Two main divisions
The human nervous system is built from two parts that work as one. The central nervous system (CNS) is the brain and the spinal cord; it receives information, makes decisions and issues instructions. The peripheral nervous system (PNS) is all the nerves that branch out from the CNS to reach receptors and effectors throughout the body.
Information from receptors travels to the CNS along sensory nerves. After the CNS processes it, instructions leave along motor nerves to muscles and glands. In this way a single stimulus can be turned into an organised, whole-body response.
The central nervous system
The brain sits inside the skull and controls thought, memory, sight, hearing, balance and many automatic actions such as breathing and heartbeat. The spinal cord runs down the backbone, protected by the vertebrae. It links the brain to the rest of the body and is also the coordinator for many reflex actions, allowing responses without waiting for the brain.
Key idea
CNS = brain + spinal cord (control centre). PNS = the nerves linking the CNS to receptors and effectors.
The peripheral nervous system
The PNS carries messages both ways. One part controls voluntary actions of skeletal muscles; another part, the autonomic nerves, controls involuntary organs such as the heart, gut and glands without conscious thought. Together the CNS and PNS ensure that a stimulus anywhere in the body can produce a coordinated response.
Example
When you read this sentence, light signals travel from the eyes along sensory nerves to the brain (CNS). The brain interprets the words and, if you decide to turn the page, sends impulses along motor nerves (PNS) to the muscles of your hand.
Remember
Sensory nerves carry impulses towards the CNS; motor nerves carry impulses away from it. The brain and spinal cord together make up the CNS.