The Brain and Nervous System: Anatomy and Function

The human nervous system is the most structurally complex biological system yet characterized by science, comprising an estimated 86 billion neurons (Azevedo et al., 2009, Journal of Comparative Neurology) alongside trillions of supporting glial cells. This page covers the anatomical divisions of the brain and nervous system, the functional mechanisms that govern signaling and integration, the clinical scenarios in which anatomy becomes diagnostically relevant, and the boundaries that distinguish normal variation from pathological change. Understanding this architecture is foundational to every topic addressed across neurological conditions, diagnostics, and treatments.

Definition and Scope

The nervous system is divided into two primary structural partitions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS encompasses all neural tissue outside that boundary — cranial nerves, spinal nerves, autonomic ganglia, and the enteric plexuses of the gastrointestinal tract.

The brain itself is further classified into three embryologically distinct regions:

  1. Forebrain (prosencephalon) — subdivided into the telencephalon (cerebral cortex, basal ganglia, limbic structures) and the diencephalon (thalamus, hypothalamus, epithalamus).
  2. Midbrain (mesencephalon) — contains the tectum, tegmentum, and the cerebral peduncles; serves as a relay for auditory and visual reflexes.
  3. Hindbrain (rhombencephalon) — comprises the cerebellum, pons, and medulla oblongata, governing balance, coordination, and autonomic vital functions.

The cerebral cortex is organized into four lobes per hemisphere: frontal, parietal, temporal, and occipital. The National Institute of Neurological Disorders and Stroke (NINDS), housed within the National Institutes of Health, maintains reference materials on brain anatomy that form the basis of clinical education in US neurology training programs (NINDS Brain Basics).

The PNS is subdivided into the somatic nervous system (voluntary motor control and sensory input) and the autonomic nervous system (ANS). The ANS branches further into the sympathetic division (arousal and stress response) and the parasympathetic division (rest and digestive functions). A third autonomic branch, the enteric nervous system, operates semi-independently within the gut wall and contains approximately 500 million neurons (Furness JB, Autonomic Neuroscience, 2012).

How It Works

Neural signaling operates through two complementary mechanisms: electrical conduction along axons and chemical transmission across synapses.

An action potential — an all-or-nothing electrochemical event — propagates along a neuron's axon when membrane depolarization exceeds a threshold of approximately −55 millivolts. In myelinated fibers, saltatory conduction between nodes of Ranvier accelerates transmission velocity to between 70 and 120 meters per second. Unmyelinated C-fibers conduct at 0.5 to 2.0 meters per second — a difference that has direct clinical relevance in conditions such as peripheral neuropathy, where demyelination slows or blocks conduction.

At synapses, neurotransmitters are released from presynaptic terminals and bind to postsynaptic receptors. Over 100 distinct neurotransmitter substances have been identified (National Institute on Drug Abuse, NIDA). Glutamate is the principal excitatory transmitter in the CNS; gamma-aminobutyric acid (GABA) is the principal inhibitory transmitter. Dopamine, serotonin, acetylcholine, and norepinephrine serve modulatory roles across circuits governing movement, mood, memory, and arousal.

The blood-brain barrier (BBB) — formed by tight junctions between cerebrovascular endothelial cells, astrocyte end-feet, and pericytes — restricts passage of pathogens, large molecules, and most pharmaceutical agents. The BBB is a central design constraint in CNS drug development and is disrupted in conditions including traumatic brain injury, multiple sclerosis, and certain brain tumors.

Cortical organization follows two mapping principles recognized in neuroscience:

Common Scenarios

Anatomy becomes clinically actionable when a structural lesion produces a predictable deficit. The localization principle — that symptom pattern maps to lesion site — drives the neurological examination and guides imaging orders.

Stroke illustrates anatomy-driven diagnosis with precision. Middle cerebral artery (MCA) territory infarction produces contralateral hemiplegia and hemisensory loss, with aphasia when the dominant hemisphere is involved. Posterior circulation strokes affecting the brainstem or cerebellum produce crossed deficits (ipsilateral cranial nerve signs with contralateral limb findings) — a pattern that localizes to structures below the tentorium. Stroke workup and treatment protocols are governed in part by guidelines from the American Heart Association / American Stroke Association (AHA/ASA Stroke Guidelines).

Seizure disorders reflect abnormal synchronous electrical discharge within cortical networks. Focal seizures originate in a discrete cortical zone; generalized seizures involve bilateral networks from onset. This distinction, codified in the International League Against Epilepsy (ILAE) 2017 classification framework (ILAE Classification), determines both diagnostic workup and drug selection.

Demyelinating disease such as multiple sclerosis preferentially attacks CNS white matter, producing lesions at the junction of gray and white matter, periventricular regions, corpus callosum, and spinal cord. The spatial dissemination of lesions — required for MS diagnosis under the 2017 McDonald Criteria — is an anatomical standard.

Decision Boundaries

Distinguishing normal anatomical variation from pathological change requires reference standards across imaging, physiology, and clinical presentation. The regulatory context for neurological conditions explains how federal agencies including the FDA and CMS define approved diagnostic thresholds and coverage criteria for neurological testing.

Key decision boundaries include:

  1. Structural vs. functional: MRI-negative epilepsy is epilepsy; absence of a visible lesion does not exclude CNS dysfunction. Electrophysiological tools (EEG, EMG and nerve conduction studies) extend beyond anatomical imaging.
  2. Central vs. peripheral localization: Upper motor neuron signs (spasticity, hyperreflexia, Babinski response) indicate CNS pathology above the anterior horn cell. Lower motor neuron signs (flaccidity, hyporeflexia, fasciculations) indicate PNS or anterior horn involvement. This binary drives triage between neurological subspecialties.
  3. Age-adjusted norms: Brain volume decreases at approximately 0.2% per year after age 60 by volumetric MRI measurement (Taki et al., NeuroImage, 2011); atrophy exceeding age-adjusted norms in specific regions (hippocampus, entorhinal cortex) is a diagnostic criterion for Alzheimer's disease under NIA-AA 2018 research framework (National Institute on Aging).
  4. Congenital vs. acquired: Structural anomalies present from birth (Chiari malformation, agenesis of the corpus callosum) follow different epidemiological and prognostic trajectories than acquired lesions. Differentiating these categories requires developmental history alongside imaging.

The classification boundary between CNS and PNS disease is not always clean — conditions such as amyotrophic lateral sclerosis (ALS) simultaneously affect upper and lower motor neurons, requiring diagnostic criteria that span both systems.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)