Question

Describe the parts of a neuron. Explain how signals move through a neuron (include where they begin and where are they transmitted).

Answer 1

Structure of neuron

Neurons have a cell body called soma. Nucleus of neuron is present in soma. There are many projections coming out from the cell body.

Dendrites are short branching processes.
Axon is single long process.

Action potential from one neuron reaches the dendrite and it passes into the cell body where it gets processed. Then it is transmitted along the actions and finally passes to the next neuron.

The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissle's granules.

Distal end of axon is branched and each branch terminated in a bulb-like structure known as synaptic knob. It possess synaptic vesicles contrasting chemicals called neurotransmitter. Axon transmit impulses away from the cell body to a synapse or to a neuron muscular junction.

Based in number of axon and dendrites, neurons are divided into 3 types;

1. Unipolar: cell body with one axon only.
2. Bipolar: with one axon and one dendrite
3. Multipolar: with one axon and two or more dendrite.

There are 2 types of axons,

1. Myelinated - enveloped with Schwann cells, which forms a myelin sheath around the axons
2. Non-myelinated - enclosed bt Schwann cells which do not form myelin sheath around the axon

Generation and conduction of nerve impulse

Neurons are excitable cells because their membranes are in a polarized state.
When a neuron is resting, axonal membrane is more permeable to potassium ions and nearly impermeable to sodium ions.
And the membrane is impermeable to negatively charged proteins inside the axoplasm.
So axoplasm inside the axons contains a high concentration of potassium ions and negative charge proteins and low concentration of sodium ions. And fluid outside the axons consists of low concentration of potassium and high concentration of sodium and thus forms a concentration gradient.

These ionic gradient across the resting membrane is maintained by sodium-potassium pump which pumps 3 Na+ outwards for 2K+ into the cell.
As a result outer surface of axon has inner surface has negative charge. And the potential difference across resting membrane is called resting membrane potential.

When a stimulus arrives at a resting membrane, it becomes freely permeable to Na+ ions and rapid influx of sodium ions occurs. As a result, parity of membrane at that site is reversed and the membrane is said to be depolarized. Electrical potential difference across the membrane at that time is termed as action potential which is the nerve impulse.

As there is reversal of polarity, inside of membrane it's positive and outside is negative. The site immediately next to that area has inside negative and outside positive (resting). As a result of these, a current flows on the inner surface and the next site depolarized. This sequence repeated throughout the entire length of axon and thus action potential is transmitted through the axon.

When it reaches the synaptic terminal, neurotransmitters stored inside the vesicles releases into synaptic cleft and they bind to specific receptors on the post-synaptic membrane. This binding open in channels allowing entry of ions which can generate a new potential in the post-synaptic neuron.