Coordination: Nervous and Endocrine Systems

Coordination: Nervous and Endocrine Systems

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Coordination is the way organisms detect stimuli and produce suitable responses using two linked systems: the nervous system (rapid, electrical, short-lived) and the endocrine system (slower, chemical via blood, long-lasting). A stimulus is detected by a receptor, the message travels via neurones, crosses a synapse using a neurotransmitter such as acetylcholine, and reaches an effector (muscle or gland). The simplest pathway is the reflex arc: receptor → sensory neurone → relay (intermediate) neurone in the spinal cord → motor neurone → effector. Key endocrine glands include the pituitary (“master gland”), thyroid (thyroxine), adrenal (adrenaline), and pancreatic islets (insulin, glucagon). NECO SSCE Biology tests reflex arc labelling, neurone structure, and hormone functions almost every year.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Neurone Structure and Function

A neurone is the functional unit of the nervous system. Each consists of dendrites (branched extensions that receive impulses), a cell body containing the nucleus, an axon that carries impulses away, and a myelin sheath (formed by Schwann cells) which insulates the axon and enables saltatory conduction — impulses “jump” between gaps called Nodes of Ranvier, increasing speed. There are three functional types: sensory (afferent) neurones carrying impulses from receptors to the CNS, motor (efferent) neurones carrying impulses from the CNS to effectors, and relay (intermediate) neurones connecting them within the CNS.

The Reflex Arc

A reflex is a rapid, involuntary, stereotyped response. The reflex arc has five components: receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (muscle or gland). At each synapse, the electrical impulse triggers synaptic vesicles to release a neurotransmitter (e.g., acetylcholine) into the synaptic cleft; this binds receptors on the postsynaptic membrane, regenerating an impulse in the next neurone. Synaptic transmission is unidirectional because only the presynaptic knob stores neurotransmitter.

Organisation of the Nervous System

The central nervous system (CNS) comprises the brain and spinal cord. The peripheral nervous system (PNS) consists of cranial and spinal nerves, divided into the somatic system (voluntary, controls skeletal muscle) and the autonomic system (involuntary, controlling smooth muscle, cardiac muscle and glands). The autonomic system splits into sympathetic (fight-or-flight, e.g., raises heart rate via noradrenaline) and parasympathetic (rest-and-digest, e.g., slows heart rate via acetylcholine) branches.

The Endocrine System

Endocrine glands are ductless and release hormones directly into the bloodstream; hormones travel to target cells bearing specific receptors. Major glands and hormones include:

Gland	Hormone	Main Function
Pituitary (anterior)	TSH, ACTH, FSH, LH, GH	Controls other endocrine glands
Pituitary (posterior)	ADH, oxytocin	Water reabsorption; uterine contraction
Thyroid	Thyroxine	Regulates metabolic rate
Adrenal medulla	Adrenaline	Increases heart rate, blood glucose
Pancreas (β cells)	Insulin	Lowers blood glucose
Pancreas (α cells)	Glucagon	Raises blood glucose
Ovary	Oestrogen, progesterone	Female secondary sexual characters, menstrual cycle
Testis	Testosterone	Male secondary sexual characters

The pituitary is the “master gland” because its anterior lobe hormones regulate the thyroid, adrenal cortex and gonads. Negative feedback maintains homeostasis: e.g., high thyroxine inhibits TSH release, lowering further thyroxine secretion.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Comparing the Two Systems

Nerve impulses are measured in milliseconds, hormone effects in minutes to hours or days. Nervous responses are localised and brief; endocrine responses are widespread and prolonged. The two systems are integrated at the hypothalamus, which contains neurosecretory cells producing ADH and oxytocin that travel down axons to the posterior pituitary for storage and release.

Synaptic Transmission in Detail

When an impulse arrives at the synaptic knob, voltage-gated Ca²⁺ channels open; Ca²⁺ entry causes vesicles to fuse with the presynaptic membrane, releasing neurotransmitter by exocytosis. The transmitter diffuses across the cleft, binds receptors, and opens ion channels on the postsynaptic membrane — producing either an excitatory (depolarising) or inhibitory (hyperpolarising) potential. Neurotransmitter is then broken down (e.g., acetylcholinesterase splits acetylcholine) or reabsorbed, terminating the signal. Drugs and toxins such as curare block receptors, while organophosphates inhibit acetylcholinesterase.

Homeostasis and Negative Feedback

Homeostasis keeps the internal environment stable (e.g., blood glucose ~90 mg/100 mL). After a meal, rising glucose stimulates pancreatic β cells to release insulin, which promotes glucose uptake by muscle and liver cells and conversion to glycogen. In fasting, glucagon from α cells triggers glycogenolysis, raising blood glucose. If insulin is deficient, blood glucose rises abnormally, producing diabetes mellitus detectable by glucose in urine.

Common Mistakes in NECO SSCE

Candidates often confuse endocrine with exocrine glands (exocrine have ducts, e.g., salivary); mislabel the reflex arc by omitting the relay neurone; mix up insulin and glucagon effects; and think hormones travel along nerves rather than in the blood.

Practice Prompts

1. Outline the sequence of events in a reflex action initiated by touching a hot object.
2. Describe how the hormones insulin and glucagon work antagonistically to regulate blood glucose.

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