1.1 Neuron Anatomy

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  Basic components ( Table 1.1, Fig. 1.1 ).

  
  
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  Synaptic junction and signal transmission:

  
  
  
  
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    Mechanism of basic chemical synapses ( Fig. 1.3 ).

    
    
    
    
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      Action potential (depolarization) reaches terminal branch of the presynaptic neuron.

      
      
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      N-type Ca²⁺ channels open, Ca²⁺ influx.

      
      
      
      
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        Associated pathologies: Lambert–Eaton myasthenic syndrome.

      
      
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      Ca²⁺ facilitates vesicle docking, neurotransmitter released into synaptic cleft.

      
      
      
      
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        Associated pathologies: botulism, tetanus (lockjaw).

      
      
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      Neurotransmitter binds neurotransmitter receptor (postsynaptic neuron).

      
      
      
      
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        Associated pathologies: myasthenia gravis.

      
      
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      Depending on its function, the neurotransmitter receptor creates either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).

      
      
      
      
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        EPSPs depolarize the postsynaptic neuron and increase the probability of action potential formation.

        
        
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        IPSPs either hyperpolarize or resist depolarization of the postsynaptic neuron and decrease the probability of action potential formation.

      
      
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      The potentials across all dendrites are integrated in the cell body and axon hillock, determining whether or not an action potential will fire in the postsynaptic neuron.

      
      
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      A variety of mechanisms, including enzymatic degradation (i.e., acetylcholine) and presynaptic reuptake (i.e., serotonin), remove neurotransmitters from the synaptic cleft to end the postsynaptic stimulus.

    
    
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    Neuromuscular junction:

    
    
    
    
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      Specialized chemical synapse between motor neuron and muscle fiber.

      
      
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      Cholinergic synapse containing mainly nicotinic acetylcholine receptors.

      
      
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      Nerve impulse results in contraction of muscle fiber(s).

    
    
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    Motor unit:

    
    
    
    
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      A single motor neuron and all muscle fibers that it innervates.

      
      
      
      
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        A small motor unit contains three to six muscle fibers and controls muscles of fine control.

        
        
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        A large motor unit contains 100 to 1,000 muscle fibers and controls muscles of crude control and strength (i.e., biceps, quadriceps).

      
      
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      All muscle fibers of a single motor unit are of the same fiber type (types 1, 2a, and 2b).

  
  
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  Neuron types ( Table 1.2 ).

  
  
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  Nerve fiber organization ( Table 1.3, Fig. 1.4 ).

  
  
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  Nervous system organization ( Fig. 1.5 ).

  
  
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  Afferent and efferent nerves ( Table 1.4, Fig. 1.6 ):

  
  
  
  
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    Afferent nerve fibers carry sensory information and arrive at the spinal cord through dorsal roots.

    
    
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    Efferent nerve fibers carry motor information and exit the spinal cord through ventral roots.

    
    
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    Efferent motor neurons ( Table 1.5, Fig. 1.7 ):

    
    
    
    
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      Upper motor neurons (UMNs)

      
      
      
      
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        Cell bodies originate within the primary motor cortex or brainstem nuclei.

        
        
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        Convey motor information by synapsing with lower motor neurons (LMNs, or interneurons) in the brainstem or spinal cord.

      
      
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      LMNs:

      
      
      
      
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        Cell bodies originate in brainstem nuclei or the ventral horn of spinal cord gray matter.

        
        
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        Convey motor information from UMNs by synapsing with skeletal muscle in the periphery via neuromuscular junctions.

    
    
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    Afferent sensory receptors ( Table 1.6 ).

    
    
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    Afferent sensory neurons ( Table 1.7 ).

  
  
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  Reflex arcs ( Table 1.8 ):

  
  
  
  
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    General principles:

    
    
    
    
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      A reflex arc is a neural pathway that controls a reflex action.

      
      
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      It involves the spinal cord only, allowing for a fast, subconscious response.

      
      
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      Sensory information is processed by the brain after the reflex has occurred.

    
    
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    Types:

    
    
    
    
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      Monosynaptic: contains two neurons (sensory and motor) with a single chemical synapse ( Fig. 1.9 ):

      
      
      
      
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        That is, patellar reflex, Achilles reflex.

      
      
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      Polysynaptic: contains one or more interneurons that connect a sensory neuron to a motor neuron:

      
      
      
      
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        Represents the majority of reflex arcs.

        
        
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        Allows for higher order processing and control.

        
        
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        That is, pain withdrawal reflex.

      
      
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      Somatic: affects skeletal muscle.

      
      
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      Autonomic: affects internal viscera.

    
    
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    Components:

    
    
    
    
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      Stimulus (muscle stretch, pain, temperature, stretch, etc.).

      
      
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      Sensory receptor (muscle spindle, free nerve ending, etc.).

      
      
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      Afferent pathway: sensory neuron (dorsal root ganglia).

      
      
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      Interneuron(s) (dorsal horn):

      
      
      
      
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        Polysynaptic reflex arcs only.

      
      
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      Efferent pathway: motor neuron (ventral horn).

      
      
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      Skeletal muscle:

      
      
      
      
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        Effector response → muscle contraction.

    
    
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    Inhibitory interneurons:

    
    
    
    
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      Activated by sensory neurons of a reflex arc.

      
      
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      Inhibit LMNs that act on antagonistic muscle groups:

      
      
      
      
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        That is, during biceps reflex, inhibitory interneurons will cause the triceps to relax.

    
    
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    UMN effects:

    
    
    
    
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      Inhibits the magnitude of LMN responses in a reflex arc:

      
      
      
      
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        This is a conscious process, and is the basis of the Jendrassik maneuver:

        
        
        
        
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          Useful to determine the effect of UMNs on clinically observed hyporeflexia.

          
          
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          Can reduce the effect of UMNs on a reflex arc by having a patient clench their teeth and hold their interlocked fingers in a hooklike configuration.

          
          
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          These maneuvers reduce the conscious activity of UMNs by providing a distraction.

    
    
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    Lesions

    
    
    
    
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      UMN lesions → hyperreflexia due to loss of inhibition.

      
      
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      LMN lesions → hyporeflexia due to loss of effector response.

Table 1.2 Basic neuron types

Type	Image	Description	Examples
Pseudounipolar		A single axon split into two branches with an adjacent cell body: Peripheral branch: periphery to cell body (contains dendrites) Central branch: cell body to spinal cord (contains synaptic terminals) Transmits sensory information from the periphery to the CNS	Sensory neurons of dorsal root ganglia Sensory ganglia of cranial nerves V, VII, IX, and X
Bipolar		Cell body centrally located between a: Dendrite: transmits signals toward cell body Axon: transmits signals away from cell body Specialized sensory neurons for the transmission of special senses (i.e., vision, hearing)	Bipolar cells, ganglion cells, horizontal cells, and amacrine cells of the retina Cochlear and vestibular ganglia of the inner ear
Multipolar		Cell body contains multiple dendrites and a single axon Able to receive and integrate abundant nerve impulses	Motor neurons (ventral horn of spinal cord) Interneurons (spinal cord gray matter) Purkinje’s cells (cerebellum) Pyramidal cells (cerebral cortex)
Abbreviation: CNS, central nervous system.