Overview of the Nervous System

Nervous System Divisions

The nervous system is a complex network responsible for coordinating the body's activities. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS), each with distinct roles in processing and transmitting information.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating sensory information and responding accordingly.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

Functions of the Nervous System

The nervous system performs three primary functions to maintain homeostasis and respond to environmental changes.

• Sensory Function: Detects changes in the internal and external environment via sensory receptors and sensory neurons. This division sends information to the CNS.

• Integrative Function: Processes and interprets sensory input, making decisions about appropriate responses. All information is integrated in the CNS.

• Motor Function: Responds to stimuli by initiating action via motor neurons. This division sends information from the CNS to effectors (muscles or glands).

Nervous System Histology

Glial Cells

Glial cells, also known as neuroglia, are non-neuronal cells that provide support, protection, and nutrition to neurons. They play essential roles in maintaining homeostasis, forming myelin, and participating in signal transmission in the nervous system.

• Astrocytes: Located in the CNS; cling to neurons and blood vessels to modulate capillary exchange and maintain the blood-brain barrier.

• Microglia: Located in the CNS; sense neuronal injury and health, performing immune functions. Once activated, they convert to phagocytic cells to remove debris.

• Ependymal Cells: Line the CSF-filled ventricles of the brain and central canal of the spinal cord; their cilia circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Located in the CNS; produce myelin sheaths that are wrapped around CNS neurons, facilitating rapid signal transmission.

• Schwann Cells: Located in the PNS; produce and wrap myelin sheaths around peripheral neuronal axons, aiding in nerve impulse conduction.

• Satellite Cells: Located in the PNS; surround cell bodies in ganglia, performing functions similar to astrocytes in the CNS.

Neurons

Neurons are the functional units of the nervous system, specialized for the transmission of electrical signals. They exhibit unique properties that enable rapid communication throughout the body.

• Properties: Electrically excitable, amitotic (do not divide), extreme longevity, and high metabolic rate.

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

Myelin Sheath: A multilayered lipid and protein covering around axons that increases the speed of nerve impulse conduction. In the PNS, myelin is not a continuous sheet; instead, there are gaps called nodes of Ranvier.

Membrane Potential

Definition and Mechanism

Membrane potential refers to the electrical potential difference across a cell's plasma membrane. This difference arises due to the unequal distribution of ions on either side of the membrane, creating potential energy that can be used for cellular signaling.

• Measurement: The potential energy is measured as voltage (millivolts, mV).

• Generation: An electric current is created in the body by the flow of ions across the plasma membrane.

• Ion Channels: Specialized proteins that allow the passage of specific ions across the membrane. Two main types are:

  • Leakage Channels: Always open, allowing ions to move down their concentration gradients.

  • Gated Channels: Open or close in response to specific stimuli:

    • Chemically-gated channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

    • Voltage-gated channels: Open in response to changes in membrane potential.

    • Mechanically-gated channels: Open in response to mechanical deformation of the receptor (e.g., pressure).

Resting Membrane Potential (RMP)

Definition and Factors

The resting membrane potential is the potential difference that exists across the membrane of a cell when it is not actively transmitting a signal. For a typical neuron, the RMP is approximately -70 mV.

• Key Factors:

  1. Differences in ion concentration across the membrane (mainly sodium and potassium).

  2. Selective permeability of the membrane to different ions.

  3. Activity of the sodium-potassium pump ( ATPase).

Equation (Nernst Equation):

Where: is the equilibrium potential for a particular ion, is the gas constant, is temperature in Kelvin, is the charge of the ion, is Faraday's constant, and and are the ion concentrations outside and inside the cell, respectively.

Changes in Membrane Potential

Neurons use changes in membrane potential to communicate. These changes are classified as depolarization or hyperpolarization.

• Depolarization: The membrane potential moves closer to zero, increasing the chance for a nerve impulse signal.

• Hyperpolarization: The membrane potential moves further from zero, decreasing the chance for a nerve impulse signal.

Alterations in membrane potential are due to ions crossing the plasma membrane through specific ion channels. The two key players are:

• Sodium (): Influx leads to depolarization.

• Potassium (): Efflux leads to hyperpolarization.

Summary Table: Glial Cells of the Nervous System

Additional info: Some details, such as the specific names of glial cells and their functions, were inferred based on standard Anatomy & Physiology curriculum.