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.