Welcome to week 3 of looking at the neural pathways involved in a karate front kick. We have taken a little diversion from that to cover some basics about the nervous system is and how it works, which we started into last week, (Starting Small. The Story of a Front Kick. Part B) with a discussion on neurons and how they communicate. (To recap, neurons, or nerve cells, form the basis of our nervous system. They transmit information with their target (be it other neurons or another type of cell) via electrical impulses called action potentials.)

This week, we will talk about neuroanatomy and functional divisions of the nervous system, so that next week we can start looking at our karate kick, hooray!

The nervous system (fig 1) is the system responsible for sending to and receiving information from all areas of the body. It has two major divisions, central and peripheral. The central nervous system, CNS, is made of the brain and spinal cord. It processes incoming sensory information and generates a response. The peripheral nervous system, PNS, is made of all the nerves in the body, sending information to the CNS via sensory nerves and carries information from the CNS to the body via (predominately) motor nerves. Say you happen to do the most painful think known to humankind, and step on a lego. The pain sensation is carried by sensory nerves from your foot to your spinal cord and on to your brain. Your brain and spinal cord process that on different levels and generate a response which is carried by motor nerves if you’re like me, your mouth, but also back down to your leg so you pick your foot up. Simple, right?

We can go into a little more detail than that though (actually, we could go into a lot more detail… later). For the most part, our nervous system is symmetric, meaning you could draw a line down the center of your body, and the left side of your nervous system- the left side of your brain, the left side of your spinal cord, the nerves running down your left arm and leg, are similar in function and a mirror image of the right side. The major exception to that is in our brains. Superficially our brains are symmetric, but each side has some specialized functions. The average adult brain weighs about 3 pounds, contains about 86 billion neurons and roughly the same number of support aka glial cells (Azevedo et al). For scale, it is thought there are about 200-400 billion stars in the Milky Way.

So what does this 3 lb mass of 180 billion cells do? Lots of stuff.  It processes incoming sensory information, plans and initiates movement, regulates other systems in the body, and most of that it does on a subconscious level, constantly in the background, only occasionally reaching conscious awareness. If you had to think about every breath or every step, there would be no time to do anything else. It is the seat of consciousness and cognition, generator of emotions and dreams, and creator of memories. Some of this we understand very well, and some not so well.

The brain is made up of substructures, the major ones being the paired cerebral hemispheres, the cerebellum, and brainstem. (Fig 2&3). The cerebral hemispheres are where we do our ‘thinking’, and play a role in most higher order functions, which are responses that are more complicated. Each cerebral hemisphere is divided into four lobes called the frontal, parietal, temporal, and occipital lobes.  The surface of each lobe (called the cortex) has a special function, occipital lobes process visual information, parietal lobes process sensory and some language (on the dominant side and serve other functions on the non-dominant), temporal lobes are involved in memory and some emotion, and frontal lobes have a dominant role in movement, speech output, emotion, and decision making. Most sensory and motor functions are crossed, meaning the right side of the brain is responsible for the left side of the body.  

The cortex and some of the deep structures (collectively called deep gray nuclei) are made up mostly of neuronal cell bodies and are called gray matter (fig 3). The deep gray nuclei, have wide spread connections, and are involved in many processes. The area below the cortex, called sub-cortical, is called the white matter and is made of axons and glial cells. It is basically the wiring in the brain, transmitting information (in the form of action potentials) via bundles of axons called tracts, to different areas in the brain and spinal cord.

The brainstem (fig 2, fig 4, fig 5) is made of 3 parts, the midbrain, pons, and medulla. This part of the brain controls our facial movements, chewing, eye movements, and processes special senses- vision, hearing, taste, smell via 12 pairs of cranial nerves, and regulates some other systems. It also has multiple white matter tracts running through it from other parts of our brain heading down to our bound via our spinal cord and vice versa. It also multiple nuclei both for the cranial nerves that arise from it, and nuclei that are involved in other processes.

The cerebellum is the last major division of the brain. Most of the neurons in the brain, about 3/4, are actually in the cerebellum, it is just packed with neurons. Like the cerebral hemispheres it has paired lobes, sulci and gyr (called folia)i, a gray matter cortex, white matter, and nuclei. Initiation and control of movement (coordination), are the most well understood functions of the cerebellum, but it also has roles in several cognitive functions, that are not yet as clear.  

That is probably enough for now. Next week we will finally finish up our neuroanatomy primer so we can get back to talking about our karate kick. Please email with questions or comments to frontal.lobe@duramatters.com.