The Nervous System: Structure and Functions

3.3 The Nervous System

Receives information from the receptors, processes and coordinates it, and generates orders for the effectors. It is made up of the central nervous system or CNS, and the peripheral nervous system or PNS.

3.4 The Central Nervous System or CNS

Coordinates all the body's functions. It is formed by the encephalon and the spinal cord.

The Encephalon

Protected by the skull or cranium and by three membranes, the meninges, between which a liquid called the cerebrospinal fluid circulates. This liquid protects the encephalon from knocks and blows. Divided into three parts:

• The brain is divided into two hemispheres, which are separated by a fissure. Its surface, the cerebral cortex, is full of folds and furrows, known as circumvolutions. The brain converts information into awareness and houses advanced functions.
• The cerebrum controls balance and voluntary movements.
• The brainstem links the encephalon to the spinal cord and controls diverse involuntary functions. For example, the medulla oblongata controls our breathing and heart rate.

The Spinal Cord

Is a long, thin bundle of nerves protected by the spine. It serves as a conduit for nerve impulses from the receptors to the encephalon. It also coordinates simple responses called reflex actions. The spinal cord has two parts: the white substance which is in the outer part and is made up of neuron axons. The inner part is called the gray substance. It has the shape of a butterfly and is made up of neuron bodies and dendrites.

3.5 The Peripheral Nervous System or PNS

It is formed by 31 pairs of nerves which stem from the spinal cord (spinal nerves) and 12 nerves which originate from the encephalon (cranial nerves). Nerves are divided into sensory nerves (which carry information to the CNS) and motor nerves (which convey orders to the effectors).

The Peripheral Nervous System is Divided Into:

• Somatic PNS consists of the sensory nerves and the motor nerves. It controls voluntary movements.
• Autonomic PNS consists of motor nerves which regulate the internal organs. It controls involuntary movements. The autonomic nervous system is divided into the sympathetic nervous system which stimulates the body to react to stress, and the parasympathetic nervous system which causes the body to relax. These two systems are antagonistic; they act in opposite ways. One causes the pupils of the eye to dilate, while the other makes them constrict.