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IB Sports, Exercise & Health Science: A.1 · Communication

Nervous and endocrine systems, motor units, feedback. 2 subtopics, 8 key terms, full free lesson below.

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In this topic: A.1.1 · A.1.2

A.1.1Nervous system and motor units

Motor unit: A motor neuron and all the muscle fibres it stimulates.
Action potential: A rapid electrical impulse that travels along a neuron.
Neuromuscular junction: Where a motor neuron meets a muscle fibre; acetylcholine crosses the gap.
All-or-none law: A motor unit either contracts fully or not at all when stimulated above threshold.

The central nervous system (brain and spinal cord) processes information; the peripheral nervous system carries sensory (afferent) signals in and motor (efferent) signals out.
At the neuromuscular junction, acetylcholine binds to receptors on the muscle fibre, triggering an action potential that leads to contraction.
Force is graded by recruiting more motor units (small units first, then larger: the size principle) and by increasing the frequency of stimulation (summation, tetanus).
Slow-twitch (type I) fibres: many mitochondria and capillaries, fatigue-resistant, aerobic (marathon). Fast-twitch (type IIx): high force, fast, fatigue quickly, anaerobic (sprint). Type IIa is intermediate.

Explain how a weightlifter produces more force when lifting a heavier bar.

More motor units are recruited, including large fast-twitch units (size principle).
Motor neurons fire at a higher frequency, so twitches summate into a stronger, sustained contraction.
Better synchronization of motor units after training also increases force.

Exam tip: Link fibre type to a named sport and to its features: mitochondria, capillaries, myoglobin, speed and fatigue.

Hormone: Chemical messenger carried in the blood from an endocrine gland to target cells.
Negative feedback: A change triggers a response that reverses it, keeping conditions stable (homeostasis).
Adrenaline: Hormone from the adrenal medulla that raises heart rate, breathing rate and glucose release.
Insulin / glucagon: Pancreatic hormones that lower / raise blood glucose.

The endocrine system is slower than the nervous system but its effects last longer.
During exercise, adrenaline and noradrenaline increase heart rate and stroke volume and mobilize fuel; glucagon rises and insulin falls to keep blood glucose up; cortisol helps mobilize fats and proteins in long exercise.
ADH (antidiuretic hormone) reduces urine output when the body is dehydrated.
Negative feedback examples: blood glucose regulation, thermoregulation (sweating, vasodilation), blood pressure control.

Describe how blood glucose is maintained during a 90-minute football match.

Muscles take up glucose, so blood glucose tends to fall.
The pancreas releases more glucagon and less insulin; glucagon stimulates the liver to break down glycogen and release glucose.
Adrenaline and cortisol also mobilize glucose and fats: negative feedback restores blood glucose.

Exam tip: For any feedback question, name the receptor, the control centre and the effector.