Our brain determines perceptions, actions, ideas and emotions, and even our character. Genetic predisposition plays as much of a role as our own experiences and the influences of the environment and people around us. Information enters our brain via the senses, such as sight, touch, hearing or taste. It’s only there that a unique individual image of the world is created. At any given point in time, countless conscious and unconscious processes are running in the human brain. The brain itself changes as a result.

Major advancements in microscopy and other imaging processes are showing with increasing precision how our brain works. But this extraordinarily complex organ continues to pose many questions for scientists. Findings in brain research are not only important in medicine, they also have effects on social areas such as education, parenting and legal practice.

How is the brain structured?
The human brain is the most complicated organ Nature has ever created. It can outperform even the most powerful supercomputers. The brain is organised in circuits at different levels: from the processes at a single synapse to networks between millions of cells. The human brain is consists of various regions, such as the cerebrum, cerebellum and brain stem, which play different roles. But for many functions, different regions of the brain have to work together. For this reason both adjacent nerve cells and cells in far-apart regions are connected to each other ©
Cortex
The cerebral cortex, known simply as the cortex, is the outermost neural tissue covering most of the other brain regions.. With its furrows and convolutions it gives the brain a walnut-like appearance. The cortex controls perception, consciousness and behaviour. It allows us to communicate and solve difficult tasks, as well as recognising and classifying objects.

Frontal lobe
The entire front part of the cortex is called the frontal lobe. It controls conscious movement, especially speed, direction and strength. Many scientists also locate the higher cognitive functions of humans here and refer to the frontal lobe as the “carrier of culture”. The frontmost area of the frontal lobe is responsible for attention, deliberation, decision and planning – and it is also considered to be where the personality is located.

Temporal lobe
The best-known function of the temporal lobe is hearing. The auditory centres occupy almost the entire surface of the temporal lobe. Language and music probably require such a high “processing power”. But the temporal lobe is also needed for many other things, for smell, speech, understanding, image recognition and forming memories.

Hippocampus
The hippocampus is a “curled up” section of the cortex and a central element of the limbic system. It’s important for storing knowledge and experiences – anyone without this won’t be able to remember new things. The hippocampus is one of the few areas of the brain in which new nerve cells are made throughout life.

Limbic system
The limbic system is a group of brain areas that are of great importance for the production and processing of emotions, and for memory processes. The most important ones are the hippocampus, the amygdala, the gyrus cinguli and the gyrus parahippocampalis. These brain areas are closely linked with each other. The limbic system controls our emotions and our sexuality, as well as evaluating the importance of information about the outside world.

Hypothalamus
The hypothalamus controls important functions such as reproduction, nutrition, temperature regulation and time measurement. It’s a superordinate centre of the autonomous nervous system that controls unconscious processes, for instance breathing or heartbeat. The rear section of the hypothalamus belongs to the limbic system.

Pituitary gland
The pituitary gland (hypophysis) is only around the size of a pea – but it’s vital. As the “master gland” it controls the body’s endocrine system. It is controlled by the hypothalamus and secretes hormones into the blood. This regulates body functions like growth and reproduction, as well as metabolism.

Cerebellum
The cerebellum is located at the back of the skull. From a perspective of evolutionary history it’s an ancient part of the brain. The connections between the nerve cells are far less complex here than they are in the cerebrum. The cerebellum coordinates motor skills such as posture and walking, but also complex motion sequences such as writing. Despite its small size the cerebellum contains four times as many cells as all the rest of the brain put together.

Brain stem
The brain stem is directly connected to the spinal cord and might be described as the brain’s “technology centre”. No larger than your thumb, the brain stem controls and regulates unconscious vital processes in the body including circulation, breathing and sleep. It is the oldest part of the brain in terms of developmental history. For this reason the differences between humans and animals are comparatively small here.

Communication is everything
Our brain is a complex network of billions of nerve cells – or neurons – in perpetual communication with each other. Connections are constantly being created or separated, strengthened or weakened. This is also the basis on which we are able to learn and forget. The nerve cells receive electrical impulses via the dendrites and send these to the neuron body. From there they are conducted via the axon to other nerve cells. Transmission from one cell to another happens at the synapses. At this point the electrical impulse is translated into a chemical impulse. There are nerve cells in the brain that receive signals from up to 10,000 other nerve cells, and neurons that pass signals on to thousands of others.
  The nerve cells in the brain are arranged in layers. These layers and their many connections are the basis for rapid processing of information.

Thought highways
The allocation of specific functions to individual brain regions does not explain the brain’s complex performance – action, emotion and attention for example depend on each other. Cognitive performance, such as arithmetic, is only possible as a result of the complicated circuitry involving different brain regions. Large bundles of nerve fibres are routed around the brain, connecting the cells in different areas to form “transregional” links. With the help of diffusion-weighted magnetic resonance imaging (DW-MRI), scientists can map the interconnectivity of the brain areas in the living human brain. The technology is non-invasive, low-risk and very accurate. The diffusion movement of water molecules in the tissue is measured. These molecules can move faster and more easily along the nerve fibre bundles than across them. Then researchers translate the measured diffusion gradients into brightly coloured patterns.
  © Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig / Ralph Schurade, Alfred Anwander / Visualisation software: Fibernavigator 2
The large nerve fibre bundles can be made visible using DW-MRI. The colours show the direction of the fibres.

More than just service for neurons
As well as the nerve cells, there’s another type of cell in the brain, known as glial cells. Without these, nothing at all would work in our heads. Glial cells form the fundamental structure of the brain and therefore help to process information fast. They transport nutrients to the nerve cells and remove their waste products. The layer providing the long nerve fibres with electrical insulation is also made up of glial cells. This is the basis of the fast neural conduction typical of vertebrates. Max Planck scientists in Göttingen are researching the importance of glial cells in neurological and psychiatric diseases. Munich-based neurobiologist Magdalena Götz has discovered that in the brain, even the nerve cells develop from glial cells. Now she is investigating whether new nerve cells could also arise from glial cells in a fully-developed brain – for instance following severe brain injury or stroke.
  © Max Planck Institute of Neurobiology, Martinsried / Volker Staiger
If the brain is injured, specific glial cells are activated: microglia (shown here in red) and astrocytes (green) support, protect and nourish the nerve cells (turquoise) to enable them to recover.

A wiring diagram for the brain
The entirety of all nerve connections in a living being is known as the connectome. This term is supposed to convey that the nerve cells are linked together in a robust network and can only be understood through the way they relate to each other. The connectome of the human brain is very complex. For this reason scientists explore the fundamental principles using brains with a simpler structure, such as in mice. In 2019 researchers at the Max Planck Institute for Brain Research were able to make the connections in a tiny slice of mouse brain more precisely visible than ever before: a wiring diagram between around 7,000 axons, with over two and a half metres of neuronal “cable”, linked together via almost 400,000 synapses. To achieve this, they use a new method of imaging based on artificial intelligence (AI). With this technique they have also been able to show for the first time that the arrangement of new synapses follows fixed patterns.
  © Reprinted with permission from A Motta et al., Science. DOI: 10.1126/science.aay3134
A small section of the cerebral cortex of a mouse – reconstructed using AI-based imaging software.