14. Coordination and response(Part 1)(Cambridge IGCSE Biology 0610 for exams in 2023, 2024 and 2025)

Organisms require coordination between organs and systems to maintain life, sense environmental changes, and respond appropriately. The nervous system is crucial for receiving, processing information, and generating responses to maintain homeostasis.

The nervous system consists of millions of neurons (nerve cells) that transmit information as electrical impulses for quick communication. A bundle of neurons is called a nerve. The mammalian nervous system is divided into the Central Nervous System (brain and spinal cord) and the Peripheral Nervous System (nerves outside the CNS).

There are three types of neurons: sensory neurons (carry impulses from sense organs to CNS), relay neurons (found in CNS, connect sensory and motor neurons), and motor neurons (carry impulses from CNS to effectors like muscles or glands). Each type has distinct structural characteristics.

A stimulus is any detectable environmental change. A receptor is a structure that detects a stimulus and generates an electrical impulse. An effector, usually a muscle or gland, responds to a signal from the nervous system by carrying out a specific action.

Voluntary responses are conscious, intentional actions originating from the brain. Involuntary responses, or reflexes, are quick, automatic reactions to a stimulus that don't involve the brain and are essential for survival, occurring without conscious thought.

A reflex arc is the pathway an involuntary nerve impulse follows. For example, touching fire: the stimulus (fire) is detected by receptors in the skin. A sensory neuron sends impulses to the spinal cord (coordinator), which passes them to a relay neuron, then to a motor neuron. The motor neuron carries the impulse to a muscle (effector) in the arm, causing it to contract and pull away.

A synapse is the gap between two neurons. Electrical impulses stimulate the release of neurotransmitter molecules from vesicles into the synaptic gap. These chemical messengers diffuse across the gap, bind to receptor proteins on the next neuron, and stimulate a new impulse. Synapses ensure impulses travel in one direction only.