The neurosensory system is the body's communication network, which transmits impulses between different parts of the body. The neurons—the basic nerve cells—are the structural and functional units of the nervous system. To understand complex neurologic disease processes, it is beneficial to review some facts and terms related to the neurosensory system first.

The nervous system consists of two subsystems:

• The central nervous system (CNS)
• The peripheral nervous system (PNS)

The CNS includes the brain and spinal cord. The brain contains three major components:

• The cerebrum, which consists of two cerebral hemispheres. The left hemisphere is responsible for language control, and the right hemisphere has greater control over nonverbal perceptual functions.
• The cerebellum, which is involved in controlling and coordinating skeletal muscle movements (in concert with other brain structures).
• The brain stem, which is composed of the midbrain, pons, and medulla oblongata.

The cerebrum controls voluntary movements and conscious thought, including the limbic system, which is associated with emotion. The cerebral hemispheres are divided into the frontal, parietal, temporal, and occipital lobes:

• The frontal lobe controls cognition and voluntary movement. Broca's area in the frontal lobe is responsible for expressive speech.
• The temporal lobe is concerned with receptive speech as well as bodily, visual, and auditory input.
• The parietal lobe integrates spatial information.
• The occipital lobe is associated with vision, processing sight stimuli.

The cerebellum functions to maintain trunk stability and coordinate voluntary movement. The brain stem, which connects to the spinal cord, consists of the midbrain, pons, and medulla oblongata. These vital centers are associated with management of respiration, vasomotor, and heart function.

The PNS consists of 12 pairs of cranial nerves; 31 pairs of spinal nerves, which contain both sensory and motor fibers; and portions of the autonomic nervous system. The 12 pairs of cranial nerves are numbered according to the order in which they emerge from the brain:

The spinal nerves are named by their location. There are 8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar spinal nerves, 5 sacral spinal nerves, and 1 coccygeal spinal nerve, corresponding to the spinal vertebrae.

The autonomic nervous system (ANS) regulates the involuntary functions of cardiac and smooth muscle as well as the glands. It consists of two divisions—sympathetic and parasympathetic—which counterbalance each other to maintain homeostasis (equilibrium).

The sympathetic division reacts to stress by releasing neurotransmitters or hormones. For example, norepinephrine is a neurotransmitter that acts on adrenergic receptors to produce the body's physiological “fight or flight” responses. A sympathetic response usually results in an increase in blood pressure (BP), pulse, systemic circulation, and mental alertness, and, simultaneously, a decrease in secretions, digestion, and urine output.

The parasympathetic division primarily dominates in nonstressful situations. Its main neurotransmitter acetylcholine (ACh) acts on cholinergic receptors to induce its effects, including conserving energy, decreasing heart rate, and increasing ­peristalsis and glandular/gastrointestinal secretions. ACh is rapidly degraded by the enzyme acetylcholinesterase.

There are two types of basic neural cells:

• Neurons
• Neuroglia or glial cells

The neurons are responsible for impulse initiation and conduction. Each neuron has three parts: a cell body, an axon, and one or more dendrites. Many axons are covered with a myelin sheath, an insulating white lipid substance. Destruction of the myelin sheath is often the cause of neurologic problems or diseases, such as in multiple sclerosis. The glial cells nourish and support the neurons.