Internuncial (Connector) Neurons & Synapse: Structure, Function and Importance
The human nervous system is a highly complex network of neurons that transmit impulses throughout the body. Among these neurons, internuncial neurons, also known as connector neurons or interneurons, play a crucial role in linking sensory and motor pathways. They form the backbone of reflex actions, neural communication, and higher-order brain functions.
In this article, we explore the structure, functions, synapses, and importance of internuncial neurons in an easy, reader-friendly way.
What Are Internuncial (Connector) Neurons?
Internuncial neurons are multipolar nerve cells found primarily in the central nervous system (CNS). They act as a bridge, transmitting nerve impulses from sensory (receptor) neurons to motor neurons.
Key Characteristics
- Their cell bodies and axons lie entirely within the CNS.
- They receive information from sensory neurons and relay it to motor neurons.
- Some interneurons are also found in the autonomic nervous system.
- Their dendrites are short, highly branched, and spread irregularly around the cell body.
- Their axons can extend up to 100 centimeters in length and branch into fine terminal filaments.
- They possess a myelin sheath, but do not have a neurolemma since they never leave the CNS.
Role of Internuncial Neurons in Reflex Arcs
Reflex actions are quick, involuntary responses to stimuli. Many spinal reflex arcs involve:
- One interneuron between sensory and motor neurons
- Or multiple interneurons, creating complex neural circuits
Some interneurons also send impulses to higher brain centers, forming ascending and descending tracts in the brain and spinal cord.
Synapse: The Connecting Point Between Neurons
A synapse is the tiny junction where the terminal ends of an axon communicate with the dendrites or cell body of another neuron.
Structure of Synapses
- Axon terminals form knob-like structures called terminaux.
- These remain in very close proximity, without structural continuity.
- Synapses may look like bulbs, rings, or loops under a microscope.
Important Facts About Synaptic Transmission
- Nerve impulses travel only in one direction across a synapse — from axon to dendrites or cell body.
- This one-way flow is called dynamic polarity.
- Synapses exhibit a slight delay in impulse transmission.
- They fatigue more quickly than axons.
- They are highly sensitive to certain chemicals and drugs like strychnine and nicotine.
Synapses in the Spinal Cord
The anterior gray column of the spinal cord contains motor neurons that form the final common motor pathway.
Each of these motor cells may receive over a thousand synapses, allowing complex coordination and integration of signals.
Two Major Features of Internuncial & Motor Neurons
- Rich Synaptic Connections – allowing efficient communication and integration.
- Collateral Branches of Axons – enabling impulses to spread to various pathways for coordinated responses.
These features help the nervous system:
- Integrate sensory (afferent) signals
- Coordinate motor (efferent) responses
- Create complex neural circuits
Neuron Theory: Independent Structure of Neurons
The neuron theory states:
- Every neuron is an independent structural and functional unit.
- There is no physical continuity between neurons; only close contact at synapses.
- Nerve impulses cross synapses but neurons remain separate.
Although once debated, neuron theory is now universally accepted.
Conclusion
Internuncial neurons are essential components of the nervous system, ensuring smooth communication between sensory inputs and motor outputs. Their extensive synaptic networks and axonal branches make them vital for reflex actions, coordination, and higher brain functions.
Understanding interneurons and synapses helps us appreciate the complexity and efficiency of the human nervous system.
