Structure of a Neuron

A neuron is the microscopic structural and functional unit of the neural system. It consists of three major parts:

1. Cell Body (Soma or Cyton):

It is the main processing center containing the nucleus, cytoplasm, and typical cell organelles.

Nissl's Granules:

These are special, highly active granular bodies found in the cytoplasm.

  • Granular bodies present in cell body and dendrites
  • They are rich in rough endoplasmic reticulum and ribosomes
  • Function: Protein synthesis (proteins are needed for growth, repair, neurotransmitter-related proteins, receptors, etc.)

Key Points:

  • They are actually rough endoplasmic reticulum and ribosomes, which means they are heavily involved in protein synthesis.
  • Nissl granules = Protein synthesis
  • Present in soma + dendrites
  • Absent in axon

2. Dendrites

  • These are the short, highly branched fibres that project out from the cell body.
  • Just like the cell body, dendrites also contain Nissl's granules.
  • Function (The Receivers): They transmit electrical impulses TOWARDS the cell body (Afferent pathway).

Key Points:

  • Dendrites = Receiving end
  • Direction: towards soma

3. Axon

  • This is the single, long fibre extending from the cell body.
  • Distal Ends: The far end of the axon branches out.
  • Synaptic Knob: Each branch ends in a bulb-like swelling called a synaptic knob.
  • Synaptic Vesicles: Inside these knobs are tiny sacs (vesicles) packed with chemical messengers called neurotransmitters.
  • Function (The Senders): The axon transmits nerve impulses AWAY from the cell body to a synapse (a gap between neurons) or to a neuro-muscular junction.

Types of Neurons

Depending on the number of extensions coming off the cell body, neurons are classified into three types:

Type Structural Features Location in Human Body
Multipolar One axon + Two or more dendrites Cerebral Cortex (Brain)
Bipolar One axon + One dendrite Retina of the Eye
Unipolar Cell body with One axon only (No dendrites) Usually found only in the Embryonic Stage

Myelinated and Non-myelinated Nerve Fibres

Axons can also be classified based on whether they have an insulating cover called a myelin sheath. Note: Both types are associated with Schwann cells, but their behavior differs.

A. Myelinated Nerve Fibres

  • The axon is enveloped by Schwann cells which actively secrete a fat-like insulating layer called the myelin sheath around the axon.
  • Nodes of Ranvier: The myelin sheath is not continuous; the unmyelinated gaps between adjacent myelin sheaths are called the Nodes of Ranvier. These gaps help speed up the impulse.
  • Location: Typically found in spinal and cranial nerves (where fast transmission is needed).

B. Non-myelinated Nerve Fibres

  • The axon is still enclosed by a Schwann cell, but this cell does NOT form a myelin sheath around the axon.
  • Naturally, there are no Nodes of Ranvier.
  • Location: Commonly found in the autonomous and somatic neural systems.

🧠 The Neuron: Structure and Function

1. The Direction Trick

  • Dendrites = Doorway (In towards the cell body).
  • Axon = Away (Out from the cell body).

2. Neuron Types & Locations (High Yield)

  • Multipolar = Mind (Cerebral Cortex). Most common.
  • Bipolar = Both Eyes (Retina).
  • Unipolar = Unborn (Embryo).

3. The Schwann Cell Trap

  • Warning: Schwann cells are present in BOTH myelinated and non-myelinated fibres in the PNS. The main difference is whether they secrete the myelin sheath or not!

💡 Questions and Answers

Q1. What are Nissl's granules?

A1: Nissl’s granules are tiny granules found mainly in the cell body and dendrites of a neuron. They help the neuron make proteins, so they are important for growth and repair.

Key points:

  • Location: Soma + dendrites
  • Function: Protein synthesis
  • Not present in axon

Q2. Where are bipolar neurons found?

A2: Bipolar neurons are mainly found in special sense organs, especially in the retina of the eye.

Key points:

  • Bipolar = 1 axon + 1 dendrite
  • Location: Retina

Q3. What is the function of the synaptic knob?

A3: It's the bulb-like structure at the very end of the axon branches. It acts like a storage unit, holding tiny bubbles (synaptic vesicles) that are filled with neurotransmitters. These chemicals are released to pass the signal to the next cell.

Key points:

  • Synaptic knob = axon terminal
  • Has vesicles with neurotransmitters
  • Helps in chemical transmission across synapse

Q4: Differentiate between myelinated and non-myelinated axons.

A4: The main difference is the "insulation." Myelinated axons have a thick myelin sheath made by Schwann cells, leaving gaps called Nodes of Ranvier (typically found in spinal and cranial nerves). Non-myelinated axons also have Schwann cells wrapped around them, but these cells don't make the myelin sheath (typically found in autonomous and somatic nerves).

Key points:

  • Myelinated: myelin sheath present (Schwann cells), Nodes of Ranvier, fast conduction
  • Non-myelinated: myelin absent, slow conduction, common in ANS

Q5. In which stage are unipolar neurons typically found?

A5: Unipolar neurons—which have a cell body and just one axon but zero dendrites—are generally only seen during the embryonic stage of human development.

Key points:

  • Unipolar = one process
  • Found mainly in embryo