Structure and Function of Nervous Systems: Neurons, Glia, and Membrane Potentials

Learning Objectives

• Describe the basic structure and function of nervous systems, neurons, and glia.

• Demonstrate how changes in membrane potentials cause transmission of action potentials across a neuron.

• Summarize how neurons communicate between themselves and other cell types using excitatory and inhibitory pathways.

Neurons

Basic Structure and Function

Neurons are the fundamental units of the nervous system, specialized for the generation and transmission of electrical signals. They are responsible for processing and transmitting information throughout the body.

• Basic unit of nervous system: Neurons are the primary signaling cells.

• Electrical signals: Neurons generate and transmit action potentials.

• Main regions of a neuron:

  • Dendrites: Receive incoming signals from other neurons.

  • Cell body (Soma): Contains the nucleus and integrates signals.

  • Axon: Conducts electrical impulses away from the cell body.

  • Axon terminals: Transmit signals to other neurons or effector cells.

• Action Potential: A rapid change in membrane potential that travels along the axon.

Example: Sensory neurons transmit information from the skin to the brain, while motor neurons send signals from the brain to muscles.

Vertebrate Neural Networks

Organization of the Nervous System

Vertebrate nervous systems are organized into two main components, each with distinct functions and structures.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Consists of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

Example: The CNS interprets sensory input, while the PNS carries out motor commands.

Neural Networks

Levels of Complexity

Neural networks vary in complexity across different animal groups, reflecting evolutionary adaptations.

• Nerve net: A simple network of interconnected neurons found in organisms like hydra.

• Ganglia: Clusters of neuron cell bodies that act as local processing centers, seen in invertebrates like earthworms.

• Brain: Large, centralized organ for complex processing, characteristic of vertebrates.

Example: Jellyfish possess a nerve net, while humans have a highly developed brain.

Glia

Types and Functions

Glial cells are non-neuronal cells that support, protect, and nourish neurons. They play essential roles in maintaining homeostasis and facilitating neural function.

• Macroglia:

  • Oligodendrocytes: Found in the CNS; produce myelin sheaths that insulate axons and speed up action potential transmission.

  • Schwann cells: Found in the PNS; also produce myelin sheaths for peripheral nerves.

  • Astrocytes: Surround blood vessels, regulate the extracellular environment, and support neuronal metabolism.

• Microglia: Act as immune cells in the CNS; remove debris and protect against pathogens through phagocytosis.

Example: Schwann cells are crucial for rapid signal transmission in peripheral nerves.

Glial Disorders

Damage or dysfunction of glial cells can lead to neurological diseases.

• Multiple sclerosis: An autoimmune disease where the immune system attacks myelin sheaths in the CNS, leading to impaired nerve conduction.

• Peripheral neuropathy: Damage to peripheral nerves, often due to diabetes, resulting in loss of sensation or motor function.

Example: Multiple sclerosis causes muscle weakness and coordination problems.

How are Action Potentials Generated?

Mechanism of Action Potential

Action potentials are rapid, transient changes in membrane potential that allow neurons to transmit signals over long distances.

• Initiation: Triggered by depolarization of the neuron's membrane.

• Propagation: The action potential travels along the axon to the axon terminals.

• Synaptic transmission: The signal is passed to the next cell via neurotransmitter release.

Example: Touching a hot surface initiates action potentials in sensory neurons, which are relayed to the brain.

Membrane Potential

Definition and Maintenance

Membrane potential is the difference in electrical charge across a cell's plasma membrane, essential for neuronal signaling.

• Electrochemical gradient: Created by differences in ion concentrations inside and outside the cell.

• Resting potential: The stable, negative charge inside the neuron when not transmitting signals (typically around -70 mV).

Equation:

$V_m = V_{inside} - V_{outside}$

where $V_m$ is the membrane potential.

Ion Transporters and Channels

Specialized proteins in the membrane regulate ion movement, maintaining and altering membrane potential.

• Ion channels: Allow specific ions (e.g., Na+, K+) to pass through the membrane.

• Ion transporters: Actively move ions against their concentration gradients.

• Na+-K+ pump: Uses ATP to move 3 Na+ ions out and 2 K+ ions in, maintaining the resting potential.

Equation:

$ext{Na}^+/ ext{K}^+ ext{ pump:} \\ 3 ext{Na}^+_{out} + 2 ext{K}^+_{in}$

Example: The Na+-K+ pump is essential for resetting the neuron after an action potential.

Summary Table: Types of Glial Cells and Their Functions

Glial Cell Type	Location	Main Function
Oligodendrocytes	CNS	Form myelin sheaths
Schwann cells	PNS	Form myelin sheaths
Astrocytes	CNS	Support neurons, regulate environment
Microglia	CNS	Immune defense, phagocytosis