Biology 30 · Nervous and endocrine
Signals in the body
Your body has two messaging systems. Nerves are fast and precise, like a text to one person. Hormones are slower and broad, like an announcement to everyone. Most questions are about how each one works.
The neuron
The idea: signals come in through the dendrites, pass the cell body, and travel out along the axon to the axon terminals.
There are three kinds of neuron: sensory neurons carry information in from receptors, motor neurons carry commands out to muscles and glands, and interneurons connect the two inside the brain and spinal cord.
The action potential
The idea: at rest the inside of a neuron is −70 mV. If a stimulus pushes it past threshold (about −55 mV), sodium rushes in, then potassium rushes out, and the signal fires.
- All-or-none: every action potential is the same size. A stronger stimulus makes impulses fire more often, not bigger.
- Sodium–potassium pump: uses ATP to move 3 Na⁺ out and 2 K⁺ in, restoring the resting state.
Crossing the synapse
Neurons do not touch. When the impulse reaches the axon terminal, calcium triggers vesicles to release a neurotransmitter such as acetylcholine into the gap. It binds to the next cell and may start a new impulse. Then cholinesterase breaks acetylcholine down so the signal stops.
Watch out: block cholinesterase and the signal does not stop — acetylcholine lingers and keeps stimulating the next cell. That is how some nerve agents and insecticides work.
How the nervous system is organized
The idea: the central nervous system (brain and spinal cord) makes decisions; the peripheral nervous system carries messages to and from it.
| Division | What it controls |
|---|---|
| Somatic | Voluntary movement of skeletal muscles |
| Autonomic: sympathetic | "Fight or flight" — faster heart rate, dilated pupils, digestion slowed |
| Autonomic: parasympathetic | "Rest and digest" — slower heart rate, digestion resumes |
| Brain region | Main job |
|---|---|
| Cerebrum | Thinking, memory, senses, voluntary movement |
| Cerebellum | Balance, coordination, posture |
| Medulla oblongata | Breathing, heart rate, blood pressure |
| Hypothalamus | Links nerves and hormones; controls the pituitary, temperature, thirst |
The reflex arc
Touch something hot and you pull away before you feel pain. The signal goes receptor → sensory neuron → interneuron in the spinal cord → motor neuron → effector (muscle), skipping the brain entirely. The brain finds out a moment later.
Hormones and negative feedback
The idea: glands release hormones into the blood, and most are controlled by negative feedback — the result of the hormone switches off its own release.
| Hormone | From | Effect |
|---|---|---|
| Insulin | Pancreas | Lowers blood glucose; liver stores it as glycogen |
| Glucagon | Pancreas | Raises blood glucose; liver releases it |
| ADH | Posterior pituitary | Kidneys reabsorb more water; less, more concentrated urine |
| TSH | Anterior pituitary | Tells the thyroid to release thyroxine |
| Thyroxine | Thyroid | Raises metabolic rate |
| Adrenaline | Adrenal medulla | Short-term stress response |
Feedback in action
Thyroxine levels drop → the hypothalamus and pituitary release more TSH → the thyroid releases thyroxine → rising thyroxine switches TSH back down. The loop holds levels steady.
Reading a "what goes wrong" question
- Thyroid tumour makes too much thyroxine: TSH will be low, because feedback is telling the pituitary to stop — the tumour just ignores it.
- No ADH (diabetes insipidus): the kidneys cannot hold water, so there are large volumes of dilute urine.
- No insulin (type 1 diabetes mellitus): blood glucose stays high and glucose appears in the urine.
Watch out: diabetes insipidus and diabetes mellitus are different diseases. Insipidus is about water and ADH; mellitus is about glucose and insulin.