Introduction to the central and peripheral nervous systems

Definitions & Key takeaways

The central nervous system (CNS) is the part of the nervous system that consists of the brain and spinal cord. The CNS controls all voluntary muscle movement and sensations in the body. The peripheral nervous system (PNS) is the part of the nervous system that transmits signals from the central nervous system to the rest of the body, and vice versa.

The PNS consists of two parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary movements, such as walking or talking. The autonomic nervous system controls involuntary movements, such as breathing and bowel movements.

Chapters:

Watch video only

Introduction0:00–0:51

The smell of a morning coffee, the memory of last christmas, and the pain of touching a hot surface. That’s your nervous system at work!
The nervous system allows each body system to communicate with each other, and monitors internal and external changes to build a proper response that keeps us alive and kicking!Now, to make things a bit easier, the nervous system is structurally subdivided into two anatomical components, called the central nervous system, or CNS or short, which consists of the brain and the spinal cord and the peripheral nervous system, or PNS, which consists of the remainder of the nervous system outside of the CNS.Both the CNS and the PNS are made up of the functional units of the nervous system, called neurons or nerve cells.

Neurons0:51–2:26

These cells have many parts, with the main part being the cell body, which houses the nucleus of the cell. From the cell body arise two types of extensions: dendrites, which are short projections and may be numerous, and an axon, or nerve fiber, which is a single long extension that looks like a thread.
Now, neurons are special because they’re able to receive and send electrical impulses that allow for communication with other neurons, as well with effector cells, like muscle cells.
This process starts at the dendrites, which receive electrical signals from other neurons and convey it towards the cell body.
Then, the cell body produces an electrical impulse and sends it through its own single axon. Many axons are covered with a layer of lipids and proteins that speed up the conduction of these impulses, called the myelin sheath.
Now, the electrical impulse eventually reaches the distal parts of an axon, called the axon terminals. Here, an axon terminal can meet up with a dendrite of another neuron, and this contact site is called a synapse which facilitates the communication between neurons.Now, there are a few types of neurons that are structurally different from one another.

Types of neurons2:26–4:05

Neurons that have two or more dendrites and a single axon, like the one we just described, are called multipolar neurons.
They’re the most common type of neurons in the entire nervous system; for example, almost all motor neurons that control voluntary muscles in the body are multipolar neurons.
Other types of neurons include bipolar and pseudounipolar neurons. Bipolar neurons are neurons with a cell body that has a single dendrite on one side and a single axon on the other side.
These cells often act as sensory neurons, meaning that they relay sensory input to the central nervous system. An example would be the bipolar cells at the roof of the nasal cavity, which aid in the transmission of special sensory information or our sense of smell to the brain.
Lastly, are pseudounipolar neurons, which have a short seemingly single, but actually double process that extends from the cell body.
This common process divides into a peripheral process that extends from a sensory receptor to the cell body, and a central process that continues from the cell body towards the CNS.
So, an electrical impulse that is generated at a receptor, like touch or pain receptors in the skin, travels through the peripheral process and then continues into the CNS through its central process.Alright, let's take a quick break o see if we can identify the different types of neurons.

Quiz4:05–4:15

Great! Now neurons are a bit like humans, they always need someone for support, and that’s where neuroglia come in.

Neuroglia4:15–4:51

Neuroglia, or glial cells, are the only other cell type found in the nervous system and, although there are several different types of neuroglia, they all function in some way to support and nourish neurons.
These cells are smaller than neurons, but fun fact; there are five times as many glial cells as there are neurons in the body!
Alright, now let’s switch gears and discuss the central nervous system, which consists of the brain and spinal cord. The main role of the CNS is to coordinate and integrate neuronal signals from the periphery as well as perform higher-order functions like learning.

Central nervous system4:51–7:11

Now, there is important terminology used when discussing different parts of the CNS. Let’s start with talking about gray and white matter.
Let’s look at a coronal section of the brain. Note that the brain tissue has two different coloured areas.
Gray matter forms the outer portion of the brain, and is the place where neuron cell bodies are found. This outer portion of the brain is often referred to as the cerebral cortex.
The white matter on the other hand, forms the inner portion of the brain and mainly consists of axons transmitting signals from one area to another.
Remember that axons are typically covered with a myelin sheath that is made up of lipids; so this is what gives white matter its lighter appearance.Now, the spinal cord also has gray and white matter, but, from a transverse section, we can see that their organization is the other way around.The gray matter lies in the center and the white matter is found in the periphery.
The gray matter of the spinal cord is shaped like a butterfly and has two major parts: an anterior part called the anterior horn, and a posterior part called posterior horn.Also, a group of neuronal cell bodies may sometimes occupy a region surrounded by white matter deep in the brain and brainstem.
In this case a group of neuron cell bodies in the CNS is called a nucleus. For example, in the brain, surrounded by white matter, are several distinct groups of cell bodies, called the basal nuclei.
Now, a bundle of axons within the CNS is called a tract, like the cortico spinal tract, which consists of the axons of motor neurons extending from the cerebral cortex to the spinal cord.
Now, the other division of the nervous system is the peripheral nervous system, which carries messages to and from the central nervous system and consists of neuron cell bodies and axons - or nerve fibers - outside of the CNS.

Peripheral nervous system7:11–8:07

The components of the PNS are similar to the CNS, but the terminology used is slightly different. In the PNS, a group of cell bodies is called a ganglion.
For example, the cell bodies of peripheral sensory neurons are located outside the spinal cord, in the dorsal root ganglion.
And a bundle of axons running together in the PNS is called a nerve.Now, there are two types of nerves in the peripheral nervous system: nerves that arise from the spinal cord, called spinal nerves, and nerves that arise from the brain, called cranial nerves.Let’s start with cranial nerves, which are 12 pairs of nerves that exit the skull through different openings - or foramina.

Cranial nerves8:07–8:42

The cranial nerves then travel to mainly innervate structures of the head and neck. These nerves can be divided into three groups depending on the type of information that they carry as either sensory, motor or mixed cranial nerves.
Now, remember, although the cranial nerves originate from the brain which is part of the CNS, they are still considered to be part of the peripheral nervous system.
The other type of nerves in the PNS are spinal nerves, which are nerves that arise directly from the spinal cord via two roots, called the ventral or anterior roots and dorsal or posterior roots.

Roots of spinal nerves8:42–9:38

Now, the ventral, or anterior, roots, consist of a number of efferent - or motor - fibers from the anterior horn of gray matter within the spinal cord.
These fibers carry motor information from the CNS to the muscles of the body. The dorsal, or posterior, roots on the other hand, consist of afferent - or sensory - fibers which carry sensory information from the periphery to the posterior horn of gray matter in the spinal cord.
You can remember these terms by the simple mnemonic - afferent fibers arrive and efferent fibers exit the spinal cord!Now, the ventral and dorsal nerve roots combine to form a mixed spinal nerve, which has both motor and sensory fibers.

Spinal nerves9:38–10:29

There are 31 spinal cord segments with 31 pairs of spinal nerves arising from them. Starting superiorly, here are: 8 pairs of cervical spinal nerves, 12 pairs of thoracic, 5 pairs in the lumbar region, another 5 pairs in the sacral region, and a single pair of coccygeal spinal nerves.
The spinal cord segments and spinal nerves are identified by a letter and a number indicating their region and level. For example, the fifth spinal nerve in the thoracic region will be abbreviated as the T5 spinal nerve.Now, each mixed spinal nerve almost immediately divides into two rami; a large anterior ramus and a small posterior ramus that supply different parts of the body.

Rami of spinal nerves10:29–11:27

Because both the anterior and posterior rami are branches of mixed spinal nerves, they carry both sensory and motor fibers These rami - particularly the anterior rami - tend to join and rebranch in complex ways as they continue distally from the spinal cord, forming other nerves and networks of intersecting nerves called plexuses.
For example, the anterior rami of spinal nerves C5 through T1 join to form a number of nerves that make up the brachial plexus, which gives branches that supply the muscles and skin of the upper limbs.Alright, before we hit our last topic, pause the video and check which nerve structures you can recall!Great, now each spinal nerve innervates a particular area of the body.

Quiz11:27–11:37

Dermatomes and myotomes11:37–12:37

The specific muscles that each spinal nerve supplies with motor innervate is called a myotome. Each spinal nerve also receives sensory information from a specific area of the skin, which is called a dermatome.
You can easily remember these terms, since ‘derma’ in Greek means ‘skin’, and ‘myo’ refers to ‘muscles’. Now, each dermatome and myotome is labeled with a letter and a number that corresponds to its associated spinal nerve.
For example, the skin around the umbilicus or belly button is innervated by the T10 spinal nerve, so we call this area the T10 dermatome.
We often use these colourful dermatome maps to help visualize the specific regions that are supplied by each pair of spinal nerves!Alright, as a quick recap, the nervous system is divided into the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, which comprises the rest of the nervous tissues.

Review12:37–14:11

The nervous tissue has two types of cells: neurons that transmit signals, and supporting glial cells. In the central nervous system, groups of neuron cell bodies form nuclei that comprise the gray matter, while myelinated axons form tracts and comprise the white matter of the CNS.
In the peripheral nervous system, a group of cell bodies is called a ganglion, and bundles of axons are called nerves. The PNS consists of 12 pairs of cranial nerves that arise from the brain and 31 pairs of spinal nerves that arise directly from the spinal cord, with each nerve being formed by the union of ventral and dorsal roots.
Efferent fibers leave the spinal cord from the anterior horn through the ventral root, and provide motor innervation to muscles, while afferent fibers enter the posterior horn of the spinal cord through the dorsal root, and provide sensory signals to the central nervous system.
The short spinal nerves then branch into anterior and posterior rami. Dermatomes and myotomes are segments of the skin and