Nerves

Nerve Tissue Study Guide

General Organization of the Nervous System

  • Anatomical Division: The nervous system is an atomically divided into two main parts:

    • Central Nervous System (CNS): Composed of the brain and spinal cord.

    • Peripheral Nervous System (PNS): Includes all cranial nerves, spinal nerves, and ganglia.

  • Functional Division: The nervous system has two major functional divisions:

    1. Sensory Division (Afferent):

    • Somatic-Sensory Input: Perceived consciously (e.g., from eyes, ears, skin, musculoskeletal structures).

    • Visceral-Sensory Input: Not perceived consciously (e.g., from internal organs, cardiovascular structures).

    1. Motor Division (Efferent):

    • Somatic-Motor Output: Controlled consciously/voluntarily (e.g., by skeletal muscle effectors).

    • Autonomic-Motor Output: Not controlled consciously (e.g., by heart or gland effectors).

      • The autonomic nervous system (ANS) has two divisions:

        • Parasympathetic Division: Ganglia within or near effector organs, maintaining normal body homeostasis.

        • Sympathetic Division: Ganglia close to the CNS, controlling emergency and excitement responses.

      • Components in the digestive tract are referred to as the enteric nervous system.

General Concepts of Nervous Tissue

  • Nervous tissue is specialized for modifying membrane electrical potentials to relay signals. Neurons form intricate circuits to:

    • Relay sensory information from various environments.

    • Integrate information among millions of neurons.

    • Transmit signals to muscles and glands.

  • Anatomical Subdivisions:

    1. CNS: Brain and spinal cord.

    2. PNS: Nerves and ganglia (singular: ganglion).

Cells of Nervous Tissue

  • Neurons:

    • Functional units of the nervous system; process information and transmit it.

    • Composed of:

    • Cell Body (Soma): Holds the nucleus and organelles.

    • Dendrites: Form complex circuits and receive information.

    • Axon: Sends information out to target cells.

    • Synapses: Connections with other neurons or effector cells.

  • Supporting Cells (Glial Cells):

    • Provide support, insulation via myelin, and maintenance of homeostasis.

    • Types:

    • Astrocytes: Metabolic support and blood-brain barrier.

    • Oligodendrocytes: Myelinate CNS axons.

    • Microglia: Phagocytic cells in the CNS.

    • Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).

    • Schwann Cells: Myelinate PNS axons.

Staining Techniques for Nervous Tissue

  • Visualization of nervous tissue is challenging; therefore, special stains are used:

    • Golgi Stains: Visualize all processes of an individual neuron but stain few neurons.

    • Nissl Stains: Stain rough endoplasmic reticulum, allowing visualization of cell bodies but not processes.

    • Myelin Stains: Highlight myelinated fibers.

    • H&E Staining: Used for pathology diagnosis.

Structure of a Neuron

  • Parts of Neuron:

    1. Cell Body (Soma): Large, euchromatic nucleus; contains Nissl bodies indicating high protein synthesis.

    2. Dendrites: Numerous extensions for receiving stimuli; dendritic spines form synapses.

    3. Axon: Long singular process that transmits impulses from cell body. Axon hillock is a specialized region for impulse initiation.

  • Axons form terminal boutons, which connect with other neurons or effector cells at synapses.

Action Potential and Nerve Impulses

  • Action Potential: Wave of depolarization initiated when the membrane reaches a certain threshold.

    • Process:

    1. When the threshold is met, voltage-gated Na+ channels open, allowing Na+ influx, causing depolarization (+30 mV).

    2. Na+ channels close and K+ channels open, repolarizing membrane.

    3. This cycle occurs rapidly, allowing action potentials to propagate along the axon.

  • Refractory Period: After an action potential, a brief period before another action potential can occur.

Synapse Structure and Function

  • Function: The synapse alters the postsynaptic membrane potential, facilitating or inhibiting stimulus propagation.

  • Types of Synapses:

    • Axodendritic: Common connection to dendrites.

    • Axosomatic: Connection to cell bodies.

    • Axoaxonic: Connects with another axon.

  • Components of Synapse:

    1. Presynaptic Terminal: Contains synaptic vesicles filled with neurotransmitters.

    2. Synaptic Clef: Gap between pre- and postsynaptic cells.

    3. Postsynaptic Membrane: Contains receptors for neurotransmitters.

  • Neurotransmitter Release Mechanism: Influx of Ca2+ ions promotes the release of neurotransmitters, which bind to postsynaptic receptors causing depolarization or hyperpolarization, depending on the type of synapse.

Neurotransmitters

  • Common Neurotransmitters:

    • Acetylcholine (ACh): Activates muscle contractions.

    • Amino Acids: Include GABA (inhibitory) and Glutamate (excitatory).

    • Monoamines: Include serotonin, dopamine, norepinephrine, and epinephrine, which regulate mood and emotional responses.

    • Neuropeptides: Short chains of amino acids that modulate neural communication.

  • Synaptic Mechanism:

    • Excitatory Synapses: Open Na+ channels, leading to depolarization.

    • Inhibitory Synapses: Open Cl- channels, leading to hyperpolarization.

Types of Neurons by Structure

  • Multipolar Neurons: Most numerous, with one axon and multiple dendrites.

  • Bipolar Neurons: One axon, one dendrite; found in sensory systems.

  • Unipolar Neurons: A single process that bifurcates; common in sensory pathways.

  • Anaxonic Neurons: Lacks a true axon, mostly involved in local circuit regulation.

Supporting Cells in Nervous System

  • CNS Glial Cells: Include astrocytes, oligodendrocytes, microglia, and ependymal cells.

  • PNS Glial Cells: Schwann cells (myelination) and satellite cells (support).

Clinical Implications & Involvement of Nervous Tissue Disorders

  • Numerous disorders affect neurological functions, including Alzheimer’s disease, multiple sclerosis, and peripheral neuropathies that arise from dysfunction or damage to neurons and supporting cells.

Myelin Sheath

  • A lipid-rich sheath formed around axons by Schwann cells in the PNS and oligodendrocytes in the CNS that facilitates faster signal conduction through saltatory conduction around Nodes of Ranvier.

Key Takeaways

  • Understanding the structure and function of neurons and supporting cells is essential for grasping how the nervous system operates and what might go wrong in various disorders.

  • The electrical and chemical signaling in neurons enables complex processes such as cognition, muscle contraction, and reflex responses.

References

  • Extensive information on nervous tissue structure, function, and pathology has been synthesized from various academic resources regarding neuroscience and neuroanatomy. Further study into details will enhance understanding of nervous disease mechanisms and treatment strategies.

Suggested Resources for Diving Deeper

  1. Review neuroanatomy charts and models for a visual understanding of neuron structures.

  2. Study histology slides of nervous tissue for recognizing different cell types and their implications in pathology.