Video Transcript
In this video, we’ll learn about
the basic anatomy of nerve cells and the important function that each part of the
cell plays in the transmission of the nerve impulse. We’ll also learn about the three
basic types of nerve cells and how to identify their distinguishing features. We’ll finish off by working through
a practice problem and reviewing some key points about nerve cells.
Santiago Ramón y Cajal is the
scientist credited with the first clear images of nerve cells as individual
cells. Using an improved cell staining
technique and his immense drawing skill, Ramón y Cajal’s observations and drawings
confirmed that nerve cells were individual cells and not a large network of tissue
as previously thought. This significant finding is one of
the reasons why Santiago Ramón y Cajal is considered the father of modern
neuroscience. Now, let’s take a page out of Ramón
y Cajal’s drawing book and dive into the anatomy of nerve cells.
This is a nerve cell, or neuron as
they’re more frequently called. Neurons are the main signaling unit
of the human nervous system. As specialized cells, the main
function of neurons is to transmit nerve impulses to other neurons, muscle cells, or
glands. Most neurons share the same basic
anatomy because the structure of a neuron is uniquely suited to aid in the
transmission of the nerve impulse. The dendrites of a neuron are the
anatomical feature which receives input signals from other neurons. Typically, these input signals take
the form of chemical messengers called neurotransmitters. The word dendrite comes from the
Greek word dendron, which means tree. So if you’ve noticed that the
dendrites look a bit like a cluster of tree branches, well done.
One dendrite can make up to 300,000
connections with other neurons. So neurons with many dendrites are
able to receive input signals from hundreds of thousands of other neurons. All of the input signals received
by the dendrites are then sent to the cell body of the neuron. The cell body of the neuron is also
often called the soma. The main function of this
anatomical feature is to process and integrate the signals received by the
dendrite. This processing determines if a
nerve impulse will be generated and transmitted down the axon to the next
neuron.
You may have noticed that there are
several organelles within the cell body. Two key organelles in the cell body
of a neuron are the nucleus and the Nissl granules. The nucleus contains genetic
information and directs protein synthesis. Nissl granules, which are shown
here as small dots, are composed of rough endoplasmic reticulum and ribosomes and
are the site of protein synthesis in the neuron. These key features and functions of
the cell body are why it is considered the control center of the neuron. The cell body of a neuron connects
to the axon. Neurons are the most asymmetrical
cell in nature, and this is due to the axon.
The axon is the threadlike
structure of the neuron that can measure up to one meter in length in one
direction. The primary function of an axon is
to transmit or carry electrical impulses from the cell body of the axon to the axon
terminals. The distance between the cell body
and the axon terminals can vary greatly. It can be as close as 0.1
millimeters or as far away as two meters. So the length of axons is highly
varied. To ensure the structural support of
the axon remains stable as it travels great distances, the axon is internally
supported by a complex mesh of structural proteins called neurofilaments. Neurofilaments are very important
for the transporter proteins made in the cell body that must travel to the axon
terminal.
Many axons are surrounded by a
fatty coating called the myelin sheath. Given the lengths of axons, the
myelin sheath helps to ensure the nerve impulse does not degrade or fizzle out
before it reaches its destination. You may notice that the myelin
sheath is not continuous, and instead there are small unmyelinated gaps between each
myelin sheath. These are the nodes of Ranvier. The nodes of Ranvier help to
increase the speed of conduction of the nerve impulse as it travels along the axon
towards the axon terminal. Myelin acts as an insulator, so the
portion of the axon that is covered by myelin is not a very good conductor of the
electrical impulse.
So in myelinated neurons, as the
nerve impulse travels down the axon, it jumps from one unmyelinated node of Ranvier
to the next unmyelinated node of Ranvier. It is this jumping action which
helps speed the nerve impulse down the axon no matter its length as well as preserve
the integrity of the nerve impulse as it reaches its final destination.
At the end of the axon are the axon
terminals, which can be considered the last structure that is transversed by the
nerve impulse in the sequence of transmission. The axon terminals are the
secretory region of the neuron that aids in communication with other neurons,
glands, and muscle cells. Each axon terminal can be
distinguished from the rest of the neuron through its enlarged club-like shape. It is here in the axon terminal
that the electrical impulse is converted into a chemical signal to be sent to the
next neuron.
So the basic anatomy of a neuron
includes the dendrites, the cell body, the axon, which may or may not be myelinated,
and ends in the axon terminal. Now that we understand the basic
anatomy of a neuron, we can discuss how the basic anatomy is arranged in different
types of neurons to carry out highly specialized and specific functions. Most neurons can be classified into
three functional groups: sensory neurons, motor neurons, and relay neurons. Let’s examine how the shape of
these neurons helps them to carry out their specialized functions.
Sensory neurons collect sensory
information from our body and the external environment. Sensory neurons have an oval-shaped
cell body and an axon that splits into two distinct branches. One branch of the axon connects the
sensory neuron to the sensory receptor, while the other branch of the axon can
extend a very long distance to connect to the central nervous system. The long axons of sensory neurons
are also myelinated. This helps to transmit the
collected sensory information to the central nervous system quickly.
Motor neurons transmit information
from the brain and spinal cord to the muscles, organs, and glands of the body. Most motor neurons have many
dendrites, but just one axon, which is myelinated. The myelinated axon helps to ensure
that the nerve impulse sent from the brain and spinal cord quickly reaches the
target muscles, organs, or glands.
Relay neurons are also known as
intermediate neurons, connector neurons, or simply just interneurons. No matter the name that is used to
refer to these neurons, the function remains the same. Relay neurons transmit nerve
signals to other neurons. This means they receive information
from other neurons, usually sensory neurons or other relay neurons, and transmit
this information to other neurons such as motor neurons or other relay neurons. Like motor neurons, relay neurons
have many dendrites and just one axon. But in contrast to motor neurons,
the axon of relay neurons is very short and unmyelinated. These shorter axons help relay
neurons to connect only to the closest sensory or motor neurons during impulse
transmission.
Now that we’re familiar with the
anatomical features of a neuron and how these anatomical features change to suit the
specialized function of our neuron, we’re ready to try a practice question.
Starting with receiving a chemical
signal from another neuron, which of the following correctly outlines the sequence
of transmission of an electrical impulse through a neuron? Dendrites, axon, cell body, axon
terminals. Dendrites, cell body, axon, axon
terminal. Cell body, dendrites, axon, axon
terminal. Axon terminal, axon, cell body,
dendrites.
To answer this question, we need to
recall the anatomy of a neuron and how this anatomy helps transmit an electrical
impulse to another neuron. This is your typical neuron. And in green, we see the chemical
signals mentioned by the question. The dendrites are the part of the
neuron that receives these chemical signals sent by other neurons. A neuron typically has many
dendrites. A key way to identify the structure
is to look for the part of the neuron that most resembles tree branches. It is at the dendrites that the
incoming chemical signal is converted into an electrical impulse to be sent to the
cell body of the neuron for processing.
The cell body of a neuron is also
called the soma. Since each neuron can have many
dendrites receiving signals from other neurons, the cell body plays a very important
role in integrating all of these incoming signals. After integration and processing,
it is the cell body that determines if an outgoing signal will be generated and
transmitted to the axon. From the cell body, the electrical
impulse is transmitted to the axon. The axon is the long threadlike
portion of the neuron which conducts the electrical impulse to the axon terminals
located at the end of the axon. Therefore, the axon is considered
the main conducting unit of the neuron. This axon is unmyelinated. This means the axon is not covered
by a myelin sheath.
In some neurons, the myelin sheath
can surround parts of the axon and helps to increase the speed of the transmission
of the electrical impulse. As the electrical impulse reaches
the end of the axon, it is transmitted to the axon terminal. The axon terminal is where the
electrical impulse is converted back into a chemical signal to be sent across the
synapse to the next neuron. So the axon terminal is the site of
secretory communication for the neuron.
Now that we’ve reviewed the anatomy
of a neuron and how this anatomy helps to transmit the electrical impulse to the
next neuron, we can answer the question. The correct outline for the
sequence of transmission of an electrical impulse through a neuron is dendrites,
cell body, axon, axon terminal.
Now that we’ve learned about the
anatomical structure of a neuron and how the anatomy of a neuron changes to suit its
specialized function and worked through a practice question, let’s take a moment to
review some of the key points from this video. Neurons are specialized cells that
transmit electrical impulses. The key anatomy of a neuron
includes the dendrites, the cell body, the axon, and the axon terminal. Each of these anatomical structures
has a key function in the transmission of a nerve impulse. Finally, neurons can be divided
into three functional groups: sensory neurons, motor neurons, and relay neurons.
