What are the cells of the nervous system?

Cells of the nervous system 

 The Nervous system primarily consists of two types of cell: 

 A. Neurons; the excitable cells, and 

 B. Neuroglia; supporting and non-excitable cells.

 Neurons: 

The nervous system comprises billions of specialised cells known as neurons. Each neuron possesses a cell body, and from the surface of the body, one or more processes emerge called neurites (often called nerve fibres). They are again of two types: the dendrites (one or more in number; receive information and conduct it toward the cell body) and the axon (single long tubular neurites; conduct impulses away from the cell bodies) 

[Cells of the Nervous system] 

 Classification of the neurons:

 A. Depending upon the number and mode of branching of the neurites (3 types)

• Unipolar neurons: Cell body + a single neurite. After a short distance, the neurite divides into two branches, one proceeds to some peripheral structures, and the other enters the CNS. E.g. neurons found in the posterior root ganglion 
• Bipolar neurons: Elongated cell body + two neurites, one from each end of the body. E.g. retinal bipolar cells, cells of the sensory cochlear and vestibular ganglia 
• Multipolar neurons: Cell body + multiple neurites ( single axon, and rests are dendrites). E.g. most neurons in the brain and the spinal cord 

 B. Depending upon the size and length neurites (2 types)

• Golgi type I: They have a long axon. In some case, it may be as long as 1 meter or even more. The axons of these neurons form the tracts of the CNS and nerve fibres of the peripheral nerve. E.g. the pyramidal cells of the cerebral cortex, the Purkinje cells of the cerebellar cortex, motor cells of the spinal cord 
• Golgi type II: They have a short axon that terminates in the neighbourhood of the cell body or is absent. They greatly outnumber the Golgi type I cells. These are numerous in the cerebral and cerebellar cortex and are often inhibitory in function 

Neuroglia: 

Generally smaller than neurons but outnumber them by 5-10 times. They contribute to half of the total volume of the brain and the spinal cord. There are 4 types of neuroglia cells:

• Astrocytes
• Oligodendrocytes
• Microglia, and 
• Ependyma 

Astrocytes: 

They form the structural framework for the neurons and control their biochemical environment, their foot processes adjoin small blood vessels and form the blood-brain barrier. These are again of 2 types: 

• Fibrous astrocytes: found mainly in the while matter. Small cell bodies with long slender processes 
• Protoplasmic astrocytes: found mainly in the gray matter. Small cells bodies with short thick processes. 

Oligodendrocytes: 

Form myelin sheath in the CNS [ in case of peripheral nerve the myelin sheath is formed by Schwann cells]

Microglia: 

Embryologically unrelated to the other neuroglial cells and are derived from the macrophages/monocytes. They have a role in fighting the infection and removing damaged cells.

Ependyma: 

They line the cavities of the brain and the central canal of the spinal cord. These also are divided into 3 groups: 

• Ependymocytes: Line the Ventricles of the brain and the spinal cord and are in contact with the CSF. They circulate and absorb the CSF.
• Tanycytes: Line the floor of the 3rd ventricle overlying the median eminence of the hypothalamus. They transport substances from the CSF to the hypophyseal-portal system. 
• Choroidal epithelium cells: Cover the surface of the choroid plexus. They produce and secrete the CSF.

[Source: Clinical neuroanatomy by Richard S. Snell; 7th Edition ||| Image from Davidson's Principles and Practice of Medicine; 23rd Edition; Fig 25.1 ]