Neuroplasticity and Neurotransmission

Synaptic Transmission

Synaptic transmission is the process by which neurons communicate with one another. It is the Fundamental mechanism underlying all neural activity, from simple reflexes to complex cognitive Processes. Understanding synaptic transmission is essential for understanding how biological Structures give rise to behaviour.

Structure of the Synapse

A synapse consists of three main components:

1. The presynaptic terminal: The end of the axon of the sending neuron, which contains synaptic vesicles filled with neurotransmitter molecules. The presynaptic terminal also contains voltage-gated calcium channels.
2. The synaptic cleft: The narrow gap (approximately 20—40 nanometres) between the presynaptic terminal and the postsynaptic membrane. Neurotransmitter molecules diffuse across this gap.
3. The postsynaptic membrane: The membrane of the receiving neuron, which contains receptor proteins specific to particular neurotransmitters. These receptors are coupled to ion channels or to intracellular signalling cascades.

Steps in Synaptic Transmission

1. An action potential arrives at the presynaptic terminal.
2. Voltage-gated calcium channels open, and calcium ions ($\mathrm{Ca}^{2+}$) flow into the presynaptic terminal.
3. The influx of calcium causes synaptic vesicles to fuse with the presynaptic membrane (exocytosis), releasing neurotransmitter molecules into the synaptic cleft.
4. Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.
5. Binding of the neurotransmitter to the receptor causes ion channels to open (or, in the case of metabotropic receptors, triggers a secondary messenger system that eventually opens ion channels).
6. If enough excitatory postsynaptic potentials (EPSPs) accumulate to reach the threshold of approximately -55 mV, an action potential is triggered in the postsynaptic neuron.
7. The neurotransmitter is removed from the synaptic cleft through reuptake (the presynaptic neuron reabsorbs the neurotransmitter), enzymatic degradation (enzymes in the synaptic cleft break down the neurotransmitter), or diffusion.

Excitatory and Inhibitory Synapses

Synaptic transmission can have either an excitatory or inhibitory effect on the postsynaptic neuron:

• Excitatory postsynaptic potentials (EPSPs): Depolarise the postsynaptic membrane (making the interior less negative), bringing it closer to the threshold for firing an action potential. The primary excitatory neurotransmitter in the brain is glutamate.
• Inhibitory postsynaptic potentials (IPSPs): Hyperpolarise the postsynaptic membrane (making the interior more negative), moving it further from the threshold. The primary inhibitory neurotransmitters are GABA and glycine.

The postsynaptic neuron integrates all incoming EPSPs and IPSPs (spatial and temporal summation) to Determine whether or not to fire an action potential.

Key Neurotransmitters

Dopamine

Dopamine is a monoamine neurotransmitter produced in several key brain regions, including the Substantia nigra, the ventral tegmental area (VTA), and the hypothalamus. Dopamine is involved in:

• Reward and motivation: Dopamine release in the nucleus accumbens is associated with the experience of pleasure and reinforcement. This mesolimbic pathway is critically involved in addiction.
• Motor control: Dopamine produced in the substantia nigra and transmitted to the striatum (the nigrostriatal pathway) is essential for smooth, coordinated movement. Degeneration of dopaminergic neurons in this pathway causes Parkinson”s disease, characterised by tremor, rigidity, and bradykinesia.
• Cognitive function: Dopamine in the prefrontal cortex (the mesocortical pathway) is involved in working memory, attention, and executive function. Abnormal dopamine activity in this pathway has been implicated in schizophrenia.
• Hormone regulation: Dopamine produced in the hypothalamus inhibits prolactin release from the pituitary gland (the tuberoinfundibular pathway).

Disorders associated with dopamine dysfunction:

Serotonin (5-Hydroxytryptamine, 5-HT)

Serotonin is produced in the raphe nuclei of the brainstem and is involved in a wide range of Functions:

• Mood regulation: Low serotonin activity is associated with depression. Most antidepressant medications (SSRIs) work by increasing serotonin availability in the synaptic cleft.
• Sleep-wake cycle: Serotonin is a precursor to melatonin, the hormone that regulates sleep.
• Appetite: Serotonin influences satiety and eating behaviour.
• Aggression and impulsivity: Low serotonin levels have been linked to increased aggression and impulsive behaviour.
• Anxiety: Abnormal serotonin activity is implicated in anxiety disorders, particularly OCD.

Acetylcholine (ACh)

Acetylcholine is produced in the basal forebrain and the brainstem. It is the primary Neurotransmitter at the neuromuscular junction (where neurons communicate with muscles) and is also Widely distributed in the brain. ACh is involved in:

• Memory and learning: ACh is critical for memory formation, particularly in the hippocampus. Alzheimer’s disease is characterised by degeneration of cholinergic neurons in the basal forebrain, leading to memory impairment.
• Attention and arousal: ACh from the brainstem contributes to cortical activation and attention.
• Motor control: ACh activates muscles at the neuromuscular junction.

GABA (Gamma-Aminobutyric Acid)

GABA is the primary inhibitory neurotransmitter in the central nervous system. Approximately One-third of all synapses in the brain use GABA. GABA functions by opening chloride channels in the Postsynaptic membrane, causing hyperpolarisation and reducing the likelihood of action potential Firing.

• Anxiety reduction: Benzodiazepines (e.g., diazepam) enhance GABA activity, producing their anxiolytic (anti-anxiety) effects.
• Epilepsy: Reduced GABA activity can lead to excessive neural excitation and seizures.
• Huntington’s disease: Degeneration of GABAergic neurons in the striatum leads to involuntary movements and cognitive decline.

Glutamate

Glutamate is the primary excitatory neurotransmitter in the central nervous system and is involved In virtually all cognitive functions. It acts on several types of receptors, including NMDA, AMPA, And kainate receptors. Glutamate is critical for:

• Long-term potentiation (LTP): The NMDA receptor is essential for LTP, the cellular mechanism underlying learning and memory (discussed below).
• Synaptic plasticity: Glutamate-mediated signalling is the basis of experience-dependent changes in synaptic strength.
• Excitotoxicity: Excessive glutamate release (as occurs during stroke or traumatic brain injury) can cause overactivation of glutamate receptors, leading to calcium influx, oxidative stress, and neuronal death.

Agonists and Antagonists

Chemical substances can affect synaptic transmission by interacting with neurotransmitter receptors Or by altering the synthesis, release, or breakdown of neurotransmitters.

Agonists

An agonist is a substance that binds to a receptor and activates it, mimicking the effect of the Natural neurotransmitter. Agonists can be direct (binding directly to the receptor) or indirect (increasing the amount of neurotransmitter available).

Antagonists

An antagonist is a substance that binds to a receptor but does not activate it, blocking the natural Neurotransmitter from binding. Antagonists reduce or block the effect of the neurotransmitter.

Understanding agonists and antagonists is essential for understanding the mechanisms of psychoactive Drugs and their therapeutic applications. For example, the effectiveness of antipsychotic medication In treating schizophrenia is attributed to its dopamine antagonist properties, while the Effectiveness of antidepressant medication is attributed to its serotonin agonist properties (via Reuptake inhibition).

Neuroplasticity

Neuroplasticity refers to the brain’s capacity to reorganise its structure and function in response To experience, learning, environmental change, or injury. It is not a single mechanism but a Collection of processes that operate at different levels, from molecular changes at individual Synapses to large-scale cortical reorganisation.

Synaptic Plasticity

Synaptic plasticity is the ability of synapses to strengthen or weaken over time in response to Changes in activity. It is the primary cellular mechanism underlying learning and memory.

Long-term potentiation (LTP): LTP is a persistent strengthening of synaptic connections Following high-frequency stimulation. When a presynaptic neuron repeatedly and persistently Stimulates a postsynaptic neuron, the connection between them becomes stronger. The mechanism Involves:

1. Glutamate is released from the presynaptic terminal and binds to AMPA receptors on the postsynaptic membrane.
2. If the postsynaptic neuron is sufficiently depolarised (from concurrent stimulation), the magnesium block on NMDA receptors is removed.
3. Calcium ions flow through the NMDA receptor into the postsynaptic neuron.
4. Calcium triggers a cascade of intracellular events (involving kinases such as CaMKII) that lead to the insertion of additional AMPA receptors into the postsynaptic membrane and structural changes (dendritic spine enlargement).
5. The synapse is now more sensitive to subsequent stimulation: the same amount of glutamate release produces a larger postsynaptic response.

LTP was first demonstrated by Bliss and Lomo (1973) in the rabbit hippocampus, and it is now widely Accepted as the primary mechanism of memory formation.

Long-term depression (LTD): LTD is the opposite of LTP: a persistent weakening of synaptic Connections following low-frequency stimulation. LTD is important for forgetting, for refining Neural representations, and for preventing synaptic saturation. Without LTD, neural circuits would Become saturated with strengthened connections, impairing the ability to form new memories.

Synaptic Pruning

Synaptic pruning is the process by which unused or weakly activated synaptic connections are Eliminated during development and learning. This process refines neural circuits, improving their Efficiency and specificity.

• At birth, the human brain contains approximately 100 billion neurons, each forming thousands of synaptic connections. The total number of synapses peaks at approximately age 3—4, at a level far exceeding adult levels.
• During childhood and adolescence, approximately half of all synapses are pruned away. This process follows the principle of “use it or lose it”: connections that are frequently activated are strengthened and retained, while those that are rarely used are eliminated.
• Synaptic pruning is particularly important in the prefrontal cortex, which continues to undergo significant pruning throughout adolescence, consistent with the protracted development of executive functions.

Dendritic Branching

Dendritic branching refers to the growth of new dendritic spines (small protrusions on dendrites That receive synaptic input) in response to learning and environmental enrichment. Increased Dendritic branching increases the number of potential synaptic connections, enhancing the Computational capacity of neural circuits.

Key Studies

Rosenzweig and Bennett (1972)

Rosenzweig, Bennett, and Diamond conducted a landmark series of experiments on the effects of Environmental enrichment on brain structure in rats. Rats were assigned to one of three conditions:

1. Enriched condition (EC): Large cage with 10—12 rats, toys, tunnels, and running wheels, with toys rotated regularly.
2. Standard condition (SC): Standard laboratory cage with 3 rats, no toys.
3. Impoverished condition (IC): Smaller cage, isolated, no toys.

After 60 days, the brains of EC rats showed:

• Increased thickness of the cerebral cortex (approximately 6—7% thicker than IC rats)
• Greater density of synaptic connections
• Higher levels of acetylcholinesterase (an enzyme involved in the breakdown of acetylcholine, indicating greater cholinergic activity)
• Larger neuron cell bodies in the cortex
• Increased glial cell density

Evaluation:

• The study used controlled experimental methods with random assignment, strengthening internal validity. However, the use of rats limits the generalisability to humans.
• Ethical concerns arise from the deprivation experienced by the IC rats and the euthanasia required for post-mortem brain analysis.
• Subsequent human research has provided converging evidence. Maguire et al. (2000) found that London taxi drivers had larger posterior hippocampi than controls, and Draganski et al. (2004) found that learning to juggle produced measurable increases in grey matter in cortical areas involved in visual motion processing after just 7 days of training.

Draganski et al. (2004)

Draganski and colleagues used voxel-based morphometry (a neuroimaging technique that measures Differences in grey matter density) to investigate whether learning a new skill could produce Structural changes in the adult human brain. Participants were taught a three-ball juggling routine And practised for 60 seconds per day over 7 days. Brain scans were taken at three time points: Before training, immediately after the 7-day training period, and 3 months later.

Key findings:

• Immediately after training, participants showed a significant increase in grey matter density in brain areas involved in visual motion processing (the hMT/V5 area in the left hemisphere and the intraparietal sulcus).
• After 3 months without practice, the increase in grey matter had partially receded, suggesting that structural changes are experience-dependent and reversible.
• A control group that did not learn juggling showed no such changes.

Evaluation:

• The study provides direct evidence that neuroplasticity operates in the adult human brain in response to relatively brief learning experiences.
• The use of voxel-based morphometry provides an objective, quantitative measure of structural brain change.
• The study does not demonstrate that the observed grey matter changes are causally related to improved juggling performance. Correlation does not imply causation, although the temporal sequence (change follows training) supports a causal interpretation.
• The study demonstrates both the adaptive potential and the reversibility of neuroplastic changes, which has important implications for rehabilitation following brain injury.

Linking to the Levels of Analysis

The study of neuroplasticity and neurotransmission connects the biological level of analysis to Other levels:

• Cognitive LOA: Neurotransmitter systems underpin cognitive processes such as memory (acetylcholine, glutamate), attention (dopamine), and decision making (serotonin, dopamine). Neuroplasticity is the biological basis of learning and memory.
• Sociocultural LOA: Social experiences (such as enriched environments, social isolation, or cultural practices) can alter brain structure and function through neuroplastic mechanisms. For example, social deprivation in childhood has been associated with reduced cortical thickness and impaired cognitive development.

For an overview of these topics at the biological level of analysis, see Biological Level of Analysis.

Common Pitfalls

1. Forgetting edge cases in algorithm design (e.g., empty input, single element, already sorted data).

2. Forgetting that $O(n \log n)$ average-case for quicksort becomes $O(n^2)$ worst-case on already sorted input.

3. Writing pseudocode that is too language-specific rather than using standard algorithmic constructs.

4. Confusing authentication (who you are) with authorisation (what you can do) in security contexts.

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.