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Mapping the organizational structure of the human nervous system and its various branches.
Imagine a bear suddenly jumps out from behind a tree. Before you even 'think' to run, your heart is already pounding and your lungs are gasping for air. How does your body coordinate this lightning-fast response while simultaneously keeping your digestion and temperature on autopilot?
The human nervous system is organized into two primary divisions. The Central Nervous System (CNS) acts as the body’s 'Command Center,' consisting of the brain and the spinal cord. It is responsible for processing information and making decisions. In contrast, the Peripheral Nervous System (PNS) consists of all the nerves that branch out from the CNS to the rest of the body. Think of the CNS as the central computer and the PNS as the cables connecting that computer to the keyboard, mouse, and monitor. The PNS's primary job is to transmit sensory information to the CNS and carry motor commands back out to the muscles and organs.
Quick Check
If a person suffers an injury to their spinal cord, which major division of the nervous system has been damaged?
Answer
The Central Nervous System (CNS).
The Peripheral Nervous System is further divided based on function. The Somatic Nervous System controls voluntary movements. When you decide to wave at a friend, your somatic system carries the signal to your skeletal muscles. On the other hand, the Autonomic Nervous System (ANS) manages 'automatic' functions that you don't consciously control, such as your heartbeat, digestion, and pupil dilation. The ANS ensures that your internal environment remains stable without you having to remind your heart to beat every second.
When you touch a hot stove, your body reacts before you feel pain. 1. Sensory neurons in the PNS detect heat. 2. The signal travels to the spinal cord (CNS). 3. The Somatic Nervous System immediately triggers your arm muscles to pull away. 4. This happens in milliseconds, bypasssing the brain's conscious thought to save time.
Quick Check
Which system is responsible for increasing your breathing rate during a sprint without you thinking about it?
Answer
The Autonomic Nervous System.
The Autonomic system is split into two competing branches that maintain homeostasis (internal balance). The Sympathetic Nervous System is your 'Fight or Flight' response. It accelerates heart rate and inhibits digestion to prepare for action. Conversely, the Parasympathetic Nervous System is the 'Rest and Digest' system. It slows the heart and stimulates digestion to conserve energy. They work like a gas pedal and a brake; if your heart rate is too high, the parasympathetic system engages to bring it back to the baseline .
Imagine standing backstage before a speech. 1. Your Sympathetic System kicks in: your palms sweat and your heart rate increases to BPM. 2. Once you start speaking and feel comfortable, your Parasympathetic System begins to dominate. 3. Your heart rate slows back toward BPM, and your dry mouth begins to produce saliva again as digestion resumes.
Consider the balance of the Autonomic Nervous System as a ratio , where is sympathetic activity and is parasympathetic activity. 1. In a state of perfect rest, . 2. During a panic attack, might increase by a factor of , making . 3. To return to homeostasis, the body must either decrease or increase until the ratio returns to . This explains why deep breathing (increasing ) can stop a racing heart.
Which part of the nervous system is responsible for relaying a 'move' command from the brain to your legs?
If your pupils are dilated and your digestion has slowed down, which system is likely dominant?
The Somatic Nervous System is a sub-division of the Autonomic Nervous System.
Review Tomorrow
In 24 hours, try to sketch a tree diagram of the nervous system from memory, starting with 'Nervous System' at the top and branching down to the 'Parasympathetic' and 'Sympathetic' systems.
Practice Activity
Monitor your body today. When you feel stressed, identify which system is active. Then, take five deep breaths and visualize your Parasympathetic system 'braking' your heart rate.