Study Notes on Neurons and Neuroglia

Definition of Multipolar Neurons
- Neurons with multiple processes extending from the cell body, generally including many dendrites and one axon.
- Essential for transmitting nerve impulses within the nervous system.
Structure of Neurons
- Dendrites
- Typically short and unmyelinated.
- Function: Gather information and receive signals from other neurons.
- Can have up to 10,000 connections.
- Axon
- Usually long and myelinated, transmitting information over distances in the body.
- Short axons are found in interneurons within the central nervous system.
- An axon's length allows for long-distance signal transmission.
- Axon Hillock
- The initial segment where a decision is made to send a signal down the axon.
- Initiates action potentials when sufficient depolarization occurs.

Resting Membrane Potential
- Typically around -70 millivolts (mV).
- Represents the electrical potential across the neuron's membrane at rest.
Action Potential
- Triggered when the membrane depolarizes to around +30 mV.
- Sodium channels open, allowing Na⁺ ions to enter the cell, leading to depolarization.
- Action potential generation involves complex interactions between different ion channels, including
- Sodium Channels: Open at threshold, leading to rapid depolarization.
- Potassium Channels: Can open allowing K⁺ to exit, repolarizing the membrane.
Average Values
- Resting potential: -70 mV
- Threshold potential: +30 mV
- Action potential peak: +54 mV (typical, but varies by neuron type).

Electrical Signal Transmission
- The axon conducts an electrical signal or action potential that travels toward axon terminals.
- Upon reaching terminals, it causes triggers the release of neurotransmitters (e.g., acetylcholine, ACH).
Signal Modulation
- Input to a neuron can be both excitatory and inhibitory.
- Excitatory Inputs: Open sodium channels to increase likelihood of firing an action potential.
- Inhibitory Inputs: Open chloride channels or potassium channels to decrease likelihood of firing.

Myelin Sheath
- Insulating layer around axons created by oligodendrocytes in the CNS and Schwann cells in the PNS.
- Facilitates faster signal conduction through saltatory conduction (action potential jumps between nodes of Ranvier).
Node of Ranvier
- Gaps in the myelin sheath allowing for regeneration of the action potential, leading to rapid transmission of electrical signals.
Comparison of Myelinated vs. Unmyelinated Axons
- Myelinated axons allow faster signal travel than unmyelinated axons, which require continuous signal regeneration along the entire length.

Mechanism
- Arrival of an action potential at axon terminals causes calcium ions to enter.
- Calcium ions facilitate the release of neurotransmitters from synaptic vesicles across the synaptic cleft to the post-synaptic neuron.
Acetylcholine (ACH)
- Major neurotransmitter involved in transmitting signals across synapses, particularly in motor pathways.

Spinal Cord Injuries
- Damage to spinal cord can sever axons, leading to loss of function, as regeneration capabilities are limited in the CNS due to degeneration of myelin and axonal growth pathways.
Axonal Regeneration
- Peripheral nerves show better regenerative capabilities due to the presence of Schwann cells that assist in regeneration.
- Regeneration possible if the cell body is intact and there is preservation of the nerve sheath (neural lemma).
Challenges
- Schwan cells create myelin for single axons, making regeneration more structured in the peripheral system than in the central nervous system where oligodendrocytes myelinate multiple axons.

Anaxonic Neurons
- Neurons without a clear distinction between axon and dendrites.
- Typically interneurons with no long axonal projection.
Multipolar Neurons
- Characterized by one long axon and multiple dendritic branches.
- Most common neuron type in the brain and spinal cord, usually involved in motor pathways.
Bipolar Neurons
- Two processes extending from opposite sides of the cell body.
- Primarily involved in sensory functions (e.g., in retina, inner ear, olfactory areas).
Unipolar Neurons
- One process projecting from the cell body that branches into a peripheral and central process.
- Mostly found in sensory pathways.

Motor Neurons
- Transmit impulses from the CNS to an effector (e.g., muscle tissue). Responsible for innervating muscles.
Sensory Neurons
- Convey sensory information from the PNS to the CNS, wiring sensory receptors to the central nervous system.
Interneurons (Association Neurons)
- Connect neurons within the same region of the CNS, facilitating communication between sensory and motor pathways.

Overview of Glial Cells
- Also referred to as neuroglia, they support and maintain neural function, creating a conducive environment for neurons.
Protoplasmic Astrocytes
- Regulate ionic environment. Control nutrient and oxygen supply to neurons.
Fibrous Astrocytes
- Form supportive scaffolding for neurons, maintain structural integrity.
Oligodendrocytes
- Form myelin sheaths in the CNS, supporting fast signal transmission across multiple neurons.
Microglia
- Act as immune cells of the CNS, removing debris and damaged cells.
Ependymal Cells
- Line the brain's ventricles and produce cerebrospinal fluid while regulating its composition.
Peripheral Nervous System Glial Cells
- Satellite Cells: Support neuron cell bodies in ganglia.
- Schwann Cells: Produce myelin in the PNS for individual axons, facilitating faster neural conduction.

Resting Membrane Potential
- Regular state of -70 mV with Na⁺ ions typically outside and negative ions inside the cell.
Action Potential Generation
- Triggered by various physical (mechanical) or chemical stimuli leading to membrane depolarization.
Propagation of Signal
- Involves sodium influx, resulting in rapid signal conduction; utilizes local graded potentials and generates action potentials.
Neural Communication Mechanisms
- Predominantly involves neurotransmitter release (e.g., ACH) and receptor-mediated sodium channel activation.