The nervous system comprises of Brain, Sensory organ, Spinal cord, and Nerves and Neurons that transmit the signal between different parts of the body and that connect the parts of nervous system with other body parts. The neurons have three components:
- Dendron (dendrites)
- Cell body
Dendron and Axon involved in the conduction of nerve impulses through the nerve cell. Both are the special cytoplasmic projection of neurons. Dendrites bring information to the cell body from the other neurons and axon takes information from the body.
Axons is a Greek word derived from the (ἄξων áxōn, axis) are also called nerve fibers, as they appear elongated and slender with uniform thickness and smooth surface throughout the length.
They are protoplasmic projections of nerve cells, arise from the discharge end of the nerve, and carry electrochemical impulses away from the cell body of neurons. Presynaptic in their function, branched only at the distal end, often producing hundreds or thousands of Presynaptic terminals.
A neuron typically contains a single axon that varies in length depending on the type of neuron. As compared to the dendrites may be several meters long above 50mm. Axons are long in order to carry information throughout the body. The axon of the sciatic nerve is the longest exon in the human body which runs from the spinal cord and reaches the big toe of each foot.
Axons do not have ribosomes, rough endoplasmic reticulum, or Golgi elements. Thus Metabolic activity (biosynthesis) in axons is limited. They are enriched with low molecule weight microtubules associated proteins.
The most plentiful proteins in the axon are the proteins forming the microtubules, neurofilaments, and actin filaments (microfilaments). Microtubules are a dominant feature of all axons.
These axonal microtubules are aligned with the long axis of the axon. They have a uniform polarity with plus ends distal to the soma. Microtubules also play key roles in the transport of metabolites and organelles in the axon. Their cytoskeleton is highly phosphorylated.
In vertebrates, some axon has the insulating layer called myelin sheath which is made of protein and fatty substances. The process of myelin sheath production is called myelination. The function of the myelin sheath to protect the axon from the electrical impulse and increase the speed of the nerve transmission. In large axons transmit impulses transmit at speeds up to 90 meters (300 feet) per second.
In axons there is the specialized region called axon hillock where axon is raised from the soma. Sometime they arise from the dendrites. The proximal part of the axon that is adjacent to the axon hillock, is called initial segment that start just after the axon hillock.
The terminal branch of the axon forms an enlarged synaptic knob. The synapse is contact between the part of one neuron (usually its axon) and the dendrites, cell body, or axon of a second neuron. Neurotransmitter is present in the synaptic knob such as acetylcholine, norepinephrine, dopamine. Axons contain neurofibrils all over but they lack Nissl’s granule’s.
Axonal transport is important for survival and growth. It is the active process by which proteins and other substances synthesized in the neurosome, moves inside the axon and transported. Axonal transport is of type of
- Fast Axonal transport (up to 400 mm/day): It is employed during development for the growth of axons and dendrite. It is elongated by adding new material to their tips.
- Slow axonal transport (5 mm/day): It is also known as axoplasmic flow.
Dendrites are the receptive surface of the neuron and also the output devices Dendrites are membranous tree-like protoplasmic projections arising from the body of the neuron, about 5–7 per neuron on average, and about 2μm in length and operate or receive signal (electrochemical impulses) from other neuron. Its name comes from the Greek word δένδρον ‘Dendron’, which means ‘Tree.
In 1889 Wilhelm His introduced the term dendrite. Around the neuron they usually form a dense canopy a-like arborization called a dendritic tree. Some dendrites are unipolar, while others are multipolar.
The size and complexity of dendritic arbors increase during development. The impulses they receive are transported inwards and towards the soma, or cell body. They are shorter, and postsynaptic as compared to axons.
Other physical characteristics that make difference between the axons and dendrites, besides the length and branching, are their shapes. Dendrites have tube-like shape usually tapers, while the radius of axons remains constant.
In the mature nervous system, the dendritic branching of a neuron is changeable either growing or retracting.
There are many dendrites per neurons and arise from the receiving end of the neuron. They receive the impulses via synapses and contain specialized proteins that receive, process, and transfer these to the cell body. Unlike axon there is no synaptic knots are formed on the tip of dendrites.
They contain both neurofibrils and Nissl’s granules. The complexity of the dendrites reflects the number of connections that a neuron receives. They occupy the large surface area of the neuron. They occupy the 300,000μm3 of the neuron (motor neuron) and provide 370,000μm3 for synaptic input.
Dendrites (and cell bodies) have high molecular weight microtubule-associated proteins designated MAP2. Dendrites also possess organelles such as neurofilaments, neurotubules, endoplasmic reticulum, mitochondria, ribosomes). These organelles are the sites of protein synthesis and involved in memory formation.
These organelles enable them to alter protein density in response to changes in the frequency of neuronal inputs and helps in the normal activity of neurons to be maintained also helps to prevent neurological disorders such as epilepsy.
Free ribosomes present throughout the cytoplasm of dendrites and cytoskeleton of dendrites is poorly phosphorylated. Sooth endoplasmic reticulum involved in the regulation of cytoplasmic calcium
There are also special dendritic organelles:
- Dendritic spines
- Dendritic swellings.
Dendritic spines are extension, membranous organelles found on the dendritic processes of neurons that deserve special mention because these are the structures that actually synapse with the axon’s terminal bulbs with narrow stalks with bulb-like head.
The length of the dendritic spine is 2μm and the volumes of the spine head can be 0.01 µm3 to 0.8 µm3 and enable to detection conjunction of presynaptic and postsynaptic activity from a single synapse of an axon.
There can be thousands of dendritic spines on a neuron. The distribution density of dendritic spines ranges from 20 to 50 spines per 10 µm stretch of the dendrite (on average, 200 000 dendritic spines per neuron).
Alterations in the spine are due to long-term alterations in afferent input. This alteration can play a role in the formation and maintenance of neuronal ensembles. The most notable classes of spine shape following based on the morphological characteristics of the spine head, neck, and length.
- Thin (long necks and a small head)
- Stubby (no neck with the postsynaptic density (PSD) of their head in close proximity to the dendritic shaft.
- Mushroom (wide head and thin neck)
Pruning of Dendrites
It is the process of trimming unused dendrites in a dendritic tree during the development of the nervous system. It is necessary to eliminate old and unused dendrites to facilitate the formation of new dendrites
Functions of dendrites
Dendrites play role in important normal neuronal functions. They also play role in physiological processes such as memory formation.
- The surface of the dendrites is filled with receptors(to which the neurotransmitters bind) that become enacted upon by neurotransmitters that traversed the synapse after the pre-synaptic neuron fired and released neurotransmitters into the synapse. Dendrites integrate this stimulation (from a multitude of receptors) and play a crucial role in determining the extent to which the received stimulation will result in an action potential.
- Enlarged surface area to receive signals from axons of other nerve cells
- Receive signals from other neurons
- Transmission of received signals
- process these signals
- Transfer the information to the soma of the neuron
- Possible contribution to memory
- The possible source of neuromodulators and tissue factors.