Basic Anatomy: Parts and Structure of the Nervous System & How the Nervous System Works: Step-by-Step Physiology

The nervous system divides into two main components: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, consisting of the brain and spinal cord, serves as the main processing center. The PNS includes all neural tissue outside the CNS, connecting the central processor to the rest of the body. This division is somewhat artificial—the nervous system functions as an integrated whole—but helps organize our understanding of this complex system.

The brain, weighing about 3 pounds (1.4 kilograms) in adults, contains approximately 86 billion neurons and even more supporting cells called glia. Protected by the skull, three layers of membranes called meninges, and cushioned by cerebrospinal fluid, the brain consumes about 20% of the body's oxygen and calories despite representing only 2% of body weight. This disproportionate energy consumption reflects the enormous metabolic demands of constant neural activity.

The cerebrum, the brain's largest part, divides into two hemispheres connected by the corpus callosum—a bridge of 200 million nerve fibers enabling interhemispheric communication. The cerebral cortex, the outermost layer of gray matter, contains neuron cell bodies arranged in six layers. This highly folded surface increases surface area dramatically—if flattened, it would cover about 2.5 square feet. The folds create characteristic gyri (ridges) and sulci (grooves) that help define brain regions.

Each hemisphere contains four lobes with specialized functions. The frontal lobe, located behind the forehead, houses the primary motor cortex controlling voluntary movement, the prefrontal cortex managing executive functions like planning and decision-making, and Broca's area essential for speech production. The parietal lobe processes sensory information, containing the primary somatosensory cortex that maps touch sensations from throughout the body.

The temporal lobe, located above the ears, processes auditory information and contains the hippocampus crucial for memory formation. Wernicke's area in the left temporal lobe enables language comprehension. The occipital lobe at the brain's back processes visual information, with different areas specialized for color, motion, and form detection. Despite this specialization, brain regions work together through extensive connections.

Beneath the cortex lies white matter—myelinated axons connecting different brain regions. Major white matter tracts include association fibers connecting areas within the same hemisphere, commissural fibers connecting the hemispheres, and projection fibers connecting the cortex with lower brain structures and spinal cord. These highways of information enable the integrated function necessary for complex behaviors.

The diencephalon, located beneath the cerebrum, contains several crucial structures. The thalamus serves as a relay station, processing and directing sensory information to appropriate cortical areas. The hypothalamus, despite weighing only 4 grams, controls vital functions including body temperature, hunger, thirst, and hormone release. It forms the crucial link between the nervous and endocrine systems via the pituitary gland. The pineal gland produces melatonin, regulating sleep-wake cycles.

The brainstem connects the brain to the spinal cord and consists of three parts. The midbrain contains centers for visual and auditory reflexes and helps control movement. The pons serves as a bridge between brain regions and contains centers regulating sleep and arousal. The medulla oblongata controls vital functions like breathing, heart rate, and blood pressure. Damage to the brainstem often proves fatal due to these life-sustaining functions.

The cerebellum, meaning "little brain," sits behind the brainstem. Though containing more neurons than the rest of the brain combined, it occupies only 10% of brain volume due to densely packed, highly organized cells. The cerebellum coordinates movement, maintains balance and posture, and contributes to motor learning. Recent research reveals cerebellar involvement in cognitive functions including attention and language.

The spinal cord extends from the brainstem through the vertebral canal to the lower back. This cylinder of neural tissue, about 18 inches long and thumb-width in diameter, contains both gray matter (neuron cell bodies) and white matter (myelinated axons). The gray matter forms an H-shaped core surrounded by white matter organized into ascending (sensory) and descending (motor) tracts. Thirty-one pairs of spinal nerves branch from the cord, serving specific body regions.

Neurons, the functional units of the nervous system, are specialized cells designed for rapid communication. A typical neuron consists of a cell body (soma) containing the nucleus, dendrites that receive incoming signals, and an axon that transmits signals to other cells. Some axons extend over three feet, like those running from your spinal cord to your toes. This polarized structure enables one-way information flow through neural circuits.

Neural communication begins with the resting potential—a voltage difference across the neuron's membrane of about -70 millivolts, with the inside negative relative to outside. This electrical gradient is maintained by the sodium-potassium pump, which actively transports three sodium ions out for every two potassium ions pumped in. This creates an unequal distribution of ions, storing potential energy like a battery.

When stimulated, sodium channels open, allowing positive sodium ions to rush into the cell. This depolarization spreads along the membrane, creating an action potential—an electrical signal that propagates down the axon at constant strength. The all-or-nothing principle means neurons either fire completely or not at all, with information encoded in firing frequency rather than signal strength.

Myelin, a fatty substance produced by specialized glial cells, wraps around many axons like insulation on electrical wire. This myelination enables saltatory conduction—the action potential "jumps" between gaps in myelin called nodes of Ranvier, dramatically increasing conduction velocity. Myelinated fibers conduct signals up to 100 times faster than unmyelinated ones. Diseases like multiple sclerosis that damage myelin severely impair nervous system function.

At the axon terminal, the electrical signal must cross the synapse—the gap between neurons. This conversion from electrical to chemical signaling allows for signal modification and integration. When an action potential reaches the terminal, calcium channels open. Calcium influx triggers vesicles containing neurotransmitters to fuse with the membrane, releasing their contents into the synaptic cleft.

Neurotransmitters diffuse across the 20-nanometer synaptic gap and bind to receptors on the receiving neuron. This binding can be excitatory (promoting firing) or inhibitory (preventing firing). Over 100 different neurotransmitters have been identified, each with specific functions. Common examples include acetylcholine (muscle contraction, memory), dopamine (reward, motivation), serotonin (mood, sleep), and GABA (primary inhibitory neurotransmitter).

Each neuron receives inputs from hundreds or thousands of other neurons. The cell body integrates these signals through spatial and temporal summation. If combined excitatory inputs exceed the threshold, the neuron fires. This integration allows for complex information processing—neurons act as computational units, not simple switches.

Neurotransmitter action terminates through reuptake (transport back into the releasing neuron), enzymatic breakdown, or diffusion away from the synapse. Many drugs work by interfering with these processes—antidepressants often block serotonin reuptake, while some pesticides inhibit acetylcholine breakdown.

Neural plasticity enables the nervous system to adapt through experience. Synapses strengthen with repeated use (long-term potentiation) or weaken with disuse (long-term depression). New synapses form while others are eliminated. This synaptic plasticity underlies learning and memory. Even adult brains generate new neurons in certain regions, a process called neurogenesis, particularly in the hippocampus involved in memory formation.