Nervous System Overview: Structure, Cells, and Reflex Pathways

The nervous system receives sensory input, integrates the information, and generates motor output. It is divided into the Central Nervous System (CNS)—the brain and spinal cord that act as the control center—and the Peripheral Nervous System (PNS), a network of nerves that carries signals to and from the CNS.

Functional Divisions of the Peripheral Nervous System

The PNS separates into sensory (afferent) pathways that bring information toward the CNS and motor (efferent) pathways that carry commands away. The motor side splits into the somatic system, which controls voluntary muscles, and the autonomic system, which regulates involuntary functions such as heart rate and digestion. The autonomic system further divides into the sympathetic division, which mobilizes the body for action, and the parasympathetic division, which promotes relaxation.

Nervous Tissue: Glial Cells and Neurons

Glial cells, also called neuroglia, outnumber neurons ten to one and make up roughly half of the brain’s mass. In the CNS they include:

• Astrocytes – anchor neurons and manage chemical exchange.
• Microglial cells – serve as immune defenders.
• Ependymal cells – line the ventricles and produce cerebrospinal fluid.
• Oligodendrocytes – create the myelin sheath that speeds axonal conduction.

In the PNS, satellite cells support neuronal cell bodies and Schwann cells form the myelin sheath around peripheral axons.

Neurons are long‑lived, amitotic cells that require a constant supply of glucose and oxygen. Each neuron consists of a soma (the cell body, described as “the neuron’s life support”), dendrites that act as listeners, and an axon that functions as the talker. Most neurons (about 99 %) are multipolar, possessing three or more processes; bipolar neurons have two processes and are rare, while unipolar neurons have a single process and are typical of sensory pathways.

Neuron Classification

Neurons can be grouped structurally (multipolar, bipolar, unipolar) and functionally:

• Sensory/Afferent neurons carry signals toward the CNS.
• Motor/Efferent neurons transmit commands away from the CNS.
• Interneurons connect sensory and motor neurons within the CNS, facilitating integration.

Applied Physiology: The “Spider on the Knee” Reflex

A classic illustration of reflex processing begins when skin receptors detect a stimulus—such as a spider landing on the knee. A unipolar sensory neuron quickly sends the impulse via its axon to the spinal cord. Inside the cord, the signal passes to multipolar interneurons, which immediately activate motor neurons. The motor neurons drive the leg muscles, producing a rapid kicking response. At the same time, interneurons relay a copy of the signal upward to the brain, allowing conscious awareness of the spider and the possibility of more complex decision‑making.

“You can’t oversell the importance of the nervous system. It controls ALL THE THINGS!”

“There is no ‘you’ without it. There is no ‘me’ without it.”

“Once considered just the scaffolding or glue that held neurons together, we now know that our different glial cell types serve many other important functions.”

“Most neurons are amitotic, so once they take on their given roles in the nervous system, they lose their ability to divide.”

“The soma, or cell body, is the neuron’s life support.”

“The bushy, branch‑like things projecting out from the soma are dendrites. They’re the listeners.”

“The neuron’s axon, meanwhile, is like the talker.”