Peripheral Nervous System. The Story of a Front Kick. Part E.

Welcome to week 5! If you’re just joining us (go back and catch up, it won’t take long), we will be talking about the neural pathways involved in a karate kick. Before we dive into that though, we are covering the basics of the nervous system and how it works so we can look at this karate kick (and all the other fun stuff we are going to cover) with a little more understanding.

Fig 1. The nervous system. Everything outside of the central nervous system makes up the peripheral nervous system.

Title: Elementary anatomy, physiology and hygiene for higher grammar grades Year: 1900 (1900s) Authors: Hall, Winfield Scott, b. 1861

This is the last installment of our neuroanatomy primer!! So far we covered the structure and function of a neuron, the central nervous system (CNS)- brain and spinal cord, and this week, we are going to cover the…peripheral nervous system!

The peripheral nervous system (PNS) is the collection of nerves running to and from your spinal cord and brain (with some exceptions that we will talk about later). Remember this handsome fella in fig 1 from a few weeks back? All the white lines outside of his brain and spinal cord are part of the peripheral nervous system.

Fig 2. Cross-section of a spinal nerve showing the inner layers around the bundles of axons called the perineurim, the outer layer, called the epineurim, and the blood vessels.

Adapted from OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.

CC BY 3.0

The central and peripheral nervous system share many of the same features. As in the central nervous system, the peripheral nervous system has neurons and neuroglial (support cells), analogous to but slightly different compared to the neuroglial cells of CNS. Most neurons of the PNS are insulated with myelin, like the CNS. Unlike the tracts of axons through the CNS, the bundles of axons that make up peripheral nerves, not only have myelin insulating the axon, but also have progressive layers of insulation (fig 2).

Now that we know a wee bit about the wee bits, lets look PNS on a larger scale. The closest part of the PNS to the CNS are the nerve roots coming off the spinal cord (fig 3). (The cranial nerves are a bit different and probably best saved for a later discussion). Each root only contains either the efferent motor neurons, carrying signals out to the muscles, or afferent sensory neurons, transmitting signals the opposite direction. Still within the bony protection of the spinal canal, the sensory nerve root joins with its corresponding motor nerve root to form a spinal nerve.

Fig 3. A spinal nerve is formed when a motor roots (the ventral root) join with its correponding sensory root (dorsal root). This immediately splits into a dorsal and ventral rami. They also split off into the white and gray rami which are part of the autonomic nervous system. The cell bodies (soma) for the sensory nerves are located outside the spinal cord in the dorsal root ganglion, where as the cell bodies for the motor nerves are located within the gray matter of the spinal cord.

By Mysid (original by Tristanb) - Vectorized in CorelDraw by Mysid on an existing image at en-wiki by Tristanb., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1420508

Because spinal nerves carry both motor and sensory information they are called mixed nerves. There are 31 pairs in total, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and one sad lonely coccygeal pair. They are numbered based on the vertebra above the opening through which the spinal nerve exits the spinal canal called the intervertebral foramina (fig 4), except the cervical spinal nerves. Because of the anatomy of the cervical spine, those are named based on the vertebra they exit above, except for the eighth pair, which exit below the 7th (and last) cervical vertebra.

Fig 4. Lateral view of the spinal column. The arrows are pointing to the intervertebral (or neural) foramen, which are the opening through which the spinal nerves travel to and from the spinal cord.

Public Domain, https://commons.wikimedia.org/w/index.php?curid=1601710

But like the weekend, spinal nerves end before they have seemly begun. After exiting the intervertebral foramina they immediately split into two major divisions, the dorsal and ventral rami (fig 3). The dorsal (or posterior) rami, innervates the muscles and skin of the back. The ventral (or anterior) rami are the larger of the two divisions, and they wrap around to the front. For most of the thoracic region, things stay simple. The nerves are called intercostal nerves and each innervate a strip of skin, intercostal muscles (between the ribs) and abdominal muscles.

Innervation to the skin and muscles from spinal nerves follows a fairly regular pattern, though it can get a little more complicated in the arms and legs. For the most part, parts of the body closer to the head are innervated by higher spinal nerves and moves distally. So the neck is innervated by some of the very first nerves off the cervical spine like C3 and C4, the upper chest is innervated by the lower cervical spinal nerves, etc, all the way down into the low back and sacrum. A sensory dermatome map (fig 5) shows how spinal nerves provide sensory input from the skin. Each level is innervated by a single pair of spinal nerves. The spinal nerves from part of the cervical and thoracic cord follow a relatively horizontal striped pattern across the chest, abdomen, and back, but the pattern gets a little stretched out in the arms and legs. A myotome map, which shows innervation to the muscles, follows a similar pattern. Unfortunately, our myotome model refused our minumum clothing requirement of a leaf, so we are not able to post the image.

Our shy dermatome map model. C represents the cervical spinal levels, so C5= 5th cervical spinal nerve. D represents the thoracic, and in most current literature has been changed to T. L represents lumbar, and S represents sacral. The ribs are labeled I-XI, and the 12th rib is not shown.

Adapted from- http://www.springerlink.com/content/h25t2658333v02mp/?p=81668d0e1acc45c785d3f06396a40075&pi=0, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2164109

Cervical, thoracic, and lumbar spinal nerves and plexuses The brachial plexus is formed from the cervical and the first thoracic spinal nerve the lubosacral plexus is formed from the lumbar and sacral spinal nerves). Adapted from Andrewmeyerson - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49411614

In addition to the myotomes and dermatomes getting stretched out over our limbs, the spinal nerves off of the cervical, thoracic, and lumbar cord don’t go straight to their targets like they do off the thoracic cord. Our limbs, especially our arms and hands, are capable of complicated movements, much more that the stabilization provided by our core, and their innervation gets a little more complicated as well. The ventral rami of the spinal nerves come together and split several times, in what is called a plexus. Each time those nerves split or come together they are renamed. Plexuses have roots, trunks, and branches, no . We have several, including the brachial plexus which innervates part of the anterior chest wall, arm, and shoulder, and the lumbosacral, which innervates the leg, lower back, and buttocks (fig 6,7).

Once past the tangle of the plexuses (seems like the plural should be plexi, but I looked it up), the peripheral nerves continue on, to innervate muscles, skin, bones, joints, blood vessels (though this is the autonomic system), tendons. Just about every single part of the body has efferent nerves running to it, transmitting information from the brain, and afferent nerves from it sending information to the brain. Our brain is constantly monitoring this sensory feed back and making adjustments, mostly without conscious awareness, which we will talk about more over the next few weeks. Thank-you, and please send questions or comments to frontal.lobe@duramatters.com.

Fig 7. Diagram of the brachial plexus. It is formed by C5-T1 spinal nerves. Each time it splits or comes together again the nerves take on a different name, The cords, which are the last major part that is still considered brachial plexus, will become the median, ulnar, axillary, and radial nerves, which will all branch again on their way to innervate the arm and hand. There are many other nerves that come off the brachial plexus at different points as well. In the first few days of medical school, we started gross dissections of the brachial plexus. I was not yet comfortable in my new role as a medical student, and so many years later, it still brings back vivid memories of formaldehyde and angst.

By Gray - Gray's Anatomy, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4245589

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!

It’s All in Your Head. The Story of a Front Kick. Part C.

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.)

Fig 1. A posterior view of the nervous system. A, The cerebral hemispheres. B. The cerebellum. C. I can’t really tell what that is pointing at, maybe the hip? (okay, per the author, it is pointing at the sciatic nerve, but it could be labeled better).

Title: Elementary anatomy, physiology and hygiene for higher grammar grades Year: 1900 (1900s) Authors: Hall, Winfield Scott, b. 1861

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.

Fig 2. A lateral view of the brain. The cerebrum is made up of the paired cerebral hemispheres. The midbrain, pons, and medulla oblongata make up the brainstem. A sulcus is a fissure on the surface of the brain, and separates the surface into ridges called gyri. The cerebellum plays a role in initiation and control of movement among other things. The diencephalon is a collection of midline nuclei that serve many functions, the hypothalamus is involved in regulation of different systemic processes like sleep and temperature, and the thalamus is involved in sensory processing among other things.

Blausen.com staff (2014) ). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Figure 3. A lateral view of the brain. The frontal lobe, parietal lobes, temporal lobes, and occipital lobes make up the cerebral hemispheres. The central sulcus is the fissure that separates the frontal and parietal lobes, the frontal gyrus is involved in control of movement, the post central gyrus is where sensory information is processed. The cerebellum plays a role in initiation and control of movement among other things. The midbrain (not shown), pons, make up the brainstem.

Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436

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.

Figure 3. MRI showing the gray matter and white matter. Adapted from Coronal T2 (grey scale inverted) MRI of the brain at the level of the the caudate nuclei. Image from Radiopaedia.org Dr Frank Gaillard.

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.

Fig 4.Human brain as viewed from the bottom showing 12 pairs of cranial nerves coming off the brainstem and their functions. As an aside, in medical school there is a mnemonic we learned the memorize the pairs of nerves. There are two, actually, one clean one that we were taught by the anatomy professor, and a less clean one that is passed on from other students. I do not remember the clean one. The other one, well, maybe after we get to know each other better.

Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436

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.

Starting Small. The Story of a Front Kick Part B.

Last week we starting talking about the pathways in the brain involved in a karate kick. Basically, a part of your brain receives a stimulus, determines that the response to that would be to kick (we were discussing a front kick), and then kick. That stimulus could be a variety of things, internal or external, or a combination thereof. An internal stimulus might happen while you are practicing kicking, and think, next I am going to kick. An external stimulus might be visual, like walking by a stack of boards that looks like they really need to be broken. Since our brain processes different stimuli in different areas, the pathways leading up to a front kick can vary but ultimately converge, as we will talk about later.

Fig. 1, a neuron. The dentrites receive information via receptors. The cell body is where the information is processed. The nucleus, which contains the genetic information for the cell, is in the cell body. The axon transmites the electrical signal, called the action potential, to other cells. The axon terminal is the part of the axon in contact with other cells. The axon is insulated by myelin, which is made by Schwann cells ( in the peripheral nervous system), and by similar cells in the central nervous system. The gaps between the myelin are called the nodes of Ranvier. The electrical signal is transmitted along the outside of the myelin, and jumps between the nodes of Ranvier.

Originally Neuron.jpg taken from the US Federal (public domain) (Nerve Tissue, retrieved March 2007), redrawn by User:Dhp1080 in Illustrator. Source: "Anatomy and Physiology" by the US National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program

First though, we should cover what the nervous system is and how it works. The nervous system is made of the brain, spinal cord, and peripheral nerves. A nerve cell or neuron, (fig 1) is the basic building block of the nervous system. A neuron is a cell that communicates information via electrical impulses to other cells. Neurons have 3 parts, dendrites- short projections from the cell body which receive signals from the environment or other cells via receptors, the cell body- which processes that information and generates an electric signal in response, and an axon- a long thin process off the cell body, which transmits the electrical impulse to target cells. The exact structure of a neuron can vary depending on its function. Neurons are not the only type of cell in the nervous system, but most other cells support them in some way. Collectively, the other cells in the nervous system are referred to as neuroglia, and we can discuss them further in a future post.

The electrical signals neurons use to communicate are called action potentials. An action potential is an all or nothing response, meaning it is identical each time the neuron fires.  An action potential is generated in response to signals from other cells or from the environment which are detected by receptors on the neuron’s dendrites. The specific signal the receptors respond to are dependent on the type of receptor, which is ultimately dependent on the function of that particular neuron. For example, olfactory neurons in your nose have scent sensitive receptors, neurons in the skin in your feet, thankfully, do not. That signal can be almost anything depending on the type of neuron, temperature,  light, waves, or certain chemicals, like neurotransmitters.

Fig 2. Sodium (Na) movement through a channel across a cell membrane. When the channel is open, sodium moves along a concentration gradient from higher concentration outside the cell to inside the cell. When the channel is closed, sodium cannot move into the cell. By Tryptofish - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=65414295

When a receptor on a dendrite is activated by a signal, it opens a channel, in this case it opens what is called a ligand gated channel, on the cell membrane (the outer layer of the cell), which allow certain electrolytes or ions in and out of the cell (fig 2). In the case of generating an action potential, channels open that allow sodium, which is usually at a higher concentration outside the cell, to enter the cell, and potassium, which is usually at a higher concentration inside the cell, to leave. This changes the charge across the neuron membrane and if it reaches a certain threshold, it triggers other Na and K channels (this time they are what are called voltage gated channels because they are activated by a change in the voltage across the cell membrane) to open and the neuron fires (fig 3 and 4). After the neuron fires, the sodium that just rushed in is pumped back out of the cell, and potassium is pumped back in via other channels and a sodium potassium pump so the cell can fire again.

Once generated, the action potential travels from the cell body down the axon towards target. Typically the axon is insulated by something called myelin (fig 1)- think of the insulation around electrical cords. The analogy breaks down there, because unlike insulation around wiring, which is designed to keep the electricity inside, the electrical current travels outside the myelin. It jumps between gaps in the myelin called nodes of Ranvier (fig 1 and 3). This process, called saltatory conduction, allows for faster conduction of the action potential down the axon. In neurons that lack myelin, either naturally or through disease, actions potentials travel much slower (fig 3).  When the action potential reaches the end of the axon, called the terminus, it triggers a response in its target by releasing chemical, called a neurotransmitter, onto the target cell. If this cell is another axon, it might generate an action potential and propagate the signal from the first neuron. If is a muscle cell, it might contract.

Action potential conduction down an unmyelinated neuron, left, and a myelinated neuron, right. The signal is able to jump from node to node on the myelinated axon, and travels much faster.

http://docjana.com/saltatory-conduction/ ; https://www.patreon.com/posts/4374048AuthorDr. Jana

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Fig 4. Action potential across a cell membrane as a function of time. The change across a cell membrane at rest is -70mV. When a stimulus is applied that open some channels across the cell membrane, the potential changes. If enough stimulus is applied, and the charge across the membrane reaches -55mV, the threshold is reached, and the the axon depolarizes, which means it fires or (generates an action potential. If the stimulus is not enough, and the membrane does not reach the threshold, it will not fire. Once a neuron fires, it must reset in preparation to fire again, callied repolarization. During this time, and the refractory period, it cannot fire. This is repolarization and the refractory period. This Wikipedia and Wikimedia Commons image is from the user Chris 73 and is freely available at //commons.wikimedia.org/wiki/File:ActionPotential.png under the creative commons cc-by-sa 3.0 license.

And that is probably enough for this week. This is meant to serve as an introduction to what a neuron is and how it functions. We can/will go into more detail with some of the topics from today in the future, like types of receptors, or the role of myelin in healthy and disease states. Next week we will cover neuroanatomy of the central and peripheral nervous system in more detail, so we can start tracking the pathways involved in a karate front kick through our nervous system. Please email with questions, comments, or suggestions for other topics at frontal.lobe@duramatters.com. Thanks!

My Not So Super Superpower. The Story of a Karate Kick, Part A

I amaze myself sometimes. Not that I have any special abilities, I can’t parkour up the side of a building, backflip my way through an olympic gymnastic competition, or get out of bed in the morning without groaning. But some movements that I, like most of us, can do with just a little practice, are more complicated than they seem on first pass. I am amazed that anyone can do something like a karate kick, for example, when we look at what is required from our nervous system alone.

I have been taking karate for a few years, and one of the first moves I learned, maybe even first day, was a four count stepping front kick. 1) Pick up your leg up thigh parallel to the floor, knee at 90. 2) Extend your leg, keeping your thigh parallel to the floor, flex your foot, extend your toes 3) Retract your leg to position (1). 4) Put your foot back on the floor. Sounds simple enough, even for me. But how do we initiate and execute that kick? How do we hear a command for a front kick, understand what that means, and then convert that into action? Or how do we know during a sparring match, for example, that we now have an opening or have created an opportunity for a stepping front kick?

Even a seemingly basic stepping front kick requires a complicated and highly coordinated set of signals within our nervous system, and from there to our muscles, more so that would be imagined from watching even in the most awkward looking of kicks. Millions of neurons in our brain down through our spinal cord and extending into our legs, all the way down to our toes, are firing long before the first muscle fibers even twitch. Then, when we finally start the kick, feedforward and feedback loops within our brain, and between our brain, muscles, and sensory nerve endings in tendons and joints fine tune the magnitude and direction of our kick. Protagonist muscles, the ones that facilitate the movements in our kick, are contracting, at the same time antagonistic muscles, the ones that directly oppose the movement are relaxing, and stabilizing muscles are keeping the kick steady. The timing has to be perfect or instead of a stepping front kick, we could end up in a twitchy floor spasm, which, as far as I know, isn’t a part of any form, unless maybe someone is being trained wrong on purpose.

The story of our front kick, this complex movement involving millions of neurons (which would stretch over miles if laid end to end) in our brain, down our spinal cord, running all the way down to our feet, and muscles in our back, abdomen, and both legs starts with just the smallest of shifts of sodium and potassium across a nerve cell membrane in our brain.

A good narrative, at some point, gets interrupted with a backstory to bring more depth and meaning; the story of our devastating front kick is no exception. Unfortunately this one does not include any radioactive bug bites or set up revenge plot, but there is still some basic neuroscience and neuroanatomy to help understand how our kick is going to happen.

How the image of our partner hits the back of our eyes and triggers a set of signals there that follow a circuitous path starting from the backs of our eyes, cross to the opposite side and, bounce to the back of our brain, splits, and bounces forward again. That signal is integrated with other sensory inputs, memories, and signals from other areas in our brain, cross again, is then sent down our spinal cord, and finally heads out to our muscles, to be constantly adjusted through feedback loops from sensory nerves.

Next week we will cover some of the neuroanatomy and neuroscience involved in our front kick.

Thumbnail image courtesy of sefcmpa at wikimedia commons