The brain is a magnificent network of neurons - over 80 billion in fact. A neuron is a special type of cell found in the brain (and spinal cord). It has a unique ability to build connections to other neurons that communicate via electric signals - like a fancy Lego set. 

The neurons link together to form various structures such as the cerebrum (with its left and right hemispheres), the cerebellum, the brain stem and the spinal cord. 

Bundles of neurons form different channels - memory, language, sensation, etc. For the purpose of a general overview - let's focus on the neuron channels that control movement. 

There are two main groups of motor neurons - Upper Motor Neurons (UMN) and Lower Motor Neurons (LMN) - simple right! The upper and lower motor neurons act as two wires, delivering a message from the motor cortex to the muscle. 
The UMN wire travels from the motor cortex (high up in the brain) all the way down to the nucleus (connection point) of the LMN and delivers the electrical signal. This signal then travels along the LMN wire to the muscle, which will cause the muscle to contract. 

These categories are worth knowing - certainly for us doctors! If a disease/injury affects the UMNs, the LMNs, or a mixture of both - a different movement disorder/disability will present itself. This helps us make an accurate diagnosis. 

Broadly speaking, if the LMN wire is faulty, the muscle cannot receive any stimulus - and so, the individual will not be able to move this muscle. The affected region or limb will be weak, and floppy. 

If the UMN wire is faulty, the nucleus of the LMN doesn't receive a signal. Interestingly, rather than the lack of input turning off the LMN wire - it does the opposite. Without signals from the UMN, the LMN goes rogue and sends a constant signal to the muscle which will stay contracted! As a result, the affected region or limb will be really tense and stiff (clinically referred to as "spastic").

Simple right? Now, in terms of the spinal cord - this is a big cylindrical bundle of neurons that run through the bones of the spine (vertebrae). If an injury happens to the spinal cord, the outcome of the trauma depends on the level and the severity. With regards to the motor neurons, the LMNs exit the spine roughly at the level of the muscle they control. The UMNs come all the way down the spinal cord to meet them. Of course, the spinal cord is a two-way street. It delivers information from the brain to the body, but also delivers information from the body to the brain!

The left hemisphere of the cortex houses the motor and sensory centres of the right side of the body. The right hemisphere houses the motor and sensory centres of the left side of the body. At a certain point near the base of the brain, the upper motor neurons and sensory fibres cross over to the other side. That is why a stroke to the left side of the brain affects the right side of the body. 

We've discussed the cerebral cortex and the spinal cord, what about the other structures? Let's fly through them. 

The cerebellum is often referred to as the 'little brain', it two has a left and right side and sits underneath the cortex at the back of the head. It controls lots of things, primarily balance and co-ordination (including speech and eyesight). The cerebellum is also a major player in muscle memory. You know when you do something really fast, but don't really know how you do it (like tying a shoelace, or playing the piano) - that's the cerebellum!
Alcohol negatively impacts the cerebellum which is why we can feel a bit off balance and slur our words. 

The brain stem is the bridge between the brain and the spinal cord. It is comprised of the midbrain, the pons and the medulla. Those fancy names aren't particularly important. What you do need to know is that the brain stem houses all the nuclei (beginning points of the LMNs) for the cranial nerves - the nerves that control our facial and eye movements. The brain stem also facilitates our taste, swallow and maintains our breathing!

Throughout all these structures; we have arteries, veins and an interesting canal system known as the ventricles - they produce and store our CSF (cerebrospinal fluid). Maybe you don't care...but I thought I'd let you know! 

As you can see, the brain's one complicated organ! We'll do our best to make it simple.