Nervous Tissue – Comprehensive Study Notes

Neurons

General Features

• Basic functional units of the nervous system; specialized for rapid transmission of electrochemical signals.

• Cell body (soma/perikaryon)

  • Large, centrally located nucleus with a prominent nucleolus, indicating high metabolic activity.

  • Abundant organelles crucial for protein synthesis and energy production:

    • Numerous mitochondria provide ATP for active processes like ion pumping and neurotransmitter synthesis.

    • Extensive rough ER and free ribosomes form dense clusters known as Nissl bodies (chromatophilic substance), responsible for the synthesis of proteins, especially neurotransmitters and membrane components. Their presence gives gray matter its characteristic appearance.

  • Cytoskeleton provides structural support and transports materials:

    • Neurofilaments (intermediate filaments) and neurotubules (microtubules) are bundled together as neurofibrils, extending into the dendrites and axon, providing structural support and maintaining cell shape.

• Dendrites

  • Typically short, highly branched processes resembling a tree, extending from the cell body.

  • Characterized by numerous small, thorny projections called dendritic spines, which significantly increase the surface area (≈ 80-90\% of neuronal surface area) for receiving synaptic input from other neurons.

  • They receive incoming signals and transmit them towards the cell body.

• Axon (nerve fiber)

  • A single, long, slender process that extends from the cell body, specialized for conducting action potentials (nerve impulses) away from the cell body.

  • The cytoplasm within the axon is called axoplasm, which contains neurofibrils, neurotubules, mitochondria, and vesicles.

  • The plasma membrane surrounding the axoplasm is the axolemma.

  • Key regions:

    • Initial segment: The unmyelinated part of the axon directly adjacent to the axon hillock, where action potentials are typically initiated due to a high concentration of voltage-gated sodium channels.

    • Axon hillock: A cone-shaped region of the cell body from which the axon originates; integrates inhibitory and excitatory signals to determine if an action potential will be generated.

    • Collaterals: Side branches that may extend from the main axon, allowing a single neuron to communicate with multiple target cells.

    • Telodendria: Fine extensions at the end of the axon and its collaterals, which terminate in specialized structures.

    • Axon (synaptic) terminals (also called synaptic boutons or terminal buttons): Knob-like swellings at the ends of telodendria containing neurotransmitters packaged in synaptic vesicles, specialized for transmitting signals across synapses.

  • Axonal (axoplasmic) transport: The active process by which materials are transported along the axon's neurotubules.

    • Anterograde transport: Movement of substances (e.g., neurotransmitters, enzymes, organelles) from the cell body towards the axon terminals, powered by the motor protein kinesin.

    • Retrograde transport: Movement of substances (e.g., recycled materials, growth factors, pathogens) from the axon terminals back towards the cell body, powered by the motor protein dynein.

Structural Classification

• Neurons are classified based on the number of processes extending from the cell body:

  • Anaxonic neurons: Small neurons where dendrites and axons are structurally indistinguishable; found in the brain (e.g., in the retina) and special sense organs, providing complex processing capabilities.

  • Bipolar neurons: Have two processes extending from the cell body — one dendrite and one axon, typically originating opposite each other. These are rare and primarily found in special sensory organs like the retina of the eye, the olfactory epithelium of the nose, and the cochlea of the inner ear.

  • Unipolar (pseudounipolar) neurons: Have a single, short process that emerges from the cell body and then divides into two longer branches: one acts as a dendrite (peripheral process) and the other as an axon (central process). The dendrite and axon are fused near the cell body. Most sensory neurons of the peripheral nervous system (PNS) are unipolar.

  • Multipolar neurons: The most common type of neuron in the central nervous system (CNS), characterized by one axon and two or more dendrites extending from the cell body. All somatic motor neurons controlling skeletal muscles, and most interneurons are multipolar.

Functional Classification

• Neurons are classified based on the direction in which they transmit nerve impulses relative to the CNS:

• Sensory (afferent) neurons:

  • Primarily unipolar (though some bipolar in special senses).

  • Transmit sensory information from receptors in the periphery towards the CNS.

  • Subdivided into somatic sensory (from skin, muscles, joints) and visceral sensory (from internal organs).

  • Receptor types based on stimulus location:

    • Interoceptors: Monitor internal systems (e.g., digestive, respiratory, cardiovascular, urinary, reproductive systems). They detect sensations of stretch, deep pressure, and pain from viscera.

    • Exteroceptors: Monitor the external environment. They detect touch, temperature, pressure, and the complex senses of sight, smell, and hearing.

    • Proprioceptors: Monitor the position and movement of skeletal muscles and joints, providing information about body posture and limb position.

• Motor (efferent) neurons:

  • Multipolar neurons that transmit commands from the CNS to effector organs (muscles and glands).

  • Somatic motor neurons: Part of the Somatic Nervous System (SNS), these neurons directly innervate and control skeletal muscle contractions (voluntary movements).

  • Visceral motor neurons: Part of the Autonomic Nervous System (ANS), these neurons innervate smooth muscle, cardiac muscle, glands, and adipose tissue (involuntary control). Their pathway typically involves two neurons: preganglionic fibers (originating in the CNS) and postganglionic fibers (originating in autonomic ganglia, outside the CNS, and innervating the effector).

• Interneurons (association neurons):

  • Predominantly multipolar neurons located entirely within the CNS (brain and spinal cord) and autonomic ganglia.

  • They form complex neural circuits, connecting sensory and motor neurons.

  • Responsible for processing and distributing sensory information, coordinating motor activity, and are crucial for higher-order CNS functions such as memory, learning, planning, and decision-making.

Neuroglia (Glial Cells)

• Non-neuronal cells in the nervous system that provide support, nourishment, and protection for neurons, greatly outnumbering neurons.

CNS Glia

• Astrocytes:

  • Star-shaped cells, making up the largest and most diverse population of glial cells (non-neuronal) in the CNS.

  • Their cytoplasmic processes radiate outward, contacting both neurons and capillaries.

  • Functions:

    • Maintain blood–brain barrier (BBB): Their processes surround CNS capillaries, inducing endothelial cells to form tight junctions, which regulate the passage of substances from the blood into the brain tissue, protecting the CNS from harmful substances.

    • Provide a 3-D structural framework for CNS neurons, acting as a scaffold for neuronal migration during development and maintaining their spatial relationships.

      Repair tissue post-injury: Form glial scars that help contain damage but can also impede axonal regrowth.

      Guide neuron development: Play a crucial role in directing the growth and interconnection of developing neurons.

      Ependymal cells
      – Form epithelium that lines central canal( with csf) of spinal cord
      and ventricles of brain
      – Produce and monitor cerebrospinal fluid (CSF)
      – Have cilia that help circulate CSF

    • Oligodendrocytes – Have small cell bodies with few processes – Many cooperate to form a myelin sheath

    • • Myelin insulates myelinated axons

    • Increases speed of action potentials

    • Makes nerves appear white

    • Internodes—myelinated segments of axon
      – Nodes (nodes of Ranvier) lie between internodes
      • Where axons may branch
      – White matter
      • Regions of CNS with many myelinated axons
      – Gray matter of CNS
      • Contains unmyelinated axons, neuron cell bodies,
      and dendrites

    • Microglia
      – Smallest and least numerous neuroglia
      – Have many fine-branched processes
      – Migrate through nervous tissue
      – Clean up cellular debris, wastes, and pathogens

    • Neuroglia of the PNS
      – Insulate neuronal cell bodies and most axons
      – Two types
      • Satellite cells
      • Schwann cells

    • Neuroglia of the PNS
      – Satellite cells
      • Surround ganglia (clusters of neuronal cell bodies)
      • Regulate interstitial fluid around neurons
      – Schwann cells (neurolemmocytes)
      • Form myelin sheath or indented folds of plasma
      membrane around axons
      • Neurolemma—outer surface of Schwann cell
      • A myelinating Schwann cell sheaths only one axon
      – Many Schwann cells sheath entire axon