Describe the major divisions of the nervous system, both anatomical and functional
- Explain differences between central, peripheral, autonomic, sympathetic, parasympathetic and somatic NS.
The human nervous system is one of the most complex biological systems in existence. The evolutionary complexity of this system gives rise to emergent properties that we take for granted, such as consciousness and memory. Because of the complexity, it is helpful to break the system down into categories. Let’s take a look at the various divisions of the nervous system.
The nervous system as a whole consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS, which is what most people are aware of, consists of the brain and the spinal cord. This is where all the information in our bodies is integrated. This is likely why the brain is often compared to a computer, although it is much more than a computer.
The PNS, which is lesser known but just as important, consists of sensory neurons and efferent neurons. These neurons branch off from the brain and spinal cord and exist all throughout the body. They are constantly monitoring and evaluating the conditions of our internal and external environments, sending the information to our CNS where it is integrated and a response is issued.
The PNS can be further divided into a Sensory Division and a Motor Division. The Sensory Division is where the body picks up on specific information, for example, feeling cold. The Sensory Division send this information to the CNS where is it processed and then Motor Division will act in order to correct the problem (put on a sweater). The Motor System consists of two parts. First, the Somatic Nervous System, which is the voluntary part of our nervous system. This part allows us to actively get up, find the sweater, and physically put it on. The other is the Autonomic Nervous System, which is the involuntary part of the nervous system. This is the part that keeps our heart beating, our lungs breathing, and our organs functioning.
The Autonomic Nervous System can be further classified into two more systems: the Sympathetic Nervous System and the Parasympathetic Nervous System. Most people know the Sympathetic Nervous System as the “fight or flight” system that enables us to respond to perceived threats. For example, when the police car gets behind you, your heart rate starts to increase, your palms sweat, you become afraid because there is the possibility that you could be pulled over. This is the Sympathetic Nervous System doing its job. Although it may not seem very useful in the modern world, it has an important purpose of helping us react instantly to the predators that our ancestors would have faced in a wilder world.
After the police vehicle pulls off the road and you are no longer being followed by a predator, the Parasympathetic Nervous System kicks in, sends your body the information that it no longer needs to be on high alert, decreases the amount of stress hormone flooding into your bloodstream, starts the process of calming you down.
- Differentiate between the structure and function of the sympathetic and parasympathetic divisions in the autonomic nervous system
- How do these neurons differ? Look at neuron length, neurotransmitters used, and overall function.
Sympathetic and Parasympathetic neurons are two sides of the same coin in that they are both critical to communication, but they are performing different jobs and doing those jobs in different ways. The role of the sympathetic neurons is to stimulate the body to action, when we must have a “fight or flight” response, it is the sympathetic nervous system that is working. These neurons are much shorter in length and faster acting, relaying a vast amount of information at a rapid pace. In the sympathetic nervous system acetylcholine communicates the message between the pre- and post-ganglionic neurons. On the other hand norepinephrine is the neurotransmitter that signals between the neuron and the muscle.
Parasympathetic neurons have a very long preganglionic neuron and a short post ganglionic neuron, whereas the Sympathetic neurons have a very short preganglionic neuron and a very long post ganglionic neuron.
The parasympathetic neurons, on the other hand, are longer and operate at a slower (relatively speaking) pace. This system is responsible for the “rest and digest” function that allows us to recuperate from stress, decreases our heart rate, allows our digestive system and other organs to function properly, and more. In the parasympathetic nervous system, acetylcholine is the neurotransmitter at both synapses.
As one might imagine, sympathetic neurons produce stimulatory neurotransmitters while the parasympathetic neurons produce calming neurotransmitters. The amino acid Glutamine is used in GABA (gamma-amino butyric acid) which is calming to the nervous system, or Glutamate, which is excitatory to the nervous system. Both are necessary for vital function but too much of one or the other can result in problems.
- Describe the general structure of a neuron, and name its important anatomical regions
- Focus on dendrites, axons, Schwann cells, myelin sheath, nodes of Ranvier
Neurons are cells that are slightly different in structure and function because they are designed to transmit information via chemical and electrical impulses. They are similar to other cells we have looked at in the sense that they have a nucleus, endoplasmic reticulum, and a Golgi apparatus that secretes a variety of chemicals. They are different in that they are some of the longest lived cells in the human body, they are considered irreplaceable, and they have a high metabolic rate; i.e. need a steady rate of glucose and oxygen in order to function properly.
Another aspect that makes the neuron different from a typical cell is that from the main cell body, known as the Soma, are multiple dendrites that extend outward and “listen” for information that is being received from other neurons. Also from the Soma is also a protruding segment known as the Axon, which is essentially a transport channel that can be a variety of lengths depending on where it is in the body. Some Axons are very short only extending a small distance, while others are very long, extending the length of your spine or legs.
The Axon is covered in what are known as Schwann Cells (some axons may have as many as 500 different Schwann cells!). A Schwann cell is unique in that it wraps around the Axon many times. In this process of wrapping itself around the Axon, the Schwann cell is actually building up layers of Myelin Sheath that are pushed to the outside of the Axon. For those who are familiar with neurodegenerative diseases like Multiple Sclerosis, the term Myelin Sheath should be familiar. MS is a disease where the immune system attacks the Myelin Sheath, exposing the Axon and interrupting the flow of information. This results in the myriad of symptoms that we see in people with MS.
Each Schwann cell only produces 1 – 1.5mm segment of the Axon, so it leaves tiny gaps between each Schwann cell. These tiny gaps between Schwann cells are referred to as the Nodes of Ranvier. Because the Myelin Sheath acts like insulation for the axon, the electrical current is able to quickly skip from node to node.
Oligodendocytes are another Glial cell that is responsible for the production of Myelin sheath, but instead of being only responsible for one axon like the Schwann cell, an oligodendrocyte “branches and forms myelin around portions of several axons”(Silverthorn, 233).
Schwann cells are just one type of Glial cells that reside in both the Central Nervous System and the Peripheral Nervous System. Schwann cells reside in the PNS, along with Satellite cells, which is considered a non-myelinating Schwann cell that provides support around nerve cell bodies.
However, within the Central Nervous System, there are four different types of Glial cells that perform various functions. Astrocytes, so called because of their star-like structure, are estimated to make up half of all cells in the brain. They come in a variety of subtypes, and their roles vary. Some take up and release chemicals, while others “provide neurons with substrates for ATP production, and they help maintain homeostasis in the CNS extracellular fluid by taking up K+ and water” (Silverthorn, 235). Some Astrocytes surround blood vessels and become part of the blood-brain barrier.
Another important category of Glial cell is referred to as Microglia, which are not actually neural tissue, but instead an immune cell that triggers the release of inflammatory cytokines in the event that there is an infection. Ideally, these microglia are only activated when there is an infection that needs to be resolved. After the microglia have dealt with the infection, they then return to a resting (ramified) state, and they produce neurotrophic factors that help rebuild the brain. However, problems can arise when the microglia become repeatedly primed and are unable to shut off the production of their inflammatory cascades.
Many of our herbal allies provide us with neurotrophic factors. Relatively new to this conversation is Lion’s Mane Mushroom (Hericium erinaceus) which is being studied for its neuroactive compounds. See the study: http://www.ncbi.nlm.nih.gov/pubmed/24266378
Another class of glial cell is Ependymal Cells, which are responsible for creating the ependymal, a selectively permeable epithelial layer that separates the fluid compartments of the CNS. The ependyma is critical because it is a source of neural stem cells. Many herbs are being studied for their ability to promote the proliferation of neural stem cells. See the study: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3991899/
These electrical impulses travel down the length of the Axon into the Pre-Synaptic Terminal, where the information is translated into chemical information known as neurotransmitters. These information molecules are released via the process of exocytosis, where they must cross the Synaptic Cleft, a tiny gap between the Pre-Synaptic Terminal and the Post-Synaptic Neuron. It is here in the Post-synaptic neuron where the neurotransmitters are re-encoded as electrical information and sent on to their final destination where they will used to convey a whole manner of information.
- Explain the events that lead to the generation of a nerve impulse and its conduction from one neuron to another
- Walk through an action potential and transmission across a synapse. Discuss the pre- and post-synaptic neurons, synaptic vesicles. You do not need to cover the details involved in action potential transmission (e.g. sodium and potassium channel specifics) unless you are ready to do so.
Our neurons are constantly in communication with each other through what are called synapses. Now that we understand the biology of the cell itself, and more specifically the neuron, we can see how this process of communication unfolds. Essentially, the neurons in our bodies are engaged in two kinds of communication: electrical and chemical. Chemicals in the body make sense, but electricity inside a living organism seems odd at first glance. Indeed all of us carry around an electrical charge in the ions that float around inside and outside out cells.
Without going into excess detail regarding this electrical communication, let’s take a second to understand what an Action Potential is, and how it occurs. Inside our cells (intracellular fluid or cytosol) is a higher concentration of Potassium (K+) ions than there is outside of the cell. Outside the cell, (extracellular fluid) there is a higher concentration of Calcium (Ca2+) ions, Sodium (Na+) ions, and Chloride (Cl-) ions.
The cell membrane is to a greater or lesser degree permeable to these ions, it just depends on a variety of factors as to which ions travel through the membrane. If a cell membrane is not permeable to a particular ion, then the permeability value for that ion is 0. Cells at rest are not normally permeable to Calcium, so the majority of the action potential is attributed to shifts in Potassium. Cells normally exist at a resting state of -70mV and in order for an Action Potential to take place, there must a a stimulus of at least -55mV. When this stimulus is reached, the voltage gated sodium channels are opened and all the positive sodium ions flood in and massively depolarize the cell. The membrane will briefly hyperpolarize in order to reestablish the gradient (known as the refractory period).
It is important to point out that ions can travel through the cell membrane through a variety of channels. There is of course, the sodium-potassium pump which we already discussed in our review of cell biology, but more specifically there are 3 ion channels that are responsible for the movement of these ions: 1) Voltage Gated, 2) Ligand Gated, and 3) Mechanically Gated.
“If the membrane suddenly increases its Na+ permeability, Na+ enters the cell, moving down its electrochemical gradient. The addition of positive Na+ to the intracellular fluid depolarizes the cell membrane and creates an electrical signal” (Silverthorn, 238). Like a domino effect, the charge is carried down the length of the axon. Another important point is that Myelinated axons will transmit the current faster than non-myelinated axons. This is because of what is known as the Nodes of Ranvier, which are essentially tiny gaps between Schwann cells. These nodes allow for a quicker transfer of electrical energy.
Once the electrical signal reaches the axon terminal, or pre-synaptic terminal, there is a conversion of electrical signal into chemical neurotransmitters. These neurotransmitters are information chemicals which are secreted via synaptic vesicles and released by exocytosis. Here, they will travel across the micro-distance, known as the synaptic cleft, where they are received by the post-synaptic neuron. It is here where this chemical information is translated back into electrical signal and goes on its merry way. This information can be readily translated from electrical to chemical and back again as the neurons are communicating with each other.
- Discuss the chemical composition of hormones and the mechanisms of hormone action
- Key terms: polypeptide, steroid and amine hormones, negative and positive feedback loops, receptors, target cells, chemical and nervous system regulation
- Summarize the site of production, regulation, and effects of the hormones of the pituitary, thyroid, adrenal, and pineal glands
In addition to the Nervous System, the body has another mode of communication known as hormones. Our hormones are chemical messengers that are produced in our glands. Glands exist all throughout the body and produce a wide range of hormones. Let’s first look at the glands that produce these hormones, and then delve deeper into the specific kinds of hormones and how they work.
Inside your head, is a tiny pea sized gland known as the pituitary gland. This gland is known as the “master gland” because it produces hormones that signal other glands like the thyroid, parathyroid, adrenal and pineal glands which make their own hormones. A hormone can only trigger a reaction in specific target cells that have the right receptors.
The thyroid produces the hormone thyroxine, which stimulates metabolism and binds to most sites in the body. There are numerous problems that can arise when the thyroid is under or over producing thyroid hormone, and even more problems can come about when doctors attempt to treat the problem with synthetic thyroid hormone. Instead of resorting to synthetic attempts at essential hormones, we can utilize dietary strategies (such as eating more organ meats or seaweeds rich in iodine and other minerals) or by implementing a protocol that incorporates herbs that will either provide essential nutrition or stimulate our natural production of these essential hormones. However, this does not rule out the possibility that a pharmaceutical intervention may be required.
The pituitary gland produces many different hormones, including Follicle Stimulating Hormone (FSH) which helps regulate growth and trigger sexual maturity. FSH only triggers specific cells in the ovaries and testes. Disruption of this gland can also result in a myriad of problems.
Most hormones are made of amino acids or derived from lipids like cholesterol. Depending on what the hormone is made of will determine whether it is water soluble or lipid soluble. Hormones can be classed as either steroid hormones or amine hormones. Typically amine hormones are water soluble and steroid hormones are fat soluble, but this is not always the case. Thyroid hormones are lipid soluble but they are not steroid hormones, putting them in a category of their own.
Since water soluble hormones are unable to pass through the cell membrane, they have specific receptor sites on the outside of the cell. Lipid soluble hormones are able to easily penetrate the phospholipid cell membrane, so these receptor sites are inside of the cell. When the hormone hits the receptor site, it alters the cell activity by either increasing or decreasing some of its functions.
Organs that are part of the endocrine system include the gonads, the pancreas, and the placenta in pregnant women. The hypothalamus is also a part of the endocrine system because it also produces and secretes hormones. The hypothalamus is unique because it is the integration center between the nervous system and the endocrine system. It is a key component in what is known as the Hypothalamus-Pituitary-Adrenal Axis (HPA Axis). The HPA Axis is a key part of the fight or flight mechanism, and is also one of the main areas of the body that are affected when we consume herbal adaptogens like Rhodiola or Eleuthero.
The fight or flight mechanism could not occur without the HPA Axis. When something stressful or potentially dangerous happens to us, action potentials in our brains (nervous system) trigger neurons in the hypothalamus which releases Corticotropic Releasing Hormone (CRH). This CRH travels to the water soluble receptor sites on the Anterior Pituitary Gland, where the release of Adrenocorticotropic hormone (ACTH) is triggered. This ACTH then travels through the bloodstream to the Adrenal Glands which reside atop the kidneys (ad renal meaning above the kidneys). ACTH binds to receptors in the adrenal glands which stimulate the synthesis of stress hormones like glucocorticoids, mineralocorticoids and DHEA. These stress hormones help us to survive a potential deadly situation by raising our blood pressure, dumping glucose into the bloodstream, and temporarily shutting down bodily functions that are not deemed necessary to immediate survival (digestion, sexual reproduction). Recent research out of the University of Pittsburgh has discovered that different hormones are produced in different areas of the adrenal glands. See below.
Here we can see that certain tissue areas of the adrenal glands will release specific hormones. This is important to understand because we are able to recognize that if we are constantly undergoing the same stressors all the time, that we are continually activating the same tissue area of the adrenal glands. By introducing new behaviors and augmenting our current behavior we can change what area of our adrenals are being stimulated and thereby which hormones are being released.
This stress reaction is an evolutionary strategy that we have developed in the event we need to fight off predators, or run away from a dangerous situation. It is good to the extent that it helps us live to fight another day, but it can also be detrimental if the system is always being activated. If the “stress button” in our brains is continually pushed and we never have a chance to “rest and digest” then we will live perpetually in a cascade of inflammatory stress hormones.
Thankfully, the hypothalamus is constantly evaluating the situation in our bodies, and when it detects an abundance of stress hormones it will eventually stop secreting CRH, which in turn will cause the other glands to stop secreting their stress hormones.
- Explain the role of the pancreatic endocrine cells in the regulation of blood glucose
One of the important jobs of the pancreas is to regulate our blood sugar level. It does this by releasing insulin and glucagon. Inside the pancreas are beta cells, which release insulin. Insulin helps lower your blood sugar by increasing the rate at which your cells store the sugar either as glycogen or fat for later use. This production and secretion of insulin is controlled via a negative feedback loop whereby insulin is secreted until the pancreas detects the shift in blood sugar levels and then turns off the production/secretion of insulin. Diabetes and similar blood sugar disorders are the result of this process going awry.
Also inside the pancreas are alpha cells, which release glucagon when we have low blood sugar. Glucagon helps raise our blood sugar levels by decreasing the storage of sugar in our cells and triggering the release of glucose back into the blood. The pancreas also produces and secretes important digestive enzymes like pancreatin, which are essential in maintaining a healthy digestion and assimilation of nutrients.
- Explain the importance of circadian rhythms and the underlying physiological mechanisms that govern them
- Key terms: Suprachiasmatic nucleus, oscillator, melatonin
The circadian rhythm is one of the most important concepts for a healthy human being to understand. Essentially, the circadian rhythm is our sleep-awake cycle. What this means is that our bodies are in tune with exposure to solar light. Many different systems in the body are activated and deactivated based on whether or not our bodies are detecting solar light. The circadian rhythm regulates a variety of functions in the human body, including: sleeping and feeding patterns, alertness, body temperature, brainwave activity, and hormone production. From an evolutionarily perspective, our sleep-awake cycles are connected to the daylight and the nighttime. What this means is we are built to be awake with the sun, and asleep with the night. Science is beginning to understand how disruption of the Circadian Rhythm via artificial light, artificial dark, etc. can result in negative consequences. See the following: http://www.cell.com/cell-metabolism/abstract/S1550-4131(16)30312-6
Inside the hypothalamus is a group of nerve cells known as the suprachiasmatic nucleus (SCN). The SCN is connected to the optic nerves in our eyes. When night falls, the amount of light getting into our eyes is getting less and less. This lack of light is detected by the SCN, which then triggers the hypothalamus to release the hormone melatonin.
As the sunrise approaches and light begins to be detected by the SCN, the hypothalamus stops producing melatonin and instead sends signals to the body which raise body temperature, heart rate, blood pressure, and delays the release of melatonin. All of these things are essentially preparing our bodies for the activity in the day ahead.
Interestingly, our desire to sleep is strongest from 2am – 4am, but this desire is also very strong around 2pm – 3pm. We ought to take under consideration the fact that we are the only species that exhibits this once-a-day sleep pattern. So perhaps incorporating a siesta into our daily routines is not a bad idea. This is just one example of how we can modify our behavior for better sleep and health.
- Describe the importance of this module’s information to the field of herbal medicine.
It should be obvious for any student of herbal medicine why this information is important and how it relates. The better we are able to understand how our body communicates with itself, the better we are able to make decisions about how best to live our lives, especially if we are suffering from a problem that can be easily remedied through a simple lifestyle change.
As we study the various communication channels that the body uses, we begin to see an overall pattern of feedback loops, a creation-destruction cycle, if you will. If we have an adequate understanding of how the various systems function and what raw materials they need to function, then we will be better suited to address problems as they arise, or even prevent them before they occur.
The melatonin example is one of my favorites. Many people these days are using melatonin much like a sleeping pill. They take large doses (1 – 5mg or more) on a daily basis. According to a 2001 study from MIT (http://news.mit.edu/2001/melatonin-1017) , the proper dose of Melatonin is 0.3mg, which is far below the dose that most people are taking when they buy a supplement form of Melatonin. Some Naturopathic Doctors are even recommending people give their children melatonin! This seems foolish for several reasons: not only is this manipulating the child’s circadian rhythm and not addressing the root cause of why the child is not sleeping, but it is unnaturally dosing a growing brain and body with a powerful hormone. There are unexpected consequences to this behavior, that much is guaranteed, and it is irresponsible for a doctor of natural medicine to utilize supplements in this fashion.
Studies are now uncovering the broad role of melatonin and the fact that it is not just related to sleep, but also to a variety of neuropsychiatric disorders: http://www.chronobiology.com/role-melatonin-variety-neuropsychiatric-disorders/ It is still largely unknown what role the artificial supplementation of melatonin is having on the human psychiatric condition, especially with the developing brains of children. This is an area that needs much more research and people would be wise to approach the situation with caution before they haphazardly dose themselves or their children with a powerful hormone like melatonin.
While I fully support the right of the individual to use whatever drugs or supplements they want, I also have to voice my opposition to such an approach. Instead of giving people melatonin as a sleeping pill, we should talk to our clients about their habits and see if they would be willing to try simple herbs like Lemon Balm, Passionflower, or Catnip tea. Instead of giving them melatonin in a pill, we could recommend they drink tart cherry juice, or even take a tart cherry supplement, in order to lessen inflammation and provide a natural food source of melatonin. See the PubMed study that concluded that “consumption of a tart cherry juice concentrate provides an increase in exogenous melatonin that is beneficial in improving sleep duration and quality” (http://www.ncbi.nlm.nih.gov/pubmed/22038497)