Unit 1.2: Overview of the Nervous System

Nervous System Organization

Definition & Components

The Central Nervous System (CNS) consists of the brain and spinal cord and serves as the command center that processes information, makes decisions, and coordinates all body systems[web:21][web:24].

The brain handles higher-level functions like thinking, memory, emotion, and awareness, while the spinal cord acts as a superhighway carrying messages between the brain and the rest of the body[web:21].

Key Functions of the CNS

• Receives and interprets sensory information from eyes, ears, skin, muscles, and internal organs[web:24]
• Processes information to make decisions about appropriate responses[web:21]
• Sends motor commands to muscles and glands throughout the body[web:24]
• Coordinates reflexes and automatic reactions for rapid responses[web:21]
• Manages thought, memory, emotion, and conscious awareness[web:24]

💡 Example: Touching a Hot Stove (Reflex Arc)

This example demonstrates how the CNS processes information rapidly[web:24]:

1. Detection: Heat-detecting sensory receptors in the hand detect high temperature
2. Transmission: Sensory neurons send electrical signals through peripheral nerves to the spinal cord
3. Processing: The spinal cord can initiate a reflex response immediately, even before the brain becomes fully aware
4. Response: Motor neurons send commands back to hand muscles
5. Action: Muscles contract, pulling the hand away from danger

Key Point: Reflex arcs allow the spinal cord to produce fast, involuntary responses without waiting for the brain, demonstrating the CNS's efficiency in protecting the body[web:21][web:24].

Definition & Function

The Peripheral Nervous System (PNS) includes all nerves extending beyond the brain and spinal cord[web:21][web:24].

It serves as the messenger network that relays information between the CNS and the rest of the body, ensuring every muscle, organ, and tissue can communicate effectively[web:21].

Voluntary Control System

The Somatic Nervous System (SNS) controls voluntary movements and carries sensory information that individuals consciously perceive[web:21][web:24].

This system enables purposeful actions like walking, talking, writing, playing sports, and any task requiring conscious control of skeletal muscles[web:24].

Key Functions

• Carries sensory information from sensory receptors (skin, muscles, joints) to the CNS[web:21]
• Transmits motor signals from the CNS to skeletal muscles to produce voluntary movement[web:21]
• Enables conscious perception of touch, temperature, pain, and body position[web:24]
• Allows fine-tuned muscle coordination for complex actions and skilled movements[web:24]

📝 Example: Raising Your Hand

When a student decides to raise their hand to answer a question[web:24]:

1. Decision: The brain consciously decides to lift the arm
2. Signal transmission: Motor commands travel from the brain down the spinal cord
3. Peripheral relay: Signals pass through peripheral nerves to arm muscles
4. Muscle contraction: Arm muscles receive the signal and contract
5. Sensory feedback: Sensory neurons report arm position back to the brain for adjustment

Involuntary Control System

The Autonomic Nervous System (ANS) controls involuntary processes that occur automatically without conscious thought, managing internal organs and maintaining homeostasis[web:21][web:24].

The ANS regulates critical life-sustaining functions like heart rate, breathing, digestion, blood pressure, pupil dilation, and glandular secretions[web:21].

🔥 Sympathetic Nervous System: "Fight-or-Flight"

The sympathetic division activates during stressful, exciting, or dangerous situations, preparing the body for action by mobilizing energy and resources[web:21][web:24].

Sympathetic Activation Effects:

• Increases heart rate and blood pressure — pumps more blood to muscles
• Dilates pupils — allows more light for better vision
• Widens airways — increases oxygen intake
• Inhibits digestion — conserves energy for immediate action
• Redirects blood flow — away from digestive organs to skeletal muscles
• Stimulates sweat glands — cools the body
• Releases adrenaline (epinephrine) — from adrenal glands for sustained arousal

🌿 Parasympathetic Nervous System: "Rest-and-Digest"

The parasympathetic division activates during calm, relaxed states, promoting recovery, energy conservation, and bodily maintenance[web:21][web:24].

Parasympathetic Activation Effects:

• Decreases heart rate and blood pressure — returns to baseline
• Constricts pupils — adjusts to normal light conditions
• Stimulates digestion — activates salivation and intestinal activity
• Promotes nutrient absorption — increases blood flow to digestive organs
• Conserves energy — supports growth, repair, and recovery
• Slows breathing rate — returns to relaxed rhythm

🧠 Memory Aid: PARA = Parachute

Think of the parasympathetic system like a parachute that helps slow down and return to a calm, balanced state[web:21].

• Sympathetic = Speed up (stress, action, alertness)
• Parasympathetic = Parachute = slow down (relaxation, recovery, rest)

Somatic vs. Autonomic Nervous Systems

Sympathetic vs. Parasympathetic Divisions

Complete Nervous System Overview

Why Two Opposing Systems?

The sympathetic and parasympathetic divisions work in opposition to maintain homeostasis — the body's stable internal environment[web:21][web:24].

Having both systems allows the body to rapidly shift into emergency mode when needed (sympathetic) and then return to baseline for recovery and maintenance (parasympathetic)[web:21].

This balance ensures quick reactions to sudden changes followed by restoration of calm, enabling survival and efficient functioning in changing environments[web:21][web:24].

Multiple Choice Tips

• Distinguish between CNS and PNS components and functions[web:21]
• Identify whether behaviors are voluntary (somatic) or involuntary (autonomic)[web:21]
• Recognize sympathetic vs. parasympathetic effects on specific body systems[web:21]
• Understand reflex arcs and how the spinal cord processes rapid responses[web:21]
• Apply concepts to real-life scenarios (exam questions, sports performance, stress responses)[web:24]

Free Response Question (FRQ) Tips

• Use precise terminology: CNS, PNS, somatic, autonomic, sympathetic, parasympathetic[web:21]
• Explain the pathway: sensory neurons → CNS → motor neurons for complete responses[web:24]
• Provide concrete examples of how each system affects specific behaviors or bodily functions[web:21]
• Contrast sympathetic and parasympathetic effects clearly (don't just list — compare)[web:24]
• Describe homeostasis as the balance between opposing autonomic divisions[web:21]

🔑 The Big Picture

The nervous system is divided into the Central Nervous System (brain + spinal cord) that processes information and the Peripheral Nervous System that relays messages between the CNS and body. The PNS includes the Somatic System (voluntary) and Autonomic System (involuntary). The Autonomic System balances Sympathetic (fight-or-flight arousal) with Parasympathetic (rest-and-digest calm) to maintain homeostasis[web:21][web:24].

💡 Remember for the Exam

• CNS = Brain + Spinal Cord
• PNS = All nerves outside CNS
• Somatic = Voluntary movement
• Autonomic = Involuntary organs
• Sympathetic = Speed up
• Parasympathetic = Slow down
• Reflex arcs bypass brain
• Homeostasis = Balance