The nervous system is divided into: Central Nervous System (CNS): brain and spinal cord — processes and integrates information. Peripheral Nervous System (PNS): all nervous tissue outside CNS — cranial nerves (12 pairs from brain), spinal nerves (31 pairs from spinal cord), and autonomic ganglia. The PNS is further divided into Somatic NS (voluntary — controls skeletal muscles, carries sensory information from skin and muscles) and Autonomic NS (involuntary — controls internal organs, glands, smooth muscle; divided into sympathetic and parasympathetic divisions). The CNS is protected by bone (skull for brain, vertebral column for spinal cord), meninges (three connective tissue membranes: dura mater, arachnoid mater, pia mater), and cerebrospinal fluid (CSF) which cushions and nourishes the CNS.

The human brain weighs approximately 1.3-1.4 kg and contains approximately 86 billion neurons (with roughly equal numbers of glial cells). Major regions: Cerebrum: largest brain region, divided into two hemispheres connected by the corpus callosum; outer layer = cerebral cortex (grey matter, 2-4 mm thick, with gyri/sulci increasing surface area); controls voluntary movement, sensation, language, memory, reasoning, personality. Diencephalon: thalamus (sensory relay), hypothalamus (homeostasis, hunger, thirst, temperature, emotional responses, controls pituitary gland), epithalamus (pineal gland — melatonin, regulates circadian rhythms). Brainstem (midbrain + pons + medulla oblongata): controls vital functions, relays signals between brain and spinal cord, houses cranial nerve nuclei. Cerebellum: coordination, balance, fine motor control.

The pericardium is a critical cardiovascular structure that both protects the heart and facilitates its function. The fibrous pericardium (outermost layer) is a tough, non-distensible sac that prevents sudden overdistension of the heart, anchors the heart in the mediastinum, and protects it from infection spreading from adjacent structures. The inner serous pericardium has two layers: the parietal layer (lining the inside of the fibrous pericardium) and the visceral layer (or epicardium, tightly adherent to the heart surface and forming its outer layer). Between these two serous layers is the pericardial cavity, which normally contains 15-50 mL of clear pericardial fluid that lubricates the heart surface during its constant contractions. Clinically important conditions include: Pericarditis (inflammation, causing chest pain that worsens with lying down and improves with sitting forward), Pericardial effusion (excessive fluid in pericardial sac, which if rapid can cause cardiac tamponade — emergency compression of the heart preventing adequate filling), and Constrictive pericarditis (fibrosis and calcification of pericardium restricting heart filling).

Just as the pericardium protects the heart, the CNS has its own protective membranes called meninges (singular: meninx). Three meningeal layers surround both the brain and spinal cord: Dura mater (outermost): tough, fibrous, double layer around the brain (outer periosteal layer fused to skull inner surface; inner meningeal layer). Forms important reflections: falx cerebri (between cerebral hemispheres), tentorium cerebelli (between cerebrum and cerebellum), falx cerebelli (between cerebellar hemispheres). Contains venous sinuses (superior sagittal, transverse, etc.). Arachnoid mater (middle): thin, delicate, spiderweb-like; subarachnoid space below it contains CSF and major cerebral arteries. Pia mater (innermost): thin, closely adheres to brain and spinal cord surface, follows all gyri and sulci. Meningitis is infection/inflammation of the meninges, a medical emergency.