The system is composed of specialized cells, termed nerve cells or neurons, that communicate with each other and with other cells in the body.  A neuron has three parts:

1. the cell body, containing the nucleus
2. dendrites, hair-like structures surrounding the cell body, which conduct incoming signals.
3. the axon (or nerve fiber), varying in length from a millimeter to a meter, which conduct outgoing signals emitted by the neuron. Axons are encased in a fat-like sheath, called myelin, which acts like an insulator and, along with the Nodes of Ranvier, speeds impulse transmission.

Somatosensory Receptors
Input (afferent system) to the nervous system is in the form of our five senses: touch/pain, vision, taste, smell, and hearing. Pain, temperature, and pressure are known as somatic senses. Sensory receptors are classified according to the type of energy they can detect and respond to. 

• Mechanoreceptors: hearing and balance, stretching. 
• Photoreceptors: light. 
• Chemoreceptors: smell and taste mainly, as well as internal sensors in the digestive and circulatory systems. 
• Thermoreceptors: changes in temperature. 
• Electroreceptors: detect electrical currents in the surrounding environment. 

• the central nervous system (CNS), consisting of of the brain and spinal cord,
• the peripheral nervous system (PNS), the vast network of spinal and cranial nerves linking the body to the brain and spinal cord. The PNS is subdivided into:
1. the autonomic nervous system (involuntary control of internal organs, blood vessles, smooth and cardiac muscles), consisting of the sympathetic NS and  parasympathetic NS
2. the somatic nevous system (voluntary control of skin, bones, joints, and skeletal muscle).

In the PNS, collections of neurons are  called ganglia.	In the CNS, collections of neurons  are called nuclei.

The brain, the body's "control central," is one of the largest of adult organs, consisting of over 100 billion neurons and weighing about 3 pounds. It is typically divided into four parts: the cerebrum; the cerebellum; the diencephalon (thalamus, hypothalamus, sometimes classed as cerebral structures); and the brain stem (medulla oblongata, pons, midbrain), which is an extension of the spinal cord. Test yourself: Name the parts of the brain.

Click image for a primer on Parts of the Brain Graphics modified from: The InnerBody: Anatomy Tutorials - Nervous System

The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex. The interior portion consists of white matter, tracts, and nuclei (gray matter) where synapses occur. Each hemisphere of the cerebral cortex is divided into four "lobes" by various sulci and gyri: The sulci (or fissures) are the grooves and the gyri are the "bumps" on the brain's surface.

Web resources: Brain-Sensory Interaction Brain-Motor Interaction

The four lobes perform specific functions:

1. frontal - controls fine movements (Betz cells)/ upper motor neuron) and smell. Also, center for abstract thinking, judgment, and language (left hemisphere)
2. parietal - coordinates afferent infromation dealing with pain, temperature, form, shape, texture, pressure, and position. Some memory functions are also found here.
3. temporal - handles dreams, memory, and emotions. Center for auditory function.
4. occipital -  governs vision

Function summary:

• "Consciousness" - awareness of one's self and one's environment
• Thought/ reasoning/ memory
• Vision, hearing, touch
• Speech, language
• Motor control (fine and gross)/ coordination
• Emotions ("limbic system" or the "emotional brain") 

Function summary:

• Balance/ Equilibrium of the trunk (See also: Vestibular System)
• Muscle tension, spinal nerve reflexes, posture and balance of the limbs
• Fine motor control, eye movement. (Incoming information is transferred from the cerebral cortext via the pons. Outgoing information goes back to the cortex via the thalamus.) 

Diencephalon
The diencephalon, located between the cerebrum and the midbrain, consists of several important structures, two of which are the:

• Thalamus: large, bilateral (right thalamus/left thalamus) egg-shaped mass of gray matter serving as the main synaptic relay center. Receives/relays sensory information to/from the cerebral cortex, including pain/pleasure centers.
• Hypothalamus: a collection of ganglia located below the thalamus and intimatetly associated with the pituitary gland. It has a variety of functions: senses changes in body temperature; controls autonomic activites and hence regulates the sympathetic and parasympathetic nervous systems; links to the endocrine system/controls the pituitary gland; regulates appetite; functions as part of the arousal or alerting mechanism; and links the mind (emotions) to the body -- sometimes, unfortunately, to the degree of producing "psychosomatic disease."

• Voluntary movement/ motor integration
• Perception/ Sensory/mind-body integration
• Temperature
• Appetite

Function summary:

• Breathing/respiration (pons, medulla)
• Heart rate/ action (medulla)
• Blood pressure (vasoconstriction)/ blood vessel diameter (medulla)
• Reflex centers for pupillary reflexes and eye movements (midbrain, pons); and for vomiting, coughing, sneezing, swallowing, and hiccuping (medulla).

Cranial Nerves

There are 12 pairs of cranial nerves. Some bring information from the sense organs to the brain; some control muscles; others are connected to glands or internal organs.  The nerves are often remembered by the mnemonic ... "On Old Olympic Towering Top  A Famous Vocal German  Viewed Some Hops" ... standing for: Ofactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Spinal accessory, Hypoglossal.    Interactive Review

Cranial Nerve Major Functions I Olfactory smell II Optic vision  III Oculomotor eyelid and eyeball movement IV Trochlear innervates superior oblique turns eye downward and laterally  V Trigeminal chewing  face & mouth touch & pain VI Abducens turns eye laterally  VII Facial controls most facial expressions  secretion of tears & saliva taste  VIII Vestibulocochlear (auditory)  hearing  equillibrium sensation  IX Glossopharyngeal taste  senses carotid blood pressure  X Vagus senses aortic blood pressure  slows heart rate  stimulates digestive organs taste  XI Spinal Accessory controls trapezius & sternocleidomastoid controls swallowing movements XII Hypoglossal controls tongue movements  Test yourself: Think you can identify the location and function of all the cranial nerves?  Try these two interactive quizzes.      1. Cranial nerve locations      2. Cranial nerve functions

Spinal Cord
Motions and Functions Available at Different Levels of Spinal Cord Injury
The spinal cord lies within the spinal cavity, consisting of the vertebral column, the meninges, spinal nerves, spinal fluid, blood vessels, and a cushion of adipose/fat tissue. The spinal cord has two general functions: 1) It provides the two-way conduction routes to/from (afferent/efferent) the brain; and 2) it serves as the reflex center for all spinal reflexes. Thirty-one pairs of nerves connect to the spinal cord. They have no special names, but are numbered according to the level of the spinal column from which they emerge. There are 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal pairs of spinal nerves. 

1. Spinothalamic tract, carrying pain and temperature impulses to the thalamus and the parietal lobe
2. Posterior columns, carrying position and pressure sense to the thalamus and the parietal lobe
3. Corticospinal (or pyrimidal) tract, carrying impulses to initiate muscle activity from the frontal lobe.

Reflexes
A reflex is an automatic, i.e."involuntary," response of the body to some stimulus, e.g. pupil dilation, to collect more light, at night. Reflexes have afferent (signals to the CNS) and efferent (signals from the CNS) components and may involve one nerve/synapse (monosynaptic reflex) or multiple synapses (polysynaptic reflex).  The simplest spinal reflex is the tendon reflex, a monosynaptic reflex. When the tendon is tapped, the sensory receptor in the muscle sends a signal to the spinal cord (afferent system), where it enters through the dorsal/posterior root, then synapses with a sensory nerve axon in the ventral/anterior horn. From there, the receiving axon carries the impulse through the ventral/anterior root back to the muscle (efferent system) whose tendon was tapped. The muscle jerks/ contracts.

Spinal/Peripheral Nerves
All cervial spinal nerves control movement of the shoulder and arm muscles and receive all sensory input from the skin, muscles, bones, and joints. After leaving the vertebral canal, they merge to form peripheral nerves. The merger of the spinal nerves in the neck region is called the brachial plexis. 
 

The thoracic spinal nerves do not recombine into plexuses. The anterior roots control the muscles of the abdomen and back; the posterior roots receive information from the abdomen, skin, and muscles and joints of the chest and upper back. The lumbar and sacral nerves, like the cervical, do combine after leaving the vertebral canal. This merger, occuring in the pelvis, is called the lumbosacral plexus, which controls the movement of, and receives information from, the lower extremities, bladder, and bowel.
 

To Learn More

Web resources: Brain Glossary (While You Are Waiting) PATTS - AT Links -  The InnerBody: Anatomy Tutorials - Nervous System Maricopa: Nervous System Exploring the Nervous System Cranial Nerves (Yale University School of Medicine) The Whole Brain Atlas Visible Human Project Probe the Brain: Map of the Motor Cortex - the "homunculus"

Acknowledgments:  Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, pages 19-30. Catherine Parker Anthony and Gary A. Thibodeau, Textbook of Anatomy & Physiology. St. Louis: Mosby, 1983, pages 328-346. Anatomy Clipart (Designs4Free) Dennis Kunkel, Electron Microscopy Gallery J. Crimando, Anatomy and Physiology Tutorials (Maricopa)