Human Central Nervous System
The human nervous system may be conveniently subdivided into two divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves extending to and from the central nervous system).
Spinal cord. The spinal cord of the central nervous system is a white cord of tissue passing through the bony tunnel made by the vertebrae. The spinal cord extends from the base of the brain to the bottom of the backbone. Three membranes called meninges surround the spinal cord and protect it. The outer tissue of the spinal cord is white (white matter), while the inner tissue is gray (gray matter). Thirty-one pairs of projections called nerve roots extend out along each side of the spinal cord. The nerve roots are sites of axons belonging to sensory and motor neurons. A central canal in the spinal cord carries a fluid, the cerebrospinal fluid, that provides for the nutrition and gaseous needs of the cord tissue. The neurons of the spinal cord serve as a coordinating center for the reflex arc and a connecting system between the peripheral nervous system and the brain.
Brain. The brain of the central nervous system is the organizing and processing center. It is the site of consciousness, sensation, memory, and intelligence.
The brain receives impulses from the spinal cord and twelve pairs of cranial nerves coming from and extending to the senses and to other organs. In addition, the brain initiates activities without environmental stimuli.
Two major hemispheres, the left and the right hemispheres, make up the tissue of the brain. The outer portion of the brain consists of gray matter, while the inner portion is white matter. Three major portions of the brain are recognized: the hindbrain, the midbrain, and the forebrain.
The hindbrain consists of the medulla, pons, and cerebellum. The medulla is the swelling at the tip of the brain that serves as the passageway for nerves extending to and from the brain. The cerebellum lies adjacent to the medulla and serves
as a coordinating center for motor activity, that is, it coordinates muscle contractions. The pons is the swelling between the medulla and midbrain. The pons acts as a bridge between various portions of the brain.
The midbrain lies between the hindbrain and forebrain. It consists of a collection of crossing nerve tracts and is the site of the reticular formation, a group of fibers that arouse the forebrain when something unusual happens.
The forebrain consists of the cerebrum, the thalamus, the hypothalamus, and the limbic system.
The cerebrum contains creases and furrows called convolutions that permit the cerebral hemisphere to accommodate more than 10 billion cells. Each hemisphere of the cerebrum has four lobes, and activities such as speech, vision, movement, hearing, and smell occur in these lobes. Higher mental activities such as learning, memory, logic, creativity, and emotion also occur in the cerebrum.
The thalamus serves as an integration point for sensory impulses, while the hypothalamus synthesizes hormones for storage in the pituitary gland. The hypothalamus also appears to be a control center for such visceral functions as hunger, thirst, body temperature, and blood pressure. The limbic system is a collection of structures that ring the edge of the brain and apparently function as centers of emotion.
Human Peripheral Nervous System
The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other parts of the body and the external environment. It is subdivided into the sensory somatic system and the autonomic nervous system.
The sensory somatic system carries impulses from the external environment and the senses. It consists of twelve pairs of cranial nerves and thirty-one pairs of spinal nerves. The sensory somatic system permits humans to be aware of the outside
environment and react to it voluntarily.
The autonomic nervous system works on an involuntary basis. It consists of two groups of motor neurons and a set of knotlike groups of cell bodies called ganglia. Motor neurons extend to and from the ganglia to the body organs. One subdivision of the autonomic nervous system is the sympathetic nervous system. Impulses propagated in this system prepare the body for an emergency. They cause the heartbeat to increase, the arteries to constrict, the pupils to dilate, and other changes to take place. The other subdivision is the parasympathetic nervous system. Impulses in this system return the body to normal after an emergency has occurred.
The senses are organs that connect the nervous system to the external environment. They are the sources of stimuli that cause a response in the nervous system, and they are the sources of all information to the human body.
Eye. In the human eye, the nerve cells are located in a single layer called the retina. The retina is located along the back wall of the eye. Light rays enter the eye through a curved, transparent structure, the cornea. Then the light rays pass through the pupil, an opening in the eyeball. The iris regulates the size of the pupil. Next, the lens focuses the light on the retina, which contains two types of light-sensitive cells, rod cells and cone cells, which detect light.
Cone cells, which detect color, are concentrated in the central portion of the retina, while rod cells, which permit vision in dim light, are concentrated at the edge of the retina. A light-sensitive pigment called rhodopsin functions in the detection of light.
From the eye, a series of impulses is generated for transmission to the brain. The optic nerve carries these impulses. The region of keenest vision, the fovea, is located at the center of the retina. When vision is poor, light is not focusing on the
fovea, and corrective lenses are prescribed.
Ear. The ear is the organ of hearing in humans. The outer ear funnels vibrations to the eardrum, or tympanic membrane. The membrane transmits the vibrations to three inner earbones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones transmit the vibrations to the inner ear where the organ of hearing, the cochlea, is located.
The cochlea is a snail-like series of coiled tubes within the skull. As the earbones vibrate, they push and pull a membrane at one end of the cochlea, causing fluid within the tubules to vibrate. The vibrations are detected by sensitive hair cells, and
nerve impulses are generated. The auditory nerve carries the impulses to the brain for interpretation.
Taste and smell. Specialized receptor cells called chemoreceptors transmit taste and smell. Chemoreceptors of the human tongue distinguish four different tastes: sweet, sour, salty, and bitter. In the human nose, chemoreceptors detect a variety of scents, including minty, floral, musky, putrid, and pungent. In both taste and smell, chemoreceptors are stimulated by molecules and ions that reach the tongue and nose. Liquid materials affect the chemoreceptors in the taste buds of the tongue, while gaseous molecules affect the chemoreceptors in the upper reaches of the nose. The olfactory nerve carries nerve impulses from the nose to the brain for interpretation.
Other senses. The other senses of the body include receptors for touch, pain, temperature, and balance. Touch and pain receptors, called Pacinian corpuscles, are located in the skin, muscles, and tendons. The sense of balance is centered in the semicircular canals of the inner ear. Visceral senses include stretch receptors in the muscles as well as carbon dioxide receptors in the arteries.