Ch. 11 Nervous System Pwr Pts(3)

Chapter 11: Nervous System

Overview of the Nervous System

  • Definition: The nervous system is the master controlling and communicating system of the body.

  • Major Functions:

    • Sensory Input: Monitoring stimuli from the environment.

    • Integration: Interpretation of sensory input.

    • Motor Output: Response to stimuli.

Organization of the Nervous System

  • Central Nervous System (CNS):

    • Composed of the brain and spinal cord.

    • Acts as the integration and command center.

  • Peripheral Nervous System (PNS):

    • Includes paired spinal (spinal nerves) and cranial nerves (cranial nerves).

    • Carries messages to and from the spinal cord and brain.

    • Links all parts of the body to the CNS.

Components of the Nervous System

  • Central Nervous System (CNS):

    • Brain

    • Spinal Cord

    • Peripheral Nervous System (PNS):

      • Cranial nerves

      • Spinal nerves

      • Ganglia

Peripheral Nervous System (PNS) Functional Divisions

  • Sensory (Afferent) Division:

    • Sensory Afferent Fibers: Transmit impulses from skin, skeletal muscle, and joints to the brain.

    • Visceral Afferent Fibers: Transmit impulses from visceral organs to the brain.

  • Motor (Efferent) Division:

    • Transmits impulses from the CNS to target organs (muscles and glands).

Motor Division Subdivisions

  • Somatic Nervous System:

    • Known as the “voluntary nervous system.”

    • Composed of somatic motor nerve fibers that conduct impulses from CNS to skeletal muscles.

    • Allows conscious control of skeletal muscles.

  • Autonomic Nervous System (ANS):

    • Known as the “involuntary nervous system.”

    • Consists of visceral motor nerve fibers.

    • Regulates smooth muscle, cardiac muscle, and glands.

    • Subdivisions: Sympathetic and parasympathetic branches.

Histology of Nerve Tissue

  • Cell Types:

    • Neurons: Excitable cells that transmit electrical signals.

    • Neuroglia: Support cells that surround and wrap neurons.

Supporting Cells: Neuroglia

  • Neuroglia Functions:

    • Provide support for neurons.

    • Insulate neurons to speed up action potential conduction.

    • Promote health and growth of neurons.

Types of Neuroglia

  • Astrocytes (CNS):

    • Most abundant and versatile glial cells.

    • Functions:

      • Support and brace neurons.

      • Anchor neurons to nutrient supplies.

      • Control the chemical environment.

  • Microglia (CNS):

    • Small, spiny cells acting as macrophages that monitor the health of neurons and phagocytize debris.

  • Ependymal Cells (CNS):

    • Line the central cavities of the brain and spinal column.

    • Contain cilia that help circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes (CNS):

    • Form the myelin sheath around CNS nerve fibers.

  • Schwann Cells (PNS):

    • Form myelin sheaths around larger PNS nerve fibers.

    • Vital for regeneration of damaged peripheral nerve fibers.

  • Satellite Cells (PNS):

    • Surround neuron cell bodies; function not fully understood.

Neurons (Nerve Cells)

  • Structure:

    • Composed of a body, dendrites, and axon.

    • Long-lived, amitotic, with a high metabolic rate.

  • Plasma Membrane Functions:

    • Electrical signaling.

    • Cell-to-cell signaling during development.

Neuron Processes

  • Dendrites:

    • Receptive regions carrying impulses towards the soma.

  • Axons:

    • Carry impulses away from the soma.

    • Only one axon, but can branch to connect with multiple cells.

Myelin Sheath

  • Formation:

    • Formed by Schwann cells in PNS and oligodendrocytes in CNS.

  • Functions:

    • Protect and electrically insulate axons.

    • Increase the speed of action potentials.

Regions of the Brain and Spinal Cord

  • White Matter:

    • Dense collections of myelinated fibers; appears white in the CNS.

  • Gray Matter:

    • Mostly somas and unmyelinated fibers; appears gray in the CNS.

Neuron Classification

  • Structural:

    • Unipolar Neuron: Single, short process, exclusively sensory.

    • Bipolar Neuron: Two processes; found in olfactory epithelium and retina.

    • Multipolar Neuron: One axon and multiple dendrites; most common type.

  • Functional:

    • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

    • Motor (Efferent) Neurons: Carry impulses away from the CNS.

    • Interneurons: Shuttle signals within the CNS pathways.

Neuronal Physiology

  • Action Potentials:

    • Electrical impulses that travel along the axons.

    • Principal means of neural communication; consistent in strength.

    • Generated by muscle cells and neurons; do not decrease over distance.

Generating an Action Potential

  • Four Main Steps:

    1. Resting State: All Na⁺ and K⁺ channels closed.

    2. Depolarization: Na⁺ channels open.

    3. Repolarization: K⁺ channels open.

    4. Hyperpolarization: Some K⁺ channels remain open; Na⁺ channels reset.

Threshold and Action Potentials

  • Threshold:

    • Membrane depolarizes by 15-20 mV for an action potential to occur.

    • All-or-None Phenomenon: Action potentials happen completely or not at all.

Conduction Velocities of Axons

  • Impulse propagation rate determined by:

    • Axon Diameter: Larger diameter leads to faster impulses.

    • Myelin Sheath Presence: Myelination greatly increases impulse speed.

Saltatory Conduction

  • Occurs in myelinated nerves.

    • Action potentials jump between nodes of Ranvier.

    • Faster than unmyelinated axon conduction (30x faster).

Nerve Fiber Classification

  • Classified by:

    • Diameter, degree of myelination, and speed of conduction.

    • Group A Fibers: Large diameter, thick myelin - 150 m/s.

    • Group B Fibers: Intermediate diameter, light myelination - 15 m/s.

    • Group C Fibers: Small diameter, no myelination - 1 m/s.

Chemical Synapses

  • Definition: Axon terminal releases neurotransmitter to communicate with the target cell.

    • Composed of:

      • Presynaptic axon terminal with synaptic vesicles.

      • Receptor region on the target cell.

Synaptic Cleft

  • Description: Fluid-filled space separating presynaptic neuron and target.

    • Transmission Process:

      • Involves release of neurotransmitter, ensuring unidirectional communication.

Termination of Neurotransmitter Effects

  • Depends on neurotransmitter removal:

    • Diffusion from the synaptic cleft

    • Reabsorption by presynaptic axon terminals

    • Degraded by enzymes (e.g., acetylcholine degraded by acetylcholinesterase).

Postsynaptic Potentials

  • Types:

    • EPSP: Excitatory postsynaptic potential; helps generate action potentials.

    • IPSP: Inhibitory postsynaptic potential; reduces ability to produce action potentials.

Summation of Postsynaptic Potentials

  • A single EPSP cannot initiate an action potential.

  • Summation Types:

    • Temporal Summation: Rapid-fire impulses from presynaptic neurons.

    • Spatial Summation: Simultaneous stimulation from multiple terminals.

Neurotransmitter Classification

  • Chemical Classifications: Acetylcholine, biogenic amines, amino acids, peptides, and dissolved gases.

Neurotransmitter Functions

  • Acts as chemical signals for communication.

  • Excitatory vs. Inhibitory:

    • Excitatory neurotransmitters cause depolarization.

    • Inhibitory neurotransmitters cause hyperpolarization.

Functional Classification of Neurotransmitters

  • Excitatory and inhibitory based on their effects on the postsynaptic neuron.

  • Example: Acetylcholine is excitatory at neuromuscular junctions but inhibitory in cardiac muscle.

Neural Integration: Neuronal Pools

  • Definition: Groups of neurons that integrate incoming information and forward to appropriate destinations.

Circuits in Neuronal Pools

  • Diverging Circuit: One input leading to many outputs; amplifies signal.

  • Converging Circuit: Many inputs leading to one output; integrates multiple signals.

  • Reverberating Circuit: A feedback circuit; controls rhythmic activity.

  • Parallel After-Discharge Circuit: Impulses reach a single output cell at different times, involved in complex processing tasks.

Patterns of Neural Processing

  • Serial Processing: Direct, single pathway to destination; all-or-none response.

  • Parallel Processing: Multiple pathways; integrates in different CNS systems; evokes various responses to a single stimulus.

Development of Neurons

  • Originates from the neural tube and neural crest.

  • Three phases of differentiation involve cell proliferation, migration, and differentiation into neuroblasts.

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