The two major divisions of the nervous system are the central and the

Central nervous system

The brain and the spinal cord make up the central nervous system. They are wrapped in a thin lining called meninges and bathed with cerebrospinal fluid (CSF).

The brain

The brain is the powerhouse of the body, even though it only makes up two per cent of the body’s weight. This soft, jelly-like organ has countless billions of neural cross-connections. The brain oversees the workings of the body, while its higher functions give us consciousness and personality.

The spinal cord

The spinal cord is connected to the brain and runs the length of the body. It is protected by the bones of the spine (vertebrae). Nerves branch off from the spinal cord into the arms, legs and torso.

The peripheral nervous system

Nerves connect the brain and spinal cord to the peripheral nervous system, which is what nerve tissue outside of the central nervous system is called. It is made up of two main parts: the autonomic and the somatic nervous systems.

The autonomic nervous system

The autonomic nervous system is part of the peripheral nervous system. One of its main roles is to regulate glands and organs without any effort from our conscious minds.

The autonomic nervous system is made up of two parts: the sympathetic and the parasympathetic. These systems act on the body in opposite ways. Together, they coordinate a multitude of adjustments required for our changing personal needs as we move through our environment. For example, the size of our pupils is adjusted automatically to allow the correct amount of light into our eyes for optimum vision, our sweat glands are turned on when we get too hot and our salivary glands produce saliva when we eat food (or even think about it!).

The somatic nervous system

The somatic nervous system is also a part of the peripheral nervous system. One of its roles is to relay information from the eyes, ears, skin and muscle to the central nervous system (brain and spinal cord). It also obeys commands from the central nervous system and makes muscles contract or relax, allowing us to move.

Problems of the nervous system

Some common problems of the nervous system include:

• Epilepsy – storms of abnormal electrical activity in the brain causing seizures
• Meningitis – inflammation of the membrane covering the brain
• Multiple sclerosis – the myelin sheaths protecting the electrical cables of the central nervous system are attacked
• Parkinson’s disease – death of neurones in a part of the brain called the midbrain. Symptoms include shaking and problems with movement
• Sciatica – pressure on a nerve caused by a slipped disc in the spine or arthritis of the spine and, sometimes, other factors
• Shingles – infection of sensory nerves caused by the varicella-zoster virus
• Stroke – a lack of blood to part of the brain.

Where to get help

Things to remember

• The nervous system uses electrical and chemical means to help all parts of the body to communicate with each other.
• The brain and spinal cord make up the central nervous system.
• Nerves everywhere else in the body are part of the peripheral nervous system.

• The nervous system, Online Biology Book, Estrella Mountain Community College, US. More information here .
• Neurological disease, Medline Plus, National Institutes of Health, National Library of Medicine, USA. More information here .

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This page has been produced in consultation with and approved by:

This page has been produced in consultation with and approved by:

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By the end of this section, you will be able to:

• Describe the difference between the central and peripheral nervous systems
• Explain the difference between the somatic and autonomic nervous systems
• Differentiate between the sympathetic and parasympathetic divisions of the autonomic nervous system

The nervous system can be divided into two major subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS), shown in Figure 1. The CNS is comprised of the brain and spinal cord; the PNS connects the CNS to the rest of the body. In this section, we focus on the peripheral nervous system; later, we look at the brain and spinal cord.

The peripheral nervous system is made up of thick bundles of axons, called nerves, carrying messages back and forth between the CNS and the muscles, organs, and senses in the periphery of the body (i.e., everything outside the CNS). The PNS has two major subdivisions: the somatic nervous system and the autonomic nervous system.

The somatic nervous system is associated with activities traditionally thought of as conscious or voluntary. It is involved in the relay of sensory and motor information to and from the CNS; therefore, it consists of motor neurons and sensory neurons. Motor neurons, carrying instructions from the CNS to the muscles, are efferent fibers (efferent means “moving away from”). Sensory neurons, carrying sensory information to the CNS, are afferent fibers (afferent means “moving toward”). Each nerve is basically a two-way superhighway, containing thousands of axons, both efferent and afferent.

The autonomic nervous system controls our internal organs and glands and is generally considered to be outside the realm of voluntary control. It can be further subdivided into the sympathetic and parasympathetic divisions (Figure 2). The sympathetic nervous system is involved in preparing the body for stress-related activities; the parasympathetic nervous system is associated with returning the body to routine, day-to-day operations. The two systems have complementary functions, operating in tandem to maintain the body’s homeostasis. Homeostasis is a state of equilibrium, in which biological conditions (such as body temperature) are maintained at optimal levels.

Figure 2. The sympathetic and parasympathetic divisions of the autonomic nervous system have the opposite effects on various systems.

The sympathetic nervous system is activated when we are faced with stressful or high-arousal situations. The activity of this system was adaptive for our ancestors, increasing their chances of survival. Imagine, for example, that one of our early ancestors, out hunting small game, suddenly disturbs a large bear with her cubs. At that moment, his body undergoes a series of changes—a direct function of sympathetic activation—preparing him to face the threat. His pupils dilate, his heart rate and blood pressure increase, his bladder relaxes, his liver releases glucose, and adrenaline surges into his bloodstream. This constellation of physiological changes, known as the fight or flight response, allows the body access to energy reserves and heightened sensory capacity so that it might fight off a threat or run away to safety.

While it is clear that such a response would be critical for survival for our ancestors, who lived in a world full of real physical threats, many of the high-arousal situations we face in the modern world are more psychological in nature. For example, think about how you feel when you have to stand up and give a presentation in front of a roomful of people, or right before taking a big test. You are in no real physical danger in those situations, and yet you have evolved to respond to any perceived threat with the fight or flight response. This kind of response is not nearly as adaptive in the modern world; in fact, we suffer negative health consequences when faced constantly with psychological threats that we can neither fight nor flee. Recent research suggests that an increase in susceptibility to heart disease (Chandola, Brunner, & Marmot, 2006) and impaired function of the immune system (Glaser & Kiecolt-Glaser, 2005) are among the many negative consequences of persistent and repeated exposure to stressful situations.

Once the threat has been resolved, the parasympathetic nervous system takes over and returns bodily functions to a relaxed state. Our hunter’s heart rate and blood pressure return to normal, his pupils constrict, he regains control of his bladder, and the liver begins to store glucose in the form of glycogen for future use. These processes are associated with activation of the parasympathetic nervous system.

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