Chapter 11 - Fundamentals of the Nervous System

Introduction

• The master controlling and communicating system of the body
• Functions
  • Sensory input - monitoring stimuli occurring inside and outside the body
  • Integration - interpretation of sensory input
  • Motor output - response to stimuli by activating effector organs

Levels of Organization

• Central Nervous System (CNS)
  • Brain and Spinal Cord
  • Integration and Control Centers
  • Interneurons (associative neurons)
• Peripheral Nervous System (PNS)-Cranial and Spinal Nerves
  • Sensory - afferent - input to CNS
    • Somatic fibers - impulses from skin, skeletal muscles, & joints
    • Visceral fibers - transmit impulses from visceral organs
  • Motor - efferent - output from CNS
    • Somatic - voluntary control (skeletal muscles)
    • Autonomic - involuntary (glands, cardiac and visceral muscles)
      • Sympathetic -stress
      • Parasympathetic - normal conditions

The two principal cell types of the nervous system are:

• Neurons - excitable cells that transmit electrical signals
• Supporting cells - cells that surround and wrap neurons
  • The supporting cells (neuroglia or glial cells):
  • Provide a supportive scaffolding for neurons
  • Segregate and insulate neurons
  • Guide neurons to the proper connections
  • Promote health and growth
  • Central Nervous System Glial Cells
    • Astrocytes
      • Most abundant, versatile, and highly branched glial cells
      • They cling to neurons and their synaptic endings, and cover capillaries
      • Functionally, they:
        • Support and brace neurons
        • Anchor neurons to their nutrient supplies
        • Guide migration of young neurons
        • Control the chemical environment
    • Microglia - small, ovoid cells with spiny processes
      • Phagocytes that monitor the health of neurons
    • Ependymal cells - range in shape from squamous to columnar
      • They line the central cavities of the brain and spinal column
    • Oligodendrocytes - branched cells that wrap CNS nerve fibers
  • Peripheral Nervous System Glial Cells
    • Schwann cells (neurolemmocytes) - surround fibers of the PNS
    • Satellite cells surround neuron cell bodies with ganglia

Histology of the Nervous System

• Neuron
  • Excitable cells that generate and conduct impulses
  • Amitotic - extreme longevity
  • High metabolic rate - require glucose and O2
  • Structure
    • Cell body - site of protein synthesis
      • nucleus with prominent nucleolus
      • no centrioles
      • Nissl bodies (chromatophilic substance)
    • Dendrites - carry impulses toward cell body
      • many per neuron
      • short and branched
      • respond to neurotransmitters
      • postsynaptic terminal
    • Axons - carry impulses away from cell body
      • long thin process, one per neuron
      • branched at end (axon terminals - presynaptic knobs)
      • secrete neurotransmitters from the axonal terminals
      • Axon conduction speed depends on::
        • Diameter of axon
        • Degree of myelination
      • Schwann cells form Myelin Sheath for peripheral nerves:
        • Envelopes an axon in a trough
        • Encloses the axon with its plasma membrane
        • Has concentric layers of membrane that make up the myelin sheath
      • Nodes of Ranvier - gaps between Schwann cells.
      • Neurilemma - remaining nucleus and cytoplasm of a Schwann cell
      • Myelin Sheath - Whitish, fatty (protein-lipoid cell membrane)
        • Protect the axon
        • Electrically insulate fibers from one another
        • Increase the speed of nerve impulse transmission
      • Unmyelinated axons
        • Schwann cell surrounds nerve fibers but coiling does not take place
        • Schwann cells partially enclose 15 or more axons
      • Axons of the CNS
        • Both myelinated and unmyelinated fibers are present
        • Myelin sheaths are formed by oligodendrocytes
      • White matter - dense collections of myelinated fibers
      • Gray matter - mostly soma and unmyelinated fibers

Classification of neurons

• Multipolar - motor, associative
  • many extensions
  • most numerous
• Bipolar - special sensory
  • two extension
  • olfactory, rods and cones
  • not common
• Unipolar - most sensory
  • one process from cell body
    
  • peripheral extension responds to a stimuli
  • cell bodies form ganglia
• Match & Match and Reflex Arc
• Neural Circuits - divergent, convergent, reverberating

Multiple Sclerosis (MS)

• Autoimmune disorder causing destruction of myelin sheaths in CNS
  • sheaths becomes scars or plaques
  • 1/2 million people in the United States
  • appears between ages 20 and 40
  • females twice as often as males
• Includes muscular weakness, abnormal sensations or double vision
• Remissions & relapses result in progressive, cumulative function loss

Epilepsy

• The second most common neurological disorder -affects 1% of population
• Characterized by recurrent attacks initiated by electrical discharges in brain
  • lights, noise, or smells may be sensed
  • skeletal muscles may contract involuntarily
  • loss of consciousness
• Epilepsy has many causes, including;
  • brain damage at birth, metabolic disturbances, infections, toxins, vascular disturbances, head injuries, and tumors

Electrical Current and the Body - Neurophysiology

• Reflects the flow of ions rather than electrons
• There is a polarized potential on either side of membranes when:
  • The number of ions is different across the membrane (ion gradient)
  • The membrane provides a resistance to ion flow until a stimulus occurs.
• Resting Membrane Potential Difference - results from distribution of ions
  • Na⁺ high outside cell
  • K⁺ high inside cell
  • Negative ions do not move across the cell membrane
  • Na⁺K⁺ pump maintains the ion concentration
  • K⁺ leak out faster than Na⁺leak in -->negative inner membrane and positive outside cell (polarized)
• For a nerve impulse to be propagated in a nerve cell, this resting potential (polarization) must be disturbed or depolarized by a stimulus.
• Action potentials, or nerve impulses, are:
  • Electrical impulses carried along the length of axons
  • Always the same regardless of stimulus
• Types of plasma membrane ion channels:
  • Found all over the neuron - maintain resting membrane potential
    • Passive, or leakage, channels - always open
  • Normally found at Synapse to regulate neurotransmission
    • Chemically gated channels (ligand gated) - open with binding of a specific neurotransmitter
  • Normally found in Axon for conduction of an Action Potential
    • Voltage-gated channels - open and close in response to membrane potential
  • Mechanically gated channels - open and close in response to physical deformation of receptors (stretch or sound)

Helpful Activities to do after you finish reading this chapter