Video: Nervous system

Ahhh! Isn't this just magical? The best way to end a day is to grab a blanket, relax, and do a bit of stargazing. Eww! Did something just touch my leg? The creepy crawlies are not ideal, but at least I know the old nerves are working, I guess. Speaking of nerves, did you know that there are actually more nerve cells in our bodies than there are stars in the Milky Way? Can you believe that? You might not quite know how they work or what our nervous system even is, but it's got to be pretty important if there are that many nerve cells, right? Maybe, we should check it out. Right, let's roll up that blanket and go learn a little bit about our nervous system.

Today, we're going to learn all about the nervous system. We'll begin by defining what the nervous system is and what it does. We will then explore the divisions of the nervous system which include the central nervous system and the peripheral nervous system. Within the peripheral nervous system, we will explore two further subdivisions – the autonomic and the somatic nervous system. It might sound a bit complicated now, but don't worry. We're going to keep it nice and simple. Along the way, we will tackle some important concepts of the nervous system such as neurons, nerves, ganglia, and finally, nervous plexuses. We'll also name some of the main nerves in each region of the body. We'll finish off this tutorial by taking a quick look at a clinical scenario surrounding the nervous system to help consolidate our knowledge.

So, what is the nervous system and why is it so important? The nervous system is a network of nerve cells which are known as neurons and nerve fibers that transmit information through the body in the form of electrical impulses. The main function of the nervous system is to pick up information from our surrounding environment and to create a reaction in response to this information. Without it, we wouldn't be able to move, sense our surroundings, taste, smell, feel emotions, or even think. The nervous system even ensures the smooth running of our vital organs such as our heart and kidneys. Without the nervous system, we wouldn't be able to function.

Before we move forward, let's take a look at the basic structure of a neuron. Every neuron consists of a cell body, which is also known as a soma, and a number of processes which project from the cell body known as neurites. The soma contains important cellular organelles and generates the neural impulses which travel along our nerves. Neurites stem from a cell body and carry impulses to and from the cell body. They connect neurons with each other and with other body cells enabling the flow of neural impulses.

There are two types of neurites which are known as axons and dendrites. Axons are the long stem which conduct impulses away from the cell body while dendrites receive impulses from other neurons and conduct the electrical signal towards the cell body. Depending on the type of neuron, the cell body can be situated at different locations and the number of neurites can vary like we see here.

Now that we have a better understanding of what the nervous system does, let's take a look at its divisions starting with the central nervous system.

The central nervous system, which is also known as the CNS, is made up of the brain and the spinal cord. The brain is a major processing center of the nervous system. Some might even call it the master organ of the CNS – it's that important. It coordinates the movement of our limbs, controls vital functions such as breathing and heart rate, and interprets the sensations that we feel.

The brain is divided into three separate regions, each with different responsibilities. These regions include the cerebrum, the cerebellum, and the brainstem. Let's take a quick look at each of these regions.

The cerebrum is the largest part of the brain and is divided into the left and right hemispheres. The division is more obvious if you look at the superior aspect of the brain. Interestingly, the left and right hemispheres control the opposite sides of the body. For example, when you move your left arm up and down, the right hemisphere of your brain is controlling that movement.

The cerebrum is involved in many processes of the body which include movement, interpreting sensations such as vision and hearing, thinking, judging, problem solving, memory and language, our emotions, our ability to learn – the list is long. The cerebrum is divided into four lobes, each of which carries out different functions. The lobes of the cerebrum include the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. If you want to find out more about the lobes of the cerebrum, check out some of our videos specifically on the brain.

Moving on, we have the cerebellum. Cerebellum is the Latin word for little brain, perfectly describing this little guy which sits tucked under the posterior aspect of the cerebrum. The cerebellum is responsible for our balance and coordination ensuring that our movements are nice and smooth. Damage to the cerebellum can lead to an uneven, unbalanced gait which looks very similar to the all-too-familiar drunk walk.

The last region of the brain is the brainstem. The brainstem connects the brain to the spinal cord. We can only see the distal portion of the brainstem in this image, so let's cut the brain in half to get a better look. The brainstem is divided into three regions which are known as the midbrain, the pons, and the medulla oblongata. These regions provide sensory and motor innervation of the head, face, and neck as the cranial nerve nuclei are located here. We'll come back to the cranial nerves when we explore the peripheral nervous system. The brainstem also controls important bodily functions such as breathing, thermoregulation, visual and auditory reflexes, digestion, heart and blood vessel function, as well as our posture.

So now that we're familiar with the brain, let's take a look at the second component of the CNS – the spinal cord.

The spinal cord is a continuation of the brainstem at the distal end of the medulla oblongata which travels all the way down our backs within the vertebral column. The spinal cord is our major delivery system. It carries messages from the brain to the peripheral nervous system and transports signals from the peripheral nervous system back up to the brain. The small exception to this rule are the cranial nerves, which carry information to and from the brainstem. As the spinal cord travels down our back, it gives off 31 pairs of spinal nerves which contribute to the formation of the peripheral nervous system. Without the peripheral nervous system, nerve impulses would only get as far as the spinal cord and never reach their desired destination.

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, also known as the PNS, comprises all elements of the nervous system that lies outside of the CNS. It is composed of two nerve types – the spinal and the cranial nerves. Its main function is to carry information from the central nervous system to peripheral organs and tissues such as muscles, skin, and viscera and vice versa.

Before we move on any further, let's hit the pause button and learn about the main component of the peripheral nervous system.

We are, of course, talking, about nerves. Nerves are cord-like structures which carry impulses around the body. They are essentially enclosed bundles of axons relaying impulses from one neuron to the next. There are two types of nerves within the peripheral nervous system – spinal nerves and cranial nerves. We'll meet these a little later on.

Now back to the peripheral nervous system. It can be further divided into the autonomic nervous system and the somatic nervous system.

The autonomic nervous system, which is also known as the ANS, is our unconscious or involuntary nervous system. The ANS receives impulses from the brain and brings about an unconscious motor reaction. It innervates our digestive system, our sweat glands, our lungs, and even the beating of our heart. The autonomic nervous system itself is divided into the parasympathetic and sympathetic nervous systems. These systems work together to unconsciously maintain a stable internal environment within the body.

The parasympathetic system is usually identified as the system that controls our resting state as it helps our body to relax. It decreases our heart rate, constricts our pupils, stimulates saliva secretion and digestion, and relaxes the sphincter muscles of the intestinal tract.

The sympathetic system is our fight or flight response. It prepares our bodies to deal with emergencies and anxious situations. It dilates our pupils, relaxes our bronchi to allow for deeper breathing, increases our heart rate, slows digestion, and stimulates adrenaline secretion. All of this happens without us even having to think about doing it.

The other division of the peripheral nervous system is the somatic nervous system. It is our voluntary nervous system carrying both motor and sensory innervation to and from the CNS. Motor innervation from the CNS carried to the peripheral nervous system elicits movement of our skeletal muscles. Motor impulses are also known as efferent nerve impulses as they enter the PNS and produce movement while sensory innervation is carried from the peripheral nervous system back to the CNS through the afferent nerve fibers which move away from the PNS. Nerve signals lead to movements of walking or talking as well as sensations of touch, pressure, and vibration are all connected by the somatic nervous system.

Autonomic and somatic nerve signals are carried by the cranial and spinal nerves. Cranial nerves, as their name suggests, relate to nerves of the head. There are 12 cranial nerves altogether which provide a mixture of sensory and motor innervation to the head and neck. All 12 cranial nerves originate from nuclei within the brain and are numbered according to their positioning. While all cranial nerves supply regions of the head and neck, the exception is the tenth cranial nerve – the vagus nerve – which travels inferiorly and also supplies the thoracic and abdominal viscera.

An example of autonomic nerve transmission via the cranial nerve is the production of saliva from the parotid gland by the parasympathetic innervation of the glossopharyngeal nerve – the ninth cranial nerve. The ability to feel our fingertips when we touch our face is brought about by the somatic sensory component of the trigeminal nerve, which is cranial nerve five.

Now that we've had a look at the nerves of the head and neck, let's have a look at the nerves of the rest of our body.

Our limbs and trunk receive their autonomic and somatic innervation via the spinal nerves. There are 31 pairs of spinal nerves which along with their branches carry motor, sensory, and autonomic innervation from the spinal cord to the rest of the body. We are now looking at a cross-section through the spinal cord. As we can see, the spinal nerve fibers emerge from the spinal cord in two locations. They can either emerge anteriorly as the anterior root or posteriorly as the posterior root. Nerve fibers which emerge through the anterior root carry motor innervation while nerve fibers of the posterior root carry sensory innervation.

As the spinal nerve containing both anterior and posterior roots emerges from the spinal column, it divides into four main branches – an anterior ramus, a posterior ramus, the meningeal branches, and the rami communicantes. The large anterior ramus supplies the motor and sensory innervation to the skin and muscles of the front and sides of the body. The smaller posterior ramus supplies the motor and sensory innervation to the skin and muscles of the back. The meningeal branches return to the spinal cord to supply components of the vertebral column, and finally, the rami communicantes carry autonomic nerve fibers to and from the spinal nerve. Each spinal nerve is named after the vertebral level from which it emerges. For example, these are the L1 to L5 spinal nerves as they emerge from the L1 to L5 vertebrae.

Sometimes, for certain information to reach the right location, a relay system is needed. Relay systems in the peripheral nervous system are known as ganglia. A ganglion is a collection of neuronal cell bodies found in both the somatic and autonomic branches of the peripheral nervous system. Information that travels along an axon enters the ganglia, is transmitted to the body of the next neuron in the ganglia, and then exits via its axon.

There are two types of ganglia within the PNS – the sensory and the autonomic ganglia. Sensory ganglia can transmit sensory information from the peripheral nervous system to the spinal cord. This type of ganglion is known as the spinal ganglion, or sometimes, the dorsal root ganglion because it is associated with the posterior or dorsal root of a spinal nerve. The other type are the sensory ganglia of the cranial nerves. Here you can see the sensory ganglion of the trigeminal or the fifth cranial nerve.

Autonomic ganglia relay motor impulses from the central nervous system to the autonomic nerves which innervate the target organs within the body. Autonomic ganglia can be divided into parasympathetic ganglia and sympathetic ganglia.

To finish up our overview of the nervous system, let's quickly go through the main nerves of each of the body regions.

What you need to know first is that the innervation of our limbs can be quite complex. For protective purposes, certain muscles and skin regions of our limbs and trunk are innervated by fibers which receive information from more than one level of the spinal cord. This is where plexuses come into play. Plexuses, or singular plexus, is a network of nerve fibers which originate from the anterior rami of numerous spinal nerves. Plexus in Latin means braid, which illustrates the intertwined nature of nerve fibers or vessels in a plexus.

Within our bodies, we have five main nerve plexuses which include the cervical plexus, the brachial plexus, the lumbar plexus, the sacral plexus, and the coccygeal plexus. They are all named after their location and can be easily identified as a large network of nerve fibers. It is through these nerve plexuses that many nerves that we might know of come about.

Let's begin with the cervical plexus. This nerve plexus is located in your neck and originates from the C1 to C4 spinal nerve roots. Nerves which originate from this plexus supply regions of the neck, back, and chest.

Next, we have the brachial plexus, which arises at the root of the neck. This plexus originates from the C5 to T1 spinal nerve roots and supplies regions of the upper limb. Three main nerves supplying the upper limb arise from this plexus. The ulnar nerve travels down the medial aspect of your arm and forearm. The radial nerve is found mostly posteriorly and laterally and supplies structures in these regions. The median nerve, as its name suggests, travels down the midline of the anterior arm.

There are no major plexuses in the thoracic region. Here, each spinal nerve gives off an intercostal nerve found between the ribs.

The lumbar plexus is located in the lower back and is formed by contributions from the nerves of the lower thoracic region and lower back or lumbar region. It is formed primarily from the L1 to L4 nerve roots. This plexus supplies regions of the lower limb. One of the main nerves of the lumbar plexus is the large femoral nerve of the thigh. This nerve supplies both motor and sensory innervation to the lower limb.

Just distal to the lumbar plexus is the sacral plexus. This plexus innervates the back of the thigh, the lower leg, the feet, and the pelvis providing them with mobility and sensation. This nerve plexus originates from the L4 to S4 spinal nerve roots. The lumbar plexus and the sacral plexus are together often also called the lumbosacral plexus. One of the main nerves which emerges from the lumbosacral plexus is the large sciatic nerve. This nerve innervates the regions of the posterior thigh and leg. The sciatic nerve splits to form the common fibular nerve and tibial nerve. These nerves travel down the leg supplying the regions within this area.

Finally, we have the coccygeal plexus which is formed within the pelvis. The coccygeal plexus originates from S4 to S5 with contribution from the anterior rami of the coccygeal nerve. This plexus innervates regions of the pelvis.

And that concludes the main elements of the nervous system. Before we wrap up though, let's go over some clinical notes.

Motor neurons allow for the transmission of motor nerve impulses from the central nervous system to muscles. Motor neuron disease is a degenerative disorder which affects the motor neurons of the central nervous system and the peripheral nervous system. It leads to decreased motor innervation to the muscles of the periphery. Its cause is not fully understood, but maybe, due to genetic inheritance or as a result of viral infection. Degeneration of the motor neurons of the spinal cord is known as the upper motor neuron disease and is noted muscle weakness and wasting as well as hypotonia, or increased muscle tone of the limb muscles, and spasticity. Degeneration of the motor neurons of the peripheral nervous system is known as lower motor neuron disease. Symptoms include muscle weakness, atrophy and muscle wasting.

There is currently no cure for motor neuron disease with treatment focusing on managing symptoms. It may include physiotherapy, respiratory therapy, as well as pharmaceutical intervention.

We've made it to the end of this tutorial. Let's just go over a quick summary to remind us of what we learnt today.

In today's tutorial, we learned about the nervous system. We divided the nervous system into the central nervous system and the peripheral nervous system. The central nervous system contains the brain and the spinal cord. When looking at the brain, we discovered that it is the major processing center of the nervous system and is divided into three regions – the cerebrum, the cerebellum, and the brainstem. The spinal cord is our chief messenger and carries nerve impulses from the brain to the peripheral nervous system and vice versa.

We then explored the peripheral nervous system, which is made up of spinal and cranial nerves. The peripheral nervous system transmits messages from the spinal cord to the periphery and from the periphery back to the spinal cord. We divided the peripheral nervous system into a somatic and autonomic nervous system. The somatic nervous system is our voluntary system while the autonomic nervous system is our involuntary system which transmits unconscious motor impulses to target organs such as the heart and lungs. Both somatic and autonomic impulses are transmitted through the spinal and cranial nerves.