Untitled Quiz

Human Physiology BIOL3205 - Neuronal Communication

• The course covers neuronal communication, specifically focusing on the lecture outline.
• Membrane potential is the voltage difference across a cell membrane, resting potential of a neuron cell is -70mV.
• Negative values are used because intracellular fluid has an excess of anions compared to the extracellular fluid, which has a slight excess of cations (polarized).
• Na+ and K+ are responsible for generating the resting membrane potential.
• The Na+-K+ pump maintains this potential.
• Channels are always open for K+ (Leaky channel).

Structure of a Neuron

• Dendrites: Receive signals from other neurons and conduct them toward the cell body.
• Dendritic spines: Formations of a synapse that can also be found in the cell body.
• Cell body: Contains the nucleus and organelles; integrates signals.
• Input zone: Receives signals from other neurons.
• Axon: Conducts action potentials to other cells; variable in length and triggered by axon hillock.
• Axon terminals: Release neurotransmitter influencing other cells.

Types of Neural Signals

• Graded potential: A change in the membrane potential that is proportional to the strength of the triggering event; it diminishes with distance from the origin. It's used as a signal over a short distance.
• Membrane potential change depends on the magnitude of the triggering event.
• Action potential: A rapid, large, and brief change in membrane potential; initiated when the membrane potential reaches threshold potential at the axon hillock. It's propagated through the entire membrane nondecrementally and serves as a long-distance signal, with a fixed threshold and magnitude, and is an all-or-none response.

Ion Channels

• Pores that open and close in an all-or-nothing fashion to provide aqueous channels through the plasma membrane for ions to traverse.
• Leaky, voltage-gated, chemically-gated, and mechanically-gated channels exist.

Change of Membrane Potential

• Only nerve and muscle tissues are excitable.
• Membrane potential can be changed.
• Polarization: more charges are separated by the membrane.
• Depolarization: fewer charges are separated by the membrane.
• Repolarization: returns to resting membrane potential.
• Hyperpolarization: becomes more polarized.
• Ion movement is mediated by channels.

Formation of Action Potential (AP)

• Caused by rapid fluxes of Na+ and K+.
• Opening and closing of voltage-gated Na+ and K+ channels is involved.
• Graded potential activates: Opening of Na+ channel (fast), closing mechanism of Na channel (slow), and opening of K channel (slow).
• Positive feedback of Na+ channel: rapid depolarization.
• Rapid repolarization caused by the slow closure of K+ channel.

Propagation of Nerve Impulse

• Once AP is initiated, no further triggering event is needed. The AP is conducted via contiguous conduction or saltatory conduction through the axon structure(myelinated or non-myelinated).

Contiguous Conduction

• Occurs in non-myelinated neurons.
• Spread of the action potential along every patch of membrane down the length of an axon.
• Active and inactive areas in this process.

Saltatory Conduction

• Occurs in myelinated neurons.
• Currents move under the myelin sheath, but diminish in amplitude.
• Na⁺ entry at the node reinforces the depolarization, maintaining and keeping a magnitude to a threshold for the next AP.
• Jumps of AP.

Refractory Period

• The time period when no new AP can be initiated.
• Absolute refractory period vs. relative refractory period.

Myelination

• Myelinate fibers cover axons with myelin along their length.
• Comprised of primarily lipids produced by oligodendrocytes (CNS) and Schwann cells (PNS).
• Prevent leakage of current (insulation).
• In between myelinated regions are nodes of Ranvier, which are concentrated with Na+ and K+ channels.
• Speed conduction 50 times faster than unmyelinated fibers, conserving energy.

Neurotoxins

• Tetrodotoxin (TTX) is a toxin that attacks the nervous system, highly enriched in the liver and gonad of pufferfish, as well as salamanders, octopuses, and gobies.
• Cannot be destroyed by cooking.
• Blocks voltage-gated Na+ channels, thus stopping AP generation.
• Symptoms can occur 10-45 minutes after eating pufferfish, starting with numbness and tingling around the mouth, followed by salivation, nausea, and vomiting.
• Can lead to paralysis, loss of consciousness, and respiratory failure, and death. An analgesic use is also noted.

Synaptic Transmission

• Differential localization of synaptic vesicles and neurotransmitter receptors (one-way transmission).
• Action potential reaches axon terminal; Ca2+ enters synaptic knob; neurotransmitter is released by exocytosis into synaptic cleft.
• Neurotransmitter binds to postsynaptic receptors, opening channels; initiates a postsynaptic potential.

Termination of Synaptic Transmission

• Endocytosis with the receptor by post-synaptic neuron.
• Re-uptake by the presynaptic terminal.
• Destroyed by the enzyme at the synaptic cleft.
• Diffused out.

Blocking of Neurotransmitter Reuptake

• Cocaine (coke) is a psychostimulant extracted from coca leaves.
• Sigmund Freud promoted cocaine as a tonic to treat depression and sexual impotence.
• A competitor of dopamine transporter.
• Prolonged activation of neural pathways and feeling of pleasure when high concentrations of Dopamine is present in the synaptic cleft, caused by cocaine interfering with the reuptake of dopamine.

Types of Neurotransmitters

• Endogenous chemicals that transmit signals across a synapse.
• Each presynaptic neuron releases only one neurotransmitter.
• Different neurons vary in neurotransmitter release as well.

Types of Synaptic Transmission (Excitatory vs Inhibitory)

• Binding of neurotransmitter causes a membrane potential change, this can be excitatory (excitatory synapse) or inhibitory (inhibitory synapse).
• Excitatory Postsynaptic potential (EPSP): A change in the postsynaptic potential that occurs at an excitatory synapse, with small depolarization.
• Inhibitory Postsynaptic potential (IPSP): A change in the postsynaptic potential that occurs at an inhibitory synapse, resulting in small hyperpolarization.
• Graded Potential vs. Action Potential.

Determination of Postsynaptic potential

• Temporal summation: Successive firings of a presynaptic neuron causing several EPSPs to occur together in time.
• Spatial summation: EPSPs from several presynaptic neurons creating simultaneous inputs, occurring together (i.e. multiple inputs from various neurons).
• Cancellation of EPSP by IPSP.
• Neuronal integration to control and regulate physiological activity (e.g. urination, etc)

Integration of Information Between Neurons

• Complex computational network
• Divergent pathway: One presynaptic neuron can branch to many post-synaptic neurons.
• Convergent pathway: Many presynaptic neurons influence one postsynaptic neuron.
• Number of neurons & synapses in the nervous system.

What Determines the Strength of a Stimulus?

• Each AP is identical.
• Frequency of AP is related to the strength of the stimulus.
• Neurotransmitter release may decrease in high frequency stimuli via desensitization.

Memory

• Memory: Storage of acquired knowledge.
• Short-term memory: Seconds to hours.
• Long-term memory: Days to years.
• Consolidation: Movement of information from short-term to long-term memory, influenced by structural and functional alterations; fixed and persistent.

Long-Term Potentiation (LTP)

• Nervous system has plasticity, able to change anatomy and function in response to changes in activity patterns.
• Repetitive stimulation of a particular synapse leads to synaptic connection strength increase for triggering AP in the postsynaptic cell. (LTP).
• Distinct from temporal summation.
• Important for the formation of long-term memory.

LTP and Memory Formation

• Example using Morris water maze.
• Treatment of rats with NMDAR antagonist AP5 blocks LTP and also blocks learning of the platform.

After the Lecture:

• Students should be able to explain membrane potential, graded and action potentials, their formation and transmission, information transmission between neurons, and what neural integration is.