Nervous Systems

The blood gets oxygenated and enters a second atrium and onto a second ventricle that pumps oxygenated blood to the tissues in a systemic circulation.

NERVOUS SYSTEMS

Animal nervous systems ultimately have some type of complex controlled system (the brain) and electrically-excitable nerve tissue that sends signals from one place to another in the organism. There are two divisions to the nervous system in animals: a central nervous system or CNS and a peripheral nervous system or PNS.

The central nervous system consists of the brain and spinal cord. Mammals will have four major lobes to the brain: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Other major structures deeper in the brain are the thalamus, hypothalamus, basal ganglia, limbic system, cerebellum, and brainstem. Most higher functions depend on the activity of more than one brain area. The spinal cord sends information from the brain to bridge the gap between the CNS and the PNS (or peripheral nerves). Motor reflexes are the responsibility of the spinal cord.

There are three protective coverings over the brain and spinal cord, called meninges. The outer layer is called the dura mater, which is primarily protective but also has veins that carry blood back to the heart. The middle layer is web-like and is called the arachnoid mater. The part that is intimately associated with the brain is referred to as the pia mater. Between the arachnoid mater and the pia mater is the cushioning cerebrospinal fluid or CSF. The CSF will circulate chemical substances throughout the brain and spinal cord.

The peripheral nervous system involves nerves that send motor signals to the muscles and organs of the body and receive sensory information, sending it to the central nervous system. The peripheral nervous system does not have a barrier between itself and the rest of the body with the PNS exposed to injury and toxins that aren’t a major factor in the CNS.

The autonomic nervous system is part of the PNS and is responsible for the unconscious control of things like the gastrointestinal system, blood pressure, heartbeat, and

breathing rate. It is important in the animal’s fight-or-flight response that is necessary for animal survival. Digestion is also a part of the autonomic nervous system.

There are two types of cells in the nervous system of animals. There are neurons, which are conducting cells, and neuroglia or glial cells, which are supporting and nonconducting cells. Neurons are non-mitotic cells that cannot be replaced if destroyed. Figure 49 shows the anatomic structure of the neuron or nerve cell:

Neurons have three cell parts: a cell body or soma, at least one dendrite, and a single axon. The cell body has the main structures that other cells have with the organelles and nucleus of the cell. They do not have centrioles, which are necessary for mitosis because these cells don’t undergo mitosis. The dendrites are the “receiving” part of the neuron. The extend out from the soma and transmit signals from the other nerve cells to the rest of the neuron. There is usually just one axon that sends a signal out to other nerves in the body. Axons can be myelinated or nonmyelinated. Myelin is a white, fatty substance made by glial cells that helps speed up the transmission of the nerve signals.

In the peripheral nervous system, the myelin as made by special glial cells called Schwann cells. It wraps tightly around the axon. The equivalent myelin-producing cell in the CNS is called the oligodendrocyte. While it is possible for nerve cells to