Edexcel A Level (IAL) Biology -8.3 The Reflex Arc- Study Notes- New Syllabus

Spinal Reflex Arc & Spinal Cord Structure

🌱 Introduction

A reflex arc is the simplest pathway for a rapid, automatic response to a stimulus.
It allows quick reactions without the delay of routing impulses through the brain.

The spinal cord contains grey and white matter which are essential for processing and transmitting these impulses.

🔍 1. Structure of a Spinal Reflex Arc

Components (5 Key Parts)

Receptor

• Detects the stimulus (e.g., heat, pressure).
• Converts it into an electrical impulse.

Sensory Neurone

• Carries the impulse from the receptor to the spinal cord.

Relay (Inter) Neurone

• Located in grey matter of the spinal cord.
• Connects sensory neurone to motor neurone.
• Integrates the response.

Motor Neurone

• Carries impulse from spinal cord to the effector.

Effector

• Muscle or gland that carries out the response (muscle contracts, gland secretes).

Tip: Reflex arcs are usually monosynaptic (sensory → motor) or polysynaptic (sensory → relay → motor).

🔍 2. Grey Matter and White Matter of the Spinal Cord

Grey Matter

• Shape: Butterfly or H-shaped in cross-section.
• Contains: Cell bodies of neurones, dendrites, and relay neurones.
• Function: Processes incoming impulses, forms synapses for reflex arcs.
• Location: Centre of spinal cord.

White Matter

• Contains: Myelinated axons of sensory and motor neurones.
• Function: Transmits impulses up and down the spinal cord.
• Location: Surrounds grey matter.

🧩 How a Reflex Works (Example: Knee-Jerk Reflex)

• Tap patellar tendon → stimulus detected by stretch receptor.
• Impulse travels along sensory neurone to spinal cord.
• Monosynaptic reflex: sensory neurone synapses directly with motor neurone.
• Motor neurone stimulates quadriceps → leg kicks.
• Happens without involving the brain → very fast.

📊 Summary Table