The Brain: Anatomy, Functions, and Common Diseases

The brain is the central organ of the nervous system, integrating sensory information, controlling movement, and shaping speech, emotions, memory, and complex behavior. It's helpful to start with three major blocks: the cerebrum, the cerebellum, and the brainstem. This "skeleton" helps us avoid getting bogged down in detail and understand why different symptoms indicate different areas of damage. [1]

Brain weight in adults varies greatly and is not, by itself, an indicator of intelligence or the "quality" of brain function. Popular literature often cites average values of around 1300-1400 g, but the clinical value of these figures is limited because what's more important is not "weight" but the connectivity of networks, the state of blood vessels, neurotransmitter systems, and plasticity. [2]

A key principle of modern understanding of the brain is its network-based nature. Even "localized" functions, such as speech or memory, are typically mediated by the interaction of multiple cortical and subcortical areas. Therefore, the same diagnosis may manifest differently in different people, and a small focal lesion at a "nodal" point in a network sometimes produces more pronounced symptoms than a larger lesion in a less critical area. [3]

In practical terms, the brain should be viewed as a system of gray matter, where information is processed, and white matter, which provides connections between areas. Disruptions to these pathways often produce "disconnected" symptoms, where individual functions are preserved but their coordination is impaired, such as in vascular lesions or myelin disorders. [4]

Table 1. 3 main parts of the brain and their basic functions

Department	Main functions	Typical symptoms of infection
Big brain	Consciousness, thinking, speech, voluntary movements, sensory integration	Weakness, speech impairment, sensitivity, attention, memory
Cerebellum	Coordination, balance, precision of movement, motor learning	Unsteadiness, nystagmus, dysmetria, scanned speech
Brain stem	Respiration, heart rate, wakefulness, pathways, cranial nerve nuclei	Impaired consciousness, swallowing, breathing, double vision, facial asymmetry

[5]

The cerebrum: the cortex, lobes, subcortical nodes, and why symptoms are “tied” to a zone

The cerebral cortex is the outer layer of gray matter, home to neural fields responsible for perception, planning, speech, and motor control. The cortex is typically described as divided into four lobes: the frontal, parietal, temporal, and occipital. This division simplifies understanding, although actual functions are often more widely distributed and overlap. [6]

The frontal lobe is important for executive functions, behavioral control, planning, and motor programing. It also contains an area critical for motor speech. Damage to the frontal lobes can manifest as weakness, personality changes, decreased critical thinking, impaired initiative, and difficulty finding words while maintaining speech comprehension. [7]

The parietal lobe is involved in sensory processing, spatial attention, and the integration of signals from different sensory systems. Damage can result in disturbances of the body schema, spatial ignorance, difficulty recognizing objects by touch, and impairments in complex skills that depend on the "assembly" of sensory information. [8]

Subcortical structures, including the basal ganglia and thalamus, are not "secondary" but rather systemically important nodes for movement, motivation, filtering sensory input, and attentional switching. Therefore, strokes in the deep regions often result in a combination of motor and sensory disturbances, as well as changes in movement rate and tone. [9]

Table 2. Cortex lobes and the most typical manifestations of damage

Share	Key Features	Common signs of damage
Frontal	Voluntary movements, planning, behavioral control, motor speech	Weakness, apathy or disinhibition, difficulties with speech production
Parietal	Sensitivity, spatial attention, integration	Numbness, ignoring part of space, disturbances of praxis
Temporal	Auditory processing, memory, speech comprehension, emotions	Memory impairment, speech comprehension, epileptic phenomena
Occipital	Vision and visual recognition	Visual field loss, visual illusions

[10]

The brainstem and cerebellum: the "autopilot" of vital functions and the precision of movements

The brainstem connects the cerebral hemispheres, the cerebellum, and the spinal cord. It includes the midbrain, pons, and medulla oblongata. The brainstem contains major pathways and the nuclei of many cranial nerves, so even a small lesion here can cause many different symptoms simultaneously. [11]

The medulla oblongata and other parts of the brainstem are involved in the regulation of breathing, heart rate, blood pressure, and alertness. This explains why brainstem lesions are considered potentially life-threatening and why sudden impairment of consciousness, breathing, or swallowing requires urgent medical evaluation. [12]

The cerebellum is traditionally associated with coordination, balance, and precision of movement, but modern evidence demonstrates its contribution to cognitive functions through connections with the cortex. Clinically, cerebellar damage often manifests as unsteadiness of gait, tremors when moving toward a target, impaired precision, nystagmus, and changes in speech rate. [13]

A useful practical hint: a "cerebellar" problem often worsens when attempting to perform a precise movement and is accompanied by impaired coordination, whereas "muscle weakness" from damage to the cortex or pathways results in the inability to perform a movement due to decreased strength. This helps to suggest at the initial examination level which area is involved. [14]

Table 3. Brainstem regions and functions critical to life

Department	What controls	Typical symptoms of damage
Midbrain	Level of wakefulness, eye movements, pathways	Impaired consciousness, double vision, pupillary asymmetry
Bridge	Connections with the cerebellum, facial functions, pathways	Weakness of facial expressions, impaired coordination, oculomotor symptoms
Medulla oblongata	Breathing, heart rate, swallowing, important reflexes	Dysphagia, respiratory distress, severe weakness

[15]

Brain protection: membranes, barriers, ventricles and cerebrospinal fluid

The brain is protected by three membranes: the dura mater, the arachnoid mater, and the pia mater. Between the membranes are spaces, one of which, the subarachnoid space, is filled with cerebrospinal fluid. These structures not only provide "packaging" but also actively contribute to mechanical protection, venous drainage, and maintenance of a stable environment. [16]

Blood and brain tissue are separated by the blood-brain barrier. It consists of endothelial cells, pericytes, a basement membrane, and astrocyte terminals. The barrier limits the penetration of many substances, helps maintain ionic balance, and protects the brain from toxins and pathogens, but damage to it is associated with many neurological diseases and complications. [17]

The ventricular system of the brain is filled with cerebrospinal fluid, which is formed primarily by the choroid plexuses. This fluid distributes mechanical stress, helps remove metabolites, and maintains chemical stability. Impaired circulation or absorption can lead to hydrocephalus, which is a pathological dilation of the ventricles with increased intracranial pressure or, in some cases, functional impairment without a significant increase in pressure. [18]

It is clinically important to recognize the signs of increased intracranial pressure and cerebrospinal fluid flow problems: progressive headache, nausea, vomiting, drowsiness, gait and vision disturbances. Infants may experience rapid growth of the head circumference and a bulging fontanelle. With such symptoms, management is determined by urgency and usually requires neuroimaging. [19]

Table 4. Cerebrospinal fluid and ventricles: what is considered clinically significant

System node	Norm and role	What happens if there is a violation?
Choroid plexuses	The main source of cerebrospinal fluid	Changes in blood-cerebrospinal fluid production or barrier
Ventricles and cerebral aqueduct	Fluid circulation paths	Flow block and ventricular dilation
Subarachnoid space	Depreciation, distribution	Impaired absorption, increased pressure
Intracranial pressure	Stable, normal	Headache, vomiting, drowsiness, visual disturbances

[20]

Blood supply and the most common brain diseases: What to know for early detection

The brain is critically dependent on constant blood flow. The primary sources of supply are the internal carotid and vertebral arteries, and at the base of the brain, their branches connect to form a collateral artery, which can provide collateral blood flow in the event of problems in one of the vascular beds. However, collaterals are not always "saving," so sudden neurological symptoms are considered an emergency. [21]

Stroke is a leading cause of death and disability. Typical symptoms include sudden weakness or numbness on one side of the body, speech impairment, sudden vision problems, coordination problems, or a severe headache with no clear cause. With these symptoms, it is important to seek emergency medical attention as quickly as possible, as the effectiveness of treatment is largely time-sensitive. [22]

Dementia is a syndrome in which memory, thinking, and the ability to function independently in everyday life are progressively impaired. The World Health Organization emphasizes that dementia has not only medical but also social consequences, and describes key risk factors and the importance of early recognition. The causes of dementia are heterogeneous: neurodegenerative and vascular mechanisms often coexist. [23]

Globally, neurological diseases pose a significant burden on public health. The World Health Organization lists stroke, migraine, dementia, epilepsy, meningitis, and tumors of the nervous system as among the most significant in terms of health loss. This provides a useful reference point for educational materials to go beyond anatomy and explain which symptoms require attention and risk factor prevention. [24]

Table 5. Common brain disorders and symptoms that require urgent evaluation

State	Typical mechanism	Warning signs
Stroke	Acute circulatory disorder	Sudden weakness, speech, vision, coordination, severe headache
Subarachnoid hemorrhage	Rupture of a blood vessel in the subarachnoid space	Sudden, very severe headache, vomiting, loss of consciousness
Hydrocephalus	Impaired circulation or absorption of cerebrospinal fluid	Progressive headache, drowsiness, gait and vision disturbances
Dementia	Progressive damage to neural networks, often of mixed causes	Increasing memory and independence problems, behavioral changes
Meningitis	Inflammation of the meninges	Fever, neck stiffness, confusion, severe weakness

[25]