Topic 15: Control and coordination
15.1 Control and coordination in mammals
Students should be able to:
1) describe the features of the endocrine system with reference to the hormones ADH, glucagon and insulin
2) compare the features of the nervous system and the endocrine system
3) describe the structure and function of a sensory neurone and a motor neurone and state that intermediate neurones connect sensory neurones and motor neurones
4) outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones
5) describe the sequence of events that results in an action potential in a sensory neurone, using a chemoreceptor cell in a human taste bud as an example
6) describe and explain changes to the membrane potential of neurones, including:
• how the resting potential is maintained
• the events that occur during an action potential
• how the resting potential is restored during the refractory period
7) describe and explain the rapid transmission of an impulse in a myelinated neurone with reference to saltatory conduction
8) explain the importance of the refractory period in determining the frequency of impulses
9) describe the structure of a cholinergic synapse and explain how it functions, including the role of calcium ions
10) describe the roles of neuromuscular junctions, the T-tubule system and sarcoplasmic reticulum in stimulating contraction in striated muscle
11) describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams
12) explain the sliding filament model of muscular contraction including the roles of troponin, tropomyosin, calcium ions and ATP
1. Features of the Endocrine System (with reference to ADH, glucagon, insulin)
The endocrine system is a collection of glands that secrete hormones directly into the bloodstream to regulate bodily functions. Hormones are chemical messengers that act on target organs to bring about specific physiological changes. For example, antidiuretic hormone (ADH) is produced by the hypothalamus and released from the posterior pituitary gland; it acts on the kidneys to increase water reabsorption and concentrate urine, helping maintain water balance. Glucagon, secreted by alpha cells of the pancreas, raises blood glucose levels by stimulating glycogen breakdown and gluconeogenesis in the liver. Insulin, secreted by beta cells in the pancreas, lowers blood glucose levels by promoting glucose uptake by cells and glycogenesis in the liver and muscles. These hormones illustrate how the endocrine system maintains homeostasis through feedback mechanisms.
2. Comparison of Nervous and Endocrine Systems
The nervous and endocrine systems both coordinate the body’s activities, but they differ significantly. The nervous system uses electrical impulses transmitted along neurones, resulting in very fast, short-term responses that are usually localized. In contrast, the endocrine system uses hormones transported via the bloodstream, producing slower, long-lasting effects that can influence the entire body. The nervous system is involved in processes like reflex actions and sensory responses, while the endocrine system regulates metabolic activities, growth, and homeostasis. For example, nerve impulses cause immediate muscle contraction, whereas insulin regulates blood glucose levels over a longer duration.
3. Structure and Function of Sensory and Motor Neurones
A sensory neurone carries impulses from sensory receptors to the central nervous system (CNS). It has a long dendron, a cell body located in the dorsal root ganglion, and a shorter axon. A motor neurone transmits impulses from the CNS to effectors such as muscles or glands. It has a large cell body at one end, many dendrites, and a long axon. Intermediate neurones, also called relay neurones, are found entirely within the CNS and connect sensory neurones to motor neurones, facilitating communication between them.
4. Role of Sensory Receptor Cells
Sensory receptor cells detect changes in the environment (stimuli) such as light, temperature, or chemicals. When stimulated, they convert this energy into electrical impulses by generating a generator potential. If this potential is large enough, it triggers an action potential in the associated sensory neurone. For instance, chemoreceptors in taste buds detect specific chemicals in food, initiating an electrical signal that is transmitted to the brain for processing.
5. Action Potential in a Chemoreceptor Cell (Taste Bud Example)
When a chemical stimulus (e.g., glucose) binds to receptors on a chemoreceptor cell in a human taste bud, it causes sodium ion channels to open. Sodium ions enter the cell, depolarizing the membrane. If the depolarization reaches a threshold, voltage-gated sodium channels open, leading to a rapid influx of Na⁺ ions and the initiation of an action potential. The action potential travels along the sensory neurone to the brain where the stimulus is perceived as taste.
6. Membrane Potential Changes in Neurones
The resting potential of a neurone is maintained at about –70 mV due to the activity of the sodium-potassium pump, which actively transports three Na⁺ ions out of the cell for every two K⁺ ions it brings in. This creates a net negative charge inside the cell. During an action potential, a stimulus causes voltage-gated sodium channels to open, allowing Na⁺ to enter the neurone, causing depolarization. At about +40 mV, sodium channels close and potassium channels open, allowing K⁺ to leave, repolarizing the membrane. The refractory period follows, during which the resting potential is restored, ensuring unidirectional impulse transmission and limiting impulse frequency.
7. Rapid Transmission in Myelinated Neurones (Saltatory Conduction)
Myelinated neurones have axons covered by a myelin sheath, interrupted at intervals by nodes of Ranvier. The myelin acts as an electrical insulator, preventing ion exchange along the axon except at the nodes. This forces the action potential to jump from node to node, a process called saltatory conduction, which significantly increases the speed of impulse transmission compared to unmyelinated neurones.
8. Importance of the Refractory Period
The refractory period is a short time after an action potential during which a neurone cannot be re-excited. This ensures that impulses travel in only one direction along a neurone and limits the maximum frequency of impulses. This control is important for accurate signal transmission and prevents the overlap of impulses.
9. Structure and Function of a Cholinergic Synapse
A cholinergic synapse is a junction between two neurones where acetylcholine is the neurotransmitter. When an action potential reaches the presynaptic knob, voltage-gated calcium channels open, and Ca²⁺ ions enter. This triggers vesicles containing acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the postsynaptic membrane, opening sodium channels and generating a new action potential. Acetylcholinesterase in the cleft breaks down acetylcholine to prevent continuous stimulation.
10. Role of Neuromuscular Junctions, T-Tubule System, Sarcoplasmic Reticulum
At neuromuscular junctions, the motor neurone releases acetylcholine, which binds to receptors on the muscle fibre’s sarcolemma, triggering an action potential. This travels along the T-tubules (invaginations of the sarcolemma) deep into the muscle fibre. The action potential stimulates the sarcoplasmic reticulum to release calcium ions, which are essential for muscle contraction.
11. Ultrastructure of Striated Muscle (Sarcomere)
Striated muscle is composed of repeating units called sarcomeres, visible under an electron microscope. Each sarcomere extends from one Z line to the next and contains thick filaments (myosin) and thin filaments (actin). The A band corresponds to the length of the myosin filaments, the I band contains only actin, and the H zone is where only myosin is present. The arrangement of these filaments gives muscle its striated appearance and is critical to contraction.
12. Sliding Filament Model of Muscle Contraction
Muscle contraction occurs when actin filaments slide past myosin filaments, shortening the sarcomere. In resting muscle, tropomyosin blocks the myosin-binding sites on actin. When calcium ions are released from the sarcoplasmic reticulum, they bind to troponin, causing a conformational change that moves tropomyosin and exposes the binding sites. Myosin heads bind to actin, forming cross-bridges. ATP is hydrolyzed to provide energy for the myosin head to pivot, pulling the actin filament. A new ATP molecule then binds to myosin, allowing it to detach and reset for another cycle. This repeated action leads to muscle contraction.
15.2 Control and coordination in plants
Students should be able to:
1) describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved
2) explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
3) describe the role of gibberellin in the germination of barley
1. Describe the rapid response of the Venus flytrap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved
The Venus flytrap exhibits a rapid and specialised response to physical stimulation of its modified leaves. Each lobe of the trap contains three sensitive trigger hairs. When two hairs are touched in quick succession, or one hair is touched twice within approximately 20 seconds, an action potential is generated. This electrical signal is transmitted across the leaf lobe, initiating a rapid cellular response.
The closure of the trap is achieved through rapid changes in turgor pressure in specialized motor cells located in the midrib of the leaf. When the action potential reaches these cells, ion channels open, leading to a sudden efflux of potassium ions (K⁺) and other solutes from the motor cells into surrounding tissues. Water follows osmotically, resulting in a rapid decrease in turgor pressure. This causes the lobes of the leaf to snap shut, trapping the prey. The mechanism is an example of thigmonasty — a non-directional plant movement in response to touch.
2. Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
Auxin, specifically indole-3-acetic acid (IAA), is a plant hormone that plays a central role in cell elongation, particularly in young growing tissues such as shoots. When auxin is distributed asymmetrically within plant tissues, it promotes differential growth that allows the plant to bend towards light (phototropism) or grow against gravity (gravitropism).
Auxin stimulates elongation growth by activating proton pumps in the plasma membranes of target cells. These pumps actively transport hydrogen ions (H⁺) into the cell wall space, lowering the pH of the cell wall to around 4.5. This acidification activates cell wall-loosening enzymes such as expansins. These enzymes disrupt the hydrogen bonds between cellulose microfibrils, loosening the structure of the cell wall.
As a result of the weakened wall structure, the turgor pressure within the cell causes it to expand, leading to cell elongation. The process is directional and tightly regulated, and it is essential for phototropic and gravitropic responses in plants.
3. Describe the role of gibberellin in the germination of barley
During the germination of barley seeds, gibberellin (specifically gibberellic acid, GA) plays a critical role in mobilising stored food reserves to support the growth of the emerging seedling. The process begins when water is absorbed by the seed, rehydrating tissues and activating metabolic pathways.
Gibberellin is synthesised in the embryo of the seed and then diffuses to the aleurone layer of the endosperm. In response to gibberellin, the cells of the aleurone layer begin to transcribe and translate genes that code for hydrolytic enzymes, particularly α-amylase.
α-amylase is secreted into the starchy endosperm, where it catalyses the hydrolysis of starch into maltose and glucose. These sugars are then absorbed by the growing embryo and used as a respiratory substrate to provide ATP for cell division and elongation.
Thus, gibberellin ensures that energy stored in the endosperm is converted into usable forms, supporting the initial stages of seedling growth during germination