What is Neuron?
A neuron is the simplest cell of the nervous system, from which the whole nervous system is formed.
A neuron is the longest cell of the body, whose primary function is to conduct the message from one part of the body to the other.
Neuron mainly has two parts-
- Cell Body / Cyton.
- Cell Processes.
Structure of a Neuron
A neuron is the long elongated cell that has many projections, some short (dendrons), and one long (axon) and a cell body / cyton (which contain nucleus and cytoplasm).
Both these projections arose from the Cyton.
Let’s see them one by one-
Also called as Cell body / Soma / Perikaryon.
The part of the neuron where all the metabolic process essential for the functioning of the cell occurs.
As it contains the nucleus, so all proteins are formed in the cyton and transferred to the rest of the cell.
Cyton also contains the Nissil’s granule and other cell organelles (except Centriole).
Nissl’s granule is the small piece of the R.E.R. clustered with ribosome, which are responsible for protein synthesis.
Note- Cyton lacks the Centriole, so neuron does not have the power to reproduce.
Still, they have power of growth and repairing itself with the help of other brain cells (Neuroglia).
2. Cell Processes
Neuron contains 2 types of processes Dendron and axon.
Their function is to transmit the impulse (just like a wire), either from brain to organ or organ to brain.
It is the anterior part of the neuron which receives the message either from other neuron or from sensory organs (thermoreceptors, tangoreceptors, olfactory receptors, etc.).
It is further divided into small projections called a dendrite.
A dendrite is attached to the axon terminal of the other neuron or work as the receptors in the sensory organs.
As soon as neurons get the stimulus the Action potential is generated in the dendrite and transferred toward the cyton then to the axon.
Note– the speed of the action potential is 120 m/s.
It is the longest part of the neuron and the main conducting area.
Axon is covered by the myelin sheath, which formed by the Schwann cells. Myelin sheath is irregular and a gap is present between 2 myelin sheath called Node of Ranvier.
A layer of Neurilemma is present around the whole axon including mylein sheath.
Axon end in the axon terminal which has the synaptic lobes, which contain the synaptic vesicles.
These synaptic vesicles are filled by the neurotransmitters.
There are 2 types of neurotransmitters Excitatory and Inhibitory.
This neurotransmitter then reaches the other neuron’s Dendron and generates the impulse through the Synaptic Cleft.
Types of Neuron
Based on the number of processes–
|Cell processes are either absent or not differentiated into axon and dendron.||Single axon arises from the Cyton.||Neuron contain only axon, but small projections arises from it which work as dendron.||2 processes arise from cyton 1 dendron and 1 axon.||This type of neuron have 1 axon and many dendrons.|
|Ex– Hydra, Amacrine cell of retina.||Nervous system of Embryo.||Dorsal root ganglia of spinal cord.||Retina (Rods and Cones)||Most the neurons in vertebrates.|
Based on functionality-
|Sensory Neuron||Inter-connecting Neuron||Motor Neuron|
|Sensory organ to CNS||From one nerve to another.||CNS to the organs.|
Transmission of Impulse
- Impulse is generated at the dendrite.
- Impulse is converted into the Action potential in Dendron and transferred to the Cyton.
- From cyton action potential is transferred to the axon.
- Action potential reaches the nerve ending (axon terminal) and polarize the membrane of the synaptic vesicles.
- Due to which the neurotransmitters are released.
- The neurotransmitter crosses the synaptic cleft.
- Neurotransmitter attaches to the receptors of the postsynaptic membrane (membrane of the next neuron’s dendron).
- Now, the action potential is generated in the next neuron and the process is repeated.
This is the case when the Excitatory neurotransmitter is released, but in the case of Inhibitory neurotransmitter the postsynaptic membrane becomes depolarized so the transmission of impulse is stopped.
Note- Only one type of neurotransmitter is present in one neuron either excitatory or inhibitory.
Mechanism of Impulse conduction
The membrane of the axon called axolemma, which contains many channels and pumps.
That is responsible for the ionic gradient across the membrane.
An axon is cylindrical the interior part of the axon is filled with axoplasm.
Outside the axon, the negatively charged chloride ions and positively charged sodium ions are in greater concentration.
Negatively charged protein molecules and potassium ions are in greater concentration in the axoplasm.
The membrane of a neuron -ve inside and +ve outside. This difference in charge called Resting potential, which varies from 70 – 90 millivolts, as a result, the membrane would be polarized.
When a stimulus is applied to a membrane of a nerve fiber. The potassium ions rush outside the membrane and sodium ions rush inside the membrane as a result polarity of the membrane is reversed.
The nerve fibers are either depolarized or they are said to be in an action potential. The potential difference is around + 30 mV.
This condition is just for a limited time and the axon membrane will obtain a resting potential again by repolarization.
Now the process takes place in reverse order. It is a reversal of the process that has taken place during a depolarization (action potential).
Here, potassium ions will be rushed inside and sodium ions will be rushed outside.
Now, the membrane is ready to take a new stimulus.
Note– Polarity of the membrane (i.e. polarization) is essential to gain the stimulus.
- Gap between the axon terminal and dendron of other neuron is called Synaptic Cleft.
- Synapse = Pre-synaptic membrane + synaptic cleft + post-synaptic membrane.
- Synapse are of 2 type Chemical synapse (most common and slow) and Electrical synapse (fast).
- Neuron do not have power of regeneration.
- Neurotransmitter can also stop the message, this type of transmitter called Inhibitory Neurotransmitter. Ex–Serotonin, Dopamine.
- Example of Excitatory neurotransmitter – Acetylcholine (Ach), Nor–adrenaline.