The nervous system is the master of all systems; it controls the functions of all other systems in the body. It is divided into two main parts: the central nervous system, or CNS; and the peripheral nervous system, or PNS. The CNS consists of the brain and spinal cord, while the PNS consists of all of the body's nerves.
The Brain
The brain is the central command station of the body, receiving information about the environment and determining the appropriate response. There are several structures in the brain that are specialized to interpret certain types of information. For example, in the very back of the brain lies the occipital lobe of the cerebrum. This area of the brain is specialized to interpret visual information coming in from the optic nerves of the eyes. There are areas associated with the other senses as well: hearing, taste, touch, smell and so on.
The Spinal Cord
The central nervous system branches off from the brain to form the spinal cord. The spinal cord is the means by which sensory signals are sent up to the brain and motor information is sent down from the brain. These types of signals are called afferent and efferent, respectively. The spinal cord is also responsible for initiating some reflexive movement, such as the withdrawal of the hand from a hot stove. Finally, the spinal cord houses a special type of cell called a central pattern generator. These specialized neurons play an important part in programming the muscles to perform repetitive, alternating motions such as walking.
The Nerves
Nerves are wire-like bundles of cells, called neurons. Nerves are part of the peripheral nervous system and serve to relay information to and from the spinal cord. There are sensory nerves, motor nerves and mixed nerves. Sensory nerves only communicate sensory information to the spinal cord and motor nerves only communicate motor information from the spinal cord. Mixed nerves are capable of sending both types of signals. There are 31 pairs of spinal nerves and 12 pairs of cranial nerves.
PNS: Sensory Division
The body contains many specialized structures called sensory receptors that are responsible for surveying the environment and the body's physiological state. Receptors convey information about touch, taste, smell, pain, body position, vision, sound and more. Sensory signals can originate from the bones, muscles and organs of the body.
PNS: Motor Division
The motor division of the PNS can be subdivided into the somatic motor division and the visceral motor division, also called the autonomic nervous system. The skeletal muscles are stimulated to contract by information exiting the central nervous system by way of the somatic motor division. This includes movement initiated both voluntarily and reflexively.
The visceral motor division functions to stimulate the body's glands, smooth muscle and heart. This results in visceral reflexes that can be classified as sympathetic and parasympathetic. Sympathetic reflexes arouse the body while parasympathetic reflexes relax it. This is commonly known as the "fight or flight" response.
References
- "Anatomy and Physiology: The Unity of Form and Function (5th Edition)"; Ken Saladin; 2010
- "Neuroscience: Exploring the Brain (3rd Edition)"; Mark Bear, Barry Connors and Michael Paradiso; 2007
