Cognitive Connie
Let's explore Nervous System Organisation & Neural Communication
Explore the architecture of the nervous system — CNS vs PNS, somatic and autonomic divisions, sympathetic and parasympathetic control — and the cellular mechanisms of neural signalling: neuron types, glial cells, action potentials, saltatory conduction, synaptic transmission, EPSPs/IPSPs, neurotransmitters, and reuptake.
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Nervous System Organisation
The nervous system is the body's primary rapid-communication network, translating external and internal stimuli into coordinated responses. Understanding its organisation — anatomically into CNS and PNS, and functionally into somatic and autonomic divisions — is foundational to all of biopsychology.
Neural Communication: Neurons, Action Potentials & Synaptic Transmission
Every thought, feeling, and behaviour depends on electrical and chemical signalling between neurons. At the cellular level, this involves maintaining a resting membrane potential, generating and propagating action potentials, and converting electrical signals into chemical messages at synapses — where neurotransmitters released from one neuron bind to receptors on another.
Neurotransmitters
Before Otto Loewi's famous 1921 experiment, the question of how one neuron communicates with the next was unresolved. Loewi filled two beating frog hearts with saline solution and stimulated the vagus nerve of the first heart, slowing it down. He then transferred the saline to the second heart — and it slowed too. The saline had carried a chemical signal: what he called "Vagusstoff", later identified as acetylcholine. This discovery, shared with Henry Dale in the 1936 Nobel Prize in Physiology or Medicine, established that neurons communicate not only by electrical impulses but by releasing chemical messengers across the synaptic cleft. These messengers — neurotransmitters — are now understood to be the molecular substrate of virtually every psychological process, from perception and memory to mood, motivation, and psychopathology.
Dopamine
Dopamine is a monoamine neurotransmitter synthesised from the amino acid tyrosine through a two-step enzymatic cascade and released from neurons concentrated in a small number of midbrain nuclei. Despite its relatively small number of neurons — approximately 400,000 in the human brain — the dopamine system exerts influence over an enormous range of psychological functions, from the moment-to-moment anticipation of reward to the planning of voluntary movement and the maintenance of working memory. Its outsized clinical significance is underlined by the fact that dopamine dysregulation has been implicated in schizophrenia, Parkinson's disease, addiction, ADHD, and depression.
Serotonin
Serotonin (5-hydroxytryptamine, 5-HT) was first isolated and named in the 1940s as a vasoconstrictor found in blood serum — hence the name. Its role as a neurotransmitter in the central nervous system was established through a series of parallel discoveries: Betty Twarog first demonstrated serotonin's presence in the mammalian brain in 1954, and Ulf von Euler and his collaborators identified the raphe nuclei of the brainstem as its primary central source. Approximately 90% of the body's serotonin is located not in the brain but in the gastrointestinal tract, where it regulates intestinal motility — a distribution that has increasingly been recognised as relevant to the gut-brain axis and the emotional consequences of gastrointestinal disease.
GABA & Glutamate
While the monoamines receive the most clinical attention in psychiatry, the brain's dominant neurotransmitters by sheer volume of synaptic traffic are glutamate and GABA. Glutamate is the primary excitatory neurotransmitter in the central nervous system — present at approximately 90% of cortical synapses and responsible for the fast, point-to-point excitatory transmission that underlies perception, learning, and memory. GABA (γ-aminobutyric acid) is the primary inhibitory transmitter, present at approximately 20% of cortical synapses and providing the tonic brake that prevents runaway excitation. Their balance — the excitation/inhibition (E/I) ratio — is one of the most fundamental regulatory parameters in neural circuit function.
Norepinephrine
Norepinephrine (also called noradrenaline outside North America) is a monoamine neurotransmitter and hormone synthesised from dopamine by the enzyme dopamine β-hydroxylase. In the peripheral nervous system it acts as the primary neurotransmitter of the sympathetic nervous system, triggering the physiological hallmarks of Cannon's fight-or-flight response — elevated heart rate, pupil dilation, glucose mobilisation, and blood flow redistribution from digestive to skeletal muscles. In the central nervous system, norepinephrine is released from a tiny nucleus in the pontine brainstem — the locus coeruleus (LC) — which, despite containing only about 1,500 neurons per hemisphere, sends ascending projections to virtually every region of the forebrain, making it arguably the single most widely projecting nucleus in the brain.
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