Structure of the nervous system

The nervous system is the part of an animal's body that coordinates its behavior and transmits signals between different body areas. Its main functions are 1) to collect sensory information from the body and external environment; 2) to process and interpret all sensory information; 3) to create an appropriate response. The nervous system is made up of nervous tissues that are, in turn, made up of many types of neurons. There are billions of neurons connected throughout the animal’s body. These neurons are connected end to end and transmit electrical impulses from one point to another.
Nervous systems are found in almost all multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges and microscopic organisms called mesozoans. The nervous systems of ctenophores (comb jellies) and cnidarians (e.g., anemones, hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types of animals, with the exception of echinoderms and a few types of worms, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system can be from a few hundred cells in the simplest worms to 100 billion cells in humans.

In vertebrate animals, the nervous system consists of two main parts, called the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. Both the brain and the spinal cord are highly protected. The brain is protected by the skull and the spinal cord - by the vertebral bones. The brain is the largest part of the central nervous system. It regulates many different functions of the body. It processes all incoming information, and tells the body what to do in response. The spinal cord controls the flow of information to and from the brain.

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. It consists of nerves connected to the brain and the spinal cord. The peripheral nervous system has two types of neurons that are constantly at work. Neurons that send impulses from the central nervous system to limbs and organs are called motor (or: efferent) neurons. Neurons that receive sensory information and transmit it to the central nervous system are called sensory (or: afferent neurons). Therefore, as the stimulus is received, the sensory (afferent) neurons work and as reaction is needed, the motor (efferent) neurons go to work.

The PNS is divided into “somatic” and “visceral” parts. The somatic nervous system controls voluntary body movements such as walking, for example. It contains all the neurons that are connected with skeletal muscles and skin. The visceral nervous system, which is also known as the autonomic nervous system, controls internal body functions that are not under conscious control (heart and respiratory rates, digestion, etc). The autonomic nervous system has two subsystems with different functions – the sympathetic systemand the parasympathetic system. They work opposite each other to maintain homeostasis.

The sympathetic system stimulates the “flight or fight”⃰ response that allows an animal to face up to an attacker or make a rapid departure. It increases the heart and respiratory rates, as well as the amount of blood flowing to the skeletal muscles while blood flow to less critical regions is reduced. It also causes the pupils of the eyes to dilate. In contrast to the sympathetic nervous system, the parasympathetic nervous system slows down the heart rate when the fight-or-flight response is no longer needed. It maintains the normal functions of the relaxed body, which are sometimes called the “housekeeping” functions. This part of nervous system promotes effective digestion, maintains a regular heartbeat and rate of breathing.

Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection or simply of ageing. The medical specialty of neurology studies disorders of the nervous system and looks for interventions that can prevent or treat them.

*fight or flight – борьба или бегство
Exercise 1. Complete the table a) on the parts of the nervous system b) and their functions.