Structure of Motor Neuron

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Introduction

Animals have a highly complicated system for communication among different parts of the body called the nervous system. This system is a complicated network of interlinked neurons. Neurons are the basic structural and functional units of the nervous system. They are responsible for carrying and processing information from different parts of the body.

Neurons are classified into three major classes based on their functions; interneurons, sensory neurons, and motor neurons. Motor neurons are the component of the peripheral nervous system that functions to carry the nerve impulses from the central nervous system to the peripheral body parts.

Some structural variations so exist among these three neuron types. In this article, we will discuss the various structural features of motor neurons and their different types.

Major Structural Components

The major structural components are common among the three neuron types. These include the following:

Cell body

It is the central part of the neuron containing nuclei and the majority of cytoplasm. It is also called perikaryon.

Cell Processes

These are the cellular processes that emerge or synapse on the cell body of the neuron. They are responsible for the transmission of information in the form of nerve impulses.

The two types of cellular processes found in motor neurons are:

  • Axons
  • Dendrites

Synapses

These are the joints among the neurons or between neurons and other cells. The nerve impulse is passed from one neuron to the next via these cellular connections.

We will have a detailed discussion on each of these structural components in the rest of the article.

Perikaryon (Cell Body)

Perikaryon or cell body is a part of motor neurons that contains the nucleus and other organelles as well as the majority of the cytoplasm. It is the part of neurons without any cellular processes. However, a large number of cell processes do converge on perikaryon and make synaptic contact with the motor neurons.

Some of the important organelles present in cell bodies of motor neurons are as follows.

Nucleus

The nucleus occupies the central position in the cell body of motor neurons. The majority of the motor neurons have a large spherical nucleus having a large and prominent nucleolus. The nucleus is pale-staining showing the euchromatic nature of chromatin. The fine chromatin threads can also be visualized. These properties of the nucleus reflect that the cell is highly synthetic. Proteins are continuously being made by these motor neurons.

Rough Endoplasmic Reticulum

Motor neurons are involved in a continuous process of protein synthesis. They have a highly developed system of rough endoplasmic reticulum for this purpose. It includes a large number of parallel cisternae of the reticulum associated with abundant polyribosomes. Polyribosome is a term for a group of ribosomes attached to a single copy of mRNA producing multiple polypeptide chains.

This system of endoplasmic reticulum indicates that the cells are involved in the active synthesis of proteins needed for cytoskeleton, transport, or secretion.

The regions of the cell body containing rough endoplasmic reticulum and associated polyribosomes appear to be highly eosinophilic upon H&E staining. These clumps of basophilic material are called Nissl substance or Nissl bodies. The amount of these basophilic bodies is abundant in highly functional motor neurons just close to the nucleus of the cell.

Golgi Apparatus

Golgi apparatus is responsible for the packaging of the proteins produced by Nissl bodies. The proteins may be packed in vacuoles for transport to other organelles or the extracellular fluid. The Golgi apparatus is present only in the cell body of motor neurons near rough endoplasmic reticulum.

Mitochondria

Mitochondria are needed to produce ATP for cellular processes occurring in the motor neurons. They are abundantly present in the cell body, although they might be present in any part of the neuron.

Cytoskeleton

The cytoskeletal framework of motor neurons is made up of intermediate filaments and microtubules. The intermediate filaments are abundant in the cell body as well as the cellular processes of the motor neuron. These intermediate filaments are called neurofilaments. The filament can be cross-linked on treatment with some fixatives. They appear as fibrils under the light microscope on silver staining.

The cytoskeleton is responsible for maintaining the shape and structure of the cell as well as cellular processes. It is also involved in intracellular transport of various substances.

Inclusion Bodies

Other than the major organelles, sometimes the inclusions of pigmented materials may also be found in the cell bodies of motor neurons. These are the residual bodies left after lysosomal degradation and are not as harmful as long as they don’t interfere with the cellular functions.

Dendrites

These are the cellular processes responsible for receiving signals from other neurons. The number of dendrites varies from cell to cell. Most of the motor neurons have a single dendrite arising from the cell body.

Dendrites are the sites of neurons that receive and process information coming from other cells. They show extensive branching and the diameter of each branch goes on decreasing as they subdivide.

Cytoplasm

The cytoplasm of dendrites is much similar to the cytoplasm present in the cell bodies of motor neurons. However, the cytoskeleton becomes more abundant in branches having less diameter.

Dendritic Spines

These are the short structures that emerge from dendrites at various points. They are the blunt structures that are visible on silver staining under a light microscope.

These are the sites of synapses. Most of the synapses impinging on dendrites occur on these dendritic spines.

Dendritic spines are present in vast numbers in the neurons responsible for processing a large amount of information coming from various body parts. However, they are not as abundant in the case of motor neurons.

The morphology of these structures is highly dependent on the amount of actin filaments present in them. They can be highly plastic and thus, play a great role in the plasticity of neurons.

Axons

Axons are the cylindrical cellular processes that carry the information away from the cell body of neurons. They are very long processes having a length in meters in the majority of the motor neurons. The length and diameter of these processes vary among different types of neurons. The cytoplasm present in axons is called axoplasm while the plasma membrane is called axolemma.

Axon Hillock

Axon hillock is a pyramid-shaped structure present at the junction of the cell body and axon of motor neurons.

The initial segment of an axon, present just beyond the axon hillock serves as a site of algebraic computation. It is a site where all the stimulatory and inhibitory signals received by a motor neuron are algebraically summed and a decision is made whether to propagate the nerve impulse or not.

There are various ion channels present in the axoplasm of this region. These ion channels help to generate the action potential and its propagation through the axon.

Axoplasm

The axoplasm has the same consistency as the cytoplasm present in the cell body of motor neurons. Organelles like mitochondria, neurofilaments, microtubules, smooth endoplasmic reticulum are also present in axoplasm just like perikaryon. However, no RER or polyribosomes are present in it. It indicates that the axons cannot produce proteins and are dependent on the cell body for their maintenance. This is the reason why the peripheral part of axon degenerates if the axon is severed.

Collaterals, Terminal Arborization and Terminal Bouton

Unlike dendrites, axons don’t show extensive branching and have a constant diameter most of their length. However, the terminal end of axons shows multiple branching termed as terminal arborization.

Axons of motor neurons do have branches at some points called collaterals. These branches synapse with the interneurons of other motor neurons to influence their activity.

Each branch arising from an axon has a dilation at its end called terminal bouton. These dilated parts make synapses with other neurons or non-neurons cells so that nerve impulses can be transferred to other cells.

Transport of Substances

Already mentioned that axons are dependent on the cell body for their maintenance. There is a bidirectional flow of substances between axon and the cell body of a motor neuron.

Organelles and macromolecules like proteins that are synthesized in the cell body of motor neurons move towards the axon terminal via anterograde transport.

Certain molecules like the ones taken up by the axon terminal from the periphery move in the opposite direction, from the axon terminal to the cell body, via retrograde transport.

Certain types of motor proteins are involved in these transport processes that move on the microtubules.

Myelination

The axons of motor neurons are insulated from the surrounding environment by the process of myelination. In this process, Schwann cells rotate around the axon of motor neurons to make myelin sheath around these processes.

Myelin sheath is majorly composed of lipids and serves as an insulator, preventing the conduction of nerve impulses. At regular intervals, some areas of axons are not covered by myelin. These are called nodes of Ranvier. In these axons, nerve impulse jumps from one node to the next which greatly increases the speed of impulse conduction.

Synapse

A synapse is a communication channel between two or more neurons or neurons and non-neurons cells. These are the structure by which a nerve impulse is transmitted from one cell to the other.

A synapse consists of three major components;

  • Pre-synaptic Terminal
  • Synaptic Cleft
  • Post-synaptic cell

A brief detail of these components is mentioned below.

Pre-Synaptic Terminal

The axon bouton makes the presynaptic terminal at the synapse. It releases certain neurotransmitters into the synaptic cleft when the nerve impulses reach the terminal. It has specialized protein channels that can take up calcium from the surrounding environment. The calcium ions are needed for the secretion of neurotransmitters.

Synaptic Cleft

It is the space between the pre-synaptic terminal and the post-synaptic neuron. The neurotransmitters diffuse through this space to act on the post-synaptic neuron. Certain enzymes are also present in the synaptic cleft that remove the neurotransmitters once they have served their function.

Post-Synaptic Neuron

It may be a neuron or non-neuron cells. Certain receptors are located on the surface of these cells. The neurotransmitters bind to these receptors and initiate an action potential in the cell by opening certain ion channels.

In the case of most of the motor neurons, the post-synaptic cell is a skeletal muscle cell and the synapse is called the neuromuscular junction.

Summary

Motor neurons are responsible for carrying information from the central nervous system to the peripheral parts of the body.

Like other neurons, they have a cell body, an axon, and one or more dendrites.

The cell body contains the nucleus of the cell and is the synthetic hub of organelles and macromolecules.

  • The nucleus is spherical, having dense euchromatin fibers and a prominent nucleolus.
  • Nissl bodies are the network of extensive RER and polyribosomes for making protein continuously needed by the motor neuron.
  • Mitochondria, Golgi apparatus, and other organelles are also present in the cell body or perikaryon.
  • Intermediate filaments called the neurofilaments and microtubules make the cytoskeleton of the motor neuron.

Dendrites are the cellular processes that carry information towards the cell body of the motor neuron.

  • They show extensive branching with a decreasing diameter.
  • The dendritic spine is the blunt processes that are the sites of synapses.
  • The cytoplasm is much like the one present in the cell body of the neuron.

Axons are the myelinated cellular processes that transmit nerve impulses away from the cell body.

  • Their cytoplasm is called axoplasm and the plasma membrane is called axolemma.
  • They originate from the cell body via a pyramidal structure called the axon hillock.
  • The axoplasm lakes RER and depends on the cell body for proteins and organelles.
  • It shows terminal branching called terminal arborization.

Motor neurons communicate with other neurons or non-neuron cells via synapses.

  • The pre-synaptic terminal releases the neurotransmitter into the synapse.
  • The neurotransmitter diffuses through the synaptic cleft.
  • It acts on receptors present on the post-synaptic cell to initiate a nerve impulse in it.