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 integrating and processing information, coordinating sensory input and motor output, and higher functions such as learning and memory.

• Peripheral Nervous System (PNS): Composed of peripheral nerves and sense organs. Transmits sensory information to the CNS and carries 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 and internal conditions.

• Controls skeletal muscle movement.

Anatomical Divisions

Central Nervous System (CNS)

• Consists of the cerebrum, cerebellum, brain stem, and spinal cord.

• Integrates and processes information.

• Coordinates sensory information and commands to organs.

• Responsible for higher brain functions such as learning and memory.

Peripheral Nervous System (PNS)

• Carries sensory information from the body to the CNS.

• Carries motor commands from the CNS to the body.

• Divided into somatic (voluntary control of skeletal muscle) and autonomic (involuntary control of smooth muscle, glands, and organs) systems.

Autonomic Nervous System

• Sympathetic Division: Prepares the body for 'fight or flight' responses; nerves exit from the middle of the spinal cord.

• Parasympathetic Division: Promotes 'rest and digest' activities; nerves exit from the brainstem and sacral spinal cord.

Cells of the Nervous System

Neurons

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

• Approximately 100 billion neurons in the human brain.

• Sense environmental changes, communicate with other neurons, and command bodily responses.

• Cannot divide or be replaced if lost due to injury or disease (lack centrioles).

Structural Components of a Neuron

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

• Dendrites: Receive information from other neurons; 'tree-like' extensions.

• Axon: Carries information toward other neurons or cells; may be short or long. Begins at the axon hillock and ends at the axon terminal, where information is passed to other cells.

• Synapse: Site where neuron communicates with another neuron or cell.

Diversity of Neuron Shapes

Neurons exhibit a variety of shapes and sizes, reflecting their specialized functions in different regions of the nervous system.

Functional Classification of Neurons

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

• Motor Neurons: Deliver information from the CNS to effectors (muscles or glands).

• Interneurons: Form connections with other neurons; most common type.

Neuroglia

Neuroglia are supporting cells essential for the survival and function of neurons. They make up 50% of neural tissue and 90% of cells in the brain, present in both CNS and PNS.

• Insulate, support, and nourish neurons.

• Important for neural tissue structure and function.

Types of Neuroglia

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, pathogens
Satellite Cells	PNS	Regulate environment around neurons
Schwann Cells	PNS	Wrap axons with myelin, insulate axons

Myelin

Structure and Function

• Myelin is a fatty substance that acts as insulation for axons.

• Oligodendrocytes (CNS) and Schwann cells (PNS) wrap around axons to form myelin sheaths.

• Myelination increases the speed of information travel along axons.

Neurotransmission

Transmission of Information Across a Synapse

Neurotransmission involves the transfer of information from one neuron to another cell via synapses. This process is dependent on the membrane potential.

Membrane Potential

Creation and Maintenance

All cells maintain ion concentration gradients across their plasma membranes, using energy (ATP) to establish these gradients. The difference in ion concentrations creates a membrane potential, which is essential for neuronal function.

• Resting Membrane Potential: The inside of the neuron is more negative relative to the outside, typically -70 mV.

• Potential Difference: The magnitude of the membrane potential.

• Polarised Cell: A cell with a resting membrane potential.

Key Proteins Involved

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

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

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

These channels work together to maintain the resting membrane potential at -70 mV.

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

• Fick's Law of Diffusion: $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Nernst Equation (for membrane potential): $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

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 and neuroglia by structure, function, and location.

Additional info: The notes are based on lecture slides and textbook references, with expanded academic context for clarity and completeness.