Nervous system| Type| function| classification| structure| characters|

  NERVE FIBRE  

A nerve fiber is a cord like structure containing bundles of axons in the peripheral nervous system. It provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons to peripheral organs. Within a nerve, each axon is surrounded by a layer of connective tissue called endoneurium. The axons are bundled together into groups called fascicles and each fascicle is wrapped in layer of connective tissue called Perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called epineurium. Nerves are bundled along with blood vessels since the neurons are of a nerve have fairly high energy requirements. A blood vessel carrying blood to a nerve is known as vasal nervosa. Within the endoneurium, the individual nerve fibers are surrounded by a low protein liquid called endo neural fluid.it constitutes a blood nerve barrier similar to the blood brain barrier. Molecules are thus prevented from crossing the blood into the endo neural fluid. During the development of nerve oedema (swelling)due to nerve injury, the amount of epineural fluid may increase at the site of injury. This can be visualized using magnetic resonance neurography.

Histologically nerves are of two kinds:

✒ MYELINATED NERVE FIBRE: -

The nerve fiber which are surrounded by a lipid rich insulating layer, so it appears white called myelin sheath. Myelin sheath is an electrical insulator, its purpose is to speed the transmission of nerve impulse. A neuron having myelin sheath means faster conduction, faster transmission and faster transfer of nerve impulses. Myelin sheath is interrupted at some places; these are called nodes of Ranvier, which is always constant in number. The space between two nodes of Ranvier is called internode. Speed of conduction of nerve impulse is faster in myelinated fiber. These nerves are more than 1µ in diameter.

✒NON-MYELINATED NERVE FIBRE: -

Nerve fibers without myelin sheath are, called non-myelinated nerve fibers. In non-myelinated nerve fibers, nodes of Ranvier are absent. They appear grey because of the absence of myelin sheath: post ganglionic sympathetic nerves are non-modulated. The nerve fibers, which carry nerve impulse from receptor organs to the central nervous system are, called afferent nerve fibers or sensory neurons. The nerve fibers, which carry nerve impulse from the central nervous system to effecter organs are, called efferent nerve fibers or motor neurons. The nerve fiber that contains, both afferent and efferent axons and thus conduct both incoming sensory information and outgoing muscle commands in the same bundle.

Types of Neurons 

The nerve cells have been classified either on the basis of their structure or function as follows: - 

✅Functional type of Neurons: On the basis of their functional or their physiological properties, the neurons may be divided into following types: -

✔Sensory or afferent neurons: These neurons carry a stimulus from the peripheral or visceral receptors to the central nervous system.

✔Motor or Efferent Neurons: These neurons carry impulses away from CNS to the 

effectors like muscles and glands. 

✔Neurosecretory neurons: These cells are specialized for producing neurohormones. 

✔Internuncial neurons: These are generally located in the central nervous system and serve to link the sensory and motor neurons.

✅Structural classification of neurons 

✔Multipolar Neurons: -

These neurons have an axon and many dendrites. Examples of multipolar neurons are motor neurons and inter neurons found in brain and spinal cord. 

 ✔Bipolar Neurons: - 

When one axon and one dendrite arise from one soma i.e. they have an axon and a dendrite. The bipolar neurons are found in retina and in the ganglia of VIII nerve. 

✔Unipolar or Pseudo unipolar Neurons: -

All developing neuroblast cells pass through a stage in which they have only one process the axon. During the later development it may be divided into two processes, one terminating in the central nervous system while another moves out. The unipolar neurons are found in the posterior roots of spinal nerves, in the mesencephalic nucleus of V cranial nerve and in the nerve roots of IX and X cranial nerve. 

✔Non-polar Neurons: -  

When many processes arise from the soma and all are equal without any distinction of axon or dendron, the neuron is called non-polar neuron. These nerve cells are unpolarized and cannot be differentiated into dendrites and axon. In non-polar neurons, nerve impulse can be conducted in any direction. Non-polar neurons are found in nerve net of coelenterates.

NEUROTRANSMITTERS 

Neurotransmitters are the endogenous chemical messengers that enable neurotransmission liberated at the nerve ending. They transmit signals across a chemical synapse from one neuron (nerve cell) to another target neuron a, muscle cell, or a gland cell. They play an important role in shaping everyday life and functions. 

✅Types of neurotransmitters: -

There are about 40 neurotransmitters and they have been classified into two types: - 

✔Rapidly acting neurotransmitters

✔Slowly acting neurotransmitters 

✅Rapidly acting neurotransmitters are small molecules and cause acute response. 

 It includes: -

i) Acetylcholine 

ii) Amines: Norepinephrine, epinephrine (Catecholamines), Dopamine, Serotonin.

iii) Amino acids like GABA (glycine and gamma amino butyric acid), glutamate, aspirate 

✅Slowly acting neurotransmitters are neuropeptides having prolonged effect: - 

It includes: -

✔Neuroactive peptides which are releasing hormones secreted by hypothalamus, for eg. TRH, LH, somatostatin 

✔Pituitary peptides like ACTH, vasopressin, oxytocin, endorphins 

✔ Peptides acting on gut and brain like leucin, methionine, cholecystokinin, neurotensin, insulin, and glucagon. 

✔ Neuropeptides from other tissues like angiotensin-II, bradykinin, and bombesin.