Introduction to Neuroscience: Structure and Function of the Nervous System

I. What is Neuroscience?

Definition and Scope

Neuroscience is the scientific study of all aspects of nervous systems, from their molecular components to their behavioral and cognitive outputs. It encompasses a wide range of subfields, each focusing on different levels of organization and function.

• Neuroscience: The study of all aspects of nervous systems.

• Neurobiology: A term often used interchangeably with neuroscience, though sometimes more narrowly focused on biological aspects.

• Goal of Neuroscience: To understand every aspect of nervous system function, from the molecular level up to behavioral and cognitive levels.

Major Subdisciplines of Neuroscience

• Neuroanatomy: Study of the gross and microscopic anatomy of the nervous system.

• Neurophysiology: Study of the electrical signaling of neurons.

• Neurochemistry: Study of how neurotransmitters are synthesized, released, and degraded.

• Neuropharmacology: Study of drug actions in the brain.

• Molecular Neuroscience: Study of gene actions as they relate to nervous system function.

• Developmental Neuroscience: Study of mechanisms involved in wiring the nervous system during development.

• Systems Neuroscience: Study of neural circuits for sensory systems, behavior, emotions, cognition, and motor control.

• Cognitive Neuroscience: Study of higher order functions such as perception, memory, and decision-making.

Key Concepts

• The real goal of neuroscience is to understand the relationships between these subdivisions.

• Neuroscience is a continuum, spanning from molecular to mind-level processes:

Molecular	Cellular	Systems	Behavioral	Mind
Genes, proteins	Neurons, glia	Neural circuits	Actions, responses	Consciousness, cognition

Additional info: This continuum highlights the integration of different levels of analysis in neuroscience research.

• A wide spectrum of animals, from jellyfish to humans, are used in neuroscience research to understand fundamental principles.

II. Gross Organization (Anatomy) of the Nervous System

Note: For detailed anatomical organization, refer to lecture slides. This section introduces the major cellular components of the brain.

III. Two Major Components of the Brain

A. Neurons

Neurons are the primary signaling cells of the nervous system, specialized for receiving, integrating, and transmitting information.

• There are approximately 100 billion neurons in the human brain.

• Each neuron receives an average of ~5,000 synaptic contacts.

B. Glia

Glia (from the Greek for "glue") are non-neuronal cells that provide support and protection for neurons. They are essential for the proper functioning of the nervous system.

• There are approximately ten times more glia than neurons in the brain.

• Three main types of glial cells:

  • Oligodendrocytes (CNS) and Schwann cells (PNS): Wrap around axons to provide insulation in the form of myelin, which increases the speed of electrical signal transmission.

  • Astrocytes: Provide structural and metabolic support for neurons, regulate extracellular potassium ($K^+$) levels, and take up glutamate released by neurons. They also help maintain the blood-brain barrier.

  • Microglia: Act as resident immune cells (phagocytes) in the CNS, clearing debris and responding to injury or infection.

IV. Structure of a Neuron

A. Parts of the Neuron

Neurons have specialized structures that enable them to transmit information efficiently.

• Soma (cell body): Contains the nucleus and metabolic machinery.

• Nucleus: Contains genetic material (DNA).

• Dendrites: Branch-like extensions that receive input from other neurons.

• Axon hillock: Region where the axon originates from the soma; site of action potential initiation.

• Axon: Long projection that transmits electrical impulses away from the soma.

• Myelin: Insulating sheath formed by oligodendrocytes (CNS) or Schwann cells (PNS).

• Node of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

• Axon collateral: Branches of the axon that can communicate with multiple targets.

• Presynaptic terminal: End of the axon containing synaptic vesicles filled with neurotransmitters.

• Synaptic vesicles: Membrane-bound sacs containing neurotransmitters.

• Synaptic cleft: Small gap between the presynaptic and postsynaptic cells.

• Postsynaptic density: Region on the postsynaptic cell rich in receptors for neurotransmitters.

B. Diversity of Neurons

• There are approximately 1,000 different classes of neurons, each with unique properties and functions.

C. Functional Implications

• Different vertebrates have virtually identical types of neurons.

• The increased functional complexity of the human brain is not due to more complex neurons, but rather to a greater number of neurons and more complex connections between them.

V. Summary Table: Types of Glial Cells

Type	Location	Main Function
Oligodendrocytes	CNS	Form myelin sheaths around axons
Schwann cells	PNS	Form myelin sheaths around axons
Astrocytes	CNS	Support neurons, regulate extracellular ions, uptake neurotransmitters
Microglia	CNS	Immune defense, phagocytosis of debris

VI. Key Terms and Concepts

• Neuron: The basic functional unit of the nervous system, specialized for communication.

• Glia: Non-neuronal cells that support, protect, and nourish neurons.

• Myelin: Fatty substance that insulates axons and increases the speed of nerve impulse conduction.

• Synapse: Junction between two neurons where information is transmitted from one to another.

• Action potential: Rapid electrical signal that travels along the axon of a neuron.

Example: Action Potential Conduction

Myelinated axons conduct action potentials more rapidly due to saltatory conduction, where the action potential "jumps" from one node of Ranvier to the next.

Equation: Nernst Equation (for equilibrium potential of ions)

The Nernst equation calculates the equilibrium potential for a particular ion:

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

Where:

• $E_{ion}$ = equilibrium potential for the ion

• $R$ = universal gas constant

• $T$ = temperature in Kelvin

• $z$ = valence of the ion

• $F$ = Faraday's constant

• $[ion]_{outside}$, $[ion]_{inside}$ = concentrations of the ion outside and inside the cell

Additional info: The Nernst equation is fundamental for understanding how membrane potentials are established in neurons.