Lesson Explainer: Nerve Cells Biology

In this explainer, we will learn how to describe the structure of different types of nerve cells and outline their functions.

The picture below is one of the first clear images of a neuron hand drawn by Santiago Ramon y Cajal, the father of modern neuroscience. In his drawing, you can see that neurons are separate, individual cells. Prior to these drawings by Ramon y Cajal, most scientists of the time believed the nervous system was a network of continuous fibers. This is because in the late 1800s, while microscopy could allow observation of the brain cells, it did not have the capability to capture these observations outside of the observer’s eye. This is why these drawings of neurons are so special! They made it obvious that neurons were individual specialized cells that seemed to communicate across a small “gap” (what we now call the synapse).

Neurons are the main signaling unit of the human nervous system. You may recall that neurons are specialized cells that transmit nerve impulses and are found throughout the central and peripheral nervous systems. By best estimates, the adult human brain contains about 86 billion neurons. It would take you over 3‎ ‎000 years to count them all!

While neurons are the main signaling unit, they are highly specialized and, therefore, need support cells to function. Neurons are supported by the other main cell type found in the human nervous system, glial cells (or glia). Unlike neurons, glia do not produce electrical impulses, and because of this, they were thought not to be critically important in nervous system function. However, glial cells, which are also called neuroglia, accomplish multiple key functions: they provide structural framework, form a barrier with blood vessels, insulate neurons, monitor and clean up the environment, and help maintain the health of neurons by repairing damaged parts and providing nutrients. This support by the neuroglia is very important since neurons cannot undergo mitosis like most other body cells. This is because neurons lack centrioles, which are important organelles in cell division.

Key Term: Neuron

A neuron is a specialized cell that transmits nerve impulses.

Key Term: Glia (Neuroglia)

Glia are nonneuronal cells that provide support to neurons.

Since neurons are the main signaling unit of the human nervous system, most neurons share the same basic anatomy. A neuron has five common features: the dendrites, cell body, axon, myelin sheath, and axon terminals. These anatomical structures are illustrated below in Figure 2. The transmission of electrical impulses involves each of these features, starting with the dendrites.

The dendrites receive chemical signals from other neurons at the level of contact formed with other neurons, called synapses. At synapses, the chemical signal, called neurotransmitter, generates an electrical signal, which is then conducted by dendrites to the cell body of the neuron. The impulses travel from the dendrites to the cell body to be integrated with other signal inputs received by the other dendrites. After integration and processing in the cell body, the electrical signal travels down the axon toward its final destination. Electrical signals never travel back from the terminals to the soma.

Some neurons are wrapped in a fatty coating called myelin sheath, which helps increase the conduction of the electrical signal down the axon. Finally, to pass the electrical impulse to another neuron or muscle cell, the axon terminals convert the impulse into chemical signals and release them across a small gap. Reception of the chemical signals by the dendrites of another neuron or the muscles helps sustain the sequence of information.

Process: Sequence of Transmission of an Electrical Impulse through a Neuron

Dendritescellbodyaxonaxonterminal

Example 1: Describing the Sequence of Transmission of an Electrical Impulse through a Neuron

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?

  1. Dendrites cell body axon axon terminals
  2. Axon terminals axon cell body dendrites
  3. Dendrites axon cell body axon terminals
  4. Cell body dendrites axon axon terminals

Answer

Most neurons share the same basic anatomy. A neuron has five common features: the dendrites, cell body, axon, myelin sheath, and axon terminals. These anatomical features are illustrated below. The transmission of electrical impulses involves each of these features, starting with the dendrites.

The dendrites receive chemical signals from other neurons at the level of synapses and convert them into an electrical signal to be processed by the cell body of the neuron. The impulses travel from the dendrites to the cell body to be integrated with other signal inputs received by the other dendrites. After integration and processing in the cell body, the electrical signal travels down the axon toward its final destination. Some neurons are wrapped in a fatty coating called myelin sheath, which helps increase the conduction of the electrical signal down the axon. Finally, to pass the electrical impulse to another neuron or muscle cell, the axon terminals convert the impulse into chemical signals and release them across a small gap. Reception of the chemical signals by the dendrites of another neuron or the muscles helps sustain the sequence of information.

Therefore, the correct outline for the sequence of transmission of an electrical impulse through a neuron is dendrites cell body axon axon terminals.

Let’s take a closer look at the features of a neuron and its role in the transmission of an electrical impulse.

The sequence transmission of an electrical impulse through neurons begins at the dendrites. Neurons interact with one another through the contact they form at the level of structures called synapses that are present on the dendrites. From the neurons shown in Figure 1 and the neuron in the illustration in figure 3 below, you may notice that the dendrites have shapes that are very similar to tree branches. This is no accident as the word dendrite comes from the Greek word déndron, which means “tree.” You can see in the illustration below, neurons can have many dendrites with many branches. This is why you may hear the term dendritic tree used to describe the multiple dendrites of a neuron.

The primary function of the dendrites is to receive and transmit electrical impulses toward the cell body. The treelike structures of the dendrites help facilitate multiple connections between multiple neurons. The dendritic trees in neurons help receive multiple input signals from multiple neurons at the same time. It is the sum of the inputs received from dendrites that helps determine whether the neuron will fire an action potential at the level of the junction between the cell body and the axon.

Definition: Dendrites

Dendrites are the tree-branch-like parts of a neuron that receive the input signals of other neurons.

Example 2: Defining the Importance of Dendrites

A key structure of nerve cells (neurons) are the dendrites. What is the primary function of the dendrites?

  1. To receive and transmit electrical impulses toward the cell body
  2. To insulate the axon and increase the speed of action potential conduction
  3. To transmit impulses from the cell body to the end of the neuron

Answer

The sequence transmission of an electrical impulse through neurons begins at the dendrites.

Neurons interact with one another through specialized points of contact called synapses that are present on dendrites. From the illustration below, you may notice that the dendrites have a unique shape, one that is very similar to tree branches. This is no accident, as the word dendrite comes from the Greek word déndron, which means “tree.” Like in the micrograph below, most neurons have many dendrites. This is why you may hear the term dendritic tree used to describe the multiple dendrites of a neuron.

The primary function of the dendrites is to bear the synapses, which are the structures where a chemical signal released by another neuron is transformed into an electrical signal. Then, dendrites transmit electrical impulses toward the cell body. The treelike structures of the dendrites help facilitate multiple connections between multiple neurons. The dendritic trees in neurons help receive multiple input signals from multiple neurons at the same time. It is the sum of the inputs received from dendrites that helps determine whether the neuron will fire an action potential.

Therefore, the primary function of the dendrites is to receive and transmit electrical impulses toward the cell body.

The summation of the types of inputs received by the dendrites is calculated by the cell body of the neuron, which is also referred to as the soma.

The cell body connects the dendrites (where input signals are received) to the axon (where impulses are conducted out). The main function of the cell body of a neuron is to integrate information received by the dendrites and transmit the signal to other cells via the axon. It is also where all the proteins for the dendrites and axons are produced. Since the cell body helps determine whether to send the information to the axon, it is often called the “control center” of the neuron.

The cell body of a neuron contains many organelles that are surrounded by neuroplasm, which is the specialized name for the cytoplasm of nerve cells. Two key features found in the cell body of a neuron are the nucleus and Nissl granules.

Key Term: Cell Body (Soma)

The cell body of a neuron integrates information received by the dendrites and transmits the signal to other cells via the axon.

Definition: Neuroplasm

Neuroplasm is the specialized name for the cytoplasm found in the cell body of neurons that holds the organelles in place.

The nucleus contains genetic information (as in DNA), directs protein synthesis, and supplies the energy for the neuron to function. The Nissl granules can be seen as the black dots in Figure 4. They are composed of rough endoplasmic reticulum and ribosomes, to be used in protein synthesis. Along with the other organelles, these two key features help the cell body provide all the energy needed by the neuron to keep it alive.

Definition: Nissl Granules

Nissl granules are a group of ribosomes used in protein synthesis in neurons.

The sequence transmission of an electrical impulse through neurons begins at the dendrites.

The axon is a tubelike structure that carries an electrical impulse from the cell body to the axon terminals where the electrical impulse will pass to another neuron. This means that the axon is the main conducting unit of the neuron. Axons help carry electrical impulses long distances.

Neurons are the most asymmetric cells in nature, and this is due to the axon. In humans, axons can measure up to a metre in length in one direction.

Definition: Axon

An axon is the long threadlike part of a neuron along which nerve impulses are conducted.

The axon is supported by a complex mesh of structural proteins called neurofilaments. Neurofilaments are an essential part of the architecture of axons and provide mechanical stability to the axon, particularly when axons reach great lengths. Early in development, axons are narrow and contain very few neurofilaments. As neurons mature, the axons acquire more neurofilaments, which drives the expansion of their length. Neurofilaments are also very important for the transport of proteins and material along the axon from the cell body to the terminal.

Definition: Neurofilaments

Neurofilaments are a complex mesh of structural proteins that provide support to the axon.

Given how long the axons can get, there is one key feature that ensures the nerve impulse does not degrade or “fizzle out” before it reaches its destination. The myelin sheath is a fatty coating made of the plasma membrane of a specialized cell that wraps in a spiral around the axon and helps to increase the speed at which the electrical impulse can travel. Since the myelin sheath can extend for one or two millimetres, depending on the diameter of the axon, it can reach a thickness that can be 1001‎ ‎000 times the diameter of the axon.

Myelin is made by two different cells, depending on the location of the axon. In the central nervous system, the myelin sheath is made by oligodendrocytes. In the peripheral nervous system, Schwann cells make the myelin sheath. Oligodendrocytes produce myelin for groups of neurons, sometimes up to 30 different neurons! Schwann cells, in contrast, only produce a myelin sheath for one neuron at a time, as shown in Figure 5. The outermost layer of the myelin sheath made by each Schwann cell is called the neurolemma.

Example 3: Describing the Function of the Myelin Sheath

Which of the following statements about the myelin sheath is correct?

  1. Changes in the structure of the myelin sheath initiate a nervous impulse.
  2. The myelin sheath is a lipid-rich layer formed by Schwann cells.
  3. The myelin sheath prevents the nerve cells from being able to undergo cell division.
  4. The myelin sheath helps slow down the speed of electrical conduction.

Answer

The myelin sheath is a fatty coating made from lipids that surrounds the axons and helps to increase the speed at which the electrical impulse can travel.

Myelin is made by two different type of cells, depending on the location of the axon. The plasma membrane of these cells is wrapped around the axon in a spiral. In the central nervous system, the myelin sheath is made by oligodendrocytes. In the peripheral nervous system, Schwann cells make the myelin sheath. Oligodendrocytes produce myelin for groups of neurons, sometimes up to 30 different neurons! Schwann cells, in contrast, only produce a myelin sheath for one neuron at a time, as shown below.

Therefore, the correct statement about the myelin sheath is that the myelin sheath is a layer of a lipid-rich layer formed by Schwann cells.

Myelin sheaths act as insulators along the length of the axon, so they prevent the electric signal from leaking sideways. Like a plastic insulator around an electric cable, the insulator keeps electrons within the path of the conducting axon. The myelin sheath is not continuous along the axon however. Instead, there are small gaps that interrupt the myelin sheath as it continues down the length of the axon. You can see this in Figure 5 on the left. These gaps, called nodes of Ranvier, enable the electric impulses to be regenerated regularly as they travel along the long axons. Without the nodes of Ranvier, electric impulses would progressively dissipate as they travel long distances down the axons.

Definition: Myelin Sheath

The myelin sheath is a fatty coating that surrounds the axons and helps increase the speed at which the electrical impulse is conducted down the axon.

The nodes of Ranvier are critical to maintaining the integrity of the nerve impulse and increasing its speed as it travels down the axon. The electrical impulse in myelinated axons thus appears to “jump” from one node to the next, increasing the speed at which the nerve impulse travels toward its destination. The movement of the nerve impulse as it travels between nodes is shown in Figure 6.

Definition: Nodes of Ranvier

The nodes of Ranvier are gaps in the myelin sheath that help increase the transmission speed of electrical impulses.

In unmyelinated neurons, impulses are conducted down axons at a speed of 0.5 m/s2.0 m/s, which is about as fast as you walk or jog. However, when axons are myelinated, the electrical impulses can travel at a speed of 70 m/s120 m/s, which is almost as fast as a speeding race car.

Example 4: Understanding the Importance of Myelin in the Conduction of Nerve Impulses

In nerve cell A, the speed of the nervous impulse is 12 m/s. In nerve cell B, the speed of the nervous impulse is 140 m/s. Which nerve cell is myelinated?

Answer

The myelin sheath is a fatty coating that surrounds the axons and helps increase the speed at which the electrical impulse can travel.

For example, in neurons with unmyelinated axons, nerve impulses travel at a speed of 0.5 m/s2.0 m/s, which is about as fast as you walk or jog. However, nerve impulses in myelinated axons travel down at a speed of 70 m/s120 m/s, which is about as fast as a speeding race car.

Myelin acts as an insulator across the length of the axon, helping the electric signal travel along the axon without losing energy by leaking sideways. However, the myelin sheath is not continuous. Instead, there are small gaps that interrupt the myelin sheath where the electric impulse is regenerated so as to continue down the length of the axon.

The gaps in the myelin sheath, known as the nodes of Ranvier, are critical to increasing the speed of electrical-impulse transmission. The electrical impulse in myelinated axons appears to “jump” from one node to the next, increasing the speed at which the nerve impulse travels toward its destination. The movement of the nerve impulse as it travels between nodes is shown in the figure below.

Since the nerve impulse in B is moving at 140 m/s, which is far greater than 12 m/s, the nerve cell that is myelinated is B.

At the end of the axon is the axon terminal, which can be considered the last structure traversed by the nerve impulse in the sequence of transmission.

The axon terminal, which is also referred to as the bouton (pronounced “boo-tōn”), is distinguished from the rest of the axon based on its enlarged club-like shape shown in Figure 7. The axon terminal is the communication site and secretory region of neurons. When the axon reaches its target destination, the neuron does not physically connect with another neuron or muscle cell. Instead, the electrical impulse is converted into chemical signals that will then cross a small gap (called the synapse) to be released to the next neuron or a muscle cell. If the chemical signal crosses the synapse to another neuron, the transmission sequence starts all over again in the next neuron.

While neurons can share some of the same anatomical features, they are highly specialized to carry out specific functions. Commonly, neurons are classified into three functional groups: sensory neurons, motor neurons, and relay neurons or interneurons. The three functional types can modify the shape of the neuron, as shown in Figure 8. Let’s examine how the shapes of neurons help them carry out their specialized functions.

Sensory neurons are typically found in the peripheral nervous system where they collect sensory information from our body and the external environment. Sometimes, these neurons are also called afferent neurons. The word afferent is Latin for “bringing toward” and describes the flow of sensory information into the central nervous system, specifically the spinal cord.

Since there are many sensory receptors conveying information to the central nervous system, the distal process of sensory neurons are often bundled together. These bundles of neurons, which are also called nerves, are held together by connective tissues called the epineurium.

Different types of sensory neurons respond to different stimuli. For example, some neurons detect temperature, others detect pain, and some are specialized for taste. Depending on the sensory stimuli to be gathered, the shape of the neuron can change. Sensory neurons are pseudounipolar or bipolar in shape.

Pseudounipolar sensory neurons have an oval-shaped cell body and a long dendrite and an axon that have fused forming a long axon as shown in Figure 9. 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. The sensory neurons found in the nose, in the retina of the eye, and in the ear are slightly different in shape, as they are bipolar neurons.

Key Term: Sensory (Afferent) Neuron

Sensory neurons transmit information from sensory receptors to the central nervous system, via the spinal cord.

Motor neurons transmit information from the brain and spinal cord to the muscles, organs, and glands of the body. They are also called efferent neurons to describe the direction that the motor commands from the central nervous system are carried in. The word efferent is Latin for “carrying away.” As motor neurons carry information to the muscles, organs, and glands of the body, they make up the motor division of the peripheral nervous system. Most motor neurons are multipolar neurons. This helps describe their structure as having one axon and many dendrites, which can be seen in Figure 10.

Key Term: Motor (Efferent) Neuron

Motor neurons transmit electrical impulses from the central nervous system to effectors, which are muscles, glands and organs.

The third functional class of neurons is called relay (or intermediate or connector) neurons, or simply interneurons. This type of neuron is the largest class of neurons because it describes any neuron that is not a sensory or motor neuron. Relay neurons receive information from other neurons (either sensory neurons or relay neurons) and transmit information to other neurons (either motor neurons or relay neurons).

Relay neurons are also multipolar neurons like motor neurons. Unlike motor neurons, relay neurons have shorter axons like the one shown in Figure 11. This helps connect only to the nearby sensory and motor neurons, helping pass signals between these neurons.

Key Terms: Relay Neuron (Interneuron)

Relay neurons transmit electrical impulses between sensory neurons and motor neurons.

A summary of the three functional classes of neurons is shown in the table below.

Example 5: Identifying the Three Functional Types of Neurons

Which of the following tables correctly summarizes the main types of neurons and their functions?

  1. TypeSensoryRelayMotor
    FunctionCarry impulses from receptors to the central nervous systemCarry impulses from the central nervous system to the muscles and glandsCarry impulses from relay to sensory neurons
  2. TypeSensoryRelayMotor
    FunctionCarry impulses from relay to motor neuronsCarry impulses from the central nervous system to the muscles and glandsCarry impulses from receptors to the central nervous system
  3. TypeSensoryRelayMotor
    FunctionCarry impulses from relay neurons to effectors like muscles and glandsCarry impulses between sensory and motor neuronsCarry impulses from receptors to relay neurons
  4. TypeSensoryRelayMotor
    FunctionCarry impulses from receptors to relay neuronsCarry impulses between sensory and motor neuronsCarry impulses from relay neurons to effectors like muscles and glands

Answer

While neurons can share some of the same anatomical features, they are highly specialized to carry out specific functions. Commonly, neurons are classified into three functional groups: sensory neurons, motor neurons, and relay neurons or interneurons. The three functional types can modify the shape of the neuron, as shown in the figure below.

Sensory neurons are typically found in the peripheral nervous system where they collect sensory information from our body and the external environment. Sometimes, these neurons are also called afferent neurons. The word afferent is Latin for “bringing toward” and describes the flow of sensory information into the central nervous system.

Motor neurons transmit information from the brain and spinal cord to the muscles, organs, and glands of the body. They are also called efferent neurons to describe the direction that the motor commands from the central nervous system are carried in. The word efferent is Latin for “carrying away.” As motor neurons carry motor commands to the muscles, organs, and glands of the body, they make up the motor division of the peripheral nervous system.

The third functional class of neurons, called relay neurons or interneurons, is the largest class of neurons because it describes any neuron that is not a sensory or motor neuron. Relay neurons receive information from other neurons (either sensory neurons or relay neurons) and transmit information to other neurons (either motor neurons or relay neurons).

Therefore, the table that correctly summarizes the main types of neurons and their functions describes the function of sensory neurons as carrying impulses from receptors to relay neurons, the function of relay neurons as carrying impulses between sensory and motor neurons, and the function of motor neurons as carrying impulses from relay neurons to effectors like muscles and glands.

Let’s summarize what we have learned in this explainer.

Key Points

  • Neurons are specialized cells found in the brain to transmit electrical impulses. They are supported by specialized cells called glial cells or neuroglia.
  • The key features of a neuron are the dendrites, cell body, and axon.
  • The dendrites receive input from other neurons and send it to the cell body of the neuron for integration with other signals received by other dendrites.
  • The cell body (also called the soma) of the neuron processes and integrates signals received by the dendrites and determines if the nerve impulse will be transmitted down the axon.
  • The axon is the main conducting unit of the neuron and is surrounded by a fatty coating called the myelin sheath that acts as an insulator, increasing the conduction of electricity.
  • The myelin sheath is not continuous and has unmyelinated gaps termed the nodes of Ranvier.
  • When an electrical impulse travels down the axon, it “jumps” from one node of Ranvier to the next, to help increase the conduction speed of the electrical impulse.
  • Neurons can be categorized by their three functions: gathering sensory information, controlling motor movements, or helping connect different neurons.

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