The control system: how a decision becomes a movement, and how the body knows it worked

The nervous system divides once, and then each part divides again. Learning it as a branching tree rather than a list is what makes the function section obvious later.

• The central nervous system is the brain and the spinal cord. It receives information, interprets it, and decides what to do. Three regions of the brain matter for this course: the cerebrum, whose motor cortex initiates voluntary movement and whose sensory cortex receives the information coming back; the cerebellum, which coordinates and fine-tunes movement, controls balance and stores learned movement patterns; and the brain stem, which runs breathing, heart rate and other functions you never think about. The spinal cord is not just a cable — it is a processor in its own right, and reflexes are decided there.
• The peripheral nervous system is everything else: the nerves running to and from the central nervous system. It splits by direction. Sensory (afferent) neurons carry information inwards from receptors, and motor (efferent) neurons carry commands outwards to muscles and glands.

The motor side then splits again, by whether you control it:

• The somatic nervous system carries voluntary commands to skeletal muscle. Every movement in Lessons 1 to 3 travels this way.
• The autonomic nervous system controls cardiac muscle, smooth muscle and glands without conscious input. It has two opposing branches: the sympathetic branch, which prepares the body for exertion, and the parasympathetic branch, which restores it afterwards.

A neuron is the cell all of this is built from. Dendrites receive incoming signals; the cell body integrates them; the axon carries the impulse away, often wrapped in a fatty myelin sheath that dramatically increases conduction speed. Where one neuron meets another, or meets a muscle fibre, they do not touch: the signal crosses a synapse as a chemical released into a tiny gap. That chemical step is the slow part of the whole pathway, which is why the number of synapses a signal must cross predicts how quickly a response can happen.

A voluntary movement follows one pathway every time, and you should be able to recite it:

Motor cortex generates the command → the impulse descends the spinal cord → it passes to a motor neuron in the peripheral nervous system → the axon reaches the neuromuscular junction → a chemical transmitter crosses the synapse → the muscle fibre is stimulated and contracts.

The unit that matters is the motor unit: one motor neuron plus every muscle fibre it supplies. Two properties of motor units explain almost everything about how movement is controlled.

• A motor unit is all or nothing. When a motor neuron fires, every fibre in its unit contracts fully. There is no such thing as a fibre contracting slightly. So the nervous system cannot make a muscle work gently by contracting each fibre a little — it must work gently by using fewer units.
• Motor units differ enormously in size. A motor unit in the muscles that move the eye may contain fewer than ten fibres; one in the quadriceps may contain more than a thousand. Small units give fine control, large units give gross force, and the size of a muscle's motor units tells you what the muscle is for.

Force is then graded two ways, and both are examinable:

• Recruitment — switching on more motor units. Small, slow-twitch units are recruited first and large, fast-twitch units last, which is why the fast fibres from Lesson 2 contribute almost nothing to easy jogging and everything to a maximal jump.
• Rate coding — firing the same units more frequently. Impulses arriving fast enough that the fibre cannot relax between them produce a smooth, sustained and much stronger contraction.

Put those two together and you have the answer to a question Lesson 2 left open. The same muscle can lift a pencil and a barbell because the nervous system chooses how many units to switch on and how fast to drive them, not because the muscle has a volume dial.

Issuing a command is only half of it. If the nervous system could not tell what happened next, every movement would be a guess, and Card 01's one-legged stand would be impossible even with your eyes open.

Proprioception is the sense that closes the loop: the continuous awareness of where your limbs are and how fast they are moving, supplied by receptors inside the body itself. Three sources matter here:

• Muscle spindles, embedded among the muscle fibres, detect how long a muscle is and how quickly its length is changing.
• Golgi tendon organs, in the tendons, detect how much tension the muscle is generating, and can inhibit contraction to protect the tendon from excessive load.
• Joint receptors and the vestibular apparatus of the inner ear report joint position and the orientation of the head, which is what balance is built on.

This is the answer to Card 01. Closing your eyes removes vision, but it does not remove proprioception — you can still stand, just less well. Vision was one input among several, and the body was cross-checking them against each other. Take one away and the remaining inputs have to carry the whole load, so the corrections become larger and later, which you experience as wobble.

Some corrections are too urgent to wait for the brain. A reflex arc routes the response through the spinal cord instead:

Receptor detects the stimulus → sensory neuron carries it to the spinal cord → a connecting neuron in the cord passes it straight across → motor neuron carries the command out → muscle responds. The brain is informed, but afterwards. That is why you have already pulled your hand off something hot before you feel the pain, and why the knee-jerk response happens whether you want it to or not.

The speed difference is worth stating precisely, because it is the most common short-answer question on this material. A reflex is faster than a voluntary response because the signal travels a shorter distance and crosses fewer synapses, not because the neurons themselves conduct faster. Every chemical crossing costs time, and the reflex route removes most of them.

Everything so far has been the somatic side. The autonomic side is what connects the nervous system to every other system in this module, and it is the reason Lessons 4, 5 and 6 behaved the way they did.

Read the last row again, because it settles something Lesson 6 raised. The sympathetic nervous system and adrenaline produce almost identical effects — deliberately. The nervous system acts within milliseconds but cannot sustain the response; the endocrine system takes seconds to arrive but holds the response for minutes. One triggers the other, so the body gets both the immediacy and the duration. That is a genuine interrelationship, and it is a far better answer than describing the two systems separately.

It also explains a measurement you will use in Lesson 14. The speed with which heart rate falls after exercise stops is largely parasympathetic reactivation, so recovery heart rate is, in part, a measure of how quickly the nervous system can switch branches — which is why it improves with aerobic training and worsens under fatigue, illness and stress.

The nervous system is usually described as if it were fixed. It is not, in either direction.

It improves with training, and it improves first. In the opening weeks of a resistance program, strength rises well before muscle size does. The gain is neural: better recruitment of the large fast-twitch units, faster firing rates, better timing between agonist and antagonist, and reduced inhibition from the Golgi tendon organs. This is why a beginner gets rapidly stronger without looking any different, and why it is a mistake to read early strength gains as evidence of hypertrophy.

It also degrades, and the causes are the ones an athlete can actually control:

Choose a movement you perform often — a netball shot, a sprint start, a tennis serve, a dance step. Then trace it through the nervous system rather than describing what it looks like.

Then the analytical part: which single link in your chain would degrade first if you performed this movement in the last minute of a hard session, and what would the spectator actually see?

Work in pairs. One partner holds a 30 cm ruler vertically, with the zero mark level with the top of the other partner's open thumb and finger. Without warning, the holder releases it; the catcher closes on it as fast as they can. Record the distance the ruler fell before it was caught. Do ten trials each.

This is a reaction time test, and the number it produces is not the point. The analysis is:

• List every structure the signal passes through between the ruler moving and the fingers closing. Start at the retina and finish at the muscle fibre. Your list should be longer than you expect, and every item on it costs time.
• Why is this not a reflex? Use the distinction from Card 06, and be precise about which part of the route makes the difference.
• Now repeat it with a warning — the holder counts down before releasing. Reaction times almost always shorten. Explain what changed, given that the pathway is identical. Think about what the nervous system can do in advance.
• Predict, then test if your teacher allows it: what would happen to the times at the end of a hard training session, and which row of the Card 08 table predicts it?

Finally, the measurement question this course keeps returning to: name two things about this test that would make the same student score differently on two different days, and say whether each is a problem with the instrument or with the person.