Nervous System Organization & Cells

Nervous System Organization & Cells

Learning Objectives (Section 1)

  • After completing Section 1, students will be able to:

    • Describe the general functions of the nervous system.

    • Compare & contrast the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

    • Differentiate between the motor and sensory components of the nervous system.

    • Describe the structure, location, & function of each of the 6 types of neuroglial (glial) cells.

    • Identify & describe the major components of a neuron. (HAPS, 2019)

Function & Organization of the Nervous System

  • Organized to detect changes in:

    • Internal environment.

    • External environment.

  • Functions include:

    • Evaluating information.

    • Initiating appropriate responses.

  • Subdivided by location into:

    • Central Nervous System (CNS).

    • Peripheral Nervous System (PNS). (Patton, 2022)

Central Nervous System (CNS) & Peripheral Nervous System (PNS)

  • CNS:

    • Composed of the brain and spinal cord.

    • Functions include:

    • Integration of sensory information.

    • Evaluation of sensory information.

    • Initiation of outgoing responses.

  • PNS:

    • Composed of nerves located in outer regions:

    • Cranial Nerves: 12 pairs.

    • Spinal Nerves: 31 pairs. (Patton, 2022)

Afferent & Efferent Organization

  • Afferent Division:

    • Comprises all incoming sensory pathways.

  • Efferent Division:

    • Comprised of all outgoing motor pathways. (Patton, 2022)

Somatic Nervous System (SNS)

  • Somatic Motor:

    • Transmits motor information to skeletal muscle.

  • Somatic Sensory:

    • Transmits sensory information to the CNS. (Patton, 2022)

Autonomic Nervous System (ANS)

  • ANS (Efferent):

    • Controls motor functions to:

    • Smooth muscles.

    • Cardiac muscles.

    • Glands.

    • Adipose and other tissues.

  • Divisions of ANS:

    • Sympathetic Division:

    • Responsible for the "fight or flight" response.

    • Parasympathetic Division:

    • Responsible for "rest & repair" functions. (Patton, 2022)

Enteric Nervous System (ENS)

  • A subdivision of the ANS.

  • Location:

    • Found within the intestinal wall.

  • Function:

    • Regulates digestive processes.

    • Exhibits some degree of independent integration. (Patton, 2022)

Neuroglial (Glial) Cells

Major Types of Glial Cells

  • Total: 6 major types of glial cells (glia), which primarily support neurons.

    • 4 Types in CNS:

    1. Astrocytes:

      • Most common glial cell type.

      • Description: Star-shaped and the largest type.

      • Functions:

        • Connect to capillaries and neurons.

        • Transfer nutrients from blood to neurons.

        • Form the Blood-Brain Barrier.

    2. Microglia:

      • Description: Small and usually stationary.

      • Functions:

        • In inflamed brain tissue, microglia enlarge and move.

        • Perform phagocytosis (removal of debris).

    3. Ependymal Cells:

      • Description: Thin sheets of epithelial-like cells.

      • Functions:

        • Line fluid-filled cavities in the CNS.

        • Some produce cerebrospinal fluid (CSF), while others help circulate it.

    4. Oligodendrocytes:

      • Description: Smaller than astrocytes with fewer processes.

      • Functions:

        • Hold nerve fibers together.

        • Produce myelin sheath around nerve fibers, creating nodes of Ranvier.

  • 2 Types in PNS:

    1. Schwann Cells (Neurolemmocytes):

      • Functions:

        • Support nerve fibers.

        • Form myelin sheath by wrapping around nerve fibers.

    2. Satellite Cells:

      • Functions:

        • Cover and support neuronal cell bodies in the PNS. (Patton, 2022)

Neurons

General Overview

  • Neurons are excitable cells responsible for initiating and conducting impulses.

Components of Neurons

  • Cell Body:

    • Contains all organelles.

  • Dendrites:

    • Function: Conduct nerve signals to the cell body.

  • Axon:

    • Function: Conduct nerve impulses away from the cell body.

    • May be myelinated.

  • Cytoskeleton:

    • Function: Aids in the rapid transport of small molecules. (Patton, 2022)

Functional Regions of Neurons

  • Input Zone:

    • Includes dendrites and cell body.

  • Summation Zone:

    • Located at the axon hillock.

  • Conduction Zone:

    • Located along the axon.

  • Output Zone:

    • Comprising telodendria and synaptic knobs. (Patton, 2022)

Structural Classification of Neurons

  • Based on the number of processes extending from the cell body. (Patton, 2022)

Functional Classification of Neurons

  • Afferent Neurons: Carry signals toward the CNS (Sensory).

  • Efferent Neurons: Carry signals away from the CNS (Motor).

  • Interneurons: Neurons that connect afferent and efferent neurons within the CNS. (Patton, 2022)

Clinical Correlation: The Blood-Brain Barrier (BBB)

  • Function: Maintains a stable environment for brain function.

  • Role of Astrocytes: Wrap around capillaries in the brain and help form the BBB.

  • Medication Considerations: Drugs for conditions such as Parkinson's must cross the BBB to be effective:

    • Dopamine: Does not cross the BBB.

    • Levodopa: Can cross the BBB and is used by brain cells to produce dopamine, thus treated as a substitute therapy. (Patton, 2022)

Learning Objectives (Section 2)

  • After completing Section 2, students will be able to:

    • Describe the major components of a reflex arc.

    • Describe the structure of a nerve, including motor & sensory elements, and connective tissue wrappings.

    • Explain how nerves may be repaired. (HAPS, 2019)

Reflex Arc

  • Described as a feedback loop comprising:

    • A 3-neuron arc (most common):

    • Sensory receptor → Afferent neuron → Interneuron → Efferent neuron → Effector tissue (muscle or gland). (Patton, 2022)

  • Type of Reflex Arc:

    • Contralateral Reflex Arc: Receptors and effectors are located on opposite sides of the body.

    • Intersegmental Reflex Arcs: Reflex arcs that may involve multiple segments of the spinal cord. (Patton, 2022)

Nerves & Tracts

  • Nerves: Bundles of nerve fibers located in the PNS.

  • Structure of Nerves:

    • Endoneurium: Connective tissue surrounding each individual nerve fiber.

    • Perineurium: Connective tissue that bundles several fibers into fascicles.

    • Epineurium: Connective tissue surrounding multiple fascicles and blood vessels, forming the nerve.

  • Types of Nerves:

    • Mixed Nerves: Contain both sensory and motor neurons.

    • Sensory Nerves: Contain sensory neurons only.

    • Motor Nerves: Contain motor neurons only. (Patton, 2022)

  • Tracts: Bundles of nerve fibers found in the CNS, lacking connective tissue coverings.

  • White Matter vs Grey Matter:

    • White Matter: Composed of myelinated nerve fibers (CNS: tracts; PNS: nerves).

    • Grey Matter: Composed of cell bodies and unmyelinated fibers (CNS: nuclei; PNS: ganglia). (Patton, 2022)

Repair of Nerve Fibers

  • Notable Facts:

    • Mature neurons typically cannot divide; thus damage may be permanent.

    • However, if certain conditions are met, PNS can repair itself:

    • Damage is not extensive.

    • Schwann cells remain intact.

    • No significant scarring is present.

  • Stages of Repair in PNS Axon:

    1. Distal portion of the axon & myelin sheath degenerate.

    2. Macrophages remove debris from the injury site.

    3. Remaining Schwann cells and endoneurium form a tunnel.

    4. New Schwann cells grow within the tunnel to help regrow the axon.

    5. The cell body synthesizes necessary proteins to extend portions of the axon.

    6. Axon sprouts appear.

    7. If a sprout reaches the tunnel, its growth rate increases.

  • CNS Repair Considerations: Similar repair is less likely to occur in the CNS. (Patton, 2022)

Clinical Correlation: Reducing Damage to Nerve Fibers

  • Indicates that crushing and bruising are the primary causes of spinal cord injuries, frequently resulting in paralysis.

  • Methylprednisolone: A promising anti-inflammatory drug that reduces additional damage when given in high doses within 8 hours of injury. (Patton, 2022)

Conditions Affecting Nervous System Cells

  • Most conditions primarily involve glial cells, not neurons.

  • Neuroma: Tumors arising in the nervous system, with glioma being common and often treated with difficulty due to their deep location within the brain. If malignant, they are frequently secondary tumors due to metastasis. (Patton, 2022)

Tumors in CNS

  • Astrocytoma: A slow-growing tumor, typically appearing in the 4th decade of life.

  • Glioblastoma Multiforme: Highly malignant, spreads within white matter, with an average survival rate of less than one year.

  • Ependymoma: The most common glioma in children, can cause cerebrospinal fluid (CSF) obstruction, leading to pressure damage.

  • Oligodendroglioma: Originates in the anterior part of the brain. (Patton, 2022)

Tumors in PNS

  • Acoustic Neuroma: A lesion associated with the sheath of Schwann cells surrounding the 8th cranial nerve.

  • Neurofibromatosis: A genetic disorder characterized by numerous neuromas and skin spots, usually inherited; while benign, they can cause disfigurement. (Patton, 2022)

References

  • Human Anatomy and Physiology Society. (2019). HAPS A&P Learning Outcomes [online]. https://www.hapsweb.org/haps-learning-outcomes/haps-ap-learning-outcomes-los/

  • Patton, K. T., Bell, F. B., Thompson, T., & Williamson, P. (2022). Anatomy & Physiology (11th ed.). Elsevier.