anatomy – All Things Body

The Nervous System II

In the previous entry I talked about the electrical signal or action potential that’s fired and acts as the initial communication in the nervous system. In this entry I’ll cover the network upon which that signal travels, as well as a few other related and interesting topics.

The Electrical Network

The network of our nervous system is not so different from the electrical system in our homes, it’s just a lot more complex. And gooey.

There is an overall structure:

There is the central nervous system (CNS), which is comprised of our spinal cord and our brain, and is sort of like our fuse box.
And then there are all the wires emanating from the CNS to our limbs and extremities. This is the peripheral nervous system (PNS) and is akin to the wiring that winds through the walls of a home. It starts at the point where nerves extend from the spinal cord and runs to the ends of our limbs. The sciatic nerve is probably the longest axon (nerve fiber), running from the lumbosacral plexus L4-S3 (you’ll understand what this means in few paragraphs believe it or not) to our big toe.

Also, the PNS is divided and categorized several times–first into the somatic (voluntary movement) and autonomic (involuntary movement) systems; and then the autonomic system is broken down into the sympathetic and parasympathetic systems. More on that later.

So, again thinking back to Part I of the Nervous System, the impetus to all activity in our bodies is a change in the action potential–a change in the electro-chemical charge of ions in, and surrounding, our cells and in this case, our neurons.

The action potential, or electrical charge, travels the length of an axon toward the brain. The signal makes connections along the way; first in the spinal cord, then in the thalamus, and lastly in the cerebral cortex.

Different types of signals have different “tracts”, or paths on which they travel. For example, the spinothalamic tract conveys touch sensory data and crosses over–decussates–to the opposite side of the spinal cord before continuing on to the thalamus. So the message of a bump to the right leg will travel up the left side of the spinothalamic tract. It is an ascending tract, meaning that the signal is moving from the PNS to the CNS.

It’s important for medical professionals to know all the tracts and possible ways that information travels to and from the brain as it helps in narrowing down the specific location of a spinal cord injury. For example, some tracts communicate touch sensors, others pain, and others temperature, etc.

Ascending and descending tracts of the nervous system.

The central nervous system, or CNS, includes the brain and the spinal cord, which travels uninterrupted from the brain stem to the lumbar vertebrae. The spinal column is made up of 33 different bones called vertebrae, each with a hole in the center, through which the spinal cord travels. Of the 33 vertebrae, 7 are in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the sacral region and 4 in the coccygeal region.

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Most of us know that spinal injuries are very serious, but do we understand why? It’s not a broken vertebrae that serious although that shouldn’t be taken lightly–bones heal. Because nerve fibers in the central nervous system (brain and spinal cord) rarely regenerate, such damage is irreversible. If the spinal cord is severed, there is no neural communication getting past that point, which is why injuries higher up on the spine are more deadly; if the cord is injured above vertebrae C4, we lose neural communication and use of all limbs, but if the injury is below T11, we lose use our upper limbs only. The phrenic nerve, which extends from the area between vertebrae C3-C5, leads to our diaphragm and controls our breathing; obviously any damage above or to that nerve could be life threatening.

So far I’ve covered a lot on voluntary sensory communication–you get cold and put on a jacket, or your finger gets burned so you move it away from the heat–but a lot of our neural communication happens behind the scenes. The autonomic system that I mentioned earlier controls all involuntary neural communication. Again, it’s divided into the sympathetic and parasympathetic systems and they are almost opposite in their affect on the body.

Most of us are familiar with the sympathetic system as it’s commonly referred to as fight-or-flight; this system is activated involunarily, in emergency situations when we feel emotionally and/or physically threatened. It causes our endocrine system (more on that later) to release the hormone adrenaline (also known as epinephrine) which causes:

accelerated heart rate,
widened bronchial passages,
decreased digestive function,
constricted blood vessels,
pupil dilation,
goose bumps,
perspiration (sweating), and
raised blood pressure.

It basically mobilizes all our resources toward functions primarily tasked with quick thought and action.

Conversely, our parasympathetic system–also known as rest-and-digest– is working behind the scenes when we’re not stressed. It’s effects include:

slower the heart rate,
increased intestinal and gland activity,
relaxed gastrointestinal tract
sexual arousal

An important aspect of how an action potential travels on an axon is called saltatory conduction, which means that the action potential leaps across the axon as it travels. This greatly speeds up transmission of the signal and is integral to survival. This is possible due to the basic structure of the axon: it’s divided into sections by a type of fatty insulation called a myelin sheath. The small space in between each myelin sheath is the node of ranvier, and acts as the hopping point while the myelin insulates and accelerates the signal.

neuron_structure

The movement of newborns is jerky because their myelin sheaths are not fully mature; as a result the stimulus or action potential travels along the axon in an irregular, uncoordinated way.

Many diseases are the result of demyelination, the most common being multiple sclerosis. In MS, the myelin sheaths in the CNS are damaged, which disrupts saltatory conduction, i.e., the ability of parts of the nervous system to communicate. The results are a range of signs and symptoms, including physical, mental, and sometimes psychiatric problems.

Parkinson’s is also a degenerative disorder of the nervous system, with progressive impairment or deterioration of neurons in an area of the brain known as the substantia nigra. This part of the brain plays an important role in reward and movement, which is why Parkinson’s sufferers not only have physical symptoms (shaking) but also commonly have problems with impulse control.

There is so much of the nervous system that I haven’t covered, but it’s time to move on. I welcome any questions, even on topics not included above–I could talk/write about this stuff all day. 🙂

The Nervous System I

A quick note to promote Coursera, an aggregator of online classes–if you’re planning to take Physiology anytime soon, I highly recommend taking it here first as a primer, especially if you have no familiarity with the material. The Duke University curriculum and instructors are excellent–and free! You should also consider it if you’ve already taken physiology but feel like you need to brush-up before taking the NLN PAX.

Now on to the nervous system–our nervous system, like the circulatory system, engages every part of our body. But instead of veins, arteries and capillaries, it’s comprised of axons, dendrites and glia, conducting electricity. Working in conjunction with the muscular system, it allows us to move and breathe. And because the heart is a muscle, it keeps the engine of the circulatory system humming. It helps to keep our vitals systems in homeostasis by communicating dangerous rises or drops in blood pressure, heart rate, pH, O2 and CO2 levels and sometimes releases neurotransmitters into the blood to move these systems back to setpoint. Lastly, it communicates a variety of sensory messages to the brain.

So how does all this communication happen? There are two main components: the electrical signal, and the network on which it travels.

The Electrical Signal
The fluid inside and surrounding our cells (intracellular fluid, ICF, and extracellular fluid, ECF) is filled with chemical elements—mostly sodium (Na), Potassium (K), Calcium (Ca), and Chloride (Cl). If an element has more or fewer electrons than protons, it has an electrical charge and is called an ion. The charge is measured, + or -, in millivolts (mV). Remember that.

A plasma membrane, the protective wrapper of the cell, separates the ICF and ECF. The mix of ions in the ICF and ECF determines its voltage, and the difference in voltage between the ICF and ECF determines the membrane potential for that cell. For example, at rest, most neurons have lots of K+ in the ICF and lots of Na+ in the ECF. The difference in voltage is about -70 mV, and this is the resting membrane potential (RMP) for the cells of our nervous system, aka neurons.

When a stimulus occurs–the smell of pancakes, a sting on your leg, even a slight breeze on the hairs of your forearm–voltage-gated Na+ channels on the plasma membrane open and Na+ rushes into the cell. This changes the difference in voltage between the ECF and the ICF and the membrane potential rises–depolarization of the neuron begins. Remember, it starts at -70 mV, the RMP. If the stimulus is strong enough, and enough Na+ moves into the cell, the membrane potential rises to +30 mV, and an action potential, or electrical signal is fired. The action potential then travels the length of the axon to your brain, where the stimulus is registered and another signal is sent from the brain to your muscles–you decide to eat pancakes, move your leg, or put on a jacket.

action_potential1334220921593 — In the center, the signature of an action potential. Around it, the phases of cell depolarization and repolarization.

Once the membrane potential reaches +30 mV and the action potential fires, the Na+ channels shut and the cell repolarizes. If the stimulus continues, another action potential is fired. Of course this all happens in femto seconds and is happening in more than one neuron–your fingertips have as many as 100 touch receptors per cm2.

An example of a medication that tweaks our nervous system, as many of our medications do, is lidocaine, which binds and inhibits voltage-gated Na+ channels. Liocaine is what the dentist injects into your gums before drilling your teeth. By inhibiting the voltage-gated ion channels of the neurons in your gums, there is no Na+ entering the cell, no cell depolarization, no action potential fired–no stimulus of the pain/nociceptors, and no pain felt.

That’s a lot of info–welcome to #nurselife! And that’s probably enough info for now. I’ll cover the network that all these signals travel upon next time–the information highway of our bodies! 😉

The Heart

Although it is not the only or heart-lores gan necessary for survival—the brain, liver, kidneys (at least one), pancreas, and lungs come to mind–we still regard the heart as most integral to life, most likely because of its central location and role in pumping blood throughout our entire body.

As children it was probably the first organ we learned about. That it was possible to press your ear to someone’s chest and hear it encouraged the attraction—bump-bump, bump-bump, or in the medical world, lub-dub, lub-dub—we know it’s beating, without the aid of special instruments. Conversely, assessing the health of other organs requires lab results, scans, or other medical diagnostics. But confirmation of the heart’s function is palpably evident.

As heart transplants become more common, our most familiar organ may lose some of its status; indeed, in cardiac units around the country, many patients today are being discharged not only with a donor heart, but increasingly, a completely artificial, mechanical heart.

Here are some facts:

5.5 L: the amount of blood in the adult body (men have a little more than women)
60-80 bpm: the number of times the heart beats in a minute

The completion of one entire circuit, meaning that the blood travels from the heart through the arteries, out to all tissues and organs, and then returns to the heart via the veins, takes about one minute. There is no area in the body for blood to be stored—it is constantly moving. So in one completed circuit, all 5.5 liters of blood is pumped through the entire body, in one minute. Impressive, right? The heart is not lazy.

Blood travels to organs and tissues primarily to keep them oxygenated. Without that key element our organs would stop working. O2 is required to by every cell in the body to complete specific functions, depending on the type and location of the cell. Our cells are like us, but smaller. They make things, require certain nutrients, and create waste. All that activity keeps our metabolism going–it is our metabolism. A byproduct of that activity—the waste–is CO2. CO2 is transported in the blood from the tissues and organs, via the veins, back to the heart, and then to the lungs, where you exhale it out of your body. More on that in circulation, which is fascinating and happens 24/7 without us ever thinking about it.

The heart has heart-lores-labeled four chambers: two halves, with two chambers each—an atrium—some call it an auricle—and a ventricle. Each atria and ventricle pair pump blood to a different part of the body. The right half supplies the lungs with blood (via the pulmonary circuit), and the left half supplies the entire body and brain with blood (the systemic circuit). Because the left ventricle pumps blood to a much larger area than the right, it is larger and much more muscular than the right ventricle.

It’s important for the blood to move in one direction through the heart–for it not to backup–and this is controlled by valves. There are four of them: one between each atria and ventricle pair and another leading out to either the pulmonary or systemic circuit. Blood fills the atria first (both left and right atria fill at the same time), pressure builds, the atria contract, and the bicuspid and tricuspid valves open.

The first sound—the lub—is the sound of the bicuspid and tricuspid valves closing. Blood then fills the ventricles, pressure builds, the pulmonary and aortic valves open and blood moves out of the heart into the pulmonary and systemic circuits. The second sound—the dub—is the sound of those valves closing. And the entire process starts over from the beginning.

The heart contracts about 80 times a minutes for our entire lives. An electrical impulse, conducted by nerve cells, or neurons, is what keeps our hearts pumping continuously throughout our lives. I’m working on a post on the nervous system next, but a quick intro is necessary when talking about the heart. Electrical impulses or signals are carried throughout the body by the nervous system. The nervous system is mostly communicating with muscle and endocrine (hormones) cells.

The heart is constructed of muscular tissue and also covered in a web of neurons. This is significant because in addition to the neural communication occurring with the individual muscle cells of the heart–the myocytes–the neurons surrounding the heart muscle are providing a separate set of instructions.

These nerve fibers converge at a few key places in the heart, into nodes. The SA node, short for sinoatrial node, is also called the pacemaker, because it is where the electrical stimulus in the heart ori heart--nerve-fibers-and-labesl ginates, and it sets the pace for the electrical impulse as it travels to several other nodes and bundles of conducting fibers. It sets the pace for the heartbeat and can be sped up by the nervous systems if we experience a surge of epinephrine and the fight or flight urge. An ECG is measuring the electrical activity of the heart and can indicate damage to the SA node and the accompanying nodes and conducting neurons.
This electrical pulse originates the contraction of the cardiac muscle; as long as the SA node is intact, the heart will continue to contract, or beat, even if lifted from its protective space in the thoracic cavity. It is timed specifically to contract the atria first (remember the atria fill with blood first, then the ventricles), then pause so that the atria have time to empty completely before the electrical pulse moves to the AV node, which controls contraction of the ventricles. Not having a pacemaker, the AV node relies on the SA node–the pacemaker–to set the heart rate. Although surgical implants of a pacemaker is high on the list of unnecessary surgeries, if it’s truly needed it’s a lifesaver.

Most neurons need a stimulus to send a signal, or “fire”. Interestingly, the SA node will fire action potentials in the absence of any stimuli. These spontaneously active cells have a precise balance of voltage-activated ion channels that allow the cells to fire without a stimulus.

Most of the ways to keep your heart healthy are also ways to keep your entire body healthy:

Adequate Sleep—7 to 8 eight hours a night
Blood pressure—high BP causes excessive stretching of arteries, which causes scarring, then plaque build-up, then narrowing, then arteriosclerosis—not a good trajectory!
Reduce Sugar intake—high glucose levels also cause arteriolosclerosis
Sit less, sweat more
High-fiber diet—also good for your colon and weight control
Dental Hygiene
Manage Stress—exercise and sweating help with this

So that’s the heart! I’d love it it you had questions, or if you’re also a nursing student, I’d love to hear what path you’re taking and where you are in your studies.