Neurones and the Reflex Arc: Quick Responses, Instant Actions

What Are Neurones?

• Neurones are the building blocks of the nervous system.
• They carry electrical signals, called impulses, around your body to make sure you can respond to changes in your environment.
• Neurones have a long section called axon. This reduces the transfer time for impulses between cells
• A fatty sheath insulates the neurone axon with small gaps which aren't insulated called nodes
• These nodes mean the impulse can jump from node to node without travelling the full axon length
• The neurone cell body has branch like structures called dendrites that allow the neurone to connect to many other neurones forming a network

 

Types of Neurones

 

Type	Function
Sensory Neurones	Carry signals from receptors (sense organs) to the CNS.
Relay Neurones	Found in the CNS (brain and spinal cord); connect sensory and motor neurones.
Motor Neurones	Carry signals from the CNS to effectors (muscles or glands).

 

 

 

Reflex Arc: A Rapid Response Pathway

There are two main types or responses to stimuli, voluntary and involuntary:

 

Voluntary Response

• A response to a change in the environment where you can react to is a conscious descision.
• You choose to respond and do an action. This starts with the brain.

 

Reflex Response (Involuntary Response)

• An involuntary response (also called a reflex response) is one that does not require thought and is instinctual.
• This means it doesn't ivolce the brain and is automatic.
• Becuase the brain isn't involved it is a faster response to stimuli and is usually essential to basic survival.
• A reflex arc is the pathway taken by nerve impulses during a reflex action.
• Reflexes are automatic and quick, helping you stay safe (e.g., pulling your hand away from something hot).

 

 

Steps in a Reflex Arc

1. Stimulus: A change in the environment (e.g., heat from a flame).
2. Receptor: Detects the stimulus (e.g., skin receptors).
3. Sensory Neurone: Carries the impulse to the CNS (spinal cord or brain).
4. Relay Neurone: Passes the impulse within the CNS.
5. Motor Neurone: Carries the impulse from the CNS to an effector.
6. Effector: The muscle or gland that responds (e.g., muscle contracts to pull your hand away).

Analogy:
Think of a reflex arc as a fire alarm system:

• Stimulus = Smoke.
• Receptor = Smoke detector.
• Sensory Neurone = Wire to the control panel.
• Relay Neurone = Control panel deciding the action.
• Motor Neurone = Wire to the sprinkler.
• Effector = Sprinkler putting out the fire.

 

 

Key Characteristics of Reflex Actions

Feature	Explanation
Involuntary	Reflexes happen automatically without thinking.
Fast	Reflexes bypass the brain to save time.
Protective	Reflexes help you avoid danger (e.g., blinking, sneezing).

 

Diagram: The Reflex Arc

Simplified Pathway:

$$\text{Stimulus} \to \text{Receptor} \to \text{Sensory Neurone} \to \text{Relay Neurone} \to \text{Motor Neurone} \to \text{Effector} \to \text{Response}$$