Neuron Structure And Classification Notes

Neuron Introduction

Neuron is defined as the structural and functional unit of the nervous system. It is otherwise called nerve cell. The neuron is like any other cell in the body having a nucleus and all the organelles in the 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.

Read And Learn More: Medical Physiology Notes

Classification Of Neuron

The 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 the axon.

 

1. 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.
      1. Unipolar Neurons: Unipolar neurons are neurons that have only one pole. From the single pole, both the processes – axon and dendrite arise. This type of nerve cells is present only in embryonic stage in human beings.
      2. Bipolar Neurons: The neurons with two poles are known as bipolar neurons. Axon arises from one pole and dendrites arise from the other pole.
      3. Multipolar neurons: Multipolar neurons are neurons that have many roles. One of the poles gives rise to the axon and, all ie other poles give rise to dendrites.
2. Depending Upon The Function: On the basis of function, the nerve cells are classified into two types:
   1. Motor neurons
   2. Sensory neurons.
      1. Motor Neurons: Motor neurons are the neurons that carry the motor impulses from central nervous system to the peripheral effector organs like muscles, glands, blood vessels, etc, Motor neurons are also known as efferent nerve cells. Generally, these neurons have long axons and arid short dendrites.
      2. Sensory Neurons: Sensory neurons are also called afferent nerve cells. These neurons carry the sensory impulses from the periphery to the central nervous system. Generally, the sensory neurons have short axon and long dendrites.
3. Depending Upon The Length Of Axon: Depending upon the length of the axon, neurons are divided into two types:
   1. Golgi type 1 neuron
   2. Golgi type 2 neurons.
      1. Gotgi Type 1 Neurons: Golgi type 1 neurons have long axons. The cell body of these neurons is in the central nervous system and their axons reach the remote peripheral organs.
      2. Golgi Type 2 Neurons: The neurons of this type have short axons. These neurons are present in cerebral cortex and spinal cord.

Structure Of Neuron

Structurally the neuron differs from other cells in the body. Each neuron is made up of three parts:

1. Nerve cell body
2. Dendrite
3. Axon.

Trie dendrite and axon together form the processes of neurons. In general, the dendrites are short processes and the axons are long processes. The dendrites and axons are usually called nerve fibers.

1. Nerve Cell Body: The nerve cell body is also known as soma or perikaryon. The nerve cell body is irregular in shape and, like any other cell 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.
   1. Nucleus: Each neuron has one nucleus which is centrally placed in the nerve cell body. The nucleus has one or two prominent nucleoli. The nucleus does not contain a centrosome. So, the nerve cell cannot multiply like the other cells.
   2. Nissl Bodies: Nissl bodies or Nissl granules are small basophilic granules found in the cytoplasm of neurons and are named after the discoverer. These bodies are present throughout the soma except in axon hillock.
      • Nissl bodies are called tigroid substances since these bodies are responsible for the tigroid or spotted appearance of soma after suitable staining.
      • The Nissl granules flow into the dendrites from soma, but not into the axon. So, the axons are distinguished from the dendrites under a microscope by the presence of Nissl granules.
      • The Nissl bodies are membranous organelles containing ribosomes. So, these bodies are concerned with the synthesis of proteins in the neurons. The proteins formed in the soma are transported to the axon by the axonal flow.
      • The 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. The granules reappear after recovery from fatigue or after the regeneration of nerve fibers.
   3. Neurofibrils: Neurofibrils are thread-like structures present in the form of a network in the soma and the nerve processes. The presence of neurofibrils is another characteristic feature of the neurons. The neurofibrils consist of microfilaments and microtubules.
   4. Mitochondria: The mitochondria are present in the soma and in the axon. The mitochondria form the powerhouse of the nerve cell, where ATP is produced.
   5. Golgi Apparatus: The Golgi apparatus of the nerve cell body is similar to that of other cells. It is concerned with processing and packaging proteins into granules.
2. Dendrite:
   • The dendrite is the branched process of the neuron and it is branched repeatedly. The dendrite may be absent or if present, it may be one or many in number. The dendrite has Nissl granules and neurofibrils.
   • Dendrite is conductive in nature. It transmits impulses toward the nerve cell body. Usually, the dendrite is shorter than the axon.
3. Axon: The axon is the longer process of the nerve cell. Each neuron has only one axon. The axon arises from the axon hillock of the nerve cell body and it is devoid of Nissl granules. The axon extends for a long distance away from the nerve cell body. The length of the longest axon is about one meter.
   • Organization of Nerve: Many axons together form a bundle called fasciculus. Many fasciculi together form a nerve. The whole nerve is covered by a tubular sheath, which is formed by a 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
     • The axon has a long central core of the cytoplasm called axoplasm. The axoplasm is covered by the tubular sheath-like membrane called axolemma which is the continuation of the cell membrane of the nerve cell body. The axoplasm along with the axolemma is called the axis cylinder of the nerve fiber.
     • Axoplasm contains mitochondria, neurofibrils, and axoplasmic vesicles. But, Nissl bodies are absent in the axon. So, the proteins necessary for the nerve fibers are synthesized in the soma and not in the axoplasm.
     • After synthesis, the protein molecules are transmitted from the soma to the axon by means of axonal flow. Some of the neurotransmitter substances are also carried by axonal flow from soma to axon.
     • The axis cylinder of the nerve fiber is covered by a membrane called a neurilemma.
   • Nonmyelinated Nerve Fiber: The nerve fiber described above is the nonmyelinated nerve fiber which is not covered by myelin sheath. The 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 the myelin sheath is absent is called the node of Ranvier. The segment of the nerve fiber between two nodes is called the internode. Myelin sheath is responsible for the white color of the 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
    • The formation of myelin sheath around the axon is called myelinogenesis. It is formed by Schwann cells in a neurilemma. In the peripheral nerve, 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.
    • The Schwann cells wrap up and rotate around the axis cylinder in many concentric layers. The concentric layers fuse to produce the myelin sheath but the cytoplasm of the cells is not deposited. The outermost membrane of the Schwann cell remains as a neurilemma.
    • The nucleus of these cells remains in between the myelin sheath and neurilemma.
  • Functions of Myelin Sheath
    1. Faster conduction: Myelin sheath is responsible for faster conduction of impulses through the nerve fibers. In the 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, the myelin sheath restricts the nerve impulse within the single nerve fiber and prevents the stimulation of neighboring nerve fibers.

Neurilemma

• A neurilemma is a thin membrane that surrounds the axis cylinder. It is also called neurilemmal sheath or sheath of Schwann. It contains Schwann cells, which have flattened and elongated nuclei. The cytoplasm is thin and modified to form the thin sheath of neurilemma.
• One nucleus is present in each internode of the axon. The nucleus is situated between the myelin sheath and neurilemma.
• In nonmyelinated nerve fiber, the neurilemma continuously surrounds the axolemma. In myelinated nerve fiber, it covers the myelin sheath. At the node of Ranvier (where the myelin sheath is absent), the neurilemma invaginates and runs up to the axolemma in the form of a finger-like process.
• Functions of Neurilemma: In nonmyelinated nerve fiber, the neurilemma serves as a covering membrane. In myelinated nerve fiber, it is necessary for the formation of the myelin sheath (myelinogenesis). Neurilemma is absent in the central nervous system. So, the neuroglial cells called oligodendroglia are responsible for myelinogenesis in the central nervous system.

Neurotrophic Factors

Neurotrophins or neurotrophic factors are the protein substances, that play an important role in the growth and functioning of nervous tissue.

• Source of Secretion: Neurotrophins are secreted by many tissues in the body particularly muscles, neuroglial cells called astrocytes, and neurons.
• Functions
  • 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.
       • Recently, it is found that the neurotrophins are capable of making the damaged neurons regrow their processes in vitro and in animal models.
       • This indicates the possibility of reversing the devastating symptoms of nervous disorders like Parkinson’s disease and Alzheimer’s disease. Commercial preparations of neurotrophins are used for the treatment of some neural diseases.
  • Mode of Action: Neurotrophin receptors are mostly situated on the nerve terminals and nerve cell body. Neurotrophins act by binding with receptors and initiating the phosphorylation of tyrosine kinase.
  • Neurotrophin Types: Nerve growth factor (NGF) was the first protein substance identified as neurotrophin. Now many neurotoxic factors have been identified.

Nerve Growth Factor: Nerve growth factor (NGF) is a neurotrophin found in many peripheral tissues.

• Chemistry: It is a peptide with 118 amino acids. Each molecule of NGF is made up of two α subunits, two β subunits, and two γ subunits. Only the β subunits have nerve growth-stimulating activity.
• Functions
  • NGF promotes the early growth and development of neurons. Its major action is on sympathetic and sensory neurons, particularly the neurons concerned with pain. Because of its major action on sympathetic neurons, it is also called sympathetic NGF. NGF also promotes the growth of cholinergic neurons in the cerebral hemispheres.
  • The commercial preparation of NGF extracted from snake venom and submaxillary glands of male mice is used to treat sympathetic neuron diseases.
  • NGF plays an important role in treating 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 Growth Factor:
   • Brain-derived neurotrophic growth factor (BDGF) was first discovered in the brains of pigs. Now it is found in the human brain and human sperm. BDGF promotes the survival of sensory and motor neurons arising from embryonic neural crest.
   • It also protects the sensory neurons in the peripheral nervous system and motor neurons of the pyramidal system. It enhances the growth of cholinergic, dopaminergic, and optic nerves. It is suggested that BDGF may regulate synaptic transmission. Commercial preparation is used for motor neuron diseases.
2. Ciliary Neurotrophic Factor (CNTF): It is present in peripheral nerves, ocular muscles, and cardiac muscle. It protects neurons of the ciliary ganglion and motor neurons.
3. Glial Cell Line Derived Neurotrophic Factor (GNDF): GDNF is found in neuroglial cells. It has a potent protective action on dopaminergic neurons. It is used for the treatment of Parkinson’s disease.
4. Fibroblast Growth Factor (FGF): FGF was first discovered as the growth factor promoting fibroblastic growth. Recently it is known to protect the neurons.
5. Neurotrophin-3 (NT-3): Neurotrophin-3 (NT-3) acts on γ motor neurons, sympathetic neurons, and neurons from sensory organs. It also regulates neurotransmitter release at the neuromuscular junction.
   • NT-3 is useful for the treatment of motor axonal neuropathy, and diabetic neuropathy.
   • Recently, a few more substances belonging to the neurotrophin family such as NT-4, NT-5, and leukemia-inhibiting factor are identified. NT-4 and NT-5 act on sympathetic neurons, sensory neurons, and motor neurons.