The nervous system is a network of nerve cells and, in most animals, a brain. In vertebrates, it also includes a spinal cord. The primary cell type found in the nervous system is the neuron , which has a cell body, containing the nucleus , and long extensions to carry information from one part of the body to another.
The nervous system has two primary functions that are critical in maintaining the life of the organism:
First, sensory receptors allow the organism to monitor its external environment and detect changes that occur (for example, an increase in temperature).
Second, the nervous system also monitors the organism's internal environment, controlling heart rate so that enough blood is delivered to organs, or measuring nutrient levels to signal when an organism needs to obtain food.
Invertebrate Nervous Systems
Although the invertebrate nervous system is usually much simpler than the nervous systems found in vertebrates, there is still a broad range in complexity depending on the type of invertebrate.The simplest type of nervous system is found in hydras and jellyfish (cnidarians) and is referred to as a "nerve net." Nerve nets do not have distinct central or peripheral regions, and lack anything that resembles a brain. Instead, the scattered nerve cells form loose networks in each cell layer of the body wall. Some of these neurons carry information from sensory organs that detect touch, light, or other changes in the environment. These neurons in turn contact neurons that control movement of the organism, such as swimming.
Invertebrates such as sea stars (echinoderms) display some centralized organization of the nervous system. A ring of neurons is located in the center of the sea star, and simple bundles of neurons called radial nerves extend from the ring to the tip of each arm. In each arm, extensions of the radial nerves form nerve nets as in the jellyfish. This arrangement permits coordinated movement of each arm and the tube feet located on the surface of the arm.
A distinct separation of peripheral and central nervous systems is found in invertebrates such as worms, insects, and mollusks, like the squid. Neuron cell bodies are grouped into clusters called ganglia , which are usually located along the animal's midline. The peripheral component of the nervous system is formed by the extensions of the cells in these ganglia; some carry sensory information from the environment to the ganglia, while others carry signals from the ganglia to produce a response (such as movement).
Vertebrate Nervous Systems
Vertebrate brains all contain three regions:
The hindbrain: is located at the junction of the brain and spinal cord, and is dedicated to coordination of motor (movement) reflexes and regulation of autonomic processes such as blood pressure and heart rate. The hindbrain consists of:
The medulla oblongata, is really a continuation of the spinal cord.
The pons carries impulses from one side of the cerebellum to the other and connects the medulla and cerebellum to other brain regions.
The cerebellum controls balance posture, and muscle coordination.
The midbrain is concerned with visual processing and some motor control.
The forebrain (the region closest to the anterior end of the organism). It can be divided into two distinct regions.
The telencephalon is concerned with associative activity, that is, combining or integrating all incoming sensory information and directing an appropriate response.
The diencephalon contains the thalamus and hypothalamus, regions important in processing sensory input and autonomic responses, respectively.
The spinal cord is similar to the invertebrate nerve cord, but is usually enclosed in a protective column of vertebrae (with the exception of the most primitive vertebrates, the lampreys and hagfishes).
Variety in Vertebrate Brains
The primitive vertebrates such as fish:
Thehindbrain is the largest of the three regions.
The cerebellum is relatively well developed for swimming and balance, although not in the lampreys and hagfishes.
Small mid-brain (just above the hindbrain) for the processing of visual information.
Small forebrain primarily concerned with the sense of smell (olfaction).
In amphibians:
The hindbrain is more enlarged compared to fish, but the cerebellum is often reduced in size, which reflects the relatively simple locomotion of amphibians.
The forebrain is still small and functions primarily in olfaction.
In reptiles and birds:
The size of the cerebellum is increased over amphibians, reaching massive proportions in birds where it regulates the complex muscle activity and spatial coordination needed for flying.
The midbrain is enlarged as well, which permits interpretation of more complex visual images. This is particularly true of birds, which also have relatively large eyes. In addition, the sense of hearing becomes more developed, and, beginning with reptiles, the midbrain shows a distinct region dedicated to auditory processing.
Reptiles and birds also possess forebrain regions that are much larger than those of more primitive vertebrates.
In mammals, including humans,
Themost striking change is in the size of the cerebellum (again for more complex movements).
The telencephalon, which may be so large that it covers the diencephalon, midbrain, and part of the cerebellum. As specialization of the telencephalon increases, the increased size is correlated with the appearance of convolutions or folds in the surface.
The highly wrinkled cerebral cortex completely covers all but the cerebellum in humans. In addition to integrating all types of sensory information and coordinating voluntary movement, all cognitive functions (speech, math, learning, memory) are located here as well.