes external genitalia (mons pubis, vulva, clitoris, labia) and internal genitalia (ovaries producing ova, fallopian tubes with fimbriae, uterus with layers: perimetrium, myometrium, endometrium, and vagina/cervix). Mammary glands in breasts produce milk through lactiferous ducts.

Physiology involves menarche (first cycle) and menopause (termination). Menstruation expels the secretory endometrial lining if no fertilization occurs. Pregnancy stages include ovulation (day $14$), conception (sperm penetrating ovum to form a zygote), embryo (weeks $1-8$), and fetus (from week $9$). Gestation lasts $280\,days$ ($40\,weeks$) in three $13$-week trimesters. Labor involves three stages: $1$ (dilation/effacement), $2$ (expulsion of child/crowning), and $3$ (expulsion of placenta/afterbirth). Disorders include erectile dysfunction, cryptorchidism (undescended testicle), and STDs (HPV, Syphilis). Female issues include endometriosis, polycystic ovary syndrome ($PCOS$), and preeclampsia (high blood pressure in pregnancy).

The Nervous System

The nervous system is comprised of neurons (nerve cells). A nerve is a bundle of neurons; a tract is a bundle in the CNS (ascending toward brain, descending away). Structure: endoneurium -> perineurium -> epineurium. billons of neurons rely on electrochemical processes. Neurons consist of a cell body, dendrites (receive impulses), and an axon (conducts away). Many axons have a myelin sheath with nodes of Ranvier. The Central Nervous System ($CNS$) includes the brain and spinal cord, protected by meninges: dura mater (outer), arachnoid membrane (middle), and pia mater (inner). Cerebrospinal fluid ($CSF$) cushions and nourishes. The brain includes the cerebrum (largest part, motor/sensory/memory), cerebral cortex (gray matter with gyri and sulci), and four lobes: Frontal (motor/behavior), Parietal (tongue/skin/muscles), Occipital (eyesight), and Temporal (hearing/smell).

The Peripheral Nervous System ($PNS$) has $12\,pairs$ of cranial nerves and $31\,pairs$ of spinal nerves. It includes autonomic (sympathetic for stress, parasympathetic for rest) and somatic (voluntary) fibers. Neurotransmitters include acetylcholine (muscle action), dopamine, epinephrine (adrenalin), norepinephrine (alertness), and serotonin (hunger/sleep). Nervous disorders include cephalalgia (headache), meningitis, Alzheimer's, Parkinson's (tremors/shuffling), and Amyotrophic Lateral Sclerosis ($ALS$). Brain injuries include concussions and hematomas. A stroke ($CVA$) can be ischemic (blocked) or hemorrhagic (bleed). Levels of consciousness include lethargy, stupor, syncope (fainting), coma, and brain dead. Seizures include Tonic-clonic (Grand mal) and Absence (petit mal).

Special Senses: Anatomy and Physiology

The eye functions as a receptor organ for sight. External structures (adnexa) include the orbit, six eye muscles, eyelids (contain cilia), conjunctiva, and the lacrimal apparatus (tears). The eyeball has three layers: sclera (white), choroid (middle), and retina (inner layer with rods for black-and-white and cones for color). The macula and fovea centralis provide sharp/color vision. The eye is divided into anterior (aqueous humor) and posterior (vitreous humor) segments. Sound is captured by the pinna (auricle), moved through the external auditory canal to the tympanic membrane (eardrum), through auditory ossicles (malleus, incus, stapes), to the cochlea and organ of Corti. Semicircular canals maintain equilibrium. The tongue uses taste buds (gustatory cell receptors) for five tastes: sweet, sour, bitter, salty, and umami. Saliva is required for chemicals (tastants) to initiate taste (gustation).

Eye disorders include ptosis (drooping), hordeolum (stye), conjunctivitis (pink eye), and glaucoma (high intraocular pressure). Macular degeneration results in loss of central vision, with dry ($90\%$ of cases) and wet (leaking blood) forms. Refractive disorders include myopia (nearsighted) and hyperopia (farsighted). Hearing disorders include impacted cerumen (earwax), otitis media (middle ear inflammation), and tinnitus (ringing). Vertigo involves dizziness; Ménière's disease is a chronic inner ear fluid increase. Hearing loss can be conductive (blockage) or sensorineural (nerve damage), including noise-induced hearing loss ($NIHL$).