Coordination is how the body keeps all its parts acting in concert. Two systems share the job — the fast, electrical nervous system and the slow, chemical endocrine system. CBSE Class 10 and 11 both dedicate full chapters to this, and NEET loves assertion-reason questions on reflexes and hormone action.
Why two control systems? Because speed and duration have trade-offs. When you touch a hot plate, your hand jerks back in 50 milliseconds — the nervous system handles this. When your body needs to grow taller over years, hormones manage that. The nervous system is the phone call; the endocrine system is the newsletter. Both are essential, neither alone is sufficient.
Core Concepts
Two control systems compared
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Signal type | Electrical impulses + neurotransmitters | Hormones via blood |
| Speed | Milliseconds | Seconds to days |
| Duration | Brief (ms to seconds) | Long-lasting (hours to years) |
| Specificity | Targeted (specific neurons → specific effectors) | Widespread (all cells with receptor respond) |
| Pathway | Neuron → synapse → neuron/effector | Gland → blood → target cells everywhere |
The neuron
The functional unit. Three parts: cell body (soma, contains nucleus and Nissl granules), dendrites (receive signals), axon (transmits signals). Myelin sheath from Schwann cells insulates the axon and speeds conduction. Nodes of Ranvier are bare gaps for saltatory conduction.
Types: sensory (afferent, toward CNS), motor (efferent, away from CNS), interneurons (within CNS).
Reflex arc
The shortest path from stimulus to response. Receptor → sensory neuron → interneuron in spinal cord → motor neuron → effector. The knee-jerk reflex (monosynaptic) and withdrawal from a hot plate (polysynaptic) are textbook examples. The brain is bypassed, which saves critical time.
Central and peripheral nervous system
CNS — brain and spinal cord, the processing centre.
| Brain part | Function |
|---|---|
| Cerebrum | Thought, memory, voluntary movement |
| Cerebellum | Balance, coordination |
| Medulla | Breathing, heart rate, BP |
| Hypothalamus | Temperature, hunger, endocrine link |
| Pons | Relay, sleep |
| Midbrain | Visual/auditory reflexes |
PNS — all nerves outside CNS:
- Somatic (voluntary) — skeletal muscles
- Autonomic (involuntary):
- Sympathetic — fight or flight (heart rate up, pupils dilate)
- Parasympathetic — rest and digest (heart rate down, digestion active)
Action potential
At rest: -70 mV. Stimulus opens Na channels → Na rushes in → depolarisation to +30 mV → Na channels inactivate → K channels open → K rushes out → repolarisation → brief hyperpolarisation → resting potential restored by Na/K pump.
Key properties: all-or-none response, refractory period for unidirectional travel, saltatory conduction in myelinated fibres (up to 120 m/s).
Synaptic transmission
Action potential reaches the synaptic knob → Ca enters → vesicles fuse with membrane → neurotransmitter released into cleft → binds postsynaptic receptors → generates new signal → neurotransmitter removed (enzymatic breakdown, reuptake or diffusion).
Key neurotransmitters: acetylcholine (muscle, parasympathetic), dopamine (reward, movement), serotonin (mood, sleep), GABA (inhibitory), glutamate (excitatory), noradrenaline (sympathetic).
Endocrine glands and their hormones
| Gland | Key hormones | Action |
|---|---|---|
| Hypothalamus | Releasing/inhibiting hormones | Controls pituitary |
| Anterior pituitary | GH, TSH, ACTH, FSH, LH, prolactin | Master gland |
| Posterior pituitary | ADH, oxytocin | Water balance, labour |
| Thyroid | T3, T4, calcitonin | Metabolism, Ca regulation |
| Parathyroid | PTH | Raises blood Ca |
| Adrenal cortex | Cortisol, aldosterone | Stress, Na balance |
| Adrenal medulla | Adrenaline | Fight or flight |
| Pancreas (islets) | Insulin, glucagon | Blood glucose |
| Testes | Testosterone | Male characters |
| Ovaries | Oestrogen, progesterone | Female characters |
| Pineal | Melatonin | Sleep-wake cycle |
| Thymus | Thymosin | T-cell maturation |
Feedback mechanisms
Negative feedback (standard): the effect inhibits further hormone release. Example: high T3/T4 → inhibits TSH → less thyroid stimulation.
Positive feedback (rare): the effect amplifies itself. Example: oxytocin during labour — contractions stimulate more oxytocin → stronger contractions → delivery.
Normal fasting blood glucose: 70-110 mg/dL. Maintained by antagonistic action of insulin (from beta cells) and glucagon (from alpha cells).
Disorders to know
| Disorder | Cause | Key feature |
|---|---|---|
| Diabetes mellitus | Insulin deficiency/resistance | High blood glucose |
| Diabetes insipidus | ADH deficiency | Massive dilute urine |
| Goitre | Iodine deficiency | Enlarged thyroid |
| Addison’s disease | Low cortisol | Low BP, skin darkening |
| Cushing’s syndrome | Excess cortisol | Moon face, obesity |
| Acromegaly | Excess GH in adults | Enlarged extremities |
| Parkinson’s | Dopamine deficiency | Tremor, rigidity |
Worked Examples
Voluntary movement involves the brain — sensory input travels up to the cortex, a decision is made, motor output travels back. A spinal reflex skips the cortex entirely, shaving off at least 100-200 ms. For touching a hot surface, this time saving prevents a serious burn.
A sudden loud noise triggers adrenaline from the adrenal medulla. Heart rate increases, pupils dilate, glucose is mobilised from liver glycogen, digestion stops, blood diverts to muscles. The sympathetic nervous system amplifies the same effects in parallel. One stimulus, many organs, all coordinated within seconds.
Both cause excessive urination (polyuria) and thirst (polydipsia). Mellitus means ‘honey-sweet’ — urine contains glucose (insulin failure). Insipidus means ‘tasteless’ — urine is dilute, no glucose (ADH failure). Different hormones, different mechanisms, similar symptoms.
Low T3/T4 → hypothalamus releases TRH → anterior pituitary releases TSH → thyroid produces T3/T4 → when levels rise, T3/T4 feeds back negatively on both TRH and TSH. In Hashimoto’s disease, autoimmune destruction of the thyroid means T3/T4 stays low despite very high TSH.
Ethanol enhances GABA (inhibitory) and reduces glutamate (excitatory) activity in the brain. Low doses suppress cortical inhibition (feeling relaxed). Higher doses impair the cerebellum (poor balance, slurred speech). Very high doses depress the medulla (risk of respiratory failure). The order of impairment — cortex then cerebellum then medulla — explains the dose-dependent clinical signs.
Common Mistakes
Saying the cerebellum controls thought. It controls balance and motor coordination. The cerebrum handles thought, memory and language.
Writing that hormones are faster than nerves. Nervous signals travel at up to 120 m/s; hormones take seconds to minutes via the bloodstream.
Confusing ADH and aldosterone. ADH conserves water (collecting duct). Aldosterone conserves sodium (DCT). Different hormones, different targets, different sources.
Calling the pituitary the master gland without noting that the hypothalamus controls it. The hypothalamus is the true master integrator.
Thinking sympathetic and parasympathetic always oppose each other. For most organs they do, but there are exceptions — male reproductive function requires both systems sequentially.
Exam Weightage and Strategy
Neural Control and Chemical Coordination together carry 8-12 marks in CBSE Class 11 boards. NEET asks 3-4 questions per year from these chapters combined. Questions cover: brain regions, action potential, reflex arc, hormone-gland matching, feedback mechanisms, and disorders.
Pair every hormone with its gland and its single-line effect. A flashcard set of 20 cards covers almost all of the endocrine PYQs. For the nervous system, memorise the action potential ion sequence and the sympathetic vs parasympathetic table.
Practice Questions
Q1. Distinguish between sympathetic and parasympathetic nervous systems (4 points).
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Situation | Stress, emergency | Rest, normal |
| Heart rate | Increases | Decreases |
| Pupils | Dilate | Constrict |
| Digestion | Inhibited | Stimulated |
| Neurotransmitter | Noradrenaline | Acetylcholine |
Q2. Why is the hypothalamus called the link between the nervous and endocrine systems?
The hypothalamus is a brain structure (part of the nervous system) that produces hormones (releasing and inhibiting factors) which control the anterior pituitary. It also directly synthesises ADH and oxytocin (stored in the posterior pituitary). It receives neural inputs (temperature, osmolarity, emotions) and converts them into hormonal outputs. No other structure bridges both systems as directly.
Q3. A patient has a tumour in the anterior pituitary that overproduces GH. Predict the effects in (a) a child and (b) an adult.
(a) In a child (before growth plate closure): gigantism — proportional but abnormally tall stature because GH stimulates longitudinal bone growth. (b) In an adult (after growth plate closure): acromegaly — bones cannot grow longer, so GH causes thickening of bones in the hands, feet and jaw, plus soft tissue enlargement. Both conditions can cause metabolic disturbances (insulin resistance, diabetes).
FAQs
What is the difference between a nerve and a neuron?
A neuron is a single nerve cell. A nerve is a bundle of many axons wrapped in connective tissue — like a cable containing many wires. The sciatic nerve contains hundreds of thousands of axon fibres from different neurons.
Can the nervous system regenerate?
PNS axons can regenerate slowly (~1-2 mm/day) if the cell body is intact. CNS regeneration is extremely limited due to inhibitory factors and glial scarring. This is why spinal cord injuries cause permanent paralysis and why CNS repair research is so important.
What is the enteric nervous system?
A semi-independent network of 100-500 million neurons in the gut wall, sometimes called the “second brain.” It controls peristalsis, enzyme secretion and blood flow locally. It communicates with the CNS via the vagus nerve (gut-brain axis) but can function without CNS input.
How do drugs affect synaptic transmission?
Many drugs work by altering neurotransmitter levels at synapses. SSRIs (antidepressants like fluoxetine) block serotonin reuptake, increasing serotonin concentration in the cleft. Caffeine blocks adenosine receptors (adenosine promotes drowsiness). Nicotine mimics acetylcholine at certain receptors. Curare blocks ACh receptors at the neuromuscular junction, causing paralysis. Organophosphate pesticides inhibit acetylcholinesterase, causing continuous ACh stimulation and muscle spasms. Understanding synapse pharmacology is increasingly important for NEET questions.
What is the role of calcium ions in synaptic transmission?
When an action potential arrives at the synaptic knob, voltage-gated Ca channels open. The influx of Ca triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter by exocytosis. Without Ca entry, vesicles do not fuse and no signal crosses the synapse. This is why Ca is called the trigger for neurotransmitter release.
Why is the hypothalamus considered the master integrator?
The hypothalamus receives sensory inputs from the nervous system (temperature sensors, osmoreceptors, emotional inputs from the limbic system) and converts them into hormonal outputs (releasing and inhibiting hormones that control the pituitary). It regulates body temperature, hunger, thirst, circadian rhythms, and the stress response. It directly synthesises ADH and oxytocin. No other structure bridges neural and endocrine control as completely.
What happens when the myelin sheath is damaged?
Demyelination (as in multiple sclerosis) exposes the axon membrane between nodes. Without insulation, ion currents leak out and the action potential weakens or fails to jump between nodes. Nerve conduction slows dramatically or stops entirely. Symptoms include numbness, weakness, vision problems and coordination difficulties, depending on which nerves are affected.
Coordination is the chapter where physiology becomes integration. Think of the nervous system as the phone call and the endocrine system as the newsletter — both necessary, different speeds, same goal.