The nervous system can be divided into several connected systems that function together. The Nervous System is divided into: The Central Nervous System and the Peripheral Nervous System Let's break the central nervous system and the peripheral nervous system into more parts. Central Nervous System The central nervous system is divided into two parts: the brain and the spinal cord. The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column. . The Central Nervous System (Brain and Spinal Cord) Peripheral Nervous System The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system. 1. Somatic Nervous System The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system AND motor nerve fibers that project to skeletal muscle. The picture on the left shows the somatic motor system. The cell body is located in either the brain or spinal cord and projects directly to a skeletal muscle. 2. Autonomic Nervous System The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands. This picture shows the general organization of the autonomic nervous system. The preganglionic neuron is located in either the brain or the spinal cord. This preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ. Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons. The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about. The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder). Here is a quick look at one way to divide the brain. Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon . From a top view, notice how the brain is divided into two halves, called hemispheres. Each hemisphere communicates with the other through the corpus callosum, a bundle of nerve fibers. (Another smaller fiber bundle that connects the two hemispheres is called the anterior commissure). Brain Structures Cerebral Cortex Functions: Thought Voluntary movement Language Reasoning Perception Cerebellum Functions: Movement Balance Posture The word "cortex" comes from the Latin word for "bark" (of a tree). This is because the cortex is a sheet of tissue that makes up the outer layer of the brain. The thickness of the cerebral cortex varies from 2 to 6 mm. The right and left sides of the cerebral cortex are connected by a thick band of nerve fibers called the "corpus callosum." In higher mammals such as humans, the cerebral cortex looks like it has many bumps and grooves. A bump or bulge on the cortex is called a gyrus (the plural of the word gyrus is "gyri") and a groove is called a sulcus (the plural of the word sulcus is "sulci"). Lower mammals, such as rats and mice, have very few gyri and sulci. The word "cerebellum" comes from the Latin word for "little brain." The cerebellum is located behind the brain stem. In some ways, the cerebellum is similar to the cerebral cortex: the cerebellum is divided into hemispheres and has a cortex that surrounds these hemispheres. Brain stem Functions: Breathing Heart Rate Blood Pressure Hypothalamus Functions: Body Temperature Emotions Hunger Thirst Circadian Rhythms Thalamus Functions: Sensory processing Movement Limbic System Functions: Emotions The brain stem is a general term for the area of the brain between the thalamus and spinal cord. Structures within the brain stem include the medulla, pons, tectum, reticular formation and tegmentum. Some of these areas are responsible for the most basic functions of life such as breathing, heart rate and blood pressure. The hypothalamus is composed of several different areas and is located at the base of the brain. Although it is the size of only a pea (about 1/300 of the total brain weight), the hypothalamus is responsible for some very important functions. One important function of the hypothalamus is the control of body temperature. The hypothalamus acts as a "thermostat" by sensing changes in body temperature and then sending signals to adjust the temperature. For example, if you are too hot, the hypothalamus detects this and then sends a signal to expand the capillaries in your skin. This causes blood to be cooled faster. The hypothalamus also controls the pituitary. The thalamus receives sensory information and relays this information to the cerebral cortex. The cerebral cortex also sends information to the thalamus which then transmits this information to other areas of the brain and spinal cord. The limbic system (or the limbic areas) is a group of structures that includes the amygdala, the hippocampus, basil ganglia, ventral tegmental area, and cingulate. These areas are important for controlling the emotional response to a given situation. The hippocampus is also important for memory. The hippocampus is one part of the limbic system that is important for memory and learning. Hippocampus Functions: Learning Memory Basal Ganglia Functions: Movement The midbrain includes structures such as the superior and inferior colliculi and red nucleus. There are several other areas also in the midbrain. Midbrain Functions: The basal ganglia are a group of structures, including the globus pallidus, caudate nucleus, subthalamic nucleus, putamen and substantia nigra, that are important in coordinating movement. Vision Audition Eye Movement Body Movement Here is where some of these Brain areas are located: Nerve Function and Drug Action: Simplified CELL SITES OF DRUG ACTION (A CARTOON VERSION OF HOW CELLS TALK TO EACH OTHER) There are millions of cells in the brain. This picture depicts two nerve cells (neurons) and their important components. Nerve Cell One is on the top, Nerve Cell Two is on the bottom. The large portion of Nerve Cell One is the working part of the cell, also known as the presynaptic area. The presynaptic area is at the end of a sending fiber called an axon, which begins outside the boundaries of the picture in a cell body called the soma. Inside the soma are manufacturing chemicals known as enzymes that manufacture chemicals called neurotransmitters. These neurotransmitters pass down the axon under the influence of a small electrical current called an action potential. The neurotransmitters are packaged in what look like cellophane envelopes (called vesicles). These vesicles release their contents (neurotransmitters) into the space between the two cells (the synapse), under the influence of small concentrations of calcium ions. Once in the synapse, one of four things can happen to the neurotransmitters. They either 1) activate an excitatory receptor (on the left), causing Nerve Cell Two to be more likely to fire, 2) activate an inhibitory receptor (middle), causing Nerve Cell Two to be less likely to fire, 3) are "gobbled up" (metabolized) by a monster enzyme (right), or 4) are taken back up into Nerve Cell One (reuptake), repackaged, and sent on down the nerve cell for use later on. Inside Nerve Cell One is another monster enzyme, known as MAO, which gobbles up the neurotransmitter molecules that accidentally leak out of the vesicles. Outside and above the nerve membrane is a small molecule known as chloride ion, which is necessary for the proper integrity of the vesicle membrane. 1. Under each of the receptor "ghosts" is a small rectangle containing globules of substances known as G proteins that are the beginning of a chemical and electrical cascade of events that make Nerve Cell Two more likely (excitation) or less likely (inhibition) to fire and carry the message of Nerve Cell One to the next nervous system component.