Before we ask what causes a neurological or mental disorder, we need to understand one simple fact:

The nervous system is a living part of the body. It does not function independently of the rest of the body.

The brain, spinal cord, and peripheral nerves are made of living cells. Like every other living tissue, they need oxygen, energy, nutrients, water, electrolytes, blood supply, hormonal support, immune regulation, and a stable internal environment.

The nervous system also needs proper structural protection and continuous maintenance.

So although we may study the nervous system separately, the body does not operate in separate compartments.

The nervous system works continuously with the cardiovascular, respiratory, digestive, endocrine, urinary, immune, musculoskeletal, integumentary, and other systems.

This gives us the first principle:

The nervous system is one system within one integrated body.

The Nervous System Depends on the Rest of the Body

The brain may regulate many activities of the body, but it cannot provide all the conditions required for its own survival and function.

It depends on other systems.

The respiratory system provides oxygen.

The digestive system provides nutrients and metabolic substrates.

The cardiovascular system transports oxygen and nutrients to neural tissue and removes metabolic products.

The urinary system helps maintain the proper balance of water, electrolytes, and acid-base conditions.

The endocrine system provides hormones that influence metabolism, development, growth, reproduction, stress responses, and brain function.

The immune system provides protection but must itself remain properly regulated because excessive or misdirected immune activity can damage neural tissue.

The skeletal system provides physical protection for the brain and spinal cord.

The integumentary system contributes to protection and temperature regulation.

Therefore, normal nervous-system function is possible only because the rest of the body continuously maintains the environment in which neural tissue can live and operate. The document describes this dependence in terms of oxygen, nutrients, water, electrolytes, blood supply, hormonal and immune regulation, structural support, and temperature regulation.

The Nervous System Also Works in the Opposite Direction

The relationship is not one-way.

The other systems support the nervous system, but the nervous system also regulates many of those systems.

It influences:

heart rate;
blood-vessel tone;
breathing;
digestion;
glandular activity;
body temperature;
skeletal-muscle activity;
reproductive functions;
and many other physiological processes.

So the relationship is better represented as:

Body systems → sustain the nervous system

Nervous system → regulates body systems

This creates a continuous feedback relationship rather than a simple chain.

That is why it is misleading to think of the brain as a completely independent "control centre."

It is part of the body it regulates and depends on the body it regulates.

The Nervous System Is One System, With Two Major Divisions

The nervous system itself is not two separate systems.

It is one system divided anatomically into:

Central Nervous System — CNS

The CNS consists of:

Brain + spinal cord

It is the major site of information processing and integration.

Peripheral Nervous System — PNS

The PNS consists of neural structures outside the brain and spinal cord, including the peripheral nerves.

It connects the CNS with the rest of the body.

Information continuously travels in both directions:

Body → PNS → CNS

and

CNS → PNS → Body

Therefore:

CNS and PNS are two divisions of one nervous system, not two independent systems.

And the nervous system itself is only one component of the larger integrated organism.

What Does the Nervous System Actually Have to Maintain?

To understand neurological disease, we need to go one step deeper.

A neuron is a living cell that must maintain a highly controlled electrical and chemical state.

It must maintain ion gradients across its membrane.

Ion channels must open and close correctly.

Electrical signals must travel along axons.

Neurotransmitters must be released at synapses.

Receptors must respond appropriately.

Neighbouring neurons must receive and process those signals.

Glial cells must maintain the environment around neurons.

Blood vessels must continuously supply the tissue.

All of these processes have to work together.

The basic chain is:

Genes

↓

Cells

↓

Molecular processes

↓

Electrical activity

↓

Synaptic communication

↓

Neural networks

↓

Nervous-system function

↓

Behaviour and experience

A disturbance anywhere along this chain can affect nervous-system function.

This is the biological foundation we need before discussing disease.

A Neurological Disorder Does Not Always Mean Visible Brain Damage

This is one of the most important points.

There are two broad ways nervous-system function can become abnormal.

1. Structural abnormality

The physical structure has been damaged, altered, lost, compressed, or otherwise changed.

Examples include:

stroke-related tissue injury;
traumatic brain or spinal injury;
tumour;
demyelination;
neuronal loss;
developmental abnormalities.

In such cases, structural changes may sometimes be demonstrated through imaging, pathology, microscopy, or other investigations.

2. Functional abnormality

The physical structure may appear relatively intact, but its operation is abnormal.

For example:

electrical activity may become abnormal;
neural networks may become excessively synchronized;
synaptic communication may change;
neuronal excitability may increase or decrease;
information may be processed abnormally.

Therefore:

Normal-looking anatomy does not necessarily mean normal physiology.

The absence of an obvious structural lesion does not mean that nothing biological is happening.

This is especially important when discussing functional neurological disorders.

"Functional" should not be understood as meaning imaginary or unreal.

It means that the abnormality is primarily expressed in how the nervous system is functioning, rather than as an explanatory structural lesion visible on conventional examination.

The Neuron Does Not Have to Die for Function to Be Lost

This is another simple but important idea.

A neuron can be physically present and still fail to function normally.

Consider electrical signaling.

A neuron depends on ion gradients across its membrane. These gradients allow it to generate electrical signals.

If ion-channel function changes, neuronal excitability can change.

If neurotransmitter release changes, communication between neurons changes.

If receptors change, the receiving neuron may respond differently.

If synaptic connections change, the behaviour of the whole network can change.

Therefore:

A neurological disorder does not necessarily require destruction of neurons. Changing how neurons operate can be sufficient to change nervous-system function.

The original document makes this distinction clearly: changes in ion channels, receptors, neurotransmitter release, or synaptic connections can alter neural function even without destruction of the neuron itself.

And the Neuron Is Not Alone

The nervous system is not made only of neurons.

It also contains several types of glial cells.

These cells help maintain the environment in which neurons function.

For example:

astrocytes contribute to metabolic and chemical regulation around neurons;
oligodendrocytes produce myelin in the CNS;
microglia provide immune surveillance;
Schwann cells produce myelin around peripheral nerves.

Therefore, normal neural function depends on more than the neuron itself.

A useful way to think about it is:

Neurons + glia + blood supply + chemical environment + immune regulation

↓

Normal neural function

A disturbance in any of these components can potentially produce neurological dysfunction.

The Nervous System Depends on a Continuous Supply of Energy

The brain is particularly dependent on continuous energy delivery.

Neurons require oxygen and metabolic substrates, particularly glucose.

Consider what happens if blood flow to part of the brain is suddenly interrupted:

Blood flow ↓

↓

Oxygen and substrate delivery ↓

↓

Energy production ↓

↓

Ion gradients cannot be maintained

↓

Electrical signaling fails

↓

Neuronal function fails

↓

Neurological dysfunction

If the problem is severe or prolonged, neuronal injury and cell death can follow.

This is the physiological basis of an ischemic stroke.

Notice something important here:

The neurological dysfunction occurs in the nervous system, but the initiating problem is vascular.

This distinction is essential for understanding the root cause.

Where the Problem Appears Is Not Always Where It Begins

This may be the most important principle in the entire section.

Suppose the nervous system is structurally intact.

But the cardiovascular system fails to deliver adequate blood flow.

The brain then receives insufficient oxygen and metabolic substrate.

The result is neurological dysfunction.

So:

Where the dysfunction appears

may be

the nervous system

while

where the disturbance began

may be

another body system.

The same principle applies to metabolic, endocrine, renal, immune, infectious, and toxic disturbances.

For example:

Severe metabolic disturbance

↓

Abnormal internal environment

↓

Neuronal function affected

↓

Neurological symptoms

Likewise:

Renal dysfunction

↓

Altered electrolyte/metabolic balance

↓

Altered neuronal excitability

↓

Neurological manifestations

And:

Immune dysregulation

↓

Damage to neural tissue, myelin, receptors, or synapses

↓

Altered nervous-system function

↓

Neurological symptoms

The nervous system is therefore dependent upon the physiological stability maintained by the rest of the body.

The Same Principle Applies to Mental Function

This is where the discussion becomes particularly important for mental disorders.

Memory, attention, emotion, perception, motivation, decision-making, consciousness, and behaviour are not abstract functions floating outside biology.

They are functions produced by the activity of the nervous system.

That activity depends upon:

neurons

synapses
neurotransmitters
receptors
electrical activity
neural networks
blood supply
energy metabolism
hormonal regulation
immune regulation
the internal chemical environment

Therefore, when mental function becomes abnormal, the question should not stop at the level of the observed behaviour.

We should ask:

What has changed in the biological processes that produce and regulate that function?

This does not mean that every mental disorder has one simple biological cause.

It means that mental functions are produced by a biological nervous system and therefore depend upon its biological operation.

A Neurotransmitter Is Not a Diagnosis

It is tempting to explain a mental or neurological disorder by saying:

"This chemical is too high."

or

"That chemical is too low."

But nervous systems are more complicated than this.

A neurotransmitter does not act in isolation.

Its effect depends on:

which receptor receives it;
where that receptor is located;
which cell receives the signal;
what intracellular pathway is activated;
which neural network is involved;
and when the signal occurs.

Therefore, nervous-system disorders often involve network physiology, rather than simply an abnormal concentration of one chemical.

This is why a biological explanation should go beyond naming a neurotransmitter.

We need to understand what the neural network is doing.

The Brain Works as a Network

The brain is not a collection of independent neurons.

It is an interconnected network.

A change in one region can influence other regions because information continuously travels through neural connections.

The sequence can therefore be:

Cellular change

↓

Altered neural signaling

↓

Altered network activity

↓

Altered communication between regions

↓

System-level dysfunction

↓

Clinical symptom

This explains why the final symptom may sometimes seem very different from the original biological disturbance.

Why Different Diseases Produce Different Symptoms

Different neural systems perform different functions.

Therefore, disturbing different parts of the nervous system produces different results.

Damage or dysfunction of motor pathways can produce:

weakness or abnormal movement

Sensory pathways can produce:

numbness, altered sensation, or pain

Visual pathways can produce:

visual disturbance

Cerebellar systems can produce:

poor coordination and balance

Autonomic pathways can produce:

disturbances in involuntary functions

Networks involved in cognition can produce:

problems with memory, attention, language, or executive function

So a symptom is an output of the nervous system.

It is not necessarily the mechanism that caused the problem.

One Symptom Can Have Many Biological Causes

Take weakness.

The same outward symptom can result from dysfunction at different levels:

Brain motor cortex

↓

Descending motor pathway

↓

Spinal cord

↓

Peripheral nerve

↓

Neuromuscular junction

↓

Muscle

The person experiences the final result as reduced movement, but the biological cause can be completely different.

That is why neurological diagnosis requires asking two questions:

Where is the function abnormal?

and

What biological mechanism produced that abnormality?

The symptom tells us what is happening.

Physiology helps us understand why it is happening.

The Major Biological Routes to Nervous-System Dysfunction

Neurological disorders can enter this system through many different routes.

Genetic

A genetic alteration can affect neural development, ion channels, receptors, proteins, metabolism, mitochondria, myelin, or cellular maintenance.

Vascular

Abnormal blood flow can deprive neural tissue of oxygen and metabolic substrates or cause bleeding.

Traumatic

Mechanical forces can damage neurons, axons, myelin, blood vessels, and supporting structures.

Infectious

Infections can directly affect neural tissue or indirectly affect it through inflammation, immune responses, blood-brain-barrier changes, or metabolic disturbances.

Immune-mediated

The immune system can mistakenly target myelin, neurons, receptors, synapses, peripheral nerves, or neuromuscular-junction components.

Degenerative

Neurons and their supporting systems progressively lose structural and functional integrity.

Metabolic

Abnormal glucose, oxygen availability, electrolytes, vitamins, mitochondrial function, or other metabolic processes can interfere with neural function.

Toxic

Certain substances can interfere with neuronal metabolism, membrane function, neurotransmission, or axonal integrity.

Neoplastic

Tumours can affect nervous-system function through invasion, compression, altered blood supply, or other biological effects.

Developmental

Abnormal development can alter the formation, migration, connectivity, or maturation of neural structures.

Functional

Neural networks can operate abnormally even when conventional structural investigations do not demonstrate an explanatory lesion.

These categories are not necessarily separate.

A single disorder can involve several of them at the same time.

For example:

Genetic susceptibility

↓

Environmental trigger

↓

Immune or metabolic disturbance

↓

Cellular dysfunction

↓

Network dysfunction

↓

Clinical symptoms

So we should not always search for one cause when several biological processes may be interacting.

From Cause to Symptom

The entire process can now be understood without making it unnecessarily complicated.

Cause or initiating disturbance

↓

Biological change

↓

Cellular dysfunction

↓

Altered neural signaling

↓

Altered network activity

↓

Nervous-system dysfunction

↓

Symptoms

The initial disturbance may be genetic, vascular, traumatic, infectious, immune, metabolic, toxic, degenerative, developmental, or functional.

The biological change may involve proteins, energy production, ion channels, myelin, neurotransmission, synapses, or inflammation.

The network may then develop abnormal electrical activity, impaired conduction, altered connectivity, abnormal synchronization, or impaired information processing.

Finally, the person experiences the result as a neurological or mental symptom.

This is the bridge between biology and what the person experiences.

The Nervous System Is Not a Fixed Machine

There is one final feature we must add.

The nervous system is dynamic.

It continuously changes in response to experience and physiological conditions.

Synaptic strength can change.

Neural circuits can reorganize.

Receptors can change.

Gene expression can change.

Metabolism can change.

Immune activity can change.

Connections can strengthen or weaken.

This ability to change is part of neuroplasticity.

This plasticity is essential for learning, adaptation, and recovery.

But it also means that abnormal patterns of neural activity can sometimes become established.

Therefore, nervous-system function at any particular moment depends on two things:

the biological structure that exists

and

how that structure is currently functioning and adapting.

The Central Principle

We can now bring everything together.

The human body is one integrated organism.

Its physiological systems are specialized, but they are not independent.

The nervous system is one of those systems.

The nervous system itself consists of the CNS and PNS, working as one interconnected system.

Neural tissue depends continuously on the rest of the body for:

oxygen

energy

nutrients

blood flow

electrolyte balance

hormonal regulation

immune regulation

temperature control

structural protection

and removal of metabolic waste.

At the same time, the nervous system regulates many of the systems that sustain it.

Therefore, the relationship is:

The body sustains the nervous system

↕

The nervous system regulates the body

A disturbance can begin within neural tissue itself, or it can begin elsewhere in the body and eventually disturb neural function.

Either way, when nervous-system structure or function becomes abnormal, neurological or mental manifestations can appear.

This leads to a more precise central statement:

Mental and neurological functions are produced by biological processes within an integrated nervous system. Disorders arise when the structure, chemistry, electrical activity, metabolism, connectivity, regulation, or adaptation of that system becomes abnormal—whether the initiating disturbance originates within neural tissue or comes from another physiological system on which the nervous system depends.

And perhaps the simplest way to remember the entire concept is:

The nervous system is not separate from the body.

It is part of the body.

The body sustains the nervous system.

The nervous system regulates the body.

And when the biological conditions required for normal neural function are disturbed, the disturbance can ultimately appear as a neurological or mental disorder.