How does the structure of a sensory neuron differ from a motor neuron regarding the cell body?

In a sensory neuron, the cell body is typically located to the side of the main axon path (unipolar or bipolar), whereas in a motor neuron, the cell body is located at one end of the axon and is surrounded by dendrites (multipolar).

What is the functional difference between dendrites and axons?

Dendrites are designed to receive incoming signals from other neurons and carry them toward the cell body, while axons are designed to carry electrical impulses away from the cell body toward the next neuron or effector.

Compare the roles of excitatory and inhibitory neurotransmitters.

Excitatory neurotransmitters increase the positive charge of the postsynaptic neuron, making it more likely to reach the threshold and fire, while inhibitory neurotransmitters increase the negative charge, making it less likely to fire.

What is a common misconception regarding the 'strength' of an action potential?

A common error is believing that a stronger stimulus produces a larger electrical charge. In reality, all action potentials are the same size; a stronger stimulus results in a higher frequency of firing.

Why is it incorrect to say that signals can travel both ways within a single neuron?

Neural transmission is strictly unidirectional due to the structural arrangement of receptors at the dendrites and neurotransmitter vesicles at the axon terminals, ensuring signals only move from input to output.

What mistake do students often make when identifying the location of relay neurons?

Students often forget that relay neurons are found exclusively within the Central Nervous System (CNS), specifically the brain and spinal cord, and do not extend into the peripheral nerves.

Define the 'Nodes of Ranvier' and their purpose.

The Nodes of Ranvier are small gaps in the myelin sheath along the axon. They allow the action potential to jump from gap to gap (saltatory conduction), which significantly speeds up the transmission of the impulse.

What is the 'Synaptic Cleft'?

The synaptic cleft is the microscopic physical gap between the presynaptic axon terminal and the postsynaptic dendrite or cell body where chemical communication occurs.

What is the function of the Myelin Sheath?

The myelin sheath is a fatty layer that insulates the axon, preventing the loss of electrical signal and increasing the speed at which the impulse travels down the neuron.

What is an 'Action Potential'?

An action potential is a brief electrical impulse that travels down the axon of a neuron when the cell's threshold of excitation is reached, caused by the movement of ions across the membrane.

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The Structure & Function of Neurons

Summary

Neurons are the fundamental building blocks of the nervous system, specialized for the rapid transmission of electrical and chemical signals. Their unique morphology—comprising dendrites for input, a soma for integration, and an axon for output—allows for complex communication networks that govern everything from basic reflexes to high-level cognition.

1. Definition & Core Concepts

A neuron is a specialized biological cell designed to process and transmit information through electrical and chemical signals. It serves as the primary functional unit of the nervous system, enabling communication between the brain, spinal cord, and the rest of the body.

The nervous system relies on three functional classes of neurons: sensory neurons (input), motor neurons (output), and relay neurons (integration). Each type possesses a specific structural layout optimized for its role in the signal pathway.

Information flow within a single neuron is strictly unidirectional, moving from the receiving dendrites, through the cell body, and down the axon to the terminal buttons.

Diagram of a typical motor neuron showing the dendrites, cell body (soma), axon with myelin sheath segments, and axon terminals.

2. Anatomical Structure

Dendrites: Branch-like extensions that act as the primary input zone, receiving chemical signals from neighboring neurons and converting them into electrical impulses directed toward the cell body.
Soma (Cell Body): The metabolic center of the neuron containing the nucleus. it integrates incoming signals and maintains the cell's biological health.
Axon: A long, slender projection that conducts electrical impulses (action potentials) away from the soma toward other neurons or effector organs.
Myelin Sheath & Nodes of Ranvier: The myelin sheath is a fatty insulating layer that prevents signal leakage. The gaps between these segments, known as Nodes of Ranvier, allow the signal to 'jump' along the axon, significantly increasing transmission speed.

3. The Action Potential & Signal Propagation

4. Synaptic Transmission

5. Key Distinctions

6. Exam Strategy & Tips

The Structure & Function of Neurons

Summary

1. Definition & Core Concepts

Information flow within a single neuron is strictly unidirectional, moving from the receiving dendrites, through the cell body, and down the axon to the terminal buttons.

Diagram of a typical motor neuron showing the dendrites, cell body (soma), axon with myelin sheath segments, and axon terminals.

2. Anatomical Structure

Dendrites: Branch-like extensions that act as the primary input zone, receiving chemical signals from neighboring neurons and converting them into electrical impulses directed toward the cell body.
Soma (Cell Body): The metabolic center of the neuron containing the nucleus. it integrates incoming signals and maintains the cell's biological health.
Axon: A long, slender projection that conducts electrical impulses (action potentials) away from the soma toward other neurons or effector organs.
Myelin Sheath & Nodes of Ranvier: The myelin sheath is a fatty insulating layer that prevents signal leakage. The gaps between these segments, known as Nodes of Ranvier, allow the signal to 'jump' along the axon, significantly increasing transmission speed.

3. The Action Potential & Signal Propagation

Communication within a neuron is electrical. When a neuron is sufficiently stimulated, it reaches a threshold of excitation, triggering an action potential—a rapid reversal of the electrical charge across the membrane.

This process follows the All-or-None Law, meaning an action potential either fires at full strength or not at all; the intensity of a stimulus is communicated by the frequency of firing, not the size of the electrical charge.

In myelinated axons, the impulse travels via saltatory conduction, where the electrical charge effectively leaps from one Node of Ranvier to the next, making the process much faster than continuous conduction in unmyelinated fibers.

4. Synaptic Transmission

When the electrical impulse reaches the axon terminal, it must cross a physical gap called the synaptic cleft. This transition requires converting the electrical signal into a chemical one.

Neurotransmitters stored in vesicles are released into the cleft and bind to specific receptor sites on the postsynaptic neuron, much like a key fitting into a lock.

The effect on the receiving neuron depends on the type of neurotransmitter: excitatory chemicals increase the likelihood of the next neuron firing, while inhibitory chemicals decrease that likelihood.

5. Key Distinctions

It is vital to distinguish between the three types of neurons based on their structure and location in the signal arc:

Feature	Sensory Neuron	Relay Neuron	Motor Neuron
Function	PNS to CNS (Input)	Within CNS (Connector)	CNS to PNS (Output)
Dendrites	Long	Short	Short
Axon	Short	Short	Long
Cell Body	Located to the side	Central	Central

6. Exam Strategy & Tips

Directional Flow: Always verify the direction of the impulse in diagrams. It moves from dendrites $\rightarrow$ soma $\rightarrow$ axon $\rightarrow$ terminal. Examiners often flip diagrams to test this.
The 'All-or-None' Principle: Remember that a stronger stimulus does NOT create a 'bigger' action potential. It creates MORE action potentials per second (higher frequency).
Structure-Function Mapping: Be prepared to explain how specific structures like the myelin sheath or Nodes of Ranvier directly contribute to the efficiency of the nervous system.
Terminology Precision: Do not confuse 'axons' with 'dendrites'. Axons Away from the cell body; Dendrites Deliver to the cell body.

Directional Flow: Always verify the direction of the impulse in diagrams. It moves from dendrites $\rightarrow$ soma $\rightarrow$ axon $\rightarrow$ terminal. Examiners often flip diagrams to test this.
The 'All-or-None' Principle: Remember that a stronger stimulus does NOT create a 'bigger' action potential. It creates MORE action potentials per second (higher frequency).
Structure-Function Mapping: Be prepared to explain how specific structures like the myelin sheath or Nodes of Ranvier directly contribute to the efficiency of the nervous system.
Terminology Precision: Do not confuse 'axons' with 'dendrites'. Axons Away from the cell body; Dendrites Deliver to the cell body.