White matter refers to the areas of the CNS with the majority of axons, the long cords that extend from neurons. Most axons are coated in myelin – a white, fatty insulating cover that helps nerve signals travel quickly and reliably. In the brain, white matter is buried under the grey surface, carrying signals across different parts of the brain. In the spinal cord, white matter is the external layer surrounding the grey core.
If the CNS is the processing center of the human body, the brain is its headquarters. It is broadly organized into three main regions: the forebrain, the midbrain, and the hindbrain. The largest of these three is the forebrain (derived from the prosencephalon in the developing brain). It contains the large outermost layer of the brain, the wrinkly cerebral cortex, and smaller structures towards its centre, such as the thalamus, hypothalamus, and pineal gland.
The midbrain (derived from the mesencephalon in the developing brain) serves as the vital connection point between the forebrain and the hindbrain. It’s the top part of the brainstem, which connects the brain to the spinal cord.
The hindbrain (derived from the rhombencephalon in the developing brain) is the lowest back portion of the brain, containing the rest of the brainstem made up of the medulla oblongata and the pons, and also the cerebellum – a small ball of dense brain tissue nestled right against the back of the brainstem.
NEURONS
Neurons (also called neurones or nerve cells) are the fundamental units of the brain and nervous system, the cells responsible for receiving sensory input from the external world, sending motor commands to our muscles, and transforming and relaying the electrical signals at every step in between. More than that, their interactions define who we are as people. Having said that, our roughly 100 billion neurons do interact closely with other cell types, broadly classified as glia (these may outnumber neurons, although it’s not known).
The creation of new neurons in the brain is called neurogenesis, and this can happen even in adults.
What does a neuron look like? A useful analogy is to think of a neuron as a tree. A neuron has three main parts: dendrites, an axon, and a cell body or soma (see image below), which can be represented as the branches, roots and trunk of a tree, respectively. A dendrite (tree branch) is where a neuron receives input from other cells. Dendrites branch as they move toward their tips, just like tree branches do, and they even have leaf-like structures on them called spines.
The axon (tree roots) is the output structure of the neuron; when a neuron wants to talk to another neuron, it sends an electrical message called an action potential throughout the entire axon. The soma (tree trunk) is where the nucleus lies, where the neuron’s DNA is housed, and where proteins are made to be transported throughout the axon and dendrites.
There are different types of neurons, both in the brain and the spinal cord. They are generally divided according to where they originate, where they project to and which neurotransmitters they use.
Axon – The long, thin structure in which action potentials are generated; the transmitting part of the neuron. After initiation, action potentials travel down axons to cause the release of neurotransmitters.
Dendrite – The receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic inputs determining whether the neuron will fire an action potential.
Spine – The small protrusions found on dendrites that are, for many synapses, the postsynaptic contact site.
Action potential – A brief electrical event typically generated in the axon that signals the neuron as ‘active’. An action potential travels the length of the axon and causes the release of neurotransmitters into the synapse. The action potential and consequent transmitter release allow the neuron to communicate with other neurons.
PARTS OF THE BRAIN
The brain’s cerebral cortex is the outermost layer that gives the brain its characteristic wrinkly appearance. The cerebral cortex is divided lengthways into two cerebral hemispheres, each of which traditionally has been divided into four lobes: frontal, parietal, temporal, and occipital.
Although we now know that most brain functions rely on many different regions across the entire brain working in conjunction, it is still true that each lobe carries out the bulk of certain functions.
FOREBRAIN
By far the largest region of your brain is the forebrain (derived from the developmental prosencephalon), which contains the entire cerebrum and several structures directly nestled within it – the thalamus, hypothalamus, pineal gland, and the limbic system.
Cerebrum and the cerebral cortex. When you picture the iconic shape of the human brain, the majority of what’s visible is the cerebrum, with its wrinkly, pinkish-grey outer appearance. It makes up around 85% of the brain and consists primarily of grey matter, divided into two hemispheres.
The cerebrum is where most of the important brain functions happen, such as thinking, planning, reasoning, language processing, and interpreting and processing inputs from our senses, such as vision, touch, hearing, taste, and smell.
The outer layer of the cerebrum is called the cerebral cortex, and in each hemisphere, it is traditionally divided into four lobes – frontal, parietal, occipital, and temporal. Communications between the two hemispheres are maintained by a fibrous bridge called the corpus callosum, which is formed in utero.
Beneath the surface of the hemispheres are large knots of neurons called basal ganglia, which specialize in programming and executing our motor functions. When basal ganglia are affected by diseases such as Parkinson’s, patients have tremors and uncontrolled movements.
Control centres for making sense of our bodies. Apart from the cerebrum, the forebrain also contains several small but highly important structures located towards the centre of the brain and are included in the limbic system. Collectively, these are called the diencephalon, and they regulate things like the body’s sensory perception, motor functions, and hormones.
The thalamus consists of two lobes of grey matter tucked away right under the cerebral cortex. It is a prime processing centre for sensory information, as it links up the relevant parts of the cerebral cortex with the spinal cord and other areas of the brain important for our senses. The thalamus also controls sleep.
The hypothalamus is quite small, only about the size of an almond. As its name suggests, it can be found right underneath the thalamus, and despite its small size, it is actually the major control centre of the autonomic motor system. It is involved in some hormonal activity and connects the hormonal and nervous systems. The hypothalamus also works to regulate blood pressure, body temperature, and overall homeostasis.
The pineal gland is even smaller than the hypothalamus – only about the length of a grain of rice – and is tucked between the two lobes of the thalamus. It is shaped like a tiny pine cone, and its main job is to produce the hormone melatonin, which regulates our sleep-wake cycles. Just like the hypothalamus, it is also involved in regulating hormonal functions.
Bumps and grooves of the brain. In humans, the lobes of the brain are divided by bumps and grooves. These are gyri (bumps) and sulci (grooves or fissures). The folding of the brain and the resulting gyri and sulci increase its surface area and enable more cerebral cortex matter to fit inside the skull.
Frontal Lobe. The frontal lobe is separated from the parietal lobe by a space called the central sulcus and from the temporal lobe by the lateral sulcus.
The frontal lobe is generally where higher executive functions, including emotional regulation, planning, reasoning, and problem-solving, occur. This is why in frontotemporal dementia, personality changes are often the first signs of the disease.
The most famous case of frontal lobe dysfunction is the story of railway worker Phineas Gage. In 1848, Gage was using a tamping iron to pack in gunpowder for blasting a tunnel through rock. While his head was slightly turned, a mistaken strike sparked an explosion that forced the rod upwards into his left eye and out through his skull.
Miraculously, Gage survived, blinded in his left eye and sustaining damage to much of his left frontal lobe. After the accident, others noticed changes in Gage’s personality: before the accident, he was known as responsible and hard-working, but afterward, he became disrespectful and foul-mouthed and had difficulty carrying out plans.
The frontal lobe also contains the primary motor cortex, the major region responsible for voluntary movement.
Parietal lobe. The parietal lobe is behind the frontal lobe, separated by the central sulcus. Areas in the parietal lobe are responsible for integrating sensory information, including touch, temperature, pressure, and pain.
Because of the processing that occurs in the parietal lobe, we can, for example, discern from touch alone that two objects touching the skin at nearby points are distinct rather than one object. This process is called two-point discrimination. Different areas of the body have more sensory receptors and so are more sensitive than others in discerning distinct points. Using calipers or a folded paperclip and asking a subject to keep their eyes closed, this test can be used to check parietal lobe function.
While a subject’s eyes are closed, a folded paper clip can be used to test two-point discrimination, which is mediated by the parietal lobe. The tester alternates using one point and two points on the area being tested (e.g. finger, shoulder, arm). The subject is asked to report whether they felt one or two points.
Temporal lobe. Separated from the frontal lobe by the lateral fissure, the temporal lobe also contains regions dedicated to processing sensory information, which is particularly important for hearing, recognizing language, and forming memories.
Auditory Processing. The temporal lobe contains the primary auditory cortex, which receives auditory information from the ears and secondary areas and processes the information so we understand what we’re hearing (e.g. words, laughing, a baby crying).
Visual Processing. Certain areas in the temporal lobe make sense of complex visual information, including faces and scenes.
Memory. The medial (closer to the middle of the brain) temporal lobe contains the hippocampus, a region of the brain important for memory, learning, and emotions.
Occipital lobe. The occipital lobe is the major visual processing centre in the brain. The primary visual cortex, also known as V1, receives visual information from the eyes. This information is relayed to several secondary visual processing areas, which interpret depth, distance, location, and the identity of seen objects.