Overview of the Nervous System

Introduction

The nervous system is the most sophisticated and complex body system because it regulates and coordinates the rest of your body systems. It controls every function of your body. Some of these functions are voluntary, such as running, jumping, and climbing. Other functions are involuntary, such as breathing, digesting, and pumping blood. But no matter what the function is, all of your body systems depend on the nervous system. The nervous system is mainly composed of the brain, spinal cord, and nerves. The brain and the spinal cord are referred to as the central nervous system (CNS) and nerves make up the peripheral nervous system (PNS). 

 

Central Nervous System

The central nervous system earned its name from its central location in your body, a good location for a system that functions as the communication headquarters. Whether you're awake or not, the central nervous system continuously receives information, makes decisions, and sends information to nerves located throughout your body.

 

The central nervous system contains the two most important structures in the body—the brain and the spinal cord.

These delicate structures are vital to our survival, and therefore have two coverings—an outer covering of bone and an inner covering of membrane.

Two types of bone form the outer covering: 

1. The cranial bone, which encases the brain

2. The vertebrae, which encase the spinal cord

Three distinct layers of tissues, also known as meninges, form the inner covering:

1. Dura mater—a tough outside layer

2. Arachnoid mater—a web of delicate fibers connecting dura mater to pia mater

3. Pia mater—a soft and tender inside layer

 

In addition to protecting the brain and the spinal cord from injury, the meninges allow nourishment to pass through a complex channel system. The dura mater is a thick, tough membrane that contains channels for blood to pass into the brain tissue.

The subdural space below the dura mater contains many blood vessels that carry nourishment to the tissue. Between the arachnoid mater and the pia mater is another space called the subarachnoid space, and this space contains the cerebrospinal fluid. The pia mater layer is made of connective tissues and a rich supply of blood vessels.

 

Cerebrospinal fluid (CSF) is a built-in shock absorber for the central nervous system. This colorless fluid circulates around and within the brain through the ventricles (cavities in the brain), the central canal of the spinal cord, and the subarachnoid space.

The ventricles and the meninges form a dynamic system that regulates the amount of CSF covering the brain. The ventricles produce the fluid which is reabsorbed by the meninges to prevent buildup of excess fluid. Therefore, the meninges and the cerebrospinal fluid not only form a physical layer of protection for the brain but also serve a circulatory role by providing nutrients and removing waste products.

The CSF is produced by a collection of blood vessels called the choroid plexus, located within each ventricle. These blood vessels also perform a special function within the brain. They help form what's known as a blood cerebrospinal fluid brain barrier (BCSFB). This barrier helps protect the brain by controlling which substances enter and leave the brain.

 

Your spinal column is the characteristic that qualifies you as a vertebrate. Inside the spinal column lies the spinal cord, a slightly hollow cylinder of neural tissue, which is attached to the brain stem. Gray matter composes its inner section. The white matter, which composes its outer region, contains sensory and motor neurons to transmit nerve impulses to and from the brain. The spinal cord begins at the medulla and continues down the vertebral column to the level of the second lumbar vertebra. At the lower end of the spinal column is the cauda equina, which is a collection of nerve roots. 

Like the brain, the spinal cord is encased by the three meninges as well as cerebrospinal fluid. The spinal cord sends out 31 pairs of spinal nerves to connect to the peripheral nervous system. 

The main function of the spinal cord is to send sensory input to the brain and receive motor output from the brain. Thus, damage to the spinal cord may affect the function of skeletal muscles even if the brain isn't affected. The spinal cord processes some signals without the involvement of the brain. These signals are responsible for primitive reflexes such as the blinking or sneeze reflex.

 

Peripheral Nervous System

The main function of the autonomic nervous system is to regulate the function of the cardiac muscles and the smooth muscles, including those in the digestive, respiratory, urinary, reproductive, and endocrine systems. The autonomic nervous system is further divided into the sympathetic and the parasympathetic nervous systems. 

Sympathetic Nervous System

This division is involved in "fight or flight" reactions which occur in a split second before the brain can consciously think about it. It kicks into high gear during stressful situations, preparing your body for physical and/or emotional conflict. Survival instincts depend on this "fight or flight" response and this response deals with the concept to fight or flee from a situation. Everyday life provides opportunity for the sympathetic nervous system to operate. If you're walking across the street and see a car coming toward you, you suddenly jump out of the way. Then you notice your heart pounding in your chest. This reaction is the result of the sympathetic nervous system. An increased heart rate is necessary for increased skeletal muscle activity.

Parasympathetic Nervous System

The parasympathetic division of the autonomic nervous system counterbalances the action of the sympathetic nervous system. The parasympathetic nervous system provides a calming counterbalance to the sympathetic nervous system by exerting opposite effects on internal organs (for example, slowing down heart rate). The parasympathetic nervous system is important for stability so that your body can proceed with normal activity. Thus, the parasympathetic nervous system activates the muscles of the digestive tract to allow digestion, a process that's unimportant during stressful situations but vital to prolonged health.

Sympathetic and Parasympathetic 

When a person is in perfect health, the sympathetic and parasympathetic nervous systems are balanced. Overactivity of either system can cause problems. High levels of stress, which stimulate the sympathetic system, may contribute to the development of heart disease and heart attacks. On the other hand, an unopposed action of the parasympathetic system may cause the heart rate to slow down and the blood vessels to dilate too much, causing a drop in blood pressure.

 

Neurons

The neuron, or nerve cell, is the basic unit of the nervous system. Its sole purpose is to conduct signals from one end of the nervous system to the other. The human body operates smoothly because hundreds of nerves communicate with the central nervous system. Neurons are divided into three major structures: dendrites, cell body, and axon. 

 

Dendrites pick up signals and send them to the body of the nerve cell. The neuron cell body nourishes the neuron. In peripheral nerves, the cell body may even grow new fibers when the old ones are cut or torn. Groups of cell bodies are called ganglions. The single axon sends the impulse from the cell body to a muscle, gland, or another nerve cell. Bundles of axons make up nerves.

Think Of This Analogy

Your fingers are the dendrites, your hand is the cell body, and your arm is the axon. All five fingers pick up sensory information, which is then relayed to your hand and up your arm. Peripheral nerves are discrete structures large enough to be seen with the naked eye. Each nerve contains a large number of fibers, which are the axons or dendrites of the neurons.

 

Sensory nerves are afferent nerves. Afferent means to carry something toward a center. These nerves receive information from the sense organs—the ears, eyes, nose, tongue, and skin—and send that information to the brain. Motor nerves are efferent nerves, meaning to carry something away from the center. These nerves receive orders from the central nervous system and carry the orders out. Mixed nerves contain both sensory and motor nerves. Most of the peripheral nerves that connect with the rest of the body are mixed nerves because they perform both sensory and motor functions. For example, the femoral nerve allows you to both feel sensations on your thigh and move it.

Myelin is a fatty white substance that covers many of the axons in the central and peripheral nervous systems. You can think of myelin as an insulator because it keeps other nearby nerves from being affected by impulses. The myelin sheath also accelerates signal transmission along the axon. Certain diseases affecting the myelin sheath can hinder or slow down the conduction of signals in the nervous system. 

The synapse is the tiny space between neurons. Chemical substances called neurotransmitters help the impulse cross the synapses and travel from axon to dendrite.

Neuroglia, the only other type of nerve cell beside neurons, literally means "nerve glue." Neuroglia cells serve to support, protect, and nourish the neurons and anchor them to the blood vessels.

 

Key Points

• The central nervous system is comprised of the two most important structures in the body: the brain and the spinal cord.
• The peripheral nervous system is comprised of the spinal and cranial nerves.
• The cerebrum is the major part of the brain that's divided into two hemispheres (right and left).
• The right hemisphere controls the body actions of the left side of the body; the left hemisphere controls the body actions of the right side of the body.
• The brain stem is a vital part of the brain because it connects the brain to the spinal cord and all nerve messages pass through the brain stem to the body.
• The main function of the spinal cord is to send sensory input to the brain and receive motor output from the brain.