Structure and Function of the Central Nervous System

The central nervous system (CNS) is comprised of the brain and spinal cord. The three broad functions of the CNS are to take in sensory information, process information, and send out motor signals.

The CNS receives sensory information from the nervous system and controls the body's responses. The central nervous system plays a primary role in receiving information from various areas of the body and then coordinating this activity to produce the body's responses.

This article discusses the structures that make up the central nervous system and how they function. It also explores some of the diseases and conditions that can affect the CNS.

Central Nervous System Structure

The CNS has three main components: the brain, the spinal cord, and the neurons (or nerve cells). Each part of the CNS plays an important role in how the body functions, and the three components of the CNS work together to take in information and control how the body responds.

The Brain

The brain controls many of the body's functions including sensation, thought, movement, awareness, and memory. The surface of the brain is known as the cerebral cortex. The surface of the cortex appears bumpy thanks to the grooves and folds of the tissue. Each groove is known as a sulcus, while each bump is known as a gyrus.

The largest part of the brain is the cerebrum. It is responsible for functions such as memory, speech, voluntary behaviors, and thought.

The cerebrum is divided into two hemispheres, the right hemisphere and the left hemisphere. The right hemisphere controls movements on the body's left side, while the left hemisphere controls movements on the body's right side.

While some functions do tend to be lateralized, researchers have found that there are not "left brained" or "right brained" thinkers, as the old myth implies. Both sides of the brain work together to produce various functions.

Each hemisphere of the brain is then divided into four interconnected lobes:

• Frontal lobes are associated with higher cognition, voluntary movements, and language.
• Occipital lobes are associated with visual processes.
• Parietal lobes are associated with processing sensory information.
• Temporal lobes are associated with hearing and interpreting sounds as well as the formation of memories.

Other important areas of the brain include the basal ganglia, cerebellum, Broca's area, corpus callosum, medulla oblongata, hypothalamus, thalamus, and amygdala.

Spinal Cord

The spinal cord connects to the brain via the brain stem and then runs down through the spinal canal, located inside the vertebrae. The spinal cord carries information from various parts of the body to and from the brain.

While it varies from one individual to the next, the spinal cord is about 18 inches in length. At the brainstem, 31 spinal nerves enter into the spinal cord. The nerves of the spinal cord consist of:

• 8 cervical nerves
• 12 thoracic nerves
• 5 lumbar nerves
• 5 sacral nerves
• 1 coccygeal nerve
  

In the case of some reflex movements, responses are controlled by spinal pathways without involvement from the brain. Examples include the Golgi tendon reflex, the crossed extensor reflex, and the stretch reflex.

Neurons

Neurons are the building blocks of the central nervous system. Billions of these nerve cells can be found throughout the body and communicate with one another to produce physical responses and actions.

Most neurons are divided into three basic sections: dendrites, cell body, and axon. These cells also differ in terms of function. The three types of neurons are afferent neurons, efferent neurons, and interneurons. 

Efferent neurons are motor neurons that carry signals from the brain to the peripheral nervous system. Afferent neurons are sensory neurons that bring information from the senses to the brain. Interneurons are association neurons that connect efferent and afferent neurons to the central nervous system.

Protective Structures

Since the CNS is so important, it is protected by a number of structures. First, the entire CNS is enclosed in bone. The brain is protected by the skull. The spinal cord is encased by the vertebrae that make up the spinal column. 

The brain and spinal cord are both covered with a protective tissue known as meninges. There are three layers of meninges protecting the brain and spinal cord:

• Dura mater: From the Latin words meaning "hard mother," this is the top layer of the meninges found directly under the bones of the skull and vertebrae. It is composed of dense connective tissue.
• Arachnoid mater: The second layer of the meninges is a spider-like, transparent membrane made up of collagen and elastic fibers.
• Pia mater: From the Latin for "soft mother," this protective layer is the innermost layer of the meninges. It is made of delicate connective tissue that is filled with tiny blood vessels that provide nourishment for the brain.

The entire CNS is also immersed in a substance known as cerebrospinal fluid, which forms a chemical environment that allows nerve fibers to transmit information effectively as well as offering yet another layer of protection from potential damage.

Diseases of the Central Nervous System

There are a number of problems and diseases that can affect the CNS. Damage or disease to the central nervous system can produce a range of effects.  Some of the conditions that can impact the CNS include:

• Degenerative diseases: Diseases such as Parkinson's disease and Alzheimer's disease can cause the degeneration of cells in pivotal areas of the brain, affecting functions such as movement and memory.
• Infections: Viruses, bacteria, and fungi can invade the central nervous system, causing symptoms and/or damage.
  
• Stroke: A blockage of blood flow to the brain prevents oxygen from reaching the tissues of the brain. This results in damage to the affected area and can lead to impairment or death.
• Trauma: Injury to the CNS can cause a number of problems ranging from paralysis to death.
• Tumors: Cancerous and benign tumors can grow in different areas of the CNS. The impact of these tumors depends on their location and size.