The Nervous System Overview

The nervous system is the body's high-speed communication and control network — the organ system that lets you feel a hot stove, pull your hand away before you consciously decide to, form a memory of the pain, and later warn a friend. For a clinician, understanding it is not optional decoration: nearly every complaint a patient brings, from a headache to a weak leg to a change in personality, is a question about where in this network something has gone wrong. Neurology rewards the student who thinks anatomically, because in few other specialties does localization — "where is the lesion?" — so directly precede diagnosis.

This page builds the scaffold you will hang the rest of neurology on: the split between central and peripheral systems, how a single neuron fires and talks to the next, the geography of the brain, and how the neurological exam turns all of that theory into bedside decisions.

Learning Objectives

• Distinguish the central nervous system (CNS) from the peripheral nervous system (PNS), and the somatic from the autonomic divisions.
• Describe the structure of a neuron and the sequence of events in an action potential and synaptic transmission.
• Name the major brain regions and match each to its principal functions.
• Outline the components of the neurological examination and explain what each tests.
• Appreciate the historical shift from Galen's ventricular doctrine to the neuron doctrine of Ramon y Cajal.

Quick Answer

The nervous system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial and spinal nerves, ganglia, and their branches). Its functional cell is the neuron, which conducts electrical signals as action potentials and communicates with other cells across synapses using neurotransmitters. The brain is organized into the cerebrum (higher functions), cerebellum (coordination), brainstem (vital reflexes and cranial nerve nuclei), and diencephalon (relay and homeostasis). The PNS carries motor commands out and sensory information in, split into a voluntary somatic division and an involuntary autonomic division (sympathetic and parasympathetic). Clinicians probe this system through the neurological examination — mental status, cranial nerves, motor, sensory, reflexes, coordination, and gait — to localize disease before naming it.

Where It Came From

For most of recorded history, the brain was not obviously the seat of thought. Aristotle (4th century BCE) argued the heart was the organ of intellect and the brain merely a radiator that cooled the blood. It was Galen of Pergamon (c. 129–216 CE), dissecting animals and treating gladiators, who located mental function in the brain. But Galen's influential model placed the "animal spirits" — the stuff of sensation and movement — in the fluid-filled ventricles, not the brain tissue itself. This ventricular doctrine dominated Western medicine for roughly 1,400 years, and you can see it in medieval diagrams that assign imagination, reason, and memory to three balloon-like chambers.

The real problem that eventually broke this model was a simple one that medicine could not answer: what is the actual physical unit of the nervous system, and how does a signal travel through it? Progress waited on tools. The microscope, and especially staining techniques, finally made nervous tissue visible. In 1873 Camillo Golgi invented a silver-chromate stain (la reazione nera, the "black reaction") that randomly darkened whole individual cells against a clear background — the first time a complete nerve cell with all its branches could be seen. Ironically, Golgi believed the results supported a reticular theory: that the nervous system was one continuous fused network, a syncytium.

The Spanish anatomist Santiago Ramon y Cajal used Golgi's own stain to reach the opposite conclusion. Through meticulous drawings, he argued that neurons are discrete, individual cells that communicate by contact, not fusion — the neuron doctrine. He also proposed the law of dynamic polarization: signals flow in one direction, from dendrites through the cell body to the axon. Cajal and Golgi shared the 1906 Nobel Prize and, famously, disagreed with each other in their acceptance lectures. Cajal was right. Decades later the electron microscope confirmed the synaptic cleft — a real gap between neurons — vindicating him. Parallel physiological work by Luigi Galvani (animal electricity, 1780s), Otto Loewi (chemical neurotransmission, 1921, "Vagusstoff"), and Hodgkin and Huxley (the ionic basis of the action potential, 1952) filled in how those separate cells actually signal.

Central and Peripheral: The Two Great Divisions

The central nervous system (CNS) comprises the brain and spinal cord. It is the integrative core — where information is processed, decisions are made, and reflex arcs are closed. It is protected by the skull and vertebral column, wrapped in three meninges (dura, arachnoid, pia), and bathed in cerebrospinal fluid (CSF). Critically, CNS neurons have very limited capacity to regenerate after injury, which is why spinal cord and brain damage tends to be permanent.

The peripheral nervous system (PNS) is everything else: the 12 pairs of cranial nerves, 31 pairs of spinal nerves, their roots, plexuses, peripheral nerves, and the sensory and autonomic ganglia. Peripheral nerves can regenerate (slowly, roughly 1 mm/day) if the cell body survives — the basis for recovery after a nerve laceration is repaired.

Functionally the PNS splits again:

• Somatic nervous system — voluntary control of skeletal muscle and conscious sensation from skin, joints, and special senses.
• Autonomic nervous system (ANS) — involuntary control of smooth muscle, cardiac muscle, and glands. It has two often-opposing arms:
  • Sympathetic ("fight or flight"): dilates pupils, speeds the heart, diverts blood to muscle, inhibits digestion. Thoracolumbar outflow, mostly noradrenergic.
  • Parasympathetic ("rest and digest"): constricts pupils, slows the heart, stimulates digestion. Craniosacral outflow, cholinergic; the vagus nerve (CN X) carries the bulk of it.

A third component, the enteric nervous system embedded in the gut wall, is large enough (hundreds of millions of neurons) and independent enough to be called a "second brain."

The Neuron and Neurotransmission

The neuron has three functional parts. Dendrites are branching antennae that receive inputs. The cell body (soma) houses the nucleus and integrates those inputs. The axon is a single long output fiber that carries the signal away, often insulated by a myelin sheath made by oligodendrocytes (CNS) or Schwann cells (PNS). Gaps in the myelin, the nodes of Ranvier, allow the impulse to jump from node to node — saltatory conduction — which is dramatically faster than conduction along bare fibers. Neurons are supported by glial cells: astrocytes (metabolic and blood-brain barrier support), microglia (immune surveillance), and the myelin-forming cells above.

The action potential — step by step:

1. Resting state: The inside of the neuron sits near −70 mV, kept negative by the Na⁺/K⁺-ATPase pump and leak channels.
2. Threshold: Summed excitatory input depolarizes the membrane to about −55 mV.
3. Depolarization: Voltage-gated Na⁺ channels snap open; Na⁺ rushes in and the inside shoots toward +30 mV.
4. Repolarization: Na⁺ channels inactivate and voltage-gated K⁺ channels open; K⁺ leaves, restoring negativity.
5. Hyperpolarization and reset: A brief overshoot below rest (refractory period) prevents the signal from running backward and sets the maximum firing rate.

The action potential is all-or-none: it either fires fully or not at all; intensity is coded by frequency, not size.

At the synapse, the electrical signal becomes chemical. The action potential reaches the axon terminal and opens voltage-gated Ca²⁺ channels. Calcium influx triggers synaptic vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft. The transmitter diffuses across and binds receptors on the postsynaptic cell, either exciting it (e.g., glutamate, the main excitatory transmitter) or inhibiting it (e.g., GABA, the main inhibitory one). Other key transmitters include acetylcholine (neuromuscular junction, parasympathetic), dopamine (reward, movement — deficient in Parkinson disease), serotonin (mood), and noradrenaline (arousal, sympathetic). The signal is then terminated by reuptake, enzymatic breakdown, or diffusion.

Clinical hook: This machinery is where much of pharmacology and disease lives. Myasthenia gravis attacks acetylcholine receptors at the neuromuscular junction (fatigable weakness). Multiple sclerosis destroys CNS myelin, slowing or blocking conduction. SSRIs block serotonin reuptake. Local anaesthetics block voltage-gated Na⁺ channels, stopping the action potential from ever forming.

The Geography of the Brain

Think of the brain in four blocks, roughly from top to bottom:

• Cerebrum — the large wrinkled hemispheres, home to conscious thought, voluntary movement, sensation, language, and memory. Its outer cortex is divided into lobes: frontal (motor control, planning, personality, Broca's speech area), parietal (somatosensation, spatial awareness), temporal (hearing, memory via the hippocampus, Wernicke's comprehension area), and occipital (vision). The two hemispheres are joined by the corpus callosum. Deep inside sit the basal ganglia, which fine-tune movement.
• Diencephalon — the thalamus (the grand relay station: almost all sensory information synapses here before reaching cortex) and the hypothalamus (master regulator of temperature, hunger, thirst, circadian rhythm, and the endocrine system via the pituitary).
• Cerebellum — the "little brain" at the back, which coordinates movement, balance, and posture. It does not initiate movement but smooths and calibrates it; damage causes clumsy, uncoordinated ataxia, not paralysis.
• Brainstem — midbrain, pons, and medulla. It carries all the tracts running between brain and cord, houses most cranial nerve nuclei, and controls the truly vital automatic functions: breathing, heart rate, blood pressure, and consciousness (via the reticular activating system). Brainstem damage is often catastrophic.

Localization in action: Sudden weakness of the right face, arm, and leg with slurred speech points to the left cerebral hemisphere (motor pathways cross). Vertigo with clumsiness on one side points to the cerebellum. Coma with pinpoint pupils suggests a pons lesion. The neurological exam is how you gather the evidence for these inferences.

The Neurological Examination

The neuro exam is a structured tour of the whole system, performed in a consistent order so nothing is missed. Each component maps onto anatomy:

1. Mental status / higher function — level of consciousness, orientation, attention, memory, language. Tests cortex and diffuse brain function.
2. Cranial nerves (I–XII) — smell, vision and pupils, eye movements, facial sensation and movement, hearing, swallowing, tongue and shoulder movement. Localizes to the brainstem and skull base.
3. Motor system — inspection (wasting, fasciculations), tone, power (graded MRC 0–5), and involuntary movements. Distinguishes upper motor neuron lesions (spasticity, brisk reflexes, weakness) from lower motor neuron lesions (flaccidity, wasting, absent reflexes).
4. Reflexes — deep tendon reflexes (biceps, triceps, knee, ankle) and the plantar response. An upgoing big toe (Babinski sign) indicates an upper motor neuron lesion — normal only in infants.
5. Sensory system — light touch, pinprick, temperature, vibration, and proprioception. Different modalities travel in different spinal tracts, so their patterns help localize cord lesions.
6. Coordination — finger-to-nose, heel-to-shin, rapid alternating movements. Tests the cerebellum.
7. Gait and stance — often the most informative single test, integrating strength, sensation, coordination, and balance. Includes the Romberg test (unsteadiness with eyes closed suggests loss of proprioception).

The exam's power is that it converts symptoms into a localization, which then narrows the differential and directs imaging — you order the MRI of the right place because the exam told you where to look.

Real-World Applications

• Stroke: The classic "face-arm-speech" screen (FAST) is applied neuro-anatomy — it detects the cortical motor and language deficits of a hemisphere stroke, and every minute of delay costs surviving neurons.
• Diabetes: A "glove and stocking" loss of sensation in the feet is a peripheral neuropathy from damaged PNS axons — a direct application of knowing that long fibers fail first.
• Anaesthesia and pain: Epidurals and nerve blocks work by putting local anaesthetic exactly where the relevant peripheral nerve or root runs.
• Everyday life: Caffeine, alcohol, and antihistamines all work by nudging neurotransmission — adenosine, GABA, and histamine systems respectively.

Common Mistakes

• "The cerebellum makes you move." It does not initiate movement — the motor cortex does. The cerebellum coordinates movement. Correction: cerebellar lesions cause ataxia and intention tremor, not paralysis. A paralysed patient has a corticospinal (motor pathway) problem, not a cerebellar one.
• "A bigger stimulus makes a bigger action potential." Action potentials are all-or-none; their amplitude is fixed. Correction: a stronger stimulus increases the frequency of firing and recruits more neurons, but each spike is the same size.
• "Peripheral nerves and CNS neurons heal the same way." They do not. Correction: PNS axons can regenerate (~1 mm/day) if the cell body survives and the sheath is intact; CNS neurons largely cannot, which is why spinal cord injury is usually permanent.
• "The autonomic nervous system is entirely automatic and beyond influence." Largely involuntary, yes, but modifiable — slow breathing raises parasympathetic (vagal) tone and lowers heart rate, the basis of many relaxation techniques.

Comparison and Connections

Feature	Central nervous system	Peripheral nervous system
Components	Brain, spinal cord	Cranial/spinal nerves, ganglia
Myelin made by	Oligodendrocytes	Schwann cells
Regeneration	Very limited	Possible if cell body survives
Protection	Skull, vertebrae, meninges, CSF	Connective tissue sheaths
Example disease	Multiple sclerosis, stroke	Guillain-Barre syndrome, neuropathy

Feature	Sympathetic	Parasympathetic
Nickname	Fight or flight	Rest and digest
Outflow	Thoracolumbar	Craniosacral (esp. vagus)
Heart rate	Increases	Decreases
Pupils	Dilate	Constrict
Main transmitter (target)	Noradrenaline	Acetylcholine

Also worth distinguishing: upper vs lower motor neuron signs (spastic and brisk vs flaccid and absent), and grey matter (cell bodies) vs white matter (myelinated axon tracts).

Practice Questions

Recall

Q: Name the two divisions of the peripheral nervous system by function, and the two arms of the autonomic division. A: Somatic and autonomic; the autonomic division has sympathetic and parasympathetic arms.

Understanding

Q: Why does myelin speed up nerve conduction, and what happens when it is lost, as in multiple sclerosis? A: Myelin insulates the axon so the impulse jumps between nodes of Ranvier (saltatory conduction), which is far faster than continuous conduction. Demyelination in MS slows or blocks conduction, producing symptoms like weakness, numbness, and visual loss that depend on which tracts are affected.

Application

Q: A patient develops sudden weakness of the right arm and leg with slurred speech but a normal-looking CT initially. Where do you localize the lesion and why? A: The left cerebral hemisphere (likely a stroke). Motor pathways decussate, so left-brain damage causes right-sided weakness; the accompanying speech disturbance also points to the dominant (usually left) hemisphere. Early CT can be normal in ischaemic stroke, so localization rests on the exam.

Analysis

Q: A patient has weakness with increased tone, brisk reflexes, and an upgoing plantar response on the right. Contrast this with weakness that shows wasting, reduced tone, and absent reflexes, and state which part of the motor system each implicates. A: The first pattern is an upper motor neuron lesion (corticospinal tract, i.e., brain or spinal cord above the anterior horn) — spasticity, hyperreflexia, Babinski. The second is a lower motor neuron lesion (anterior horn cell, root, or peripheral nerve) — wasting, hypotonia, hyporeflexia. Separating them is the first fork in localizing any weakness.

FAQ

Is the spinal cord part of the brain? No, but it is part of the CNS. The brain and spinal cord together make the central nervous system; the cord is the main highway carrying signals between brain and body, and it closes many reflexes on its own.

What is the difference between grey matter and white matter? Grey matter is mostly neuron cell bodies and synapses (the cortex and deep nuclei); white matter is myelinated axons connecting regions. The whiteness comes from the fatty myelin.

How is a nerve different from a neuron? A neuron is a single cell. A nerve is a bundle of many axons (from many neurons) wrapped in connective tissue, running together in the periphery — like a cable made of many wires.

Why do strokes on one side of the brain affect the opposite side of the body? Because the major motor and sensory pathways cross (decussate) as they travel between brain and body — most in the brainstem. So the left brain controls and senses the right body.

Do we really only use 10% of our brains? No — this is a myth. Functional imaging shows essentially all regions are active over the course of normal activity, and there is no large dormant reserve. Even small areas of damage typically cause noticeable deficits.

Can nerve cells grow back? Peripheral nerves can regenerate slowly if the neuron's cell body survives and the supporting sheath guides regrowth. CNS neurons (brain and spinal cord) have very limited regenerative capacity, which is a major reason spinal cord injuries are usually permanent — and an active frontier of research.

Quick Revision

• CNS = brain + spinal cord; PNS = cranial/spinal nerves + ganglia.
• PNS splits into somatic (voluntary) and autonomic (involuntary → sympathetic + parasympathetic).
• Neuron parts: dendrites (in), soma (integrate), axon (out); myelin enables fast saltatory conduction.
• Action potential is all-or-none: Na⁺ in (depolarize), K⁺ out (repolarize); intensity coded by frequency.
• Synapse: Ca²⁺ influx → neurotransmitter release → excite (glutamate) or inhibit (GABA).
• Brain blocks: cerebrum (thought/movement/sensation), diencephalon (thalamus relay, hypothalamus homeostasis), cerebellum (coordination), brainstem (vital functions).
• Neuro exam order: mental status → cranial nerves → motor → reflexes → sensory → coordination → gait.
• Upper motor neuron = spastic, brisk reflexes, upgoing toe; lower motor neuron = flaccid, wasted, absent reflexes.
• History: Galen's ventricles → Golgi's stain → Cajal's neuron doctrine (Nobel 1906).

Related Topics

Prerequisites

• Neurology Branch Overview
• Basic cell biology and membrane transport

Related Topics

• Physiology
• Anatomy
• Pharmacology

Next Topics

• Cranial nerves in detail
• The motor and sensory pathways
• Stroke and cerebrovascular disease