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

    • Differentiate between the motor (efferent) and sensory (afferent) components of the nervous system.

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

    • Identify and describe the major components of a neuron.
      (Source: HAPS, 2019)

Function & Organization

  • The nervous system is organized to:

    • Detect changes, both internal and external.

    • Evaluate the information received.

    • Initiate an appropriate response.

  • It is subdivided based on location:

    • Central Nervous System (CNS)

    • Peripheral Nervous System (PNS)
      (Source: Patton, 2022)

Organization of Nervous System: Central (CNS) & Peripheral (PNS)

  • Central Nervous System (CNS)

    • Consists of the brain and spinal cord.

    • Functions:

    • Integrates sensory information.

    • Evaluates the information.

    • Initiates an outgoing response.

  • Peripheral Nervous System (PNS)

    • Consists of all nerves located in the outer regions.

    • Comprises:

    • 12 pairs of cranial nerves.

    • 31 pairs of spinal nerves.
      (Source: Patton, 2022)

Organization of Nervous System: Afferent & Efferent

  • Afferent Division

    • Comprised of all incoming sensory pathways.

  • Efferent Division

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

Organization of Somatic Nervous System (SNS)

  • Somatic Nervous System (SNS)

    • Somatic Motor:

    • Carries motor information to skeletal muscles.

    • Somatic Sensory:

    • Carries sensory information to the CNS.
      (Source: Patton, 2022)

Organization of Autonomic Nervous System (ANS)

  • Autonomic Nervous System (ANS)

    • Efferent (motor) pathways to:

    • Smooth and cardiac muscles.

    • Glands.

    • Adipose and other tissues.

    • Divided into two efferent divisions:

    1. Sympathetic Division:

      • Known as the "fight or flight" system.

    2. Parasympathetic Division:

      • Known as the "rest and repair" system.
        (Source: Patton, 2022)

Organization: Autonomic (ANS) and Enteric Nervous System (ENS)

  • Afferent Division of ANS

    • Comprises visceral sensory pathways that relay information to autonomic centers in the CNS.

  • Enteric Nervous System (ENS)

    • A special division of the ANS.

    • Location:

    • Found within the walls of the intestines.

    • Function:

    • Regulates digestion.

    • Possesses some independent integration capabilities.
      (Source: Patton, 2022)

Somatic & Autonomic Nervous Systems

  • Classification within the overall nervous system:

    • Nervous System

    • Central (CNS):

      • Brain and spinal cord.

      • Somatic integration centers.

    • Peripheral (PNS):

      • Somatic sensory division (afferent) (cranial and spinal nerves).

      • Somatic motor division (efferent).

      • Autonomic nervous system (ANS):

      • Autonomic integration centers.

      • Visceral sensory division (afferent).

      • Sympathetic division (efferent) and Parasympathetic division (efferent).

    • Somatic effectors (skeletal muscle) respond to stimuli.

    • Autonomic effectors (cardiac and smooth muscle, glands, adipose tissue) respond to stimuli.
      (Source: Patton, 2022)

Neuroglial (Glia) Cells

  • There are six major types of glial cells.

  • Glial cells make up most of CNS tissue and support neurons.

  • Types of Glia Found in CNS:

    1. Astrocytes:

    • Most common and largest type.

    • Star-shaped with extensions connecting capillaries and neurons.

    • Transfers nutrients from blood to neurons and forms the Blood-Brain Barrier.

    1. Microglia Cells:

    • Small and stationary.

    • In inflamed brain tissue, they enlarge, move, and perform phagocytosis.

    1. Ependymal Cells:

    • Thin sheets of epithelial-like cells lining fluid-filled cavities in the CNS.

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

    1. Oligodendrocytes:

    • Smaller than astrocytes with fewer processes.

    • Hold nerve fibers together and produce the myelin sheath, with gaps known as Nodes of Ranvier.

  • Types of Glia Found in PNS:

    1. Schwann Cells (Neurolemmocytes):

    • Support nerve fibers and form the myelin sheath by wrapping around the nerve.

    1. Satellite Cells:

    • Schwann cells that cover and support neuron cell bodies in the PNS.
      (Source: Patton, 2022)

Neurons

  • Definition:

    • Neurons are excitable cells that initiate and conduct impulses.

  • Components of Neurons:

    • Cell Body:

    • Contains all organelles of the neuron.

    • Dendrites:

    • Conduct nerve signals toward the cell body.

    • Axon:

    • Conducts nerve impulses away from the cell body and may be myelinated.

    • Cytoskeleton:

    • Facilitates rapid transport of small molecules.
      (Source: Patton, 2022)

Functional Regions of Neurons

  • Input Zone:

    • Comprised of dendrites and cell body, where inputs are received.

  • Summation Zone:

    • Located at the axon hillock, where inputs are integrated to determine if an impulse is propagated.

  • Conduction Zone:

    • The axon, along which the nerve impulse is conducted.

  • Output Zone:

    • Comprised of telodendrions and synaptic knobs, where neurotransmitters are released to communicate with other neurons or effectors.
      (Source: Patton, 2022)

Structural Classification of Neurons

  • Based on the number of processes:

    • Multipolar Neurons:

    • Have multiple dendrites and one axon.

    • Bipolar Neurons:

    • Have one dendrite and one axon.

    • Unipolar Neurons (Pseudo-unipolar):

    • Have one process that branches into peripheral and central processes.
      (Source: Patton, 2022)

Functional Classification of Neurons

  • Neurons are categorized into three functional classes:

    1. Afferent Neurons (Sensory Neurons):

    • Carry signals towards the CNS from sensory receptors.

    1. Efferent Neurons (Motor Neurons):

    • Carry signals away from the CNS to effectors (muscles or glands).

    1. Interneurons:

    • Found within the CNS; connect afferent and efferent neurons.

  • Reflex Arc:

    • A feedback loop often comprising three neurons:

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

Clinical Correlation: Blood-Brain Barrier (BBB)

  • The BBB maintains a stable environment for the brain.

  • Astrocytes wrap around capillaries in the brain, contributing to the formation of the BBB.

  • Important drugs for diseases such as Parkinson's must cross the BBB.

  • Dopamine:

    • Helps alleviate Parkinson's symptoms but cannot cross the BBB.

  • Levodopa:

    • Can cross the BBB and is used by brain cells to synthesize dopamine.
      (Source: Patton, 2022)

Review Question Section 1

  • Question: help form the blood-brain barrier.

    • a. Oligodendrocytes

    • b. Microglial cells

    • c. Astrocytes

    • d. Schwann cells

    • e. Satellite cells

    • Correct Answer: c. Astrocytes
      (Source: 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 and sensory elements and connective tissue (CT) wrappings.

    • Describe how nerves may be repaired.
      (Source: HAPS, 2019)

Reflex Arc

  • A reflex arc is a feedback loop with a common structure:

    • Three-Neuron Arc:

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

Reflex Arc Variations

  • Contralateral Reflex Arc:

    • Involves receptors and effectors located on opposite sides of the body.

  • Intersegmental Reflex Arcs:

    • Involve multiple segments of the spinal cord.
      (Source: Patton, 2022)

Nerves & Tracts

  • Nerves:

    • Bundles of nerve fibers in the PNS.

  • Structure of Nerves:

    • Endoneurium:

    • Connective tissue surrounding each individual nerve fiber.

    • Perineurium:

    • Connective tissue that ties together fascicles (bundles of nerve fibers).

    • Epineurium:

    • Connective tissue encompassing many fascicles along with blood vessels to form the whole nerve.
      (Source: Patton, 2022)

Types of Nerves

  • Types of Nerve Classifications:

    • Mixed Nerves:

    • Contain both sensory and motor neuron fibers.

    • Sensory Nerves:

    • Contain only sensory fibers.

    • Motor Nerves:

    • Contain only motor fibers.
      (Source: Patton, 2022)

Tracts in CNS

  • Tracts:

    • Bundles of nerve fibers found in the CNS.

    • Lacking connective tissue coverings found in nerves.

    • White Matter:

    • In PNS = myelinated nerves.

    • In CNS = myelinated tracts.

    • Grey Matter:

    • Composed of cell bodies and unmyelinated fibers.

    • In CNS = nuclei; in PNS = ganglia.
      (Source: Patton, 2022)

Repair of Nerve Fibers

  • Mature neurons are incapable of cell division, and damage may lead to permanent effects.

  • The ability to repair depends on:

    • The extent of the damage.

    • The integrity of Schwann cells.

    • The absence of significant scarring.

  • PNS nerve fibers can be repaired if those conditions are met.
    (Source: Patton, 2022)

Stages of Repair of Axon in PNS

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

  2. Macrophages remove debris.

  3. Remaining Schwann cells and endoneurium form a tunnel for guiding regrowth.

  4. New Schwann cells grow within the tunnel to facilitate the regrowth of the axon.

  5. The cell body synthesizes the necessary proteins to assist in extending the healthy part of the axon.

  6. Axon sprouts begin to appear.

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

  • Note: Similar repair processes in the CNS are unlikely.
    (Source: Patton, 2022)

Clinical Correlation: Reducing Damage to Nerve Fibers

  • Spinal cord injuries often arise from crushing or bruising, leading to paralysis.

  • A promising anti-inflammatory medication, methylprednisolone, can reduce additional nerve fiber damage.

  • Administration must occur in high doses within 8 hours of the injury for maximum effect.
    (Source: Patton, 2022)

Conditions of Nervous System Cells

  • Many conditions affect glia rather than neurons.

  • Neuroma:

    • Tumors in the nervous system.

    • Glioma:

    • A common type; presents treatment challenges due to deep brain location and potential malignancy; often secondary tumors from metastasis.
      (Source: Patton, 2022)

Tumors in CNS

  • Astrocytoma:

    • Slow-growing, usually diagnosed in the fourth decade of life.

  • Glioblastoma Multiforme:

    • Highly malignant and tends to spread within the white matter, with an average survival of less than 1 year.

  • Ependymoma:

    • The most common type of glioma in children, causing obstruction of cerebrospinal fluid (CSF) and leading to pressure-induced neurological damage.

  • Oligodendroglioma:

    • Typically located in the anterior portion of the brain.
      (Source: Patton, 2022)

Tumors in PNS

  • Acoustic Neuroma:

    • A lesion of the sheath of Schwann cells surrounding the 8th cranial nerve (CN).

  • Neurofibromatosis:

    • A genetic disorder characterized by multiple neuromas and skin spots.

    • Typically inherited, resulting in benign but potentially disfiguring tumors.
      (Source: Patton, 2022)

Review Question Section 2

  • Question: Nerves are found in the nervous system and tracts are found in the nervous system.

    • Correct Answer: peripheral, central
      (Source: 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.