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What is the nervous system?

Overview of the central and peripheral nervous systems.

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Hello everyone! This is Joao from Kenhub, and in this tutorial, we will be looking at the nervous system as a whole and answering the question, what is the nervous system?

Before we begin discussing the nervous system, the obvious question to answer first is what is the nervous system? The nervous system is a rather complex system responsible for information processing in your body. I realize that this answer doesn’t quite give a complete picture. Basically, the only way to answer this question is to look at the parts that make up the nervous system.

The first thing you should note is that the nervous system is comprised of two main parts – the central nervous system or CNS and the peripheral nervous system or PNS. We will start by talking about the first item of that list which is the central nervous system.

The central nervous system which as I mentioned before is abbreviated as CNS. So, the CNS is made up of the brain which is located in the skull and responsible for most of the functions and processing of the nervous system. Also, the brainstem which consists of the midbrain, the pons and the medulla oblongata as well as the cerebellum which is located at the base of the brain in the back and the spinal cord which is an extension of the brainstem that travels through the vertebral column. All of these components of the central nervous system are surrounded by the meninges and the cerebrospinal fluid also known as CSF in addition to be enclosed in the skeletal structures of either the cranial vault or the vertebral column which we see a little bit here in this close-up image of the brain in the coronal view.

So to continue our discussion on the central nervous system, let's first look a little more closely at the brain. When we talk about the brain, we are referring to the cerebrum. Now, let's first flip here the brain so we can look at its inferior or basal view and we're cutting it in half to show you that the cerebrum is divided into two hemispheres. You can see that on this image of the midsagittal cut of the brain that the hemispheres are partially connected to each other in the sagittal plane via the corpus callosum – this structure that you see now highlighted in green.

The cerebrum consists of the outer cerebral cortex and the subcortical structures, but we will not look at these structures in this tutorial. Instead, I just want you to keep in mind that as part of the central nervous system, the brain is responsible for coordinating the function of our muscles and limbs for activities like running, climbing and paddling a boat and the brain is also connected to the release of hormones that contribute to eating, emotions and feelings of love that we need to adapt, grow and change with our environment.

As I just mentioned before, the brainstem is comprised of the midbrain also known as the mesencephalon, the pons seen here highlighted in green and the medulla oblongata which is responsible for the regulation of the heart rate, blood pressure and also breathing rate. The medulla oblongata also contains the vomiting center.

If we flip the brain over, we can see that the brainstem is attached to the inferior aspect of the brain and lies within the cranial cavity against the clivus on the inferior aspect of the cranial vault. The brainstem gives rise to the majority of the cranial nerves which we can see from this inferior view. You can see the cranial nerves on this image as these yellow structures coming out of the brainstem. The brainstem is structurally continuous with the spinal cord as you can see on this image. We see the beginning – for lack of better word – of the spinal cord here.

If we return to the sagittal view and zoom in, we will see the next structure we'll look at – the cerebellum – seen here highlighted in green. The term cerebellum comes from the Latin meaning "little brain" – this is easy to remember since it looks like a little brain when turned. If we switch to a superior view which is looking down into the skull, we can see where the cerebellum is. It's situated in the posterior cranial fossa which is highlighted in green. Here we can even see the cerebellum highlighted in situ in a transverse cross-section.

The cerebellum is responsible for balance, coordination and fine tuning of our movements which is important in activities like gymnastics, wakeboarding and drawing. The cerebellum reprograms itself with a feed forward system according to the stimuli it faces.

The final component of the central nervous system is the spinal cord. The structure is enclosed within the vertebral column as you can see on this image and it extends from the level of the foramen magnum of the skull which is encircled here all the way down to about the first or second lumbar vertebrae. Along the length of the vertebral column, the spinal cord gives rise to thirty one spinal nerves which exit the vertebral column through the intervertebral foramina as you can see with these arrows. These nerves go on to merge and form plexuses which innervate muscles and viscera as well as give off cutaneous branches like the green structure of the image showing the lateral cutaneous nerve of the forearm.

Following the spinal cord branches takes us to the peripheral nervous system which is the next topic we're going to be talking about. Now that we have looked at the central nervous system, let's move on to look at the peripheral nervous system which is another major component of the nervous system as a whole.

The peripheral nervous system is abbreviated as PNS. It is then comprised of the spinal nerves and the cranial nerves. The PNS provides a line of communication between the central nervous system and the rest of your body. The nerves of the peripheral nervous system consist of sensory afferents which bring information from the world to the brain as a stimulus and the motor efferents which bring information from the brain to the muscles like reacting to a stimulus.

The peripheral nervous system can be then further subdivided into the somatic and the autonomic nervous systems which we will discuss next.

First let's look at the somatic nervous system abbreviated as SoNS. The term somatic means "relating to the body" and this gives us a clue as to what the somatic nervous system is. The nerves in this part of the peripheral nervous system are responsible for voluntary movement like lifting weights and sensory information like feeling the soft fur of a dog. These nerves receive contributions from both the cranial nerves and the spinal nerves. They innervate voluntary muscles like muscles of the neck and the trunk as well as the arms and the legs.

The nerves of the somatic nervous system carry both motor and sensory information. Here in this illustration, you can see the hypoglossal nerve which is the twelfth cranial nerve which has primarily somatic motor function. If we follow its path, we can see that this nerve innervates most of the muscles of the tongue which is a structure responsible for sensing taste as well as movement. This is clearly a good example of a nerve that belongs to the somatic nervous system.

The other part of the peripheral nervous system is the autonomic nervous system which is abbreviated as ANS. This division of the peripheral nervous system can in turn be divided into the sympathetic nervous system which we see here – this is the lumbar ganglion of the sympathetic trunk that we're seeing now – and the sympathetic nervous system prepares the body for fight, flight and fright responses also known as the three F's. The ANS is then also divided into the parasympathetic nervous system which is represented here in an image of the course of the vagus nerve which plays a role – a big role actually – in the digestive system. The parasympathetic nervous system coordinates the rest and digest functions of the body.

Now that we're finished talking about the nervous system, let's discuss some clinical notes. As we saw earlier on, the brainstem is part of the central nervous system and it gives rise to the majority of the cranial nerves. Here we see the facial nerve highlighted in green and these nerves exit the skull through openings known as foramina to then supply the head and neck regions. In this illustration, we see the highlighted internal acoustic meatus from a superior view of the skull where the facial nerve passes through.

When the foramina or openings that the nerve exits through are narrowed or if the nerves are constricted along their course, this can result in a condition known as cranial nerve palsy which restricts the function of the nerve. An example of cranial nerve palsy is a condition known as Bell's palsy. Here you can see a picture of a man suffering from this condition and as you can see, he is only able to raise one side of his face when prompted by the physician to do so on both sides. In this case, the seventh cranial nerve – the facial nerve – is affected.

As you may know, the facial nerve innervates the muscles of facial expression as well as the anterior two-thirds of the tongue and several glands of the head and neck regions. A patient suffering from Bell's palsy where the facial nerve is affected can experience symptoms which include affected taste in the anterior two-thirds of the tongue, drooping of the face on the side that is affected due to paralysis also known as hemiplegia, and absence of corneal or blink reflex and even overload hearing. This condition can bet treated with the use of corticosteroids, antivirals, surgery and even physiotherapy depending on the cause. Bell's palsy can also go away on its own with time.

Before we end this tutorial, let's quickly, quickly look at what we have covered. We saw that the nervous system is incredibly complex and the sum of its parts are all vital to the proper function of the body as a whole. We have seen that the nervous system can be divided into the central nervous system which includes the brain, brainstem, cerebellum and the spinal cord and the peripheral nervous system which comprises of the spinal nerves and the cranial nerves, and can also be further subdivided into the somatic nervous system and the autonomic nervous system.

Hope you enjoyed this tutorial and thank you for watching and I will see you on the next one.

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