Critical Thinking Questions About The Nervous System

In the 1800s a German scientist by the name of Ernst Weber conducted several experiments meant to investigate how people perceive the world via their own bodies (Hernstein & Boring, 1966). It is obvious that we use our sensory organs—our eyes, and ears, and nose—to take in and understand the world around us. Weber was particularly interested in the sense of touch. Using a drafting compass he placed the two points far apart and set them on the skin of a volunteer. When the points were far apart the research participants could easily distinguish between them. As Weber repeated the process with ever closer points, however, most people lost the ability to tell the difference between them. Weber discovered that the ability to recognize these “just noticeable differences” depended on where on the body the compass was positioned. Your back, for example, is far less sensitive to touch than is the skin on your face. Similarly, the tip of your tongue is extremely sensitive! In this way, Weber began to shed light on the way that nerves, the nervous system, and the brain form the biological foundation of psychological processes.

In this module we will explore the biological side of psychology by paying particular attention to the brain and to the nervous system. Understanding the nervous system is vital to understanding psychology in general. It is through the nervous system that we experience pleasure and pain, feel emotions, learn and use language, and plan goals, just to name a few examples. In the pages that follow we will begin by examining how the human nervous system develops and then we will learn about the parts of the brain and how they function. We will conclude with a section on how modern psychologists study the brain. 

It is worth mentioning here, at the start, that an introduction to the biological aspects of psychology can be both the most interesting and most frustrating of all topics for new students of psychology. This is, in large part, due to the fact that there is so much new information to learn and new vocabulary associated with all the various parts of the brain and nervous system. In fact, there are 30 key vocabulary words presented in this module! We encourage you not to get bogged down in difficult words. Instead, pay attention to the broader concepts, perhaps even skipping over the vocabulary on your first reading. It is helpful to pass back through with a second reading, once you are already familiar with the topic, with attention to learning the vocabulary.

Nervous System development across the human lifespan

As a species, humans have evolved a complex nervous system and brain over millions of years. Comparisons of our nervous systems with those of other animals, such as chimpanzees, show some similarities (Darwin, 1859). Researchers can also use fossils to study the relationship between brain volume and human behavior over the course of evolutionary history. Homo habilis, for instance, a human ancestor living about 2 million years ago shows a larger brain volume than its own ancestors but far less than modern homo sapiens. The main difference between humans and other animals-- in terms of brain development-- is that humans have a much more developed frontal cortex (the front part of the brain associated with planning). 

Interestingly, a person’s unique nervous system develops over the course of their lifespan in a way that resembles the evolution of nervous systems in animals across vast stretches of time. For example, the human nervous system begins developing even before a person is born. It begins as a simple bundle of tissue that forms into a tube and extends along the head-to-tail plane becoming the spinal cord and brain. 25 days into its development, the embryo has a distinct spinal cord, as well as hindbrain, midbrain and forebrain (Stiles & Jernigan, 2010). What, exactly, is this nervous system that is developing and what does it do?

The nervous system can be thought of as the body’s communication network that consists of all nerve cells. There are many ways in which we can divide the nervous system to understand it more clearly. One common way to do so is by parsing it into the central nervous system and the peripheral nervous system. Each of these can be sub-divided, in turn. Let’s take a closer, more in-depth look at each. And, don’t worry, the nervous system is complicated with many parts and many new vocabulary words. It might seem overwhelming at first but through the figures and a little study you can get it.

The Central Nervous System, or CNS for short, is made up of the brain and spinal cord (see Figure 1). The CNS is the portion of the nervous system that is encased in bone (the brain is protected by the skull and the spinal cord is protected by the spinal column). It is referred to as “central” because it is the brain and spinal cord that are primarily responsible for processing sensory information—touching a hot stove or seeing a rainbow, for example—and sending signals to the peripheral nervous system for action. It communicates largely by sending electrical signals through individual nerve cells that make up the fundamental building blocks of the nervous system, called neurons. There are approximately 100 billion neurons in the human brain and each has many contacts with other neurons, called synapses (Brodal, 1992). 

If we were able to magnify a view of individual neurons we would see that they are cells made from distinct parts (see Figure 2). The three main components of a neuron are the dendrites, the soma, and the axon. Neurons communicate with one another by receiving information through the dendrites, which act as an antenna. When the dendrites channel this information to the soma, or cell body, it builds up as an electro-chemical signal. This electrical part of the signal, called an action potential shoots down the axon, a long tail that leads away from the soma and toward the next neuron. When people talk about “nerves” in the nervous system, it typically refers to bundles of axons that form long neural wires along which electrical signals can travel. Cell-to-cell communication is helped by the fact that the axon is covered by a myelin sheath—a layer of fatty cells that allow the signal to travel very rapidly from neuron to neuron (Kandel, Schwartz & Jessell, 2000)

 If we were to zoom in still further we could take a closer look at the synapse, the space between neurons (see Figure 3). Here, we would see that there is a space between neurons, called the synaptic gap. To give you a sense of scale we can compare the synaptic gap to the thickness of a dime, the thinnest of all American coins (about 1.35 mm). You could stack approximately 70,000 synaptic gaps in the thickness of a single coin!

As the action potential, the electrical signal reaches the end of the axon, tiny packets of chemicals, called neurotransmitters, are released. This is the chemical part of the electro-chemical signal. These neurotransmitters are the chemical signals that travel from one neuron to another, enabling them to communicate with one another. There are many different types of neurotransmitters and each has a specialized function. For example, serotonin affects sleep, hunger and mood. Dopamine is associated with attention, learning and pleasure (Kandel & Schwartz, 1982) 

It is amazing to realize that when you think—when you reach out to grab a glass of water, when you realize that your best friend is happy, when you try to remember the name of the parts of a neuron—what you are experiencing is actually electro-chemical impulses shooting between nerves!

The Central Nervous System: Looking at the Brain as a Whole

If we were to zoom back out and look at the central nervous system again we would see that the brain is the largest single part of the central nervous system. The brain is the headquarters of the entire nervous system and it is here that most of your sensing, perception, thinking, awareness, emotions, and planning take place. For many people the brain is so important that there is a sense that it is there—inside the brain—that a person’s sense of self is located (as opposed to being primarily in your toes, by contrast). The brain is so important, in fact, that it consumes 20% of the total oxygen and calories we consume even though it is only, on average, about 2% of our overall weight.

It is helpful to examine the various parts of the brain and to understand their unique functions to get a better sense of the role the brain plays. We will start by looking at very general areas of the brain and then we will zoom in and look at more specific parts. Anatomists and neuroscientists often divide the brain into portions based on the location and function of various brain parts. Among the simplest ways to organize the brain is to describe it as having three basic portions: the hindbrain, midbrain and forebrain. Another way to look at the brain is to consider the brain stem, the Cerebellum, and the Cerebrum. There is another part, called the Limbic System that is less well defined. It is made up of a number of structures that are “sub-cortical” (existing in the hindbrain) as well as cortical regions of the brain (see Figure 4).

The brain stem is the most basic structure of the brain and is located at the top of the spine and bottom of the brain. It is sometimes considered the “oldest” part of the brain because we can see similar structures in other, less evolved animals such as crocodiles. It is in charge of a wide range of very basic “life support” functions for the human body including breathing, digestion, and the beating of the heart. Amazingly, the brain stem sends the signals to keep these processes running smoothly without any conscious effort on our behalf.

The limbic system is a collection of highly specialized neural structures that sit at the top of the brain stem, which are involved in regulating our emotions. Collectively, the limbic system is a term that doesn’t have clearly defined areas as it includes forebrain regions as well as hindbrain regions. These include the amygdala, the thalamus, the hippocampus, the insula cortex, the anterior cingulate cortex, and the prefrontal cortex. These structures influence hunger, the sleep-wake cycle, sexual desire, fear and aggression, and even memory.

The cerebellum is a structure at the very back of the brain. Aristotle referred to it as the “small brain” based on its appearance and it is principally involved with movement and posture although it is also associated with a variety of other thinking processes. The cerebellum, like the brain stem, coordinates actions without the need for any conscious awareness. 

The cerebrum (also called the “cerebral cortex”) is the “newest,” most advanced portion of the brain. The cerebral hemispheres (the left and right hemispheres that make up each side of the top of the brain) are in charge of the types of processes that are associated with more awareness and voluntary control such as speaking and planning as well as contain our primary sensory areas (such as seeing, hearing, feeling, and moving). These two hemispheres are connected to one another by a thick bundle of axons called the corpus callosum. There are instances in which people—either because of a genetic abnormality or as the result of surgery—have had their corpus callosum severed so that the two halves of the brain cannot easily communicate with one another. The rare split-brain patients offer helpful insights into how the brain works. For example, we now understand that the brain is contralateral, or opposite-sided. This means that the left side of the brain is responsible for controlling a number of sensory and motor functions of the right side of the body, and vice versa.

Consider this striking example: A split brain patient is seated at a table and an object such as a car key can be placed where a split-brain patient can only see it through the right visual field. Right visual field images will be processed on the left side of the brain and left visual field images will be processed on the right side of the brain. Because language is largely associated with the left side of the brain the patient who sees car key in the right visual field when asked “What do you see?” would answer, “I see a car key.” In contrast, a split-brain patient who only saw the car key in the left visual field, thus the information went to the non-language right side of the brain, might have a difficult time speaking the word “car key.” In fact in this case, the patient is likely to respond “I didn’t see anything at all.” However, if asked to draw the item with their left hand—a process associated with the right side of the brain—the patient will be able to do so! See the outside resources below for a video demonstration of this striking phenomenon.

Besides looking at the brain as an organ that is made up of two halves we can also examine it by looking at its four various lobes of the cerebral cortex, the outer part of the brain (see Figure 5). Each of these is associated with a specific function. The occipital lobe, located at the back of the cerebral cortex, is the house of the visual area of the brain. You can see the road in front of you when you are driving, track the motion of a ball in the air thanks to the occipital lobe. The temporal lobe, located on the underside of the cerebral cortex, is where sounds and smells are processed. The parietal lobe, at the upper back of the cerebral cortex, is where touch and taste are processed. Finally, the frontal lobe, located at the forward part of the cerebral cortex is where behavioral motor plans are processed as well as a number of highly complicated processes occur including speech and language use, creative problem solving, and planning and organization. 

One particularly fascinating area in the frontal lobe is called the “primary motor cortex”. This strip running along the side of the brain is in charge of voluntary movements like waving goodbye, wiggling your eyebrows, and kissing. It is an excellent example of the way that the various regions of the brain are highly specialized. Interestingly, each of our various body parts has a unique portion of the primary motor cortex devoted to it (see Figure 6). Each individual finger has about as much dedicated brain space as your entire leg. Your lips, in turn, require about as much dedicated brain processing as all of your fingers and your hand combined!

Because the cerebral cortex in general, and the frontal lobe in particular, are associated with such sophisticated functions as planning and being self-aware they are often thought of as a higher, less primal portion of the brain. Indeed, other animals such as rats and kangaroos while they do have frontal regions of their brain do not have the same level of development in the cerebral cortices. The closer an animal is to humans on the evolutionary tree—think chimpanzees and gorillas, the more developed is this portion of their brain.

The Peripheral Nervous System

In addition to the central nervous system (the brain and spinal cord) there is also a complex network of nerves that travel to every part of the body. This is called the peripheral nervous system (PNS) and it carries the signals necessary for the body to survive (see Figure 7). Some of the signals carried by the PNS are related to voluntary actions. If you want to type a message to a friend, for instance, you make conscious choices about which letters go in what order and your brain sends the appropriate signals to your fingers to do the work. Other processes, by contrast, are not voluntary. Without your awareness your brain is also sending signals to your organs, your digestive system, and the muscles that are holding you up right now with instructions about what they should be doing. All of this occurs through the pathways of your peripheral nervous system.

How we study the brain

The brain is difficult to study because it is housed inside the thick bone of the skull. What’s more, it is difficult to access the brain without hurting or killing the owner of the brain. As a result, many of the earliest studies of the brain (and indeed this is still true today) focused on unfortunate people who happened to have damage to some particular area of their brain. For instance, in the 1880s a surgeon named Paul Broca conducted an autopsy on a former patient who had lost his powers of speech. Examining his patient’s brain, Broca identified a damaged area—now called the “Broca’s Area”—on the left side of the brain (see Figure 8) (AAAS, 1880). Over the years a number of researchers have been able to gain insights into the function of specific regions of the brain from these types of patients. 

An alternative to examining the brains or behaviors of humans with brain damage or surgical lesions can be found in the instance of animals. Some researchers examine the brains of other animals such as rats, dogs and monkeys. Although animals brains differ from human brains in both size and structure there are many similarities as well. The use of animals for study can yield important insights into human brain function.

In modern times, however, we do not have to exclusively rely on the study of people with brain lesions. Advances in technology have led to ever more sophisticated imaging techniques. Just as X-ray technology allows us to peer inside the body, neuroimaging techniques allow us glimpses of the working brain (Raichle,1994). Each type of imaging uses a different technique and each has its own advantages and disadvantages.

Positron Emission Tomography (PET) records metabolic activity in the brain by detecting the amount of radioactive substances, which are injected into a person’s bloodstream, the brain is consuming. This technique allows us to see how much an individual uses a particular part of the brain while at rest, or not performing a task. Another technique, known as Functional Magnetic Resonance Imaging (fMRI) relies on blood flow. This method measures changes in the levels of naturally occurring oxygen in the blood. As a brain region becomes active, it requires more oxygen. This technique measures brain activity based on this increase oxygen level. This means fMRI does not require a foreign substance to be injected into the body. Both PET and fMRI scans have poor temporal resolution , meaning that they cannot tell us exactly when brain activity occurred. This is because it takes several seconds for blood to arrive at a portion of the brain working on a task.

One imaging technique that has better temporal resolution is Electroencephalography (EEG), which measures electrical brain activity instead of blood flow. Electrodes are place on the scalp of participants and they are nearly instantaneous in picking up electrical activity. Because this activity could be coming from any portion of the brain, however, EEG is known to have poor spatial resolution, meaning that it is not accurate with regards to specific location. 

Another technique, known as Diffuse Optical Imaging (DOI) can offer high temporal and spatial resolution. DOI works by shining infrared light into the brain. It might seem strange that light can pass through the head and brain. Light properties change as they pass through oxygenated blood and through active neurons. As a result, researchers can make inferences regarding where and when brain activity is happening.

Conclusion

It has often been said that the brain studies itself. This means that humans are uniquely capable of using our most sophisticated organ to understand our most sophisticated organ. Breakthroughs in the study of the brain and nervous system are among the most exciting discoveries in all of psychology. In the future, research linking neural activity to complex, real world attitudes and behavior will help us to understand human psychology and better intervene in it to help people. 

2. Which structures make up the nervous system?

The structures that form the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

The organs of the CNS are the brain (cerebrum, brainstem and cerebellum) and spinal cord. The PNS is made of nerves and neural ganglia. In addition to these organs, the meninges (dura-mater, arachnoid and pia-mater) are also a part of the nervous system, since they cover and protect the encephalon and the spinal cord.

Cells of the Nervous System

3. What are the main cells of the nervous system?

The main cells of the nervous system are neurons. In addition to neurons, the nervous system is also made up of glial cells. 

4. What are the functional differences between neurons and glial cells?

Glial cells and neurons are the cells that form the nervous system. Neurons are cells that have the function of receiving and transmitting neural impulses whereas glial cells (astrocytes, microgliacytes, ependymal cells and oligodendrocytes) are the cells that support, feed and insulate (electrically) the neurons. The Schwann cells that produce the myelin sheath of the peripheral nervous system can also be considered glial cells.

Neurons and Synapses

5. What are the three main parts into which a neuron can be divided? What are their respective functions?

The three mains parts into which a neuron can be divided are: dendrites, the cell body and the axon.

Dendrites are projections of the plasma membrane that receive the neural impulse from other neurons. The cell body is where the nucleus and the main cellular organelles are located. The axon is the long membrane projection that transmits the neural impulse at a distance to other neurons, to muscle cells and to other effector cells. 

6. What is the name of the terminal portion of the axon?

The terminal portion of the axon is called the presynaptic membrane. Through this membrane, neurotransmitters are released into the synaptic junction. 

  • The Nervous System Review - Image Diversity: synapse

7. What are synapses?

Synapses are the structures that transmit a neural impulse between two neurons.

When the electric impulse arrives, the presynaptic membrane of the axon releases neurotransmitters that bind to the postsynaptic receptors of the dendrites of the next cell. The activated state of these receptors alters the permeability of the dendritic membrane and the electric depolarization moves along the plasma membrane of the neuron to its axon.

Neural Transmission

8. What is an example of a situation in which the cell body of a neuron is located in one part of the body while its axonal terminal portion is located in another distant part of the body? Why does this happen?

Most neurons are located within the brain and the spinal cord (the central nervous system) in places known as neural nuclei. Neural ganglia, or simply ganglia, are structures of the peripheral nervous system located beside the spine or near certain organs, in which neuron cell bodies are also located.

Neurons located at specific points can may have distant axonal terminations and can also receive impulses from the axons of distant neurons. An example of this are the inferior motor neurons in the spinal cord, since their axons can transmit information to the extremities of the lower limbs, triggering foot contractions.

9. What are the types of neurons in terms of the function of the impulses they transmit? How different are the concepts of afference and efference in terms of neural impulse transmission?

There are three types of neurons: afferent neurons, efferent neurons and interneurons. Afferent neurons only transmit sensory information from the tissues to neural nuclei and ganglia (where they come into contact with interneurons or effector neurons). Efferent neurons transmit commands for tasks to be performed in several parts of the body. Interneurons, also known as association neurons or relay neurons, serve as a connection between the other two types of neurons.

Afference is the conduction of sensory impulses and efference is the conduction of effector impulses (impulses that command some action in the body).

Nerves and Ganglia

10. What are nerves?

Axons extend throughout the body inside nerves. Nerves are axon-containing structures which are home to a large number of axons and which are covered by connective tissue. Nerves connect neural nuclei and ganglia with tissues.

Nerves may contain only sensory axons (sensory nerves), only motor axons (motor neurons) or both types of axons (mixed nerves).

  • The Nervous System Review - Image Diversity: nerves

11. What are ganglia?

Ganglia (singular ganglion), or neural ganglia, are structures located outside the central nervous system (for example, beside the spine or near viscera) made of a concentration of neuron bodies.

Examples of neural ganglia are the ganglia that contain the cell bodies of sensory neurons in the dorsal roots of the spinal cord and the ganglia of the myenteric plexus, which are responsible for the peristaltic movements of the digestive tract.

In the central nervous system (CNS), concentrations of neuron bodies are called nuclei and not ganglia. 

  • The Nervous System Review - Image Diversity: ganglia

12. What is meant by the peripheral nervous system (PNS)?

The peripheral nervous system is made up of the nerves and ganglia of the body.

The Myelin Sheath

13. What is the function of the myelin sheath? Do all axons have a myelin sheath?

The function of the myelin sheath is to improve the safety and speed of neural impulse transmission along the axon. The myelin sheath serves as an electrical insulator, preventing the dispersion of the impulse into other adjacent structures. Since the myelin sheath has gaps called Ranviers’ nodes along its length, the neural impulse “jumps” from one node to another, thus increasing the speed of the neural transmission.

Not all neurons have a myelin sheath. Axonal fibers may be myelinated or unmyelinated.

14. Which cells produce the myelin sheath? What is the myelin sheath made of?

In the central nervous system (CNS), the myelin sheath is made of an apposition of oligodendrocyte membranes. Each oligodendrocyte may cover portions of the axons of several different neurons. In the peripheral nervous system (PNS), the myelin sheath is made of consecutive Schwann cell membranes covering segments of a single axon. The Ranviers’ nodes appear in the intercellular space between these cells.

The myelin sheath is rich in lipids but also contains proteins. 

15. What are some diseases in which the axonal myelin sheath is progressively destroyed?

Multiple sclerosis is a severe disease caused by progressive destruction of the myelin sheath of the central nervous system. Guillain-Barré disease is due to the destruction of the myelin sheath in the peripheral nervous system caused by autoimmunity (attacks carried out by the immune system of the body). A genetic deficiency in the formation or preservation of the myelin sheath is an X-linked inheritance called adrenoleukodystrophy. The movie “Lorenzo’s Oil” featured a boy with this disease and his father's dramatic search for a treatment.

Meninges

16. What are meninges and cerebrospinal fluid?

Meninges are the membranes that enclose and protect the central nervous system (CNS). Cerebrospinal fluid is the fluid that separates the three layers that form the meninges. It has the functions of nutrient transport, defense and the mechanical protection of the CNS.

Cerebrospinal fluid fills and protects cavities of the brain and the spinal cord.

  • The Nervous System Review - Image Diversity: meninges

The Functions and Anatomy of the Brain

17. What is the difference between the brain and the cerebrum? What are the main parts of these structures?

The concept of brain, or encephalon, includes the cerebrum (mostly referred to as the hemispheres, but in reality, the concept also includes the thalamus and the hypothalamus), the brainstem (midbrain, pons and medulla) and the cerebellum. The brain and spinal cord form the central nervous system (CNS).

18. How is the cerebrum anatomically divided?

The cerebrum is divided into two cerebral hemispheres, the right and the left. Each hemisphere is composed of four cerebral lobes: the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe.

Each cerebral lobe contains gray matter and  white matter. Gray matter is the outer portion and is made of neuron bodies; gray matter is also known as the cerebral cortex. White matter is the inner portion and is white because it is in the region where the axons of cortical neurons pass.

19. Which region of the brain is responsible for coordination and balancing of the body?

In the central nervous system, the cerebellum is the main controller of motor coordination and balance. (Do not confuse this with muscle command, which is performed by the cerebral hemispheres).

  • The Nervous System Review - Image Diversity: cerebellum

20. Why is the cerebellum more developed in mammals that jump or fly?

The cerebellum is the main structure in the brain that coordinates the movement and balance of the body. For this reason, it appears to be more developed in mammals that jump or fly (such as bats). The cerebellum is also very important for the flight of birds.

21. Which region of the brain is responsible for the regulation of breathing and blood pressure?

The neural regulation of breathing, blood pressure and other physiological parameters such as heartbeat, digestive secretions, peristaltic movements and transpiration is performed by the medulla.

The medulla, together with the pons and the midbrain, is part of the brainstem.

22. Which region of the brain receives conscious sensory information? Which region of the brain triggers voluntary motor activity?

In the brain, conscious sensory information is received by neurons located in a special region called the postcentral gyrus (or sensory gyrus). Gyri are the convolutions of the cerebrum. Each of the two postcentral gyri are located in one of the parietal lobes of the cerebrum.

Voluntary motor activity (voluntary muscle movement) is commanded by neurons located in the precentral gyrus (or motor gyrus). Each of the two precentral gyri are located in one of the frontal lobes of the cerebrum.

The names post- and pre-central refer to the fact that the motor and sensory gyri are spaced apart in each cerebral hemisphere by the sulcus centralis, a fissure that separates the parietal and frontal lobes.

The Spinal Cord and Reflex Arc

23. What is the spinal cord? What elements make up the spinal cord?

The spinal cord is the dorsal neural cord of vertebrates. It is the part of the central nervous system that continues into the trunk to facilitate the nervous integration of the whole body.

The spinal cord is made of groups of neurons located in its central portion forming gray matter, and axon fibers in its exterior portion forming white matter. Neural bundles connect to both lateral sides of spinal cord segments to form the dorsal and ventral spinal roots that join to form the spinal nerves. Dorsal spinal roots contain a ganglion with neurons that receive sensory information; ventral spinal roots contain motor fibers. Therefore, dorsal roots are sensory roots and ventral roots are motor roots. 

  • The Nervous System Review - Image Diversity: spinal cord

24. Which regions of the brain are associated with memory?

According to researchers, some of the main regions of the nervous system associated with the phenomenon of memory are the hippocampus, located in the interior portion of the temporal lobes, and the frontal lobe cortex, both of which are part of the cerebral hemispheres.

25. How can the fact that the motor activity of the left side of the body is controlled by the right cerebral hemisphere and the motor activity of the right side of the body is controlled by the left cerebral hemisphere be explained?

The cerebral hemispheres contain neurons that centrally command and control muscle movements. These neurons are called superior motor neurons and are located in a special gyrus of both frontal lobes known as the motor gyrus (or precentral gyrus). These superior motor neurons send axons that transmit impulses to the inferior motor neurons of the spinal cord (for neck, trunk and limb movements) and to the motor nuclei of the cranial nerves (for face, eyes and mouth movements).

The fibers cross to the other side in specific areas of those axon paths. About 2/3 of the fibers that go down the spinal cord cross at the medullar level forming a structure known as the pyramidal decussation. The other (1/3) of fibers descend on the same side as their original cerebral hemisphere and cross only within the spinal cord at the level where their associated motor spinal root exits. The fibers that command the inferior motor neurons of the cranial nerves cross to the other side just before the connection with the nuclei of these nerves.

The motor fibers that descend from the superior motor neurons to the inferior motor neurons of the spinal cord form the pyramidal tract. Injuries to this tract caused by spinal sections or by central or spinal tumors, for example, may lead to paraplegia and tetraplegia.

26. What is meant by the reflex arc?

In some situations, the movement of skeletal striated muscles does not depend on commands from superior motor neurons, meaning that it is not triggered by volition.

Involuntary movements of those muscles may occur when sensory fibers that make direct or indirect contact with inferior motor neurons are unexpectedly stimulated in situations that suggest danger to the body. This happens, for example, in the patellar reflex, or knee jerk reflex, when a sudden percussion on the knee patella (kneecap) triggers an involuntary contraction of the quadriceps (the extension muscle of the thigh). Another example of the reflex arc occurs when someone steps on a sharp object: one leg retracts and the other, through the reflex arc, stretches to maintain the balance of the body.

  • The Nervous System Review - Image Diversity: reflex arc

27. Which types of neurons participate in the spinal reflex arc? Where are their cell bodies located?

In a reflex arc, first a sensory neuron located in the ganglion of a dorsal spinal root collects the stimulus information from the tissue. This sensory neuron makes a direct or indirect (through interneurons) connection with the inferior motor neurons of the spinal cord. These motor neurons then command the reflex reaction. Therefore, sensory neurons, interneurons and inferior motor neurons participate in the reflex arc.

28. What is the grey and white matter of the spinal cord made of?

The gray matter of the spinal cord mainly contains neuron bodies (inferior motor neurons, secondary sensory neurons and interneurons). The white matter is mainly made up of axons that connect neurons of the brain with spinal neurons. 

29. Is the neural impulse generated by the stimulus that triggers the reflex arc restricted to the neurons of this circuit?

The sensory fiber that first triggers the reflex arc connects with neurons of the reflex arc as well as with secondary sensory neurons of the spinal cord that transmit information on to other neurons of the brain. This is obvious, since the person that received the initial stimulus (for example, something hitting his/her kneecap) perceives it (meaning that the brain became conscious of the fact).

30. How is it possible that a person with a spinal cord severed at the cervical level is still able to perform the patellar reflex?

The reflex arc only depends on the integrity of the fibers at a single spinal level. In the reflex arc, the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. Therefore, it happens even if the spinal cord is damaged at other levels.

31. How does poliomyelitis affect neural transmission within the spinal cord?

The poliovirus is parasitic to and destroys spinal motor neurons, causing the paralysis of the muscles that depend on these neurons. 

Somatic and Autonomic Nervous Systems

32. Concerning volition, how can the reactions of the nervous system be classified?

The efferences (reactions) of the nervous system can be classified into voluntary, when they are the result of volition, and involuntary, when they are not consciously controlled. Examples of reactions triggered by volition are the movements of limbs, the tongue and respiratory muscles. Examples of involuntary efferences are those that command peristaltic movements, the heartbeat and arterial wall muscles. Skeletal striated muscles are voluntarily contracted; whereas cardiac striated and smooth muscles are involuntarily contracted. 

33. What are the functional divisions of the nervous system?

Functionally, the nervous system can be divided into the somatic nervous system and the visceral nervous system.

The somatic nervous system includes the central and peripheral structures that constitute the voluntary control of efferences. Central and peripheral structures that participate in the control of the vegetative (unconscious) functions of the body are included in the concept of the visceral nervous system.

The efferent portion of the visceral nervous system is called the autonomic nervous system.

34. What are the two divisions of the autonomic nervous system?

The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system includes the nerves that extend from the ganglia of the neural chains lateral to the spine (near the spinal cord) and therefore are located at a distance from the tissues they innervate. The central and peripheral neurons associated with those neurons are also a part of the sympathetic nervous system.

The parasympathetic nervous system is made up of nerves and central or peripheral neurons related to the visceral ganglia, neural ganglia located near the tissues they innervate.

35. What is the antagonism between sympathetic and parasympathetic neural actions?

In general, the actions of the sympathetic and the parasympathetic nervous systems are antagonistic, meaning that when one stimulates something, the other inhibits it and vice versa. The organs, with few exceptions, receive efferences from these two systems and the antagonism between them serves to balance their effects. For example, the parasympathetic system stimulates salivation while the sympathetic system inhibits it; the parasympathetic system constricts pupils while the sympathetic system dilates them; the parasympathetic system contracts the bronchi while the sympathetic system relaxes them; and the parasympathetic system excites the genital organs while the parasympathetic system inhibits the excitation.

Cephalization and the Evolution of the Nervous System

36. Using examples of invertebrate nervous systems, how can the process of evolutionary cephalization be described?

Considering invertebrates, it is possible to observe that evolution accompanies the increasing complexity of organisms with the convergence of nervous cells at special structures for controlling and commanding: the ganglia and the brain. In simple invertebrates, such as cnidarians, nervous cells are not concentrated rather they are found dispersed in the body. In platyhelminthes, the beginning of cephalization with an anterior ganglion concentrating neurons is already verified. In annelids and arthropods the existence of a cerebral ganglion is evident. In cephalopod molluscs, the cephalization is even greater and the brain controls the nervous system.

37. What are some of the main differences between vertebrate and invertebrate nervous systems?

In vertebrates, the nervous system is well-defined, with a brain and dorsal neural cord protected by rigid skeletal structures. In most invertebrates, the nervous system is predominantly ganglial, with ventral neural cords.

38. What are the protective structures of the central nervous system present in vertebrates?

In vertebrates, the brain and the spinal cord are protected by membranes, the meninges, and by osseous structures, the skull and the spine, respectively. These protective structures are fundamental in maintaining the integrity of these important organs, which control the functioning of the body. 

Neurophysiology

39. What is the nature of the stimuli received and transmitted by neurons?

Neurons receive and transmit chemical stimuli through neurotransmitters released in the synapses. However, the impulse transmission is electrical along the neuron body. Therefore, neurons conduct electrical and chemical stimuli.

40. What are the two main ions that participate in electrical impulse transmission in neurons?

The two main ions that participate in electrical impulse transmission in neurons are the sodium cation (Na⁺) and the potassium cation (K⁺). 

41. Is the electric charge between the two sides of the neuron plasma membrane positive or negative? What is the potential difference (voltage) generated between these two sides? What is that voltage called?

As in most cells, the region just outside the surface of the neuron plasma membrane has a positive electrical charge compared to the region just inside it, which is therefore negative.

The normal (resting) potential difference across the neuron membrane is about –70 mV (millivolts). This voltage is called the resting potential of the neuron.

42. How sodium and potassium ions maintain the resting potential of neurons?

When at rest, the plasma membrane of a neuron maintains an electric potential difference between its external and internal surfaces. This voltage is called resting potential. A resting potential around –70 mV indicates that the interior is more negative than the exterior (negative polarization). This condition is maintained by the transport of sodium and potassium ions across the plasma membrane.

The membrane is permeable to potassium ions but not to sodium ions. At rest, the positive potassium ions exit the cell in favor of the concentration gradient, since within the cell the potassium concentration is higher than in the extracellular space. However, the positive sodium ions cannot enter the cell. Positive potassium ions exit the cell and not enough compensatory positive ions enter the cell, causing the intracellular space to become more negative and making the cell remain polarized.

43. How is the depolarization of the neuronal plasma membrane caused? How does the cell return to its original resting state?

When the neuron receives a stimulus via the binding of neurotransmitters to specific receptors, sodium channels open and the permeability of the plasma membrane in the postsynaptic region is altered. Sodium ions then enter the cell, causing a decrease (less negative) in the potential difference of the membrane. If the reduction in the potential difference of the membrane reaches a level called the excitation threshold, or threshold potential, around –50 mV, the action potential is generated, meaning that the depolarization intensifies until reaching its maximum level. The depolarization current is then transmitted along the remaining length of the neuronal membrane.

If the excitation threshold is reached, voltage-dependent sodium channels in the membrane open, allowing more sodium ions to enter the cell in favor of the concentration gradient, and an approximate level of –35 mV of positive polarization of the membrane is achieved. The voltage-dependent sodium channels then close and more voltage-dependent potassium channels open. Potassium ions then exit the cell in favor of the concentration gradient and the potential difference of the membrane decreases. This process is called repolarization.

The action potential triggers the same electrical phenomenon in neighboring regions of the plasma membrane and the impulse is therefore transmitted from the dendrites to the terminal region of the axon.

44. What is the excitation threshold of a neuron? How does this threshold relate to the “all-or-nothing” rule of neural transmission?

The excitation threshold of a neuron is the depolarization level that must be caused by a stimulus to be transmitted as a neural impulse. This value is about –50 mV.

The transmission of a neural impulse along the neuronal membrane obeys an all-or-nothing rule: either it happens at maximum intensity or nothing happens. Only when the excitation threshold is reached does the depolarization continue, causing the membrane to reach its maximum possible positive polarization, about +35 mV. If the excitation threshold is not reached nothing happens.

45. How does the depolarization of the neuronal membrane start?

The primary cause of neuronal depolarization is the binding of neurotransmitters released in the synapse (by the axon of the neuron that sent the signal) to specific receptors in the membrane of the neuron that is receiving the stimulus. The binding of neurotransmitters to those receptors is a reversible phenomenon that alters the membrane permeability of the region, since the binding causes sodium channels to open. When positive sodium ions enter the cell in favor of their concentration gradient, the voltage of the membrane increases, thus decreasing its negative polarization. If this depolarization reaches the excitation threshold (about –50 mV), the depolarization continues, the action potential is reached and the impulse is transmitted along the cell membrane.

46. In terms of neurons, how different are the concepts of action potential, resting potential and excitation threshold?

Action potential is the maximum positive voltage level achieved by the neuron during the process of neuronal activation, around + 35 mV. The action potential triggers the depolarization of the neighboring regions of the plasma membrane and therefore the propagation of the impulse along the neuron.

The resting potential is the voltage of the membrane when the cell is not excited, about –70 mV.

The excitation threshold is the voltage level, about –50 mV, which the initial depolarization must reach for the action potential to be attained.

47. In chemical terms, how is neuronal repolarization achieved?

Repolarization is the return of the membrane potential from the action potential (+35 mV) to the resting potential (-70 mV).

When the membrane reaches its action potential, voltage-gated sodium channels close and voltage-gated potassium channels open. As a result, sodium stops entering the cell and potassium starts to exit it. Therefore, the repolarization is due to the exit of potassium cations from the cell.

The repolarization causes the potential difference to temporarily increase under –70 mV, below the resting potential, in a phenomenon known as hyperpolarization.

48. What is the mechanism by which the a neural impulse is transmitted along the axon?

A neural impulse is transmitted along the neuronal membrane through the depolarization of consecutive neighboring regions. When a region on the internal surface of the membrane is depolarized, it becomes more positive in relation to the neighboring internal region. As a result, positive electrical charges (ions) move towards this more negative region and voltage-gated sodium channels are activated and opened. The action potential then linearly moves along the membrane until reaching the presynaptic region of the axon.

49. Through which structure is a neural impulse transmitted from one cell to another? What are its parts?

The structure through which a neural impulse passes from one cell to another is the synapse. The synapse is composed of the presynaptic membrane in the terminal portion of the axon of the transmitter cell, the synaptic cleft (or synaptic space) and the postsynaptic membrane in the dendrite of the receptor cell.

50. How does synaptic transmission between neurons take place?

The propagation of the action potential along the axon reaches the region immediately anterior to the presynaptic membrane, causing its permeability to calcium ions to change and causing these ions to enter the cell. In the presynaptic area of the axon, there are a large amount of neurotransmitter-filled vesicles that, by means of exocytosis activated by the calcium influx, release the neurotransmitters into the synaptic cleft. The neurotransmitters then bind to specific receptors of the postsynaptic membrane. (The binding of neurotransmitters to their receptors is reversible, that is, the neurotransmitters are not consumed during the process.) With the binding of the neurotransmitters to the postsynaptic receptors, the permeability of the postsynaptic membrane is altered and the depolarization that will lead to the first action potential of the postsynaptic cell begins.

51. What are some important neurotransmitters?

The following are important neurotransmitters: adrenaline (epinephrine), noradrenaline (norepinephrine), acetylcholine, dopamine, serotonin, histamine, gaba (gamma aminobutyric acid), glycine, aspartate and nitric oxide.

52. Since neurotransmitters are not consumed during the synaptic process, what mechanisms are used to reduce their concentrations in the synaptic cleft after they have been used?

Since the binding of neurotransmitters to postsynaptic receptors is reversible, after these neurochemicals carry out their role, they must be eliminated from the synaptic cleft. Neurotransmitters then bind to specific proteins that carry them back to the axon they came from in a process called neurotransmitter re-uptake. They can also be destroyed by specific enzymes, such as acetylcholinesterase, an enzyme that destroys acetylcholine. In addition, they can simply diffuse out of the synaptic cleft.

53. Fluoxetine is an antidepressant drug that uses an action mechanism related to synaptic transmission. What is that mechanism?

Fluoxetine is a substance that inhibits the reuptake of serotonin, a neurotransmitter that acts mainly in the central nervous system. By inhibiting the reuptake of the neurotransmitter, the drug increases its availability in the synaptic cleft, thus improving neuronal transmission.

54. What is the neuromuscular synapse?

The neuromuscular synapse is the structure through which a neural impulse passes from the axon of a motor neuron to a muscle cell. This structure is also known as the neuromuscular junction, or motor end plate.

Like with the nervous synapse, the axonal terminal membrane releases the neurotransmitter acetylcholine into the cleft between the two cells. Acetylcholine binds to specific receptors of the muscle membrane, dependent sodium channels then open and the depolarization of the muscle membrane begins. The impulse is then transmitted to the sarcoplasmic reticulum, which releases calcium ions into the sarcomeres of the myofibrils, thus triggering the contraction.

Sensory Receptors

55. How does the nervous system get information about the external environment, organs and tissues?

Information about the conditions of external and internal environments, such as temperature, pressure, touch, spatial position, pH, metabolite levels (oxygen, carbon dioxide, etc.), light, sounds, etc., are collected by specific neural structures (different types for each type of information) called sensory receptors. Sensory receptors are distributed throughout tissues according to their specific roles. The receptors obtain information and transmit it through their own axons or through dendrites of neurons connected to them. The information reaches the central nervous system, which interprets it and uses it to control and regulate the body.

56. What are sensory receptors?

Sensory receptors are structures specialized in the acquisition of information, such as temperature, mechanical pressure, pH, chemical environment and luminosity, which transmit them to the central nervous system. Sensory receptors may be specialized cells, such as the photoreceptors of the retina, or specialized interstitial structures, such as the vibration receptors of the skin. In this last case, they transmit information to the dendrites of the sensory neurons connected to them. There also exist sensory receptors that are specialized terminations of neuronal dendrites (such as olfactory receptors).

57. According to the stimuli they obtain, how are sensory receptors classified?

Sensory receptors are classified according to the stimuli they obtain: mechanoreceptors are stimulated by pressure (touch or sound); chemoreceptors respond to chemical stimuli (olfactory, taste, pH, metabolite concentration, etc.); thermoreceptors are sensitive to temperature changes; photoreceptors are stimulated by light; nocireceptors send pain information; and proprioceptors are sensitive to the spatial position of muscles and joints (they generate information for the balance of the body).

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