Overview
This lecture introduces the fundamental structure and function of the nervous system, focusing on neurons, synapses, neural circuits, and motifs, while highlighting key neurological disorders and principles of neuronal communication.
Introduction to Neuroscience and Brain Complexity
- Neuroscience studies the structure and function of the nervous system using tools from anatomy, physiology, biochemistry, and related fields.
- The brain contains over 100 billion neurons, each specialized for different functions (e.g., vision, memory, planning, motor control).
- Unlike organs like the liver, neurons form extensive, highly interconnected circuits, making the brain uniquely complex.
Major Neurological Disorders
- Alzheimer's Disease: cognitive and memory loss due to neuronal degeneration.
- Epilepsy: seizures from uncontrolled electrical activity.
- Huntington's Disease: abnormal movements from Huntington gene mutation.
- Multiple Sclerosis: sensory/motor loss from demyelination (autoimmune).
- Myasthenia Gravis: muscle weakness due to acetylcholine receptor loss (autoimmune).
- Parkinson's Disease: movement disorder from dopamine neuron degeneration.
- Schizophrenia: delusions/hallucinations due to dopamine/glutamate imbalance.
- Stroke: function loss from disrupted blood supply.
- Depression and anxiety also affect millions, together impacting nearly 10% of the US population.
Neuron Anatomy and Function
- Neurons are polarized cells with four main parts: soma (cell body), dendrites (input), axon (conducts electrical signals), and synapses (output).
- Dendrites receive signals; axons transmit action potentials; myelin insulates axons; synapses enable neuron-to-neuron communication.
Synapses and Electrical Signaling
- Synaptic vesicles store neurotransmitters, released into the synaptic cleft when an action potential arrives.
- Neurotransmitters bind to receptors on the postsynaptic cell, changing its membrane potential.
Resting Potential and Action Potential
- All cells have a resting membrane potential (~-60 mV, inside negative).
- Neurons generate action potentials (nerve impulses) in an all-or-none manner once a threshold is reached.
- The frequency (not size) of action potentials encodes stimulus intensity—this is frequency coding.
Excitatory and Inhibitory Connections
- Excitatory synapses depolarize the postsynaptic cell (excitatory postsynaptic potential, EPSP).
- Inhibitory synapses hyperpolarize the postsynaptic cell (inhibitory postsynaptic potential, IPSP), reducing the likelihood of firing.
Neural Circuit Motifs
- Feed-forward excitation: one neuron excites the next.
- Feed-forward inhibition: excitatory neuron activates an inhibitory interneuron, which inhibits a third neuron.
- Convergence: multiple neurons connect to one postsynaptic neuron.
- Divergence: one neuron connects to multiple postsynaptic neurons.
- Lateral inhibition: neurons inhibit neighbors, enhancing sensory contrast (edge enhancement).
- Feedback/recurrent inhibition: output neuron inhibits its own input via interneurons, acting as a braking mechanism.
- Feedback/recurrent excitation: circuits sustain activity, functioning as a switch.
- Such motifs underlie reflexes, sensory processing, rhythmic behaviors, and complex actions.
Examples and Functional Consequences
- Monosynaptic stretch reflex: feed-forward excitation for rapid muscle response; feed-forward inhibition prevents opposing muscle activation.
- Lateral inhibition in the retina enhances edge detection but can create visual illusions (e.g., Mach bands).
- Feedback inhibition and oscillatory circuits enable rhythmic behaviors (like breathing) and circadian rhythms (via the suprachiasmatic nucleus).
- Multiple interconnected microcircuits form macrocircuits for complex cognitive and sensory functions.
Key Terms & Definitions
- Neuron — nerve cell specialized for transmitting information.
- Synapse — junction where neurons communicate via neurotransmitters.
- Resting Potential — stable membrane voltage in unstimulated neurons (~-60 mV).
- Action Potential — rapid, all-or-none electrical signal along the axon.
- EPSP — excitatory postsynaptic potential, depolarizing current.
- IPSP — inhibitory postsynaptic potential, hyperpolarizing current.
- Feed-forward Excitation — direct activation of a neuron.
- Feed-forward Inhibition — inhibition via an intervening interneuron.
- Lateral Inhibition — inhibition of neighboring neurons, enhancing contrast.
- Convergence — multiple inputs to one neuron.
- Divergence — one neuron connecting to many.
Action Items / Next Steps
- Review online electronic syllabus animations on nerve cell signaling and circuit motifs.
- Prepare for the next lecture on neural circuit development and wiring by Dr. Bean.