Central Nervous System, Parts, Definition, and Its Functions

The Human Neural System is Divided into Two Main Parts:

1. Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)

Central Nervous System

The central nervous system (CNS) includes the Brain and the Spinal cord and is the site of information processing and control including awareness, movements, sensations, thoughts, speech, and memory.

The Human Brain

The brain (weighing 1220 to 1400 gm) is the central information-processing organ of our body. The brain lies in the cranium of the skull. The brain and spinal cord are surrounded by connective tissue membranes called meninges.

There are three meninges in humans, an outer layer called duramater, a middle layer called an arachnoid membrane, and an inner thin layer called piamater.

Parts of Brain

The Human Brain is Divided into Three Parts:

1. Forebrain (prosencephalon)
2. Midbrain (mesencephalon)
3. Hindbrain (rhombencephalon)

1. Forebrain or Prosencephalon

The forebrain consists of the olfactory lobes, cerebrum, and diencephalon.

1. The olfactory lobes are concerned with the sense of smell.
2. The cerebrum forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, termed the left and right cerebral hemispheres.
   • A tract of nerve fibers connects the hemispheres called the corpus callosum.
     • The cerebrum’s outer portion, called the cerebral cortex, makes up the grey matter of the cerebrum.
     • Beneath the grey matter, there are millions of medullated nerve fibers that give an opaque white appearance, hence they are collectively called white matter.
   • Each cerebral hemisphere is divided into four lobes:
     1. Frontal (monitors complex thoughts, actions, and ideas and control intellectual ability)
     2. Parietal (registers sensory perception and takes in information from the environment, organizes it, and communicates it to the rest of the brain),
     3. Temporal (decodes and interprets sound, smell, memory, and emotion) and
     4. Occipital lobes (decodes and interprets visual information, shape, and color).
3. Diencephalon is completely covered by cerebral hemispheres. Its main parts are:
   • Epithalamus, Thalamus, and Hypothalamus.
     • Epithalamus is a (dorsal) posterior segment of the diencephalon.
       • The epithalamus serves as a connecting point for the dorsal diencephalic conduction system, which is responsible for carrying information from the limbic forebrain to limbic midbrain structures.
     • Thalamus is a major coordinating center for sensory ranging from relaying sensory and motor signals, as well as regulation of consciousness and alertness.
     • Hypothalamus lies at the base of the thalamus and contains a number of centers that control body temperature, urge to eat and drink, growth and sexual behavior, etc.
       • Hypothalamus is an essential link between the neural and endocrine systems (neuroendocrine role).

2. Midbrain or Mesencephalon

Midbrain is located between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.

• The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina.
• Corpora quadrigemina control visual and auditory reflexes and are equivalent to the optic lobes of lower animals.

3. Hindbrain or Rhombencephalon

Hindbrain comprises pons Varolii, cerebellum, and medulla oblongata.

• Pons consists of fiber tracts that interconnect different regions of the brain.
• The cerebellum is the second largest part of the human brain.
  • It consists of two lateral cerebellar hemispheres.
  • The cerebellum controls rapid muscular activities, such as running, typing and even taking.
• The medulla oblongata contains centers that control respiration, cardiovascular reflexes, and gastric secretions.

The Brain Stem

The brain stem forms the connections between the brain and the spinal cord. Three major regions make up the brain stem; midbrain, pons, and medulla oblongata.

The Spinal Cord

The spinal cord (40 to 50 cm long) extends from the medulla oblongata and is continuous to the level of the second lumbar vertebra.

It conducts impulses to and from the brain and controls most of the reflex activities and provides a means of communication between the spinal nerves and the brain.

• The spinal cord is formed of two types of neural tissue:
• Internal grey matter and outer white matter. The area within the vertebral column below the second lumbar vertebra contains spinal nerves that the collectively called, the cauda equina. The spinal cord ends at the conus medullaris from which a fine filament called filum terminate arises that anchors the spinal cord within the vertebral column.
• Reflex action: It is an immediate involuntary action of any organ or part of the body in response to a particular stimulus. The nervous pathway taken by nerve impulses in a reflex action is called the reflex arc. The components that mediate a reflex, usually include a receptor, afferent pathway, integrating center, efferent pathway, and effector. Some common examples of reflexes are closing of eyes when strong light is flashed, salivation on seeing some favorite food, etc.
• Reflexes are categorized into unconditioned and conditioned reflexes. Unconditioned reflexes are the inborn, unlearned response to a stimulus or any environmental change. Whereas, conditioned reflexes are not genetic but acquired from past experience, training, or learning. Ivan Pavlov first demonstrated conditioned reflexes.