Coordination, Response & Homeostasis
Your body has two communication systems (nervous and hormonal) that detect environmental changes and regulate internal conditions so your cells can function properly.
Quick Overview
- Nervous system: Fast electrical signals via neurones
- Reflex arc: Automatic response that bypasses the brain
- Synapse: Junction between neurones; uses chemicals
- Sense organs: Contain receptors that detect stimuli
- The eye: Complex sense organ; lens focuses, iris controls light
- Hormonal system: Slower, longer-lasting chemical messengers
- Blood glucose control: Insulin and glucagon regulate via negative feedback
- Homeostasis: Maintaining stable internal conditions
- Temperature control: Skin sweats, blood vessels dilate/constrict, muscles shiver
- Plant responses: Tropisms (phototropism, gravitropism) controlled by auxin
1. The Nervous System
Structure and Role
The nervous system has two main parts:
- Central Nervous System (CNS): Brain and spinal cord — the control centre
- Peripheral Nervous System (PNS): All the nerves in your body — carry messages to and from the CNS
Its job is to:
- Detect changes in your surroundings (stimuli)
- Coordinate your body's response to those changes
- Regulate body functions like heart rate and digestion
Information travels as nerve impulses — electrical signals that pass along nerve cells called neurones. A bundle of neurones is called a nerve.
Key Concept: The nervous system uses electricity to communicate. This is why it's fast — electrical signals travel at speed, and the whole reflex can happen in milliseconds without you even thinking about it.
Types of Neurones
There are three types of neurone, each with a different job:
1. Sensory Neurones
Carry impulses FROM sense organs TO the CNS (brain or spinal cord).
Structure: Long, with cell body branching off the middle of the axon.
2. Relay Neurones (Intermediate)
Found inside the CNS. Connect sensory neurones to motor neurones.
Structure: Short, with small cell body at one end and many dendrites.
3. Motor Neurones
Carry impulses FROM the CNS TO effectors (muscles or glands).
Structure: Long, with large cell body at one end and long dendrites.
Memory Trick: Sensory neurones connect to your sense organs. Motor neurones cause movement (or gland secretion). The relay stays in the middle, connecting the two.
Special Features of All Neurones
Neurones are designed for speed and connectivity:
- Long axon: Reduces time wasted transferring the impulse between cells
- Fatty myelin sheath: Insulates the axon, with small gaps called nodes
- Saltatory conduction: The impulse jumps from node to node (faster than travelling the whole length)
- Dendrites: Many branches allow the neurone to connect with hundreds of other neurones, creating a network for complex communication
Practice Question 1
Describe the structure of a sensory neurone and explain how its structure makes it well-suited to its function.
2. The Reflex Arc