The next biological system we will examine is the nervous system. The sophistication of the nervous system is one of the adaptations that has contributed to the success of insects. The insect nervous system is similar to that of other animals, in that it is characterized by the presence of special cells called neurons. Neurons are cells that receive and send out electrochemical impulses. These impulses travel along neurons due to an electrochemical gradient across the cell membrane. This transmission is called an action potential. There are three major types of neurons. Sensory neurons receive stimuli from the environment and send this information to the central nervous system. Motor neurons receive stimuli from the central nervous system and transmit signals that stimulate muscles. Finally, there are interneurons, bridging cells which connect two or more neurons of any type. The neuron itself has three main parts; the dendrite, the cell body, and the axon. The dendrite receives stimuli from the environment or other cells. The stimulus is then transmitted as an action potential from the dendrite through the cell body which contains the cell's nucleus. From here, the signal travels along the axon to another neuron or the target tissue. The space between neurons where a dendrite and an axon meet is called a synapse. At the synapse, neural signals are passed from one neuron to the next via chemical messengers known as neurotransmitters. These chemicals can either stimulate or inhibit other neurons and muscles. An important excitatory neurotransmitter in insects is acetylcholine, an organic chemical released by neurons to stimulate other neurons in tissues. The insect nervous system has two main components. The peripheral nervous system perceives information from the environment and sends action potentials to the central nervous system where information is processed and behaviors are initiated. The central nervous system contains the brain and the ventral nerve cord. The peripheral nervous system register stimuli from the environment. Most insects can detect visual or factory, auditory, and mechanical cues. Some can even sense heat cues from a distance. For example, Fire Chaser Beetles in the Genus Melanophila, lay eggs in freshly burned trees that they sense with sensory organs on their legs. These receptors can detect infrared radiation produced by hot objects from up to 130 kilometers away. Neurons in the peripheral nervous system that detect external environmental stimuli are enclosed in sensory structures called sensory receptors, that are located on various parts of the insect's body. They can be present in large concentrations on sensory structures like the antennae, the compound eyes, or the mouth parts. They are also located on an insect's tarsi or feet, and even on a female's ovipositor so that she can sense the environment before laying eggs. Sensory receptors come in many forms. A common one is the trichoid sensillum, which has a single hair-like seta that is associated with a neuron. Neurons within most trichoid sensilla register physical movement. A process known as mechanoreception. During mechanoreception, movement of the seta stimulates the neuron within the sensillum and generates an action potential. This is transmitted to the central nervous system which allows the insect to register physical cues in its vicinity. Some trichoid sensilla sends chemical stimuli, a process known as chemoreception. Chemo receptive trichoid sensilla have one or more microscopic pores along the length of the seta which allow liquid or gaseous chemicals to enter. The compounds dissolve in a fluid within the seta and bind to the sensory neuron which in turn sends a signal to the insect's central nervous system. There are two types of trichoid sensilla that receive chemical cues. Contact chemo receptors which detect substances dissolved in fluids and olfactory chemo receptors which detect volatile substances in the air. Let's move on to the central nervous system, which you'll remember, receive signals from the peripheral nervous system and is made up of the brain and the ventral nerve cord. In insects, the nerve cord is located along the ventral side of the body, unlike vertebrates in which the nerve cord is dorsally located. Within the central nervous system, the nerve cells are bundled into interconnected masses of neurons known as ganglia. These neural bundles located along the length of the nerve cord are joined by connectives, giving the insect central nervous system a distinctive ladder-like appearance. Ancestrally, each insect body segment had a pair of ganglia that received stimuli from receptors and transmitted signals to muscles and other organs within each segment. This is why if a primitive insect like a cockroach is beheaded, it can continue to function as long as water balance is maintained. In most insects today, fusion of ganglia has resulted in a reduction in the overall number of ganglia. This mirrors the reduction of the number of segments that we also see in derived insects. The insect's brain is formed by the fusion of three pairs of ganglia located in the head. Each pair of ganglia forms a distinct part of the brain with different functions. The brain, as it does in vertebrates, integrates and transmits information that controls behavior. In adult insects, the largest and most anterior part of the brain receives stimuli from the compound eyes inner cell eye. It also receives and processes messages from other parts of the brain. It contains dense neural structures known as mushroom bodies that sorts and coordinate information, enabling some insects to learn particularly from olfactory cues. The mushroom bodies are most highly developed in social insects like the honeybee. The second part of the insect's brain receives signals mainly from receptors on the antennae. Olfactory signals are processed here and inter neurons transmit this information to higher brain centers to influence insect behavior. Finally, the third part of the brain receives information from the rest of the body, primarily, the foregut and the labrum. Posterior to the brain, lies the subesophageal ganglion, which communicates with muscles that control movements of the mouth parts. It is the bridge between the insect's brain and the rest of the nervous system. Neurosecretory cells are found throughout the insect nervous system but occur in especially large concentrations in the brain. These neurosecretory cells are responsible for the production and release of insect hormones and neurohormones. Neurohormones may promote further hormone production in other non neural tissues like the endocrine glands. Examples of hormones that are regulated by neurohormones include ecdysteroids and juvenile hormone, which influence physiological processes in other tissues. Remember, ecdysteroids are important in the molting process of insect growth and development. Neurohormones produced and released by the corpora cardiaca and sometimes for the corpora alatta in lapendectomies, stimulate the production and release of ecdysteroids by pro-thoracic glands. Another important hormone released by endocrine glands is juvenile hormone, an important regulator of metamorphosis in reproduction. Juvenile hormone is produced by the corpora alatta. The insect's nervous system is often the target of chemical insecticides. Some chemical insecticides called axonic poisons, inhibit the neurons ability to transmit an action potential along the axon. Insects effected by these poisons exhibit tremors and have a general loss of control of their motor functions. A very well-known and controversial axonic poison widely used in North America after the Second World War is DDT. DDT or Dichlorodiphenyltrichloroethane, don't worry we won't be testing you on this, is a colorless and odorless chemical that was developed for its insecticidal properties during World War II. Widespread use of DDT against agricultural pests in the years following the war resulted in environmental poisoning and human toxicity issues. DDT was considered the major cause of sharp declines in Bald Eagle and Peregrine Falcon populations in the United States in the 1940s and '50s. Fortunately, the use of DDT has since been banned in most developed countries. It is still used against insect vectors to reduce malaria in some parts of the world. We will discuss how DDT affects insects in detail when we discuss pest control in a future module. Insecticides that act at the synapse between nerve cells are called synaptic poisons. These poisons result in rapid nerve firing in the affected insects which often results in restlessness, tremors, paralysis and finally death. The neurons of insects and humans function in the same way. As such, most insecticides that target the insect nervous system can also affect people and other non-target animals. We hope you enjoyed this lesson on the remarkably complex insect nervous system. The next biological system we'll look at maybe even more remarkable and sometimes downright bizarre, the reproductive system.
