A neuron (nerve cell) is the structural and functional unit of the nervous system. It is a highly specialised cell capable of receiving, processing, and transmitting electrochemical signals.
Every neuron has three fundamental structural components:
- Contains the nucleus and most cellular organelles.
- Contains Nissl's granules — clusters of rough endoplasmic reticulum (rER) and free ribosomes responsible for protein synthesis (including neurotransmitters).
- Also contains mitochondria, Golgi apparatus, and a cytoskeleton.
- The cell body integrates incoming signals.
- Short, highly branched cytoplasmic extensions of the cell body.
- Function: receive incoming nerve impulses from other neurons or receptors and carry them toward the cell body.
- Increase the surface area for receiving signals.
- A single, long cytoplasmic extension arising from the axon hillock of the cell body.
- Function: transmits nerve impulses away from the cell body toward other neurons, muscles, or glands.
- Ends in axon terminals (synaptic knobs) that release neurotransmitters.
- May be covered by a myelin sheath.
| Feature | Description |
|---|
| Myelin Sheath | A fatty (lipid-rich) insulating layer wrapped around the axon |
| Function | Insulates the axon and greatly increases the speed of nerve impulse conduction |
| Produced by | Schwann cells in the PNS; Oligodendrocytes in the CNS |
| Nodes of Ranvier | Unmyelinated gaps between adjacent Schwann cells where the axon membrane is exposed |
| Significance of Nodes | Enable saltatory conduction — the impulse 'jumps' from node to node, increasing speed and energy efficiency |
Neurons are classified by function into three types:
- Location: Cell body in dorsal root ganglia (outside CNS); long dendrites extend to receptors.
- Function: Carry nerve impulses from receptors (sense organs, skin) to the CNS for processing.
- Structure: Typically unipolar — a single process that divides into a peripheral branch (to receptor) and a central branch (to CNS).
- Example: Neurons carrying pain signals from the skin to the spinal cord.
- Location: Entirely within the CNS (brain and spinal cord).
- Function: Receive signals from sensory neurons, process and integrate information, and relay signals to motor neurons.
- Structure: Multipolar — many dendrites and a short axon; highly branched for complex integration.
- Example: Neurons in the spinal cord grey matter that connect sensory and motor pathways in a reflex arc.
- Location: Cell body in the CNS (ventral horn of spinal cord); long axon extends to effectors.
- Function: Carry nerve impulses from the CNS to effectors (muscles and glands), producing a response.
- Structure: Multipolar — large cell body, many dendrites, and a very long axon (up to 1 metre).
- Example: Neurons stimulating skeletal muscle contraction.
| Structural Feature | Function |
|---|
| Many dendrites | Receive signals from multiple sources simultaneously |
| Long axon (motor neuron) | Transmit impulses over long distances to distant effectors |
| Myelin sheath | Insulates axon; increases conduction speed |
| Nodes of Ranvier | Enable saltatory conduction for rapid, energy-efficient transmission |
| Nissl's granules (rER + ribosomes) | Protein synthesis — neurotransmitters, enzymes, structural proteins |
| Axon terminals / synaptic knobs | Release neurotransmitters into the synapse to communicate with next cell |
| Axon hillock | Site where action potential is initiated |
| Feature | Myelinated | Non-Myelinated |
|---|
| Myelin sheath | Present | Absent |
| Nodes of Ranvier | Present | Absent |
| Conduction type | Saltatory (fast) | Continuous (slow) |
| Diameter | Larger | Smaller |
| Example | Motor neurons, sensory neurons | Autonomic post-ganglionic fibres |