Your brain is the organ that allows for interaction with the outside world. It takes in outside stimulation through special sensory organs like the eyes, the nose and our ears. Our brains respond to the outside world mainly through the motor system. Voluntary movement is the result of a muscle contraction happening at the command of a corresponding motor neuron. This all takes place in a matter if milliseconds so that you are not aware of it.
Our sensory system has several components. Mainly receptors that become activated by changes in the environment.
The magnitude of sensation is related to the number of receptors activated. And the greater the stimulus, the more receptors become activated and the longer the effect. Non-painful stimuli cause what we call sensory adaptation, so you don't constantly feel your socks, for example. Whereas painful stimuli cause potentiation or sensitization, meaning the pain gets worse as it goes on. This is useful, as it protects the skin from damage. The fundamental units of our brains that receive and transmit messages across our body are neurons. Neurons are composed of a single cell body that is responsible for the transmission of communication signals between cells and hundreds of shorter processes that come out of the cell body.
So how do neurons communicate with each other? They do this through a mechanism called synaptic transmission.
One neuron communicates with other neurons or a muscle cell at a synapse. A typical neuron has a cell body, dendrites or branches, and an axon that carries electrical signals away from the neuron towards another neuron. Electrical signals carried by axons are called action potentials. The action potential is converted into a chemical message, which in turn interacts with the recipient neuron or effector. Surprisingly, neurons do not actually touch each other to communicate.
Rather, there is a space between cells where neurotransmitters are secreted that allows the transmission to appen. Though we often only hear of neurons, glial cells, or simply called glia, are important in maintaining equilibrium and provide support and protection for neurons.
As the Greek name implies, glia are commonly known as the glue of the brain. For the longest time, these cells were thought to serve as just a glue for neurons. Recent advances in neuroscience currently identify four main functions of glial cells. To surround neurons and hold them in place. To supply nutrients and oxygen to neurons. To insulate one neuron from another. And to destroy pathogens and remove dead neurons.
There are many transmitters in the brain that chemically facilitate messages between neurons. The two that account for most of your brain's neurotransmitters are glutamate and GABA. Glutamate is an excitatory neurotransmitter and acts as an accelerator. While GABA, which is an inhibitory neurotransmitter, acts like the brakes. Other important neurotransmitters found in the brain include serotonin, which you hear of in depression. Norepinephrine, which is produced in the brain stem and travels to different parts of the brain. And dopamine, involved in reinforcing behavior and can also be involved in addiction.
Dopamine is also key in motor modulation. A lack of dopamine, for example, is what is seen in Parkinson's disease. A lack of dopamine causes poverty of movement and a rigidity that is characteristic of the disease. Vision is carried into the brain through another specialized pathway called the visual pathway. The visual pathway, of course, starts in the retina in the eyes. The visual signal then gets converted to an electrical signal and travels along the optic nerve, the chiasm, the optic tracts, the optic radiations. And finally makes it to the primary visual cortex in the occipital lobes all the way in the back of the brain.
The visual cortex is responsible for decoding that signal into crude information that looks like a very pixelated picture. The information then gets carried from the primary visual cortex to the association visual cortex where the picture gets interpreted with colors, and depth, and edges and such. The motor cortex is in the most posterior part of the frontal lobe. Once this area activates, a neuron carries that information through the depths of the brain, the brain stem, and the spinal cord, all the way to the effector muscles that are being targeted.
The motor cortex is responsible for carrying out the outgoing signal. This signal is modulated by two loops, the basal ganglia and the cerebellar loop. These two loops control the precision speed and other characteristics of movement, making it smooth and purposeful.
That's why when the basal ganglia are affected, for example, in Parkinson's disease, the movements are irregular, slow, and broken. Near the motor cortex is the speech area. In the vast majority of people, this is located in the left hemisphere. The speech area is interesting because it has a sensory area and a motor area. The pathway of speech and conversation starts with first with hearing a sound. This gets conveyed through the ear to the hearing centers where sounds are deciphered into words and comprehended. From there, the signal gets carried through a thick bundle of fibers to Broca's area. Broca's area is the motor center of speech. This is what moves your mouth in precise ways to be able to make speech sounds. A lesion along any of these structures causes speech impairment.
The interior frontal area is where your executive functions are located. In a lot of ways, that is what differentiates us from animals. That is where behavioral control, personality, emotions, thinking, and planning are housed. These circuits are involved in many neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. But also in neuropsychiatric disorders, including schizophrenia, depression, OCD, and so on.
Moving on deeper inside the brain is the brain stem. The brain stem is thought to be the oldest part of the brain. In MaClean's triune of the brain, it represents the reptilian brain. The brain stem is where a lot of the cables coming from those higher cortical areas converge. If you remember, your motor cortex and sensory areas, those axons all go through the brain stem on their way to or from the spinal cord. This is also where respiration, heart rate, and other autonomic functions are controlled. Lastly, the cerebellum, or small brain in Latin, takes up only 10% volume in your brain, yet contains 80% of your brain's neurons. Traditionally, the cerebellum was thought of as a motor coordination structure, although recent studies show involvement in cognition as well.
From a motor standpoint, the cerebellum acts as a comparator. It compares the movement detected from sensory feedback to the movement intended from the cortex and adjusts accordingly.
We used to think that the brain was a static organ. You were born with a set of number of neurons, and that is all you got. But as it turns out, the brain is a malleable organ constantly changing. The other term for that is plastic. The brain is very plastic. Just like a muscle, the more you use the muscle, the bigger it gets. If you don't use it, you lose the bulk of that muscle, same thing happens in the brain. So how can you improve your brain's plasticity? Let's begin exploring how nutrition, sleep, exercise and other factors can help.