Functional Organization of Nervous Tissue

Functional Organization of Nervous Tissue

Introduction to Nervous Tissue

  • The nervous system consists of two main divisions:
      - Central Nervous System (CNS): Brain and spinal cord.
      - Peripheral Nervous System (PNS): Nerves, ganglia, and receptors.

Nervous System Overview

  • Nerve: Bundle of axons outside the brain and spinal cord.
      - Cranial Nerves: 12 pairs that originate from the brain.
      - Spinal Nerves: 31 pairs that arise from the spinal cord.

  • Ganglion: Collection of neuron cell bodies outside the brain and spinal cord.

  • Plexus: Extensive network of axons, and sometimes neuron cell bodies, located outside the CNS.

  • Glial Cells: Supportive cells with various functions, including regulating the extracellular environment and producing myelin.

Functions of the Nervous System

  1. Maintaining Homeostasis: Regulates and coordinates physiological activities to maintain balance.

  2. Receiving Sensory Input: Monitors internal and external stimuli through sensory receptors.

  3. Integrating Information: The brain and spinal cord process sensory inputs and initiate appropriate responses.

  4. Controlling Muscles and Glands: Coordinates the activities of muscles and glands in response to stimuli.

  5. Establishing and Maintaining Mental Activity: Engages in consciousness, thinking, memory, and emotion.

Divisions of the Nervous System

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS):
      - Sensory Division (Afferent): Carries signals from sensory receptors to the CNS.
      - Motor Division (Efferent): Transmits signals from the CNS to effectors (muscles and glands).
        - Somatic Nervous System: Voluntary control of skeletal muscles.
        - Autonomic Nervous System (ANS): Involuntary control over cardiac and smooth muscles; further divided into sympathetic, parasympathetic, and enteric.

Cells of the Nervous System

Neurons

  • Structure: Three major parts:
      - Neuron Cell Body (Soma): Contains Nissl bodies for protein synthesis; performs cellular housekeeping functions
      - Dendrites: Extensions that receive signals; short, branching, conduct currents toward the cell body.
      - Axon: Long extension that transmits signals; begins at the axon hillock (trigger zone) where action potentials are generated; ends at presynaptic terminals with neurotransmitter-filled synaptic vesicles.

Axonic Transport Mechanisms

  • Anterograde Transport: Movement of materials from the cell body to the axon terminal (e.g., cytoskeletal proteins, mitochondria).

  • Retrograde Transport: Movement toward the cell body, carrying damaged organelles and recycled materials.

  • Note: Certain viruses (e.g., rabies, herpes) can use retrograde transport to reach the CNS.

Types of Neuronsas

  • Functional Classification:
      - Sensory (Afferent): Transmit action potentials toward the CNS.
      - Motor (Efferent): Transmit action potentials away from the CNS.
      - Interneurons: Connect neurons within the CNS.

  • Structural Classification:
      - Multipolar Neurons: Common in CNS; have multiple dendrites and one axon.
      - Bipolar Neurons: Sensory neurons found in the retina and nasal cavity.
      - Pseudo-Unipolar Neurons: Single process that branches into two; part that extends to the periphery has dendrite-like sensory receptors.
      - Anaxonic Neurons: Only dendrites, no axons; primarily found in the brain and retina.

Glial Cells of the CNS

  1. Astrocytes: Star-shaped cells that support neurons and regulate the extracellular environment; form blood-brain barrier.

  2. Ependymal Cells: Line ventricles and central canal of the spinal cord; specialize in producing cerebrospinal fluid (CSF).

  3. Microglia: Act as CNS macrophages; respond to inflammation and clear away dead tissue.

  4. Oligodendrocytes: Form myelin sheaths around CNS axons, insulating them.

Glial Cells of the PNS

  1. Schwann Cells: Form myelin sheaths around single axons; cytoplasm and organelles reside in the outer layer (neurilemma).

  2. Satellite Cells: Surround neuron cell bodies in sensory and autonomic ganglia; provide support and nutrients.

Myelination and its Effects

  • Myelinated Axons: Insulate axons, facilitating rapid signal transmission and aiding in repair.
      - Nodes of Ranvier: Gaps between myelin sheaths; crucial for saltatory conduction.

  • Unmyelinated Axons: Do not have myelin sheathed around them; found in gray matter.

  • Disorders: Conditions like multiple sclerosis are associated with myelin degeneration.

Nervous Tissue Response to Injury

  • Nerve injuries can heal or become permanently damaged. Key processes include:
      - Degeneration: Distal axon segments break and die, Schwann cells start to degenerate, and macrophages clean up debris.
      - Regeneration: Schwann cells proliferate and form a column that guides regenerating axons, facilitating reconnection to target tissues (limited in the CNS).

Organization of Nervous Tissue

  • Gray Matter: Contains unmyelinated axons, cell bodies, and dendrites; primarily involved in integrative functions (e.g., CNS cortex).

  • White Matter: Composed of myelinated axons; involved in propagating action potentials.

  • Nuclei and Tracts: In the CNS, clusters of cell bodies are referred to as nuclei, while bundles of myelinated axons are termed tracts; in the PNS, clusters are ganglia and bundles are nerves.

Electrical Signals in Neurons

  • Action Potentials: Electrical signals produced by cells, essential for perception and response to stimuli; enabled by movement of ions across membranes.

Membrane Potential Basics

  • Resting Membrane Potential: Typically ranges from -70 mV to -90 mV. Established due to:
      - Ion Concentration Differences: High external Na+ and K ext{+}; high internal K ext{+}. Negatively charged proteins contribute to inside negativity.
      - Permeability Characteristics: Differential permeability to various ions; increased permeability to K ext{+} through leak channels contributes to resting potential.

Ion Concentration Dynamics

  • Sodium-Potassium Pump: Active transport mechanism that maintains the ionic concentration gradients by pumping Na ext{+} out and K ext{+} in across the membrane; contributes directly to resting potential.

Types of Ion Channels

  1. Leak Channels: Always open, responsible for passive ion movement at resting potential (predominantly for K ext{+}).

  2. Gated Ion Channels: Include ligand-gated and voltage-gated channels which open in response to specific stimuli (ligand binding or voltage changes).

Action Potentials

  • Creation: Action potentials are all-or-nothing responses that manifest once a threshold potential is reached.

  • Phases:
      - Depolarization: Rapid influx of Na ext{+} when voltage-gated sodium channels open.
      - Repolarization: K ext{+} channels open, allowing K ext{+} outflow, returning the membrane potential to resting levels.
      - Hyperpolarization: Can occur past resting potential due to delayed closure of K ext{+} channels.

Propagation of Action Potentials

  • Continuous Conduction: Occurs in unmyelinated axons; action potentials propagate along every segment.

  • Saltatory Conduction: Occurs in myelinated axons; action potentials jump from one node of Ranvier to the next, increasing conduction speed.

Refractory Period

  • Period during which neuron is less responsive to stimulus.
      - Absolute Refractory Period: No action potential can be generated regardless of stimulus strength.
      - Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

Synapse Structure and Function

  • Synapses: Junctions that allow communication between neurons. Types include:
      - Electrical Synapses: Allow graded currents via gap junctions, allowing rapid synchronization of cellular activity.
      - Chemical Synapses: Involve neurotransmitter release from pre-synaptic neurons into the synaptic cleft and binding to post-synaptic receptors.

Neurotransmitter Mechanisms

  • Synaptic Transmission: Involves neurotransmitter release, binding to receptors, and inducing graded potentials in the post-synaptic membrane.

  • Neurotransmitter Removal: Essential for reset between signals; methods include enzymatic degradation (e.g., acetylcholine) and reuptake.

Excitatory and Inhibitory Postsynaptic Potentials

  1. Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic neuron's membrane, potentially leading to an action potential.

  2. Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of action potential generation.

Summation of Postsynaptic Potentials

  • Spatial Summation: Synaptic inputs from multiple presynaptic neurons combine to reach the threshold potential.

  • Temporal Summation: Successive synaptic inputs from a single presynaptic neuron combine to reach threshold potential.

Neuronal Pathways and Circuits

  • Pathways: Neurons across the CNS are organized in various pathways:
      - Serial Pathways: Direct linear information flow.
      - Convergent Pathways: Multiple neurons synapse on fewer neurons for data synthesis.
      - Divergent Pathways: One neuron connects to many, spreading information broadly.
      - Reverberating Circuits: Enable repeating or cyclical activity, e.g., rhythmic functions like breathing.
      - Parallel After-Discharge Circuits: Facilitate complex processing from several neurons converging on an output cell.

Summary of Key Concepts

  • The organization and function of nervous tissue underlie all bodily activities through complex signal processing and the integration of sensory input, motor control, and higher mentative functions. Proper functioning depends on the integrity of both glial and neuronal cells and their interconnections, evidenced in conditions of injury and disease.