Nervous System: Synapses and Neural Integration

The Synapse

• Neurons connect via synapses to transfer information.
• Presynaptic neuron: Sends impulses toward synapse.
• Postsynaptic neuron: Receives signals.
• Can be neuron, muscle, or gland.

Synaptic Connections

• Axodendritic: Axon to dendrite.
• Axosomatic: Axon to cell body (soma).
• Less common: axoaxonal, dendrodendritic, somatodendritic.
• Two main types:
  • Chemical.
  • Electrical.

Chemical Synapses

• Most common synapse type.
• Specialized for neurotransmitter release and reception.
• Components:
  • Presynaptic axon terminal with synaptic vesicles.
  • Postsynaptic receptor region (usually on dendrite or cell body).
  • Synaptic cleft separates the two parts.
• Electrical impulse converted to chemical signal across synapse, then back to electrical.
• Synaptic cleft ensures unidirectional communication.

Steps of Information Transfer Across Chemical Synapses

1. Action potential arrives at presynaptic axon terminal.
2. Voltage-gated $Ca^{2+}$ channels open, $Ca^{2+}$ enters.
3. $Ca^{2+}$ entry triggers neurotransmitter release via exocytosis.
4. Neurotransmitter diffuses across synaptic cleft and binds to postsynaptic receptors.
5. Ion channels open, creating graded potentials.
6. Neurotransmitter effects are terminated by:
   • Reuptake.
   • Degradation.
   • Diffusion.

• Synaptic delay is rate-limiting step (0.3-5.0 ms).

Electrical Synapses

• Less common; neurons electrically coupled.
• Joined by gap junctions for rapid, bidirectional communication.
• Found in brain regions for eye movement, hippocampus (emotions, memory).
• Abundant in embryonic nervous tissue.

Postsynaptic Potentials

• Graded potentials influenced by neurotransmitter amount/duration in cleft.
  • EPSP: Excitatory postsynaptic potential.
  • IPSP: Inhibitory postsynaptic potential.

Excitatory Synapses and EPSPs

• Neurotransmitter binding opens channels, allows simultaneous $Na^{+}$ influx and $K^{+}$ efflux.
• Net depolarization (EPSP) occurs, triggering AP if threshold is reached.

Inhibitory Synapses and IPSPs

• Neurotransmitter binding opens channels for $K^{+}$ efflux or $Cl^{-}$ influx, causing hyperpolarization.
• Reduces ability to generate action potential.

Summation

• EPSPs can summate to reach threshold.
• Two types:
  • Temporal: Rapid-fire impulses from one neuron.
  • Spatial: Simultaneous stimulation by many terminals.

Synaptic potentiation

• Repeated synapse use increases presynaptic cell's ability to excite postsynaptic neuron.
• More neurotransmitter release, leading to more EPSPs.
• Long-term potentiation: learning and memory.

Presynaptic inhibition

• Inhibition occurs via axoaxonal synapse, reducing neurotransmitter release and EPSPs.

Neurotransmitters

• Over 50 identified; neurons make two or more.
• Classified by chemical structure and function.

Classification by Chemical Structure

• Acetylcholine (ACh): At neuromuscular junctions; degraded by acetylcholinesterase.
• Biogenic amines:
  • Catecholamines (dopamine, NE, epinephrine from tyrosine).
  • Indolamines (serotonin from tryptophan, histamine from histidine).
• Amino acids: Glutamate, aspartate, glycine, GABA.
• Peptides (neuropeptides): Substance P, endorphins, gut-brain peptides.
• Purines: ATP, adenosine.
• Gases and lipids:
  • Gasotransmitters (NO, CO, $H_{2}S$).
  • Endocannabinoids.

Classification by Function

• Effects: Excitatory (depolarizing) or inhibitory (hyperpolarizing); determined by receptor.
• Actions:
  • Direct: binding opens ion channels (ACh, amino acids).
  • Indirect: acts through second messengers (biogenic amines, neuropeptides, gases).
  • Neuromodulator: affects synaptic transmission strength.

Receptor Types

• Channel-linked receptors: ligand-gated ion channels for rapid, brief action.
• G protein–linked receptors: indirect, slow, prolonged responses involving second messengers.

Neural Integration

• Neurons function in groups.

Neuronal Pools

• Integrate and forward information.
• Simple pool: discharge and facilitated zones.

Patterns of Neural Processing

• Serial: Input along one pathway to a destination (e.g., reflexes).
• Parallel: Input travels along several pathways; important for higher-level mental function.

Types of Circuits

• Diverging, converging, reverberating, parallel after-discharge.

Developmental Aspects

• Nervous system from neural tube and neural crest.
• Growth cone guides axon using adhesion proteins, neurotropins, nerve growth factor, and filopodia.
• Apoptosis prunes neurons; synapse formation requires astrocytes.
• Learning reinforces/prunes synapses.
• Limited neuron division post-birth (olfactory neurons, hippocampus).