Structure and Function of the Central Nervous System

The central nervous system (CNS) is the body’s command center: the brain and spinal cord working together to control movement, sensation, thought, and emotion. It processes information from the senses and coordinates everything from voluntary actions like walking to automatic functions like breathing and heartbeat.

CNS Diagram

CNS Functions

The Central Nervous System (CNS), composed of the brain and spinal cord, functions as the body’s primary control centre.

It is responsible for receiving and interpreting sensory information, making decisions, and directing the body’s muscular and physiological responses.

The CNS operates through a continuous cycle of receiving input, processing that data, and initiating output.

1. Core Physiological Functions

The CNS is responsible for both high-level cognitive processes and the maintenance of internal stability (homeostasis).

• Reflex Coordination: The spinal cord facilitates rapid, involuntary responses to stimuli, such as the withdrawal reflex, bypassing the brain for immediate action.
• Sensory Processing: The system receives and interprets data gathered by the peripheral nervous system regarding the environment and the body’s internal state.
• Integration (Processing Phase): The brain combines current sensations with existing emotions and memories to form a complete picture of the environment.
• Cognition: The cerebral cortex turns this integrated information into consciousness, reasoning, and language, letting us solve problems and communicate.
• Motor Command: The brain initiates signals that travel through the spinal cord to trigger voluntary movements (e.g., speech) and regulate involuntary muscle actions.
• Homeostatic Regulation: The hypothalamus maintains internal equilibrium (body temperature, hunger, thirst) by working with the autonomic nervous system.

2. Component Roles

The CNS is divided into two primary anatomical components with specialized roles:

The Brain

• Acts as the primary site for higher-order functions including intelligence, personality, and emotion.
• Manages the interpretation of complex sensory data and the production of speech.

The Spinal Cord

• Functions as the vital communication relay, carrying messages bidirectionally between the brain and the rest of the body.
• Serves as the independent center for coordinating spinal reflexes.

3. Protection and Metabolic Support

Because the CNS is composed of delicate neural tissue, it requires extensive physical and physiological protection:

• Physical Armor: The brain is encased within the skull, and the spinal cord is protected by the vertebral column (vertebrae).
• Meninges: Three protective membranes (dura mater, arachnoid mater, and pia mater) surround the CNS to provide a barrier against injury.
• Nutrient Dependency: The CNS has a high metabolic rate and is heavily reliant on a constant supply of nutrients and oxygen to function.

The Brain

The brain is the CNS’s most complex structure. It processes sensory information, regulates bodily functions, and enables higher-order abilities like thinking, memory, and reasoning.

Although the brain is anatomically continuous with the spinal cord, it stands apart for its structural complexity and specialization.

To understand its function, biologists categorize the brain into three main developmental divisions: the forebrain, midbrain, and hindbrain.

• Forebrain (Prosencephalon): This is the largest brain region. It includes the cerebrum (responsible for reasoning and memory) and the diencephalon, which houses the thalamus and hypothalamus.
• Midbrain (Mesencephalon): This region serves as a connector between the higher forebrain and the lower hindbrain, managing visual and auditory reflexes.
• Hindbrain (Rhombencephalon): This evolutionarily older region controls vital life-support functions through the medulla oblongata (breathing/heart rate) and coordinates motor control via the cerebellum.

The Brain Stem

The brain stem is located at the base of the brain and is one of the most primitive regions of the brain; and is made up of the midbrain, pons, and medulla oblongata.

The brain stem functions are correspondingly basic and physiological, including automatic behaviors such as breathing and swallowing.

The Cerebellum

Sitting just above the brainstem, the cerebellum regulates motor activity, balance, and posture.

It helps fine-tune movements, especially those we perform automatically, like walking or catching a ball.

The cerebellum holds most of the brain’s neurons. Direct cell counts put the human brain at roughly 86 billion neurons, with most packed into the cerebellum rather than the cortex, as long assumed (Herculano-Houzel, 2009).

In some animals, like amphibians, the brain consists mainly of the brainstem and cerebellum.

The Cerebrum

The cerebrum is the brain’s largest and most recently evolved area, making up about 85% of its mass.

Divided into two hemispheres, it supports higher-level functions including speech, voluntary movement, and problem-solving.

Each hemisphere controls the opposite side of the body.

Within the cerebral hemispheres, there are four areas, or lobes, that each serve different functions:

• Frontal lobes – these are positioned at the forefront of the brain and are responsible for higher cognitive functioning, language development, attention, decision-making, and problem-solving.
• Occipital lobes – positioned at the back of the brain, these lobes are responsible for processing and encoding different visual information such as color, orientation, and motion.
• Parietal lobes – situated at the top of the brain, are responsible for processing sensory information, attentional awareness, visuospatial processing, and integrating somatosensory information (e.g., touch, temperature, and pain).
• Temporal lobes – located just behind the ears, the temporal lobes are responsible for the recognition, perception (hearing, vision, smell), understanding of language, and forming memories.

The Cerebral Cortex: Thinking Surface

The surface of the cerebrum is called the cerebral cortex, also known as grey matter.

It’s a 3mm-thick layer packed with billions of neurons.

This is where memories are stored, sensory data is interpreted, and thinking occurs.

The folds and grooves (called gyri and sulci) increase the cortex’s surface area, allowing for more complex neural networks.

White Matter and Connectivity

Beneath the cortex lies white matter – bundles of nerve fibers that connect different brain regions and speed up communication using myelin insulation.

Spinal Cord

The spinal cord is a long, thin collection of neurons attached to the base of the brain (brain stem), running the length of the spinal column.

The spinal cord contains circuits of neurons that can control some of our simple reflexes, such as moving a hand away from a hot surface without participation from the brain.

It serves two primary functions:

1. Transmission: It routes messages to and from the brain, connecting the CNS to the peripheral nervous system.
2. Reflexes: The spinal cord can initiate automatic, involuntary responses (such as the knee-jerk reaction) without input from the brain, allowing for faster reactions in survival situations.

How the Reflex Arc Works

Picture touching a very hot pan. Your hand pulls back before you consciously feel any pain. This reflex shows the spinal cord doing real work of its own.

A sensory neuron relays the signal to the cord. Inside the cord, this signal crosses to a motor neuron, sometimes through a connecting interneuron, which then passes the command back out to the muscle.

Because the whole loop runs inside the cord, the reflex is very fast. The brain only learns what happened after the movement is already done: the felt pain follows the withdrawal by roughly a second.

The brain can still speed up or dampen these reflexes. Doctors still test them, like the knee-jerk reflex, to check the nervous system’s whole pathway.

Segments of the Spinal Cord

The spinal cord is divided into 30 segments, each serving a specific body area:

• Cervical – 8 segments serving the head, neck, shoulders, arms, and hands.
• Thoracic – 12 segments serving the arms, chest, and abdomen.
• Lumbar – 5 segments serving the legs and feet.
• Sacral – 5 segments serving the lower back, pelvic organs, genitals, and parts of the legs and feet.
• Coccyx – the small base of the spinal cord.

How the CNS Communicates with the Body

Neurons

The fundamental unit of the CNS is the neuron, or nerve cell.

The CNS contains about 86 billion neurons, each designed to transmit electrical signals.

Neurons are designed to send and receive information through a specific structure:

• Dendrites: Branch-like fibres that receive messages from other neurons.
• Soma (Cell Body): Contains the nucleus and genetic information; it processes the incoming signals.
• Axon: A tube-like extension that carries electrical impulses (action potentials) away from the cell body toward other neurons.
• Myelin Sheath: A white, fatty covering on axons that provides insulation and increases the speed of signal transmission.

Communication between neurons occurs at the synapse, a gap between the axon terminal of one neuron and the dendrite of another.

When an electrical impulse reaches the end of an axon, it triggers the release of chemical messengers called neurotransmitters (e.g., dopamine, serotonin, acetylcholine).

These chemicals cross the synaptic cleft and bind to receptors on the receiving neuron, delivering excitatory (fire) or inhibitory (don’t fire) messages.

Glial Cells

Although glial cells don’t send signals themselves, they are vital for maintaining brain health.

Glial cells are roughly as numerous as neurons, not the nine-to-one ratio once assumed (Herculano-Houzel, 2009). Key types include:

• Astrocytes: Supply nutrients, clean up toxins, and support neuron survival.
• Microglia: Act as the CNS’s immune system, cleaning up waste and protecting against infection.
• Oligodendrocytes: Produce myelin, a fatty sheath that wraps around axons to speed up signal transmission.

Disorders & Damage

Disorders and damage associated with the CNS range from acute injuries caused by external forces to progressive neurodegenerative diseases and complex psychiatric conditions rooted in biological dysfunction.

Because the CNS controls vital functions, sensation, movement, and cognition, damage to specific areas often results in distinct, sometimes profound, behavioral and functional deficits.

1. Acute Trauma and Vascular Disorders

These disorders involve sudden physical or circulatory failure within the brain.

Traumatic Brain Injury (TBI)

TBI results from biomechanical forces, such as blunt impact or rapid acceleration, that deform the brain and vasculature.

• Pathophysiology: Primary injury causes mechanical deformation, while secondary cascades (inflammation/metabolic changes) occur over time.
• Tissue Vulnerability: Damage often occurs at the interface between gray matter (cell bodies) and white matter (axons) due to their different elastic properties.
• Historical Case (Phineas Gage): Damage to his frontal lobe illustrated that this region governs executive function and personality; he became impulsive and irresponsible following his injury.

Cerebrovascular Disorders (Stroke)

A stroke occurs when blood supply is disrupted, leading to cell death.

• Types: Ischemic (blockage) or Hemorrhagic (rupture).
• Localization: Symptoms depend on the affected artery. For example, Middle Cerebral Artery (MCA) occlusion causes face/arm deficits and dysphasia, while Posterior Cerebral Artery (PCA) damage affects the occipital lobe, causing visual deficits.

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2. Neurodegenerative Diseases

These conditions involve the progressive loss of neuronal structure or function.

• Alzheimer’s Disease (AD): Involves amyloid plaques and neurofibrillary tangles, leading to memory loss and shrunken hippocampi.
• Parkinson’s Disease: Degeneration of dopamine-producing neurons in the substantia nigra, causing tremors and difficulty initiating movement.
• Multiple Sclerosis (MS): An autoimmune disorder causing demyelination in the CNS, which slows or blocks nerve impulses.
• Huntington’s Chorea: A genetic disorder causing degeneration of the striatum (caudate nucleus and putamen), leading to uncontrollable movements.

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3. Complex Processing Disorders (Aphasia & Agnosia)

Damage to “higher-order” association areas of the cortex produces syndromes where sensation remains intact, but recognition fails.

• Broca’s Aphasia: Damage to the left inferior frontal cortex results in halting, laborious speech, though comprehension is often intact.
• Wernicke’s Aphasia: Damage to the left temporal lobe results in fluent but nonsensical “word salad” and poor comprehension.
• Visual Agnosia: Inability to recognize objects despite seeing them, often due to damage in the ventral visual stream.
• Prosopagnosia: Specific inability to recognize faces, linked to lesions in the fusiform gyri.

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4. Psychiatric and Developmental Disorders

Biological dysfunctions within brain networks often underpin psychiatric conditions.

• Epilepsy: A disorder of neural excitability. In severe cases, severing the corpus callosum (split-brain surgery) is used to prevent seizure spread, resulting in hemispheres that cannot communicate.
• Schizophrenia: Structural abnormalities include enlarged cerebral ventricles and reduced gray matter in the frontal lobes, often involving dopamine and glutamate imbalances.
• Mood Disorders: Linked to dysfunction in regions regulating emotion, such as the amygdala and prefrontal cortex.

How the central nervous system is protected

The CNS is vital for survival and everyday function. It is exceptionally well protected as a result.

A skull encases the brain, and the spinal cord runs through the middle of a column of hollow bones known as vertebrae.

Three layered membranes, the meninges, wrap around both structures for extra protection. From outside in, they are the dura mater, arachnoid mater, and pia mater.

The brain and spinal cord never touch bone directly. Both float in a clear liquid called cerebrospinal fluid (CSF), cushioning them from the skull and vertebrae.

CSF fills the space between the meninges and circulates through the CNS’s ventricles. This cushions the brain and spinal cord, protecting them from damage.

The Blood-Brain Barrier

The brain needs its own kind of shield. A tight-fitting cell layer called the blood-brain barrier lines its blood vessels, blocking most substances from crossing out of the blood and into brain tissue.

This barrier protects neurons from toxins, infections, and chemical swings they cannot tolerate. It also blocks many potentially helpful drugs from reaching the brain, a real obstacle in treating brain disease.

No one knew exactly how it worked.

Aim: Reese and Karnovsky (1967) had one goal. They wanted to find exactly where the blood-brain barrier sits, tracking a protein marker as it tried to move from the bloodstream into brain tissue.

Method: The researchers injected horseradish peroxidase into the bloodstream of mice. This enzyme is large enough to be stopped by a genuine barrier, and it produces a visible marker under an electron microscope, showing exactly where it was blocked.

Findings: The peroxidase passed through capillaries elsewhere in the body. In brain capillaries, though, it stopped exactly at the tight junctions between neighbouring cells lining the vessel wall.

Conclusion: The blood-brain barrier is a real physical structure. It sits at the tight junctions between the cells that form brain capillary walls, not in some separate membrane, settling a long-running question.

Protecting the central nervous system

Although many CNS disorders, like Alzheimer’s or multiple sclerosis, may not have a cure, there are ways to reduce risks and support long-term brain and spinal cord health.

Simple ways to protect your CNS:

• Wear safety gear to prevent head and spine injuries during sports, cycling, or driving.
• Stay active, both mentally and physically, to support brain function and circulation.
• Eat a brain-healthy diet, rich in omega-3s, fruits, and vegetables.
• Limit alcohol and avoid harmful substances, which can damage nerve tissue.
• Manage blood pressure and cholesterol to lower the risk of stroke.
• Stay up to date on vaccinations to help prevent infections like meningitis.

Taking care of your nervous system doesn’t mean you can prevent all conditions, but it can help protect what matters most: your ability to move, think, and live independently.

Critical Evaluation of the CNS

The CNS/PNS framework, and the brain’s own anatomical divisions, are powerful tools in biopsychology, but a balanced view weighs their real power against their limits.

How Useful Is the CNS/PNS Distinction?

The CNS/PNS split has strong anatomical and clinical grounding. The CNS sits inside bone and the meninges, is shielded by the blood-brain barrier, and is built from cells that regenerate far less readily than peripheral nerves. A severed peripheral nerve can sometimes regrow; a severed CNS tract generally cannot.

This gives the framework real predictive power. Knowing whether an injury is central or peripheral helps a clinician predict what functions will be lost and how reversible that loss is likely to be.

But the CNS and PNS are not really so separate. Sensory feedback from the body shapes what the CNS does.

CNS output means nothing without the peripheral nerves and muscles that carry it out. This framework organises study and reasoning; it is not a claim that one side dictates behaviour alone.

The Limits of Localisation and Reductionism

The brain’s three-region scheme, and localisation of function more generally, are useful but incomplete. Some structures do map onto specific, testable jobs: the substantia nigra and Parkinson’s disease, the hippocampus and certain kinds of amnesia, the medulla and the vital reflexes.

But most “higher” functions do not have one address. Beyond primary sensory and motor cortex, they emerge from networks of regions working together.

Even the thalamus, once seen as a simple relay, is now known to actively shape the information passing through it. Confident claims that one psychological function has a single anatomical address tend to overreach.

The same caution applies to explanation, not just anatomy.

Explaining memory, emotion, or motivation purely through one brain circuit risks reductionism. That means treating a single biological cause as the whole story, while ignoring learning, social context, and personal history.

The CNS supplies the machinery for behaviour, but rarely the whole explanation on its own.

Contemporary Research

The idea that CNS damage cannot be undone is also being revised.

The Brain’s Own Immune Drainage System

Aim: Louveau et al. (2015) tested whether the meninges contain lymphatic vessels, the drainage network that clears fluid and immune cells in every other organ. For over a century, the brain had been assumed to lack this system entirely.

Method: The team imaged mouse meninges for lymphatic markers. They then tracked whether immune cells and fluid travelled through these vessels to the lymph nodes in the neck.

Findings: They found a genuine network of lymphatic vessels running through the dura mater. These vessels carried immune cells and fluid from the brain to the neck’s lymph nodes.

Conclusion: The brain is not immunologically isolated after all. Its own protective membranes double as a drainage route linking brain fluid and immune signalling to the rest of the body. This reshapes how researchers think about neurodegenerative and neuroinflammatory disease.

Restoring Movement After Spinal Cord Injury

A second advance shows the spinal cord itself can recover more than assumed.

Aim: Wagner et al. (2018) had a bold target. They tested whether people with long-term, incomplete spinal cord injury, too severe to walk unaided, could regain voluntary stepping using targeted spinal stimulation.

Method: An implanted device delivered electrical pulses to specific spinal segments. These were precisely timed to match the muscle activity needed for each phase of a step, rather than constant background stimulation.

Findings: The stimulation immediately enabled voluntary stepping and posture control. After training with the device, some participants kept part of this improvement even with the stimulation switched off.

Conclusion: Even a badly damaged spinal cord retains working circuits that targeted technology can recruit, challenging the old assumption that CNS injury is simply permanent.

Together, the two findings paint a CNS that is more connected, and more capable of recovery, than older textbooks assumed.

References

Brodal, P. (2004). The central nervous system: structure and function. Oxford University Press.

Herculano-Houzel, S. (2009). The human brain in numbers: A linearly scaled-up primate brain. Frontiers in Human Neuroscience, 3, 31. https://doi.org/10.3389/neuro.09.031.2009

Louveau, A., Smirnov, I., Keyes, T. J., Eccles, J. D., Rouhani, S. J., Peske, J. D., Derecki, N. C., Castle, D., Mandell, J. W., Lee, K. S., Harris, T. H., & Kipnis, J. (2015). Structural and functional features of central nervous system lymphatic vessels. Nature, 523(7560), 337–341. https://doi.org/10.1038/nature14432

Noback, C. R., Ruggiero, D. A., Strominger, N. L., & Demarest, R. J. (Eds.). (2005). The human nervous system: structure and function (No. 744). Springer Science & Business Media.

Reese, T. S., & Karnovsky, M. J. (1967). Fine structural localization of a blood-brain barrier to exogenous peroxidase. The Journal of Cell Biology, 34(1), 207–217. https://doi.org/10.1083/jcb.34.1.207

Wagner, F. B., Mignardot, J.-B., Le Goff-Mignardot, C. G., Demesmaeker, R., Komi, S., Capogrosso, M., Rowald, A., Seáñez, I., Caban, M., Pirondini, E., Vat, M., McCracken, L. A., Heimgartner, R., Fodor, I., Watrin, A., Seguin, P., Paoles, E., Van Den Keybus, K., Eberle, G., Schurch, B., Pralong, E., Becce, F., Prior, J., Buse, N., Buschman, R., Neufeld, E., Kuster, N., Carda, S., von Zitzewitz, J., Delattre, V., Denison, T., Lambert, H., Minassian, K., Bloch, J., & Courtine, G. (2018). Targeted neurotechnology restores walking in humans with spinal cord injury. Nature, 563(7729), 65–71. https://doi.org/10.1038/s41586-018-0649-2