Heredity and environment work together
We get genes from our parents. Genes are instructions in our cells. This is heredity (nature). We also grow up in a home, a school and a culture. This is environment (nurture).
Psychologists do not ask "nature or nurture?" They ask how the two work together. Tools they use:
- Twin studies: identical twins share all their genes; fraternal twins share about half. If identical twins are more alike in a trait, genes likely play a part.
- Adoption studies: compare adopted children with their birth parents (genes) and adoptive parents (home).
- Family studies: see how a trait runs in families.
Heritability tells how much of the differences between people in a group link to genes. It does not describe one person. Epigenetics shows that life events can switch genes on or off. The evolutionary view asks how a behaviour helped our ancestors survive.
Overview of the nervous system
The central nervous system (CNS) is the brain and the spinal cord. The peripheral nervous system (PNS) is every other nerve.
The PNS has two parts:
- Somatic: controls muscles you move on purpose and brings in touch, sight and sound.
- Autonomic: runs things on its own, like heartbeat and digestion. Its sympathetic branch gets you ready for action ("fight or flight"): faster heart, wider pupils. Its parasympathetic branch calms you down ("rest and digest").
A reflex is a quick, automatic answer. A sensory neuron carries the signal to the spinal cord, an interneuron passes it on, and a motor neuron moves the muscle. The brain hears about it a moment later.
The neuron and neural firing
A neuron is a nerve cell. Its parts: dendrites (receive), cell body or soma (adds up the inputs), axon (sends), axon terminals (pass the message on). A fatty myelin sheath wraps many axons and speeds the signal. Glial cells support, feed and protect neurons.
At rest, the inside of a neuron is about −70 mV (resting potential). If the inputs push it past the threshold, gates open, positive ions rush in and an action potential races down the axon (about +40 mV inside). This is all-or-none: a neuron fires fully or not at all. A stronger stimulus makes it fire more often, not bigger. After firing there is a short refractory period when it cannot fire again.
Some inputs are excitatory (push towards firing) and some are inhibitory (hold it back).
Neurotransmitters, drugs and hormones
Neurons do not touch. Between them is a tiny gap, the synapse. The axon terminal releases neurotransmitters, which cross the gap and fit into receptors. Leftover molecules are taken back in (reuptake).
| Neurotransmitter | Main roles |
|---|---|
| Acetylcholine | muscle movement, memory |
| Dopamine | reward, movement |
| Serotonin | mood, sleep, hunger |
| Norepinephrine | alertness |
| Glutamate | main "go" signal; learning |
| GABA | main "stop" signal; calming |
| Endorphins | natural pain relief |
| Substance P | pain signals |
A drug that copies or boosts a neurotransmitter is an agonist; one that blocks it is an antagonist. Some drugs block reuptake, so the messenger stays longer.
The endocrine system uses hormones carried in the blood. They are slower but last longer: adrenaline (stress), melatonin (sleep), leptin and ghrelin (fullness and hunger), oxytocin (bonding). The hypothalamus controls the pituitary, the "master gland".
The brain: regions, plasticity and how we study it
- Brainstem (medulla, pons): breathing, heartbeat. The reticular activating system keeps you awake and alert.
- Cerebellum: balance, smooth movement, skill learning.
- Thalamus: relay station for senses (except smell). Hypothalamus: hunger, thirst, temperature, hormones. Amygdala: fear and emotion. Hippocampus: forming new memories.
- Cerebral cortex: the wrinkled outer layer. Frontal lobe: planning, decisions, personality; it holds the motor cortex and Broca's area (speaking). Parietal lobe: touch, with the somatosensory cortex. Temporal lobe: hearing and Wernicke's area (understanding words). Occipital lobe: vision.
The two hemispheres are joined by the corpus callosum. Split-brain studies show each half has some special strengths, though they normally work as a team.
Neuroplasticity: the brain can rewire itself, especially in childhood and after injury or practice.
How we look inside: EEG (electrical waves), fMRI (blood flow to active areas), PET (uses a tracer to show activity), CT and MRI (structure), and case studies of brain injury.
Key formulas and definitions
- Message path: dendrites → cell body → axon → terminals → synapse → next neuron
- Resting potential ≈ −70 mV; action potential peak ≈ +40 mV
- All-or-none: stronger stimulus = more frequent firing, not bigger pulses
- Sympathetic = arouse (fight or flight); parasympathetic = calm (rest and digest)
- Agonist mimics/boosts; antagonist blocks; reuptake recycles the neurotransmitter
Worked examples
1. A patient after a stroke can understand speech but struggles to produce words. Which area is likely damaged?
Broca's area in the left frontal lobe, which helps produce speech. Understanding (Wernicke's area) is intact.
2. A runner feels less pain near the end of a long race. Which neurotransmitter helps?
Endorphins, the body's natural pain relievers, released during stress and hard exercise.
3. Identical twins raised apart are still very similar in height and fairly similar in some personality traits. What does this suggest?
Genes play a strong part in these traits, because the twins share all their genes but not their homes. It does not mean environment has no effect.
4. A stimulus of strength 3 does not make a neuron fire, but strength 5 does. What happens at strength 9?
The neuron still fires the same-sized action potential (all-or-none), but it fires more often, signalling a stronger stimulus.
Common mistakes
- Thinking a stronger stimulus makes a bigger action potential. The size is fixed; only the firing rate changes.
- Mixing up sympathetic (arousing) and parasympathetic (calming).
- Saying heritability tells how much of one person's trait comes from genes. It describes differences in a group.
- Thinking neurons touch each other. They pass messages across a gap, the synapse.