Lesson Explainer: Structures of the Nervous System Biology

In this explainer, we will learn how to describe the main components of the nervous system as the brain, spinal cord and neurons, and relate their structure to their function.

The human nervous system plays a very important role in receiving information about our environment, allowing us to respond correctly to things that are happening around us and keeping our internal environment constant. It does this by rapidly transmitting huge amounts of information as electrical impulses around the body, using specialized cells called neurons.

The nervous system of humans can be divided into two parts: the central nervous system and the peripheral nervous system. These are outlined in figure 1.

The central nervous system is made up of the brain and spinal cord, and the peripheral nervous system is composed of nerves that connect the central nervous system to the rest of the body. Peripheral nerves extend into structures like the limbs, the skin, and other internal organs, apart from the brain.

Information is passed through the nerves, spinal cord, and brain by electrical signals, or “impulses.” These impulses travel extremely quickly and allow the body to respond rapidly.

If there is a sudden change in your environment, for instance, a football flying toward your face, you may need to respond quickly and duck! Your nervous system helps you do this by detecting a stimulus, which is a change in your environment (in this case, the football moving toward you).

Definition: Stimulus

A stimulus is a change in the internal or external environment of the body.

The human body is well adapted to detecting stimuli by possessing lots of receptors. A receptor is any structure within the body that detects a stimulus. Examples of receptor organs include ears, which detect sound; eyes, which detect light; and skin, which detects changes in temperature, pressure, and pain.

Definition: Receptor

A receptor is a specialized cell, organ, or structure in the body that detects a stimulus and generates electrical signals in response.

Example 1: Defining a “Stimulus”

What is a stimulus?

  1. A coordinated response to a change in the environment
  2. Messages passed through the central nervous system
  3. A change in your external or internal environment
  4. The structure that detects changes in the environment
  5. Information transmitted by motor neurons

Answer

Let’s look at an example. Imagine the temperature of the room you are in suddenly increases—we refer to this as a change in your environment, or a stimulus. There are structures in your body that detect this change, called receptors, for instance, heat receptors in your skin. Your body can then carry out a coordinated response to this change to reduce any risk of damage; this may mean you will start to sweat more, the blood vessels in your skin will dilate to increase heat loss from the body, and you will probably need to urinate less. The body brings about this coordinated response by communicating with itself, and this can be through electrical impulses that travel through neurons.

Therefore, a stimulus is a change in the external or internal environment of an organism.

Information is passed around the nervous system by neurotransmission—the transmission of nerve impulses from one cell to another in the nervous system. Nervous impulses are a type of electrical impulse that travels incredibly quickly. Specialized cells called neurons carry these impulses around the body.

Definition: Neurons

Neurons are specialized cells that transmit nerve impulses throughout the nervous system.

The human body contains different types of neurons that have different functions to allow rapid neurotransmission. Let’s have a look at three types of neurons.

Type of NeuronSensoryRelayMotor
FunctionSensory neurons transmit impulses from receptor cells or structures to the central nervous system.Relay neurons are found in the central nervous system and transmit impulses from sensory neurons to the motor neuron.Motor neurons transmit impulses from relay neurons to effectors.

You may not have encountered effectors before, but these are structures in the body—most commonly muscles, or glands that release hormones—that bring about a response.

Example 2: Describing the Function of a Sensory Neuron

Which of the following is the correct function of the sensory neurons?

  1. To transmit electrical impulses from receptor cells to the central nervous system
  2. To transmit electrical impulses from relay neurons to motor neurons
  3. To transmit electrical impulses from relay neurons to effectors
  4. To transmit chemical signals across the gap where two neurons meet
  5. To coordinate responses such as movement

Answer

Sensory neurons are important for passing information about the external environment to the central nervous system. The structures that pick up information from the external environment are receptors, for example, your eyes or your ears. The sensory neurons then receive this information from the receptors as an electrical impulse and carry it to the central nervous system. The central nervous system contains relay neurons, which pass electrical impulses to motor neurons, which work with effectors to bring about a response.

Therefore, the correct function of sensory neurons is that they are responsible for transmitting impulses from receptors to the central nervous system.

Each type of neuron has a unique structure that allows it to perform its function. Let’s have a look at the structure of each of these neurons.

A basic outline of the structure of a sensory neuron is provided in figure 2. The direction the nerve impulse travels in is shown by the red arrow. The dendrites of a sensory neuron receive information from receptor, and this information travels as a nerve impulse down through the axon and the axon terminals.

The myelin sheath is made up of specialized cells that surround the axon. These cells are very rich in fats, and they act as an electrical insulator, which increases the speed of transmission.

Example 3: Describing the Function of the Myelin Sheath

Choose the best description for the function of a myelin sheath.

  1. A myelin sheath surrounds the dendrites and speeds up the transmission of chemical signals.
  2. A myelin sheath acts as an electrical insulator and speeds up the transmission of impulses along the axon.
  3. A myelin sheath acts as an electrical conductor and slows down the transmission of impulses along the axon.
  4. A myelin sheath acts as a chemical conductor and speeds up the transmission of impulses.
  5. A myelin sheath acts as an insulator to regulate the neuron’s temperature.

Answer

The myelin sheath is a fat-rich layer that surrounds the axons of neurons (or nerve cells). The first thing to remember is that the signals that travel down an axon are electrical impulses, so we can rule out the answers that mention “chemical signals” or “chemical conduction.” The myelin sheath is made up of specialized cells that are rich in fats, which act to insulate the axons of nerve cells. Unlike insulation in the skin of the human body, this is not to keep us warm, but to act as an electrical insulator to make the speed of the electrical impulses even faster.

So the myelin sheath acts as an electrical insulator and helps speed up the transmission of impulses along the axons of neurons.

A relay neuron, shown in figure 3, has many dendrites that receive electrical impulses from sensory neurons. The transmission of these impulses is shown by the orange arrows. This information is then transmitted to motor neurons via the axon, as shown by the red arrow. Relay neurons are found within the brain and the spinal cord.

Motor neurons receive information from relay neurons via their dendrites. This information travels as an electrical impulse down the axon and into the axon terminals. The motor neuron axon is surrounded by a myelin sheath, which acts as an electrical insulator to increase the speed at which the impulse travels. The basic outline of a motor neuron is provided in figure 4.

By looking at the different types of neurons, we can see that the structure of the human nervous system helps the body coordinate responses to stimuli. The sensory neurons receive information from the receptors, which is processed by the relay neurons in the brain and spinal cord. Then, it is transmitted to the motor neurons, which can cause the effectors to bring about a response.

This process requires communication between different neurons. It is important to remember that, for most of the neurons in the human body, when two neurons communicate, they do not physically connect. Instead, there is a small gap between them. The place where neurons meet is called a synapse.

Definition: Synapse

A synapse is the junction between two neurons.

When an electrical nerve impulse reaches this gap between neurons, it causes the release of a neurotransmitter from the axon terminal.

Definition: Neurotransmitter

A neurotransmitter is a chemical substance that is released from an axon terminal of a neuron to stimulate a nerve impulse in the following neuron.

Figure 5 shows a simplified outline of the events that occur at a synapse.

The nerve impulse travels down the axon of the presynaptic neuron to the axon terminal. Here, it initiates the release of a neurotransmitter into the synaptic gap. The neurotransmitter is detected by receptors on the neuron that is after the gap (called the postsynaptic neuron). This initiates a new electrical impulse in this neuron.

Example 4: Describing the Series of Events That Occur at a Synapse

Which of the following best explains what usually happens at a synapse in the human nervous system?

  1. When an impulse reaches the end of an axon, it stimulates the release of neurotransmitters into the synaptic gap. The neurotransmitters diffuse across and are detected by the next neuron. This neuron then generates a new electrical impulse.
  2. When an impulse reaches the end of a dendrite, it stimulates the release of an electrical signal into the synaptic gap. This moves across and is detected by the axon of the next neuron, and a chemical signal is generated.
  3. When an impulse reaches the end of a dendrite, the dendrite releases neurotransmitters into the synaptic gap, which diffuse into the surrounding cell membranes.
  4. When an impulse reaches the end of a dendrite, it releases neurotransmitters into the gap. The neurotransmitters travel across the gap by osmosis and are detected by the next neuron. This neuron generates a new chemical impulse.
  5. Electrical impulses are passed between neurons by the connections that form in the cell wall of the axons where two neurons meet.

Answer

The first thing to remember is that when the large majority of neurons meet, they do not physically touch. Instead there is a small gap between them, and the junction created here is called a synapse. A nervous impulse is an electrical signal, but this cannot be directly transmitted over this gap. Instead, when the electrical impulse reaches the end of an axon of a neuron, it stimulates the release of a chemical signal, called a neurotransmitter, from the axon terminals. This diffuses across the synaptic gap and is picked up by the next neuron, which can then generate a new electrical impulse.

Therefore, we can describe the events that occur at a synapse as an impulse reaching the end of an axon and stimulating the release of neurotransmitters into the synaptic gap. The neurotransmitters then diffuse across this gap and are detected by the next neuron. This neuron then generates a new electrical impulse.

We will conclude with a summary of the key points about the structures involved in the human nervous system.

Key Points

  • The nervous system can be organized into the central nervous system and the peripheral nervous system.
  • The central nervous system is the brain and spinal cord.
  • The peripheral nervous system includes the nerves that extend into the other organs and limbs.
  • There are three major types of neurons, which have different functions: sensory, relay, and motor.
  • A synapse occurs when two neurons meet, creating a junction.
  • Different neurons work together to bring about a coordinated response to a stimulus.

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