How neurons give the nervous system its powerful abilities and how neurological conditions lead to impaired nervous system function

 

By Tré LaRosa
NeuLine Health

Neurological conditions are conditions that have a directly negative impact on the nervous system. But not every neurological condition is due to the same pathophysiology or affects the same part of the nervous system, nor do all neurological conditions present identically (or even similarly) in the clinic. Some neurological conditions directly affect a person’s cognition, some affect a person’s mobility, and sometimes both are affected. For these reasons and more, it’s not exactly helpful to view every neurological condition in the same way. A more helpful way of viewing neurological conditions is that they all result from damage to neurons, which are the cells that make up the nervous system. From this basic level, we can zoom outwards to better understand how neurological conditions are similar and why they are different.

The neuron and nerve cell communication

Every part of the nervous system is made up of specialized cells called neurons. Neurons, like all human cells, have smaller structures that have specific functions; these are structures like the nucleus, Golgi apparatus, endoplasmic reticulum, and mitochondria. These structures function nearly identically in every cell, yet cells have different functions. This is due to cellular differentiation which is the process by which a cell goes from a stem cell (or unspecialized) to a cell with a specific function. Specialized cells contain the same basic organelles but they usually have adaptations that allow them to work in concert with other identical specialized cells as well as other types of cells to perform functions. That means that a skin cell does not look or function like a neuron or kidney cell, though they both contain basic organelles (i.e., nucleus, mitochondria). Understanding neurons and how they give rise to the rest of the nervous system’s critical functions is a core responsibility of neuroscientists and neurologists.

Neurons are remarkable cells, not only for their function but also for their structure. Structurally, neurons have a central body that contains the aforementioned basic organelles, called the cell body or soma, and then they have tendrils — called axons and dendrites — coming out of them in all directions. Neurons look a bit like a small rock embedded within a spider web. The dendrites and axons are critical for the function of the neuron’s specialized capabilities and therefore for healthy functioning of the nervous system. Let’s step back for a second and discuss the basic structure and function of the nervous system. The brain, spinal cord, and nerves make up the nervous system and, at its most basic, the nervous system is what controls the other organ systems (pulmonary, cardiovascular, muscular, gastrointestinal, skeletal, etc). It controls our body movements, it’s what controls our breathing, our sleep, sex, hunger, and thirst drives, and it’s what allows us to sense and process the world. The machinery required to operate such a complex role is intricate, which brings us to neurons. Above, I mentioned the tendrils that extend outward from the cell bodies of neurons. There are two types of tendrils: dendrites and axons. All neurons have only one axon with varying amounts of dendrites. Axons function as the conduit by which a neuron can send a message to another neuron; dendrites, on the other hand, function as the methods by which neurons receive signals. In practice, that looks like one neuron sending a message through its axon to a neighboring neuron’s dendrite, and this recipient neuron then distributes its messages through its axon to another neuron’s dendrites, and so on and so forth. Axons and dendrites are not intertwined however; there is an infinitesimal junction connecting them called the “synaptic cleft.” It’s in the synaptic cleft that brain chemistry comes into the picture. Once the electrical signal reaches the end of the axon, its terminal vesicles filled with certain neurotransmitters (depending on the message sent) are released into the synaptic cleft. The released neurotransmitters then bind to receptor molecules on the target cell’s dendrites, which transfer an electrical signal to the target cell. When your brain is sending a signal to your finger to move it, it requires the transfer of information from the brain to your spinal cord to the target cell.

The nervous system and neurological conditions

Altogether, the nervous system is an electrochemical system where it uses electricity to trigger the release of chemicals in different ratios to send signals to target cells and control the rest of the body. This electrochemical cascade happens for every biological function, including our conscious and unconscious processes. Your brain tells your lungs to breathe and eyes to blink, and it ensures your heart remains beating while your digestive tract catalyzes and metabolizes the nutrients that nourish and sustain the rest of your body. My brain is actively sending signals to my hands to write these very words you’re now reading, which you understand because your eyes send signals to your brain that your brain can process. 

There are two major divisions of the nervous system: the central nervous system and peripheral nervous system, which are always communicating with each other. The central nervous system (CNS), the brain and the spinal cord, coordinates the activities of the rest of the body through nerves which make up the peripheral nervous system (PNS). Though your central nervous system controls the movements of the rest of the body, the peripheral nervous system still must transmit messages to the brain which triggers the brain to respond and send responses back to the peripheral nerves. You know not to touch an active stove, but if you were to accidentally touch one, the nerves in your fingers send an almost instantaneous signal to your brain alerting it to the danger from the temperature of the stove. Your brain replies by firing a signal back to your finger telling it to come off the stove. In the meantime, there is damage done to the epithelial cells in your finger. As you experience this moment, it happens in such rapid succession that the time that passes between the nerves in your finger sending the signal to your brain and your brain’s response to move your finger is a fraction of a second. Consider that your nervous system did that while at the same time controlling the muscles in your legs, heart, GI tract, and arms. This multifaceted responsibility of the nervous system is why there are two subdivisions of nervous system function called autonomic and somatic nervous systems. Your autonomic nervous system is what controls your involuntary actions, such as your breathing while you’re sleeping (or not consciously thinking about it), your heartbeat, and other routine movements that we don’t have to think about. Your somatic system controls your sensory input (all except your sight) and the movements you choose to make. When you go to play a sport or go for a walk or brush your teeth, you are voluntarily choosing to make those movements; you can’t simply stop your own heart rate like you could stop moving your hand while brushing your teeth. Further, there’s the enteric nervous system that lines your digestive tract which underlies the also-fascinating gut-brain axis.

The higher-level view of the subdivisions of the nervous system relies on communication between neurons. Neurological conditions are the result of damage to the nervous system, but where that damage occurs is what results in the phenotype of the condition. 

There are other components to the nervous system that improve its efficiency. For example, there is the insulating myelin sheath that wraps the axon which allows for increased conductance of electrical signals down the length of the axons, thus increasing the speed at which a signal reaches the axon terminal. Without the myelin sheath, a much higher amount of energy is required to transfer a signal in unmyelinated fibers. Multiple sclerosis, one of the most common neurological conditions, is such a pervasive condition for this very reason: In people with MS, their immune system attacks the myelin sheath of their neurons, resulting in inflammation and destroyed nerve cell processes. The myelin sheath is so effective at what it does that “if nerves were not myelinated and equivalent conduction velocities were maintained, the human spinal cord would need to be as large as a good-sized tree trunk.”

Conditions like Parkinson’s, Alzheimer’s dementia, stroke, and epilepsy are conditions of the central nervous system. Despite these conditions affecting the same subdivision of the nervous system, they present in the clinic much differently. This is because the damaged neurons are in different parts of the brain; neurons function similarly as one another, but still, different parts of the brain are responsible for different functions. Parkinson’s results in damage to a part of the brain that controls movement, the basal ganglia, while early Alzheimer’s usually results in damaged neurons in the hippocampus and the entorhinal cortex, parts of the brain involved in forming memories (and thus why memory loss is one of the first clinical symptoms of Alzheimer’s). Yet, there is also Parkinsonian dementia, though not everybody with Parkinson’s will also develop dementia. The brain is a complex organ, and sometimes damage to one part of the brain results in effects seen in one person but not another. When neurons in a certain part of the brain die, that part of the brain is no longer capable of sending the signals it had previously sent, thus leading to the symptoms we see once a condition becomes severe enough, such as tremors in Parkinson’s and memory loss in Alzheimer’s. Many behaviors can lead to neuronal death, so while this is not the only consideration for neurological disease prevention, preventing the destruction of neurons should be a priority for anybody who wants to reduce their likelihood of developing Alzheimer’s or Parkinson’s. And since aging is such a pronounced risk factor, a current priority in neurology is neuroprotection and neurogenesis.

Neurons are intricate, microscopic structures that give rise to all the functions that we see in the nervous system. Neurons, the organs that make up our nervous system, and the intricacy of the different subdivisions of the nervous system is just one of the many reasons for why the human body is such a marvel. Maintaining the health of our nervous system is one of the most profound things we can do to keep ourselves healthy and safe, and it’s paramount that we continue advancing medicine and research as fast as possible to improve the quality of life of every person affected by neurological conditions.  

Resources

  1.  Organization of the Nervous System | SEER Training. (n.d.). Retrieved from https://training.seer.cancer.gov/anatomy/nervous/organization/
  2. The Principles of Nerve Cell Communication. (1997). Alcohol Health and Research World, 21(2), 107–108.
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