Neuronal Signaling and Synaptic Transmission: Study Notes for ANP 1105A

Study Guide - Smart Notes

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Neurons: Structure and Function

Overview of Neuron Regions

Neurons are the fundamental units of the nervous system, specialized for the reception, propagation, and transmission of nerve impulses. Each region of a neuron has distinct structural and functional roles.

Dendrites: Receive incoming signals from other neurons or sensory receptors.
Cell Body (Soma): Integrates incoming signals and contains the nucleus and organelles.
Axon: Propagates electrical impulses (action potentials) away from the cell body toward other neurons, muscles, or glands.
Axon Terminals: Transmit signals to target cells via synapses.

Example: Sensory neurons have long dendrites to receive stimuli, while motor neurons have long axons to reach distant muscles.

Electrical Activity of Neurons

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane when the cell is not actively transmitting a signal. It is primarily established by differences in ion concentrations and membrane permeability.

Key Ions: Sodium (Na+), Potassium (K+), Chloride (Cl-).
Na+/K+ Pump: Maintains concentration gradients by pumping 3 Na+ out and 2 K+ in.
Typical Value: -70 mV (inside negative relative to outside).

Equation:

Additional info: This is the Nernst equation for potassium; the Goldman-Hodgkin-Katz equation considers multiple ions.

Graded Potentials

Graded potentials are brief, localized changes in membrane potential that occur in dendrites and the cell body. They vary in amplitude depending on the strength of the stimulus.

Short-distance signals: Affect only the region near the site of stimulation.
Summation: Multiple graded potentials can combine to influence action potential generation.

Action Potentials

Action potentials are rapid, long-distance electrical signals that travel along the axon. They are all-or-none events triggered when the membrane potential reaches a threshold.

Phases: Depolarization (Na+ influx), Repolarization (K+ efflux), Hyperpolarization.
Propagation: Action potentials move unidirectionally along the axon.

Equation:

Additional info: This represents ionic current, where g is conductance and E is equilibrium potential.

Synapses: Structure and Function

Types of Synapses

Synapses are specialized junctions where neurons communicate with other neurons, muscle fibers, or glands. There are two main types:

Chemical Synapses: Use neurotransmitters to relay signals across a synaptic cleft.
Electrical Synapses: Directly connect cytoplasm of adjacent neurons via gap junctions, allowing rapid, bidirectional communication.

Table: Comparison of Chemical and Electrical Synapses

Feature	Chemical Synapse	Electrical Synapse
Signal Transmission	Neurotransmitter release	Direct ion flow
Speed	Slower (synaptic delay)	Faster (minimal delay)
Directionality	Unidirectional	Bidirectional or unidirectional
Plasticity	High (modifiable)	Low
Location	Most CNS synapses	Some CNS, heart, smooth muscle

Chemical Synapse Mechanism

At chemical synapses, neurotransmitters stored in vesicles are released from the presynaptic neuron and bind to receptors on the postsynaptic cell.

Action potential arrives at axon terminal.
Voltage-gated Ca2+ channels open; Ca2+ influx triggers vesicle fusion and neurotransmitter release by exocytosis.
Neurotransmitter diffuses across synaptic cleft and binds to postsynaptic receptors.
Binding opens ion channels, generating graded potentials (EPSPs or IPSPs).
Signal termination by reuptake, diffusion, or enzymatic degradation.

Postsynaptic Potentials

Excitatory Postsynaptic Potentials (EPSPs)

EPSPs are depolarizing graded potentials that increase the likelihood of action potential generation in the postsynaptic neuron.

Caused by opening of non-selective cation channels (Na+ influx > K+ efflux).
Main excitatory neurotransmitter: Glutamate.
Often occur on dendritic spines, which are dynamic structures involved in synaptic plasticity.

Inhibitory Postsynaptic Potentials (IPSPs)

IPSPs are hyperpolarizing graded potentials that decrease the likelihood of action potential generation.

Caused by opening of channels permeable to Cl- or K+ (influx of Cl- or efflux of K+).
Main inhibitory neurotransmitter: GABA (in CNS).

Synaptic Integration

Summation of Synaptic Inputs

Neurons integrate multiple excitatory and inhibitory inputs to determine whether to fire an action potential. Summation can be temporal or spatial.

Temporal Summation: Multiple EPSPs/IPSPs from a single presynaptic neuron occur in rapid succession.
Spatial Summation: EPSPs/IPSPs from multiple presynaptic neurons occur simultaneously at different locations on the postsynaptic neuron.

Table: Types of Summation

Type	Description	Effect
Temporal	Rapid, repeated signals from one neuron	Can reach threshold for AP
Spatial	Simultaneous signals from multiple neurons	Combined effect may reach threshold

Initial Segment and Action Potential Generation

The initial segment of the axon (axon hillock) has a high concentration of voltage-gated sodium channels, making it highly sensitive to changes in membrane potential. It is the primary site for action potential initiation.

Threshold for AP generation is lower than other regions.
Integration of all synaptic inputs occurs here.

Regulation of Synaptic Strength

Mechanisms of Modulation

Synaptic strength can be modulated by several mechanisms, affecting the efficacy of neurotransmission.

High-frequency stimulation increases neurotransmitter release (facilitation).
Axonic input can facilitate or inhibit neurotransmitter release.
Presynaptic receptors provide feedback to regulate release.
Desensitization: Receptors may become less responsive despite continued neurotransmitter presence.
Drugs can affect neurotransmitter synthesis, release, reuptake, or degradation (e.g., SSRIs).

Developmental Aspects of Neurons

Neural Development

Neurons originate from the neural tube (CNS) and neural crest (PNS). Development involves proliferation, migration, axon outgrowth, and synapse formation.

Growth Cone: Specialized structure at the tip of growing axons, senses environmental cues and guides axon to target.
Guidance Cues: Attractive, repulsive, or stop signals direct axon pathfinding.
Neurotrophic Factors: Substances like nerve growth factor (NGF) promote growth and survival.

Chemical Affinity Hypothesis

Proposed by Roger Sperry, this hypothesis suggests that axons find their correct targets based on specific chemical cues, ensuring precise synaptic connections.

Axons recognize and bind to target cells via molecular markers.
Critical for proper neural circuit formation.

Summary Table: Key Concepts in Neuronal Signaling

Concept	Description
Resting Membrane Potential	Established by ion gradients and membrane permeability
Graded Potential	Local, variable amplitude signals
Action Potential	All-or-none, long-distance signal
Synapse	Junction for neuron-to-neuron communication
EPSP	Depolarizing, increases AP likelihood
IPSP	Hyperpolarizing, decreases AP likelihood
Summation	Integration of multiple inputs
Plasticity	Ability to modify synaptic strength