The central nervous system (CNS) consists of the brain and spinal cord and acts as the body's main processing and control centre. The peripheral nervous system (PNS) includes nervous tissue outside the CNS and carries information between the CNS and the rest of the body.
How the Two Systems Differ
| Feature | Central nervous system | Peripheral nervous system |
|---|---|---|
| Main structures | Brain and spinal cord | Cranial nerves, spinal nerves and ganglia |
| Main function | Integrates information, makes decisions and coordinates responses | Transmits sensory information and motor commands |
| Protection | Skull, vertebral column, meninges and cerebrospinal fluid | Less physical protection than the CNS |
| Functional divisions | Brain and spinal cord | Sensory and motor divisions; motor division includes somatic and autonomic systems |
The systems work together through inter-system communication. The sensory, or afferent, division of the PNS carries information from receptors toward the CNS. For example, thermoreceptors in the skin detect heat and sensory neurons transmit impulses to the spinal cord and brain.
The CNS processes this information and organizes an appropriate response. The motor, or efferent, division of the PNS then carries impulses away from the CNS to effectors, such as skeletal muscles or glands.
The motor division has two important subdivisions. The somatic nervous system primarily controls voluntary skeletal muscle activity, while the autonomic nervous system regulates involuntary functions such as heart rate, breathing adjustments and digestion. The autonomic system includes sympathetic and parasympathetic divisions.
A common misconception is that the PNS makes all responses while the CNS merely receives information. In fact, the CNS is primarily responsible for integration, whereas the PNS provides the communication pathways. However, some rapid reflexes are coordinated by the spinal cord before conscious awareness occurs.
Exam Technique
For an IB SEHS comparison, identify structures and functions, then explain the direction of communication using afferent, CNS integration, efferent and effector. Do not simply state that the CNS is “inside” and the PNS is “outside”; link both systems in a complete stimulus-response pathway.