Neurotransmitters and Brain Imaging

Neurons talk to each other with chemicals. Each neurotransmitter has a distinct personality: some excite, some inhibit, some modulate mood or attention. Then we’ll turn to how we see the brain in action - the imaging methods you need to keep straight for the MCAT.

The Major Neurotransmitters

Glutamate

• Main excitatory neurotransmitter in the CNS.
• Diffuse projections (reticular activating system) keep the cortex aroused.
• Too much glutamate → excitotoxicity; kainic acid experiments exploit this to kill neurons.
• Involved in learning, memory (long-term potentiation), consciousness.

Mnemonic: GLU = eGLUcites.

GABA and Glycine

• Main inhibitory neurotransmitters. GABA in the brain, glycine in the spinal cord.
• Alcohol, benzodiazepines, barbiturates all enhance GABA-A receptor activity → sedation.
• Loss of inhibition produces seizures - why anticonvulsants often boost GABA.

Acetylcholine (ACh)

• Released by all motor neurons at the neuromuscular junction - essential for muscle contraction. Mnemonic: ACE flexes his muscles.
• Used by the autonomic nervous system (both sympathetic pre-ganglionic and all parasympathetic).
• In the CNS: attention and arousal. Basal forebrain (nucleus basalis, septal nuclei) projects ACh widely to cortex - degeneration is a feature of Alzheimer’s disease.

Dopamine

• Reward, motivation, motor control.
• Substantia nigra → striatum (nigrostriatal pathway): motor planning. Degeneration = Parkinson’s.
• VTA → nucleus accumbens (mesolimbic pathway): reward and reinforcement. Hyperactivity associated with positive symptoms of schizophrenia (hallucinations, delusions).
• VTA → prefrontal cortex (mesocortical pathway): cognition, affect. Hypoactivity associated with negative symptoms of schizophrenia.
• Mnemonic: Low dopamine → Parkinson’s; high dopamine → schizophrenia (“ski-zophrenia”).

Serotonin (5-HT)

• Released from raphe nuclei in the brainstem. Projects widely.
• Regulates mood, appetite, sleep, arousal.
• Low serotonin linked to depression (SSRIs boost serotonin). Mnemonic: “Sir Rotten” is depressed.
• Also implicated in anxiety and aggression.

Norepinephrine (Noradrenaline)

• Released from the locus coeruleus in the pons.
• Promotes alertness, attention, arousal.
• Low levels → depression; high levels → anxiety/mania.
• Also the main transmitter for sympathetic post-ganglionic synapses (with epinephrine).

Histamine

• From the hypothalamus to the cortex.
• Wakefulness, attention. Antihistamines cause drowsiness by blocking this pathway.

Endorphins (Endogenous Opioids)

• Peptide neurotransmitters.
• Block pain sensations; produce “runner’s high.”
• Mimicked by opiates (morphine, heroin, oxycodone).
• Mnemonic: “A running door with fins” (happy because endorphins are happy).

Types of Neurotransmitters by Chemistry

• Amino acid neurotransmitters. Glutamate, GABA, glycine. Fast-acting, ligand-gated channels.
• Monoamines (biogenic amines). Serotonin, histamine, dopamine, norepinephrine, epinephrine. Slower-acting, G-protein-coupled receptors. Involved in attention, cognition, emotion. Dopamine, norepinephrine, and epinephrine are also catecholamines (share a catechol group).
• Peptide neurotransmitters. Endorphins, enkephalins. Small peptides.
• Acetylcholine. In its own chemistry class. ANS and motor neurons.

Brain-Imaging Techniques

Two broad aims: structure (what does the brain look like?) and function (what is it doing?).

Structural Imaging

• CT (Computerized Axial Tomography). Uses X-rays to build a 3D image. Fast; good for tumors, bleeding, and skull fractures. Lower resolution than MRI for soft tissue. Mnemonic: CATs use X-rays.
• MRI (Magnetic Resonance Imaging). Uses powerful magnets and radio waves. Higher resolution for soft tissue than CT. Slower. Good for detailed brain anatomy. Does NOT show function.

Functional Imaging

• EEG (Electroencephalogram). Scalp electrodes measure summed electrical activity of cortical neurons. High temporal resolution (milliseconds); poor spatial resolution. Non-invasive, inexpensive. Used for sleep studies, seizures, cognitive tasks. Mnemonic: Electro = EEG = electricity.
• MEG (Magnetoencephalogram). Measures magnetic fields produced by brain currents using SQUIDs (Superconducting Quantum Interference Devices). Better spatial resolution than EEG, similar temporal resolution. Rare and expensive.
• PET (Positron Emission Tomography). Inject radioactive glucose; active brain regions take up more. Shows metabolic activity. Invasive (radiation exposure); low spatial and temporal resolution. Mnemonic: PETs like glucose.
• fMRI (Functional MRI). Measures changes in blood oxygenation (BOLD signal) - active regions use more oxygen. High spatial resolution; moderate temporal resolution (seconds). Non-invasive. Most popular research tool in cognitive neuroscience. Mnemonic: f = function.