The nervous system of the human anatomy is responsible for sending, receiving, and processing nerve impulses. All of the body's muscles and organs rely upon these nerve impulses to function. Three systems work together to carry out the mission of the nervous system: the central, the peripheral, and the automonic nervous systems.
The central nervous system is responsible for issuing nerve impulses and analyzing sensory data and includes the brain and the spinal cord. The peripheral nervous system is responsible for carrying nerve impulses to and from the body's many structures and includes the many craniospinal nerves which branch off of the brain and spinal cord. The autonomic nervous system is composed of the sympathetic and parasympathetic systems and is responsible for regulating and coordinating the functions of vital structures in the body.
Of all of these components, the brain is the primary component of the nervous system, occupying the crainal cavity. Without its outermost protective membrane, the dura mater, the brain weights an average of three pounds (approx. 1.4kg), comprising about 97% of the entire central nervous system. The brain is connected to the upper end of the spinal cord (which connects through the foramen magnum of the skull) and is responsible of issuing nerve impulses, processing nerve impulse data, and engaging in the higher order thought processes. The brain is divided into three parts: the large cerebrum, the smaller cerebellum, and the brainstem leading to the spinal cord. The brainstem is also descriptively divided into the medulla oblongata, the midbrain, and the pons.
The autonomic nervous system is responsible for the self-controlling aspects of the body's nervous network, and is under the control of the cerebral cortex, the hypothalmus, and the medulla oblongata. Working in tandem with the central nervous system, the autonomic nervous system features two subsystems which regulate body functions such as involuntary smooth muscle movement and heart rate. These two subsystems are called the sympathetic nervous system and the parasympathetic nervous system, and their functions operate in opposition to one another, delicately balancing the bodily functions which they control The sympathetic nervous system causes "fight or flight" responses in moments of stress stimulus, such as increased heart rate, saliva flow and perspiration. The parasympathetic system counterbalances these effects by slowing the heart rate, dilating blood vessels, and relaxing involuntary smooth muscle fibers.
Viewed individually, the sympathetic nervous system features a series of nerves which branch out of the spinal cord betwen the first thoracic vertebra and the second lumbar vertebra. These nerve fibers join into a long trunk of fibers, called the sympathetic trunk, on each side of the spinal cord. Along the sympathetic trunk are enlarged clusters of nerve fibers, called ganglia. From these ganglia, a number of nerve fibers extend throughout the body's tissues. Many of these nerves create additional ganglia, such as the celiac ganglia and the mesenteric ganglia. The sympathetic nerves are responsible for contracting involuntary smooth mucles fibers, viscera, and blood vessels, speeding up the heart rate, and dilating the bronchial tubes in moments of stress.
The parasympathetic nervous system, features ganglia in the midbrain, in the medulla oblongata, and in the sacral region. The cranial ganglia of the parasysmpthetic system, give pass impulses to the facial, oculomotor, glossopharyngeal, and vagus nerves. The parasympathetic nerves originate at the second, third, and fourth vertebrae and extend nerves to the bladder, the distal colon, the rectum, and the genitals. The nerves of the parasympathetic nervous system are responsible for conserving and restoring energy in the body following a sympathetic response to stress.
The term 'plexus" refers to a network of nerves or blood vesels. The nervous system features a number of these networks, where autonomic and voluntary nerve fibers join together. These networks include the brachial plexus (shoulder), the cervical plexus (neck) the coccygeal plexus (coccyx), and sacral or lumbar plexus (lower back).
The cerebellum is the second smaller division of the brain, located below the cerebrum and in the posterior of the brain. The cerebellum features a central portion, called the vermis and two side portions, or hemispheres-one on each side. It is the responsibility of the cerebellum to coordinate and modify the resultant activity of impulses and orders sent from the cerebrum. It does this by receiving information from nerve endings all over the body, such as the balance and equilibrium centers in the inner ear, and adjusts and fine-tunes these actions by passing the regulating signals to the motor neurons of the brain and spinal cord. Damage to the cerebellum therefore results in loss of ability to maintain precise muscular coordination and fine cooperative actions of the motor processes(ataxia).
The cerebrum, or forebrain, forms the bulk of the brain, made up of a large mass of white and grey neural fibers in the upper cranium. It is responsible for the higher thought processes (memory, judgement, reason), processing sensory data, and with initiating willful motor processes, such as voluntary muscle flexion. The cerebrum is composed of two lateral halves, or hemispheres, which feature a number of folds (gyri) and furrows (sulci) and which are connected in the middle at the medulla. The cerebrum is descriptively divided into four sections, or lobes named for the cranial bones which they are nearest: the frontal lobe, the occipital lobe, the parietal lobe, and the temporal lobe. Cerebrospinal fluid, used to support the brain and buffer it, is transmitted to these lobes by means of lateral ventricles which project branches, or horns, into the frontal, occipital, and temporal lobes. The functions of each lobe are coordinated by connecting fibers, the largest and most dense of these is the corpus callosum which connects the two hemispheres and connects through branches to the rest of the surface, or cortex, of the cerebrum. The two smaller commissural fibers are the anterior commissure, which contains olfactory fibers as well as other temporal connections, and the hippocampal commissure which runs transversely below the rear of the corpus callosum and is specifically related to the olfactory centers of the brain. Containing about a trillion neurons, the human brain is the most complex mechanism known and the least understood.