Nerves, hormones, and immunity
The nervous and endocrine systems coordinate the body, while the immune system defends it.
Neurons
Neurons are cells that carry electrical signals. Dendrites receive signals, the cell body processes them, and the axon sends an impulse along to its terminals. A fatty myelin sheath insulates many axons, speeding impulses to over 100 metres per second.
Synapses
Neurons communicate across tiny gaps called synapses. When an impulse arrives, the neuron releases neurotransmitters, chemicals that cross the gap and bind receptors on the next cell. Many drugs work at synapses: some antidepressants increase serotonin levels; nicotine mimics acetylcholine.
The nervous system
The central nervous system is the brain and spinal cord; the peripheral nervous system connects them to the rest of the body. Reflexes, like pulling a hand from a hot stove, travel through the spinal cord before the brain is even aware, saving precious time.
The brain
The cerebrum handles thought, memory, language, and voluntary movement; it is divided into lobes with different roles. The cerebellum coordinates balance and fine movement. The brainstem controls automatic functions like breathing and heart rate.
The endocrine system
Endocrine glands release hormones into the blood, which carry them to target cells anywhere in the body. Hormonal signals are slower than nerve signals but last longer. The pituitary gland, under the hypothalamus, controls many other glands. Adrenaline from the adrenal glands prepares the body for fight or flight.
Innate immunity
The first lines of defence are barriers such as skin, mucus, and stomach acid. If microbes get in, innate immune cells such as phagocytes engulf them, and inflammation brings more cells to the site, causing redness, heat, and swelling. Fever can also slow pathogens.
Adaptive immunity
Adaptive immunity targets specific invaders. B cells make antibodies, proteins that bind specific antigens and mark them for destruction. T cells kill infected cells and coordinate the response. After an infection, memory cells remain, so a second exposure triggers a faster, stronger response.
Vaccines
Vaccines expose the immune system to a harmless form or piece of a pathogen, such as an inactivated virus or an mRNA instruction for one viral protein. The body builds memory cells without having the disease. When enough people are immune, spread is limited, protecting those who cannot be vaccinated (herd immunity).
Key takeaways
Neurons carry electrical impulses and signal each other with neurotransmitters at synapses. The brain and spinal cord form the central nervous system; reflexes save time. Hormones are slower, longer-lasting chemical signals. Innate immunity provides barriers and general responses; adaptive immunity uses antibodies, T cells, and memory. Vaccines build memory safely.
Flashcard
Nerve cell carrying electrical signals
Check your understanding
Why does a second infection by the same virus usually cause milder illness?