In a dog's nervous system, electrical impulses travel via nerve fibers, which deliver messages to cells and organs. Chemical transmitters are also used for communication between different nerve cells and other tissues they communicate with. The nervous system is a very complex network.
Central Nervous System: In mammals, the nervous system is divided into several segments. The central nervous system (CNS) is made up of the brain, brain stem, and spinal cord. The peripheral nervous system (PNS) includes the nerves that run from the brain to areas of the head and neck, and also those nerves exiting and entering the spinal cord. These nerves carry messages from the CNS to other body areas such as the legs and tail. Nerve impulses travel from the brain down the spinal cord, out the peripheral nerves, to the tissues and back again.
Peripheral Nervous System: Peripheral nerves that go from the brain or spinal cord are called motor nerves. These nerves affect muscles, i.e., they control movements, posture, and reflexes. Peripheral nerves that return to the brain or spinal cord are referred to as sensory nerves. These nerves carry information (such as pain sensation) from the body's structures back to the central nervous system.
Autonomic Nervous System: Another set of nerves comprise the autonomic nervous system (ANS). The ANS (which arises from the CNS) contains nerves which control involuntary movements of organs such as the intestines, heart, blood vessels, bladder, etc. Dogs have no voluntary control over the autonomic nervous system; it functions automatically.
Development of the nervous system
Coordinated Movement: A puppy is born without a fully developed nervous system. The brain, spinal cord, and associated nerves are present at birth but lack the capacity to adequately transmit electrical impulses in a coordinated fashion. As the nervous system matures in the initial weeks of life, a series of nerve controlled events begin to become evident. During the first week of life, it seems that puppies do little but eat and sleep. They do have some motor activity, moving even while seemingly sound asleep. By the second week of life a puppy still spends a great deal of time sleeping, but the sleep becomes quieter or more restful with fewer body movements. Awake moments are typically spent nursing. By three weeks of age, most puppies can maintain an upright posture and begin to spend more time awake. They attempt to move by pushing or sliding, as they are still unable to stand and walk. The initial attempts at 'crawling' are usually short as the muscles are not strong. After three weeks of age, the puppy will develop the ability to stand and perhaps walk short distances. Eventually, over the next few weeks, the puppy becomes fully mobile and able to walk and even run in a clumsy sort of fashion.
Vision: Puppies are born blind with closed eyelids. The eyelids open by fourteen days of age, exposing the eyeball which now is only mildly sensitive to light. Most puppies will have vision by three to four weeks of age, but it will not be fully developed until after ten weeks of age.
Hearing: Puppies are born deaf as well as blind. Just like the eyelids, the ear canals remain closed until about two weeks of age. At about two weeks of age, most puppies can hear some noises. At this age, they are easily startled by sharp noises. By four weeks of age, most puppies will hear the sounds without becoming startled. Puppies over four weeks of age can hear quite well.
All of the above developments, the walking, vision, and hearing, are controlled by the nervous system. The exact age at which these abilities develop is variable. The ages mentioned are the average, not the rule.
Disorders of the nervous system
Nervous system disorders may result from improper development of the nervous tissue and its associated organs, or from damage due to trauma or infections. Many conditions exist which are genetic in origin.