Into the Brain

The brain is the central part of our nervous system. It regulates many bodily functions and processes the inputs from our sensory organs.

It is arguably the most complex organ. It can be divided into regions. The largest ones visible from the outside being the cerebral cortex, the cerebellum and the brain stem. There are many more regions on the inside not visible in this depiction.

Each of these regions serve different functions. The cerebral cortex for instance takes part in sensory perception, generation of motor commands, conscious thought, and language. It is also the region we are going to dive into during this brief lesson.Scroll down to zoom in!

Scroll down to zoom in!

Cerebral Cortex

The neocortex makes up 90 % of the cerebral cortex. It can be divided into the frontal, temporal, and parietal lobe, the motor strip, sensory strip, and the occipital lobe.

Sulcus and Gyrus

Most non-primate species have a rather smooth brain with small crests and folds.  

The reason our cortex developed these folds is linked to our evolutionary past. As the brains of ancient human ancestors grew larger relative to skull volumes, the neocortex developed greater folding to fit inside the head.

The general pattern oft he gyri stays more or less the same in most brains, though they can vary in size and location. Gyri have been carefully mapped across the brain and are in some cases linked to specific functions, such as the control of certain body parts. Other areas such as the one in the center of this image do not have a clearly defined, single function.

White vs. Grey Matter

The brain consists of grey matter and white matter. As can be seen here the grey matter is on the surface at the brain. It owes its darker color to the numerous cell bodies it contains.

The white matter in contrast is primarily made up of bundles of neural fibers called axons, which transmit electrical information from one location to another. White matter fibers are wrapped in an electrically insulating sheath of lipids called myelin, which give this tissue its whitish hue.

The myelination along nerve tracts allows for fast and efficient electrical signaling from one brain region to another.

Cortical Layers

The neocortex is composed of six cortical layers, labeled “1” through “6” starting with the topmost layer closest to the surface and moving downwards towards the center of the brain. Each layer contains neurons of a characteristic size and shape, arranged with a certain density. These differences allow neurobiologists to distinguish between layers.   

The thickness of each layer varies depending on the region where it is found and its function in the neocortex.

Staining

The neuron is not naturally distinguishable from the background like these images would suggest. Brain tissue needs to be stained to make individual neurons visible.

In addition to neurons, there are blood vessels and specialized non-neuronal braincells called glial cells that occupy a significant fraction of the cortical mass. There are multiple types of glial cells, each serving various important functions, such as supplying neurons with nutrients from the blood stream, disposing of waste, and forming the electrically isolating myelin sheaths that surround some axons.

Several different techniques for staining brain tissue exist, each with various strengths and weaknesses. This particular image is a digital recreation of a staining method called “Nissl staining”.

Neuron

This is a neuron. Neurons can be identified by their unique morphology, or shape. Neuronal morphologies are usually separated into three parts: soma, dendrites, and axon.

Soma

The soma is the middle part connecting all the branches. It contains the nucleus and cell organelles.

Dendrites

Dendrites are tree-like that receive electrochemical information sent from other cells.

Axon

Axons are thin branch-like structures that extend from the soma to contact other cells. It serves as the outgoing cable through which the neuron can transmit signals to other neurons.

Dendrites

The mushroom-like knobs extending from the dendrites are called dendritic spines. They are the locations where the axons of other neurons can connect.

An average neuron has around 1’000 to 3’000 connections to other neurons. In an average human brain, there are around 86 billion neurons. This means that there may be hundreds of trillions of synaptic connections in the brain.

Synapse

A synapse is the region where the axon of a neuron approaches the dendrite on another and over which electrochemical signals are sent.

Usually this communication only goes one way. The neuron sending the signal is referred to as the presynaptic neuron and the neuron receiving it is called the postsynaptic neuron. The space between the two neurons is called the synaptic cleft and is between 20 and 500 nm large

When an electrical impulse traveling down the axon of the presynaptic neuron reaches the synapse, small membrane bubbles filled with neurotransmitters are released into the synaptic cleft, activating receptors in the postsynaptic neuron.

Membrane

The neuron has, like every other cell, a protective membrane that regulates what can enter and exit the cell.

In particular, neural membranes control the relative concentrations of electrically charged ions inside and outside the cell. The difference in the number of charged particles across the cell membrane determines the voltage, or ‘membrane potential’. By manipulating this potential, the neuron can create electrical pulses that travel like waves along its branches.

The charged ions involved in this electrical potential are usually sodium, potassium and calcium ions.

Membrane protein

This is our final stop on this brief introduction to the human brain.These small structures visible on the surface of the membrane are receptorproteins; in particular, ‘NMDA receptors’ and ‘AMPA receptors’. 
They are both activated by the release of glutamate and allow positivelycharged ions to enter the cell once they have been activated. Once the ionsenter the cell, they depolarize it and have successfully transmitted the signalfrom one neuron to the next.

The reason we stop at this point of magnification is that the visual appearance of anything beyond this point is mostly speculative. Even the image visible right now should be seen as a model—there is no imaging technique to prove it right.