Understanding the Peripheral and Central Nervous Systems

The Peripheral Nervous System

The peripheral nervous system is responsible for connecting the sensory organs and nerves with the effector organs. It consists of nerves, which are like strands formed by several axons. To prevent the passage of current, the axons are electrically isolated from each other by the myelin sheath.

The nerves are classified as:

  • Sensory nerves: transmit sensory information from the sensory organs.
  • Motor nerves: send commands to the effector organs.
  • Mixed nerves: contain axons of both types.

According to their origin in the nervous system, they can be distinguished as:

  • Cranial Nerves: originate from the brain. There are 12 pairs of sensory and motor nerves.
  • Spinal Nerves: originate from the spinal cord. There are 31 pairs of mixed nerves. Along the course of certain nerves, small bumps called nerve ganglia, clusters of neuron bodies, can be observed.

Central Nervous System

The Central Nervous System (CNS) consists of the spinal cord and brain, and it processes information.

The central nervous system is protected by:

  • The skull and spine.
  • Three membranes, called meninges. Among the meninges is the cerebrospinal fluid, which cushions the effect of shock on the nerve centers.

In the central nervous system, there are two different tissue areas:

  • Gray matter: made up of nerve cell bodies and dendrites, and it functions as the control center.
  • White matter: contains only extensions of neurons, axons, covered by a layer called myelin.

The spinal cord is a nerve cord that runs inside the channel formed by the spine. The gray matter is in the center of the cord with a butterfly-like shape, and the white matter is in its outer zone. In the gray matter, there is a conduit, the ependyma, which runs from top to bottom and contains fluid.


Spinal Cord Functions:

  • Initiates reflexes.
  • Conducts sensory impulses to the brain and motor commands from the brain to the effector organs.

The Brain

The brain is the central nervous system organ located inside the skull or cranium. The brain has four main regions:

  • Medulla Oblongata: is an extension of the spinal cord. It contains several areas of gray matter surrounded by white matter.
  • Brain Stem: includes several parts of the brain between the medulla oblongata and the brain. It regulates sleep-wake states, visual and auditory reflexes, and communication between the brain. A part of the brainstem, the hypothalamus, is connected to the pituitary gland, which it controls. The pituitary gland is an endocrine gland that secretes hormones.
  • Cerebellum: is located at the back of the brain. Its external appearance is similar to the brain. It consists of gray matter in the outer zone and white matter in its inner area that looks like a branched “tree of life”.
  • Cerebrum: is the largest part of the brain and the most important nervous organ. It consumes 18% of glucose and 20% of oxygen. It has white matter in its interior and gray matter in the outer cortex, forming a layer of only 2 mm thick, which represents the most important evolutionary acquisition of the nervous system of animals, as it directs learned behaviors. Without the cerebral cortex, learning or making voluntary movements is impossible.

Cerebrum Features:

  • Receives information from all sensory organs, except the organ of equilibrium, and produces corresponding sensations.
  • Processes the information received and prepares responses and orders for effector organs.
  • Coordinates and controls the operation of all parts of the nervous system.
  • Hosts the intellectual faculties and higher nervous functions.