MNB.10_The_nervous_system_at_cellular_level_HMA

Function of the Nervous System

  • Main Functions:

    • Coordination of muscle activity

    • Monitoring and regulation of organs

    • Control of sensory input (sight, hearing, touch, smell, taste)

    • Initiation of actions.

Composition of the Nervous System

  • Key Components:

    • All parts composed of nervous tissue.

    • Receptors: Cells that receive sensations.

    • Effectors: Cells that respond to sensations.

Divisions of the Nervous System

  • Two Main Subdivisions:

    1. Central Nervous System (CNS):

      • Composed of the brain and spinal cord.

    2. Peripheral Nervous System (PNS):

      • Encompasses all nerves and neurons outside of the CNS.

Development of the CNS

  • Neurulation:

    • Development from the ectodermal neural plate.

  • Neurogenesis:

    • Process of generating neurons from neural stem and progenitor cells.

Neural Tube Disorders (NTDs)

  • Malformations: Result from neural tube closure failures.

    1. Anencephaly: Neural tube fails to close, leading to incomplete brain development.

    2. Encephalocele: Neurological defect with unknown causes but related to neural tube closure failure.

    3. Spina Bifida: Combination of genetic and environmental factors, often related to folic acid deficiency.

Maternal Folic Acid Supplements

  • Folic Acid (Vitamin B9):

    • Essential for DNA synthesis and needed before and during pregnancy (400mg daily recommended).

    • Can reduce neural tube defects by ~70% when taken periconceptionally.

Autonomic and Somatic Nervous Systems

  • Components of PNS:

    • Autonomic Nervous System (ANS): Controls involuntary actions.

      • Sympathetic: Prepares the body for fight or flight.

      • Parasympathetic: Restores body to calm state.

      • Enteric: Involves both sympathetic & parasympathetic actions.

    • Somatic Nervous System: Controls voluntary movements.

Somatic Nervous System Details

  • Functions:

    • Afferent Nerves: Carry sensory information from receptors to spinal cord.

    • Efferent Nerves: Transmit commands from spinal cord to muscles.

Neuron Structure

  • Fundamental Unit: Neuron, comprised of four parts:

    1. Dendrites

    2. Soma (cell body)

    3. Axon

    4. Synaptic terminal

  • Soma contains a centrally located large nucleus.

Neuron Cell Body Composition

  • Within Soma:

    • Nissl Bodies: Clusters of rough ER for protein synthesis.

    • Neurofilaments: Microfilaments that provide structural support.

    • Neurotubules: Microtubules found in axons and dendrites.

Types of Neuron

  • Classification of Neurons:

    1. Unipolar/Pseudounipolar: Sensory neurons, primarily in PNS.

    2. Bipolar: Specialized sensory neurons (eye, inner ear).

    3. Multipolar: Most common, numerous dendrites.

Length of Neuron Components

  • Dendrites: Typically short (<1mm).

  • Axons: Can be up to 1 meter long, with the sciatic nerve being the longest.

Structure of Pseudounipolar Neurons

  • Pseudounipolar Neuron:

    • Sensory neuron linking sensory organs to spinal cord with one long dendrite connecting to the spinal cord and a short axon.

Bipolar Neurons

  • Bipolar Neurons:

    • Involved in the transmission of specialized senses (e.g., smell, sight).

    • Have one dendrite and one axon.

Multipolar Neurons

  • Multipolar Neurons:

    • Possess many dendrites and one long axon, receiving significant information.

    • Largely located in the CNS and include motor neurons and interneurons.

Neuron Organization

  • Nerve Cell Bodies:

    • Most neuron cell bodies reside in the CNS, with others forming ganglia.

    • Ganglia: Masses of neuron cell bodies, interconnected to form complex nerve systems.

Ganglia Defined

  • Ganglation: Composed mainly of somata and dendritic structures, providing relay points for different neurological structures.

Brachial Plexus Injury

  • Types of Injury: Results in limp or paralyzed arms, often from childbirth.

    • Severity influences rehabilitation options such as physiotherapy or surgery.

CNS Gray and White Matter

  • Composition:

    • Gray Matter: Contains somata and dendrites.

    • White Matter: Composed primarily of axons.

Glial Cells

  • Supporting Cells: Surround nerve cells providing insulation and structural support.

  • Types:

    • CNS: Neuroglia (with three types).

    • PNS: Schwann Cells, which form myelin sheaths around axons.

Myelination Process

  • Myelinated Nerve Fibers: Wrapped in myelin by Schwann cells, providing insulation.

  • Non-Myelinated Fibers: Embedded within Schwann cell cytoplasm without myelin wrapping.

  • Nodes of Ranvier: Gaps between myelinated sections, influencing conduction speed.

Peripheral Nerves Structure

  • Anatomy: Composed of nerve fibers with somata in CNS or ganglia.

  • Fascicles: Bundles of nerve fibers, each surrounded by connective tissue layers (endoneurium, perineurium, epineurium).

Neuroglia in CNS

  • Supporting Structures: About 86 billion neurons present; neuroglia support neurons in various capacities.

Microglial Cells

  • Role: White blood cell types in CNS that can transform into phagocytic cells and function in defense.

Macroglial Cells

  • Types: Including astrocytes, oligodendrocytes, ependymal cells that serve various supportive roles in CNS.

  • Ependymal Cells: Line brain ventricles and circulate cerebrospinal fluid.

Astrocyte Functions

  • Astrocytes: Abundant in gray matter, forming the blood-brain barrier to regulate substance passage from blood to brain.

Oligodendrocytes

  • Myelination: Responsible for the myelination of CNS axons, supporting many fibers per cell.

Multiple Sclerosis Overview

  • Pathophysiology: Autoimmune disease targeting oligodendrocytes, leading to patchy myelin loss and various neurological deficits.

  • Demographics: Primarily affects individuals aged 20-40; higher incidence in temperate zones.

Summary of Learning Outcomes

  • Final recap of key concepts:

    • Features of vertebrate nervous systems.

    • Neuron structure and classification.

    • Understanding of myelinated vs non-myelinated fibers, ganglia, and nervous system anatomy.


Key Features of Vertebrate Nervous Systems

  • CNS (Central Nervous System): Includes the brain and spinal cord; processes information and coordinates actions.

  • PNS (Peripheral Nervous System): Consists of all nerves outside the CNS; connects CNS to limbs and organs.

Nerve Cell Structure

  • Dendrites: Tree-like structures that receive signals from other neurons.

  • Axons: Long, slender projections that carry impulses away from the cell body to other neurons or muscles.

Types of Neurons

  1. Multipolar Neurons: Most common type with many dendrites and one axon; mainly located in the CNS.

  2. Pseudounipolar Neurons: Sensory neurons with a single process that splits into two branches; found in the PNS.

  3. Bipolar Neurons: Feature one dendrite and one axon; associated with special senses (like sight and smell).

Important Neurological Structures

  • Ganglia: Clusters of neuron cell bodies in the PNS; act as relay points.

  • Gray Matter: Contains neuron cell bodies and unmyelinated axons; site of synapse formation.

  • White Matter: Made up of myelinated axons; facilitates communication between brain regions.

  • Glial Cells: Support cells in the nervous system that maintain homeostasis, create myelin, and protect neurons.

Nerve Fiber Types

  • Myelinated Fibers: Encased in a myelin sheath by Schwann cells, allowing faster impulse conduction.

  • Non-Myelinated Fibers: Lacking a myelin sheath; conduct impulses more slowly.

Peripheral Nerve Structure

  • Comprised of nerve fibers grouped into fascicles and enclosed by connective tissue layers: endoneurium (individual fibers), perineurium (fascicles), and epineurium (entire nerve).