Essay: The nervous system

The nervous system allows us to communicate with the outside world while controlling many internal mechanisms at the same time. It is made up of all the nerve cells in your entire body, and by taking in information through our senses, it processes the information and triggers reactions, allowing your muscles to move or causing you to feel pain. Metabolic processes are also controlled by the nervous system. Nervous systems are exceedingly complex: it has been determined that as much as 70% of an animal’s genome is expressed in a single nerve cell.
There are billions of nerve cells, known as neurons, in the nervous system — the brain alone has about 100 billion neurons. Each neuron has a cell body and various extensions; the shorter extensions (dendrites) act like antennae, receiving signals from other neurons and passing them on to the cell body through a long extension (the axon), which can be up to a meter long.
The nervous system is made of two parts: the central nervous system and the peripheral nervous system. The central nervous system (CNS) includes the nerves in the brain and the spinal cords, contained within the skull and vertebral canal of the spine. All the other nerves in the body are part of the peripheral nervous system (PNS). The neurons that carry out integration form the CNS, while neurons that carry information into and out of the CNS form the PNS.
The central nervous system is referred to as such because it combines information from the entire body and coordinates activity across the whole organism. The brain is the most complex organ in the body and uses 20% of the total oxygen we breathe in. It can be divided into four main lobes: temporal, parietal, occipital, and frontal. The brain is protected by the skull (the cranial cavity), and the spinal cord travels from the back of the brain to the center of the spine, stopping in the lumbar region of the lower back. In vertebrates, the spinal cord runs inside the vertebral column, conveying information to and from the brain. It generates basic patterns of locomotion, but it can also act independently as part of the simple nerve circuits that produce certain reflexes.
Both the brain and the spinal cord contain grey and white matter. Grey matter is made up of neuron cell bodies. White matter consists of bundled axons, making up the outer layer of the spinal cord. It links the CNS to sensory and motor neurons of the PNS. In the brain, white matter is located mostly in the interior.
The CNS also contains fluid-filled spaces, called the central canal in the spinal cord and ventricles in the brain. The cerebrospinal fluid is formed in the brain by filtering arterial blood, supplying the CNS with nutrients and hormones and carrying away waste. It circulates through the ventricles and central canal before draining into the veins.
While the CNS consists of, arguably, the most important organ in the body, it does not work alone. The PNS transmits information to and from the CNS and helps regulate an animal’s movements and its internal environment. Sensory information reaches the CNS along afferent PNS neurons. Following information processing within the CNS, instructions travel to muscles, glands, and endocrine cells along efferent PNS neurons. The CNS controls the activity of many other systems in the body as well, including the respiratory system (refer to page #) and the digestive system (refer to page #). The brain monitors respiratory volume and blood gas levels and regulates respiratory rate. The brain also controls the tone of the digestive tract and the digestive system sends sensory information to the brain, while also providing the building blocks for some neurotransmitters.
A major function of the nervous system is to control the relative constancy of the internal environment of an organism and provide the right chemical environment for living processes to take place, known as homeostasis. Any disturbances in the internal environment are monitored by sense organs; chemoreceptors within the central nervous system are sensitive to hydrogen ion concentration or to various hormones, while other chemoreceptors and mechanoreceptors regulate blood pressure. Sense organs feed information to the central nervous system, where it is processed. Appropriate outputs are sent to the effectors to counteract a disturbance.
Although the complexity of the CNS and its role in the body seems like a given in modern-day vertebrates, there was a time when animals did not have a simple nervous system. The evolutionary origin of the first nerve cells among metazoa is widely accepted, but how and from what is still debated. One assumption is that sensory and nerve cells originated from neuromuscular cells, while another postulates an independent origin of nerve cells from epithelial cells. Although we don’t know much about the origin of nerve cells, more is known about evolution of the brain. In amphibians and reptiles, the cerebrum is small and has a smooth surface, and the optic lobe carries out the integration of most sensory information input. In birds and mammals, the cerebrum developed to perform the same functions of the optic lobe due to the increased needs for information in processing, especially with the development of senses. In humans, the neocortex makes up a large area of the brain, pushing aside the olfactory brain and hippocampus, which have both been present since ancient times.
Because of the important role the CNS plays in the body, the diseases that impact the central nervous system are often degenerative. Although some diseases, such as encephalitis and polio, are caused by infections of the CNS, some are organic disorders, like autism and ADHD. While autism and ADHD are both early-onset neurological disorders, there are also late-onset neurodegenerative diseases, such as Alzheimer’s and Parkinson’s. All of these are linked to structural and biochemical imbalances in the brain, specifically low levels of dopamine. However, some diseases are linked to structural damage in the central nervous system. Multiple sclerosis (MS) is caused by the destruction of myelin sheaths of neurons, inflammation, and the formation of lesions, which together produce the breakdown of nerve tissue.
Although the central nervous system has been studied by anatomists and physiologists, we are still not sure of its full capabilities. It controls our thoughts, movements, emotions, and desires, while also controlling breathing, heart rate, the release of some hormones, and body temperature. There is still much more to learn, and with the improving technology, we will be able to discover what else the central nervous system controls and possibly find cures to the diseases that impact it.