Coordination and Control — Nervous and Chemical Coordination
Animals integrate information through two parallel systems: the nervous system (fast, electrical, short-lived) and the endocrine system (slower, chemical, long-lasting). This chapter focuses on where neurons communicate — the synapse — the major regions of the brain, the endocrine glands and their hormones, and feedback regulation. Neuron structure, action potentials, and the spinal cord are covered in the companion Neurons and Nervous Signaling notes. This is one of the highest-yield areas for the Bio/Biochem section.
A signal arriving at the end of one neuron cannot simply flow into the next — the cells are separated by a tiny gap. That gap, and the chemical machinery that bridges it, is the synapse, where the electrical language of neurons is converted into a chemical message.
The Synapse
A synapse is the junction between a neuron and its target (another neuron, a muscle, or a gland). Most are chemical synapses that use a neurotransmitter; a few are electrical (gap junctions) that pass current directly.
- An action potential reaches the presynaptic axon terminal.
- Voltage-gated Ca2+ channels open; Ca2+ enters the terminal.
- Synaptic vesicles fuse with the membrane and release neurotransmitter into the synaptic cleft (~20 nm wide) by exocytosis.
- Neurotransmitter diffuses across and binds receptors on the postsynaptic membrane.
- Ligand-gated ion channels open → a postsynaptic potential (EPSP or IPSP).
- The signal is terminated by degradation, reuptake, or diffusion (see below).
The postsynaptic response depends on which channels the neurotransmitter opens:
- EPSP (excitatory) — e.g., glutamate opens Na+ channels, depolarizing the cell toward threshold.
- IPSP (inhibitory) — e.g., GABA opens Cl− (or K+) channels, hyperpolarizing the cell away from threshold.
A neuron adds up many inputs by spatial summation (many synapses at once) and temporal summation (rapid repeats). It fires only if the net depolarization at the axon hillock reaches threshold.
Transmission must stop quickly so the synapse can reset:
- Enzymatic degradation — e.g., acetylcholinesterase breaks down acetylcholine in the cleft.
- Reuptake — transporters pump the neurotransmitter back into the presynaptic terminal (e.g., serotonin; this is the target of SSRIs).
- Diffusion away from the cleft.
Major neurotransmitters
- Acetylcholine — neuromuscular junction, parasympathetic.
- Noradrenaline — sympathetic.
- Dopamine — reward, motor control (deficient in Parkinson's).
- Serotonin — mood, sleep.
- GABA — main inhibitory transmitter in the brain.
- Glutamate — main excitatory transmitter in the brain.
Parts of the Brain
The human brain has three primary divisions: forebrain (cerebrum, thalamus, hypothalamus), midbrain, and hindbrain (pons, medulla, cerebellum). The brainstem (midbrain + pons + medulla) links the brain to the spinal cord. The whole organ weighs ~1.4 kg.
Largest part — ~85% of brain mass. Two hemispheres connected by the corpus callosum. The outer gray matter is folded into gyri and sulci. Four lobes:
- Frontal — voluntary motor control, planning, speech (Broca's area).
- Parietal — somatic sensation, taste.
- Temporal — hearing, smell, language (Wernicke's area), memory.
- Occipital — vision.
The cerebrum integrates higher functions: thought, reasoning, language, and conscious memory.
Located behind the medulla. Coordinates voluntary muscular activity, balance, posture, and fine motor learning. Damage → ataxia (clumsy, uncoordinated movement).
- Midbrain — visual and auditory reflexes (e.g., the pupillary reflex).
- Pons — bridge between the cerebellum and the rest of the brain; helps regulate breathing.
- Medulla oblongata — continuous with the spinal cord; houses vital centers for heartbeat, breathing, blood pressure, vomiting, swallowing, and coughing. Damage is rapidly fatal.
The hypothalamus (below the thalamus) is the master regulator of homeostasis: thermoregulation, hunger, thirst, and the sleep-wake cycle. It controls the autonomic nervous system and the pituitary gland — the key bridge between the nervous and endocrine systems.
- Thalamus — sensory relay station to the cerebral cortex.
- Limbic system — emotion and memory (hippocampus, amygdala).
Endocrine System
The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. Hormones act on distant target cells that carry the matching receptor. Endocrine signaling is slower than nervous signaling but produces sustained, widespread effects.
How a hormone acts depends on whether it can cross the plasma membrane:
- Peptide / amine hormones (e.g., insulin, glucagon, ADH, epinephrine) are water-soluble and cannot cross the membrane. They bind cell-surface receptors and act through second messengers (e.g., cAMP), triggering fast, short-lived responses via enzyme cascades. They travel free in the blood.
- Steroid hormones (e.g., cortisol, aldosterone, testosterone, estrogen) are lipid-soluble and diffuse through the membrane. They bind intracellular (cytoplasmic/nuclear) receptors; the complex acts as a transcription factor to alter gene expression — a slower but longer-lasting effect. They travel bound to carrier proteins.
Thyroid hormone is the classic exception: although amine-derived, it is lipophilic and, like a steroid, acts on intracellular receptors to change gene expression.
The hypothalamus controls the pituitary ("master gland") via releasing/inhibiting hormones.
- Anterior pituitary — secretes GH (growth hormone), TSH, ACTH, FSH, LH, and prolactin.
- Posterior pituitary — stores and releases ADH (vasopressin) and oxytocin, which are produced in the hypothalamus.
Disorders: gigantism / acromegaly (excess GH), dwarfism (deficient GH), diabetes insipidus (deficient ADH).
The thyroid (anterior neck) secretes thyroxine (T4) and triiodothyronine (T3) — which raise the basal metabolic rate — and calcitonin (lowers blood Ca2+). Iodine deficiency → goiter; hyperactivity → Graves' disease; underactivity → cretinism (children) or myxedema (adults).
The parathyroid glands secrete PTH, which raises blood Ca2+ — the antagonist of calcitonin.
β-cells secrete insulin — lowers blood glucose by promoting cellular uptake and glycogen synthesis. α-cells secrete glucagon — raises blood glucose by glycogenolysis. Type 1 diabetes is autoimmune destruction of β-cells; type 2 is insulin resistance.
The adrenal glands sit atop the kidneys. The cortex secretes mineralocorticoids (aldosterone — Na+ retention), glucocorticoids (cortisol — stress, raises glucose), and a little sex steroid; the medulla secretes adrenaline (epinephrine) and noradrenaline — the "fight-or-flight" hormones. The gonads secrete sex steroids: ovaries make estrogen and progesterone; testes make testosterone, all under FSH/LH control.
| Hormone | Gland | Main action | Hyper / hypo disorder |
|---|---|---|---|
| Growth hormone (GH) | Anterior pituitary | Stimulates growth, protein synthesis | Hyper: gigantism / acromegaly · Hypo: dwarfism |
| TSH | Anterior pituitary | Stimulates thyroid to release T3/T4 | — |
| ACTH | Anterior pituitary | Stimulates adrenal cortex (cortisol) | — |
| FSH / LH | Anterior pituitary | Gametogenesis, ovulation, sex hormones | Infertility |
| ADH (vasopressin) | Posterior pituitary | Water reabsorption in the kidney | Hypo: diabetes insipidus |
| Oxytocin | Posterior pituitary | Uterine contractions, milk ejection | — |
| Thyroxine (T4) | Thyroid | Raises basal metabolic rate | Hyper: Graves' · Hypo: cretinism / myxedema; iodine deficiency → goiter |
| Calcitonin | Thyroid (C-cells) | Lowers blood Ca2+ | — |
| PTH | Parathyroid | Raises blood Ca2+ (antagonist of calcitonin) | Hypo: tetany · Hyper: bone demineralization |
| Insulin | Pancreas (β-cells) | Lowers blood glucose → glycogen | Hypo: diabetes mellitus (type 1, type 2) |
| Glucagon | Pancreas (α-cells) | Raises blood glucose by glycogenolysis | — |
| Cortisol | Adrenal cortex | Stress response, raises glucose, anti-inflammatory | Hyper: Cushing's · Hypo: Addison's |
| Aldosterone | Adrenal cortex | Na+ retention, K+ excretion (kidney) | Affects BP regulation |
| Adrenaline | Adrenal medulla | "Fight-or-flight" — ↑ HR, ↑ BP, ↑ glucose | — |
| Testosterone | Testes (Leydig) | Male sex characters, spermatogenesis | Hypogonadism |
| Estrogen / Progesterone | Ovaries | Menstrual cycle, pregnancy maintenance | Cycle disorders |
Feedback Mechanism
The endocrine and nervous systems regulate themselves through feedback loops.
The output reverses the original change, restoring the set point. Examples:
- High blood glucose → insulin release → glucose uptake → glucose falls → insulin release stops.
- Low T4 → hypothalamus releases TRH → pituitary releases TSH → thyroid releases T4 → rising T4 inhibits TRH/TSH.
- Body temperature, blood pressure, and blood Ca2+.
The output amplifies the change. Examples:
- Childbirth — oxytocin causes uterine contraction → pushes the baby's head onto the cervix → more oxytocin.
- Blood clotting — activated platelets recruit more platelets.
- LH surge at ovulation.
Positive feedback always ends with a definite event (delivery, a sealed vessel, ovulation), or it would run away.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter.
Q1. Arrival of an action potential at the axon terminal triggers neurotransmitter release by causing:
Depolarization of the terminal opens voltage-gated Ca2+ channels; the Ca2+ influx triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter by exocytosis. Reuptake instead terminates the signal afterward.
Q2. Which part of the brain coordinates posture and balance?
The cerebellum coordinates voluntary movement, posture, balance, and fine motor learning; damage causes ataxia. The medulla controls vital autonomic centers, and the hypothalamus regulates homeostasis.
Q3. Insulin is secreted by which cells of the pancreas?
β-cells of the islets of Langerhans secrete insulin in response to high blood glucose. α-cells secrete glucagon (raises blood glucose). Acinar cells of the exocrine pancreas secrete digestive enzymes — not hormones.
Q4. Steroid hormones such as cortisol act primarily by:
Steroids are lipid-soluble, cross the plasma membrane, and bind cytoplasmic/nuclear receptors; the hormone-receptor complex regulates gene transcription — a slower, longer-lasting effect. Peptide/amine hormones (e.g., insulin, epinephrine) instead use surface receptors and second messengers like cAMP.
Q5. Childbirth (parturition) is regulated by which kind of feedback?
Oxytocin causes uterine contractions, which push the baby's head onto the cervix; cervical stretch signals the hypothalamus to release more oxytocin. Each round amplifies the next — classic positive feedback. The loop ends when the baby is delivered.
Quick Recap
- Synapse: action potential → Ca2+ influx → vesicle fusion → neurotransmitter release; cleared by enzymes, reuptake, or diffusion.
- EPSP (excitatory, e.g. glutamate) vs IPSP (inhibitory, e.g. GABA); spatial/temporal summation at the axon hillock decides firing.
- Major neurotransmitters: ACh, noradrenaline, dopamine, serotonin, GABA, glutamate.
- Brain: cerebrum (cognition), cerebellum (balance), medulla (vital reflexes), hypothalamus (homeostasis + pituitary control), thalamus (relay).
- Endocrine glands: hypothalamus, pituitary (FLAT PiG anterior; OA posterior), thyroid, parathyroid, pancreas, adrenals, gonads.
- Peptide/amine hormones → surface receptors + second messengers (fast); steroid hormones → intracellular receptors + gene transcription (slow).
- Feedback: negative (most homeostasis — glucose, thyroid axis); positive (childbirth, blood clotting, LH surge).