INTRODUCTION
Neuron or nerve cell is defined as the structural and functional unit of nervous system. Neuron is similar to
any other cell in the body, having nucleus and all the organelles in cytoplasm. However, it is different from
other cells by two ways:
1. Neuron has branches or processes called axon and dendrites
2. Neuron does not have centrosome. So, it cannot undergo division.
CLASSIFICATION OF NEURON
Neurons are classified by three different methods.
1. Depending upon the number of poles
2. Depending upon the function
3. Depending upon the length of axon.
DEPENDING UPON THE NUMBER OF POLES
Based on the number of poles from which the nerve fibers arise, neurons are divided into three types:
1. Unipolar neurons
2. Bipolar neurons
3. Multipolar neurons.
Unipolar Neurons
Unipolar neurons are the neurons that have only one pole. From a single pole, both axon and dendrite arise
(Fig. 134.1). This type of nerve cells is present only in embryonic stage in human beings.
Bipolar Neurons
Neurons with two poles are known as bipolar neurons. Axon arises from one pole and dendrites arise from the
other pole.
Multipolar Neurons
Multipolar neurons are the neurons which have many poles. One of the poles gives rise to axon and all other
poles give rise to dendrites.
DEPENDING UPON THE FUNCTION
On the basis of function, nerve cells are classified into two types:
· Motor or efferent neurons
· Sensory or afferent neurons.
1. Motor or Efferent Neurons
Motor or efferent neurons are the neurons which carry the motor impulses from central nervous system to
peripheral effector organs like muscles, glands, blood vessels, etc. Generally, each motor neuron has a long
axon and short dendrites.
2. Sensory or Afferent Neurons
Sensory or afferent neurons are the neurons which carry the sensory impulses from periphery to central
nervous system. Generally, each sensory neuron has a short axon and long dendrites.
DEPENDING UPON THE LENGTH OF AXON
Depending upon the length of axon, neurons are divided into two types:
1. Golgi type I neurons
2. Golgi type II neurons.
Golgi Type I Neurons
Golgi type I neurons have long axons. Cell body of these neurons is in different parts of central nervous system
and their axons reach the remote peripheral organs.
Golgi Type II Neurons
Neurons of this type have short axons. These neurons are present in cerebral cortex and spinal cord.
STRUCTURE OF NEURON
Neuron is made up of three parts:
1. Nerve cell body
2. Dendrite
3. Axon.
Dendrite and axon form the processes of neuron. Dendrites are short processes and the
axons are long processes. Dendrites and axons are usually called nerve fibers.
NERVE CELL BODY
Nerve cell body is also known as soma or perikaryon . It is irregular in shape. Like any other cell, it is constituted
by a mass of cytoplasm called neuroplasm, which is covered by a cell membrane. The cytoplasm contains
a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus. Nissl bodies and neurofibrils are
found only in nerve cell and not in other cells.
Nucleus
Each neuron has one nucleus, which is centrally placed in the nerve cell body. Nucleus has one or two prominent
nucleoli. Nucleus does not contain centrosome. So, the nerve cell cannot multiply like other cells.
Nissl Bodies
Nissl bodies or Nissl granules are small basophilic granules found in cytoplasm of neurons and are named
after the discoverer. These bodies are present in soma and dendrite but not in axon and axon hillock.
Nissl bodies are called tigroid substances, since these bodies are responsible for tigroid or spotted
appearance of soma after suitable staining. Dendrites are distinguished from axons by the presence of Nissl
granules under microscope. Nissl bodies are membranous organelles containing ribosomes. So, these bodies are concerned with
synthesis of proteins in the neurons. Proteins formed in soma are transported to the axon by axonal flow.
Number of Nissl bodies varies with the condition of the nerve. During fatigue or injury of the neuron, these
bodies fragment and disappear by a process called chromatolysis . Granules reappear after recovery from
fatigue or after regeneration of nerve fibers.
Neurofibrils
Neurofibrils are thread-like structures present in the form of network in the soma and the nerve processes.
Presence of neurofibrils is another characteristic feature of the neurons. The neurofibrils consist of microfilaments
and microtubules.
Mitochondria
Mitochondria are present in soma and in axon. As in other cells, here also mitochondria form the powerhouse
of the 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 packing
of proteins into granules.
DENDRITE
Dendrite is the 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 granules and neurofibrils.
Dendrite transmits impulses towards the nerve cell body. Usually, the dendrite is shorter than axon.
AXON
Axon is the longer process of nerve cell. Each neuron has only one axon. Axon arises from axon hillock of the
nerve cell body and it is devoid of Nissl granules. Axon extends for a long distance away from the nerve cell
body. Length of longest axon is about 1 meter. Axon transmits impulses away from the nerve cell
body.
Organization of Nerve
Each nerve is formed by many bundles or groups of nerve fibers. Each bundle of nerve fibers is called a
fasciculus .
Coverings of Nerve
The whole nerve is covered by tubular sheath, which is formed by a areolar membrane. This sheath is called
epineurium . Each fasciculus is covered by perineurium and 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 the tubular sheathlike
membrane called axolemma . Axolemma is the continuation of the cell membrane of nerve cell body.
Axoplasm along with axolemma is called the axis cylinder of the nerve fiber (Fig. 134.4).
Axoplasm contains mitochondria, neurofibrils and axoplasmic vesicles. Because of the absence of Nissl
bodies in the axon, proteins necessary for the nerve fibers are synthesized in the soma and not in axoplasm.
After synthesis, the protein molecules are transported from soma to axon, by means of axonal flow. Some
neurotransmitter substances are also transported by axonal flow from soma to 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 fiber which is insulated by myelin sheath is 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 . Segment of the 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. The lipids are cholesterol, lecithin
and cerebroside (sphingomyelin).
Formation of Myelin Sheath| Myelinogenesis
Formation of myelin sheath around the axon is called the myelinogenesis. It is formed by Schwann cells in
neurilemma. In the peripheral nerve, the myelinogenesis starts at 4th month of intrauterine life. It is completed
only in the second year after birth. Before myelinogenesis, Schwann cells of the neurilemma
are very close to axolemma, as in the case of unmyelinated nerve fiber. The membrane of the
Schwann cell is double layered. Schwann cells wrap up and rotate around the axis
cylinder in many concentric layers. The concentric layers fuse to produce myelin sheath but cytoplasm of the cells
is not deposited. Outermost membrane of Schwann cell remains as neurilemma. Nucleus of these cells remains
in between myelin sheath and 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. This type of transmission of impulses is called
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 the stimulation of neighboring nerve fibers.
NEURILEMMA
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 is thin
and modified to form the 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, the neurilemma surrounds axolemma continuously. In myelinated
nerve fiber, it covers the myelin sheath. At the node of Ranvier (where myelin sheath is absent), 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 the formation of myelin sheath (myelinogenesis). Neurilemma is absent in central
nervous system. So, the neuroglial cells called oligodendroglia are responsible for myelinogenesis in
central nervous system.
