Divisions and Cells of the Nervous System: Structure, Function, and Membrane Potential

Nervous System Overview

Major Functional and Anatomical Divisions

The nervous system is a complex network responsible for communication, integration, and regulation of bodily functions. It is divided into anatomical and functional regions, each with distinct roles.

• Central Nervous System (CNS): Includes the brain and spinal cord. Responsible for processing, integrating, and coordinating sensory data and motor commands. Higher functions such as learning and memory are also managed here.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS. It carries sensory information to the CNS and motor commands from the CNS to the body.

Functions of the Nervous System

• Maintains homeostasis by controlling organ systems

• Supports cognition and memory

• Provides and interprets sensory information about external conditions

• Controls skeletal muscle movement

Divisions of the Peripheral Nervous System

Somatic and Autonomic Divisions

The PNS is further divided into the somatic and autonomic nervous systems, each with specific functions:

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions, including the activity of smooth muscle, cardiac muscle, and glands. It is subdivided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Cells of the Nervous System

Neurons

Neurons are the primary functional units of the nervous system, specialized for communication and information processing.

• Number: Approximately 100 billion in the human brain.

• Function: Sense environmental changes, process information, communicate with other neurons, and command body responses.

• Cell Division: Typical neurons cannot divide due to the lack of centrioles; they cannot be replaced if lost to injury or disease.

Structural Components of a Neuron

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

• Dendrites: Receive information from other neurons; the term is derived from the Greek word for 'tree.'

• Axon: Carries information away from the cell body to other neurons or cells.

  • Axon Hillock: The initial segment of the axon.

  • Axon Terminal: The end of the axon where information is passed to other cells.

• Synapse: The site where a neuron communicates with another neuron or cell.

Functional Classification of Neurons

• Sensory Neurons: Deliver information from sensory receptors to the CNS.

• Motor Neurons: Transmit commands from the CNS to effectors (muscles or glands).

• Interneurons: Connect neurons within the CNS; most common type.

Neuroglia (Glial Cells)

Neuroglia are supportive cells essential for neuron survival and function. They constitute about 90% of cells in the brain and 50% of neural tissue, present in both CNS and PNS.

• Insulate, support, and nourish neurons

• Do not conduct electrical impulses

Types of Neuroglia and Their Functions

Neuroglia Type	Location	Function
Astrocytes	CNS	Structural support, regulate environment, maintain blood-brain barrier, form scar tissue after injury
Ependymal Cells	CNS	Line central canal and ventricles, produce/secrete/monitor cerebrospinal fluid (CSF)
Oligodendrocytes	CNS	Wrap axons with myelin sheaths, insulate axons
Microglia	CNS	Phagocytic cells, clean up debris, waste, and pathogens
Satellite Cells	PNS	Regulate environment around neurons, similar to astrocytes
Schwann Cells	PNS	Wrap axons with myelin, insulate axons, similar to oligodendrocytes

Myelin and Its Role

Structure and Function

Myelin is a fatty substance produced by oligodendrocytes (CNS) and Schwann cells (PNS) that insulates axons and increases the speed of electrical signal transmission.

• Acts as insulation

• Enables rapid conduction of action potentials

Neurotransmission and Membrane Potential

Membrane Potential

Membrane potential is the electrical potential difference across the plasma membrane, resulting from the uneven distribution of ions.

• All cells maintain ion concentration gradients using energy (ATP).

• The inside of the neuron is more negative than the outside, creating a resting membrane potential.

• Resting Membrane Potential: Typically -70 mV in neurons.

• The cell is said to be polarised at rest.

Key Proteins Involved

• Sodium-Potassium Exchange Pump: Uses 1 ATP to pump 2 K+ in and 3 Na+ out, maintaining the negative charge inside the cell.

• K+ Leak Channel: Allows potassium to leak out of the cell.

• Na+ Leak Channel: Allows sodium to leak back into the cell.

Ion Concentration Table

Ion	Extracellular Fluid (mM)	Cytosol (mM)
Potassium (K+)	5	100
Sodium (Na+)	150	15
Calcium (Ca2+)	2	0.0002
Chloride (Cl-)	150	13

Relevant Equations

The Nernst equation can be used to calculate the equilibrium potential for each ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where: Eion = equilibrium potential for the ion R = universal gas constant T = temperature in Kelvin z = charge of the ion F = Faraday's constant [ion]outside = concentration of ion outside the cell [ion]inside = concentration of ion inside the cell

Neurotransmission

Neurotransmission is the process by which information is transferred across a synapse from one neuron to another cell. This involves changes in membrane potential and the movement of ions through membrane channels.

• Action potentials are generated by changes in membrane potential.

• Neurotransmitters are released at the synapse to communicate with the postsynaptic cell.

Summary Table: Neuroglia Types

Type	Location	Function
Astrocytes	CNS	Support, regulate environment, maintain blood-brain barrier
Ependymal Cells	CNS	Produce and monitor CSF
Oligodendrocytes	CNS	Myelinate axons
Microglia	CNS	Phagocytosis
Satellite Cells	PNS	Regulate environment
Schwann Cells	PNS	Myelinate axons

Key Learning Outcomes

• State the functions of the nervous system.

• Describe the anatomy and function of neurons and neuroglia.

• Explain the creation and maintenance of resting membrane potential.

• Classify neurons by function and describe the roles of sensory, motor, and interneurons.

• Describe the structure, function, and location of astrocytes, ependymal cells, Schwann cells, and microglia.

Example Application

Damage to oligodendrocytes in the CNS can result in demyelinating diseases such as multiple sclerosis, leading to impaired signal transmission and neurological deficits.

Additional info: The notes are adapted from a lecture and reference the 11th edition of 'Fundamentals of Anatomy & Physiology,' Chapter 12, Sections 12-1 to 12-4.