The Nervous System: Overview & Organization

What Is the Nervous System?

Major Divisions

1. Central Nervous System (CNS)

• Components: Brain (including cranial nerve II and the retina) and spinal cord
• Covered by three meninges: dura mater (outer), arachnoid (middle), and pia mater (inner, delicate)
• Within the CNS, neurons with similar functions cluster into nuclei
• Divided into gray matter (neuron cell bodies) and white matter (myelinated axons)
  • In the brain: gray matter is on the cortical surface; white matter is deep
  • In the spinal cord: the opposite - white matter is superficial, gray matter is central
• Myelination is provided by oligodendrocytes
• Axons in the CNS cannot regenerate after injury

2. Peripheral Nervous System (PNS)

• Components: Everything outside the dura mater - sensory receptors, peripheral portions of spinal and cranial nerves (except CN II), and peripheral portions of the autonomic nervous system
• Afferent nerves carry signals toward the CNS (sensory/input)
• Efferent nerves carry signals away from the CNS (motor/output)
• Peripheral ganglia are clusters of nerve cell bodies outside the CNS
• Myelination is provided by Schwann cells
• Axons in the PNS can regenerate after injury

3. Autonomic Nervous System (ANS)

• A functionally distinct division that regulates visceral functions: heart rate, blood pressure, digestion, temperature regulation, and reproductive function
• Anatomically composed of both CNS and PNS components
• Works via reflex arcs: visceral afferent neurons → CNS control centers → visceral motor output
• Further divided into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) subdivisions

Functional Organization

The Brain: Structural Overview

• The surface is folded into gyri (ridges) and sulci (grooves), dramatically increasing surface area
• Each hemisphere is divided into four lobes:
  • Frontal - separated from parietal by the central sulcus (sulcus of Rolando)
  • Parietal - separated from frontal anteriorly; from occipital by the parieto-occipital sulcus medially
  • Temporal - separated from frontal by the lateral sulcus (fissure of Sylvius)
  • Occipital - posterior visual processing lobe
• The two hemispheres are separated by the longitudinal (interhemispheric) fissure
• The insula is a small cortical region concealed by the operculum, visible only when the lateral sulcus is opened

White Matter Pathways

• Corona radiata: broad white matter sheet beneath the cortex
• Internal capsule: V-shaped condensation of white matter containing axons to/from cortex and deep structures
  • Divided into anterior limb, genu, and posterior limb

Cells of the Nervous System

1. Neurons - the primary signaling units; vary greatly in shape but all conduct electrical signals
   • The human brain contains approximately 100 billion (10¹¹) neurons
   • Each neuron may connect with thousands of others
   • The neuron doctrine (formalized by von Waldeyer, 1891; proven by Ramón y Cajal using Golgi's silver staining) established that neurons are discrete individual cells
2. Neuroglial cells (glia) - support cells; slightly outnumber neurons
   • Oligodendrocytes (CNS) and Schwann cells (PNS): form myelin sheaths
   • Astrocytes: structural and metabolic support
   • Microglia: immune defense of the CNS

Regional Specialization and Clinical Significance

• Damage to a specific brain area causes predictable, localizable symptoms
• Neurological diagnosis relies heavily on matching symptoms to anatomy (e.g., stroke localization)
• Any recovery reflects enhancement of existing circuits or recruitment of latent ones - not true regeneration of CNS tissue

The Reproductive System: Overview & Anatomy

MALE REPRODUCTIVE SYSTEM

Testes

1. Spermatogenesis - production of sperm (male gametes)
2. Steroidogenesis - synthesis of androgens, primarily testosterone

• Paired ovoid organs located in the scrotum - outside the body cavity
• The scrotal position is essential because spermatogenesis requires a temperature lower than core body temperature
• Each testis contains approximately 250 lobules, each holding 1-4 tightly coiled seminiferous tubules
• Development is triggered by the SRY gene on the Y chromosome, which produces testis-determining factor (TDF)
• Under testosterone and Müllerian-inhibiting factor (MIF), testes descend from the posterior abdominal wall into the scrotum during fetal development
• Dihydrotestosterone (DHT) drives development of external genitalia and accessory sex glands

Inside the Seminiferous Tubules

Spermatogenesis (Overview)

1. Spermatogonial phase - stem cells (spermatogonia) divide by mitosis, producing primary spermatocytes
2. Spermatocyte phase - primary spermatocytes undergo meiosis I (22 days) → secondary spermatocytes → meiosis II (hours) → haploid spermatids
3. Spermiogenesis - spermatids remodel into mature spermatozoa in association with Sertoli cells

Excurrent Duct System

• Rete testis → Efferent ductules → Epididymis (site of sperm maturation and storage) → Ductus (vas) deferens → Ejaculatory duct → Urethra

Accessory Sex Glands

External Genitalia: Penis

• Corpus cavernosum (x2) - dorsal erectile bodies
• Corpus spongiosum (x1) - ventral, surrounds the urethra, expands distally to form the glans penis

FEMALE REPRODUCTIVE SYSTEM

Internal Organs

Ovaries

1. Oogenesis - production of oocytes (female gametes)
2. Steroidogenesis - secretion of estrogens and progestogens

• Paired, almond-shaped structures (~3 cm × 1.5 cm × 1 cm in nulliparous females)
• Attached to the posterior surface of the broad ligament by the mesovarium
• Superior (tubal) pole attached to pelvic wall by the suspensory ligament of the ovary (carries ovarian vessels and nerves)
• Inferior (uterine) pole attached to uterus by the ovarian ligament (remnant of the gubernaculum)
• Surface becomes progressively scarred after each ovulation; shrinks to ~1/4 its reproductive-life size after menopause
• Covered by germinal epithelium (actually a mesothelium); underlain by a dense tunica albuginea

• Cortex (peripheral): contains ovarian follicles embedded in cellular stroma
• Medulla (central): loose connective tissue with large contorted blood vessels, lymphatics, and nerves

• Estrogens - promote growth and maturation of sex organs; responsible for female secondary sex characteristics; stimulate mammary ductal and stromal growth
• Progestogens - prepare the uterus (especially the endometrium) for pregnancy; if implantation does not occur, the endometrium degenerates and menstruation follows

Uterine (Fallopian) Tubes

• Paired muscular tubes that transport the oocyte from the ovary to the uterus
• Segments from lateral to medial: infundibulum (funnel-shaped opening with fimbriae that sweep the ovum) → ampulla (widest part; usual site of fertilization) → isthmus → uterine portion (enters the uterine wall)
• Attached to broad ligament via the mesosalpinx

Uterus

• Fundus - dome-shaped top, above the entry of the uterine tubes
• Body - main cavity
• Cervix - inferior narrow portion connecting to the vagina via the cervical canal (with internal os above and external os below)

Vagina

• Fibromuscular tube connecting the uterine cervix to the exterior
• Lined by mucosal folds
• Serves as the birth canal and receives the penis during intercourse

External Genitalia (Vulva)

• Mons pubis - fatty eminence over the pubic symphysis
• Labia majora - outer folds of skin
• Labia minora - inner mucosal folds
• Clitoris - erectile structure, homologous to the penis
• Vestibule - space between labia minora containing vaginal and urethral openings
• Hymen - thin mucosal fold partially covering the vaginal opening in virgins

Mammary Glands

Reproductive Lifespan (Female)

The Lymphatic System: Overview & Organization

What Is the Lymphatic System?

1. Fluid homeostasis - drains excess interstitial fluid from tissues and returns it to the bloodstream
2. Immune defense - transports antigens and immune cells, initiating adaptive immune responses

Components at a Glance

1. Lymphatic Vessels

Lymphatic Capillaries

• Blind-ended channels found in the skin, epithelia, and parenchymal organs of almost all vascularized tissues
• Exceptions: superficial skin, muscle endomysium, bone (but these have prelymphatics that eventually drain into lymphatics)
• Lined by overlapping endothelial cells with no tight junctions and no continuous basement membrane - this allows free uptake of interstitial fluid, proteins, and even large particles like bacteria
• Attached to surrounding connective tissue by elastin anchoring filaments that pull them open when tissue swells
• One-way flap valves at the endothelial cell overlaps prevent backflow once fluid enters

How Lymph Flows

• Contraction of perilymphatic smooth muscle cells
• Pressure from musculoskeletal movement
• One-way intraluminal valves throughout all lymphatic vessels

Major Lymphatic Ducts

2. Lymph Composition

• Protein concentration: ~2 g/dL in most tissues
• Liver lymph: up to 6 g/dL (highest protein)
• Intestinal lymph: 3-4 g/dL
• Thoracic duct lymph (mixture of all): typically 3-5 g/dL
• After a fatty meal, thoracic duct lymph may contain up to 1-2% fat - the lymphatics are the primary route for dietary fat absorption (as chylomicrons via lacteals in the intestinal villi)

3. Lymph Nodes

Structure

• Surrounded by a fibrous capsule
• Afferent lymphatics deliver lymph → subcapsular sinus → medullary sinus → efferent lymphatics
• Macrophages in the subcapsular sinus phagocytose and destroy infectious organisms

Internal Zones

Follicles - Key Distinction

• Primary follicles (no germinal center): contain naive, resting B cells
• Secondary follicles (with germinal center): contain activated B cells; sites of B cell proliferation, affinity maturation, and generation of memory B cells and plasma cells
  • Dark zone: proliferating B cells (centroblasts)
  • Light zone: non-proliferating cells (centrocytes) being selected for high-affinity antibody production

Immune Role

• Dendritic cells (DCs) in tissues capture microbial antigens, enter lymphatics, and carry them to nodes
• T and B cells in the node then mount the adaptive immune response
• Each lymph node drains a specific body territory - this is why palpable lymphadenopathy in a region often indicates local infection or malignancy

4. The Spleen

Three Core Functions (Harrison's Principles)

1. Quality control of erythrocytes (red pulp) - removes senescent and defective red blood cells
2. Antibody synthesis (white pulp) - generates humoral immune responses to blood-borne antigens
3. Clearance of opsonized microbes - removes antibody-coated bacteria and blood cells

Structure: Red vs. White Pulp

5. The Thymus (Primary Lymphoid Organ)

• Located in the anterior mediastinum
• Most active in childhood; involutes with age (replaced by fat)
• Site of T lymphocyte maturation (thymocytes → mature T cells)
• Divided into cortex (densely packed thymocytes, darker staining) and medulla (lighter, contains Hassall's corpuscles - concentric whorls of epithelial cells unique to the thymic medulla, function not fully understood)
• The blood-thymus barrier in the cortex shields developing T cells from self-antigens until they are fully tolerant

6. Other Lymphoid Tissues

Mucosa-Associated Lymphoid Tissue (MALT)

• GALT (gut-associated): Peyer's patches in the small intestine, appendix
• BALT (bronchus-associated): airways
• NALT (nasopharynx-associated): tonsils and adenoids (Waldeyer's ring)

The Brain's Lymphatic System - A Recent Discovery

• CSF flows into brain parenchyma along periarterial spaces (glial cell-lined)
• Exits via perivenous routes, carrying interstitial fluid and waste products (including amyloid-beta)
• Drains into meningeal lymphatics → blood

Clinical Relevance