chap-41-nerves-sm17-lecture-notes-1

Neural Signaling Overview

Processes Involved:

1. Reception: Detection of external (e.g., light, sound) and internal (e.g., blood pressure) stimuli.

2. Transmission: Afferent (sensory) neurons relay information to the CNS.

3. Integration: Interneurons process information and formulate responses.

4. Action by Effectors: Efferent (motor) neurons transmit impulses to muscles and glands, leading to observable responses (e.g., muscle movement, physiological changes).

Neuron Structure Comparison:

• Unipolar Neurons: Single process extending from the cell body; mainly sensory.

• Bipolar Neurons: One dendrite and one axon; typically found in sensory organs like the retina.

• Multipolar Neurons: Most common; multiple dendrites and a single axon; primarily involved in motor function.

Typical Neuron Structure:

• Dendrites: Receive signals toward the cell body.

• Cell Body (Soma): Contains the nucleus and organelles.

• Axon: Transmits impulses away from the cell body.

• Myelin Sheath: Insulates the axon, speeding up transmission.

• Axon Terminals: Release neurotransmitters to communicate with other neurons or effectors.

Main Types of Glial Cells and Their Functions:

• Astrocytes: Support neurons, maintain blood-brain barrier, and regulate nutrient supply.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin in the PNS.

• Microglia: Act as immune cells in the CNS.

Resting Potential Maintenance:

• A neuron at rest maintains a resting potential of about -70 mV, primarily due to the sodium-potassium pump and selective permeability of the membrane.

• Changes in ion concentration or permeability can disrupt resting potential.

Ion Channel Comparison:

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically Gated Channels: Open in response to neurotransmitter binding.

• Leak Channels: Allow ions to flow freely according to concentration gradients.

Action Potential Generation::

• Depolarization: Sodium channels open, allowing Na+ influx.

• Repolarization: Potassium channels open, K+ exits the neuron.

• Blocked Sodium/Potassium Channels: Blockage prevents action potential generation or alters the ap.

Feedback in Nerve Conductance:

• Positive feedback: Sodium influx triggers further depolarization.

• Negative feedback: Potassium efflux restores resting potential.

Refractory Periods:

• Absolute Refractory Period: No new action potential can occur regardless of stimulus strength.

• Relative Refractory Period: A stronger than normal stimulus is required to generate an action potential.

All-or-None Response:

• Action potentials either occur fully or not at all; intensity of sensation is determined by frequency of action potentials.

Conduction Types:

• Continuous Conduction: Action potentials propagate along unmyelinated axons.

• Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons.

• Action potentials are self-propagating due to the local depolarization leading to the opening of adjacent voltage-gated channels.

Factors Affecting Action Potential Velocity:

• Axon diameter, myelination, temperature.

Synapse Comparison:

• Electrical Synapses: Fast, bidirectional communication via gap junctions.

• Chemical Synapses: Slower, unidirectional, involving neurotransmitter release and diffusion.

IPSPs vs. EPSPs:

• EPSPs (Excitatory Postsynaptic Potentials): Depolarizations that promote action potentials.

• IPSPs (Inhibitory Postsynaptic Potentials): Hyperpolarizations that inhibit action potentials.

• Temporal Summation: Multiple stimuli in succession.

• Spatial Summation: Several stimuli from different axons contributing to the response.

Neural Circuits:

• Convergence: Multiple inputs lead to a single output.

• Divergence: One input spreads its influence across several outputs.