Like any other cell in the body, neuron has nucleus and all the organelles in the cytoplasm.

However, neuron is different from other cells by two ways:

1. Neuron has branches or processes called axon and dendrites.
2. Neuron has no centrosome. So, it cannot undergo division.

Classification of Neuron

Neurons are classified by three methods:

I. Classification Depending Upon Number of Poles

Based on number of poles from which nerve fibers arise, neurons are divided into three types:

1. Unipolar neurons have only one pole from which both the axon and dendrite arise.
2. Bipolar neurons have two poles. Axon arises from one pole and dendrites arise from the other pole.
3. Multipolar neurons have many poles. One of the poles gives rise to axon and all other poles give rise to dendrites.

II. Classification Depending Upon Function

Based on function, nerve cells are classified into two types:

1. Motor or efferent neurons carry the motor impulses from central nervous system to peripheral effector organs such as muscles, glands and blood vessels.
2. Sensory or afferent neurons carry the sensory impulses from periphery to central nervous system.

III. Classification Depending Upon Length of Axon

Depending upon length of axon, neurons are divided into two types:

1. Golgi type I neurons have long axons. Cell bodies of Golgi type I neurons are in central nervous system and their axons leave central nervous system and reach the remote peripheral organs.
2. Golgi type II neurons have short axons. Golgi type II neurons are present in large numbers in cerebral cortex and cerebellar cortex.

Each neuron is made up of three parts:

1. Nerve cell body
2. Dendrite
3. Axon

Dendrite and axon together form the processes of neuron.

In general, dendrites are short processes and axons are long processes. Dendrites and axons are usually called nerve fibers.

1. Nerve Cell Body

Nerve cell body is also known as soma or perikaryon.

It is irregular in shape and is constituted by a mass of cytoplasm called neuroplasm which is covered by a cell membrane.

Neuroplasm contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus.

Nissl bodies, neurofibrils, mitochondria and Golgi apparatus are found only in the nerve cell body and not in other cells.

Centrioles are absent in nerve cells. So, nerve cells cannot multiply with other cells of the body.

Nucleus

Each neuron has one nucleus which is centrally placed in the cell body.

Nucleus has one or two prominent nucleoli.

Nissl Bodies or Nissl Granules

Nissl bodies are small basophilic granules found in neuroplasm of neurons and are named after the discoverer Nissl.

Nissl bodies are present in the soma except axon hillock.

These bodies are responsible for the spotted or tigroid appearance of soma after suitable staining.

Nissl bodies flow into the dendrites from soma but not into axon.

So, Nissl bodies are distinguished from axons by the presence of Nissl bodies under microscope.

Nissl bodies are membranous organelles containing ribosomes.

So, Nissl bodies are concerned with synthesis of proteins in the soma of neurons.

Proteins formed in soma are transported to the axon through axonal flow.

Neurofibrils

Neurofibrils are thread-like structures present in the form of network in soma and nerve processes.

Presence of neurofibrils is another characteristic feature of neurons.

Mitochondria

Mitochondria are present in the soma and in axon.

As in other cells, mitochondria form the powerhouse of nerve cell, where ATP is produced.

Golgi Apparatus

Golgi apparatus of nerve cell body is similar to that of other cells.

It is concerned with processing and packaging of proteins into granules.

2. Dendrite of Neuron

Dendrite is a branched process of neuron and it is branched repeatedly.

Dendrite may be present or absent.

If present, it may be one or many in number.

Dendrite has Nissl bodies and neurofibrils.

Dendrite is conductive in nature.

It transmits impulses towards the nerve cell body.

3. Axon of Neuron

Axon is longer than dendrite.

Each neuron has only one axon.

Axon arises from axon hillock of the nerve cell body.

Axon extends for a long distance away from the nerve cell body.

Length of the longest axon is about 1 meter.

Organization of Nerve

Many axons together form a bundle called fasciculus.

Many fasciculi together form a nerve.

Coverings of Nerve

Whole nerve is covered by a tubular sheath called epineurium.

Each fasciculus is covered by perineurium.

Each nerve fiber (axon) is covered by endoneurium.

Internal Structure of Axon: Axis Cylinder

Axon has a long central core of cytoplasm called axoplasm.

Axoplasm is covered by a tubular sheath-like membrane called axolemma, which is the continuation of cell membrane of nerve cell body.

Axoplasm along with axolemma is called the axis cylinder of nerve fiber.

Axoplasm contains mitochondria, neurofibrils and cytoplasmic vesicles.

But Nissl bodies are absent in the axon.

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

Non-myelinated Nerve Fiber

Nerve fiber described above is the non-myelinated nerve fiber which is not covered by myelin sheath.

Myelinated Nerve Fiber

Nerve fibers which are insulated by myelin sheath are called myelinated nerve fibers.

Myelin Sheath

Myelin sheath is a thick lipoprotein sheath that insulates the myelinated nerve fiber.

Myelin sheath is not a continuous sheath.

It is absent at regular intervals.

The area where 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 color of nerve fibers.

Chemistry of Myelin Sheath

Myelin sheath is formed by concentric layers of proteins alternating with lipids.

Lipids in myelin sheath are cholesterol, lecithin and cerebroside (sphingomyelin).

Formation of Myelin Sheath: Myelinogenesis

Formation of myelin sheath around axon is called myelinogenesis.

It is formed by Schwann cells in neurilemma.

Functions of Myelin Sheath

1. Faster conduction: Myelin sheath is responsible for faster conduction of impulse through the nerve fibers. In myelinated nerve fibers, the impulses jump from one node to another node by saltatory conduction.
2. Insulating capacity: Myelin sheath has a high insulating capacity. Because of this quality, myelin sheath restricts the nerve impulse within single nerve fiber and prevents stimulation of neighboring nerve fibers.

Neurilemma or Sheath of Schwann

Neurilemma is a thin membrane which surrounds the axis cylinder.

It is also called neurilemmal sheath or sheath of Schwann.

It contains Schwann cells, which have flattened and elongated nuclei.

Cytoplasm of Schwann cells is thin and modified to form a thin sheath of neurilemma.

One nucleus is present in each internode of the axon.

Nucleus is situated between myelin sheath and neurilemma.

In non-myelinated nerve fiber, neurilemma surrounds the axolemma continuously.

In myelinated nerve fiber, it covers the myelin sheath.

At the node of Ranvier, where myelin sheath is absent, the neurilemma invaginates and runs up to axolemma in the form of a finger-like process.

Functions of Neurilemma

In non-myelinated nerve fiber, the neurilemma serves as a covering membrane.

In myelinated nerve fiber, it is necessary for myelinogenesis (formation of myelin sheath).

Source of Secretion of Neurotrophins

Neurotrophins are secreted by many tissues in the body, particularly muscles, neurons and neuroglial cells such as astrocytes.

Functions of Neurotrophins

1. Facilitate initial growth and development of nerve cells in central and peripheral nervous system.
2. Promote survival and repair of the nerve cells.
3. Play an important role in the maintenance of nervous tissue and neural transmission.

Commercial preparations of neurotrophins are used for the treatment of neural diseases.

Types of Neurotrophins

Many types of neurotrophic factors are identified.

Nerve Growth Factor

Nerve growth factor (NGF) is found in many tissues.

It promotes early growth and development of neurons.

Commercial preparation of NGF extracted from animals is used to treat many nervous disorders such as Alzheimer’s disease, neuron degeneration in aging and neuron regeneration in spinal cord injury.

Other Neurotrophins

1. Brain-derived neurotrophic factor (BDNF) found in brain and human sperm.
2. Ciliary neurotrophic factor (CNTF).
3. Glial cell line-derived neurotrophic factor (GDNF).
4. Fibroblast growth factor (FGF).
5. Neurotrophin-3, 4 and 5 (NT-3, NT-4 and NT-5).

Nerve fibers are classified by six methods.

I. Classification Depending Upon Structure

Based on structure, nerve fibers are classified into two types:

1. Myelinated nerve fibers which are covered by myelin sheath.
2. Non-myelinated nerve fibers which are not covered by myelin sheath.

II. Classification Depending Upon Distribution

Nerve fibers are classified into two types depending upon distribution:

1. Somatic nerve fibers that supply skeletal muscles of the body.
2. Visceral or autonomic nerve fibers that supply internal organs of the body.

III. Classification Depending Upon Origin

Based on origin, nerve fibers are divided into two types:

1. Cranial nerves arising from brain.
2. Spinal nerves arising from spinal cord.

IV. Classification Depending Upon Function

Based on functions, nerve fibers are classified into two types:

1. Sensory or afferent nerve fibers carry sensory impulses from different parts of the body to central nervous system.
2. Motor or efferent nerve fibers carry motor impulses from central nervous system to different parts of the body.

V. Classification Depending Upon Secretion of Neurotransmitter

Depending upon neurotransmitter substance secreted, nerve fibers are divided into two types:

1. Adrenergic nerve fibers that secrete noradrenaline.
2. Cholinergic nerve fibers that secrete acetylcholine.

VI. Classification Depending Upon Diameter and Conduction of Impulse: Erlanger-Gasser Classification

Erlanger and Gasser classified nerve fibers into three major types based on diameter of fibers and rate of conduction of impulses:

1. Type A nerve fibers
2. Type B nerve fibers
3. Type C nerve fibers

Among all the fibers, type A nerve fibers are the thickest fibers and type C nerve fibers are the thinnest fibers.

Type A nerve fibers are divided into subtypes.

Except C’ fibers, all the nerve fibers are myelinated.

Velocity of impulse through a nerve fiber is directly proportional to the thickness of the fibers.

Types of Nerve Fibers

Excitability

Stimulus is defined as an external agent, which produces excitability in the tissues.

When the nerve fiber is stimulated, action potential develops.

Response to Stimulus

When a nerve fiber is stimulated, based on strength of stimulus, two types of responses develop:

1. Action potential or nerve impulse.
2. Electrotonic potential or local potential.

Action Potential or Nerve Impulse

Action potential or nerve impulse develops in a nerve fiber when it is stimulated by a stimulus with adequate strength.

Adequate strength of stimulus, necessary for producing action potential in a nerve fiber, is known as threshold or minimal stimulus.

Action potential is propagated.

Action potential in a nerve fiber is similar to that in a skeletal muscle, except for some minor differences.

Electrical Values

Electrotonic Potential or Local Potential

When a stimulus with subliminal strength is applied, only electrotonic potential develops and the action potential does not develop.

Electrotonic potential is non-propagated.

Conductivity

Nerve impulse is transmitted through nerve fiber in the form of action potential.

Normally, action potential is transmitted through the nerve fiber in only one direction.

However, in experimental conditions, when a nerve is stimulated, action potential travels through the nerve fiber in either direction.

Mechanism of Conduction of Action Potential

Depolarization occurs first at the site of stimulation in nerve fiber.

This causes depolarization of the neighboring areas.

Like this, depolarization travels throughout the nerve fiber.

Depolarization is followed by repolarization.

Conduction Through Myelinated Nerve Fiber: Saltatory Conduction

Saltatory conduction is a form of conduction of nerve impulse by which the impulse jumps from one node to another.

Conduction of impulse through a myelinated nerve fiber is about 50 times faster than through a nonmyelinated fiber.

It is because action potential jumps from one node to another node of Ranvier instead of travelling through the entire nerve fiber.

Mechanism of Saltatory Conduction

Myelin sheath is not permeable to ions.

So, entry of sodium from extracellular fluid into nerve fiber occurs only in the node of Ranvier, where myelin sheath is absent.

It causes depolarization in node, and not in the internode.

Thus, depolarization occurs at intermediate nodes.

So, action potential jumps from one node to another.

This is called saltatory conduction (saltare = jumping).

Refractory Period

Refractory period is of two types:

1. Absolute refractory period
2. Relative refractory period

Absolute Refractory Period

Absolute refractory period is the period during which nerve does not show any response at all, whatever may be the strength of stimulus.

Relative Refractory Period

It is the period during which nerve fiber shows response, if strength of stimulus is increased to maximum.

Absolute refractory period corresponds to a period from the time when firing level is reached till the time when 1/3 of repolarization is completed.

Relative refractory period extends through rest of repolarization period.

Summation

One subliminal stimulus does not produce any response in the nerve fiber because subliminal stimulus is very weak.

However, if two or more subliminal stimuli are applied within a short interval of about 0.5 msec, the response is produced.

It is because subliminal stimuli are summed up together to become strong enough to produce the response.

This phenomenon is known as summation.

Adaptation

While stimulating a nerve fiber continuously, excitability of the nerve fiber increases in the beginning.

Later the response decreases slowly and finally the nerve fiber does not show any response at all.

This phenomenon is known as adaptation or accommodation.

Causes for Adaptation

1. When a nerve fiber is stimulated continuously, depolarization occurs continuously.
2. Continuous depolarization inactivates the sodium pump and increases the efflux of potassium ions.

Infatigability

A nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time.

The reason for this is the nerve fiber can conduct only one action potential at a time.

At that time, it is completely refractory and does not conduct another action potential.

All-or-None Law

All-or-none law states that when a nerve is stimulated by a stimulus, it gives maximum response or does not give any response at all.

Causes for Injury

1. Obstruction of blood flow.
2. Local injection of toxic substances.
3. Crushing of nerve fibers.
4. Transection of nerve fiber.

Degrees of Injury

Sunderland had classified injury to nerve fibers into five types based on order of severity.

First Degree Injury

First degree injury is very common type of injury.

It is caused by pressure applied over a nerve for a short period leading to occlusion of blood flow and hypoxia.

Axon loses the function temporarily for a short time, which is called conduction block.

The function returns within four hours to few weeks.

First degree injury is called Seddon neuropraxia.

Second Degree Injury

Second degree is due to prolonged severe pressure, which causes Wallerian degeneration.

However, the endoneurium is intact.

Repair and restoration of function take about 18 months.

Second degree of injury is called axonotmesis.

Third Degree Injury

In this case, the endoneurium is interrupted.

Epineurium and perineurium are intact.

After degeneration, the recovery is slow and poor or incomplete.

Third, fourth and fifth degrees of injury are called neurotmesis.

Fourth Degree Injury

This type of injury is more severe.

Epineurium and perineurium are also interrupted.

Fasciculi of nerve fibers are disturbed and disorganized.

Regeneration is poor or incomplete.

Fifth Degree Injury

Fifth degree of injury involves complete transection of the nerve trunk with loss of continuity.

Useful regeneration is not possible unless the cut ends are rearranged and approximated by surgery.

Degenerative Changes in Neuron

Degeneration means deterioration or impairment or pathological changes of an injured tissue.

When a peripheral nerve fiber is injured, degenerative changes occur in the nerve cell body and nerve fiber of same neuron.

Accordingly, degenerative changes are classified into three types:

1. Wallerian degeneration.
2. Retrograde degeneration.
3. Transneuronal degeneration.

Wallerian Degeneration

Wallerian or orthograde degeneration is the pathological change that occurs in distal cut end of nerve fiber (axon).

It is named after discoverer Dr. Waller.

Wallerian degeneration starts within 24 hours of injury.

Change occurs throughout the length of distal part of nerve fiber simultaneously.

Changes in Nerve Fiber

1. Axis cylinder swells and breaks up into small pieces. After few days, broken pieces appear as debris in the space occupied by axis cylinder.
2. Myelin sheath is slowly disintegrated into fat droplets. Changes in myelin sheath occur from 8th to 35th day.
3. Neurilemmal sheath is unaffected, but Schwann cells multiply rapidly. Macrophages invade from outside. Macrophages remove debris of axis cylinder and fat droplets of disintegrated myelin sheath.
4. Now neurilemmal tube becomes empty. Later it is filled by the cytoplasm of Schwann cell.

Above changes take place for about 2 months from the day of injury.

Retrograde Degeneration

Retrograde degeneration is the pathological changes which occur in nerve cell body and axon proximal to cut end.

Changes in Nerve Cell Body

Changes in nerve cell body commence within 48 hours after section of nerve.

1. Nissl bodies disintegrate into fragments by chromatolysis.
2. Golgi apparatus is disintegrated.
3. Nerve cell body swells due to accumulation of fluid and becomes round.
4. Neurofibrils disappear followed by displacement of the nucleus towards the periphery.
5. Sometimes nucleus is extruded out of the cell. In this case, death of neuron occurs and regeneration of the injured nerve is not possible.

Changes in Axon Proximal to Cut End

In the axon, changes occur only up to first node of Ranvier from site of injury.

Degenerative changes that occur in proximal cut end of axon are similar to those changes occurring in distal cut end of the nerve fiber.

Transneuronal Degeneration

If an afferent nerve fiber is cut, degenerative changes can occur in another neuron with which the afferent nerve fiber synapses.

It is called transneuronal degeneration.

Injured and degenerated nerve fiber can regenerate.

It starts as early as 4th day after injury, becomes more effective only after 30 days and is completed in about 80 days.

Criteria for Regeneration

Regeneration is possible only if certain criteria are fulfilled by degenerated nerve fiber:

1. Gap between cut ends of the nerve should not exceed 3 mm.
2. Neurilemma should be present.
3. Nucleus must be intact.
4. Two cut ends should remain in the same line.

Stages of Regeneration

1. First, some pseudopodia-like extensions grow from proximal cut end of the nerve. Such extensions are called fibrils or regenerative sprouts. Number of fibrils is up to 100.
2. Fibrils move towards distal cut end of the nerve fiber.
3. Some fibrils enter the neurilemmal tube of distal end and form axis cylinder.
4. Schwann cells line up in neurilemmal tube and actually guide the fibrils into the tube. Schwann cells also synthesize nerve growth factors, which attract the fibrils from proximal segment.
5. Axis cylinder is fully established inside the neurilemmal tube. Above processes are completed in about 3 months after injury.
6. Myelin sheath is formed by Schwann cells slowly. Myelination is completed in 1 year.
7. Diameter of the nerve fiber gradually increases. However, the regenerated nerve fiber has only 80% of original diameter. Newly formed internodes are also shorter than the original ones.
8. In the nerve cell body, first Nissl bodies appear followed by Golgi apparatus.
9. Cell loses the excess fluid and nucleus occupies the central position.

Though anatomical regeneration occurs in the nerve, functional recovery occurs after a long period.

Classification of Neuroglial Cells

Neuroglial cells are distributed in central nervous system (CNS) as well as peripheral nervous system (PNS).

Central Neuroglial Cells

Neuroglial cells in CNS are of three types:

1. Astrocytes
2. Microglia
3. Oligodendrocytes

1. Astrocytes

Astrocytes are star-shaped neuroglial cells present in brain.

Astrocytes are of two types namely:

1. Fibrous astrocytes
2. Protoplasmic astrocytes

2. Microglia

Microglia are the smallest neuroglial cells and are derived from monocytes.

These phagocytic cells are often called the macrophages of CNS.

3. Oligodendrocytes

Oligodendrocytes or oligodendroglia are the neuroglial cells which produce myelin sheath around nerve fibers in CNS.

Peripheral Neuroglial Cells

Neuroglial cells in PNS are of two types:

1. Schwann cells which are the major glial cells present in PNS.
2. Satellite cells which are the glial cells present on the exterior surface of neurons in peripheral nervous system.

Functions of Neuroglial Cells

Neuron

• Unipolar
• Bipolar
• Multipolar
• Motor or efferent
• Sensory or afferent
• Golgi type I
• Golgi type II

Nerve Fibers

• Myelinated
• Non-myelinated
• Somatic
• Visceral or autonomic
• Cranial
• Spinal
• Sensory or afferent
• Motor or efferent
• Adrenergic
• Cholinergic
• Type A
• Type B
• Type C

Degeneration

• Wallerian degeneration
• Retrograde degeneration
• Transneuronal degeneration

Neuroglia

• Astrocytes
• Microglia
• Oligodendrocytes
• Schwann cells
• Satellite cells