The Nervous System is the system of cells, tissues, and organs that regulates the body’s responses to internal and external stimuli. In vertebrates it consists of the brain, spinal cord, nerves, ganglia, and parts of the receptor and effector organs.
Your nervous system is composed of the central nervous system, the cranial nerves, and the peripheral nerves. The brain and spinal cord together form the central nervous system. The cranial nerves connect the brain to the head. The four groups of nerves that branch from the cervical, thoracic, lumbar, and sacral regions of the spinal cord are called the peripheral nerves.
The peripheral nervous system consists of sensory neurons running from stimulus receptors that inform the Central Nervous System of the stimuli. It also consists of motor neurons running from the Central Nervous System to the muscles and glands, called effectors, which take action. The peripheral nervous system is further subdivided into an sensory division and an motor division. The sensory division transmits impulses from peripheral organs to the Central Nervous System. The motor division transmits impulses from the Central Nervous System out to the peripheral organs to cause an effect or action.
The Somatic nervous system of the motor division supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, it is sometimes called the voluntary nervous system. The autonomic nervous system supplies motor impulses to cardiac muscle, to smooth muscle, and to glandular epithelium. Because the autonomic nervous system regulates involuntary or automatic functions, it is called the involuntary nervous system.
The brain is the source of all our behavior, thoughts, feelings, and experiences. In humans, the brain weighs about 3 pounds. Differences in weight and size don’t have anything to do with differences in mental ability. The brain is a pinkish mass that is composed of about 10 billion nerve cells. The nerve cells are linked to each other and together are responsible for the control of all mental functions.
The brain is divided into three major parts, the hindbrain, the midbrain, and the forebrain. The cerebrum occupies the topmost portion of the skull. It is by far the largest part of the brain. It makes up about 85% of the brain’s weight. The cerebrum is split vertically into left and right hemispheres. Its upper surface, the cerebral cortex, contains most of the master controls of the body. In the cerebral cortex ultimate analysis of sensory data occurs, and motor impulses originate that initiate, reinforce, or inhibit all of the muscle and gland activity. The left half of the cerebrum controls the right side of the body; the right half controls the left side.
The Midbrain joins the diencephalons and the medulla oblongata. It has centers that control movements of the eyes and of other part of the body. The hindbrain lies toward the back and base of the skull. It includes the medulla oblongata and cerebellum. The cerebellum consists of two hemispheres. Although it represents only 10% of the weight of the brain, it contains as many neurons as all the rest of the brain combined. Its most clearly understood function is to coordinate body movements. People with damage to their cerebellum are able to perceive the world as before and to contract their muscles, but their motions are jerky and uncoordinated. So the cerebellum appears to be a center for learning motor skills.
Three protective layers called the meninges surround the delicate human brain. The dura mater is the most superior of the meningeal layers. This tissue forms several structures that separate the cranial cavity into compartments and protect the brain from displacement. The arachnoid mater is the middle layer of the meninges. The pia mater is the innermost layer of the meninges. Unlike the other layers, this tissue is close to the brain.
The spinal cord carries out two main functions: It connects a large part of the peripheral nervous system to the brain. Information reaching the spinal cord through sensory neurons is transmitted up into the brain. Signals arising in the motor areas of the brain travel back down the cord and leave in the motor neurons.