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Did you know that hammerhead sharks and platypuses share a special type of sensory neuron that humans and most other animals don’t have? In this lesson, you’ll find out what that sensory neuron and a whole lot of others can do.

You’ll also learn how the nervous system is organized and the differences between the central and peripheral nervous systems.

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Central and Peripheral Nervous System

The nervous system is quite complex and includes many types of neurons that have many different functions. However, it is very well organized, and despite its complexity, the organization is actually quite elegant and reflects the functions of its component neurons. The central nervous system is composed of the brain and spinal cord and functions mainly to process information and determine the appropriate responses.The peripheral nervous system is composed of all of the sensory and motor neurons of the body and functions to gather sensory information and to control the actions of our bodies.

Sensory Neurons

The peripheral nervous system includes two basic types of neurons: sensory neurons and motor neurons.

You may remember that sensory neurons are neurons that collect sensory input and send it to the brain, and motor neurons are neurons that transmit signals to responsive tissues.When Timmy sees a butterfly in his backyard, his eyes collect a lot of detailed information about the light they receive. This information is turned into neural impulses by specialized sensory neurons in the eye called photoreceptors, which are sensory neurons that sense light. These neural impulses are then carried by the axons of the photoreceptors to neurons in the brain. But photoreceptors aren’t the only type of sensory neuron; in fact, humans probably have more types of sensory neurons than you think. For example, people have neurons that sense temperature called thermoreceptors.

We also have pain receptors that are sensitive to excess heat, pressure or specific molecules that are released from injured tissues.Another whole class of sensory neurons is the mechanoreceptors, which are sensory neurons that are stimulated by physical forces. There are many different types of mechanoreceptors. Some mechanoreceptors are located near the skin surface and sense very small changes in pressure that we interpret as touch. Others, located deeper in the skin, respond to stronger pressure and inform us about the strength of the pressure. Other mechanoreceptors are sensitive to stretch.

We have lots of these stretch-responsive receptors in our muscles that monitor the lengths of all of our muscles, which is important for the coordination of movement. Even our hearing is dependent on mechanoreceptors. Specialized cells in the ear, called hair cells, are sensitive to vibrations of membranes in the ear which allow us to hear sounds.Chemoreceptors are yet another class of sensory neurons that respond to specific molecules. In humans, chemoreceptors are responsible for the senses of taste and smell. Chemoreceptors are also used by the body to monitor the concentration of various substances in the blood which is necessary for maintaining homeostasis. As you can see, humans have quite a variety of sensory neurons, but there are many more types of sensory neurons found in other animals that humans don’t have.

For example, honeybees and some other animals have photoreceptors that can sense ultraviolet light, which we cannot. What appears to be a plain yellow flower to us sometimes turns out to be a yellow and ultraviolet striped flower to the bee, like this flower here. The photograph on the left was taken with a standard camera. The photograph on the right is of the same flower but was taken with an ultraviolet camera and reveals that the flower is actually yellow with ultraviolet stripes. We cannot detect this with our eyes, but bees can.

Bees have photoreceptors that cause objects to look ultraviolet
Flower as seen by humans and bees

Hammerhead sharks and platypuses have electroreceptors that they use to sense the electric fields of animals that they might not be able to see in dark water or buried under mud and sand. And many snakes have infrared sensors that allow them to locate warm-blooded animals based on the infrared radiation that they produce. As you can see, there is a wide variety of sensory neurons that exist in the animal kingdom.

Motor Neurons

The other half of the peripheral nervous system is composed of motor neurons which have a completely different job from the sensory neurons. While sensory neurons monitor the status of the body and the environment around us and send this information to the central nervous system, motor neurons receive signals from the central nervous system, which they then use to tell responsive tissues what to do.

Motor neurons aren’t normally classified into different groups, and they are most often associated with controlling muscle actions, but some motor neurons also signal to glandular tissues and control secretion of various substances in the body, like saliva and gastric juice.

The Central Nervous System

The main functions of the central nervous system are to process sensory information and determine what the appropriate reaction is. Neurons of the central nervous system only interact with other neurons and are called interneurons. Unlike sensory and motor neurons, interneurons both send and receive signals to and from other neurons. For the most part, sensory neurons throughout the body synapse with interneurons in the spinal cord.

The spinal cord neurons will then send the signal to the brain for further processing, but sometimes an immediate reaction before thorough processing can be completed is advantageous.

Reflexes

In these special cases, the spinal cord neurons send the signal out to the motor neurons to cause an automatic reaction to very specific stimuli without any involvement of the brain. This type of reaction is called a reflex which is basically an automatic reaction to a stimulus that is not consciously controlled.

A well-known reflex is the knee-jerk reaction. This is a reaction that doctors use to test people’s reflexes. The doctor will tap on the tendon just below the kneecap which causes a reflexive muscle contraction to straighten the leg. Even though we don’t have control over it, we’re still consciously aware of what’s going on. We can still feel the tap on our tendon, and we can feel our leg move, so the signals are being sent to our brain too.

Review

So let’s do a quick review.

The central nervous system is composed of the brain and spinal cord and functions mainly to process information and determine the appropriate responses.The peripheral nervous system is composed of all of the sensory and motor neurons of the body and it functions to gather sensory information and to control the actions of our bodies.The peripheral nervous system includes two basic types of neurons: sensory neurons and motor neurons. Sensory neurons are neurons that collect sensory input and send it to the brain.

There are lots of different types of sensory neurons that are specifically designed to gather different types of information and send it to the central nervous system. Motor neurons are neurons that transmit signals to responsive tissues. These neurons receive signals from the central nervous system and transmit them to responsive tissues to appropriately respond to the situation.

The central nervous system is composed of two major organs: the brain and the spinal cord. The spinal cord is responsible for receiving signals from most of the sensory neurons throughout the body and passing these signals along to the brain. It’s also responsible for transmitting signals to motor neurons. Some specialized immediate reactions, called reflexes, are automatic reactions to stimuli that are not consciously controlled don’t involve the brain, but are mediated by the spinal cord instead. However, all of the more complex processing of sensory information and decision making is performed by the brain.

Key Points

Motor and sensory neurons are how the central and peripheral nervous systems control the body
The nervous system
Peripheral Nervous System Central Nervous System
Composed of sensory and motor neurons Composed of the brain and spinal cord
Gathers information from specialized sensory neurons and transmits it to the central nervous system Receives signals from the peripheral nervous system, passes them to the brain, and sends signals to the motor neurons

Learning Outcomes

Working your way through this lesson should assist you in accomplishing the following:

  • Describe the central and periphery nervous systems and how they function
  • Identify the types of sensory neurons and state their functions
  • Explain how motor neurons link the peripheral and central nervous systems
  • Recall how reflexes differ from normal reactions
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