Title: InkBiology / Nervous coordination, receptors, muscles and behaviour
On every note of this deck, unless a note says otherwise:
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Front: CHECK YOURSELF
Explain how the arrival of an impulse at a neuromuscular junction leads to the shortening of a sarcomere, and state what happens to the width of the A band, the I band and the H zone as it does so.
Back: Acetylcholine released at the neuromuscular junction binds to receptors on the sarcolemma and depolarises it. The depolarisation spreads along the sarcolemma and down the transverse tubules into the fibre, and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
Calcium ions bind to troponin, changing its shape so that it moves tropomyosin away from the myosin binding sites on the actin. Myosin heads, already carrying ADP and phosphate, attach to those sites and form cross-bridges.
Each head then bends in a power stroke, releasing ADP and phosphate and pulling the thin filament about 10 nm towards the centre of the sarcomere. ATP binds to the head, which detaches, and hydrolysis of that ATP returns the head to its original position so it can attach further along. With heads working out of step, the thin filaments are drawn steadily inwards and the Z lines move closer together.
The A band is unchanged, because it is the length of the thick filaments and they neither shorten nor move. The I band and the H zone both get shorter, because the thin filaments now occupy more of the A band and leave less of it unoccupied. The sarcomere shortens by the amount those two regions lose.

Front: DEFINE
Define: Transducer.
Back: Something that converts energy from one form into another; a receptor converts the energy of a stimulus into a change in membrane potential.

Front: DEFINE
Define: I band.
Back: The light band, containing thin filaments only, spanning a Z line.

Front: DEFINE
Define: Synapse.
Back: The junction between the end of one neurone and another neurone or an effector cell, across which chemical transmission occurs.

Front: CHECK YOURSELF
A postsynaptic neurone receives inputs from three presynaptic neurones. Each excitatory input depolarises it by 6 mV; one inhibitory input hyperpolarises it by 9 mV. The resting potential is −70 mV and threshold is −55 mV. Determine whether an action potential is produced when all three excitatory neurones fire together, and when the inhibitory neurone fires alongside them. Explain what the neurone is doing in each case.
Back: Threshold requires a depolarisation of 15 mV, from −70 mV to −55 mV. Three excitatory inputs together give 3 × 6 = 18 mV, which takes the membrane to −52 mV. That is past threshold, so voltage-gated sodium channels open and an action potential is produced. This is spatial summation: no single input would have done it, since 6 mV alone reaches only −64 mV.
With the inhibitory input firing as well, the total is 18 − 9 = 9 mV of depolarisation, reaching −61 mV. That is short of threshold, so no action potential is produced.
The inhibitory synapse achieves this by opening chloride or potassium channels, making the inside of the postsynaptic neurone more negative and so raising the excitatory input needed to reach threshold.
Read together, the two cases describe what a postsynaptic neurone is for. It is not a relay that passes on whatever it receives; it adds up positive and negative inputs arriving at that moment and fires only if the total clears a fixed bar. A single neurone in the brain may receive thousands of such inputs, and the decision it makes is this same arithmetic on a larger scale.

Front: DEFINE
Define: Sarcoplasmic reticulum.
Back: The membrane system inside a muscle fibre that stores and releases calcium ions.

Front: DEFINE
Define: Stimulus.
Back: A detectable change in the internal or external environment.

Front: CHECK YOURSELF
A neurone is treated with a drug that blocks voltage-gated potassium channels but leaves the sodium channels and the sodium-potassium pump working. Predict the effect on the shape of an action potential in that neurone, and explain each part of your prediction.
Back: The rising phase would be unchanged. Depolarisation depends on voltage-gated sodium channels opening and sodium ions entering, and neither is affected, so the membrane would still reach about +40 mV at the same rate.
Repolarisation would be much slower. Normally potassium ions leave through the voltage-gated potassium channels, carrying positive charge out and bringing the potential down quickly. With those channels blocked, the fall would depend on the pump and on whatever leakage remains, both far slower, so the action potential would be prolonged.
The hyperpolarisation would be reduced or absent altogether. The undershoot exists because potassium continues to leave after the resting value is passed, and if that exit is blocked there is nothing to carry the potential below −70 mV.
One knock-on effect is worth adding: a prolonged action potential lengthens the refractory period, so the neurone could not fire as often. Its maximum frequency would fall, and with it the intensity of stimulus it can report.

Front: DEFINE
Define: Generator potential.
Back: The graded depolarisation produced in a receptor by a stimulus, whose size depends on the strength of that stimulus.

Front: SPOT THE ERROR
Say what is wrong with this claim:
The impulse travels along the neurone like electricity down a wire.
Back: Nothing flows along the axon. What moves is a sequence of events across the membrane, ions crossing at one point and depolarising the patch next door. The fastest human axon manages about 100 m s⁻¹, roughly a millionth of the speed of electricity in copper.

Front: DEFINE
Define: Resting potential.
Back: The potential difference across the membrane of a neurone that is not conducting an impulse, about −70 mV inside relative to outside.

Front: DEFINE
Define: Excitatory postsynaptic potential.
Back: A depolarisation of the postsynaptic membrane that makes an action potential more likely.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Muscles shorten because the filaments in them shorten.
Back: Nothing shortens; the filaments slide. Under an electron microscope the dark A band is the same width in a contracted muscle as in a relaxed one, and the thick filaments that make it are the same length. What changes is how far the thin filaments have been pulled between them.

Front: DEFINE
Define: Reflex arc.
Back: The pathway of neurones taken by a reflex. In the examined withdrawal reflex: receptor, sensory neurone, relay neurone, motor neurone, effector.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Your fingertip detects heat and sends the heat up your arm; your eye detects colour and sends colour along the optic nerve.
Back: Nothing about the stimulus travels anywhere. A receptor is a transducer: it converts the energy of one stimulus into a change in membrane potential. Pressure, light and sound are then told apart by which neurone the impulses arrive along and how often.

Front: DEFINE
Define: Sarcomere.
Back: The functional unit of a myofibril, running from one Z line to the next.

Front: DEFINE
Define: Receptor.
Back: A cell or structure that detects one kind of stimulus and converts it into a change in membrane potential.

Front: DEFINE
Define: Threshold.
Back: The membrane potential, around −55 mV, that must be reached before an action potential is triggered.

Front: DEFINE
Define: H zone.
Back: The lighter region at the centre of the A band, containing thick filaments only.

Front: DEFINE
Define: A band.
Back: The dark band, the full length of the thick filaments, including any overlap with thin filaments.

Front: SPOT THE ERROR
Say what is wrong with this claim:
The impulse crosses the synapse.
Back: The impulse arrives at the presynaptic membrane and stops there. A chemical is released, diffuses across a gap about 20 nm wide and binds on the far side, where, if enough arrives, a new action potential begins in a different cell. The message continues; the impulse does not.

Front: DEFINE
Define: Voltage-gated channel.
Back: A channel protein that opens or closes in response to a change in the potential difference across the membrane.

Front: DEFINE
Define: Polarised.
Back: Describes a membrane with a potential difference across it; at rest the axon membrane is polarised with the inside negative.

Front: DEFINE
Define: Neurotransmitter.
Back: A chemical released from a presynaptic neurone that diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Front: DEFINE
Define: Effector.
Back: A muscle or gland that brings about a response.

Front: DEFINE
Define: Cholinergic synapse.
Back: A synapse that uses acetylcholine as its neurotransmitter.

Front: DEFINE
Define: Reflex.
Back: A rapid, automatic response to a stimulus, involving a fixed pathway of neurones and no conscious decision.

Front: CHECK YOURSELF
A student presses gently on the back of their hand and feels nothing; pressing harder in the same place, they feel it clearly. Using what a Pacinian corpuscle does, explain both observations, and explain why the harder press does not produce larger impulses in the sensory neurone.
Back: Gentle pressure deforms the lamellae only slightly, so few stretch-mediated sodium channels are opened, few sodium ions enter and the generator potential is small. It does not reach the threshold of about −55 mV, no action potential is triggered, and nothing is sent to the central nervous system.
The harder press deforms the lamellae much more, so more channels open, more sodium ions diffuse in and the generator potential is larger. This time the membrane reaches threshold and action potentials are triggered in the sensory neurone.
Their size, though, is fixed. An action potential is all-or-nothing: once threshold is passed, the voltage-gated channels open fully and the same peak is reached every time, whatever the stimulus. What a stronger stimulus changes is the frequency of impulses, and how many neurones are firing at once.
The graded step and the all-or-nothing step are different stages of the same journey, receptor first and neurone second, and questions on this topic are usually testing whether you have kept them apart.