Biopsychology

This is Unit Two of Psychology, Part One: the cell, the signal, and the wiring. Unit One was about how psychology finds anything out. This unit is about the hardware — and it starts at the smallest level, with a single cell, and works outward. The organising fact of this part is a law that sounds like a technicality and is not. A neuron either fires at full strength or does not fire at all. There is no partial firing and no bigger spike for a stronger stimulus. Which raises an obvious problem. If every spike is identical, how does a nervous system carry the difference between a light touch and a hard one? The answer is in this part, and it changes how you think about the whole system. Then the gap between one neuron and the next, where chemistry takes over from electricity — and where nearly every drug that affects the mind does its work. And the part closes on the wiring diagram: the divisions of the nervous system, nested inside each other, ending with a response organised entirely below the brain. By the end you will be able to do five things. One: name the neuron's parts and what each contributes. Two: say what the all-or-none law rules out, and what codes intensity instead. Three: trace a signal across a synapse in order. Four: say what agonists and antagonists do. Five: place any nerve in the right division.

Five entries on the unit of the whole system. Four of them are parts of the neuron, in the order a signal passes through them — and the fifth is about the cells that are not neurons, which outnumber them and do the maintenance.

The cell.

Psych 0141 The Neuron

A cell that receives, processes and transmits information through electrical and chemical signals. Both kinds of signal, in that order, within one cell.

The receiving end.

Psych 0143 Dendrites and the Cell Body

Dendrites receive incoming signals and carry them to the cell body, which integrates them and holds the nucleus. Integrates is the operative word — many inputs, one decision.

The sending end.

Psych 0145 The Axon and Terminal Buttons

The axon carries the outgoing signal away from the cell body to the terminal buttons at its end. One route out, however many routes came in.

The coating.

Psych 0147 The Myelin Sheath

A fatty coating on the axon that speeds transmission and insulates it from neighbouring axons. It does two jobs — faster signals, and signals that stay in their own wire.

The other cells.

Psych 0149 Glial Cells

Supporting neurons by supplying nutrients, producing myelin, and clearing waste from around the synapse. Not signalling cells, and the signalling would stop without them.

Three entries taking one neuron from rest to firing. There is a stable state, a line that has to be crossed, and then something that happens very quickly — and the three are worth keeping in order, because the third depends entirely on the second.

At rest.

Psych 0151 The Resting Potential

A stable negative charge inside the neuron relative to outside, held while the neuron is not firing. Stable is the point — resting is maintained, not passive.

The line.

Psych 0153 The Threshold of Excitation

The level of depolarisation a neuron has to reach before it fires. Below it nothing happens; at it, everything does.

Crossing it.

Psych 0155 The Action Potential

A rapid electrical spike travelling down the axon once the threshold is crossed. One event, always the same event, whatever pushed the cell over the line.

Three entries, and this is the conceptual centre of the part. The law, then the error people make because the law feels wrong, and then the answer to the question the law creates — how a system of identical spikes carries the difference between a whisper and a shout.

The law.

Psych 0157 The All-or-None Law

A neuron either fires at full strength or does not fire at all. No partial firing, no gradation within the single spike.

The error.

Psych 0158 The "Stronger Signal, Bigger Spike" Error

A more intense stimulus does not produce a larger action potential in a single neuron. The intuition is reasonable and the biology does not work that way.

And the answer.

Psych 0159 Rate and Population Coding

Intensity is coded by how often neurons fire and how many of them fire — never by the size of one spike. The information is in the pattern across cells and time, not inside any single event.

Four entries on where one neuron meets the next. The electrical signal stops at the gap and chemistry takes over — and everything from an antidepressant to a nerve agent operates somewhere in these four entries.

The junction.

Psych 0161 The Synapse

The junction where one neuron's terminal button communicates with the next cell across a narrow gap. The cells do not touch, which is why chemistry is needed.

What crosses.

Psych 0163 Neurotransmitters

Chemical messengers released into the gap, carrying the signal to the receiving neuron. The electrical signal becomes a chemical one and then becomes electrical again.

What receives it.

Psych 0165 Receptor Specificity

Each neurotransmitter binds only to receptor sites shaped to receive it, the way a key fits one lock. Specificity is what keeps many messages in one space from becoming noise.

What clears it.

Psych 0167 Reuptake

The neurotransmitter is pumped back into the neuron that released it, clearing the gap. Clearance matters as much as release — a signal that never ends is not a signal.

Two entries naming the two directions a substance can push. This pair is the vocabulary underneath every psychoactive drug in the rest of the course, so it is worth having exactly rather than approximately.

Upward.

Psych 0169 Agonists

A substance increasing a neurotransmitter's activity, either by mimicking it or by amplifying its effect. Two different mechanisms, one direction.

Downward.

Psych 0170 Antagonists

A substance decreasing a neurotransmitter's activity, usually by occupying its receptor without activating it. It works by taking the space, which is a more elegant mechanism than blocking.

One procedure, and it assembles the last four topics into a single sequence. Threshold, spike, release, binding, clearance — with every structure named as the signal reaches it, in order, without gaps.

Watch one signal cross.

Psych 0172 Tracing a Signal Across the Synapse

Following a synaptic signal from threshold to spike to release to binding to clearance, naming each structure in order. Every step of the chain, with nothing assumed between them.

Six entries covering seven neurotransmitters. Most of them are named for what they are involved in — and the pair in the middle is different, because those two set the overall balance of excitation and inhibition everything else operates inside.

The first.

Psych 0174 Acetylcholine

Acting at the junction between motor neurons and muscle, and involved in memory formation. Two roles that sound unrelated and are carried by one molecule.

The second.

Psych 0176 Dopamine

Involved in movement, reward learning, and motivation toward an anticipated outcome. Anticipated is doing work — it is about the pursuit more than the having.

The third.

Psych 0178 Serotonin

Involved in mood, sleep, appetite, and the regulation of arousal. A wide portfolio, which is why interventions touching it have wide effects.

The fourth.

Psych 0180 Norepinephrine

Involved in alertness, arousal, and the body's mobilisation during stress. It reappears in this course's stress material, doing the same job under a different heading.

The balance.

Psych 0182 GABA and Glutamate

One is the brain's main inhibitory transmitter and the other its main excitatory one. Everything else in this topic operates inside the balance these two set.

And the last.

Psych 0184 Endorphins

Reducing pain and producing feelings of pleasure or euphoria. Produced internally, which is what makes the external substances resembling them work.

Seven entries laying out the whole diagram, and the divisions nest — each one is inside the one before it. The last entry is the exception that proves the design: a response organised entirely in the spinal cord, which happens before the brain knows anything about it.

The centre.

Psych 0186 The Central Nervous System

The brain and the spinal cord. Two structures, and everything else in the body is defined by not being them.

Everything else.

Psych 0188 The Peripheral Nervous System

Every nerve outside the brain and spinal cord. Defined by exclusion, and then divided by what it does.

The first division.

Psych 0190 The Somatic Nervous System

Carrying sensory information inward and voluntary motor commands to skeletal muscle. Voluntary is the word that separates it from the next one.

The second.

Psych 0192 The Autonomic Nervous System

Regulating involuntary activity in organs, glands and smooth muscle. Continuous, and almost entirely outside awareness.

Mobilising.

Psych 0194 The Sympathetic Division

Mobilising the body for action, raising heart rate and redirecting energy to the muscles. One half of a pair that works by opposition.

Restoring.

Psych 0195 The Parasympathetic Division

Restoring the body toward rest, slowing heart rate and resuming digestion. Not the absence of the other one — an active process in the other direction.

And the shortcut.

Psych 0197 The Spinal Reflex

A response organised in the spinal cord, occurring before the signal reaches the brain. Not a fast brain response — a response the brain was not consulted about.

Five things you can now do, and Part Two takes up the brain. You can name the neuron's parts in signal order — dendrites in, cell body integrating, axon out, myelin making it faster and keeping it in its own wire. You can say what the all-or-none law rules out — a bigger spike for a stronger stimulus — and that intensity is coded by how often and how many neurons fire. You can trace a signal across a synapse: threshold, spike, release, binding to a specific receptor, and clearance back into the sending cell. You can say what agonists and antagonists do — one increases a transmitter's activity by mimicking or amplifying, the other decreases it by occupying the receptor without activating it. And you can place a nerve in the right division: central or peripheral, then somatic or autonomic, then mobilising or restoring.