PSYC10006 Chap.11 Neurons, Synapses and Measuring the Brain
Neurons, Synapses and Measuring the Brain
The neuron is the unit of information processing in the nervous system, and its shape is its function: many branches collecting input, one process carrying an answer away, and a direction of travel that never reverses. This module defines firing by three voltage landmarks rather than by a description, and then draws out a consequence that is easy to state and easy to under-explain.
Because every action potential is the same size, intensity cannot be carried by amplitude and has to be carried by rate. The synapse turns the electrical signal into a chemical one, delivering excitation or inhibition, and a receiving neuron fires only if the sum of what it is being told exceeds the sum of what it is being told not to do.
The chapter ends with four ways of measuring all of this, and the trade-off that separates them.
What this chapter covers
- 01
Dendrites, soma, axon and terminals, and the direction the signal always takes
- 02
Why myelin has gaps in it, and why complete insulation would be worse
- 03
The resting potential, the threshold and the peak, in millivolts
- 04
Sodium in to depolarise, potassium out to repolarise, and the overshoot that follows
- 05
All-or-none firing, and why it forces intensity to be carried by rate
- 06
Stimulus strength, time to threshold, firing rate and transmitter released
- 07
The four release steps, from cell body to receptors on the far side
- 08
Reuptake and recycling, and why it is a point a drug can act on
- 09
The primary excitatory and inhibitory transmitters and the potentials they produce
- 10
Neural integration: summed excitation against simultaneous inhibition
- 11
Receptors as locks, and agonists and antagonists as two kinds of key
- 12
Four measurement techniques, positioned by spatial and temporal precision
Choose a measurement technique and defend the choice
- 2For the first, the demand is chemical. Only tracer imaging can index receptor binding and transmitter function; the others measure electrical or blood-flow activity rather than which molecules are present, so its weaker spatial resolution is irrelevant here.
- 2For the second, the demand is timing. Electrical recording from the scalp is the choice, because discriminating very brief events in time is its strength, and its poor spatial resolution costs nothing because the question does not ask where.
- 2For the third, the demand is anatomy, so structural magnetic imaging, whose high spatial resolution suits distinguishing tissue types and locating structures. In each case the decisive property is the one the question demands and the technique's weaknesses are ones the question does not touch.
Key terms
- Action potential
- The rapid change in membrane potential produced by ions crossing the membrane, which is what firing means. It is defined in this subject by three voltages: a resting value, a threshold and a peak.
- Threshold potential
- The membrane value at which the action-potential cascade triggers. Everything before it is graded and everything after it is not, which is why it is the most important of the three voltages.
- Rate law
- The principle that signal intensity is carried by how frequently a neuron fires, since every action potential is the same size. It follows from the all-or-none property rather than being an independent finding.
- Synaptic cleft
- The gap between the sending and receiving neurons that a neurotransmitter must cross. Its existence is why an electrical signal has to become a chemical one at every junction.
- Neural integration
- The summing of excitatory inputs against simultaneous inhibitory ones, with the neuron firing only if the difference passes threshold. It is a computation performed by a single cell rather than a relay.
- Antagonist
- A compound that binds a receptor without properly activating it, thereby blocking the natural compound. It is contrasted with an agonist, which activates the receptor as the natural compound would.
- Blood oxygen signal
- The principle behind functional magnetic imaging, which indexes activity indirectly through blood flow. It works because haemoglobin that has given up its oxygen responds more strongly to a magnetic field than haemoglobin still carrying it, so the ratio between the two can be detected.
Neurons, Synapses and Measuring the Brain FAQ
Which ion moves which way, and how do I stop reversing it?
Sodium moves into the cell during depolarisation and potassium moves out during repolarisation. The check that catches an inversion is the sign: depolarisation has to raise the inside voltage towards the outside, so it must be positive charge entering. Deriving it that way is safer than recalling it.
Why does the rate law follow from all-or-none firing?
Because if every signal is identical in size, the system cannot say how strong something is by making the signal bigger. Intensity therefore has to be carried by frequency. A question asking you to explain the rate law is usually asking why it must be true rather than what it states.
Are inhibitory signals just weaker excitatory ones?
No. Both are influences on the probability of reaching threshold, in opposite directions. An inhibitory input moves the membrane further from threshold, and a large one is a strong signal pushing away from firing. Describing inhibition as partial excitation gets the sign wrong, and the sign is the whole distinction.
What resolution figures can I quote for the techniques?
None. The subject describes resolutions only qualitatively, as good or poor and high or low, and gives no millimetre or millisecond value for any of the main techniques. Quoting a figure would be inventing one, so compare them in relative terms and say what each is best suited to.
Which techniques are outside what I will be assessed on?
The subject states that computed tomography, magnetoencephalography and autoradiography are given as context and will not be examinable, and directs particular attention to magnetic imaging, tracer imaging, single-neuron recording and scalp electrical recording. Knowing that saves preparation time.
Assessment move
Fix the three voltages first and rehearse the sequence between them as a story about ions rather than as a shape, because the shape can be recalled with the ions reversed and the ions cannot. Then practise deriving the rate law rather than stating it, since the mark is for the inference.
Build a single table for the transmitter material with four columns, the transmitter, the potential it produces, the membrane change and the effect on firing, and drill it in both directions, because items in this area are usually mirror pairs.
For the measurement material, learn each technique by the one property that makes it the right choice and the one weakness that usually does not matter, and practise method-selection items rather than technique descriptions, because selection is what the subject asks for.
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