The Nervous System Lesson: Parts, Functions & More

Lesson Overview

• Major Parts of the Nervous System
• Functions of the Nervous System
• Central vs. Peripheral Nervous System
• Neurons – The Basic Unit of the Nervous System
• The Myelin Sheath and Schwann Cells
• The Brain: Cerebrum and Cerebellum
• Cranial and Spinal Nerves
• The Nervous System in Action: Reflexes and Stress Responses

The nervous system is the body's control and communication network, linking the brain with every part of the body. In this introductory section, we'll ease into understanding how the nervous system works and why it's so important. Imagine touching a hot stove and pulling your hand away immediately – that's your nervous system in action! It's constantly sensing what's happening both inside and outside your body and deciding how to react.

Can you think of other moments in daily life where you react without even thinking (like blinking or dodging something)? These quick responses are possible thanks to your nervous system's rapid communication.

Major Parts of the Nervous System

The nervous system has three major parts: the brain, the spinal cord, and the nerves. Together, these form the body's communication highway. The brain is like the command center or the "supercomputer" of your body – it processes information and issues commands. The spinal cord is a long bundle of nerves inside your spine that connects your brain to the rest of your body, acting like a main highway for signals. 

The nerves are numerous wire-like structures (bundles of nerve fibers) that branch out everywhere, carrying messages to and from all body regions.

• Brain: Inside your skull, it controls thought, memory, emotion, and many automatic functions (like breathing or heart rate).
  
• Nerves: These extend out from the brain and spinal cord to the rest of the body. They reach your eyes, ears, skin, muscles, organs – everywhere! Nerves make sure the commands from your brain and spinal cord reach the target, and they bring back sensory information to the central system.
  

Functions of the Nervous System

The nervous system's main jobs are to sense changes, process information, and coordinate responses. In short, it does a lot, but there are some things it doesn't do. Your nervous system is constantly sensing what's going on: it monitors the outside world (through sight, hearing, touch, etc.) and the inside conditions (like temperature, hunger, etc.). It then processes that information in the brain and spinal cord, deciding what to do. 

Finally, it responds by sending signals through nerves to muscles (to move) or glands (to adjust things like sweating, heart rate, etc.). The nervous system allows you to sense changes, interpret them, and respond appropriately. 

For example, it senses touch or temperature, processes that information in your brain/spinal cord, and then triggers a response (like moving your hand or shivering). However, one thing the nervous system does not do is store water or nutrients – tasks like that are handled by other systems (for instance, the kidneys in the urinary system store and regulate water).

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Central vs. Peripheral Nervous System

The nervous system has two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). CNS is just the brain and spinal cord (the central command center that processes information).

PNS is all the nerves outside the CNS. In fact, by definition the PNS includes every nerve that branches off from the brain or spinal cord to reach the rest of the body.

Neurons – The Basic Unit of the Nervous System

Neurons (nerve cells) are the building blocks of the nervous system. They carry electrical signals called nerve impulses throughout the body. When we talk about "nerves," we're usually referring to bundles of many neuron fibers. But let's break down a single neuron's structure:

• Cell Body (Soma): Contains the nucleus – it's the neuron's "control center" where incoming signals are processed.
  
• Dendrites: Branch-like extensions that receive signals from other neurons. Neurons may have many dendrites to collect information.
  
• Axon: A long fiber that sends signals away from the cell body to other cells. Typically, each neuron has one axon, which can be very long.
  

The Myelin Sheath and Schwann Cells

Many neurons have an insulating layer called the myelin sheath around their axons to increase signal speed. If you think of an electrical wire, myelin is like the plastic insulation around it that helps the signal travel fast without leaking out. Myelin is not continuous along the axon; it has gaps (Nodes of Ranvier) which help the impulse jump faster down the axon. 

This fatty coating insulates the axon and speeds up the transmission of electrical impulses, allowing signals to travel much faster. In summary, the myelin sheath is made by Schwann cells in the PNS. 

Synapses – How Neurons Communicate

A synapse is the tiny junction where a nerve signal passes from one cell to the next. It includes the presynaptic terminal (end of the sending neuron's axon), the small gap in between, and the postsynaptic membrane on the receiving cell. 

When an impulse reaches a presynaptic terminal, neurotransmitter chemicals are released into the gap and bind to the postsynaptic side, carrying the signal forward. Thus, a synapse is made up of both the presynaptic and postsynaptic elements.

The Brain: Cerebrum and Cerebellum

The brain's two largest regions are the cerebrum and the cerebellum, each with different roles.

• Cerebrum: The largest part of the brain, located on top. Responsible for thinking, reasoning, memory, and sensing. (If a question asks, cerebrum is the largest region – not the smallest or second largest.)
  
• Cerebellum: A smaller section at the back of the head (near the base of the brain). It controls balance and coordination of movements. (It is the second largest part of the brain, and it sits behind and below the cerebrum.)
  

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Cranial and Spinal Nerves

Nerves are bundles of neuron fibers in the PNS, and they come in two varieties: cranial (from the brain) and spinal (from the spinal cord).

• Cranial Nerves (12 pairs): These nerves arise from the brain and mostly serve the head and neck. Some carry sensory info (like vision or smell), others carry motor signals (to move muscles), or do both. For example, the 12th cranial nerve (hypoglossal nerve) controls tongue movement, which means it's a motor nerve – not purely sensory.
  
• Spinal Nerves (31 pairs): These nerves branch out from the spinal cord to the rest of the body (neck, trunk, limbs, and internal organs). Each spinal nerve has both sensory and motor fibers (mixed). They are named by the region of the spine they come from. For instance, nerves from the chest region are called thoracic nerves (not lumbar – lumbar are lower back). In total there are 31 pairs of spinal nerves.
  

The Nervous System in Action: Reflexes and Stress Responses

Our nervous system not only handles conscious actions but also automatic responses, such as reflexes and how our body reacts to stress. For example, a reflex is a fast, involuntary action (like pulling your hand away from a hot object) that the spinal cord directs without needing the brain's input. When you experience stress or fear, the autonomic nervous system (specifically the sympathetic branch) kicks in to trigger the fight-or-flight response.

For example, if you're very nervous before a test, you might feel your heart pound and your palms sweat – that's your nervous system releasing stress hormones (like adrenaline) and activating organs. Understanding these nervous system responses is important for therapists, because it helps them see how stress affects the body (heart rate, blood pressure, breathing, etc.) and how calming techniques can engage the opposite, relaxation response. By studying the nervous system, therapists learn how stress can lead to physical symptoms and how to help people manage those effects.

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