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Nervous system

Main organs and nerves of the nervous system.

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Transcript

Hey everyone! It's Nicole from Kenhub, and welcome to our tutorial on the nervous system. Before we begin, let me just first give you a quick overview of what we’re going to be talking about in today’s tutorial. First of all, we’ll need to establish what exactly the nervous system is, and once we’ve defined the nervous system, we’re going to move on to the cells that make up the nervous system. We’ll also talk about the central nervous system, which is often simply referred to as the CNS, and the peripheral nervous system, which is otherwise known as the PNS. Finally, you can look forward to a brief clinical excursion making this exciting new information relevant to clinical and daily practice.

So before we look at the nervous system in detail, let’s first define what the term actually means. The nervous system is a network of nerve cells and nerve fibers that transmit information all over the body in the form of electrical impulses. You can see the nerves of the body highlighted in yellow in this image of our lovely lady here. All in all, the nervous system is an essential component of the human body, and without it, you would not be able to move, sense your surroundings, feel emotions, think or make decisions. Even our vital organs such as the heart, lungs and digestive system rely on it. As such, it’s a very complex system with many different components without which we don’t be able to function, let alone live!

There are two main parts of the nervous system. These are the central nervous system and the peripheral nervous system. We’ll look at these in further detail a little bit later on in the tutorial.

So now that I’ve briefly introduced you to the nervous system, let’s have a look at the building blocks of the nervous system which are the cells of the nervous system.

So, first we’re going to be talking about the nerve cells which can also be called neurons. We can see a cartoon depiction of a neuron here, and in this image, we can see that it’s characterized by an axon, a cell body also known as the soma, and dendrites. The structures we can see branching from the body – the cell body and the dendrites – are involved in receiving and processing information, whilst the axon transfers the information to other cells.

A neuron can have multiple dendrites, but usually only has one axon, and a nerve is made up of bundles of many, many axons. There are three main types of neurons and each fulfills a slightly different role. For example, sensory neurons obtain information from your surroundings through mechanism such as touch, whereas motor neurons transmit electrical impulses to organs, muscles and glands around the body to give a signal for action. Interneurons are found between neurons transmitting information from neuron to neuron. Specifically, they form a communication network between the sensory and motor neurons.

Neurons communicate with each other via gaps called synapses. Synapses can be classified as chemical or electrical. Chemical synapses are more common, therefore, we’re going to be focusing on them in this portion of the tutorial, and they’re involved the release of chemical messengers known as neurotransmitters.

Neurotransmitters carry information or electrical impulses from the ascending neuron to the receiving neuron. Based on what I told you just earlier that the cell body and dendrites are involved in receiving and processing information and that the axons transfer the information, you’ll be able to figure out that a synapse usually forms between the axon of ascending neuron and the dendrites or cell body of the receiving neuron.

Neurons are one of two cell types found within the nervous system. The other type of cell is referred to as glial cells. These cells are supporting cells and have various functions. For example, protection and providing insulation which speeds up the conduction of electrical impulses along the nerves.

In the central nervous system, there are four types of glial cells, and these include astrocytes, oligodendrocytes, microglia, and ependymal cells, whereas in the peripheral nervous system, there are two types of glial cells and these are satellite cells and Schwann cells.

Many axons are surrounded by a special substance called myelin which acts as an insulator and helps to transfer information more quickly. In the peripheral nervous system, myelination of axons is done by Schwann cells whereas in the central nervous system, it’s done by oligodendrocytes. In our image, we can see how these cells surround and insulate the axon.

So now that we’re familiar with the cells of the nervous system, let’s move on to talk about the divisions of the nervous system, starting with the central nervous system.

The central nervous system comprises the brain and the spinal cord, each of which play important roles in the overall functioning of the nervous system. First, we’ll have a look at the brain’s role in the central nervous system and then we’ll move on to the spinal cord. The brain is the master organ of the central nervous system. It coordinates the movement of our limbs, vital functions such as breathing and heart rate as well as the release of hormones needed for processes such as growth.

The brain is made up of three different parts – the cerebrum which accounts for the largest part of the brain and is made up of the right and left cerebral hemispheres, the cerebellum which is Latin for the little brain and sits posteriorly in and a little below the cerebrum, and the brainstem which is continuous with the spinal cord. Let’s look at these three parts of the brain in more detail, starting with the cerebrum.

As I previously mentioned, the cerebrum consist of a right and a left hemisphere and each controls functions for the opposite side of the body. For example, when you move your right thumb, an area in the left cerebral hemisphere is responsible for this movement. The surface of the cerebrum is characterized by gyri, which are elevations, and sulci and fissures, which are depressions or furrows. This help increase the surface area and play an important role in the localization of specific functional areas of the brain. Each hemisphere is divided into several components known as lobes. These are the frontal lobe, the two temporal lobes, the two parietal lobes, and the occipital lobe. Let’s take a look at these lobes in a bit more detail.

The first lobe we’re going to be looking at is the frontal lobe. The frontal lobe contains the orbitofrontal cortex which inhibits impulsive behaviors, the precentral gyrus which is the primary motor cortex, and Broca’s area which allows us to produce speech and comprehend and process language.

The temporal lobes contain the primary auditory cortex. They’re also generally involved in the processing of sensory stimuli and, therefore, play an important role in hearing, language, and visual interpretation and storage. Where the temporal lobes meet the parietal lobes, we can find another important area called Wernicke’s area, which is important in the understanding of written and spoken language. The somatosensory cortex can be found in this lobe as well. Finally, there’s the occipital lobe which is situated at the back of the head. It contains the primary visual cortex.

Let’s move on to the next part of the brain – the cerebellum – which we can see here highlighted in green on the dorsal aspect of the brainstem. If the cerebellum is damaged or dysfunctional, you may see the individual walk as if they’re drunk as the cerebellum is responsible for our balance and coordination ensuring that our movements are nice and smooth.

If we cut the brain in half, we can see the final part of the brain that we’re going to talk about. The brainstem connects the brain to the spinal cord and is divided into three parts – the midbrain, the pons and the medulla oblongata. The cell bodies of most of the cranial nerves that innervate the skin and the muscles of our face, head and neck are situated in the three parts of the brainstem. We’ll have a closer look at the cranial nerves a little later on in this tutorial.

We’ll talk about the three parts of the brainstem in superior to inferior order starting with the midbrain, which you can see here highlighted in green. The midbrain is associated with vision, hearing, arousal, the sleep/wake cycle, motor control, and thermoregulation. Next, we have the pons which contains nuclei that help to control sleep, respiration, swallowing, the bladder, hearing, balance, taste, eye movement, posture, and facial expressions. Finally, most inferior, you can see the medulla oblongata which helps to regulate breathing, digestion, swallowing, sneezing, and heart and blood vessel function. Due to these regulating functions of the brainstem, an injury to the brainstem due to, for example, a fractured neck or a gunshot to the mouth as often seen in the movies always end fatally.

The spinal cord is a continuation of the brainstem and it is found within the vertebral canal of the spine. It has thirty one pairs of spinal nerves which exit via the intervertebral foramina on either side of the spine. The intervertebral foramina are lateral openings formed by the two vertebra sitting on top of each other, and if we zoom in on our image, we can see them a little bit more clearly. Now that we’re familiar with the central nervous system, let’s move on to look at the peripheral nervous system.

The peripheral nervous system refers to the nervous system outside of the central nervous system. Its main function is to carry information from the central nervous system to peripheral organs and tissues such as muscles, skin and sweat glands and vice versa. The peripheral nervous system comprises the spinal nerves and the cranial nerves.

An important part of the peripheral nervous system is the autonomic nervous system. The autonomic nervous system, also known as the ANS, is our unconscious nervous system. This part of the peripheral nervous system controls all the things in our body that function without us noticing it. It innervates glands, helps to dilate or constrict our blood vessels, and controls our digestive and pelvic organs as well as our heart and lungs.

Another part of the peripheral nervous system we should briefly mention is the somatic nervous system. The word somatic means relating to the body and it explains the function of this system well. The nerves that supply our arms and legs as well as our neck muscles and trunk all originate from this system. As I just said, it is considered a part of the peripheral nervous system and is responsible for carrying sensory and motor information. Both cranial and spinal nerves contribute to the somatic nervous system.

Let’s now move on to discuss the twelve cranial nerves which are, as I previously mentioned, part of the peripheral nervous system. The sensation and movement of muscles of our face, head and part of the neck as well as the conduction of special senses such as taste and vision are carried by the twelve pairs of cranial nerves. These nerves originate in the brainstem and brain so you may expect them to be part of the central nervous system, but they’re actually part of the peripheral nervous system. I’ll show you now each of these nerves using an inferior view of the brain.

The first six cranial nerves are the olfactory nerves, which give us our sense of smell; the optic nerve, which allows us to see; the oculomotor nerve, which allows to move our ices and eyelids and control the size of our pupils; the trochlear nerves which are also involved in the moving of the eyes; the trigeminal nerves, which provide feeling in our face and help us to chew food; and the abducens nerve, which allows us to move our eyes from side to side.

The next six pairs of the cranial nerves are the facial nerves, which gives us facial movement and are involved in taste sensations; the vestibulocochlear nerves which control hearing and balance, the glossopharyngeal nerves, which allows to taste and give sensation to the pharynx; the vagus nerve, which are involved in our cough reflex and autonomic regulation of upper abdominal and thoracic organs; the accessory nerves, which innervate certain muscles like the trapezius muscles in the back; and finally, the hypoglossal nerves, which innervate the muscles in the tongue.

Since we’ve already looked at the nerves that innervate our face, head and part of the neck, you might be wondering about the rest of our body. Our limbs and trunk receive their innervation by the spinal nerves. There are thirty one pairs of spinal nerves, all of which have both a sensory component and a motor component. Once the leave the intervertebral foramen of the spine, they form both anterior and posterior rami. The posterior rami supply the skin and muscles of the back whereas the anterior rami supply the skin and muscles of the front and sides of the body as well as the upper and lower limbs.

The innervation of our limbs is via a very complex system. For protective purposes, specific muscles and skin regions of our limbs are innervated by fibers, which receive information from more than one level of the spinal cord. To achieve this, the anterior rami are mixed and reorganized in structures called plexi, singular plexus. Plexus means braid in Latin and describes a network of nerves or vessels. We can see an example of a nerve plexus in our image on the right – the brachial plexus. These networks of nerves are always formed by the anterior rami of our spinal nerves.

Today, we’re going to be talking about the five plexi – the cervical plexus, the brachial plexus, the lumbar plexus, the sacral plexus, and the coccygeal plexus. So, let’s start with the cervical plexus.

The cervical plexus is located in the neck, as shown in our illustration on the right. The nerves that arise from this plexus innervate some of the muscles found in the back, neck and diaphragm as well as the skin of the neck, head, ears and thorax. The phrenic nerve, which arises from the cervical plexus and innervates the diaphragm will be a structure likely covered at length in your anatomy lectures.

The plexus you can now see here highlighted in green is the brachial plexus. It is a nerve network extending from the parts of the spinal cord located in the lower neck and the upper thoracic region. It extends through the neck and shoulder region and into the armpit, also referred to as the axilla. Nerves arising from this network innervate almost all muscles and skin of the shoulders, arms, forearms and hands.

We’ll now have a look at three peripheral nerves that originate from the brachial plexus and can be found within the upper limb before we move on to look at the other plexi. The nerves we’ll be focusing on here are the median nerve, the ulnar nerve, and the radial nerve. We can see the median nerve highlighted in green in this image, and as the name already suggests, it travels down the midline of the anterior arm. The median nerve innervates most of the flexor muscles of the forearm, the muscles of the thenar eminence and the radial two lumbricals. It is the only nerve that passes through the carpal tunnel which connects the forearm to the hand. If the median nerve becomes compressed in the carpal tunnel, carpal tunnel syndrome can occur.

The radial nerve is found mostly on the posterior aspect of the arm, and we can see it here highlighted in green. It innervates various muscles that help us to stretch or extend the elbow, wrist and fingers. Examples of these called the triceps brachii in the arm and the muscles of the posterior compartment of the forearm. The radial nerve also innervates the skin of the posterior arm, forearm and hand, providing it with sensation.

Finally, we have the ulnar nerve which is located close to the ulnar bone running along the medial side of the arm and the forearm. You can see it highlighted in green in the image on our right. The ulnar nerve can broadly be described as the nerve of the hand as it innervates the vast majority of the intrinsic hand muscles. The ulnar nerve is often known for causing the pain associated when hitting your elbow. This painful sensation occurs when the ulna nerve becomes trapped within a groove of the humerus on the posteromedial side of the elbow. If this happens to you and you feel closely where the tingling occurs, you’ll be able to recognize the areas of skin innervated by the ulnar nerve.

As we’ve just looked at the plexi found in the upper part of the spinal cord and their relation to the upper limb, we’re now going to be looking at the plexi found in the lower parts of the spinal cord, which play a role in pelvic and lower limb innervation.

The first one which you can see here, highlighted in green, is called the lumbar plexus. It’s located in the lower back and is formed by contributions from the nerves of the lower thoracic region and lower back. The lumbar plexus innervates the skin and musculature of the lower limb. When somebody complains of a herniated disc in their lower back, nerves of the lumbar plexus as well as nerves of the sacral plexus might be affected and carry the pain sensation as far as the toes.

The sacral plexus is found in the lower region of the trunk. You can see it here highlighted in green. It innervates the back of the thigh, the lower leg, the feet and the pelvis, providing them with mobility and sensation. The lumbar plexus and sacral plexus are together often also called the lumbosacral plexus. The nerves arising from the lumbosacral plexus carry the longest axons of the body. These can be up to one meter in length depending on your leg length, of course.

We’ll now have a look at some of the nerves which arise here and travel within the lower limbs.

The nerve highlighted in green here is called the obturator nerve. It arises from the lumbar plexus and travels through an opening in the plexus called the obturator foramen, hence, its name. In our next illustration, we can see the obturator nerve passing through the obturator foramen. This nerve innervates the muscles in the medial compartment of the thigh commonly called the adductor muscles. When you press your legs together, you’re adductor muscles are active and your obturator nerve is firing information to move.

The sciatic nerve is the largest peripheral nerve of the body. It branches from the lumbosacral plexus and innervates the posterior thigh, legs and feet. Therefore, it’s sometimes referred to as the nerve of the lower limb. Within the thigh, the sciatic nerve splits into two nerves – the common peroneal nerve and the tibial nerve which innervate the legs and feet. We’ll have a closer look at these two nerves right now.

The common peroneal nerve is found laterally within the leg as you can see in our image on the right. It innervates the anterior and lateral compartments of the legs which enable dorsiflexion and ankle eversion respectively. The tibial nerve is the nerve we can see next to the common peroneal nerve that continues posteriorly into the leg from the sciatic nerve. You can see it here highlighted in green. The tibial nerve’s function is also to provide sensation and motility to the calf and foot muscles. The tibial nerve, for example, innervates the muscles that make it possible for us to stand on our toes, or in more technical terms, that create flexion or plantarflexion of the foot. Specifically, it innervates the triceps surae, plantaris, popliteus, tibialis posterior, flexor digitorum longus and flexor hallucis longus muscles.

Now that we’ve looked at the largest plexi of our body and some of their peripheral nerves, we’ll have a brief look at our last and smallest nerve plexus – the coccygeal plexus.

The coccygeal plexus is named after a bone called the coccyx which you might know better as the tailbone. The few of you that have actually experienced a fractured coccyx will remember that it’s extremely painful and leads to an inability to sit. The coccygeal plexus innervates the skin around our tailbone.

Now that we’ve learned that the anterior rami of the cervical, brachial, lumbosacral, and coccygeal spinal nerves hence the nerves of your neck, upper limbs and lower back form plexi, you may be wondering what happens to the spinal nerves of the thoracic region. Well, these nerves form the intercostal nerves.

The intercostal nerves run along the ribs in the thorax. They are the anterior rami of the twelve spinal nerves originating in the thoracic region. You can see them here in green. The first two of these nerves innervate the arms while the other nerves remain restricted to the thorax, and the lower five also supply the abdomen. The intercostal nerves are involved in muscle contraction and gaining sensory information from the skin.

The subcostal nerve, which you can see here in green, is a division of the twelfth thoracic nerve. It runs along the lower region of the twelfth rib in the thorax. The subcostal nerve innervates some of the abdominal muscles and branches into the skin providing it with some sensation.

And, finally, let’s talk about the tail of the spinal cord. Since the spinal cord is actually shorter than the spine itself, the lower peripheral nerves continue to travel within the vertebral canal before exiting forming the cauda equina. The cauda equina is Latin for horse’s tail and it comprises a bundle of spinal nerves and spinal nerve roots that look a little like a horse’s tail at the caudal end of the spinal cord. The cauda equina is made up of a number of nerves. Some of these are the second to fifth lumbar nerve pairs, the first to fifth sacral nerve pairs, and the coccygeal nerve.

As you’ve probably already noticed, some clinical scenarios were mentioned throughout this video. It shows the complexity in the many things that can go wrong with the nervous system. We’re going to go into a bit more depth now with our clinical notes.

An injury to the peripheral nerve can lead to loss or disturbance of sensation and movement. For example, steppage gait can result from injury to the common peroneal nerve. Steppage gait is characterized by foot drop due to a loss of dorsiflexion. The foot hangs with the toes pointing down causing the toes to scrape the ground while walking. This requires the individual to lift their leg higher than normal when walking.

Another example of this can occur with spinal disc herniation. Spinal disc herniation is a condition affecting the spine in which a tear in the annulus fibrosus of an intervertebral disc allows the soft central portion to bulge out beyond the damaged outer rings. This can result in the irritation of nerve roots that are compressed by the herniated material. For example, if the herniated disc is in the lumbar region, the patient may experience leg pain due to irritation of one of the nerve roots of the sciatic nerve.

Another example which we’ve already mentioned but we’re going to talk about it again anyway is the nerve pain felt when you hit your funny bone. This will be something you can all relate to. I know I’ve hit my funny bone many times and it’s a pretty uncomfortable feeling; however, it’s not actually a bone you hit. Rather, this painful sensation occurs when the ulnar nerve becomes trapped within a groove of the humerus. If you feel closely where the tingling occurs next time you bang your elbow, you’ll be able to recognize the areas of skin innervated by the ulnar nerve.

We’ve arrived at the end of the tutorial. But before I let you go, let me first summarize what we’ve spoken about today.

First, we defined the nervous system as a network of nerve cells and nerve fibers that transmit information all over the body in the form of electrical impulses. We then had a look at the cells of the nervous system. These included the neuron with its three main parts – the axon, dendrites and cell body – and the glial cells, which included the Schwann cells and the oligodendrocytes. These are responsible for the myelination of axons in the PNS and CNS respectively.

We then looked at the central nervous system, which consists of the brain and the spinal cord. The brain is divided into three parts – the cerebrum, the cerebellum and the brainstem. We saw that the spinal cord is a continuation of the brainstem and that it is found within the vertebral canal of the spine.

Next, we moved on to the peripheral nervous system and talked about the twelve pairs of cranial nerves and the thirty one pairs of spinal nerves. We saw that the anterior rami of the spinal nerves are mixed and reorganized into structures called plexi focusing on the five specific plexi. These were the cervical plexus, which is located in the neck; the brachial plexus, which innervates almost all muscles of the skin of the upper limb; the lumbar plexus, which comes together with the sacral plexus to form the lumbosacral plexus; and finally, the coccygeal plexus, which innervates the skin around our tailbone.

We also mentioned some of the peripheral nerves that arise from these plexi. The nerves found in the upper limb that we talked about are the median nerve, which travels down the midline of the anterior arm; the radial nerve, which is mostly found within the posterior aspect of the arm; and the ulnar nerve, which runs along the medial side of the arm and the forearm.

The nerves found in the lower limb that we talked about are the obturator nerve, which passes through the obturator foramen to innervate the lower limb; and the sciatic nerve, which divides into the common peroneal nerve and the tibial nerve. We then looked at the nerves which arise from the thoracic region of the spinal cord and these travel within the ribs and are called the intercostal nerves, and the subcostal nerve. Finally, we spoke about the cauda equina which is Latin for the horse’s tail and forms the lowest part of the spinal cord.

I hope you enjoyed this introduction to the nervous system and will think of it next time you hit your funny bone. See you later!

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