Study Notes on Neurons and the Nervous System

INTRODUCTION TO NEURONS

OVERVIEW

  • Course Title: K416: Cell & Molecular Neurobiology

AGENDA

  • Cells of the nervous system

  • Glial Cells

  • Neurons

  • Anatomy of a neuron

BIG QUESTIONS IN NEUROSCIENCE

  • How does the brain represent and process information?

  • What is the brain’s "code"?

  • How do neural circuits generate behavior?

  • How does activity at the cellular level translate into action?

  • What similarities exist between human brains and the brains of other organisms?

  • What factors contribute to human uniqueness?

  • How does the brain give rise to consciousness?

  • How does subjective experience arise from neural activity?

INTERESTING FINDINGS

CELLS CAN BE SMART

  • Slime molds, a type of protist (eukaryotic organism), demonstrate intelligence in cell communication and decision-making.

CELLS CAN COMMUNICATE TO EACH OTHER

  • Quorum Sensing in Bacteria: Communication evolves among cells to determine population density. This allows bacterial populations to coordinate behavior based on number.

COMPONENTS OF THE NERVOUS SYSTEM

GENETICS AND GENOMICS IN THE NERVOUS SYSTEM

  • The nervous system (NS) is the product of differential gene expression.

  • Gene Types:

    • Coding Genes: Directly code for proteins.

    • Non-coding Genes: Control levels and timing of gene expression; include introns, 5' and 3' regions.

  • Current genetic estimates:

    • Total Genes: Approximately 20,000

    • Genes in NS: 14,000 (70% of total)

INDIVIDUAL GENE EXPRESSION

  • Genes are regulated differentially throughout the NS. This is illustrated by the varying expression levels of mRNA in different regions of the NS.

IMPACT OF MUTATIONS

  • Mutations in genes crucial to the NS can result in malformations or dysfunction, such as:

    • ASPM Gene: Associated with microcephaly; affects function of proteins involved in cell division.

CELLS OF THE NERVOUS SYSTEM: GLIAL CELLS

  • Glial cells are often referred to as "glue" cells for their support functions.

  • Characteristics:

    • Non-conductive cells.

    • Responsible for nourishing and protecting neurons.

    • There are 11 major types of glial cells in the NS.

TYPES OF GLIAL CELLS

ASTROCYTE

  • Star-shaped cells maintaining microenvironment.

  • Role includes:

    • Contributing to the blood-brain barrier.

    • Construction of new synapses.

    • A subgroup maintains stem cell properties in adults.

OLIGODENDROCYTE

  • Responsible for myelination in the CNS.

  • Increases speed of electrical conduction along axons.

SCHWANN CELLS

  • Responsible for myelination in the PNS.

MICROGLIAL CELL

  • Role in immune defense within CNS.

  • Functions akin to macrophages, secreting cytokines that facilitate communication between cells.

EPENDYMAL CELLS

  • Line the ventricles of the CNS.

  • Produce cerebrospinal fluid (CSF) and facilitate movement through the ventricles.

  • Specialized ependymal cells in the choroid plexus generate most of the CSF.

SATELLITE CELLS

  • Support cells found in the PNS, particularly around dorsal root ganglia housing sensory neurons.

NEURON ANATOMY

STRUCTURAL COMPONENTS

AXON

  • Axon Hillock: Initial region where the cell body transitions into the axon.

  • Myelination: Increases the conduction speed of action potentials.

  • Synaptic Endings (Boutons): Comprises presynaptic and postsynaptic components.

  • Node of Ranvier: Gaps in the myelin sheath that facilitate rapid signal propagation through saltatory conduction.

NEURON CLASSIFICATION

BASED ON MORPHOLOGY

  • Unipolar Neurons: Single process from the soma used for both input and output, common in invertebrates; not in mammals.

  • Bipolar Neurons: One axon and one dendrite; found in sensory systems (e.g., retina).

  • Multipolar Neurons: Multiple dendrites and a single axon; predominant in the human nervous system.

BASED ON FUNCTION

  • Sensory Neurons: Afferent neurons that gather information from the environment and deliver it to the CNS; detect stimuli including light, chemicals, and physical changes.

  • Motor Neurons: Efferent neurons carrying signals from the CNS to effectors (muscles, glands); includes:

    • Somatic motor neurons for skeletal muscle control.

    • Autonomic motor neurons targeting smooth/cardiac muscles or glands.

  • Interneurons: Relay between neurons, facilitating communication within the CNS.

NEURONS STRUCTURAL COMPONENTS

DENDRITES

  • Receive signals using specialized structures called spines, which may be sites of synaptic input and are integral to neuroplasticity.

  • A single neuron, such as a pyramidal cell, can have over 30,000 spines, illustrating its capacity for input from numerous sources.

SOMA (CELL BODY)

  • Contains organelles similar to mammalian cells but has a higher concentration of rough ER, ribosomes, and Golgi apparatus to support neuron-specific functions.

AXON

  • Long, single projection originating from the axon hillock and ending at the presynaptic terminal.

  • Myelin sheath surrounds many axons, enhancing conduction speed and signal durability.

AXON-CYTOPLASMIC TRANSPORT

  • Axon diameter and myelination affects conduction speed.

  • Large-diameter axons typically have thicker myelin, resulting in faster signal transmission.

  • Microtubules in the axonal cytoskeleton facilitate transport; motor proteins such as:

    • Kinesin: Mediates anterograde transport (from the cell body to the axon terminal).

    • Dynein: Mediates retrograde transport (from the axon terminal back to the cell body).

NEURONS: SYNAPSE

TYPES OF SYNAPSES

ELECTRICAL SYNAPSES

  • Characterized by direct cytoplasmic connections between neurons via connexons.

CHEMICAL SYNAPSES

  • Utilize neurotransmitters for communication over a synaptic gap of 15-40 nm, separating the pre- and postsynaptic neurons.

  • Chemical synapses do not allow for direct cytoplasmic exchange.

SYNAPTIC STRUCTURE

  • Chemical synapses notably appear at junctions, such as neuromuscular junctions, where a motor neuron communicates with muscle fibers.

REVIEW QUESTIONS

  • Name labeled structures from provided diagrams.

  • Identify structures specialized for various neuronal functions, including:

    • Sending chemical signals

    • Increasing signal propagation speed

    • Receiving communications from presynaptic neurons.