Ch 1 · 15.1 Control and coordination in mammals · AS & A Level Biology 9700
Topic 1 of 2 in Control and coordination — AS & A Level Biology 9700 — 108 lessons.
Introduction
The mammalian nervous system coordinates a fast response to a stimulus by generating and conducting electrical impulses along neurones and passing them between cells at synapses. This lesson compares the nervous and endocrine systems, then covers neurones and receptors, the ionic basis of the resting and action potentials, synapses, and how a nerve impulse makes a striated muscle contract. (Syllabus section 15.1.)
Key ideas — The endocrine system and the nervous system
The endocrine system is made of glands that secrete hormones straight into the blood. A hormone travels all round the body but acts only on target cells that have the matching receptor. Examples from this syllabus:
🧑🏫 Teacher's explanation
Think of your nervous system like a fiber-optic internet cable: it sends rapid-fire electrical signals to specific locations almost instantly. In contrast, the endocrine system is like a broadcast radio signal: it sends chemical messages through the bloodstream that take longer to arrive but can affect many different parts of the body at once.
📖 Definitions
🔍 Worked example — Easy
A resting neurone has a membrane potential of -70 mV. During an action potential, the potential rises to +30 mV. Calculate the total change in membrane potential.
🔍 Worked example — Standard
Describe the role of calcium ions (Ca^2+) at a cholinergic synapse.
🔍 Worked example — Exam-style
Explain how the refractory period affects the frequency of nerve impulses.
⚠️ Common errors
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🎯 Exam question
Key ideas — Neurones
Many axons and dendrons are wrapped in a myelin sheath made by Schwann cells, with gaps called nodes of Ranvier.
🧑🏫 Teacher's explanation
Think of a neurone as a biological electrical cable. In a house, a wire carries an electrical signal from a switch to a lightbulb. In your body, the neurone carries an electrical signal (an impulse) from a sensor (like your skin) to your brain.
📖 Definitions
🔍 Worked example — Easy
A student is asked to identify the components of a sensory neurone. List the three main structural features of a sensory neurone and state the location of its cell body.
🔍 Worked example — Standard
Compare the structural differences between a sensory neurone and a motor neurone regarding the position of the cell body and the length of the dendron/axon.
🔍 Worked example — Exam-style
Explain the advantage of saltatory conduction in a myelinated neurone compared to an unmyelinated neurone.
⚠️ Common errors
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🎯 Exam question
(a) Describe the role of the myelin sheath in the transmission of an impulse. [2] (b) Explain how the presence of nodes of Ranvier affects the speed of impulse transmission. [2] [4 marks]
Key ideas — Sensory receptors: the taste bud
A sensory receptor cell detects a stimulus and converts its energy into electrical changes, starting impulses in a sensory neurone. Chemoreceptor cells in taste buds on the tongue detect chemicals. For salt (sodium ions):
🧑🏫 Teacher's explanation
Think of a sensory receptor cell like a doorbell. The chemical stimulus (the food you eat) is like someone pressing the button. The receptor cell doesn't just 'know' there is food; it converts that chemical signal into an electrical 'ding' (a receptor potential). If the person presses the button harder (a higher concentration of salt), the 'ding' is louder (a larger depolarization). If the 'ding' is loud enough to reach a certain volume (the threshold), the doorbell's internal wiring triggers a formal signal (an action potential) that travels through the wires (the neurone) to the main control panel (the brain).
📖 Definitions
🔍 Worked example — Easy
A chemoreceptor cell in a taste bud is exposed to a low concentration of sodium ions. The resulting receptor potential is 5 ext mV. If the threshold for generating an action potential is 15 ext mV, will an action potential be generated? Explain why.
🔍 Worked example — Standard
Describe the sequence of events that occurs in a taste bud chemoreceptor when a salt stimulus is detected, specifically focusing on the role of calcium ions.
🔍 Worked example — Exam-style
A student observes that increasing the concentration of a chemical stimulus increases the frequency of action potentials in a sensory neurone. Explain the mechanism that allows this change in frequency to occur.
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🎯 Exam question
(a) Describe the sequence of events that leads to the generation of an action potential in the associated sensory neurone. [4] (b) Explain why the frequency of action potentials increases as the concentration of salt increases. [2] [6 marks]
Key ideas — The resting potential
In a resting axon, the membrane potential is maintained at about −70 mV (inside negative relative to outside). The sodium-potassium pump actively transports 3 Na⁺ out for every 2 K⁺ in, using ATP. At rest, the membrane is far more permeable to K⁺ than to Na⁺ because more potassium "leak" channels are open, so K⁺ diffuses out down its concentration gradient faster than Na⁺ diffuses in, leaving the inside negatively charged relative to the outside. This resting potential is maintained until a stimulus arrives. The pump also keeps the concentration gradients that make the next action potential possible.
🧑🏫 Teacher's explanation
Imagine a crowded nightclub where the security guards (the sodium-potassium pump) are constantly working to keep the 'cool kids' (sodium ions) outside and the 'party animals' (potassium ions) inside. Even though people are constantly slipping through the doors (leak channels), the guards use energy to maintain a specific balance. Because the party animals (K+) are much more eager to leave the club than the cool kids (Na+) are to enter, the inside of the club becomes increasingly negative compared to the street outside.
📖 Definitions
🔍 Worked example — Easy
A resting neurone has an internal charge of -70 mV and an external charge of 0 mV. Calculate the magnitude of the potential difference across the membrane.
🔍 Worked example — Standard
A sodium-potassium pump moves 3 Na^+ ions out and 2 K^+ ions in per ATP molecule. If a neurone uses 500 ATP molecules to maintain its gradient, how many net positive charges have been moved out of the cell?
🔍 Worked example — Exam-style
A student observes a graph of membrane potential. At time t_1, the potential is -70 mV. At time t_2, it is -70 mV. During the interval, the potential reached +30 mV. Describe the process that occurred between t_1 and t_2 and explain why the potential returned to -70 mV.
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🎯 Exam question
Key ideas — The action potential and its propagation
A stimulus opens some voltage-gated Na⁺ channels, depolarising the membrane. At the threshold (about −55 mV), many more Na⁺ channels open, rapidly depolarising the membrane to about +40 mV. These channels then close and voltage-gated K⁺ channels open, so K⁺ diffuses out and the membrane repolarises; a brief overshoot below −70 mV, hyperpolarisation, precedes the restored resting potential. The pump then restores the used ionic gradients. An action potential is all-or-nothing: it fires at full size or not at all.
🧑🏫 Teacher's explanation
Think of an action potential like a stadium wave. To start the wave, you need a stimulus to trigger the first person. Once triggered, the wave moves rapidly from person to person. However, once a person has stood up and sat back down, they cannot immediately stand up again for a second wave; they need a moment to recover. This 'recovery time' is exactly like the refractory period in a neurone.
📖 Definitions
🔍 Worked example — Easy
A neurone has a resting potential of -70 mV. During an action potential, the membrane potential reaches +40 mV. Calculate the total change in membrane potential during the depolarisation phase.
🔍 Worked example — Standard
Describe the sequence of ion movements during the repolarisation phase of an action potential.
🔍 Worked example — Exam-style
Explain why an action potential propagates in only one direction along a neurone.
⚠️ Common errors
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🎯 Exam question
(a) Explain how saltatory conduction in the myelinated neurone results in a faster impulse transmission. [3] (b) State the role of the refractory period in determining the frequency of impulses. [1] [4 marks]
Key ideas — Transmission across a cholinergic synapse
When an action potential reaches the presynaptic knob, it opens voltage-gated Ca²⁺ channels; the influx of Ca²⁺ causes synaptic vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis. ACh diffuses across the cleft and binds to specific receptors on ligand-gated Na⁺ channels in the postsynaptic membrane, which open and allow Na⁺ influx, depolarising the postsynaptic membrane; if threshold is reached, a new action potential is generated. Acetylcholinesterase in the cleft hydrolyses ACh into choline and ethanoic acid, which are reabsorbed into the presynaptic neurone and used to resynthesise ACh using ATP. This breakdown prevents continuous stimulation of the postsynaptic membrane, and because vesicles are only in the presynaptic neurone and receptors only on the postsynaptic membrane, transmission across a synapse is unidirectional.
🧑🏫 Teacher's explanation
Think of a synapse like a gap between two islands. You cannot jump the gap yourself, so you must send a boat (the neurotransmitter) across the water to deliver a message to the next island. The message isn't electrical while it is in the water; it becomes chemical.
📖 Definitions
🔍 Worked example — Easy
A student is asked to list the sequence of events occurring at a synapse when an action potential reaches the presynaptic knob. List the first three steps.
🔍 Worked example — Standard
Explain why neurotransmission across a synapse is unidirectional.
🔍 Worked example — Exam-style
Calculate the percentage decrease in acetylcholine concentration in the synaptic cleft if the initial concentration is 5.0 imes 10^-6 mol/L and the concentration after acetylcholinesterase action is 0.5 imes 10^-6 mol/L.
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🎯 Exam question
Key ideas — The refractory period and impulse frequency
After an action potential, there is a short refractory period (a few milliseconds) when the voltage-gated sodium channels are closed and cannot reopen, so no new action potential can start in that part of the membrane. This matters because:
🧑🏫 Teacher's explanation
Think of a neuron like a single-lane road with a temporary construction barrier that only lifts once a car has passed. Once a car (the action potential) passes a certain point, the road behind it is blocked for a few seconds. This 'blockage' is the refractory period. Because the road behind the car is blocked, the car cannot turn around or move backward, ensuring it only moves forward.
📖 Definitions
🔍 Worked example — Easy
A neuron is stimulated with a weak stimulus that results in 5 impulses per second. A stronger stimulus results in 20 impulses per second. Explain why the magnitude of the action potential does not increase with the stronger stimulus.
🔍 Worked example — Standard
Calculate the maximum frequency of impulses if the refractory period of a neurone is 2.5 ms.
🔍 Worked example — Exam-style
A student observes a nerve impulse traveling along a myelinated axon. Explain how the refractory period ensures the impulse travels in one direction only.
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🎯 Exam question
Key ideas — The neuromuscular junction, T-tubules and sarcoplasmic reticulum
A neuromuscular junction is a synapse between a motor neurone and a muscle fibre. It works like a cholinergic synapse: an impulse arrives, Ca²⁺ enters the motor neurone ending, acetylcholine is released, diffuses across and binds to receptors on the muscle fibre membrane (the sarcolemma), opening sodium channels and depolarising it.
🧑🏫 Teacher's explanation
Think of a muscle contraction like a massive factory assembly line. The motor neurone is the manager sending a signal. The neuromuscular junction is the office where the manager's memo (acetylcholine) is sent out. However, the factory floor is huge, so the memo alone isn't enough; you need a high-speed electrical wiring system to reach every corner instantly. The T-tubules act like these internal electrical conduits, ensuring the signal reaches the very center of the cell, while the sarcoplasmic reticulum acts like a specialized storage warehouse for calcium ions, ready to be released the moment the signal arrives.
📖 Definitions
🔍 Worked example — Easy
Identify the specific neurotransmitter released at a neuromuscular junction and state its effect on the sarcolemma.
🔍 Worked example — Standard
Explain the role of calcium ions (Ca^2+) in the context of the sarcoplasmic reticulum during muscle stimulation.
🔍 Worked example — Exam-style
A student observes an electron micrograph of a muscle fibre. They note deep invaginations of the sarcolemma that run parallel to the myofibrils. Name this structure and explain its functional importance for rapid contraction.
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🎯 Exam question
Key ideas — The ultrastructure of striated muscle
A muscle fibre is packed with myofibrils, each made of repeating units called sarcomeres, joined end to end at Z lines. Each sarcomere contains two kinds of protein filament:
🧑🏫 Teacher's explanation
Think of a muscle contraction like a rowing team in a boat. The myosin filaments are the rowers, and the actin filaments are the oars. To move the boat (the muscle), the rowers must grab the oars and pull them toward the center. In a muscle, the 'rowers' (myosin) grab the 'oars' (actin) and pull them toward the middle of the sarcomere, shortening the distance between the ends of the muscle fiber.
📖 Definitions
🔍 Worked example — Easy
A sarcomere has a total length of 2.5 mum. The distance between the two Z lines is 2.5 mum. If the actin filaments are 1.8 mum long and the myosin filaments are 2.0 mum long, calculate the width of the H zone in the relaxed state.
🔍 Worked example — Standard
During muscle contraction, the Z line distance decreases from 3.0 mum to 2.4 mum. Calculate the percentage decrease in sarcomere length.
🔍 Worked example — Exam-style
A student observes an electron micrograph of a striated muscle. They note that the A band is 2.2 mum wide and the I band is 0.8 mum wide. Determine the total length of one sarcomere.
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🎯 Exam question
Key ideas — The sliding filament model
The filaments slide past each other; they do not shorten. So during contraction the sarcomere, I band and H zone shorten, while the A band stays the same length. When stimulation stops, Ca²⁺ is pumped back into the sarcoplasmic reticulum using ATP and the muscle relaxes.
🧑🏫 Teacher's explanation
Imagine a heavy curtain being pulled across a rod. The rod is the actin filament, and the curtain is the myosin filament. However, the curtain is initially stuck to the wall by small clips (tropomyosin) that prevent it from sliding. To move the curtain, you need a key (calcium ions) to release the clips, and you need a motor (ATP) to provide the energy to pull the curtain along the rod.
📖 Definitions
🔍 Worked example — Easy
A student observes an electron micrograph of a contracting muscle sarcomere. If the initial length of the I band is 4 mum and the final length after contraction is 2.5 mum, calculate the percentage decrease in the I band length.
🔍 Worked example — Standard
Describe the role of calcium ions in the process of muscle contraction.
🔍 Worked example — Exam-style
Compare the changes in the lengths of the A band, I band, and H zone during muscle contraction.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student investigated the effect of a local anaesthetic drug that blocks voltage-gated Na⁺ channels in the axon membrane. (a) Explain why an axon treated with this drug cannot conduct a nerve impulse. [3] (b) A stronger stimulus does not produce bigger action potentials. Explain how the nervous system shows the strength of a stimulus. [2]
🔍 Worked example — Easy
A resting neurone has a membrane potential of -70 ext mV. During an action potential, the potential reaches +30 ext mV. Calculate the total change in membrane potential during this depolarization phase.
🔍 Worked example — Standard
A student observes a nerve impulse traveling along a myelinated axon. If the distance between two nodes of Ranvier is 2 ext mm and the impulse travels at 50 ext m/s, how long does it take for the impulse to travel between these two nodes?
🔍 Worked example — Exam-style
Explain the role of the refractory period in determining the frequency of nerve impulses. [2]
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