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Studying the evolutionarily older parts of the brain responsible for survival and basic motor control.
While you sleep, your heart keeps beating and your lungs keep breathing without a single conscious thought. Who is the 'autopilot' running your body while 'you' are offline?
The hindbrain is the evolutionarily oldest part of our brain, often called the 'reptilian brain' because it handles basic survival. At the very base sits the medulla, which controls autonomic functions like heart rate, blood pressure, and breathing. Just above it is the pons (Latin for 'bridge'). The pons acts as a relay station between the cerebellum and the cerebral cortex. It is also critical for regulating sleep cycles and facial expressions. Because these structures handle 'low-level' processing, they operate entirely outside of your conscious awareness. If the medulla is compromised, the body's basic life-support systems fail almost instantly.
Think about a sudden sneeze. This is a complex, coordinated physical event that you don't 'plan.' 1. Sensory nerves detect an irritant. 2. Signals travel to the medulla. 3. The medulla triggers a massive, involuntary coordination of your diaphragm and facial muscles to expel the air.
Quick Check
If a patient suffered a stroke in their medulla, why would they likely require a mechanical ventilator?
Answer
Because the medulla is responsible for the autonomic control of breathing; damage there stops the body's 'autopilot' respiratory drive.
The cerebellum (meaning 'little brain') sits at the back of the brainstem. Although it accounts for only about of the brain's total volume, it contains more than of the brain's total neurons. Its primary job is motor coordination and balance. It doesn't initiate movement, but it smooths it out. When you learn to ride a bike or play the piano, the cerebellum stores those 'muscle memories.' It also helps us judge time and discriminate sounds. When you consume alcohol, the cerebellum is one of the first areas affected, which is why coordination and balance become impaired.
Consider a gymnast on a balance beam. 1. The motor cortex sends the 'move' signal. 2. The cerebellum constantly compares the intended movement with actual body position. 3. It sends micro-corrections to muscles to prevent a fall, calculating adjustments in milliseconds.
Quick Check
Why would damage to the cerebellum make it difficult to play a rhythm-based game like 'Guitar Hero'?
Answer
The cerebellum is responsible for fine motor coordination and the precise timing of movements.
Running through the middle of the medulla and pons and extending into the midbrain is the reticular formation. This is a nerve network that acts as the brain's 'gatekeeper' for arousal and attention. It filters incoming sensory stimuli and decides what is important enough to wake up the rest of the brain. If you are sleeping and someone whispers your name, your reticular formation recognizes the significance and alerts the cortex. It is the reason you can sleep through a loud fan but wake up to a crying baby. In the midbrain, it also helps coordinate simple movements with sensory input, such as turning your head toward a sudden loud noise.
In a famous 1949 experiment, researchers severed a cat's reticular formation from higher brain regions. 1. The cat lapsed into a permanent coma. 2. Even loud noises or painful stimuli could not wake it. 3. This proved that without the 'arousal' signal from the reticular formation, the rest of the brain remains 'dark' regardless of sensory input.
Which structure is most likely damaged if a person can move their limbs but their movements are jerky, uncoordinated, and lack balance?
A patient is in a deep coma and cannot be aroused by any stimuli. Which part of the brainstem is likely injured?
The medulla is considered an 'evolutionarily old' structure because it performs functions that are also found in simpler animals like reptiles.
Review Tomorrow
In 24 hours, try to sketch the brainstem and label the 'Big Four': Medulla, Pons, Cerebellum, and Reticular Formation. Can you list one 'failure' symptom for each?
Practice Activity
Observe your own body: The next time you trip and catch your balance, or wake up suddenly to a noise, identify which part of your hindbrain or midbrain just saved the day.