Biology (9700) | Control and coordination

🧠 Chapter 15: Control and Coordination – Comprehensive A Level Study Notes 🧠

Hello future biologists! This chapter is where we explore how complex organisms, like mammals and plants, manage their internal systems and respond to the outside world. Think of it as studying the body’s highly efficient communication network—some messages are instant emails (nervous system), and others are long-term policy changes (endocrine system). Mastering this topic is key to understanding the physiological complexity of life!

---

1. Control Systems in Mammals: Nervous vs. Endocrine

Multicellular organisms need two major systems to coordinate their activities: the fast, electrical system and the slower, chemical system.

1.1 Comparing Coordination Systems (Syllabus 15.1.2)

It’s essential to know the key differences between these two "command centres":

• Nervous System: Uses electrical impulses (action potentials) transmitted along neurones.
• Endocrine System: Uses chemical messengers (hormones) transported via the blood.

Table: Key Comparison

Feature | Nervous System | Endocrine System
--- | --- | ---
Signal Type | Electrical impulses & neurotransmitters | Chemical hormones
Speed | Very rapid (milliseconds) | Slow (seconds to days)
Duration | Short-lived, instantaneous effects | Long-lasting effects
Transmission Path | Dedicated nerve pathways | Bloodstream
Target | Highly localised (specific muscles or glands) | Widespread (any cell with receptors)
Examples | Reflex actions, muscle movement | Growth, homeostasis (e.g., control of blood sugar, ADH action)

(Reminder: The syllabus requires knowledge of ADH, glucagon, and insulin in the context of the endocrine system, linking back to Topic 14 Homeostasis.)

Key Takeaway

The nervous system provides rapid, precise control (like slamming on the brakes), while the endocrine system provides slow, broadcast, and long-term control (like regulating metabolism or growth).

---

2. The Nervous System: Structure and Signals

2.1 Neurone Structure and Function (Syllabus 15.1.3)

Neurones are the basic functional units of the nervous system. You need to know three main types:

• Sensory Neurones: Transmit impulses from receptors (detecting stimuli) to the Central Nervous System (CNS).
  Structure note: Cell body often found in the middle of the axon, outside the CNS.
• Motor Neurones: Transmit impulses from the CNS to effectors (muscles or glands).
  Structure note: Cell body usually at one end, long axon leads to the effector.
• Intermediate Neurones (Relay Neurones): Connect sensory neurones to motor neurones, found entirely within the CNS.

2.2 Sensory Receptors (Syllabus 15.1.4)

Sensory receptor cells are essential because they act as transducers—they convert one form of energy (the stimulus, e.g., light, heat, chemical) into electrical energy (the nerve impulse).

The role of receptors is to detect the stimulus and initiate the transmission of an impulse in the sensory neurone.

2.3 Maintaining the Resting Potential (Syllabus 15.1.6)

A neurone that is not transmitting an impulse is at its resting potential, typically around -70 mV (millivolts). This potential difference is maintained by:

1. The Sodium-Potassium Pump (\(Na^+/K^+\) Pump): This is an active transport mechanism (requires ATP). It pumps 3 \(\mathbf{Na^+}\) ions out for every 2 \(\mathbf{K^+}\) ions in. This maintains a higher concentration of \(Na^+\) outside and \(K^+\) inside.
2. Differential Membrane Permeability: The membrane is far more permeable to \(K^+\) ions (through leak channels) than to \(Na^+\) ions. As \(K^+\) leaks out, it makes the outside relatively more positive and the inside relatively more negative.

The resulting potential difference is called the polarised state.

---

3. Nerve Transmission: Action Potentials and Synapses

3.1 The Action Potential (Syllabus 15.1.5, 15.1.6)

An action potential (AP) is a brief, rapid reversal of the membrane potential, moving from negative (-70 mV) to positive (+30 mV or +40 mV) before returning to rest. It is an "all-or-nothing" event.

We can trace the sequence of events using the example of a chemoreceptor in a human taste bud:

Step 1: Stimulation and Generator Potential
The taste molecule (stimulus) binds to the chemoreceptor cell, causing ion channels to open and allowing positive ions to enter. This generates a small change in potential called the generator potential (or receptor potential).

Step 2: Threshold Reached
If the generator potential is strong enough to reach the threshold potential (around -55 mV), voltage-gated sodium channels open rapidly, triggering the action potential.

Step 3: Depolarisation
Massive influx of \(\mathbf{Na^+}\) ions. The inside of the neurone becomes positive relative to the outside, reversing the potential to approximately +40 mV.

Step 4: Repolarisation
At the peak, \(\mathbf{Na^+}\) channels close and voltage-gated \(\mathbf{K^+}\) channels open. \(\mathbf{K^+}\) ions rapidly diffuse out of the cell. This restores the negative charge inside the cell.

Step 5: Hyperpolarisation (Undershoot)
\(\mathbf{K^+}\) channels are slow to close, causing too much \(\mathbf{K^+}\) to leave. The potential momentarily drops below the resting potential (e.g., to -80 mV).

Step 6: Restoration
The \(\mathbf{Na^+/K^+}\) pump and \(K^+\) leak channels return the potential to the normal resting potential of -70 mV.

3.2 The Refractory Period and Impulse Frequency (Syllabus 15.1.6, 15.1.8)

The period during and immediately after an action potential, during which a new impulse cannot be generated, is called the refractory period.

There are two parts:

1. Absolute Refractory Period: It is impossible to generate a new AP (because \(Na^+\) channels are inactivated).
2. Relative Refractory Period: It is possible, but difficult, to generate a new AP (only if the stimulus is much stronger than usual).

Importance of the Refractory Period:

• Ensures that impulses are transmitted in one direction only (they cannot travel backwards).
• Limits the frequency at which impulses can be generated, ensuring that individual action potentials remain separate.

3.3 Saltatory Conduction (Syllabus 15.1.7)

In myelinated neurones (those wrapped in an insulating fatty layer formed by Schwann cells), the impulse transmission is much faster. This is achieved by saltatory conduction.

• The myelin sheath prevents ion flow, meaning action potentials can only occur at the exposed gaps in the myelin sheath called the Nodes of Ranvier.
• The impulse effectively jumps from one node to the next, which is significantly faster than continuous conduction along an unmyelinated axon.

3.4 The Cholinergic Synapse (Syllabus 15.1.9)

A synapse is the junction between two neurones (or a neurone and an effector). A cholinergic synapse uses the neurotransmitter acetylcholine (ACh).

Functioning of a Cholinergic Synapse (Step-by-Step):

1. The action potential arrives at the presynaptic terminal.
2. Depolarisation causes voltage-gated calcium ion channels (\(\mathbf{Ca^{2+}}\)) to open, and \(\mathbf{Ca^{2+}}\) rapidly diffuses into the presynaptic knob.
3. The influx of \(\mathbf{Ca^{2+}}\) causes synaptic vesicles containing ACh to fuse with the presynaptic membrane (exocytosis).
4. ACh is released into the synaptic cleft.
5. ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane.
6. This binding causes ligand-gated \(\mathbf{Na^+}\) channels to open, allowing \(\mathbf{Na^+}\) to rush into the postsynaptic neurone, creating a postsynaptic potential (PSP).
7. If the PSP reaches the threshold, a new action potential is initiated in the postsynaptic neurone.
8. The enzyme acetylcholinesterase (AChE) rapidly hydrolyses ACh into acetate and choline, stopping the signal and allowing the synapse to be ready for the next impulse (preventing continuous stimulation).

Did you know?

Synapses are crucial for filtering information. They allow multiple signals to be integrated (summation) before a decision is made to pass on the impulse.

Key Takeaway

The nervous signal converts from electrical (AP) to chemical (neurotransmitter) at the synapse, before becoming electrical again in the next neurone.

---

4. Control of Striated Muscle Contraction

The nervous system often signals to muscles to produce movement. We must understand the control and mechanism of contraction in striated (skeletal) muscle.

4.1 Ultrastructure of Striated Muscle (Syllabus 15.1.11)

Striated muscle cells (muscle fibres) are long and multinucleated. They contain numerous myofibrils, which are made up of repeating contractile units called sarcomeres.

• A sarcomere runs between two adjacent Z-lines.
• It contains thick filaments (myosin) and thin filaments (actin).
• A-band: Contains thick filaments (myosin), overlaps with actin. (Always stays the same length during contraction).
• I-band: Contains only thin filaments (actin). (Shortens during contraction).
• H-zone: Centre of the A-band, containing only myosin. (Shortens during contraction).

4.2 Neuromuscular Junctions and Control (Syllabus 15.1.10)

The synapse between a motor neurone and a muscle fibre is called the neuromuscular junction (NMJ). It works similarly to a cholinergic synapse, using ACh.

Key structures involved in signal transfer to the entire muscle fibre:

• T-tubules (Transverse Tubules): Inward folds of the muscle cell membrane (sarcolemma). They carry the electrical impulse deep into the muscle fibre.
• Sarcoplasmic Reticulum (SR): A specialised type of endoplasmic reticulum in muscle cells. It stores high concentrations of \(\mathbf{Ca^{2+}}\) ions. When stimulated by the impulse from the T-tubules, the SR releases \(\mathbf{Ca^{2+}}\) into the cytoplasm (sarcoplasm).

4.3 The Sliding Filament Model (Syllabus 15.1.12)

Muscle contraction occurs when the thin actin filaments slide past the thick myosin filaments, shortening the sarcomere.

The process requires \(\mathbf{Ca^{2+}}\) and \(\mathbf{ATP}\):

1. Resting State: The proteins tropomyosin and troponin are associated with the actin filament, physically blocking the myosin-binding sites.
2. Activation: An action potential arrives, and the Sarcoplasmic Reticulum releases \(\mathbf{Ca^{2+}}\).
3. \(\mathbf{Ca^{2+}}\) Binding: \(\mathbf{Ca^{2+}}\) ions bind to the troponin protein.
4. Shifting: The binding of \(\mathbf{Ca^{2+}}\) causes troponin to change shape, pulling tropomyosin away from the myosin binding sites on the actin filament.
5. Cross-Bridge Formation: The myosin head (which contains ATPase) binds to the exposed site on the actin, forming a cross-bridge.
6. Power Stroke: ATP hydrolysis (using energy from ATP) allows the myosin head to pivot, pulling the actin filament towards the centre of the sarcomere. ADP and \(P_i\) are released.
7. Detachment: A new ATP molecule binds to the myosin head, causing it to detach from the actin.
8. Recocking: The cycle repeats as long as \(\mathbf{Ca^{2+}}\) and \(\mathbf{ATP}\) are present, rapidly pulling the Z-lines closer together.

Key Takeaway

Muscle contraction is a rapid, active process driven by the cycling of myosin heads powered by ATP, controlled entirely by the availability of calcium ions.

---

5. Coordination in Plants

Plants also coordinate their activities, although typically without a fast nervous system. Their responses range from very rapid turgor changes to slower, hormone-controlled growth changes.

5.1 Rapid Response: The Venus Fly Trap (Syllabus 15.2.1)

The Venus fly trap (Dionaea muscipula) exhibits a rapid, non-growth movement to catch prey. The closure mechanism is incredibly fast.

Mechanism of Closure:

1. Stimulation: The insect touches the sensitive trigger hairs (mechanoreceptors) located on the inner surface of the modified leaves (lobes). Usually, two contacts within a 20-second interval are needed to prevent closure from accidental stimuli.
2. Electrical Signal: Contact generates an electrical signal (an action potential-like event) that spreads rapidly across the leaf lobe cells.
3. Ion Flux: This electrical signal triggers rapid changes in membrane permeability in the cells along the hinge region (midrib).
4. Turgor Loss: Water moves rapidly by osmosis out of the turgid cells on the *outer* side of the hinge and into the surrounding tissue.
5. Closure: The rapid loss of turgor on the outer side causes the outer cell walls to collapse slightly, leading to the sudden, explosive closure of the trap.

5.2 Growth Response: Auxin (Syllabus 15.2.2)

Auxin is a crucial plant hormone (specifically indoleacetic acid, IAA) involved in elongation growth (e.g., in shoots during phototropism).

Role of Auxin in Cell Elongation (Acid Growth Hypothesis):

1. Auxin binds to receptors on the cell surface membrane of target cells.
2. This stimulates the activation of proton pumps (\(\mathbf{H^+}\) pumps) located in the cell membrane.
3. The proton pumps actively transport \(\mathbf{H^+}\) ions out of the cell and into the cell wall space.
4. The resulting decrease in pH (acidification) activates proteins called expansins.
5. Expansins loosen the bonds between cellulose microfibrils and other components in the cell wall.
6. With the cell wall loosened, the high turgor pressure within the plant cell is sufficient to drive the uptake of water and cause the cell to rapidly expand/elongate.

5.3 Growth Response: Gibberellin (Syllabus 15.2.3)

Gibberellin (GA) is important in processes like seed germination and stem elongation.

Role of Gibberellin in Barley Germination:

1. The barley seed absorbs water and the embryo releases gibberellin.
2. GA travels to the aleurone layer (the outermost layer of the endosperm).
3. GA acts as a signalling molecule to activate genes for the synthesis of enzymes (like amylase) necessary for breaking down stored starch.
4. The molecular mechanism involves GA causing the breakdown of DELLA protein repressors, which normally inhibit the factors that promote gene transcription. (This removes the "brake" on gene expression.)
5. The synthesised amylase breaks down starch into maltose (a sugar), providing glucose for embryo growth.

Quick Review Box: Plant Coordination

• Venus Fly Trap: Fast response; relies on rapid turgor pressure changes.
• Auxin: Slower response; relies on acidifying the cell wall via proton pumps to allow expansion.
• Gibberellin: Slower response; controls gene expression (amylase synthesis) in seeds.