4 Physiological Psychology · 4.1 Physiological psychology
4.1.1 Neuron, synapse, neurotransmitters and the nervous system
The neuron is the basic information-processing unit of the nervous system; it fires an all-or-none electrical impulse down its axon and passes the message across a synapse with chemical neurotransmitters; neurons form the central (brain, spinal cord) and peripheral (somatic, autonomic) nervous systems, and the autonomic system has a sympathetic (fight or flight) and a parasympathetic (rest and digest) branch.
- APPSC Paper I
- Standard level
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Core facts
- Neuron = nerve cell = the basic structural and functional (information-processing) unit of the nervous system. The human brain has about 86 billion neurons.
- Parts of a neuron: dendrites (receive messages), cell body / soma (holds the nucleus, keeps the cell alive, adds up incoming signals), axon (carries the impulse away from the cell body), axon terminals / terminal buttons (release neurotransmitters).
- Myelin sheath: fatty white coat on many axons; speeds the impulse; made by Schwann cells (PNS) and oligodendrocytes (CNS). Gaps in it = nodes of Ranvier.
- Direction of a message: dendrite → cell body → axon → axon terminal → synapse.
- Kinds by function: sensory (afferent) neurons carry messages to the CNS; motor (efferent) neurons carry commands from the CNS to muscles and glands; interneurons (association neurons) connect neurons inside the CNS and are the most numerous.
- Kinds by shape: unipolar, bipolar (e.g. in the retina), multipolar (most motor neurons and interneurons).
- Glial cells (glia): support, nourish, insulate and clean up neurons; textbooks say they outnumber neurons (recent counts put them at roughly equal numbers in the human brain).
- Resting potential: about −70 mV (inside negative). Action potential (nerve impulse): a brief reversal to about +30 to +40 mV when the neuron fires.
- All-or-none law: a neuron either fires fully or not at all; a stronger stimulus raises the rate of firing, not the size of each impulse.
- Refractory period: a short time after firing when the neuron cannot fire again (absolute) or needs a stronger stimulus (relative).
- Synapse: the junction between two neurons; term coined by Charles Sherrington (1897). The tiny gap = synaptic cleft.
- First neurotransmitter identified: acetylcholine, by Otto Loewi (1921), in frog-heart experiments.
- CNS = brain + spinal cord. PNS = all nerves outside them: 12 pairs of cranial nerves and 31 pairs of spinal nerves.
- PNS divisions: somatic (voluntary; skeletal muscles and senses) and autonomic (involuntary; heart, glands, gut, blood vessels).
- Autonomic divisions: sympathetic = arousal, emergency, fight or flight (Walter Cannon); parasympathetic = calming, conserves energy, rest and digest.
| Neurotransmitter | Main role in behaviour | When it goes wrong |
|---|---|---|
| Acetylcholine (ACh) | muscle movement, attention, learning and memory | low in Alzheimer's disease |
| Dopamine | voluntary movement, reward, pleasure, motivation | low → Parkinson's disease; excess linked to schizophrenia |
| Serotonin | mood, sleep, appetite, impulse control | low → depression |
| Norepinephrine (noradrenaline) | alertness, arousal, vigilance | low → depressed mood |
| GABA | main inhibitory transmitter; calms neurons | low → anxiety, seizures |
| Glutamate | main excitatory transmitter; learning and memory | excess → overstimulation, seizures |
| Endorphins | natural pain relief and pleasure | released in pain, stress, hard exercise |
Explained
1. How a neuron is built and what each part does.
- Dendrites are short, branched fibres that pick up signals from other neurons or from sense receptors. More branches mean more connections.
- The soma adds up all the "fire" and "don't fire" signals it receives. If the total crosses the threshold (about −55 mV), the neuron fires.
- The axon is a single long fibre (from a fraction of a millimetre to about a metre, as in the nerves to the toes). Its start, the axon hillock, is where the impulse begins.
- Myelin insulates the axon. The impulse jumps from one node of Ranvier to the next (saltatory conduction), so myelinated axons conduct much faster (up to about 120 m/s) than bare ones. Loss of myelin in the CNS causes multiple sclerosis.
- Terminal buttons hold small sacs, the synaptic vesicles, filled with neurotransmitter.
- Myelinated axons look white (white matter); cell bodies look grey (grey matter).
2. The nerve impulse, step by step.
- At rest the inside of the neuron is negative compared with the outside: there are more sodium ions (Na⁺) outside and more potassium ions (K⁺) inside. The sodium-potassium pump keeps it so. The membrane is polarised.
- When the threshold is reached, sodium channels open and Na⁺ rushes in: depolarisation; the inside turns positive (the action potential).
- Potassium channels then open and K⁺ flows out: repolarisation. A brief overshoot below −70 mV is hyperpolarisation.
- The action potential travels down the axon like a falling row of dominoes. It is electrical within the neuron and chemical between neurons.
- Because of the all-or-none law, the brain codes stimulus intensity by the frequency of impulses and by how many neurons fire.
3. What happens at the synapse.
- The impulse reaches the terminal → vesicles release neurotransmitter into the cleft → the molecules bind to receptor sites on the next neuron, like a key in a lock.
- The effect is either excitatory (makes the next neuron more likely to fire) or inhibitory (less likely).
- The transmitter is then cleared by reuptake (pumped back into the sending neuron) or broken down by enzymes (e.g. acetylcholinesterase breaks down ACh).
- Drugs act at the synapse. An agonist mimics or boosts a transmitter (morphine mimics endorphins; nicotine mimics ACh). An antagonist blocks it (curare blocks ACh receptors and causes paralysis). Many antidepressants (SSRIs) block the reuptake of serotonin.
- Most synapses are chemical; a few are electrical (direct contact, very fast).
4. Neurotransmitters and behaviour (more detail).
- Acetylcholine: carries the message from motor neurons to skeletal muscles; in the brain it helps memory (the hippocampus). The ACh-producing neurons decline in Alzheimer's disease.
- Dopamine: the brain's reward signal (food, praise, addictive drugs). The dopamine neurons of the substantia nigra die in Parkinson's disease (tremor, rigid slow movement; treated with L-dopa). Drugs that block dopamine reduce hallucinations in schizophrenia.
- Serotonin: steadies mood, sleep and appetite; low levels go with depression, aggression and impulsive behaviour.
- Norepinephrine: works with the sympathetic system for alertness and emergency arousal; also released as a hormone (noradrenaline) by the adrenal medulla.
- GABA: brakes neural activity; anti-anxiety drugs (benzodiazepines) and alcohol enhance GABA, which is why they calm and sedate.
- Glutamate: the accelerator; vital for forming memories (long-term potentiation).
- Endorphins ("endogenous morphine"): the body's own opioids; reduce pain and give the "runner's high".
5. Organisation of the nervous system.
| Division | Made of | Job |
|---|---|---|
| Central (CNS) | brain, spinal cord | processes information, decides, stores |
| Peripheral: somatic | sensory and motor nerves to skin, muscles, joints | voluntary movement, sensation |
| Peripheral: autonomic | nerves to heart, lungs, gut, glands | automatic body functions, emotion |
| Autonomic: sympathetic / parasympathetic | two opposing branches | arouse / calm the body |
- Spinal cord: carries messages between the brain and the body (ascending sensory tracts, descending motor tracts) and controls reflexes.
- Reflex arc: receptor → sensory neuron → interneuron in the spinal cord → motor neuron → muscle. The response (e.g. pulling the hand from a flame, the knee-jerk) happens before the brain is aware; the knee-jerk has no interneuron (monosynaptic).
- The hypothalamus is the chief brain centre that controls the autonomic nervous system.
6. Sympathetic vs parasympathetic.
| Body part | Sympathetic (fight or flight) | Parasympathetic (rest and digest) |
|---|---|---|
| Pupils | dilate | constrict |
| Heart rate, blood pressure | increase | decrease |
| Breathing, bronchi | faster, bronchi widen | slower, bronchi narrow |
| Salivation | decreases (dry mouth) | increases |
| Digestion | slows down | speeds up |
| Blood sugar | rises (liver releases glucose) | falls; energy is stored |
| Sweating | increases | no direct effect |
| Adrenal medulla | stimulated (adrenaline) | not stimulated |
| Bladder | relaxes (holds urine) | contracts (empties) |
| Overall | spends energy | conserves and restores energy |
- Sympathetic nerves leave from the thoracic and lumbar spinal cord (thoracolumbar); parasympathetic from the brainstem and sacral cord (craniosacral); the vagus nerve is the main parasympathetic nerve.
- The two usually work in opposition (one speeds up an organ, the other slows it), which keeps the body in balance (homeostasis). In strong emotion such as fear or anger the sympathetic branch dominates; after the danger passes the parasympathetic branch restores calm.
Common traps
- ✗ "Dendrites carry the impulse away from the cell body." ✓ Dendrites receive; the axon carries the impulse away.
- ✗ "A stronger stimulus produces a bigger impulse." ✓ All-or-none: the impulse is the same size; stronger stimuli raise the firing rate.
- ✗ "Sensory neurons are efferent." ✓ Sensory = afferent (to the CNS); motor = efferent (from the CNS).
- ✗ "The synapse was named by Loewi." ✓ Sherrington (1897) named the synapse; Loewi (1921) found the first transmitter, acetylcholine.
- ✗ "Parkinson's disease is due to too little acetylcholine." ✓ Dopamine loss → Parkinson's; ACh loss → Alzheimer's.
- ✗ "GABA is excitatory." ✓ GABA is the main inhibitory transmitter; glutamate is the main excitatory one.
- ✗ "The CNS includes the cranial and spinal nerves." ✓ CNS = brain and spinal cord only; the nerves belong to the PNS.
- ✗ "The sympathetic system conserves energy." ✓ The parasympathetic conserves energy; the sympathetic spends it (raises heart rate and blood sugar).
- ✗ "Sympathetic activity increases digestion and salivation." ✓ It slows digestion and reduces saliva; the parasympathetic increases them.
- ✗ "The autonomic system controls voluntary movement." ✓ Voluntary movement = somatic system.
Quick revision
- Neuron = basic information-processing unit; dendrite → soma → axon → terminal.
- Myelin speeds conduction; nodes of Ranvier; saltatory conduction.
- Sensory = afferent; motor = efferent; interneurons most numerous.
- Resting −70 mV; action potential: Na⁺ in, then K⁺ out; all-or-none.
- Synapse: Sherrington 1897; first transmitter ACh: Loewi 1921.
- ACh: memory, muscles (Alzheimer's); dopamine: movement, reward (Parkinson's, schizophrenia).
- Serotonin: mood, sleep (depression); GABA inhibitory; glutamate excitatory; endorphins pain relief.
- CNS = brain + spinal cord; PNS = 12 cranial + 31 spinal nerve pairs.
- PNS: somatic (voluntary) and autonomic (involuntary).
- Sympathetic: fight or flight, spends energy; parasympathetic: rest and digest, conserves energy.
Go deeper: Deeper (beyond what APPSC Paper I usually asks)
Deeper Advanced
1. Why the all-or-none law does not make the brain all-or-none. A single neuron receives thousands of synapses. Excitatory and inhibitory inputs are summed (in space and in time) at the soma; only if the total crosses the threshold does an impulse start. So decisions are graded at the level of the network even though each impulse is fixed in size.
2. The same chemical can be a transmitter and a hormone. Norepinephrine and epinephrine act as neurotransmitters at synapses and as hormones when the adrenal medulla releases them into the blood. The sympathetic branch thus produces a fast neural effect and a slower, longer hormonal one, which is why the pounding heart continues for some minutes after a fright.
3. Transmitters at the organs. Parasympathetic fibres act on organs through acetylcholine; most sympathetic fibres act through norepinephrine (sweat glands are an exception: ACh). This is why drugs that block ACh produce a dry mouth and fast heart (sympathetic-like effects).
4. Neurons and newer ideas. The adult brain can form new connections throughout life (neuroplasticity) and, according to some studies, adds a few new neurons in the hippocampus. Artificial neural networks and the parallel distributed processing (connectionist) models of cognition are loosely modelled on networks of neurons with weighted connections.
5. Lie detection and emotion. The polygraph records sympathetic signs (heart rate, breathing, skin conductance / sweating). It measures arousal, not lying itself, which is why its results are disputed.
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