The human nervous system is one of the most highly developed and evolved organismal mechanisms known. This system functions with the help of the brain, spinal chord, nerves, tissues, and reflexive responses—just to name a few of the major parts to this highly specialized ‘computing system.’ Made up of the central, peripheral, and autonomic nervous systems, this conglomerate has a lot to offer, as well as a lot to understand…

 

 

 

 

Understanding the Basic Background:

 

The nervous system is comprised of the Central Nervous System (CNS), and the Peripheral Nervous System (PNS). The CNS is concerned with the brain and spinal chord, while the rest is left to the PNS—such as nerves in the appendages. The nervous system is very unique in that it works both chemically and electrically. This makes it fast, specific, short-lived, and very adaptive. To further separate between the PNS and CNS, the CNS uses tracts and nuclei, while the PNS uses nerves and ganglion.

 

The PNS:

 

The PNS has two divisions: the sensory (afferent) and motor (efferent). Both sensory and motor divisions deal with both visceral and somatic aspects. Within the motor division’s visceral (ANS) facet there is another split into the sympathetic or parasympathetic divisions. Sympathetic is commonly referred to as “fight or flight,” while the parasympathetic is dominant in calm times—such as digestion, resting, etc. (this can be observed visually to the right of the page).

 

All About The Neuron:

 

The neuron is involved with sensory and motor function. The motor neurons carry out their info to the visceral and somatic departments, and the sensory neurons bring info back to the CNS. In addition, interneurons are located within the CNS and help process and filter information. The neuron has some interesting properties as well. It contains excitability, which allows it to receive stimuli; conductivity, which allows it to take the info to a source; secretion, which allows it to pass its info from one neuron to the next; dendrites, which allows the neuron to receive the info from another neuron. Lastly, the impulsive travels from neuron to neuron via the axon, which is a long membranous ‘chord.’ This membranous chord is insulated by myelin, which produces greater velocity, as it passes from one Node of Ranvier to the next. Neurons can also be classified as multi or bipolar, depending on whether or not it has two or more extensions.  

 

Cells of the PNS and CNS:

 

In the CNS, neuroglia cells such as: oligodendrocytes, ependymal cells, microglia, and astrocytes are present. Oligodendrocytes function to create myelin sheaths for axons—which increase conductivity. Ependymal cells produce CSF and line cavities. Microglia (macrophages) that look for pathogens in CNS. Astrocytes create barriers between capillaries and axons (blood-brain barrier), and is the most common glial cell.

In the PNS, Schwann cells and satellite cells are of importance. Schwann cells are similar to oligodendrocytes in the CNS, except in the PNS they wrap around only axon. Satellite cells are there for protection, regeneration, and re-wiring.

 

But how exactly does an impulse get generated and how does it work?

 

Beginning at resting membrane potential (-70mV), there is a high concentration of sodium, and chlorine outside of the cell, and a high concentration of phosphate, and potassium within the cell. The sodium potassium pump works to take 3 sodium ions from the inside of the cell to the outside and 2 potassium ions from the outside of the cell to the inside, therefore this allows the outside of the cell to stay more positive. In addition, there are ‘leaky’ channels which allow the previous ions to pass down the gradient, however more potassium leaks, which moves more positive charge to the outside. In the next part (local depolarization) a ligand (usually Ach) binds to a ligand gated channel and opens it, thus allowing for a massive influx of sodium and positive charge—which then creates an effect that travels down the axon. It should be noted that this occurs around (-55mV). As this is occurring at the relative area, potassium will then begin to leave the cell and make the outside more positive—thus repolarizing the cell around (+35mV). Once this has been accomplished, the rest of the ligands will be broken down by Ach-esterase, which will produce acetic acid and cholines to be recycled by the presynaptic knob. While the outside of the cell is getting more positive, more potassium is outside of the cell and sodium is within. The solution is utilization of the sodium potassium pump to bring the respective ions to their respective sides, such as in resting membrane potential. It should also be understood that when repolarizing, the channel is left open slightly longer, and therefore allows more potassium to leave, and creates a hyperpolarized state below (-70mV).

 

Now that you understand the general mechanism, lets dive into it a bit more. First off, the impulse that travels down the axon is called an ‘action potential.’ To get an action potential to fire, the cell utilizes excitatory postsynaptic potentials to increase the likelihood of an action potential. On the flip side, inhibitory postsynaptic potentials reduce the likelihood of an action potential. These postsynaptic potentials can either be temporal, where they come from one source, or they can be spatial—where there are more numerous sources acting upon the system. Action potentials have characteristics such as: once threshold is reached, the entire membrane reacts and cannot go back, they are also non-detrimental and keep their magnitude throughout, and they are irreversible. After the action potential passes a segment of the axon, the ‘refractory period’ makes it so that the membrane cannot depolarize right away (absolute refractory), however shortly after in the ‘ relative refractory period’ there is the capability for another action potential to be formed.

 

Your mother told you to have good posture for a reason—to protect your spine! Now, lets see why the spine and its assets are so important:

 

The spine or ‘backbone,’ is essential in supporting the human body. Its primary functions are conduction, locomotion, and reflexes. It is made up 31 spinous processes as well as 31 pairs of spinal nerves—as the nerves drop below L1, the spinal nerves branch off into numerous ones—this is called the Cauda Equinae. The processes are account for as: 8 in the cervical region, 12 in the thoracic region, 5 in the lumbar region, and 5 in the sacral region—getting larger as they move down the spine. Each spinous process has a sub-aracnoid space that contains CSF for protection. It also has grey matter, which is involved with somal synapses, as well as dorsal, ventral, and lateral horns. In addition, a grey commissure is present which allows for left and right communication of spinal info—called decasation. There is another type of matter called white matter, which works via the ventral, lateral, and dorsal columns to ascend and descend information.

 

Information enters and leaves the spinal chord via the spinal nerves. Afferent or sensory information is received in the dorsal horn, and must pass the dorsal root ganglion, which is a cluster of nerve cell bodies. The anterior horns deal with efferent or motor function. In the grand scheme, these impulses are then taken up through the grey matter, and travel up the white matter until they reach the thalamus, which directs it further within the brain. Neurons can also be categorized into orders (1st, 2nd, 3rd). First order neurons would be from one’s finger to the medulla, 2nd would from the medulla through the midbrain, then to the thalamus, and a 3rd would be from the thalamus to the destination. It is also important to mention that nerves are bundled similarly to muscle with an internal endoneurium, which covers each fiber, then a perineurium that surrounds the fascicle, and lastly the epineurium that covers the nerve.

 

Reflexes are very important to humans. Myotatic reflex of the muscle occurs when the cell recognizes a stretch and increases tone to compensate. Reciprocal inhibition is also used to prevent muscles from working against one another. The polysynaptic flexor withdrawal is when the hip flexor and hamstrings are dorsiflexed, which inhibits the glutes, vastus, and plantars—which allow other leg muscles to hold balance due to decussation through the spinal nerves. Also the golgi tendon reflex, is where excessive tension on the tendon inhibits the motor neuron—which inhibits the contraction of that muscle.

 

The ANS, and why it is perfect for lazy people:

 

The ANS does not require thought, therefore it lazy people would most likely love this aspect of it. Within the ANS, motor neurons control glands; cardiac and smooth muscle, and maintain homeostasis. The lateral horn (visceral) sends a myelinated signal to call for Ach, which the autonomic ganglion then forwards through the unmyelinated postsynaptic fiber, and releases Ach or norepinephrine to the visceral effector organ. The ANS has two neurons from CNS to effectors: the presynaptic neuron cell body in CNS and postsynaptic neuron cell body in peripheral ganglion. This can be seen by the image, which provides an easier avenue to understand the concept at hand.

 

Returning back to sympathetic and parasympathetic, it is important to know that the sympathetic systems preganglionic neurons originate in the thoracolumbar region, and the parasympathetic systems preganglionic neurons originate in the cranial-sacral region. Sympathetic, which has longer lasting effects, has two types of receptors: Adrenergic and Cholinergic. It is important to note that the effects of the ANS are determined by the type of neurotransmitter released, and the type of receptors on target cells. For instance, this may make a heart beat fast, but relax the lungs.

 

 

Its time to explore the brain and its nerves! Let’s see if we can make some ‘connections’:

 

The brain is a lobular mass of tissue. To separate the lobes, fissures are present, these separate gyri, which can be thought of as the thick brain folds. Sulci are the shallow grooves in between gyri, which is the most superficial aspect of the cerebral cortex. At the bottom of the brain, the brainstem is present with the diencephalon, midbrain, pons, and medulla. These work together with the help of white matter, which allows for input to be manipulated up or down its tracts; it should also be noted that grey matter is more involved with the receiving on an impulse. The brain has many other parts of it that will be noted. The Choroid Plexus is a mass of capillaries on the wall of the ventricles. The ventricles also add CSF at the 3rd and 4th. CSF is contained via the meningeal layers (Pia-, Arachnoid-, Dura-mater) from deep to superficial. These layers help protect the brain from injury as the CSF creates buoyancy, and the layers also protect against toxins. This is better understood by the Blood-brain barrier, which is only permeable to certain molecules or ions.

 

Nerves from the brain pass through the medulla. The medulla functions for sensory and motor functions, temperature, taste, pain, and regulates heartbeat. Also, nerves 9-12 begin or end here. The other nerves deal with the pons (5-8) and midbrain (1-4). The image to the right depicts the nerves and their function. The pons is important as it relays motor and sensory info and has peduncles that attach it to the cerebellum. The brain stem is a very special area as you may come to find. With the midbrain and their superior/inferior coliculi, visual attention and hearing result. Another interesting thing is reticular formation, which grey matter that runs along the brainstem, and functions in somatic motor control, cardio control, pain modulation, and allow what the brain wants to detect. Another key part of the brain in the thalamus, which is the gateway to the cerebral cortex. It is involved in memory and emotion of limbic system. Just under the thalamus is the hypothalamus, which relays signals from the limbic system to the thalamus. The hypothalamus controls the endocrine system and ANS (This is done through pituitary secretions), and has a thermoregulatory and homeostatic role. On the other side, the epithalamus contains the pineal gland, which helps regulate the sleep cycle. Lastly, it is important to mention the corpus callosum, a commissure, which acts to connect two hemispheres of the brain. All of these structures work in synergy and together to allow the brain to function and to recognize and transmit information to and from the ‘control centers.’