Coordination and Response in Humans, revision notes

Overview

The body coordinates its actions through two systems working side by side: the nervous system, which sends fast electrical impulses along neurones to produce an immediate, short-lived response, and the endocrine system, which releases chemical hormones into the blood to produce a slower, longer-lasting and often more widespread response. This chapter covers the structure of neurones, how information crosses the synapse, the difference between voluntary and involuntary actions, the main hormones and the glands that release them, and what happens when either system malfunctions.

The reflex arc and the comparison between nervous and hormonal coordination are two of the most frequently examined ideas in this chapter, often appearing as a labelling diagram, a comparison table, or a short-answer question asking a student to trace the pathway of an impulse from stimulus to response. Getting the order of the five parts of a reflex arc exactly right, in the correct direction, is one of the easiest ways to secure full marks on this topic.

Both coordination systems can also malfunction, and this chapter includes two health issues as examples: a nervous system disorder such as stroke or Parkinson's disease, in which neurones in the brain are damaged or lost, and an endocrine disorder such as diabetes mellitus, in which the body produces too little insulin or its cells stop responding properly to the insulin available, causing blood glucose levels to rise dangerously high.

Learning the healthy pathway thoroughly before studying the disorder makes the disorder far easier to explain, because most exam answers on a health issue simply ask which specific part of the normal system has stopped working correctly and what effect that has on the body's ability to coordinate and respond.

Two coordination systems compared (Content Standard 12.1)

Content Standard 12.1 introduces the idea that the body coordinates its actions through two systems that work side by side. The nervous system sends fast electrical impulses along neurones to produce an immediate, short-lived and precisely targeted response.

The endocrine system releases chemical hormones into the blood to produce a response that begins more slowly, spreads more widely and lasts much longer.

A stimulus is a change detected by a receptor, and a response is the action of an effector, usually a muscle or a gland. In the nervous system a receptor sends an impulse through the neurones to an effector; in the endocrine system a gland releases a hormone that travels in the blood to its target cells.

The body uses both because some situations need a split-second reaction while others need a steady, sustained change such as growth.

The comparison in the table below is one of the most frequently examined ideas in the whole chapter, and the safest answers give the speed, the duration and the route of the signal for each system rather than a single difference.

The nervous system: central and peripheral (Content Standard 12.2)

The nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system is the brain and the spinal cord, where most information is processed and decisions are made.

The peripheral nervous system is the network of nerves that carries impulses between the central nervous system and every receptor and effector in the body.

Three parts of the brain are examined at this level. The cerebrum controls conscious thought, memory, reasoning and voluntary actions.

The cerebellum controls balance and the coordination of muscle movement. The medulla oblongata controls involuntary actions such as heartbeat, breathing rate and the diameter of blood vessels.

The spinal cord links the brain to the rest of the body and acts as the processing centre for many reflexes.

A precise answer names the correct part of the brain for a given function; a common loss of marks is naming the cerebrum for balance, which is the job of the cerebellum.

Neurones and the synapse (Content Standard 12.3)

There are three types of neurone, and each carries an impulse in a fixed direction. A sensory neurone carries an impulse from a receptor towards the central nervous system.

A relay neurone, found inside the brain and spinal cord, connects one neurone to another. A motor neurone carries an impulse away from the central nervous system to an effector such as a muscle or a gland.

A synapse is the tiny gap between the end of one neurone and the start of the next. An impulse cannot jump the gap electrically.

Instead, when the impulse reaches the end of the first neurone, it triggers the release of a chemical called a neurotransmitter, which diffuses across the gap and binds to receptors on the next neurone, starting a new impulse. Because the neurotransmitter is released on only one side and detected on the other, the synapse makes sure an impulse travels in one direction only.

The table below sets the three neurones side by side. Stating the direction of travel for each one is the key to answering questions on the nervous pathway correctly.

The reflex arc: a fast, automatic pathway

A reflex action is a fast, automatic, involuntary response that protects the body without waiting for the brain to decide. The pathway it follows is the reflex arc, and the order of its five parts is one of the most reliable sources of full marks in this chapter.

The five parts, in order, are: receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, and effector. A stimulus such as a pin prick is detected by a receptor in the skin; the sensory neurone carries the impulse to a relay neurone in the spinal cord; the relay neurone passes it to a motor neurone; and the motor neurone triggers the effector, a muscle, to contract and pull the hand away.

All of this happens in a fraction of a second.

The reason a reflex is so fast is that the spinal cord acts as the processing centre, so the impulse does not have to travel to the brain and back before the effector acts. The brain may be informed afterwards, which is why a person feels the pain a moment after the hand has already moved.

Voluntary and involuntary actions (Content Standard 12.4)

A voluntary action, such as picking up a pen or writing a sentence, is consciously decided and controlled by the brain, specifically the cerebrum, and can be started, changed or stopped at will. An involuntary action happens automatically, without conscious control, and is usually either a protective reflex or a process that keeps the body working, such as the heartbeat.

A reflex action is a clear example of an involuntary action: it uses the spinal cord rather than conscious thought, so it is faster and cannot easily be suppressed. The beating of the heart and the movement of food along the gut are also involuntary, but these are controlled by the medulla oblongata and continue whether a person thinks about them or not.

The difference most often tested is the control centre and whether conscious choice is involved: a voluntary action involves the cerebrum and conscious decision, while an involuntary action bypasses conscious thought and protects or maintains the body automatically.

The endocrine system and negative feedback (Content Standard 12.6)

The endocrine system is a group of glands that release hormones directly into the bloodstream. A hormone travels all over the body but only affects target cells that carry the matching receptor.

Four glands and their hormones are examined closely, and the table below links each gland to its hormone and effect.

Hormone levels are usually regulated by negative feedback. When a condition such as blood glucose concentration rises away from normal, the pancreas releases insulin, which lowers it by making cells take up glucose and the liver store it as glycogen.

When blood glucose concentration falls too low, the pancreas releases glucagon, which raises it by making the liver release glucose. Insulin and glucagon are antagonistic hormones because they have opposite effects, and together they hold blood glucose within a narrow range.

Negative feedback is the core idea to state in an explanation: a change away from normal triggers a response that pushes the condition back towards normal, and once normal is restored the correcting hormone is reduced.

Health issues of the nervous and endocrine systems (Content Standards 12.5 and 12.7)

Both coordination systems can malfunction, and each disorder is examined by asking which part of the normal system has stopped working. A stroke occurs when the blood supply to part of the brain is interrupted, so the neurones in that region are starved of oxygen and die, and the functions they controlled, such as movement or speech, are impaired.

Parkinson's disease develops when neurones that produce a chemical needed for smooth movement gradually die, causing tremors and slow, stiff movement.

Diabetes mellitus is the endocrine disorder studied here. It develops when the body produces too little insulin, or when body cells stop responding normally to the insulin that is present.

Because insulin normally signals cells to take up glucose from the blood, when it is missing or ineffective the glucose is not taken up efficiently and the blood glucose concentration rises above the normal range, which can damage blood vessels and organs over time.

The nervous and endocrine systems are not fully separate. Part of the brain, the hypothalamus, monitors conditions inside the body and controls the pituitary gland, which then releases hormones that influence other glands.

A nervous signal can therefore trigger a hormonal response, which is why the two systems are described as working together rather than in isolation.

Key concepts to master

• Nervous system, The nervous system is made up of the central nervous system, the brain and spinal cord, where most information is processed and decisions are made, and the peripheral nervous system, the network of nerves that carries electrical impulses between the central nervous system and every receptor and effector in the body.
• Neurones, Sensory neurones carry impulses from a receptor towards the central nervous system, motor neurones carry impulses away from the central nervous system to an effector such as a muscle or gland, and relay neurones inside the brain or spinal cord connect the two, allowing impulses to be processed or redirected.
• Synapse, A synapse is a tiny gap between the end of one neurone and the start of the next. When an impulse arrives, it triggers the release of a chemical neurotransmitter that diffuses across the gap and binds to receptors on the next neurone, generating a new impulse and ensuring signals travel in one direction only.
• Reflex arc, A reflex arc is a fast, automatic, involuntary pathway that does not require conscious thought: a stimulus is detected by a receptor, carried by a sensory neurone to a relay neurone in the spinal cord, passed to a motor neurone, and finally reaches an effector, which responds within a fraction of a second.
• Voluntary vs involuntary actions, A voluntary action, such as picking up a pen, is consciously decided and controlled by the brain, involves the cerebrum, and can be started, changed or stopped at will. An involuntary action, such as a reflex or the beating of the heart, happens automatically without conscious control and usually protects the body or maintains an internal process.
• Endocrine system, The endocrine system is a collection of glands that release hormones directly into the bloodstream rather than through a duct. Each hormone travels throughout the body but only affects target cells that carry the matching receptor, producing effects that develop more slowly than a nerve impulse but often last for a much longer time.
• Main hormones and glands, Four glands and their hormones are frequently examined: the pituitary gland releases growth hormone controlling body growth, the thyroid gland releases thyroxine controlling metabolic rate, the pancreas releases insulin and glucagon controlling blood glucose level, and the adrenal glands release adrenaline that prepares the body for a stressful or dangerous situation.
• Negative feedback and homeostasis, Hormone levels are usually regulated by negative feedback: when a condition such as blood glucose level rises or falls away from normal, a gland releases a hormone that pushes it back towards normal, and once normal is restored the gland reduces hormone release again. This feedback loop is central to how the endocrine system maintains a stable internal environment.
• Health issues of the nervous system, Damage to the nervous system disrupts coordination: a stroke occurs when blood supply to part of the brain is interrupted, killing the neurones in that region and impairing the function they controlled, while Parkinson's disease develops when neurones that produce a chemical needed for smooth movement gradually die, causing tremors and slow, stiff movement.
• Health issues of the endocrine system, Diabetes mellitus develops when the body produces too little insulin or when body cells stop responding normally to the insulin that is present, so glucose is not taken up from the blood into cells efficiently and blood glucose level rises above the normal range, which can damage blood vessels and organs over time.

Quick recall checklist

1. Can you define and explain Nervous system?
2. Can you define and explain Neurones?
3. Can you define and explain Synapse?
4. Can you define and explain Reflex arc?
5. Can you define and explain Voluntary vs involuntary actions?
6. Can you define and explain Endocrine system?
7. Can you define and explain Main hormones and glands?
8. Can you define and explain Negative feedback and homeostasis?
9. Can you define and explain Health issues of the nervous system?
10. Can you define and explain Health issues of the endocrine system?

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