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
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!