7a

Chapter Overview

7.1 Overview of the Nervous System

  • Central nervous system (CNS)

    • Components

    • Brain

    • Spinal Cord

  • Peripheral nervous system (PNS)

    • Information Flow

    • Afferent Information (Input)

      • Senses

      • Internal

      • External

    • Efferent Information (Output)

      • Somatic Nervous System

      • Autonomic Nervous System

      • Sympathetic Nervous System

      • Parasympathetic Nervous System

7.2 Cells of the Nervous System

  • Two Major Categories

    • Neurons

    • Definition: Excitable cells that transmit information throughout the nervous system.

    • Glial Cells

    • Definition: Support cells that provide assistance and protection for neurons.

Components of a Neuron

  • Soma

    • Definition: The cell body of the neuron that contains the nucleus and most organelles.

  • Dendrites

    • Function: Responsible for the reception of incoming information from other neurons.

  • Axon

    • Action Potentials (AP): Electrical signals that travel along the axon.

    • Axon Hillock

    • Function: The region where action potentials are initiated.

    • Axon Collaterals

    • Definition: Branches of the axon that extend to communicate with other neurons.

    • Axon Terminal

    • Function: Releases neurotransmitters that carry signals to adjacent neurons.

  • Synapse

    • Presynaptic Neuron: The neuron sending the signal.

    • Postsynaptic Neuron or Effector Cell: The neuron or muscle cell receiving the signal.

Transport in Neurons

  • Axonal Transport

    • Definition: The process by which materials are transported within the neuron.

    • Microtubules and Neurofilaments: Structures that support and facilitate transport.

    • Transport Rates:

    • Slow Transport: Ranges from 0.5 to 40 mm/day.

    • Fast Transport: Ranges from 100 to 400 mm/day.

    • Vesicles & Kinesins: Proteins involved in transport processes.

    • Anterograde Transport: Transport from soma to axon terminal.

    • Retrograde Transport: Transport from axon to soma.

Ion Channels: Neurons

  • Leak Channels

    • Definition: Always open channels allowing ions to flow freely throughout the neuron.

    • Function: Contributes to resting membrane potential.

  • Ligand-Gated Channels

    • Definition: Channels that open or close in response to specific chemical signals (ligands).

    • Location: Found in dendrites and cell body, involved in generating synaptic or graded potentials.

  • Voltage-Gated Channels

    • Definition: Channels that open or close in reaction to changes in membrane potential.

    • Sodium and Potassium Channels: More prevalent in the axon, especially at the axon hillock, crucial for generating action potentials.

    • Calcium Channels

    • Location: Found in the axon terminal, important for the release of neurotransmitters.

Structural Classes of Neurons

  • Bipolar Neurons

    • Characteristics: Have two projections from the cell body.

    • Function: Typically involved in olfaction (smell) and vision.

  • Pseudo-Unipolar Neurons

    • Characteristics: Most sensory neurons exhibit this structure, having a modified dendritic process and a central axon.

    • Function: Transmits sensory information from the peripheral nervous system (PNS) to the central nervous system (CNS).

  • Multipolar Neurons

    • Characteristics: Have multiple projections from the cell body (many dendrites and a single axon).

    • Function: Most common type of neuron, particularly for integration and processing within the CNS.

Functional Classes of Neurons

  • Afferent Neurons

    • Type: Pseudo-unipolar.

    • Function: PNS to the CNS, often found in ganglia.

  • Efferent Neurons

    • Type: Multipolar.

    • Function: CNS to the PNS, controlling muscle movements and glandular responses.

  • Interneurons

    • Type: Multipolar.

    • Function: Located entirely within the CNS, these account for approximately 99% of neurons, facilitating communication between sensory and motor pathways.

Structural Organization of Neurons

  • Central Nervous System (CNS)

    • Components: Includes nuclei, pathways, tracts, and commissures which connect different parts of the brain and spinal cord.

  • Peripheral Nervous System (PNS)

    • Components: Includes ganglia (clusters of neuron cell bodies) and nerves (bundles of axons).

Glial Cells

  • Quantity: Glial cells constitute approximately 90% of all cells in the nervous system.

  • Five Types of Glial Cells:

    • Astrocytes

    • Function: Maintain the blood-brain barrier, support neuronal metabolism.

    • Microglia

    • Function: phagocytosis of debris.

    • Ependymal Cells

    • Function: cerebrospinal fluid (CSF).

    • Oligodendrocytes

    • Function: Form myelin sheaths in the CNS, facilitating faster signal transduction.

    • Schwann Cells

    • Function: Form myelin sheaths in the PNS, also facilitating faster signal transduction.

Myelin-Forming Cells/ lipid

  • CNS: Oligodendrocytes

    • Characteristics: multiple myelin sheaths to multiple axons.

    • Nodes of Ranvier: Gaps in the myelin sheath that facilitate the conduction of action potentials.

  • PNS: Schwann Cells

    • Characteristics: one myelin sheath

    • Nodes of Ranvier: non-myelinated sections between myelin

      • are important, as positive ions can enter these sections to continue the axon potential to travel down the axon

  • axons are white matter