The brain and the spinal cord, along with the nerves found all over the body, constitute the nervous system. They form a complex network connected by different types of circuits that enable the animals to experience various sensations and respond to them in different ways. The reflex system is one such circuit that brings about sudden, automatic reactions in animals. The immediate closing of the eyelid when something comes towards an eye or touches it, and the automatic pulling away of a limb when something sharp pricks it, are reflex actions.
Various types of infections, injuries, inflammations, poisoning, and degenerative disease conditions can cause damage to the nervous system of a dog. Nutritional and metabolic disorders and congenital abnormalities also affect it. Tumors and cancers of the nervous system are not uncommon.
Many of the diseases affecting the nervous system of dogs are often typical of a sex or a specific breed. Some of them usually appear at a particular age too.
The nervous system of dogs may be differentiated into the central nervous system that consists of the brain and the spinal cord, and the peripheral nervous system under which are included all the nerves found in the other parts of the body.
The Central Nervous System
The brain has three distinct functional areas. The cerebrum is the seat of conscious thinking. It helps the animal analyze situations and take decisions. The cerebellum controls motor functions and movement. The brain stem takes care of the basic functions of the body.
The spinal cord has several divisions corresponding to the sections of the vertebral column through which it passes, such as the cervical and thoracic sections that cover the neck and the chest, the lumbar and sacral segments covering the rest of the body, and the caudal segment in the tail. The brain and the spinal cord have the protection of tissue layers called the meninges which cover them and the cerebrospinal fluid that occur in between them.
Nerve cells or Neurons
The cells that constitute the nervous system are called neurons. There are several billion neurons in the brain, spinal cord, and in the nerve fibers of the peripheral nervous system. The neurons are connected with one another, forming a neurological network that facilitates the different circuits of the nervous system to establish communication channels of electrical signals.
Each neuron has a cell body that contains the nucleus of the cell. Two types of extensions arise from this cell body. The longer extension is the axon that carries electric signals to other cells, while the shorter extensions called dendrites bring the electrical signals into the cell from other neurons. While sending a signal from the neuron, an electric current first passes along the axon, and when it reaches its end, certain chemical compounds known as neurotransmitters get released. These neurotransmitters are then picked up by the dendrites of other neurons or by the target glands or muscles that receive the signals.
The Peripheral Nervous System
It has paired nerves made up of neurons. The pairs arising from the spinal cord at different points send out the axons to internal organs such as the bladder and the anal muscles as well as to the limbs and the tail. These spinal nerves branch out to form a network that reaches every area of the body, both inside and on the surface. The cranial nerve pairs connect the brain to the head and the face.
The Different Types of Neurons
There are mainly two types of neurons, the sensory neurons and the motor neurons.
They carry sensory information from the sense organs and the different parts of the body to the brain stem or to the spinal cord. The sensory information, such as taste, pain, sight, hearing, temperature, touch and smell reaching the brain stem is sent to the cerebrum and the cerebellum for decoding.
On receiving the sensory stimuli, the brain and the spinal cord send appropriate responses to specific parts of the body through the motor neurons. The axons of these neurons come together to form bundles called tracts inside the spinal cord. They transmit the responses to the peripheral nerves that control the muscles in the hands and the legs to initiate the required reaction to the sensation. Control of the muscles and voluntary motion are brought about by these motor neurons.
The Autonomic Nervous System
Involuntary movements such as the rhythmic contraction and relaxation of the heart, the peristaltic movements of the digestive system etc., are controlled by another set of neurons that form the autonomic nervous system which is controlled by the brain stem. The axons of these neurons extend from the brain stem to the heart muscles, the muscles in the eye that control the movement of the pupils and to the walls of the gastrointestinal tract.