3.2 Human Nervous, Endocrine & Excretory Systems

3.2 Human Nervous, Endocrine & Excretory Systems

Integration of metabolic processes, environmental sensing, and fluid-electrolyte balance depends on the coordinated actions of neural circuits, hormonal messengers, and renal filtration units. A rigorous command of neurophysiology, endocrinology, and renal excretion is essential for success in the Pakistan Army Medical Cadet examination.

---

1. Human Nervous System & Neurophysiology

The human nervous system is divided structurally into the Central Nervous System (CNS) (brain and spinal cord) and the Peripheral Nervous System (PNS) (cranial and spinal nerves).

Nervous System Hierarchy:
├── Central Nervous System (CNS)
│   ├── Brain (Forebrain, Midbrain, Hindbrain)
│   └── Spinal Cord
└── Peripheral Nervous System (PNS)
    ├── Sensory (Afferent) Division
    └── Motor (Efferent) Division
        ├── Somatic Nervous System (Voluntary skeletal muscle control)
        └── Autonomic Nervous System (ANS - Involuntary visceral control)
            ├── Sympathetic Division ("Fight or Flight"; thoracolumbar origin)
            └── Parasympathetic Division ("Rest and Digest"; craniosacral origin)

Neuron Anatomy & Glial Support

• Neuron Structure: Comprises a cell body (soma) containing Nissl granules (rough ER and ribosomes), multiple branched dendrites receiving input, and a single long axon originating at the axon hillock (the trigger zone for action potential generation).
• Myelination: In the PNS, Schwann cells wrap around axons producing a lipid-rich myelin sheath, interrupted at regular intervals by Nodes of Ranvier. In the CNS, myelination is performed by oligodendrocytes.

Resting Membrane Potential & Action Potential Dynamics

• Resting Membrane Potential (RMP): Neurons maintain an RMP of $-70 \text{ mV}$ (inside negative relative to outside). This is established by:
  1. The $Na^+/K^+$ ATPase pump ($3 \text{ } Na^+$ pumped OUT for every $2 \text{ } K^+$ pumped IN).
  2. High intracellular permeability to $K^+$ via ungated leak channels compared to $Na^+$.
  3. Non-diffusible intracellular organic anions (proteins, organic phosphates).

Action Potential Phases:
1. Resting State (-70 mV)
2. Depolarization Threshold (-55 mV) --> Voltage-gated Na+ channels OPEN (Rapid Na+ influx)
3. Peak Action Potential (+30 mV to +40 mV) --> Na+ channels INACTIVATE; Voltage-gated K+ channels OPEN
4. Repolarization --> K+ efflux restores negative potential
5. Hyperpolarization (-80 mV) --> Delayed closure of K+ channels
6. Return to RMP via Na+/K+ pump

• Saltatory Conduction: In myelinated axons, ionic currents cannot pass through the insulating lipid myelin. Depolarization skips between non-insulated Nodes of Ranvier, accelerating impulse transmission speeds up to $120 \text{ m/s}$ while conserving ATP energy.

Synaptic Transmission

1. Action potential reaches the presynaptic terminal, opening voltage-gated $Ca^{2+}$ channels.
2. Influx of $Ca^{2+}$ triggers exocytosis of synaptic vesicles containing neurotransmitters (e.g., acetylcholine, norepinephrine, GABA, glutamate) into the synaptic cleft ($20\text{--}30 \text{ nm}$ wide).
3. Neurotransmitters bind specific ligand-gated ion channels on the postsynaptic membrane:
   • Excitatory Postsynaptic Potential (EPSP): Influx of $Na^+$ depolarizes membrane toward threshold.
   • Inhibitory Postsynaptic Potential (IPSP): Influx of $Cl^-$ or efflux of $K^+$ hyperpolarizes membrane away from threshold.

---

2. Human Endocrine System & Hormonal Control

The endocrine system regulates growth, metabolism, stress responses, and reproductive cycles through chemical messengers transported in blood plasma.

Hormone Classification:
1. Peptide/Protein Hormones: Water-soluble, bind cell-surface receptors (e.g., Insulin, Glucagon, TSH, FSH, LH, ADH).
   --> Act via Second Messengers: cAMP, IP3/DAG, Ca2+.
2. Steroid Hormones: Lipid-soluble, derived from cholesterol, cross cell membranes, bind intracellular/nuclear receptors (e.g., Cortisol, Aldosterone, Testosterone, Estrogen, Progesterone).
   --> Alter Gene Expression directly.
3. Amino Acid Derivatives: Derived from Tyrosine (e.g., Epinephrine, Norepinephrine, Thyroxine T3/T4).

Hypothalamic-Pituitary Axis

The hypothalamus serves as the master neuroendocrine integrator, regulating the pituitary gland (hypophysis).

Key Endocrine Glands & Metabolic Regulation

• Thyroid Gland: Follicular cells produce Thyroxine ($T_4$) and Triiodothyronine ($T_3$), which regulate basal metabolic rate (BMR), protein synthesis, and heat production. Parafollicular (C-cells) produce Calcitonin, which lowers blood $Ca^{2+}$ by inhibiting osteoclasts and promoting bone deposition.
• Parathyroid Glands: Secrete Parathyroid Hormone (PTH), which elevates blood $Ca^{2+}$ by stimulating osteoclasts, increasing renal $Ca^{2+}$ reabsorption, and activating Vitamin D ($1,25-(OH)_2D_3$) for intestinal calcium uptake.
• Adrenal Gland:
  • Adrenal Cortex: Outer zone secretes Aldosterone (mineralocorticoid: $Na^+$ retention, $K^+$ excretion), middle zone secretes Cortisol (glucocorticoid: gluconeogenesis, anti-inflammatory), inner zone secretes adrenal androgens.
  • Adrenal Medulla: Chromaffin cells secrete catecholamines (Epinephrine and Norepinephrine) during acute stress (sympathetic stimulation).
• Pancreas (Islets of Langerhans):
  • Alpha ($\alpha$) Cells: Secrete Glucagon (elevates blood glucose via glycogenolysis and gluconeogenesis).
  • Beta ($\beta$) Cells: Secrete Insulin (lowers blood glucose via cellular uptake, glycogenesis, and lipogenesis).

---

3. Human Excretory System & Renal Physiology

The kidneys filter blood, eliminate nitrogenous metabolic wastes (primarily urea), regulate plasma osmolality ($~300 \text{ mOsm/L}$), and maintain acid-base equilibrium.

Nephron Microanatomy & Filtration Mechanics

The nephron is the functional filtration unit of the kidney (~1 million per kidney). It comprises a vascular component (glomerulus) and a tubular component (Bowman's capsule, PCT, Loop of Henle, DCT, Collecting Duct).

Renal Blood Flow:
Renal Artery -> Afferent Arteriole -> Glomerulus (Capillaries) -> Efferent Arteriole -> Peritubular Capillaries / Vasa Recta -> Renal Vein

• Ultrafiltration at Glomerulus: Driven by Glomerular Hydrostatic Pressure ($~55 \text{ mmHg}$) opposing Capsule Hydrostatic Pressure ($~15 \text{ mmHg}$) and Blood Colloid Osmotic Pressure ($~30 \text{ mmHg}$), yielding a Net Filtration Pressure (NFP) of $+10 \text{ mmHg}$.
• Proximal Convoluted Tubule (PCT): Site of mandatory reabsorption. Reabsorbs 100% of glucose and amino acids (via $Na^+$-cotransporters), ~65% of $Na^+$, $Cl^-$, and $H_2O$, and secretes $H^+$, $NH_4^+$, and organic drugs.

Countercurrent Multiplier System & Henle's Loop

The Loop of Henle establishes an osmotic gradient in the renal medulla ($300 \text{ mOsm/L}$ at cortex to $1200 \text{ mOsm/L}$ at inner medulla):

• Descending Thin Limb: Highly permeable to water (via aquaporin-1), impermeable to solutes. Water leaves tubular fluid, making it hypertonic.
• Ascending Thick Limb: Impermeable to water. Actively transports $Na^+$, $K^+$, and $2Cl^-$ out of lumen into medullary interstitium via the $NKCC_2$ symporter, rendering tubular fluid hypotonic ($~100 \text{ mOsm/L}$).
• Vasa Recta: Loop-shaped capillary network acting as a countercurrent exchanger, preventing washout of the medullary gradient.

Cortical Interstitium (300 mOsm/L)
       |
       v   [ Descending Limb: H2O Out ] ----> [ Ascending Limb: Na+/K+/2Cl- Out (No H2O) ]
       |
       v
Deep Medullary Interstitium (1200 mOsm/L)

Endocrine Regulation of Renal Function

1. Renin-Angiotensin-Aldosterone System (RAAS): Low blood pressure or reduced $Cl^-$ at macula densa triggers juxtaglomerular (JG) cells to release Renin. Renin converts angiotensinogen (from liver) to Angiotensin I, converted to Angiotensin II by ACE in lungs. Angiotensin II causes vasoconstriction and stimulates Aldosterone release.
2. Antidiuretic Hormone (ADH / Vasopressin): Released by posterior pituitary during dehydration (high plasma osmolality). Binds $V_2$ receptors on collecting duct principal cells, inserting Aquaporin-2 water channels to produce concentrated urine.