SD

Neural Histology Study Notes

Introduction to Neural Histology

  • Overview of muscle tissue superpower: ability to shorten and generate tension, enabling movement and force generation.

  • Neural tissue superpower: rapid and complex communication throughout the body, essential for coordinating all bodily functions.

Functions of the Nervous System

  • Sensation: Continuously monitors both the internal environment (e.g., blood pressure, internal organ status) and the external environment (e.g., temperature, touch, light, sound) through specialized sensory receptors.

  • Integration: Processes and interprets diverse sensory information received from various sources (eyes, ears, skin, internal organs, etc.) within the CNS. This involves filtering, prioritizing, and analyzing data to form a coherent perception of the environment.

  • Memory: Compares current sensory experiences and integrated information with past experiences stored in memory. This function is crucial for learning, recognition, and adapting behavior.

  • Prediction and Decision-Making: Involves evaluating potential future outcomes based on current sensory input, integrated information, and memories, leading to conscious or subconscious choices and behavioral adjustments.

  • Response Control: Commands voluntary actions, such as skeletal muscle movements, and involuntary actions, like regulating cardiac and smooth muscle activity and controlling glandular secretions, to maintain homeostasis or react to stimuli.

Anatomical Subdivisions of the Nervous System

  • Central Nervous System (CNS):

    • Composed of the brain (the primary control center, responsible for higher cognitive functions) and the spinal cord (a major pathway for information between the brain and the rest of the body, also capable of reflex actions).

    • Functions include information integration, complex sensory processing, and higher brain functions such as memory, emotions, language, and consciousness.

  • Peripheral Nervous System (PNS):

    • Everything outside the CNS, consisting of nerves and ganglia that serve as communication links between the CNS and the rest of the body.

    • Peripheral Nerves: Bundles of axons that connect the CNS to sensory receptors, muscles, and glands in the limbs and organs.

    • Spinal Nerves: Typically 31 pairs that originate from the spinal cord, innervating the trunk and limbs.

    • Cranial Nerves: Typically 12 pairs that emerge directly from the brain or brainstem, innervating the head, neck, and some visceral organs.

Types of Neurons

  • Sensory Neurons (Afferent Neurons): Carry information (sensory input) from sensory receptors in the periphery towards the CNS. They detect changes in the internal and external environment (e.g., sensation from skin, blood pressure, taste).

  • Motor Neurons (Efferent Neurons): Carry motor commands from the CNS to effector organs (skeletal muscle, smooth muscle, glands) to initiate a response.

    • Different types of motor output:

      • Somatic Motor Neurons: Provide voluntary control over skeletal muscles, responsible for conscious movements.

      • Visceral Motor Neurons (Autonomic Motor Neurons): Provide involuntary control over cardiac and smooth muscles, as well as glands, regulating internal organ functions.

  • Interneurons (Association Neurons): Located entirely within the CNS, they connect sensory and motor neurons, facilitating integration and complex processing. They are the most numerous type of neuron.

Functional Subdivisions of the Nervous System

  • Somatic Nervous System (SNS):

    • Deals with sensations received from the skin, muscles, and joints; includes both conscious inputs (e.g., pain, touch, temperature, proprioception) and subconscious inputs (e.g., reflex actions, muscle spindle activity).

    • Output primarily leads to voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS):

    • Regulates the involuntary functions of internal organs (viscera) and glands, maintaining internal homeostasis. It operates largely subconsciously.

    • Comprises:

      • Sympathetic Nervous System: Often termed the "fight or flight" system; increases heart rate, dilates pupils, inhibits digestion, mobilizes energy reserves, preparing the body for stressful situations.

      • Parasympathetic Nervous System: Often termed the "rest and digest" system; decreases heart rate, constricts pupils, stimulates digestion, conserves energy, promoting relaxation and vegetative functions.

    • Inputs often come from subconscious sensations (e.g., stretch receptors in the gut, chemoreceptors detecting blood chemistry).

Neural Tissue Components

  • Neurons: The fundamental functional units of the nervous system, specialized for rapid communication through the generation and transmission of electrical signals (action potentials) and chemical signals (synaptic transmission).

  • Neuroglia (Glial Cells): Support neurons and perform various crucial functions, vastly outnumbering neurons.

    • Astrocytes: Star-shaped cells that provide structural support, regulate the chemical environment by buffering ions (especially potassium) and neurotransmitters, form part of the blood-brain barrier, and assist in nutrient transport to neurons. They are critical for functional synapses.

    • Oligodendrocytes: Found in the CNS, these cells form the myelin sheath around multiple axons, which insulates the axons and significantly increases the speed of action potential conduction.

    • Microglia: Act as the immune cells of the CNS, functioning as phagocytes that remove cellular debris, waste products, and pathogens, playing a role in inflammation and neuroprotection.

    • Ependymal Cells: Epithelial-like cells that line the fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid (CSF) and its circulation through their cilia.

    • Schwann Cells: Found in the PNS, these cells form the myelin sheath around a single axon, or enclose multiple unmyelinated axons, similar to oligodendrocytes in function but in the peripheral system.

    • Satellite Cells: Surround neuron cell bodies in peripheral ganglia, providing structural support and regulating the chemical environment around the neurons.

Neuron Structure

  • Dendrites: Branching, tree-like extensions that serve as the primary receptive input structures of the neuron. They receive graded electrical signals (postsynaptic potentials) proportional to the strength of the incoming stimulus through neurotransmitter binding.

  • Cell Body (Soma): The metabolic center of the neuron, containing the nucleus with a prominent nucleolus, and most of the neuron's organelles (e.g., rough endoplasmic reticulum, Golgi apparatus). It integrates incoming signals.

  • Axon: A single, long extension that conducts action potentials (all-or-none electrical impulses) away from the cell body towards other neurons or effector cells. It begins at the axon hillock and the initial segment, which is typically the site of action potential generation.

    • Axons conduct signals in one direction, from the cell body towards the synaptic terminals.

    • Can be myelinated (covered by a myelin sheath) or unmyelinated. Myelinated axons transmit signals much faster than unmyelinated ones due to saltatory conduction.

  • Axon Terminals (Synaptic Terminals): The ends of the axon where neurotransmitters are released into the synaptic cleft to communicate with target cells.

  • Axon Collaterals: Side branches that extend from the main axon, allowing a single neuron to influence multiple target cells simultaneously or communicate with different regions.

Types of Neurons Based on Structure

  1. Multipolar Neurons:

    • The most common type of neuron in the CNS and PNS; characterized by having one axon and multiple dendrites extending from the cell body.

    • Include most motor neurons (regulating muscle contraction) and interneurons (involved in complex CNS processing).

  2. Bipolar Neurons:

    • Relatively rare; characterized by having one axon and one dendrite extending from opposite ends of the cell body.

    • Typically found in special sensory organs (e.g., retina of the eye for vision, inner ear for balance and hearing, olfactory epithelium for smell).

  3. Pseudo-unipolar Neurons (Unipolar Neurons):

    • Characterized by having a single process that emerges from the cell body and then bifurcates into two branches: one extending to the periphery (acting as a dendrite) and one extending into the CNS (acting as an axon).

    • Primarily function as general sensory neurons, conveying sensation (e.g., touch, pain, temperature, proprioception) from the periphery to the CNS.

Ganglia

  • Defined as aggregates or clusters of neuron cell bodies located in the PNS.

  • Sensory Ganglia: Contain the cell bodies of pseudo-unipolar neurons responsible for sensory input (e.g., dorsal root ganglia).

  • Motor Ganglia: Contain the cell bodies of multipolar neurons involved in autonomic control (e.g., sympathetic chain ganglia, parasympathetic ganglia) which regulate involuntary functions.

The Blood-Brain Barrier (BBB)

  • A highly selective semipermeable barrier that separates the circulating blood from the brain extracellular fluid in the CNS electrodes. It is comprised of specialized endothelial cells with extremely tight junctions, surrounded by the basal lamina and the end-feet (astrocytic foot processes) of astrocytes.

  • Controls the passage of substances from the blood into the CNS, protecting the brain from toxins and pathogens, while allowing essential nutrients to pass.

  • Blood-CSF Barrier: Created by ependymal cells of the choroid plexus in the brain ventricles, which have tight junctions. This barrier regulates the composition of the cerebrospinal fluid (CSF), ensuring a stable environment for the brain and spinal cord.

White Matter vs Gray Matter

  • Gray Matter: Regions of the CNS primarily composed of neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It is involved in synaptic transmission, integration of information, and processing.

    • Its darker color is due to the lack of myelin and the presence of neuron cell bodies and capillaries.

  • White Matter: Regions of the CNS primarily composed of myelinated axons, which are organized into tracts or pathways. It functions to carry signals rapidly over longer distances between different gray matter areas.

    • Its lighter color is due to the high lipid content of the myelin sheaths.

  • Functional Terms (CNS):

    • Cortex: An outer layer of gray matter found in the cerebrum and cerebellum, involved in higher-level processing.

    • Nuclei: Distinct collections or clusters of neuronal cell bodies located deep within the gray matter of the CNS, often associated with specific functions.

    • Tracts/Trajectories: Bundles of myelinated axons (white matter) in the CNS that transmit specific directional signals between different brain regions or between the brain and spinal cord.

Peripheral Nerve Structure

  • Peripheral nerves are complex bundles composed of numerous sensory (afferent) and motor (efferent) nerve fibers (axons), as well as blood vessels and various layers of connective tissue.

  • Epineurium: The outermost, tough fibrous connective tissue sheath that encloses the entire peripheral nerve, providing structural support and protection.

  • Perineurium: A layer of connective tissue that surrounds individual bundles of nerve fibers called fascicles, forming a protective barrier that maintains the internal environment of the fascicle.

  • Endoneurium: A delicate layer ofloose connective tissue that encloses individual axons (nerve fibers), surrounded by their Schwann cells (myelinated or unmyelinated). It plays a role in nerve regeneration after injury and the features of its tissue type are apparent in histological sections.