Topic 1.2 Notes – Overview of the Nervous System

How the Nervous System Is Organized

The nervous system takes in information, processes it, and sends commands. That means it helps you sense the world, think about it, and respond with muscles, organs, and glands.

Here’s the hierarchy you need to know:

• Nervous system
  • Central nervous system
    • brain
    • spinal cord
  • Peripheral nervous system
    • somatic nervous system
    • autonomic nervous system
      • sympathetic nervous system
      • parasympathetic nervous system

One trap shows up constantly. CNS does not mean just the brain. It means brain + spinal cord. Also, sympathetic and parasympathetic are not their own separate top-level systems. They are divisions of the autonomic system, which is part of the PNS.

Central and Peripheral Nervous Systems

Central nervous system

The central nervous system is the brain and spinal cord. It is the main processing and coordinating center.

• Brain organizes sensory input and supports thinking, memory, emotion, and awareness.
• Spinal cord carries messages between the brain and the rest of the body.
• The spinal cord also helps create reflexes, which are fast automatic responses. For this topic, the point is simple. A reflex shows that the spinal cord is part of the CNS.

Peripheral nervous system

The peripheral nervous system includes all nerves outside the brain and spinal cord. It connects the CNS to the rest of the body.

• Sensory messages travel from receptors toward the CNS.
• Motor commands travel from the CNS to muscles, organs, and glands.

The PNS is not a second brain. It does not do the main decision-making. It is the communication link.

Central versus peripheral

A quick way to separate them:

The Two Divisions of the Peripheral Nervous System

Inside the PNS, you split things by what kind of job is being done.

Somatic nervous system

The somatic nervous system handles voluntary processes and controls skeletal muscles.

• walking
• reaching
• turning your head
• speaking

It also carries sensory information from your skin, muscles, and joints to the CNS. If the action is a deliberate skeletal movement, think somatic.

Autonomic nervous system

The autonomic nervous system handles largely involuntary internal processes. It regulates organs and glands.

• heart rate
• digestion
• sweating
• pupil size
• breathing-related adjustments

If it is automatic internal regulation, think autonomic. A common trap is breathing. You can control it for a bit, but it is still mainly an autonomic function.

Somatic versus autonomic

• Somatic = voluntary + skeletal muscles + bodily sensation
• Autonomic = involuntary + organs/glands + internal regulation

Sympathetic and Parasympathetic Divisions

Both of these are parts of the autonomic nervous system, and they often have opposite effects on the same organs. Together, they help maintain homeostasis, which means keeping internal conditions within a healthy range.

Sympathetic nervous system

The sympathetic division is fight-or-flight. It prepares your body for action.

• heart rate increases
• pupils dilate
• sweating increases
• alertness rises
• digestion slows

This is not only for emergencies. It also shows up before a race, test, or performance.

Parasympathetic nervous system

The parasympathetic division is rest-and-digest. It calms the body and restores routine functioning.

• heart rate decreases
• pupils return toward normal
• digestion resumes or increases
• the body recovers

Their relationship

They are not good vs bad, and they are not simple on/off switches. Your body shifts their influence depending on what you need.

How the Systems Work Together in Real Situations

One event can involve several systems at once. Think about a runner:

1. Hearing the start signal. Sensory messages travel through the PNS to the CNS.
2. The brain and spinal cord process the signal and coordinate a response. That is CNS.
3. Pushing off to run uses the somatic system because skeletal muscles are moving voluntarily.
4. During the race, rising heart rate and arousal show sympathetic activation.
5. After the race, falling heart rate during recovery shows parasympathetic influence.

The graph below puts those last two steps into one quick timeline, with heart rate rising during exertion and then dropping back toward baseline during recovery.

Heart rate during exertion and recovery

On tests, the best clue is the specific process. Ask what exactly is happening, then match it to the subsystem.