By Rene Fester Kratz

Animals are the only living things on Earth with complex nervous systems that first receive and interpret sensory signals from the environment and then send out messages to direct the animal’s response. The complexity of an animal’s nervous system depends on its lifestyle and body plan.

  • Animals whose bodies don’t have a defined head or tail have nerve nets, which are weblike arrangements of nerve cells that extend throughout the body.

  • Animals with a defined head possess a two-part nervous system:

    • The central nervous system (CNS) consists of the animal’s brain and central neurons. It’s housed in the head and may continue along the back.

    • The peripheral nervous system (PNS) consists of all the nerves that travel from the CNS to the rest of the animal’s body.

In all animals with a backbone, including you, the CNS consists of a brain and a spinal cord. The brain contains centers that process information from the sense organs, centers that control emotions and intelligence, and centers that regulate the physiological balance of the body (homeostasis). The spinal cord controls the flow of information to and from the brain.

Both the brain and the spinal cord are highly protected. First of all, they sit within a liquid called cerebrospinal fluid that guards the CNS against shocks caused by movement and they’re protected by the bones of the skull and vertebrae. CNS also supports the brain and spinal cord by supplying nutrients and helping to remove wastes.

The blood-brain barrier, which is created by the capillaries surrounding the brain, provides yet another layer of protection because the capillaries are highly selective about what they allow to enter the brain or cerebrospinal fluid. A final layer of protection is the meninges, two layers of connective tissue that surround the brain and spinal cord.

From the CNS, the nervous system branches off into the PNS, which is divided into two systems:

  • Somatic nervous system: This part of the PNS carries signals to and from the skeletal muscles. It controls many of an animal’s voluntary responses to signals in its environment.

  • Autonomic nervous system: This part of the PNS controls the mostly involuntary internal processes in the body, such as heartbeat and digestion. It has two divisions that work opposite each other to maintain homeostasis:

    • The sympathetic nervous system automatically stimulates the body when action is required. This is the part of the nervous system responsible for the fight-or-flight response, which stimulates a surge of adrenaline to give the body quick energy so it can escape danger.

      The sympathetic nervous system also quickens the heart rate to move blood through the blood vessels faster and releases sugar from the liver’s glycogen stores into the blood so fuel is readily available to the cells.

    • The parasympathetic nervous system stimulates more routine functions, such as the secretion of digestive enzymes or saliva. In contrast to the sympathetic nervous system, the parasympathetic nervous system slows down the heart rate after the fight-or-flight response is no longer needed.

For numbers 13, use the following terms to identify the type of nervous system that would be involved in each example.

a.    Somatic nervous system

b.    Sympathetic nervous system

c.    Parasympathetic nervous system

  1. You’re about to give a presentation in front of a class. Your heart is pounding and you feel a little lightheaded.

  2. You reach into a refrigerator to get yourself a drink.

  3. The heart rate of a black bear slows as it settles into dormancy.

  4. Print and mark the following figure, using two different colored pencils or highlighters. With one color, highlight the CNS. With the other color, highlight the PNS.


The following are answers to the practice questions presented.

  1. The answer is b. Sympathetic nervous system.

  2. The answer is a. Somatic nervous system.

  3. The answer is c. Parasympathetic nervous system.

  4. You should have colored the brain and spinal cord as one color and all the other nerves as a different color.