The peripheral nervous system: somatic and autonomic
The peripheral nervous system links the brain and spinal cord with the body. Its somatic part controls voluntary muscles; its autonomic part runs the internal organs.
What the peripheral nervous system does
The peripheral nervous system (PNS) connects the central nervous system with the sense organs, muscles and glands. It is made of the sensory and motor nerves that arise from the brain and spinal cord. It has two parts. The somatic nervous system carries orders to the skeletal muscles, the muscles we move by will, so it controls voluntary actions. The autonomic nervous system controls the involuntary work of the internal organs, smooth muscles, heart muscle and glands.
The two parts of the PNS
| Part | Made of | Controls |
|---|---|---|
| Somatic nervous system | 12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord | Voluntary actions of the skeletal muscles |
| Autonomic nervous system (ANS) | A pair of chains of nerves and ganglia (clusters of nerve cell bodies), one on each side of the backbone | Involuntary actions of the internal organs, smooth muscles, heart and glands |
Sympathetic vs parasympathetic nervous system
Their effects look opposite, but they complement each other: one prepares the body for action, and the other restores calm.
Which part is at work?
- Your heart pounds and your palms sweat before an exam: the sympathetic system.
- A learner who usually oversleeps wakes up on her own on exam mornings: the sympathetic system, preparing her for action.
- After a meal, as you rest, saliva flows and food is digested: the parasympathetic system.
- You decide to kick a ball, and your leg moves: the somatic system, carrying orders to the skeletal muscles.
- You see a bright light: the optic nerve, a cranial nerve, carries the message from the eye to the brain.
How to draw a chart of the nervous system
- At the top, write Nervous system and draw two branches: Central (CNS) and Peripheral (PNS).
- Under CNS, draw two boxes: Brain and Spinal cord. Under Brain, add cerebrum, cerebellum and medulla oblongata, each with one job.
- Under Spinal cord, write: controls reflex actions; carries impulses to and from the brain.
- Under PNS, draw two boxes: Somatic and Autonomic (ANS).
- Under Somatic, write cranial nerves (12 pairs) and spinal nerves (31 pairs). Under ANS, write sympathetic and parasympathetic, each with one effect.
Before a job interview or a cricket match, your heart races and your palms sweat as the sympathetic system gets you ready. A few slow, deep breaths and a calm walk afterwards let the parasympathetic system bring you back to normal.
1 marks: “How many pairs of spinal nerves are there in the human body?” 31 pairs.
2 marks: “Give two differences between the sympathetic and the parasympathetic nervous systems.” Sympathetic: active in stress; prepares the body for fight or flight, speeds up the heartbeat and widens the pupils. Parasympathetic: brings the body back to normal; controls unhurried actions such as making saliva and digesting food.
2 marks: “Name the two parts of the peripheral nervous system and the kind of actions each controls.” Somatic nervous system: voluntary actions of the skeletal muscles. Autonomic nervous system: involuntary actions of the internal organs, heart and glands.
Sympathetic for Stress (fight or flight); Parasympathetic for Peace (rest and digest). Cranial 12, spinal 31.