Neuroscience Chapter on Memory & Synapses

Human Physiology BIOL3205 - Neuronal Communication

• Course instructor: Prof. Chi Bun Chan
• Contact information: [email protected], 39173823
• Location: 5N10 Kadoorie Biological Sciences Building
• Course topic: Neuronal communication

Lecture Outline

• Membrane potential
• Action potential formation and conduction
• Synapses and neurotransmitters
• Neural integration and networking

Structure of a Neuron

• Dendrites: Receive signals from other neurons, directed towards the cell body. Dendritic spines are also found in the cell body, forming synapses.
• Cell body: Contains the nucleus and organelles, integrating signals.
• Input zone: Receives incoming signals.
• Axon: Conducts action potentials to other cells; variable in length triggered by the axon hillock.
• Axon terminals: Release neurotransmitters influencing other cells.

Membrane Potential

• Membrane potential: Voltage difference across a cell membrane.
• Resting membrane potential of a neuron cell is -70 mV.
• Expressed as a negative value due to a slight excess of anions inside the cell.
• Maintained by the Na+-K+ pump, which expends energy.
• K+ channels are always open (leaky channels).
• Na+ and K+ ions are responsible for generating the resting membrane potential.

Change of Membrane Potential

• Only nerve and muscle tissues are excitable.
• Membrane potential can be changed through:
  • Polarization: Separation of charges across the membrane.
  • Depolarization: Decrease in membrane potential (less negative).
  • Repolarization: Return to resting membrane potential.
  • Hyperpolarization: Increase in membrane potential (more negative).
• Ion movement is mediated by channels.

Ion Channels

• Pores that open and close in an all-or-nothing fashion.
• Types:
  • Leaky channels
  • Voltage-gated channels
  • Chemically-gated channels
  • Mechanically-gated channels
  • Thermally-gated channels

Generation of Electrical Signals in Neurons

• Electrical signals in neurons occur via changes in membrane potential.
• Two basic forms of electrical signals:
  • Graded potentials: Changes in membrane potential that are localized.
  • Action potentials: Large, rapid changes in membrane potential that are used for long-distance signaling.

Graded Potentials

• Depolarization of membrane in small, specialized regions.
• Provoked by Na+ entry.
• Bi-directional over a short distance.
• Decreases ("current loss") gradually.
• Signal for short distances only.
• Magnitude depends on triggering event.

Action Potentials

• Brief, rapid, large changes in membrane potential.
• Initiated by a triggering event (membrane potential reaching threshold potential, often at the axon hillock).
• Generated and propagated non-decrementally throughout the entire membrane.
• Long-distance signals; fixed threshold.
• All-or-none response: either generated or not.

Conformation of Voltage-gated Na+ and K+ Channels

• Na+ channels:

  • Open rapidly at threshold.
  • Inactivated.
  • Close slowly after depolarization.

• K+ channels:

  • Open more slowly than Na+ channels.
  • Responsible for repolarization and then hyperpolarization.

Formation of Action Potentials

• Caused by rapid fluxes of Na+ and K+.
• Opening and closing of voltage-gated Na+ and K+ channels.
• Positive feedback of Na+ channels (rapid depolarization).
• Rapid K+ outflow (repolarization).
• Hyperpolarization by the slow closure of K+ channels.

Propagation of Nerve Impulse

• Once initiated, no further triggering is needed.
• Conducted through:
  • Contiguous conduction (non-myelinated).
  • Saltatory conduction (myelinated).
  • Depends on axon structure (myelinated or non-myelinated).

Contiguous Conduction

• Occurs in non-myelinated neurons.
• Spreads along every patch of membrane.
• Active and inactive areas.
• Self-perpetuating cycle.
• Original action potential is not carried along, new action potentials are formed.

Refractory Period

• Time when a new action potential cannot be initiated by a region that has just undergone one.
• Types:
  • Absolute refractory period: Impossible to generate another action potential.
  • Relative refractory period: More difficult to generate an action potential.

Myelination

• Myelin sheath surrounds axons, insulating them.
• Produced by oligodendrocytes (CNS) and Schwann cells (PNS).
• Concentrates Na+ and K+ channels at the nodes of Ranvier.
• Saltatory conduction: Faster signal transmission.
• Energy conservation.

Saltatory Conduction

• APs produced only at Nodes of Ranvier.
• Saltatory conduction: Current movement through the myelinated sheath diminished.
• Na+ entry at the node reinforces depolarization to keep the magnitude at threshold for the next AP.
• AP "jumps" between nodes.

Synaptic Transmission

• Different localization of synaptic vesicles and neurotransmitter receptors.
• One-way transmission.
• Action potential triggers neurotransmitter release.Neurotransmitter binds to receptors, causing potential changes.

Termination of Synaptic Transmission

• Endocytosis of neurotransmitter receptors by the post-synaptic neuron.
• Re-uptake by the presynaptic terminal (re-use).
• Enzyme destruction within the synaptic cleft.
• Diffusion out of the cleft.

Blocking of Neurotransmitter Reuptake (Example: Cocaine)

• Cocaine blocks neurotransmitter reuptake and prolongs the activation of neural pathways (e.g., pleasure).
• Cocaine is a competitor of dopamine transporter.
• Higher concentration of dopamine in synaptic cleft.

Types of Neurotransmitters

• Endogenous chemicals that transmit signals across synapses.
• Each Presynaptic neuron releases only one neurotransmitter.
• Different neurons vary in neurotransmitter release.

Excitatory and Inhibitory Synaptic Transmission

• Binding of neurotransmitters causes a membrane potential change in the post-synaptic neuron.
• Can be excitatory (EPSP) or inhibitory (IPSP).
• EPSP: Small depolarization.
• IPSP: Small hyperpolarization.

Determination of Postsynaptic Potential

• Temporal summation: Several EPSPs occurring close together in time.
• Spatial summation: EPSPs originated simultaneously.
• Cancellation of EPSPs by IPSPs.
• Neuronal integration: Neuronal integration controls physiological activities. (e.g., urination).

Integration of Information Transfer between Neurons

• Complex computational network.
• Convergent pathway: Many pre-synaptic neurons input a signal to a postsynaptic neuron.
• Divergent pathway: One presynaptic neuron branches to large numbers of postsynaptic neurons

What Determines Strength of Action Potential

• Action potential frequency.
• Neurotransmitter release, which may decrease during high frequency.

Memory

• Memory is the storage of acquired knowledge.
• Short-term memory (seconds to hours).
• Long-term memory (days to years).
• Consolidation: Process of transferring short-term to long-term memory, often in the hippocampus, cerebellum, and prefrontal cortex.
• Structural changes: Elongation of dendrites and formation of new synapses.
• LTP: Long-term potentiation.

Long-Term Potentiation (LTP)

• Repetitive stimulation leads to an increase in synaptic strength.
• Triggering an AP in the postsynaptic cell for prolonged period.
• Distinguished from temporal summation.
• Crucial for long-term memory formation.

LTP and Memory Formation

• Related to the Morris water maze.
• NMDAR (N-methyl-D-aspartate receptor) antagonist blocks this process.

After-Lecture Explanations

• Students should be prepared to explain membrane potential, graded potential, action potential formation and transmission, information transmission between neurons, and neural integration.

Multiple Sclerosis

• Autoimmune disease attacking the nervous system.
• Loss of myelin.
• Genetic/environmental factors.
• Slows nerve impulse transmission.
• Symptoms varied.
• No cure.