Chapter Overview: Nervous System and Hormones
Welcome to your study notes for Coordination and Control! In this chapter, we will explore how your body detects changes in the surrounding environment and sends messages from one part of the body to another. Think of your body like a busy school or workplace: without effective communication, everything would turn into chaos. In humans, communication is managed by two main control systems: the nervous system (fast, electrical signals) and the hormonal system (slower, chemical signals in the blood). We will also look at how plants use hormones to respond to light.
Don't worry if this seems like a lot of information at first! We will break everything down into bite-sized steps with helpful memory tricks and real-world examples.
1. Coordination and the Nervous System
To survive, living organisms must detect changes in their environment (known as stimuli) and react appropriately (known as a response).
The General Pathway of Communication
Every nervous response follows this universal sequence:
Stimulus \(\rightarrow\) Receptor \(\rightarrow\) Coordinator (CNS) \(\rightarrow\) Effector \(\rightarrow\) Response
• Stimulus: A detectable change in the environment (e.g., bright light, a sharp pin, heat, sound).
• Receptor: Specialised cells that detect the stimulus (e.g., light receptor cells in the eye, temperature receptors in the skin).
• Coordinator: The Central Nervous System (CNS), which is made up of the brain and the spinal cord. It processes information and decides what action to take.
• Effector: A muscle or gland that brings about the response (e.g., a bicep muscle contracting, or a salivary gland releasing saliva).
• Response: The resulting action (e.g., pulling your hand away, squinting, or secreting a chemical).
Types of Neurones (Nerve Cells)
Neurones are specialised cells adapted to carry electrical messages, called nerve impulses, rapidly across the body. There are three main types you must know for your exam:
1. Sensory Neurone: Carries electrical impulses from receptors (like the eye or skin) to the CNS (coordinator).
2. Relay (Association) Neurone: Found entirely within the CNS (brain or spinal cord). It connects sensory neurones to motor neurones.
3. Motor Neurone: Carries electrical impulses away from the CNS to an effector (a muscle or a gland).
Memory Trick: Remember the order with the mnemonic SAM:
Sensory \(\rightarrow\) Association (Relay) \(\rightarrow\) Motor.
Neurone Adaptations
• Axon: A long, thin extension of the cell cytoplasm that carries the electrical impulse over long distances quickly.
• Myelin Sheath: A fatty insulating layer surrounding the axon that speeds up the transmission of electrical impulses and protects the neurone.
The Synapse: How Impulses Cross Gaps
Neurones do not physically touch each other. There is a tiny microscopic gap between two neurones called a synapse.
Because an electrical impulse cannot jump across this physical gap, the signal must be converted into a chemical message:
1. An electrical impulse reaches the end of the first neurone (the presynaptic neurone).
2. This triggers the release of special chemicals called neurotransmitters.
3. The neurotransmitter chemical diffuses across the microscopic synaptic gap (moving from high concentration to low concentration).
4. The chemical binds to specific receptor molecules on the surface of the next neurone.
5. This binding triggers a brand new electrical impulse in the second neurone.
Did you know? Synapses ensure that nerve impulses travel in one direction only, because neurotransmitters are only released from one side of the gap, and receptor sites are only located on the opposite side!
Reflex Actions and the Reflex Arc
A reflex action is an automatic, rapid, and involuntary response to a stimulus. It does not involve conscious thought from the brain. Examples include blinking when dust flies toward your eye, the knee-jerk reflex, or snatching your hand away from a hot stove.
Why are reflex actions important? They protect the body from damage by producing an immediate reaction.
The pathway that the electrical impulse travels along during a reflex action is called a reflex arc:
1. Stimulus: A hot plate touches the skin.
2. Receptor: Heat/pain receptors in the skin generate an electrical impulse.
3. Sensory Neurone: Carries the impulse to the spinal cord (part of the CNS).
4. Synapse: Neurotransmitter diffuses across the gap to the relay neurone.
5. Relay (Association) Neurone: Passes the impulse across the spinal cord to the motor neurone.
6. Synapse: Neurotransmitter diffuses across the gap to the motor neurone.
7. Motor Neurone: Carries the impulse out of the spinal cord to the effector muscle.
8. Effector: Bicep muscle contracts.
9. Response: Hand is pulled away rapidly from the heat source.
Key Takeaway: The nervous system uses electrical impulses carried along neurones and chemical neurotransmitters across synapses to provide rapid, short-lived responses. Reflex arcs bypass conscious thought to protect the body.
2. The Hormonal (Endocrine) System
While the nervous system provides super-fast communication, the body also relies on a chemical communication network called the endocrine system.
Hormones are chemical messengers produced by specialised organs called endocrine glands. They are secreted directly into the bloodstream and carried around the body in blood plasma to reach specific target organs, which possess complementary receptors.
Comparing Nervous and Hormonal Control
Exam questions often ask you to contrast these two systems. Here is a clear comparison:
• Type of signal: Nervous system uses electrical impulses (and chemical neurotransmitters at synapses); Hormonal system uses chemical hormones.
• Transmission pathway: Nervous impulses travel along neurones (nerve cells); Hormones travel dissolved in the bloodstream (blood plasma).
• Speed of transmission: Nervous system is very fast / instantaneous; Hormonal system is slower.
• Duration of effect: Nervous responses are short-lived; Hormonal responses are long-lasting.
• Target area: Nervous impulses go to a very specific localised point (e.g., one single muscle); Hormones can act on widespread target organs or tissues throughout the body.
3. Homeostasis and Blood Glucose Control
Homeostasis is the maintenance of a constant internal environment in the body regardless of external changes. Maintaining constant levels of water, body temperature, and blood glucose are all vital for cells to function properly.
Control of Blood Glucose
Your cells need glucose for respiration to release energy. However, having too much or too little glucose in your blood can be dangerous. The organ responsible for monitoring and controlling blood glucose concentration is the pancreas, acting alongside the liver.
Scenario A: When Blood Glucose Rises (e.g., after eating a carbohydrate-rich meal)
1. The pancreas detects the high concentration of glucose in the blood.
2. The pancreas secretes the hormone insulin into the bloodstream.
3. Insulin travels in the blood to target organs, mainly the liver and muscle cells.
4. Insulin causes liver and muscle cells to absorb glucose from the blood and convert soluble glucose into an insoluble storage carbohydrate called glycogen.
5. Glucose is also used up more quickly in respiration.
6. As a result, the blood glucose level decreases back down to the normal resting level.
Scenario B: When Blood Glucose Falls (e.g., during exercise or fasting)
1. The pancreas detects the low concentration of glucose in the blood.
2. The pancreas secretes the hormone glucagon into the bloodstream.
3. Glucagon travels to the liver.
4. Glucagon causes the liver cells to break down stored glycogen back into soluble glucose.
5. The glucose is released back into the bloodstream.
6. As a result, blood glucose levels rise back up to normal.
Memory Alert - Don't Mix These Up!
• Glucose: The small, soluble sugar carried in the blood.
• Glycogen: The large, insoluble storage carbohydrate stored in the liver.
• Glucagon: The hormone released by the pancreas when glucose is "all gone"!
Negative Feedback
This control mechanism is an example of negative feedback. Whenever a factor changes from its normal set point (higher or lower), the body detects this change and triggers corrective mechanisms to reverse the change and restore the balance back to normal.
Diabetes
Diabetes is a chronic medical condition where a person is unable to regulate their blood glucose levels effectively.
Type 1 Diabetes:
• Cause: The pancreas fails to produce sufficient insulin because the body's immune system attacks its own insulin-producing cells.
• Onset: Usually develops in children or young adults.
• Treatment: Regular daily insulin injections before meals, combined with monitoring blood glucose levels and eating a balanced diet.
Type 2 Diabetes:
• Cause: The pancreas still produces insulin, but the body's target cells (liver and muscle cells) become resistant and do not respond to it, or the pancreas does not make enough insulin.
• Risk factors: Strongly linked to obesity, lack of regular exercise, and an unhealthy diet high in refined sugar and fats.
• Treatment: Managed primarily through lifestyle changes: eating a healthy, low-sugar/low-carbohydrate diet, exercising regularly, losing weight, and sometimes oral medications or insulin injections if needed.
Symptoms of Undiagnosed Diabetes:
• Glucose present in the urine (the kidneys cannot reabsorb all the excess glucose).
• Persistent excessive thirst (due to water lost in urine).
• Frequent urination.
• Lethargy and fatigue (cells cannot take in enough glucose for respiration to release energy).
• Unexplained weight loss.
Key Takeaway: Blood glucose is regulated by negative feedback. Insulin lowers blood glucose by converting it to glycogen; glucagon raises blood glucose by converting glycogen back to glucose.
4. Osmoregulation and Water Balance
Osmoregulation is another key homeostatic process: it is the control of the water and salt balance in the blood and bodily fluids.
The kidneys are the organs responsible for osmoregulation and the excretion of waste products (like urea). The hormone responsible for controlling how much water the kidneys reabsorb is Anti-Diuretic Hormone (ADH), which is produced by the brain and released by the pituitary gland.
How ADH Works
• When blood is too concentrated (low water content / dehydration, e.g., on a hot day or after heavy sweating):
1. The brain detects that blood water potential is too low.
2. The pituitary gland releases more ADH into the blood.
3. ADH travels to the kidney tubules, making their walls more permeable to water.
4. More water is reabsorbed from the urine back into the blood.
5. Result: A small volume of concentrated (dark yellow) urine is produced, and blood water level returns to normal.
• When blood is too dilute (high water content, e.g., after drinking lots of water):
1. The brain detects the high water level in the blood.
2. The pituitary gland releases less ADH.
3. The kidney tubules become less permeable to water.
4. Less water is reabsorbed into the blood; more remains in the kidney tubules.
5. Result: A large volume of dilute (pale) urine is produced, removing excess water from the body.
Memory Trick: Diuresis means producing lots of urine. Therefore, Anti-Diuretic Hormone (ADH) does the opposite—it stops you from producing lots of urine!
5. Plant Hormones: Phototropism
Plants do not have a nervous system, but they must still respond to their environment to survive. A plant's growth response to a directional stimulus is called a tropism.
Phototropism is the growth response of a plant in response to directional light. Shoots are positively phototropic because they grow towards light, maximising light absorption for photosynthesis.
The Role of Auxin
Phototropism is controlled by a plant growth hormone called auxin.
1. Auxin is produced in the tip of the shoot.
2. When the shoot receives unidirectional light (light coming from one side only), auxin diffuses away from the light and accumulates on the shaded side of the shoot.
3. In shoot cells, higher concentrations of auxin cause cell elongation (the cells grow longer).
4. Because cells on the shaded side elongate more rapidly than cells on the illuminated side, the shoot bends and grows towards the light source.
Classic Experiments with Shoot Tips (Coleoptiles):
• Tip left intact: Shoot bends towards light.
• Tip removed (decapitated): No auxin produced \(\rightarrow\) shoot does not grow or bend.
• Tip covered with an opaque (light-proof) cap: Light cannot be detected \(\rightarrow\) auxin diffuses evenly down all sides \(\rightarrow\) shoot grows straight upwards without bending.
• Tip covered with a transparent cap: Light penetrates \(\rightarrow\) shoot bends towards light.
Key Takeaway: Auxin is made in shoot tips and moves to the shaded side, stimulating cell elongation on that side and causing shoots to bend towards light for photosynthesis.
Quick Review Summary: Common Exam Traps to Avoid!
• Trap 1: Confusing insulin and glucagon. Remember: Insulin is released when blood glucose is high (to bring it down); Glucagon is released when blood glucose is low (to bring it up).
• Trap 2: Confusing glucose (soluble sugar) and glycogen (insoluble storage molecule).
• Trap 3: Forgetting that ADH increases kidney tubule permeability, leading to more water reabsorption and less urine.
• Trap 4: Saying reflex actions involve the brain making a decision. Reflexes are involuntary and automatic; the brain is informed only after the action occurs.
• Trap 5: Stating that electrical impulses cross the synapse. Impulses cannot cross the gap—they trigger the diffusion of chemical neurotransmitters.