Nervous System: Nervous Tissue

Nervous System

It is the Central Processing Unit (CPU) of the human body. It controls all the activities of the body.

It consists of two principal types of cells: neurons and neuroglia.

Fig.1: Anatomy of a neuron



Neuron:

Also known as nerve cells

These are Excitable cells

Sensitive to various stimuli

Convert stimuli into electrical signals (Action Potentials) or (Nerve Impulse)

Action potentials can travel along the plasma membrane of a neuron due to the presence of specific voltage-gated ion channels

Neuron conduct these Action potential to other neurons, to muscle tissue or to glands with the help of a chemical, called Neurotransmitter


Classification of neurons


Fig.2: Different types of neurons. A: Multipolar neuron; B: Bipolar neuron; C: Unipolar or pseudounipolar neuron

Neuroglia or glia

Initially, it was believed that these cells were the glue that held the nervous tissue together.

Smaller than neurons, but 5-50 times more numerous

Do not generate or propagate an action potential

In case of injury or disease, neuroglia multiply to fill in the spaces


Six types of neuroglia


    CNS has four types of neuroglia


       1. Astrocytes: Form Blood-Brain Barrier (BBB)

Fig.3: Blood-Brain Barrier is formed by the astrocytes


        2. Oligodendrocytes: Form Myelin sheath around CNS neuron


        3. Microglia: Perform phagocytosis


        4. Ependymal cells: Produce Cerebro-spinal Fluid (CSF)


    PNS has two types of neuroglia


        5. Schwann cells: Form Myelin sheath around CNS neuron

 

Fig. 4: Schwann Cells

 

       6. Satellite cells: surround the cell bodies of neurons of PNS ganglia


Electrical Signals in Neurons

Neurons are electrically excitable. They communicate with one another using two types of electrical signals:

    (1) Graded potentials are used for short-distance communication only. 

    (2) Action potentials allow communication over long distances within the body.


The production of graded potentials and action potentials depends on two basic features of the plasma membrane of neurons


1. The existence of a resting membrane potential

The plasma membrane of neurons exhibits a membrane potential, an electrical potential difference (voltage) across the membrane. 

In excitable cells, this voltage is termed the resting membrane potential.

It is due to

(a)Unequal distribution of ions in the extracellular fluid (ECF) and cytosol 

(b)The inability of most anions to leave the cell

(c)Electrogenic nature of the Na+/K+ ATPases: Throw out the Na+ ions and bring in K+ ion into the cell


2. The presence of specific types of ion channels

When ion channels are open, they let specific ions to move across the plasma membrane, down their electrochemical gradient

Ions move from areas of higher concentration to areas of lower concentration (the chemical gradient). 

Also, positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area (the electrical gradient)

four types of ion channels

(a) Leakage ion channel: ion channels that open or close randomly

(b) Ligand-gated ion channel: ion channel that open upon binding of a ligand (chemical) stimulus

(c) Mechanically gated ion channel: ion channel that opens in response to a mechanical stimulus like touch, pressure, vibration, etc.

(d) Voltage-gated ion channel: ion channel that opens in response to a particular voltage (change in membrane potential)


Fig.5: Resting Membrane Potential across the plasma membrane of a neuron 



Acknowledgment

The images were created on biorender.com


References

Tortora, Gerard J., and Bryan H. Derrickson. Principles of anatomy and physiology. John Wiley & Sons, 2018.



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