The Nervous System: Structure and Function

Neuron Structure and Types

Neurons are the functional units of the nervous system, specialized cells that generate and transmit electrical signals. A typical neuron consists of a cell body (soma) containing the nucleus and most organelles, dendrites that receive incoming signals from other neurons, and an axon that carries signals away from the cell body to other neurons, muscles, or glands. Axons range from less than a millimeter to over a meter in length, with the longest running from the base of the spinal cord to the toes.

Three functional categories of neurons serve distinct roles. Sensory (afferent) neurons carry information from sensory receptors toward the CNS. Motor (efferent) neurons carry commands from the CNS to muscles and glands. Interneurons, which make up over 99% of all neurons, process and integrate information within the CNS, forming the complex circuits that underlie thought, memory, decision-making, and coordinated movement. A single interneuron in the cerebral cortex may form synaptic connections with 10,000 or more other neurons.

Glial cells (neuroglia) outnumber neurons and provide essential support functions. Astrocytes regulate the chemical environment around neurons, form the blood-brain barrier, and provide metabolic support. Oligodendrocytes in the CNS and Schwann cells in the PNS produce myelin, the insulating lipid sheath that wraps around axons and dramatically increases conduction speed. Microglia act as the immune cells of the CNS, clearing debris and responding to injury and infection. Ependymal cells line the brain ventricles and produce cerebrospinal fluid.

The Central Nervous System

The brain, weighing about 1.4 kilograms, consumes approximately 20% of the body's total oxygen and glucose despite representing only about 2% of body weight. The cerebrum, the largest brain region, consists of two hemispheres connected by the corpus callosum and is responsible for conscious thought, voluntary movement, sensory perception, language, and memory. Its outer surface, the cerebral cortex, is a 2 to 4 millimeter thick layer of gray matter (neuron cell bodies) folded into gyri (ridges) and sulci (grooves) that increase surface area to about 2,500 square centimeters.

Each cerebral hemisphere contains four lobes with generally distinct functions. The frontal lobe handles motor control, planning, reasoning, judgment, and speech production (Broca's area). The parietal lobe processes somatosensory information (touch, pressure, temperature, pain) and spatial awareness. The temporal lobe handles auditory processing, language comprehension (Wernicke's area), and certain aspects of memory. The occipital lobe processes visual information. Beneath the cortex, deep structures including the thalamus (sensory relay), hypothalamus (autonomic and endocrine control), hippocampus (memory formation), amygdala (emotional processing), and basal ganglia (motor coordination) perform critical integrative functions.

The spinal cord, a cylindrical structure about 45 centimeters long and 1 to 1.5 centimeters in diameter, extends from the brainstem through the vertebral canal to approximately the first or second lumbar vertebra. It contains gray matter (interneurons and motor neuron cell bodies) arranged in a butterfly shape centrally, surrounded by white matter (myelinated axon tracts) that carry signals up to the brain (ascending tracts) and down from the brain (descending tracts). The spinal cord also functions as an independent processing center for reflexes, completing simple sensory-motor circuits without requiring brain involvement.

Both the brain and spinal cord are protected by three meningeal layers (dura mater, arachnoid mater, pia mater) and cushioned by cerebrospinal fluid (CSF), which fills the ventricles inside the brain and the subarachnoid space surrounding both the brain and spinal cord. The choroid plexuses in the ventricles produce about 500 milliliters of CSF daily, maintaining a total volume of roughly 150 milliliters at any given time. CSF provides buoyancy (reducing the effective weight of the brain from 1,400 grams to about 50 grams), delivers nutrients, removes waste, and acts as a shock absorber.

The Peripheral Nervous System

The peripheral nervous system (PNS) connects the CNS to the rest of the body through 12 pairs of cranial nerves (originating from the brain) and 31 pairs of spinal nerves (originating from the spinal cord). Cranial nerves serve the head and neck, with the exception of the vagus nerve (cranial nerve X), which extends into the thorax and abdomen to innervate the heart, lungs, and digestive organs. Spinal nerves exit the vertebral column through intervertebral foramina and branch repeatedly to reach every region of the body.

The PNS divides functionally into the somatic nervous system and the autonomic nervous system. The somatic division controls voluntary skeletal muscle movements and transmits sensory information from the skin, muscles, and joints to the CNS. Each motor neuron in the somatic system releases acetylcholine at the neuromuscular junction, always producing an excitatory (stimulating) response in the muscle fiber.

The autonomic nervous system (ANS) controls involuntary functions of smooth muscle, cardiac muscle, and glands. It subdivides into the sympathetic division ("fight or flight") and the parasympathetic division ("rest and digest"). Most organs receive dual innervation from both divisions, which typically have opposing effects. Sympathetic activation increases heart rate, dilates bronchioles, diverts blood to skeletal muscles, dilates pupils, and releases glucose from the liver. Parasympathetic activation slows the heart, constricts bronchioles, promotes digestion and absorption, constricts pupils, and stimulates urination and defecation. A third division, the enteric nervous system, functions semi-independently within the walls of the GI tract.

Nerve Impulse Transmission

Neurons communicate through action potentials, rapid changes in membrane electrical potential that propagate along the axon. At rest, the neuron's membrane is polarized at about -70 millivolts (inside negative relative to outside), maintained by the sodium-potassium pump, which continuously moves 3 sodium ions out and 2 potassium ions in, and by potassium leak channels. When a stimulus depolarizes the membrane to threshold (about -55 mV), voltage-gated sodium channels open rapidly, sodium ions rush in, and the membrane potential swings to about +30 mV. Within a millisecond, sodium channels inactivate and voltage-gated potassium channels open, allowing potassium to flow out and restoring the resting potential.

Action potentials follow an all-or-nothing principle: if threshold is reached, a full action potential fires; if not, nothing happens. Signal strength is encoded by firing frequency, not amplitude. A louder sound or stronger touch produces more action potentials per second, not larger action potentials. In myelinated neurons, action potentials jump from one node of Ranvier (gap in the myelin sheath) to the next, a process called saltatory conduction that increases speed from about 1 meter per second in unmyelinated fibers to up to 120 meters per second in large myelinated fibers.

At synapses, electrical signals convert to chemical signals. When an action potential reaches the axon terminal, voltage-gated calcium channels open, calcium influx triggers synaptic vesicles to fuse with the presynaptic membrane, and neurotransmitter molecules are released into the synaptic cleft (about 20 nanometers wide). Neurotransmitters bind to receptors on the postsynaptic neuron, producing either excitatory postsynaptic potentials (EPSPs, pushing toward threshold) or inhibitory postsynaptic potentials (IPSPs, pushing away from threshold). A postsynaptic neuron integrates thousands of simultaneous EPSPs and IPSPs to determine whether to fire its own action potential.

Reflexes

Reflexes are rapid, automatic, involuntary responses to specific stimuli that protect the body from harm and maintain posture and balance. The simplest reflex arc involves five components: a receptor (detects the stimulus), a sensory neuron (carries the signal to the CNS), an integration center (usually interneurons in the spinal cord), a motor neuron (carries the command to the effector), and an effector (a muscle or gland that produces the response).

The patellar (knee-jerk) reflex is a monosynaptic stretch reflex involving only two neurons and one synapse. Tapping the patellar tendon stretches the quadriceps muscle, activating muscle spindle receptors. Sensory neurons carry the signal to the spinal cord, where they synapse directly on motor neurons that stimulate the quadriceps to contract, extending the knee. Simultaneously, inhibitory interneurons suppress the antagonist hamstring muscles. This reflex completes in about 50 milliseconds, far faster than any voluntary response. Physicians test reflexes to assess the integrity of specific spinal cord segments and peripheral nerves.

The withdrawal reflex is a polysynaptic reflex that protects against painful stimuli. Stepping on a sharp object activates pain receptors, sensory neurons relay the signal to spinal cord interneurons, and motor neurons contract the flexor muscles to lift the foot away from the stimulus. The crossed-extensor reflex simultaneously extends the opposite leg to maintain balance. Both reflexes are processed entirely in the spinal cord, which is why the foot begins moving away from the painful stimulus before the brain registers the sensation of pain.

Common Neurological Conditions

Multiple sclerosis (MS) is an autoimmune disease in which the immune system attacks myelin in the CNS, disrupting nerve conduction and producing symptoms including vision problems, numbness, weakness, fatigue, and cognitive impairment. It affects approximately 2.8 million people worldwide and typically presents between ages 20 and 40. Demyelinated areas develop scar tissue (sclerosis) that further impairs signal transmission. Disease-modifying therapies can reduce relapse frequency and slow progression but cannot restore lost myelin.

Parkinson's disease results from the progressive loss of dopamine-producing neurons in the substantia nigra, a midbrain structure involved in motor control. Cardinal symptoms include resting tremor, rigidity, bradykinesia (slowness of movement), and postural instability. Symptoms typically appear after 60% to 80% of dopaminergic neurons have been lost. Levodopa, a dopamine precursor that can cross the blood-brain barrier, remains the most effective symptomatic treatment. Deep brain stimulation, which delivers electrical pulses to specific brain regions, provides significant relief for patients who no longer respond adequately to medication.

Peripheral neuropathy, damage to peripheral nerves, affects an estimated 20 million people in the United States alone. Diabetes is the leading cause, with approximately 50% of diabetic patients developing neuropathy. Other causes include B12 deficiency, alcohol abuse, autoimmune diseases, infections, and chemotherapy. Symptoms typically begin in the feet and hands (glove-and-stocking distribution) and include numbness, tingling, burning pain, and weakness. Treatment focuses on managing the underlying cause and symptomatic relief.