Nervous System


The nervous system is an organ system that contains a network of specialized cells called neurons. This is the master controlling and communicating system of the body. It coordinates the action of an animal and transmits signals between the different parts of the body. Every thought, movement and emotions reflect the activity of the nervous system.

Functions of the NERVOUS SYSTEM
  1. To monitor changes that takes place inside and outside the body. The nervous system utilizes the million sensory receptors to carry out this function. Any changes or stimuli occurring are noted by the nervous system and the gathered data is now called a sensory input.
  2. Another important function of the nervous system is to process and interpret the sensory input or gathered data. It is the working of this system to make decision about what should be done at each moment. This is the process known as INTEGRATION.
  3. As the nervous system has reached a decision of what response and appropriate action to be done in response to the stimuli, it theneffects a response by activating muscles or glands through motor output.
Structural Classification of the Nervous system

Structurally, the nervous system is classified into the central nervous system and the peripheral nervous system.

  • Central nervous system. The CNS consists of the brain and the spinal cord. These organs occupy the dorsal body cavity and act as the INTEGRATING and COMMAND CENTERS of the nervous system. It is the CNS that interprets an incoming sensory information and sends and instruction basing on the past experience and current condition.
  • Peripheral Nervous System. The PNS is consisting of the nerves that extend from the brain and the spinal cord. It is the part of the nervous system outside the CNS. There are varieties of nerves. The spinal nerves carry impulses to and from the spinal cord. The cranial nerves, on the other hand, carry impulses to and from the brain. These nerves serve as the communication lines of the body.
Functional Classification of the Nervous System

The functional classification of the nervous system is only concerned about the structures of the peripheral nervous system (PNS). The PNS in this classification is divided into two principal subdivisions:

  • Sensory or afferent division. This subdivision is composed of the nerve fibers that convey impulses to the central nervous system (CNS) from the sensory receptors. These sensory receptors are located in the different parts of the body. With the presence of these sensory fibers the CNs is constantly informed of the events going on both inside and outside the body.
  1. The fibers responsible for delivering impulses from the skin, skeletal muscles and joints are called the somatic sensory fibers.
  2. Fibers that transmit impulses from the visceral organs are called the visceral sensory fibers.
  • Motor or efferent division. This division is responsible for carrying impulses from the CNS to the effector organs, muscles and glands. In response these impulses, activate muscles and glands and they effect a motor response. The two classification of motor or efferent division are:
  1. Somatic nervous system. This subdivision is also referred as the voluntary nervous system. The somatic NS allows a person to consciously or voluntarily control aperson’s skeletal muscles.
  2. Autonomic nervous system (ANS). The ANS regulates the events that are automatic or INVOLUNTARY such as the activity of the smooth and cardiac muscles and glands. The two parts of the ANS are the sympathetic and the parasympathetic systems.
Function and  Structure of the Nervous System

If you think of the brain as a central computer that controls all bodily functions, then the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.

The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Even though it is complex, nervous tissue is made up of two principal types of cells namely, the supporting cells and the neurons.


The supporting cells in the CNS are “lumped together” as NEUROGLIA or GLIAL CELLS. Glial Cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin and participate in signal transmission in the nervous system. In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.

The functions of glial cells are:

  1. to support neurons and hold them in place
  2. to supply nutrients to neurons
  3. to insulate neurons electrically
  4. to destroy pathogens and remove dead neurons
  5. to provide guidance cues directing the axons of neurons to their targets

Characteristics of Glial Cells:

  1. Lumped together.
  2. Not able to transmit impulses.
  3. Never lose their ability to divide.

The CNS glia include:

  • Astrocytes. These are star-shaped cells that account nearly half of the neural tissue. Astrocytes form a living barrier between capillaries and neurons and play a role in making exchanges between the two. This is to prevent harmful substances in the blood from entering the neurons. Aside from that, astricytes are also important in controlling the chemical environment in the brain. This is done by picking up excess ions and recapturing released neurotransmitters.
  • Microglia. These are spiderlike phagocytes that dispose debris including dead brain cells and bacteria.
  • Ependymal cells. These cells line the cavities of the brain and the spinal cord. Aside from lining the cavities of certain organs, these cells are very important in helping the CSF through their cilia to circulate and fill those cavities and form a protective cushion around the CNS.
  • Oligodendrocytes. These are glial cells that wrap their flat extensions tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths.
Anatomy of the Neuron

The nervous system is defined by the presence of a special type of cell—the neuron (sometimes called “neurone” or “nerve cell”). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via SYNAPSES,  which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an AXON,  a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts. Axons frequently travel through the body in bundles called nerves.

  • Cell body – the metabolic center of the neuron. This part of neuron contains the usual organelles except for the centrioles. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. This is well protected and is located in the bony skull or vertebral column and is essential to well-being of the nervous system. The cell body carries out most of the metabolic functions of a neuron.
  • Nissl substance and Neurofibrils – found in the cell body that is essential in maintaining cell shape.
  • Dendrites – neuron processes that covey incoming messages TOWARD the cell body.
  • Axons – neuron processes that generate nerve impulses AWAY from the cell body.
  • Axon hillock – a cone-like region of the cell body where the axon arises.
  • Axon terminals – – located at the terminal end of the axons that contains tiny vesicles or membranous sacs that contains chemicals called neurotransmitters. When impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular spaces.
  • Synaptic cleft – a tiny gap that separates axon terminal from the next neuron.
  • Myelin – a whitish, fatty material that covers long nerve fibers. It has a waxy appearance that protects and insulates the fibers and increases the rate of nerve impulses.
  • Schwann cells – myelinates the axon outside the nervous system. Schwann cells are specialized supporting cells that enclose themselves tightly around the axon jelly-roll fashion.
  • Myelin sheath – a tight coil of wrapped membranes created after the Schwann cells enclose the axon.
  • Neurilemma – part of the Schwann cell external to the myelin sheath.
  • Nodes of Ranvier – gaps or indentations between the myelin sheaths.