The Nervous System: Organization and Tissue

I. Overview

Function:

1. Uses sensory receptors to monitor changes (stimulosensory input)
2. Processes and interprets sensory input (integration)
3. Effects a response (motor input)
   

II. Nervous System Organization Diagram

A. Central Nervous System (CNS)

• Brain and spinal cord
• Integrative and control centers

B. Peripheral Nervous System (PNS)

• Cranial and spinal nerves
• Communication lines between CNS and the rest of body
• Divisions:
  • Sensory (afferent) division
    • Somatic (skin, muscle, joints) and visceral (organs) sensory neurons
    • Conducts impulses from receptors to the CNS
  • Motor (efferent) division
    • Motor neurons
    • Conducts impulses from the CNS to effectors (muscles and glands)
    • Divisions:
      • Somatic Nervous System
        • Voluntary
        • Conducts impulses from CNS to skeletal muscles
      • Autonomic Nervous System
        • Involuntary
        • Conducts impulses from CNS to cardiac muscles, smooth muscles, and glands
        • Divisions:
          • Sympathetic
          • Parasympathetic

III. Histology of Nervous Tissue

A. Nervous Tissue

• Densely packed and intertwined
• Composed of neurons and supporting cells

B. Supporting Cells (Neuroglial Cells)

• PNS:
  • Schwann Cells and Satellite Cells Diagram
  • Schwann Cells - forms myelin sheath around large nerve fibers in PNS and is also phagocytic (engulfs damaged and dying nerve cells).
  • Satellite Cells - surround nerve cell body and may aid in controlling chemical environment of neurons.
• CNS:
  • Astrocytes, Microglia, Ependymal Cells and Oligodendrocytes Diagram
  • Astrocytes - half of neural volume; projections with bulbous ends that cling to neurons and capillaries (therefore connecting neurons to blood/nutrient supply); controls chemical environment around neurons (buffer K⁺ in extracellular space and/or recapture neurotransmitters released).
  • Microglia - ovoid cells with highly branched processes; macrophage that engulfs microbes and dead neural tissue.
  • Oligodendrocytes - few branches (less than astrocytes and microglia) line up along thicker neuron fibers in CNS and wraps extensions around nerve fibers (myelin sheaths).
  • Ependymal Cells - line central cavities of brain and spinal cord, creating a barrier between CNS cavities and tissues surrounding cavities; their cilia circulates the cerebrospinal fluid.
    

C. Neurons (nerve cells)

Characteristics

• Conducts messages in form of nerve impulses
• Has longevity
• Amitotic
• Have a high metabolic rate
• Functionally composed of:
  • A receptive (input) region
  • A conducting component (generates and propagates an action potential)
  • A secretory (output) component
• Structurally composed of a cell body and one or more processes Diagram
  • Cell Body
    • Most neuron cell bodies located within CNS
    • Clusters of cell bodies in CNS are called nuclei
    • Few/clusters of cell bodies in PNS are called ganglia
  • Processes
    • Cellular processes are called either tracts (in CNS) or nerves (in PNS)
    • Dendrites - have large surface area to receive chemical signals as well as conduct electrical signals (graded potentials)
    • Axons - single in each neuron, transmit graded potential away from cell body to axonal terminal
• Neuron repair: some neurons can be repaired through the processes of fragmentation, proliferation. elongation Diagram
  
• Electrochemical signals transmitted with the aid by myelin sheath (protein-lipoid) which insulates nerve fibers (long axons) and increases the transmission
  • Nodes of Ranvier also aid in the transmission of nerve impulses
  • Myelinated processes form the white matter of nervous tissue and unmyelinated processes form the gray matter of nervous tissue.

D. Classification of Neurons

• Structural Classification - grouped according to the number of processes extending from their cell body: Diagram
  • Anaxonic - axons and dendrites are indistinguishable; found in brain; functions poorly understood
    
  • Multipolar neurons - three or more processes (usually with a single axons); most common type, major neuron in CNS.
  • Bipolar neurons - two processes (axon and dendrite) extend from opposite sides of neuron; rare in adult but may be found in retina and olfactory mucosa.
  • Unipolar neurons - one process extending from cell body and forms central and peripheral processes
    • Central process associated with secretory region
    • Peripheral process associated with sensory region (receptor)
    • Unipolar also referred to as Pseudounipolar (unipolar, originally bipolar but processes fuse in development).
      
• Functional Classification - according to direction in which nerve impulses travel relative to the CNS
  • Sensory (afferent) neurons - transmit impulses from sensory receptors toward CNS
    • Unipolar neurons - skin or internal organs to CNS for interpretation
    • Bipolar neurons - special sense organs, retina
  • Motor (efferent) neurons - carry impulses away from CNS to organs
    • Multipolar neurons - cell body located within CNS and neurons form neuromuscular junctions with effector cells
  • Association neurons (interneurons) - transmit impulses within CNS (usually sensory to motor); found in CNS only; mostly multipolar and 99% of neurons in body

IV. Neurophysiology

A. Principles of Electricity

• Body is electrically neutral but some sites one charge may predominates
• Opposite charges attract and energy must be used to separate them
• Conversely when opposite charges come together, energy is liberated
• Therefore separated electrical charges have potential energy
• Measure of potential energy = voltage
• Voltage is always measured between two points and is called the potential difference or potential between the two points
• Flow of electrical charge from one point to another is called current
• The amount of charge moving between two points depends on voltage and resistance (hindering of flow of charge)
• Ohm's law states that "current = voltage/resistance"
• Current is proportional to voltage (the greater the voltage, the greater the current) and is inversely proportional to resistance (the greater the resistance, the less the current)
• In the body, electrical currents correspond to the flow of ions across cellular membranes and any resistance is provided by the membrane themselves.
• Variety of ion channels occur in the plasma membranes and classified as being either passive or active
  • Passive (leakage) channels are always open
  • Active (gated) channels that are made up of one or more proteins capable of undergoing changes to open or close
    • Chemically gated channels - respond to neurotransmitters
    • Voltage-gated channels - respond to membrane potential changes
    • Other gated channels - mechanical, pressure, light
• Ions move along chemical gradients (due to diffusion) and along electrical gradients (move toward an opposite charge); therefore ions flow along electrochemical gradients

B. Resting Membrane Potential Diagram

• Voltage - approximately -70 millivolts (negative indicates cytoplasmic side of neuron's membrane)
• Membrane is referred to as being polarized Diagram
• Two forces that establish and maintain resting membrane potential: Passive (diffusion via channels) and Active (sodium-potassium pump) Diagram
• Cells (neurons and muscle) use changes in their membrane potential as communication signals for receiving, integrating, and sending information; change in membrane potential can be produced by:
  • Anything that changes membrane permeability to any type of ion
  • Anything that alters ion concentration on the two sides of the membrane.
• Two types of signals produced by membrane permeability changes:
  • Graded potential (signal over short distances)
  • Action potential (long distance signals)

C. Graded Potentials

• Local changes in membrane potential (on dendrites and along cell body) Diagram 
  • Depolarization = reduction in membrane potential (become less negative, -70 to -60mV)
  • Hyperpolarization = increase in membrane potential (become more negative, -70 to -80mV)
• Depolarization and hyperpolarization -- collectively are changes that cause local flows of current that decrease with distance traveled
• "Graded" - magnitude of potential varies directly with the intensity or strength of stimulus; therefore increase stimulus, increase change in voltage, increase current flow
• Graded potentials - triggered by various stimuli
• Ions only pass in and out of cell membrane at the point of stimulus
  • Positive ions migrate toward more negative areas and negative ions simultaneously move toward more positive areas
  • Inside the cell, positive ions move away from active area and accumulate on neighboring membrane areas, where they displace negative ions
  • Due to various permeabilities, most charge is lost quickly and flow or current is decremental

D. Action Potentials

• Communication between cells with excitable membranes (neurons and muscles)
• They do not decrease in strength with distance and are referred to as nerve impulse in neurons
• Stimulus changes permeability of neuron's membrane by opening specific voltage-regulated gated channels located on axons
• These channels open and close in response to local changes in membrane potential and activated by local currents (graded potentials) that spread toward the axon along the dendritic and cell body membranes
• Axons only are capable of generating action potentials!
• Generation of Action Potential (induced by depolarization) due to three sequential changes:
  1. Increase in sodium permeability and reversal of the membrane potential
     • Axonal membrane depolarized by local currents
     • Voltage-dependent sodium ion channels open
     • Sodium rushes in
     • Depolarization increases until a critical level (threshold) is reached
     • Depolarization then is self-generating (depolarization is driven by the ionic currents created by ion influx)
  2. Decrease in sodium permeability
     • Sodium permeability as a result of action potential lasts a millisecond
     • Sodium gates close after passing 0 mv (due to increase positive charge)
  3. Increase in potassium permeability and repolarization
     • As sodium entry declines, voltage-regulated potassium gates open and potassium leaves cell, following electrochemical gradient
     • Cell interior becomes progressively less positive and membrane potential tends toward resting level, causing repolarization
• Repolarization restores electrical conditions not original ionic distribution of resting state
• It is the activation of the sodium-potassium pump that reestablishes ionic distribution
• Propagation of Action Potential:
  • Propagation = transmission of action potential
  • Non-myelinated (bipolar) neurons - depolarization occurs locally which causes adjacent membrane areas to open voltage dependent channels, triggering an action potential, therefore impulse propagates away from point of origin
  • Following depolarization each segment of axon undergoes repolarization (restoring resting membrane potential)
  • Repolarization wave chases the wave of depolarization down the length of axon - therefore action potential is a self propagating event
• Types of action potentials
  
  • Continuous conduction - propagation of an action potential in a step-by-step depolarization of each adjacent area of an axon membrane Diagram
  • Saltatory conduction- propagation of an action potential along exposed portions of a myelinated nerve fiber; "jumping" node to node Diagram

E Transmission at Synapses

• Synapse, composed of Diagram Diagram
  
  • Presynaptic axon terminal
    • Presynaptic action potential causes calcium gates to open and causes calcium to flow into the axonal terminal
    • Calcium promotes the fusion of synaptic vesicles with the presynaptic membrane
    • Vesicles release (via exocytosis) neurotransmitters into the synaptic cleft
  • Synaptic cleft
  • Postsynaptic membrane (neuron or muscle)
  • Receptor and ion channel
• Types of synapses: neuron/neural, neuron/muscular, and neuron/glandular Diagram
  
• Excitatory and Inhibitory Postsynaptic Potential
  • Excitatory postsynaptic potentials (EPSP) - depolarizes the postsynaptic membrane (bringing the membrane closer to threshold)
  • Stimulatory postsynaptic potentials (IPSP) - hyperpolarization of the postsynaptic membrane (bringing the membrane further away from threshold)

F. Neurotransmitters Diagram

Classes of Neurotransmitters:

• Acetylcholine
• Biogenic amines (contains one or more amine groups)
  • Dopamina, Norepinephrine, and epinephrine
  • Serotonin
  • Histamine
• Amino acids
  • GABA (gamma-aminobutyric acid)
  • Glutamate
  • Glycerine
• Peptides
  • Endorphins and enkephalins
  • Somatostatin