🧠 Coordination and Response: Your Body's Control Centre 🧠
Welcome to one of the most exciting topics in Biology! This chapter is all about how living organisms, especially humans, detect changes around them and react to stay safe and healthy. Think of your body as a massive company—you need a super-fast internal communication system (the nervous system) and a slower, more general announcement system (the hormonal system) to keep everything coordinated.
Let's dive into how you react, see, feel, and keep your temperature perfectly constant!
14.1 The Nervous System: The Body's Electrical Network
The nervous system provides a rapid, electrical pathway for messages to travel. These messages are called electrical impulses and they travel along specialized cells called neurones.
Structure of the Mammalian Nervous System
The nervous system is split into two main parts:
- The Central Nervous System (CNS): This is the "control room." It consists of the brain and the spinal cord. This is where all the information is processed and decisions are made.
- The Peripheral Nervous System (PNS): This is the "road network." It consists of all the nerves found outside the brain and spinal cord. The PNS carries impulses to and from the CNS.
The overall role of the nervous system is the coordination and regulation of body functions.
Types of Neurones
There are three types of neurones involved in carrying impulses:
- Sensory Neurone: Carries impulses from the receptor (like the skin) to the CNS (spinal cord/brain).
- Relay Neurone: Found entirely within the CNS. It links the sensory neurone to the motor neurone.
- Motor Neurone: Carries impulses from the CNS to the effector (a muscle or gland).
Quick Tip (Mnemonic): Remember the path of the impulse using this simple phrase:
Really Smart Readers Must Exert
(Receptor → Sensory → Relay → Motor → Effector)
The Simple Reflex Arc and Reflex Action (Core)
A reflex action is an automatic, rapid, and involuntary response to a stimulus. These are vital for survival, preventing damage before the brain even registers the event (e.g., quickly pulling your hand away from a hot stove).
The pathway an impulse takes during a reflex action is called the reflex arc:
- A stimulus is detected by a receptor (e.g., pain receptors in your finger).
- An impulse travels along the sensory neurone to the spinal cord.
- The impulse passes to a relay neurone within the spinal cord.
- The impulse passes to a motor neurone.
- The impulse travels to the effector (e.g., a muscle in your arm).
- The effector produces a response (e.g., your arm muscle contracts, pulling your hand away).
Synapses: The Tiny Gaps (Core & Supplement)
A synapse is the small junction (gap) between two neurones (or between a neurone and an effector).
Why do we need a gap? Neurones don't actually touch. The synapse ensures the electrical signal is converted into a chemical signal to jump the gap.
Structure of a Synapse (Supplement)
The synapse contains:
- Vesicles: Small sacs in the first neurone containing neurotransmitter molecules (chemical messengers).
- Synaptic Gap (or cleft): The tiny space between the two neurones.
- Receptor proteins: Found on the membrane of the second neurone, ready to bind to the neurotransmitter.
Events at a Synapse (Supplement - Step-by-Step)
- An electrical impulse arrives at the end of the first neurone.
- The impulse stimulates the release of neurotransmitter molecules from the vesicles into the synaptic gap.
- The neurotransmitter molecules rapidly diffuse across the gap.
- The neurotransmitter molecules bind to the receptor proteins on the membrane of the next neurone.
- This binding stimulates a new electrical impulse in the second neurone.
Key Fact: Synapses are crucial because they ensure that impulses travel in one direction only. They prevent impulses from flowing backwards.
Key Takeaway (14.1): The CNS processes information transmitted via sensory neurones. The motor neurones carry the response to effectors. Reflexes are fast, involuntary responses mediated by the synapse, which uses chemical messengers to transmit signals one way.
14.2 Sense Organs: Windows to the World
Sense organs are groups of receptor cells that respond to specific stimuli in the environment. These stimuli include: light, sound, touch, temperature, and chemicals.
The Structure and Function of the Eye (Core)
The eye is our primary sense organ for detecting light stimuli. You need to know the structure and function of the following parts:
| Part of the Eye | Function |
|---|---|
| Cornea | Transparent outer layer; refracts (bends) light into the eye. |
| Iris | Coloured part; controls the size of the pupil, therefore controlling how much light enters the eye. |
| Pupil | Hole in the centre of the iris through which light enters. |
| Lens | Focuses light onto the retina. |
| Retina | Contains light receptor cells (rods and cones). |
| Optic Nerve | Carries electrical impulses from the retina to the brain. |
| Blind Spot | Area where the optic nerve leaves the eye; contains no receptor cells. |
The Pupil Reflex (Core & Supplement)
The pupil reflex is an involuntary action that protects the retina from damage and allows vision in dim light. It is a response to changes in light intensity.
- In bright light: The pupil gets smaller (constricts). This limits the amount of light entering.
- In dim light: The pupil gets larger (dilates). This lets more light in so you can see better.
Antagonistic Muscle Action (Supplement)
The iris uses two sets of muscles that work in opposition (antagonistically):
1. Circular muscles: Run around the pupil. When they contract, the pupil gets smaller. (Used in Bright Light).
2. Radial muscles: Run outwards like spokes on a wheel. When they contract, they pull the pupil wider. (Used in Dim Light).
Accommodation: Focusing on Objects (Supplement)
Accommodation is the process of changing the shape of the lens to focus on objects at different distances. Think of it like a camera lens zooming in or out.
Focusing on Distant Objects:
- Ciliary muscles relax.
- Suspensory ligaments become tense (tight).
- The lens is pulled thin and flat.
- Light is refracted less.
Focusing on Near Objects:
- Ciliary muscles contract.
- Suspensory ligaments become slack (loose).
- The lens becomes thicker and more convex.
- Light is refracted more.
Rods and Cones (Supplement)
The retina contains two types of light receptor cells:
- Rods: Are very sensitive to light. They are essential for night vision (seeing in low light). They only see in shades of grey.
- Cones: Are less sensitive to light but respond to different wavelengths (colours). There are three kinds of cones, allowing for colour vision. They require high light intensity to work.
The Fovea (or yellow spot) is the area of the retina where light focuses when you look directly at something. It contains the highest concentration of cones and gives the sharpest image. The rods are concentrated in the peripheral areas of the retina.
Key Takeaway (14.2): The eye works by refracting light onto the retina. The iris controls light intensity (pupil reflex), and the ciliary muscles control lens shape (accommodation). Rods help in low light; cones help in colour vision.
14.3 Hormones: The Body's Chemical Messengers
While the nervous system uses fast electrical signals, the hormonal system uses chemical signals transported in the blood. This system is slower but its effects last longer.
What is a Hormone? (Core)
A hormone is a chemical substance, produced by an endocrine gland, carried by the blood, which alters the activity of one or more specific target organs.
Key Endocrine Glands and Hormones (Core & Supplement)
Endocrine glands secrete hormones directly into the blood (ductless glands).
- Adrenal glands secrete adrenaline.
- Pancreas secretes insulin and glucagon (Supplement).
- Testes secrete testosterone (male sex hormone).
- Ovaries secrete oestrogen (female sex hormone).
Adrenaline: The Fight or Flight Hormone (Core & Supplement)
Adrenaline is released in situations of stress, danger, or excitement (the "fight or flight" response). Its role is to prepare the body for intense physical activity by increasing metabolic activity (Supplement).
Effects of Adrenaline:
- Increased breathing rate.
- Increased heart rate.
- Increased pupil diameter (for better vision).
- Increased blood glucose concentration (Supplement - providing more fuel for muscles).
Nervous vs. Hormonal Control (Core)
It is important to compare these two systems:
| Feature | Nervous Control | Hormonal Control |
|---|---|---|
| Type of signal | Electrical impulses | Chemicals (hormones) |
| Speed of action | Very fast (milliseconds) | Slower (seconds/minutes) |
| Duration of effect | Short-lived | Long-lasting |
| Pathway | Neurones | Bloodstream |
Key Takeaway (14.3): Hormones are slow, long-term chemical signals in the blood. Adrenaline prepares the body for action. Hormonal control is slower but its effects persist longer than the rapid, short-term electrical nervous control.
14.4 Homeostasis: Keeping Internal Conditions Constant
Homeostasis is the maintenance of a constant internal environment. This is crucial because enzymes and metabolic reactions work best within narrow ranges of temperature and pH.
Analogy: Homeostasis is like the thermostat in your house, constantly checking conditions and making adjustments to keep the temperature (or blood sugar/water level) perfect.
Control of Blood Glucose (Core & Supplement)
The body needs to keep blood glucose levels stable. The pancreas is responsible for this, using two hormones: insulin and glucagon.
Negative Feedback (Supplement): Homeostatic control typically uses negative feedback. This means if a factor (like glucose concentration) rises above a set point, the body initiates a process to lower it, and vice versa. This keeps the factor oscillating around the set point.
Insulin and Glucagon Roles (Core & Supplement)
- If blood glucose is too high: The pancreas secretes insulin (Core). Insulin travels to the liver and muscles, causing them to take up glucose and convert it into glycogen for storage. This decreases blood glucose concentration (Core).
- If blood glucose is too low: The pancreas secretes glucagon (Supplement). Glucagon signals the liver to break down stored glycogen back into glucose and release it into the blood. This increases blood glucose concentration (Supplement).
Type 1 Diabetes (Supplement): This condition occurs when the pancreas fails to produce enough insulin. Treatment usually involves regular monitoring of blood glucose and the injection of insulin.
Thermoregulation: Controlling Body Temperature (Supplement)
Mammals maintain a constant internal body temperature, even when the external temperature changes. The brain (specifically the hypothalamus) detects temperature changes and coordinates responses.
Key Skin Structures Involved (Supplement)
The skin contains several structures essential for thermoregulation:
- Receptors (for temperature detection).
- Sweat glands (produce sweat).
- Blood vessels (arterioles supplying skin capillaries).
- Hair erector muscles (control hair position).
- Fatty tissue (acts as insulation).
Mechanisms to Maintain Constant Temperature (Supplement)
1. When the body is too HOT:
- Sweating: Sweat glands produce sweat. The evaporation of sweat carries heat away from the skin.
- Vasodilation: Arterioles supplying skin surface capillaries widen. This increases blood flow near the skin surface, increasing heat loss to the environment. (Think of opening a radiator valve to let heat out.)
2. When the body is too COLD:
- Shivering: Rapid, involuntary muscle contractions generate heat through increased respiration.
- Insulation: Hairs stand up (raised by hair erector muscles) trapping a layer of insulating air next to the skin.
- Vasoconstriction: Arterioles supplying skin surface capillaries narrow. This reduces blood flow near the skin surface, reducing heat loss. (Think of closing a radiator valve to keep heat in.)
Key Takeaway (14.4): Homeostasis keeps internal conditions stable, often using negative feedback. Blood glucose is balanced by insulin (lowers) and glucagon (raises). Body temperature is regulated mainly by the skin via sweating/vasodilation (cooling) and shivering/vasoconstriction (warming).
14.5 Tropic Responses in Plants
Plants also respond to stimuli, but usually much slower than animals. These responses, involving growth towards or away from a stimulus, are called tropisms.
If the plant part grows towards the stimulus, it is a positive tropism. If it grows away, it is a negative tropism.
Types of Tropisms (Core)
- Gravitropism (or geotropism): A response to gravity.
- Shoots show negative gravitropism (grow upwards, away from gravity).
- Roots show positive gravitropism (grow downwards, towards gravity).
- Phototropism: A response to the direction of a light source.
- Shoots show positive phototropism (grow towards the light).
- Roots show negative phototropism (grow away from the light).
Investigation Tip: To study these, you can place seedlings horizontally (for gravitropism) or expose them to light from only one direction (for phototropism).
The Role of Auxin (Supplement)
Tropisms are controlled chemically by a plant hormone called auxin (an example of the chemical control of plant growth).
How Auxin Controls Shoot Growth (Supplement - Step-by-Step)
1. Auxin Production: Auxin is made in the shoot tip (apex).
2. Diffusion: Auxin diffuses backwards through the plant from the shoot tip.
3. Cell Elongation: Auxin stimulates cell elongation (making cells longer) in the shoot.
In Phototropism:
If light hits the shoot from one side (e.g., the right), the auxin moves to the shaded side (the left). This causes an unequal distribution.
Since the shaded side has more auxin, the cells on the shaded side elongate faster than the cells on the lighted side, causing the shoot to bend towards the light (positive phototropism).
In Gravitropism (Shoots):
If a shoot is horizontal, gravity causes auxin to accumulate on the lower side.
Since the lower side has more auxin, the cells here elongate faster, causing the shoot to bend upwards (negative gravitropism).
Common Mistake to Avoid: Auxin causes cells to elongate, which leads to growth. It does *not* cause cells to divide or multiply.
Key Takeaway (14.5): Tropisms are growth responses. Shoots are positively phototropic and negatively gravitropic. These movements are controlled by the unequal distribution of auxin, a hormone that stimulates cell elongation.