Introduction to the Nervous System, Neurons, and Neuroglia

General Overview and Functions of the Nervous System

  • Perception and Personality:

    • The nervous system defines individual identity, personality, likes, dislikes, and behavioral preferences by processing both external sensory input and internal physiological signals.
    • Controls how individuals perceive, experience, and react to their environment (such as preferences for spicy foods or scary movies).
    • Regarded as the seat of consciousness, personality, learning, and memory.
    • Directs all voluntary movements, speech, and conscious actions.

  • Interaction with the Endocrine System:

    • Functions in close coordination with the endocrine system to maintain internal regulation and chemical signaling.
    • Coordinates endocrine responses such as the release of releasing hormones, stimulating hormones, testosterone, growth hormone, and insulin.

  • Homeostatic Control:

    • Primary goal of the nervous system is maintaining homeostasis.
    • Homeostasis is dynamic rather than static, characterized by continuous fluctuations around set points.
    • Key physiological parameters regulated include:
    • Dynamic blood pressure regulation.
    • Sleep-wake cycle management.
    • Core body temperature maintenance.
    • Respiratory rate adjustments.
    • Blood pH regulation.

  • Functional Categories:

    • Sensory (Afferent) Function:
    • Gathers information concerning internal and external environments.
    • Internal Environment: Evaluates internal tissue integrity, body temperature, hunger status, and fluctuations in blood pH.
    • Integrative Function:
    • Processes, analyzes, and interprets sensory inputs within the central nervous system to determine appropriate responses.
    • Approximately 99%99\% of sensory data gathered is filtered out and discarded as unimportant; only critical information triggers a motor response.
    • Motor (Efferent) Function:
    • Carries out decisions formulated through integration by sending signals to effector organs, muscles, or glands.

Structural Organization of the Nervous System

  • Central Nervous System (CNS):

    • Located within the dorsal body cavity; comprised of the brain and spinal cord.
    • Acts as the central integration, decision-making, and command center.
    • Contains approximately 1×1091 \times 10^9 (1 billion1\text{ billion}) neurons or nerve cells.
    • Spinal Cord:
    • Serves as the primary communication path between the brain and the body below the head and neck.
    • Merges with the brain at the foramen magnum (the large opening at the base of the skull).
    • Extends downward through the vertebral foramina of the spinal column and terminates at the level of the first or second lumbar vertebra (L1–L2L1\text{--}L2).

  • Peripheral Nervous System (PNS):

    • Composed of all nervous structures situated outside the brain and spinal cord.
    • Consists of bundles of long neuron axons packed together with supportive blood vessels.
    • Relays incoming sensory signals toward the CNS and outgoing motor signals away from the CNS.
    • Structural elements:
    • Cranial Nerves: 1212 pairs originating directly from or traveling to the brain.
    • Spinal Nerves: 3131 pairs originating from or entering the spinal cord to supply peripheral body regions.

  • Subdivisions of the Peripheral Nervous System:

    • Sensory (Afferent) Division:
    • Relays information toward the CNS.
    • Somatic Sensory Division: Transmits inputs from skeletal muscles, skin, bones, and joints ("soma" referring to body/muscle structures), as well as special senses (vision, hearing, taste, smell, and balance/equilibrium).
    • Visceral Sensory Division: Transmits inputs from internal thoracic and abdominal organs (viscera), including the heart, lungs, stomach, intestines, kidneys, and urinary bladder (e.g., monitoring stretch or pressure in the bladder wall).
    • Motor (Efferent) Division:
    • Transmits output signals away from the CNS to effector tissues.
    • Somatic Motor Division: Provides voluntary motor output to control skeletal muscle contraction.
    • Visceral Motor Division (Autonomic Nervous System / ANS): Provides involuntary motor output to thoracic and abdominal viscera, cardiac muscle, smooth muscle, and exocrine/endocrine glands.
      • Sympathetic Nervous System: Active state mobilization system ("fight or flight"); prepares the body for physical activity or acute stressors.
      • Parasympathetic Nervous System: Energy conservation system ("rest and digest"); maintains baseline function during non-stressful conditions.

Structure and Physiology of Neurons

  • General Characteristics:

    • Excitable cells capable of generating and propagating action potentials.
    • Responsible for processing sensory input, performing integration, and executing motor functions.
    • Exceptionally long-lived cells that are amitotic (incapable of cell division/mitosis).
    • Resting membrane potential baseline is standardly set at −70 mV-70\,\text{mV} (referred to as the core golden baseline number for cellular action potentials).
    • Sizes vary drastically: lengths range from 1 mm1\,\text{mm} in the CNS to over 1 m1\,\text{m} extending down the extremities.

  • Anatomical Components of a Neuron:

    • Cell Body (Soma):
    • Ranges from 5 μm5\,\mu\text{m} to 100 μm100\,\mu\text{m} in diameter.
    • Serves as the metabolic center housing key organelles: nucleus, prominent rough endoplasmic reticulum (Nissl bodies), Golgi apparatus, and dense populations of mitochondria.
    • Dendrites:
    • Short, highly branched, tree-like processes protruding from the soma.
    • Function as the primary receptive fields that accept chemical inputs (neurotransmitters) or sensory stimuli, converting them into electrical impulses conveyed toward the soma.
    • Axon Hillock:
    • The conical region where the axon originates from the soma.
    • Functions as the critical "gatekeeper" or trigger zone; integrates incoming action potential voltages to determine whether an electrical signal is propagated down the axon (a prime target for laboratory identification).
    • Axon:
    • Single, elongated process conducting nerve impulses away from the cell body.
    • Enclosed by the axolemma (the specialized plasma membrane of the axon).
    • Contains axoplasm (the cytoplasm of the axon), through which motor proteins move organelles and chemical substances toward or away from the cell body.
    • Axon Collaterals and Telodendria:
    • Side branches along the axon pathway are known as axon collaterals.
    • Distal end splits into fine terminal branches called telodendria.
    • Axon Terminals (Synaptic Knobs):
    • Enlarged swollen endings of telodendria that communicate with target cells (neurons, muscle cells, or gland cells).
    • Contain membrane-bound vesicles filled with neurotransmitter chemicals.
    • Myelin Sheath:
    • Multi-layered lipid/protein covering formed around axons by neuroglial cells (neurolemmocytes/Schwann cells in PNS, oligodendrocytes in CNS).
    • Insulates the axon membrane, allowing action potentials to jump rapidly between unmyelinated gaps rather than traveling continuously across every segment of membrane.

Classifications of Neurons

  • Structural Classifications:

    • Multipolar Neurons:
    • Represent over 99%99\% (and conservatively 88–90%88\text{--}90\%) of all body neurons.
    • Feature a single axon and multiple branching dendrites extending from the soma.
    • Example: Spinal motor neurons and CNS interneurons.
    • Bipolar Neurons:
    • Possess two distinct processes extending from opposite sides of the soma: one axon and one dendrite.
    • Found in specialized sensory organs within the PNS, specifically the retina of the eye and the olfactory epithelium of the nasal cavity.
    • Pseudo-unipolar Neurons:
    • Possess a single short process extending from the soma that bifurcates into two long axonal processes.
    • One branch extends peripherally to sensory receptors in skin, joints, or muscles; the other branch extends centrally into the spinal cord.
    • Mediates somatic sensory sensations including touch, pressure, and pain.

  • Functional Classifications:

    • Sensory (Afferent) Neurons:
    • Conduct nerve impulses from sensory receptors located in peripheral tissues toward the CNS.
    • Interneurons (Association Neurons):
    • Confined entirely within the CNS; form complex internal networks that analyze, store, and integrate incoming sensory data before formulating responses.
    • Motor (Efferent) Neurons:
    • Conduct nerve impulses away from the CNS soma out to peripheral effector tissues (muscles and glands).

Neuroglia (Supporting Cells)

  • General Features of Neuroglia:

    • Non-excitable supporting cells that outnumber neurons and maintain structural integrity.
    • Functions include anchoring neurons, maintaining the chemical environment, cleaning debris, protecting against pathogens, and facilitating metabolic support.
    • Unlike neurons, neuroglial cells retain the ability to divide rapidly via mitosis throughout life.
    • Total of 66 functional types: 44 in the CNS and 22 in the PNS.

  • Central Nervous System Neuroglia (44 Types):

    • Astrocytes:
    • Star-shaped neuroglia featuring a central body and numerous radiating processes.
    • Primary Functions:
      • Anchor neurons directly to nearby capillary blood vessels.
      • Facilitate transport of essential nutrients and dissolved gases between blood vessels and neurons.
      • Regulate the extracellular environment by removing excess extracellular potassium ions (K+K^+) and recycling spilled neurotransmitters.
      • Assist in the structural formation and maintenance of the Blood-Brain Barrier (BBB).
      • Repair damaged neural tissue; however, rapid proliferation can form glial scar tissue that impedes functional neuron axon regeneration.
    • Clinical Correlation (Astrocytoma):
      • Primary brain tumor resulting from the uncontrolled mitotic division of astrocytes.
      • Example case: An individual developed an astrocytoma; after surgical resection and chemotherapy, the tumor recurred 2 years2\,\text{years} later, illustrating the high malignancy and rapid cell division rate characteristic of glial tumors.
    • Oligodendrocytes:
    • Possess radiating processes with flattened ends that wrap around segments of multiple CNS axons.
    • Produce and form the myelin sheath around axons located within the brain and spinal cord.
    • Microglia:
    • Small, highly branched cells that act as resident wandering phagocytes in response to CNS inflammation or injury.
    • Ingest cellular debris, dead tissue, and foreign infectious agents (e.g., viral or bacterial invaders).
    • Ependymal Cells:
    • Ciliated epithelial-like cells lining the internal cavities (ventricles and central canal) of the CNS.
    • Produce, monitor, and assist in circulating cerebrospinal fluid (CSF) via ciliary movement.

  • Peripheral Nervous System Neuroglia (22 Types):

    • Neurolemmocytes (Schwann Cells):
    • Wrap around individual peripheral axon segments in the PNS to form myelin sheaths (functionally equivalent to oligodendrocytes in the CNS).
    • Satellite Cells:
    • Flattened cells that surround and envelop neuron cell bodies within peripheral ganglia.
    • Provide structural support, enclose soma, and regulate the surrounding chemical microenvironment.