The Central Nervous System

Impulses travel through the central nervous system, which proc­esses incoming sensory impulses and sends out responding im­pulses. The brain and spinal cord, which make up the central nervous system, each control specific tasks.

The Brain

The brain is the control center for the human body. Its 100 billion nerve cells not only coordinate and regulate body activi­ties but also enable humans to think. The human brain weighs only about 1.4 kg (3 lb.), but it is the most complex structure on Earth. The surface is gray matter, which consists of about 6 million cell bodies and their dendrites packed into each cubic centimeter (0.06 cu. In.). Under the gray matter is white matter, formed from myelinated axons.

The brain is composed of three major structures: the cere­brum, the cerebellum, and the brain stem. Each area seems to control separate functions. However, it is not the independence but rather the interdepend­ence of its parts that makes the brain so effective.

The Cerebrum

The cerebrum makes up about seven-eighths of the total brain weight. Its two sides, called cerebral hemi­spheres, are joined by a bridge of 200 million nerve fibers. This bridgelike structure between the cerebral hemispheres is known as the corpus callosum. Deep grooves mark off four areas on each hemisphere. The four areas are the frontal, parietal, temporal, and occipital lobes.

The gray matter of the cerebrum is called the cerebral cor­tex. Its main function is to receive sensory impulses from the body and coordinate motor responses to them. Its many ridges and valleys, called convolutions, greatly increase the surface area of the brain. Each area on the section called the motor cortex controls the movement of muscles in a specific part of the body. Each area of the sensory cortex receives impulses from specific part of the body. The area devoted to each body part is proportional to its sensitivity or motor capability, not to its size. For example, a large area is devoted to the hand, a sensitive area.

Each hemisphere controls the actions and sensations of the opposite side of the body. For example, the left side controls movement of the right hand; the right side controls movement of the left hand. Scientists have discovered that in most people each side also has exclusive control over certain functions.

Several important structures lie within the cerebrum. On each side of the brain is the thalamus, a small organ that acts as a relay center for impulses. The thalamus processes incoming sensory impulses before sending them to appropriate parts of the cortex. It also sorts out and combines impulses from the cortex and other areas of the brain. Below the thalamus is the hypothalamus. Research has indicated that this structure controls body temperature, thirst, hunger, salt and water balance, and emotional behavior in general. Near the cor­pus callosum is a network of neurons called the limbic system. The limbic system is thought to translate a person’s drives and emotions into actions.

The Cerebellum

The cerebellum is located beneath the oc­cipital lobe. The white matter that composes most of the cere­bellum is covered by a thin layer of gray matter. The cerebellum coordinates voluntary muscle movements and maintains muscle vigor and body balance. Damage to the cerebellum may result in jerky, awkward movements, although the ability to make the movements is not affected.

The Brain Stem

The brain stem contains all the nerves that connect the spinal cord with the cerebrum. The principal divi­sions of the brain stem are the medulla oblongata, pons, and midbrain. The medulla oblongata is the enlarged portion of the spinal cord that enters the lower skull. It controls breathing, swallowing, digestive processes, and action of the heart and blood vessels. In the medulla, many nerve fibers criss­cross. As a result, each hemisphere receives impulses from and sends impulses to the opposite side of the body. The pons con­nects the two hemispheres of the cerebellum and links the cere­bellum with the cerebrum. The midbrain lies above the pons. It controls responses to sight, such as movements of the eyes and size of the pupils.

A complex network of nerve fibers called the reticular for­mation runs through the brain stem and thalamus. This structure plays an essential role in consciousness, awareness, and sleep. The reticular system activates the rest of the brain when a stimu­lus is received. However, it first filters every stimulus. For example, people can sleep through loud noises such as traffic sounds but be awakened instantly by the ring of a telephone. Researchers do not know exactly how the reticular formation functions during sleep, but they know that a lack of sleep can seriously affect a person’s well-being. A person deprived of sleep becomes quick-tempered, lacks concentration and energy, and is easily distracted. Too little sleep can eventually affect sight and hearing.

Protection of the Brain

The brain is protected in three ways. First, the skull helps prevent serious injury from blows to the head. Second, the brain is cushioned inside the skull by cerebrospinal fluid. Cerebrospinal fluid is tissue fluid that cir­culates constantly around the brain and spinal cord. Third, three layers of tissue known collectively as the meninges protect the surface of the brain. The innermost layer, called the pia maier) follows all brain convolutions. Its rich blood supply carries nutrients and oxygen to brain cells and carries waste products away. The middle layer, the arachnoid, is a delicate weblike structure. Fluid between the pia mater and arachnoid serves as the pathway for exchange of nutrients and waste products. The outenjiost layer is a tough fibrous mem­brane called the dura mater.

The Spinal Cord

The spinal cord is a column of nerve tissue extending from the brain through the spinal column. In adults it is about 43 cm (17 in.) long and as thick as a pencil. The spinal cord links the brain with nerves to all parts of the body and controls involuntary movements known as reflexes.

The center of the cord is filled with gray matter with a cross section shaped somewhat like the letter H. Cell bodies of motor neurons and intemeurons are in the gray matter. The cell bodies of sensory neurons form small masses called ganglia outside the spinal cord. White matter around the gray matter consists of myelinated axons. Vertebrae, mening and cerebrospinal fluid protect the spinal cord.