2 Main types:

1. Microglia – microgliocytes (main CNS phagocytes).

2. Macroglia:

• astrocytes;

• ependimocytes;

• oligodendrogliocytes.

Astrocytes are the biggest gliocytes. Protoplasmic astrocytes are located in grey matter; fibrous astrocytes are situated in white matter.

Functions:

1. supporting – CNS carcass forming especially during embryogenesis; directing – synthesis of substances determining neurons differentiation;

2. demarcating, transport and barrier – is directed to neurons optimal microenvironment providing:

   1. hemato-encephalic barrier (HEB) – it separates CNS neurons from blood and internal environment tissues and includes:

• capillaries endothelium;

• basal membrane;

• perivascular membrane (near-vessel) formed by astrocytes processuses;

1. neuro-liquor barrier – it separates neurons from liquor;

2. formation of perineuronal sheathes surrounding neurons bodies in synaptic area (isolating function);

1. metabolic and regulatory – one of the most important astrocytes functions – K⁺-ions and mediators optimal level supporting in neurons surrounding;

2. protective:

• phagocytic;

• immune (antigen-presenting cells);

• reparative (they form scar at ending stages of inflammation in CNS while their growth).

Ependimocytes – (“ependyma” – “street cloth” or “covering”) cover brain ventricles and cerebral sheathes.

Functions:

1. supporting;

2. forming of barriers:

• neural-liquor (with high permeability);

• hemato-liquor;

3. liquor components ultrafiltration.

Tanicites – cover the III-rd ventricle walls. They absorb substances from liquor and transport them through their process to vessels lumen thus providing connection between liquor in ventricles lumen and blood.

Oligodendrogliocytes – (Greek – “oligo” – “a little”, “dendron” – “tree” and “glia” – “glue” - glia with little processuses number – they surround neurons bodies, are in nervous endings compounds; they are observed both in central and peripheral nervous system.

Functions:

1. myeline sheath formation;

2. neurons metabolism.

Neurotrophins

In a developing fetal brain, chemicals called neurotrophins, promote neuron growth. Nerve growth factor (NGF) was the first neurotrophin to be identified; others include brain-derived neurotrophic factor (BDNF); glial-derived neurotrophic factor (GDNF); neurotrophin-3 and neurotrophin 4/5 (the numbers depend on the species). NGF and neutrophin-3 are known to be particularly important in the embryonic development of sensory neurons and sympathetic ganglia.

Neurotrophins also have important functions in the adult nervous system. NGF is required for the maintenance of sympathetic ganglia, an there is evidence that neurotrophins are required for mature sensory neurons to regenerate after injury. In addition, GDNF may be needed in the adult to maintain spinal motor neurons and to sustain neurons in the brain that use the chemical dopamine as a neurotransmitter.

Experiments suggest that neurons of the CNS can regenerate if they are provided with the appropriate environment. While neutrophins promote neuron growth, some chemicals including myelin-associated inhibitory proteins, have bben shown to inhibit axon regeneration. Research in this area, with its important implications for the repair of spinal cord and brain injury, is ongoing.

NERVOUS FIBERS PHYSIOLOGY

Structure of Myelinated Nerve Fiber

Axis cylinder of nerve fiber is covered by a membrane called neurilemma.

In myelinated nerve fiber, axis cylinder is covered by a thick sheath called myelin sheath. Myelin sheath in turn is covered by neurilemma.

Fig.10. Myelinated nerve fiber

Myelin sheath

In a myelinated nerve fiber, axis cylinder is covered by a thick tubular sheath called myelin sheath. Myelin sheath does not form a continuous sheath and is absent at regular intervals. The area where the myelin sheath is absent is called node of Ranvier. The segment of nerve fiber between two nodes is called internode. Myelin sheath is responsible for white colour of nerve fibers. Myelin is lipoproteid.