Neurons are the fundamental components of the nervous system that perform a specific task by receiving, conducting, and transmitting the signal or action potential to the other parts of the body. An electrical signal produced by the neurons refers as Nerve impulse that propagates via continuous and saltatory conduction.
A term neuron is sometimes interchangeable with the term nerve-cell and nerve-fibre. A neuron includes three functional elements like a cell-body, dendrite and an axon. Its size and shape can vary in different species.
Definition of Neuron
Neurons can define as the morphologically and functionally distinct nerve fibres, by having root-like dendrites that receive a signal, irregular cell body that conducts the signal, and axons are the specialized projections that processes and transmit a signal via synapse from one cell to the other. Despite being unique from the other cells, it shares some common features like presence of cell membrane, cell-nucleus, cell-organelles etc. Neurons merely refer as the excitable cells, which can respond to the stimulus, by generating a nerve impulse or action potential.
Important Facts About Neurons
- Neurons are the indispensable units in our nervous system that controls all the actions or moves of our body, by producing a nerve impulse.
- A single neuron comprises around 1000-10,000 synapses.
- A nerve impulse is merely a signal produced as a result of synaptic transmission that reaches our brain, and then a brain converts that signal into an action.
- The propagation of nerve impulse from the neural surface ranges between the speeds of 0.5-130 meters per second.
- Neurons cannot replace or regenerate once they die.
- In the year 1998, an article was published where it has mentioned that a hippocampus region of our brain can produce new neurons in adults. The research is still continued on the process of neurogenesis.
- Neurons contribute up to 10% of the brain.
- DRG (Dorsal Root Ganglion) has the most prolonged axon up to 1metres in length that carries the information from the skin cell receptor to the brain.
- Unlike other cells, neurons can communicate with one and all via electrical and chemical synapses.
- Electrical synapse is very uncommon, but works faster and occurs during the time of defensive reflexes. It can flow the information bidirectionally.
- Chemical synapse can trigger complex reactions but is slower than the electrical synapse.
Parts of Neuron
It also refers as “Soma” that is usually 10-25 mm in diameter. Cell body functions as a “Core of neuron” that provides energy to generate a nerve impulse and structural integrity. It contains genetic information inside the nucleus. A nucleus is encircled by a membrane and contains nucleolus that helps in the synthesis of ribosomes to mediate protein translation.
A cell body is quite irregular in appearance that comprises cell organelles like endoplasmic-reticulum, Golgi body, and mitochondria that helps in the release of protein precursors, packaging of proteins and producing energy to fuel the cellular activities respectively.
Nissl ribosomal bodies are the structural elements present within the cell body that also participates in the protein synthesis. Microfilaments or neurotubules are the structures that contribute structural support to the nerve fibre.
It also refers as “Dendritic tree” as it emerges out from the surface of the cell body in a vast number, and further branched to form a structure refers as “Dendritic spine”. Dendrites can define as the cellular extensions of the cell body which functions as an “Input system” by receiving a signal from the other neuron via dendritic spines.
It can receive multiple signals all through their dendritic tree, which can be one of the two signals (excitatory or inhibitory). It contains ribosomes but lacks myelin insulating layer. The number of dendrites per neuron can be many.
It is a tubular, long, and filamentous structure that is protruded outwards from the cell body. Like dendrites, axons also form branches towards an end refers as axon terminal. The endings of the axon give rise to a structure, known as “Bulbous swelling”. An axon terminal contains synaptic vesicles from where the neurotransmitters are released to excite the adjacent neuron.
It lacks ribosomes but contains myelin insulating membrane. A neuron constitutes a single axon whose length and diameter varies within species to species. The function of axon is to conduct the signal from the axon terminal to the target cells. An axon comprises of the following specialized regions like:
Axon hillock: It is the junction element between the cell body and axon, which contains voltage-dependent sodium channels and having negative threshold potential.
Glial cells: These are the non-neuronal cells. When it forms myelin in the peripheral nervous system, it is known as Schwann cells (also refers as “Neurilemma cells”). When the glial cells form myelin in the central nervous system, then it is known as oligodendrocytes. Thus its functional role is to form an insulating layer of myelin around the axon tail.
Myelin insulating membrane: The outer layer of axon also refers to as “Myelin sheath” that protects it from degeneration and accelerates the rate of information flow. The slow breakdown of myelin can cause multiple sclerosis.
Nodes of Ranvier: These are the small periodic gaps (1-2mm) in the myelin coating in the middle of adjoining Schwann cells. It speeds up the flow of the signal by allowing jumping of a signal from one gap to the other.
Types of Neuron
Neurons can classify into the following kinds, based on the number of extensions from the cell body and based on their functional role.
Based on Number of Extensions
Unipolar: It has a single extension originated from the cell body, which is also called as “Neurite”. Unipolar kind of neuron only found in invertebrates, commonly in insects. It is electrically inactive.
Pseudounipolar: It also consists of a single extension that bifurcates into two individual branches. One branch extends into the direction of the peripheral nervous system, and others into the direction of the central nervous system. These are generally sensory or afferent neurons, passes a signal from the peripheral system to the CNS.
Example: Dorsal root ganglion neuron.
Bipolar: In this kind, a cell body contains two extensions one axon and one dendrite. These are the specialized sensory neurons responsible for the transmission of sensations.
Example: Retinal and olfactory epithelium cells.
Multipolar: It constitutes majorly in the central nervous system, and comprises of motor neurons and interleukins.
Example: Motor and interneurons are of a multipolar type.
Based on Function
Sensory-neurons: It is a pseudounipolar neuron whose function is to convey the message from what’s going in our surrounding to the sensory receptors of the central nervous system like a brain and spinal cord. It can transmit both physical and chemical signal input throughout the central nervous system. Based on the neural network, it also refers to as an afferent nerve fibre. These are the arriving nerve fibres extending towards the central nervous system from the sensory cell receptors.
Motor-neurons: It is a multipolar neuron whose function is to convey a message from the CNS through the interneurons to the peripheral effector cells like muscle, gland and organs. Thus, its primary purpose is to conduct the signal from CNS to the skeletal and smooth muscles. Based on the neural network, it also refers to an efferent nerve fibre.
These are the exiting nerve fibres that take a signal from the central nervous system and deliver it to the peripheral effector cells. Motor neurons are basically of two types:
Upper motor nerve: It extends between the brain and spinal cord.
Lower motor nerve: It conducts the signal from the spinal cord to the muscle cell.
Interneurons: It also refers to “Relay neurons”. Inter-neurons function as neural intermediaries. Interneurons connect with the other neurons to transfer the action potential between the sensory and motor neurons.
Neurons or nerve fibres are specialized for the tasks like processing and transmission of cellular signals, coordination between the cells for performing different cellular activities etc. In the structure of a neuron, all the components play a distinct role:
Dendrites functions as a receiver of the signal.
Cell body functions as an integrator of the signal.
Axon performs conduction of the signal to the axon terminal.
Axon terminal performs transmission of the signal to other neurons.