Fundamentals of the Nervous System: Structure, Function, and Physiology

Chapter 11 Part 1: Fundamentals of the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

• CNS (Central Nervous System): Consists of the brain and spinal cord. It is the main control center for processing information.

• PNS (Peripheral Nervous System): Composed of nerves outside the CNS, connecting it to limbs and organs.

Functions of the Nervous System

The nervous system performs three primary functions to maintain homeostasis and coordinate bodily activities.

• Sensory Function: Detects changes in the internal and external environment through sensory receptors. Information is transmitted to the CNS for processing.

• Integrative Function: Analyzes and interprets sensory information, allowing for decision-making and response planning.

• Motor Function: Initiates actions by sending signals to muscles and glands, resulting in movement or secretion.

Nervous System Histology

The nervous system is composed of two main cell types: neurons and glial cells. Glial cells support and protect neurons, while neurons transmit electrical signals.

Glial Cells

• Astrocytes: Located in the CNS. Regulate the chemical environment and help maintain the blood-brain barrier.

• Microglia: CNS immune cells. Remove debris and protect against injury and infection.

• Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord. Produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: CNS cells that produce myelin sheaths, which insulate axons and increase signal transmission speed.

• Schwann Cells: Located in the PNS. Produce myelin sheaths around peripheral axons.

• Satellite Cells: Found in the PNS. Support neuron cell bodies, similar to astrocytes in function.

Neurons

Neurons are the functional units of the nervous system, specialized for electrical excitability, communication, and integration.

• Properties: Electrically excitable, extreme longevity, high metabolic rate.

• Structure: Consist of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).

• Axon: Conducts impulses away from the cell body. May be myelinated for faster transmission.

Example: A motor neuron transmits signals from the CNS to muscles, causing contraction.

Myelination

Myelination is the process of wrapping axons with a lipid-rich myelin sheath, which increases the speed of nerve impulse conduction.

• Myelin Sheath: Composed of lipids and proteins. In the CNS, oligodendrocytes form myelin; in the PNS, Schwann cells do.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission via saltatory conduction.

Membrane Potential

Neurons maintain a difference in electrical charge across their plasma membrane, known as the membrane potential. This is essential for nerve impulse transmission.

• Potential Difference: The voltage across the membrane due to ion distribution.

• Ion Channels: Proteins that allow specific ions to diffuse across the membrane. Types include:

  • Leakage Channels: Always open, allowing passive ion movement.

  • Chemically-Gated Channels: Open in response to chemical signals.

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

  • Mechanically-Gated Channels: Open in response to physical deformation.

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Equation:

$RMP = -70\,\text{mV}$

• Maintained by the sodium-potassium pump and differential permeability of the membrane to ions.

• More potassium (K+) inside the cell, more sodium (Na+) outside.

Alterations of Resting Membrane Potential

Changes in membrane potential are crucial for nerve impulse transmission.

• Depolarization: The cell becomes less negative, increasing the chance for a nerve impulse.

• Hyperpolarization: The cell becomes more negative, decreasing the chance for a nerve impulse.

Alterations occur via opening or closing of ion channels, allowing ions to move across the membrane.

• Sodium (Na+): More outside the cell; influx causes depolarization.

• Potassium (K+): More inside the cell; efflux causes hyperpolarization.

Summary Table: Glial Cells and Their Functions

Cell Type	Location	Function
Astrocytes	CNS	Regulate chemical environment, support neurons, maintain blood-brain barrier
Microglia	CNS	Immune defense, remove debris
Ependymal Cells	CNS	Produce and circulate CSF
Oligodendrocytes	CNS	Produce myelin sheath
Schwann Cells	PNS	Produce myelin sheath
Satellite Cells	PNS	Support neuron cell bodies

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.