Lesson 1.6 · Unit 1 · CED topic 1.2
The nervous system and its divisions
Every thought, movement, and feeling depends on a communication network that reaches from your brain to your fingertips and organs. The nervous system is that network. It collects information from the body and the world, decides what to do with it, and sends instructions back out. This lesson maps the system's divisions and what each does. It is a short topic, but the map is used constantly in later lessons: the autonomic nervous system returns in the study of stress and emotion, and the brain structures come in Lesson 1.9.
Learning objective.
- 1.2.A. Differentiate among the subsystems of the human nervous system and their functions.
Essential knowledge.
- 1.2.A.1. The central nervous system includes the brain and the spinal cord and interacts with all processes in the body.
- 1.2.A.2. The peripheral nervous system relays messages from the central nervous system to the rest of the body and includes the autonomic and somatic nervous systems.
- 1.2.A.2.i. The autonomic nervous system governs processes that are involuntary and includes the parasympathetic and sympathetic nervous systems.
- 1.2.A.2.ii. The somatic nervous system governs processes that are voluntary. 1.A 3.A
Two main parts: central and peripheral
The central nervous system (CNS) is the brain and the spinal cord. The brain is the control center that interprets information and directs responses, and the spinal cord is a bundle of neurons running down the back that carries messages between the brain and the rest of the body and also handles some simple reflexes by itself. The CNS interacts with all the processes in the body.
The peripheral nervous system (PNS) is made up of all the nerves outside the brain and spinal cord. Its job is to relay messages between the CNS and the rest of the body. Information flows both ways: sensory (afferent) neurons carry information from the senses and organs toward the CNS, and motor (efferent) neurons carry commands away from the CNS to muscles and glands. A simple memory aid is that afferent neurons arrive at the CNS and efferent neurons exit it.
The two divisions of the peripheral nervous system
The PNS has two parts, which differ in whether you control them.
- The somatic nervous system governs processes that are voluntary. It carries sensory information from the skin, eyes, and other sense organs and sends the motor commands that move your skeletal muscles, as when you lift your hand to answer a question, walk to class, or write a sentence.
- The autonomic nervous system (ANS) governs processes that are involuntary: the activity of organs and glands that run without your deciding, such as heart rate, breathing rate, digestion, and pupil size. ("Autonomic" is related to "automatic.")
The autonomic nervous system: sympathetic and parasympathetic
The autonomic nervous system has two subdivisions that usually work in opposite directions and balance each other.
The sympathetic nervous system prepares the body for action and responds to stress or danger. When it is active the heart beats faster, breathing quickens, pupils widen, digestion slows, and stress hormones, notably adrenaline, are released. This is the fight, flight, or freeze response: the body mobilizes its resources to confront a threat, escape it, or hold still. The parasympathetic nervous system calms the body and conserves energy after the threat has passed. It slows the heart, lowers breathing, and restarts digestion, sometimes described as "rest and digest."
| Division | Voluntary? | What it does | Example |
|---|---|---|---|
| Central nervous system | Both | Brain and spinal cord process information and direct responses | Deciding what to say |
| Somatic nervous system | Voluntary | Senses and skeletal muscle movement | Raising your hand |
| Sympathetic nervous system | Involuntary | Arouses the body for action | Pounding heart before a speech |
| Parasympathetic nervous system | Involuntary | Calms the body and conserves energy | Heart rate settling after the speech |
The two sides do not work like an on/off switch. Most of the time both are active, and their balance shifts with circumstances. Before a presentation the sympathetic side dominates; afterward the parasympathetic side restores calm. A person who stays chronically aroused, with the sympathetic system active for long periods, is at higher risk for stress-related health problems, which Unit 5 returns to.
The American physiologist Walter Cannon studied how animals' bodies change when they are frightened or enraged. In experiments with cats and other animals he observed that threat produced a coordinated set of changes, including faster heartbeat, release of adrenaline, widened pupils, and slowed digestion, which prepared the animal either to fight or to flee. He named this the "fight-or-flight" response, and later researchers added "freeze." Cannon argued that the response is adaptive because it mobilizes energy when action is needed.
Reading the research: a set of controlled laboratory observations in animals, linking a stimulus (threat) to measured bodily responses. A limitation is that the work was done on animals, so researchers had to confirm that humans respond similarly.
Name the division most directly involved in each: (a) lifting a pencil to write, (b) your pupils widening when you walk into a dark room, (c) your heart racing when you hear a loud noise behind you, (d) your digestion resuming and your heart slowing down once you realize the noise was a dropped book, (e) a message from your skin to your brain telling you the pan is hot.
Step 1, (a) is voluntary muscle movement, so the somatic nervous system (motor neurons).
Step 2, (b) is involuntary control of an organ (the iris), so the autonomic nervous system.
Step 3, (c) is an involuntary arousal response to a sudden threat, so the sympathetic division.
Step 4, (d) is the return to calm, so the parasympathetic division.
Step 5, (e) is sensory information traveling toward the CNS, carried by sensory (afferent) neurons of the peripheral nervous system, which are part of the somatic system when they carry information from the skin.
A researcher measures the heart rate (beats per minute) of 6 students at three times: resting at baseline, during a surprise oral presentation, and 10 minutes after it. The averages are 72, 98, and 76. Interpret the data.
Step 1, identify the variable: heart rate in beats per minute, a measure of autonomic activity, in a repeated-measures sample of 6 students.
Step 2, during the presentation the mean rose by \(98-72=26\) beats per minute, which is consistent with sympathetic activation.
Step 3, after 10 minutes the mean fell to 76, only \(76-72=4\) above baseline, consistent with the parasympathetic system restoring calm.
Step 4, limitations: only 6 participants, so this cannot be generalized widely. Without a control group (students who did not present) the researchers cannot be sure that the presentation, not something else such as caffeine or exercise, caused the change, although the stress of presenting is a plausible explanation.
- CNS = brain + spinal cord. PNS = everything else; it relays messages and has somatic (voluntary) and autonomic (involuntary) parts.
- The autonomic system has the sympathetic (arousal, fight-flight-freeze) and parasympathetic (calming, rest and digest) divisions.
- Sensory (afferent) neurons carry information toward the CNS; motor (efferent) neurons carry commands away from it.
- Match behaviors to systems: voluntary movement is somatic; heartbeat, digestion, and pupil size are autonomic.
- Which nervous system divisions make up the peripheral nervous system, and how do they differ?
Show answer
The somatic nervous system (voluntary: skeletal muscles and senses) and the autonomic nervous system (involuntary: organs and glands, including the sympathetic and parasympathetic divisions).
- After a near-miss in traffic, a driver's heart pounds and her palms sweat. Which division is responsible, and what does it do?
Show answer
The sympathetic division of the autonomic nervous system. It arouses the body for a threat (fight, flight, or freeze) by speeding the heart, quickening breathing, and releasing adrenaline.
- Why is the parasympathetic nervous system important after a stressful event?
Show answer
It calms the body: it slows the heart rate and breathing and restores digestion, conserving energy and returning the body to its resting state.
- A person touches a hot stove. Describe the direction of messages carried by sensory and motor neurons.
Show answer
Sensory (afferent) neurons carry the pain message from the skin to the central nervous system. Motor (efferent) neurons carry the command away from the CNS to the muscles that pull the hand back.
- A person with a spinal cord injury cannot move their legs but still digests food normally. Which divisions might explain the difference?
Show answer
Movement of the legs depends on motor commands in the somatic system that travel through the spinal cord, which was damaged. Digestion is controlled by the autonomic system, which can operate without voluntary commands.
- State one limitation of drawing a conclusion about humans from Cannon's animal work.
Show answer
Animal bodies and brains differ from humans', so the same pattern of responses must be confirmed in people before it can be generalized.