What’s Back There. The Story of a Front Kick. Part D.

Week 4 already! Time flies when you getting your learning on and having fun. So far in our brief history, we have been tracking the neural pathways involved in a karate kick. Basically, how do you see something, decide ‘gosh, that really needs to be kicked’, and then, well, kick it. We took a look at that in broad terms the first week, but for the last several weeks, we have put that aside to cover some basics about the nervous system. Week two (Starting Small) we covered how neurons use electrical impulses to transmit information. Last week, (It’s All in Your Head), we talked about some of the basic anatomy and function of the brain. This week, we’re going to finish up our basic anatomy review of the central nervous system with the spinal cord. Next week we will cover the peripheral nervous system, so we can start back to our original topic.

Fig 1. The spinal cord showing 31 pairs of nerves exiting the cord. By BruceBlaus - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=27796969

 We probably should quickly define some terms. Axons frequently travel in bundles. A bundle of axons (the wiring) in your brain is called a fasicle, those some bundles in our spinal cord are called tracts (and are named for where they are coming from to where they are going, and once they leave the spinal cord they are called nerves. Okay, back to the spinal cord.

Fig 2. Cross section of the spinal cord. The gray matter is central with the white matter surrounding the outside. The cells that will form the motor component of the peripheral nervous system are located in the ventral horn of the gray matter, their axons travel out through the ventral root to join with axons from sensory neurons that enter the spinal cord through the dorsal root to make a spinal nerve as part of the peripheral nervous system. Cell bodies (soma) for sensory nerves are located not in the spinal cord, but in what is called the dorsal root ganglion. Tracts of axons running to and from the brain travel through the white matter. Not pictured are cell bodies for the sympathetic nervous system. http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction

The spinal cord (fig 1) is the second part of central nervous system (the other being the brain). It is an elongated structure also made of nerves and neuroglia running down the spinal canal of the vertebra from the medulla (the last part of the brainstem) to the lumbar region. (the bones that make up the spine). Like the brain, it has a hard candy shell for protection in the form of the spine. It also has gray and white matter with neuron cell bodies making up the center gray portion, and axons going to and from the brain and nerves around the outside making up the white matter (fig 2). Three segments (fig 3) called the cervical, lumbar, and thoracic make up the spinal cord. Information is carried from the spinal cord to and from the body via 31 pairs of spinal nerves which we will discuss in more detail next week.  Those nerves are named by what vertebral body they exit above in the cervical spine, and below in the thoracic and lumbar (more on that next week). Spinal cord levels are identified by where their nerve roots exit, which does not always correlate to the vertebral level by that part of the cord. (T1 would be the part of the thoracic cord that has its nerve root exiting below the first thoracic vertebra). This gets a little more confusing further down in the spinal cord as the actual spinal cord ends by the first lumbar vertebra, but there are nerve roots that come off of the cord that exit through lumbar vertebra, and even the sacrum.

The spinal cord has several functions; it transmits information to and from the brain, controls part of the autonomic nervous system (the part of the nervous system that controls bodily functions, even the gross ones), and coordinates some reflexes.

There are two basic types of information the spinal cord carries to and from the brain, motor and sensory. There are multiple white matter tracts that run almost the entire length of the spine, carrying sensory information cephalad (towards the brain), and motor commands caudad (away from the brain) The tracts are named by where the come from to where they are going. For example, one of the main motor tracts is called the corticospinal tract because it comes from the cortex and terminates at different levels along the spinal cord (we will talk about this more later). From the spinal cord, the motor signals that travel along that pathway exit the spinal cord via nerves bound for muscles. Sensory information travels in to opposite direction. It travels to the spinal cord through nerves, up tracts through the spinal cord to the brain.

Fig 2. Cervical, thoracic, and lumbar segments of the spinal cord.

Adapted from Andrewmeyerson - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49411614

Reflexes coordinated at the level of the spinal cord, creatively named ‘spinal reflexes’ are yet another function of the spinal cord. A reflex is a nearly instantaneous involuntary movement in response to a stimulus. One of the most famous spinal reflexes is the knee jerk, or patellar reflex, where the doctor taps you knee and you kick. There are more complicated movements controlled by the spinal cord, like… walking! Yep, the basic rhythmic action of walking is coordinated at the level of the spine (much of it still requires some higher input, like stepping around obstacles).

The last major function of the spinal cord is control of part of the autonomic nervous system (fig 4). The autonomic nervous system regulates many bodily functions, controls heart rate, digestion, sweating, etc. It can be further subdivided into the sympathetic (fight or flight), or parasympathetic (rest and digest or feed and breed, hehe) nervous system. Anyway, cell bodies for the sympathetic nervous system can be found at most thoracic and some lumbar levels of the spinal cord (T1-L2).

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Fig 4. Summary of the sympathetic nervious system as mediated by the spinal cord.

Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=28086441

Well, that is it for this week. Next week we will cover the peripheral nervous system, so we can get back to the fun stuff. As always please send questions or ideas to frontal.lobe@duramatters.com. Thanks!