Control and Coordination

Your body is doing a million things right now — breathing, balancing, blinking, digesting — and you're not thinking about any of it. That's because your nervous system and endocrine system handle everything behind the scenes. This chapter is about the control room (brain) and the messaging systems (nerves and hormones) that keep you alive. The nervous system is built from specialized cells called neurons. Each neuron has dendrites (receivers), a cell body (processor), an axon (wire), and axon terminals (output). The signal travels as an electrical impulse along the axon. In myelinated neurons, the signal jumps from node to node (saltatory conduction) — much faster than in unmyelinated neurons. When the signal reaches the end of a neuron, it crosses a tiny gap called the synapse. Neurotransmitters (chemical messengers) are released, float across the gap, and bind to receptors on the next neuron. Then the signal continues. Some drugs work by messing with this process. The brain has three main parts. The cerebrum is the biggest part — responsible for thinking, memory, voluntary movement, and sensation. The cerebellum handles balance and coordination. The brainstem (medulla) handles automatic stuff — heartbeat, breathing, digestion — the things you never think about. The brain uses 20% of your body's oxygen despite being only 2% of your weight. The spinal cord runs from the brainstem down the spine and serves as the information highway between the brain and the rest of the body. It's also the center for reflex actions. A reflex arc is the fastest response in the body. When you touch something hot, you pull your hand back before you even feel the pain. The signal goes from sensory neuron to spinal cord to motor neuron — straight back, no brain involvement. The brain gets the message a split second later as pain. This delay saves your skin from burning. The peripheral nervous system (PNS) connects the CNS to the rest of the body. It has two divisions: the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs). The autonomic system has two branches: sympathetic (fight or flight — speeds up heart, dilates pupils, slows digestion) and parasympathetic (rest and digest — slows heart, constricts pupils, stimulates digestion). They work antagonistically to maintain balance. Sense organs are the windows to the world. The eye has a lens that focuses light onto the retina (where photoreceptors — rods for dim light, cones for color — convert light to nerve signals). The ear converts sound waves into nerve signals (in the cochlea) and helps maintain balance (through the vestibular apparatus). The endocrine system is the slow messenger. While nerves act in milliseconds, hormones act in seconds to hours but last longer. Glands like the pituitary (master gland), thyroid, parathyroid, adrenal, pancreas, and gonads release hormones into the blood. These chemical messengers travel everywhere but only affect cells with the right receptors. The hypothalamus controls the pituitary, which in turn controls other glands — this is the hypothalamic-pituitary axis. Feedback mechanisms maintain homeostasis. Negative feedback is most common: when blood sugar goes up, insulin is released to bring it down. When you're cold, shivering generates heat. Positive feedback is rarer — like during childbirth, oxytocin makes contractions stronger, which triggers more oxytocin, until the baby is born.