10th Class Science Control and Coordination Nervous System

         Introduction

In the previous chapter we have studied about the various activities involved in the maintenance of life process. In this chapter, we will discuss "how our body controls these various activities"?. How do we coordinate and response to the external stimulus? There must be a system which provides the control and coordination activities in the multicellular organisms.

The ability of the organism to detect changes and make appropriate response is called sensitivity. Anything to which an organism responds and reacts is called a stimulus. Animals response to the stimulus either by moving towards it, or by moving away from it. Response and coordination in animals involve the sense organs, nervous system and hormones.

         Nervous System

The nervous system is an organ system containing a network of specialized cells, called neurons. It coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system of vertebrates contains the brain, spinal cord, and retina. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. These regions are all interconnected by means of complex neural pathways.

The enteric nervous system, a subsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the vagus nerve, to function independently in controlling the gastrointestinal system.

Neurons send signals to other cells as electrochemical waves travelling along thin fibers called axons. This causes chemicals called neurotransmitters to be released at junctions, called synapses. A cell that receives a synaptic signal may be excited, inhibited, or otherwise modulated.

Sensory neurons are activated by physical stimuli impinging on them, and sends signals that inform the central nervous system of the state of the body and the external environment. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect the nervous system to muscles or other effector organs. Central neurons, which in vertebrates greatly outnumber the other types, make all of their input and output connections with other neurons. The interactions of all these types of neurons form neural circuits that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells, called glial cells, which provide structural and metabolic support.

The central nervous system (CNS) is the largest part, and includes the brain and spinal cord. The spinal cavity contains the spinal cord, while the head contains the brain. The CNS is enclosed and protected by meninges, a three- layered system of membranes, including a tough, leathery outer layer called the dura mater. The brain is also protected by the skull, and the spinal cord by the vertebrae.

The peripheral nervous system (PNS) is the collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS. Even though when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system.

           Function

At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others parts. There are multiple ways that a cell can send signals to other cells.

• One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this broadcast mode of signaling, the nervous system provides point-to-point signals.
• Neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster. The fastest nerve signals travel at speeds that exceed 100 meters per second.

At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors:

• Sending signals that encode this information into the central nervous system.
• Processing the information to determine an appropriate response,
• Sending output signals to muscles or glands to activate the response.

The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.