Introduction

The nervous system is itself divided into two sections:

• The Peripheral Nervous System (PNS): sensory receptors and motor effectors
• The Central Nervous System (CNS): Brain and spinal cord

In summary, the nervous system is organized as follows:

Peripheral Nervous System (PNS)

The peripheral nervous system consists of nerve cells that lie outside the brain case or spinal vertebrae. Sensory receptors detect changes in the external environment (visual, auditory, olfactory, sensory, etc.) and communicates them to the CNS (via afferent sensory nerves). The CNS, in turn, communicates desired behavior such as muscle movement to the PNS (via efferent motor nerves) using motor effectors.

The PNS is divided into two sections: an Autonomic Division, which controls smooth muscles (involuntary), such as the stomach and heart, and a Somatic Division, which controls striated muscles (voluntary), which are the skeletal muscles.

In summary, the PNS consists of:

• Sensory (afferent nerves)
• Motor (efferent nerves)
• Somatic Division
• Autonomic Division

The Autonomic Division can itself be divided into Sympathetic and Parasympathetic divisions:

• Parasympathetic
• Maintains bodily functions such as digestion and blood flow at a balanced level
• Maintains homeostasis

• Sympathetic
• 'Fight-or-flight' system
• Diverts energy away from sustaining activities
• Instead, mobilizes the delivery of oxygen (via the blood) to the extremities and brain
• Increases responsiveness

The central nervous system integrates information arriving from the PNS, processes this information to generate behavior, which it communicates back to the PNS.

The CNS is divided into two sections: the spinal cord, which connects directly with the PNS, and the brain, which contains the higher brain functions. Both are encased within bone. The brain communicates its behavior to the PNS by way of the spinal cord. In cases of very quick reflexes, such as pain response, the PNS-spinal cord-PNS pathway is invoked without first processing information in the brain (e.g., knee jerk response). The brain itself consists of three major divisions: cerebral hemispheres, cerebellar hemispheres, and the brain stem.

In summary, the CNS consists of:

• Spinal cord
• Brain