13.4 Neurons

• Neurons, functional units of NS, transmit information from one part of NS to another by electrical impulses
• The cell body of a representative neuron contains large/round nucleus with prominent nucleolus 
• Cytoplasm of neuron is perikaryon
• Cytoskeleton of perikaryon contains neurofilaments and neurotubules, similar to intermediate filaments and microtubules of other types of cells
• Neurofilament bundles, neurofibrils, extend into dendrites/axons, internal support
• Perikaryon contains organelles provide energy/synthesize organic materials
• Nissl bodies, dark color regions, where there are clusters of free ribosomes and RER
• Most neurons lack a centrosome
• Neurons lose centrosomes and centrioles during differentiation unable to divide 
• Neurons lose to injury or disease cannot be replaced
• Axon is long cytoplasmic process capable of propagating action potential 
• In multipolar neuron, specialized region, axon hillock, connects initial segment (base) of axon to cell body 
• Axoplasm, cytoplasm of axon, contains neurofibrils, neurotubules, lysosomes, mitochondria, etc 
• Axon branch along length, producing side branches called collaterals
• Axon and collaterals end in fine terminal extensions, telodendria
• Telodendria end in axon terminals, where neuron contacts another cell
• Molecules move between cell body and axon terminals by process axoplasmic transport
• Molecules move between cell body and axon terminals by process axoplasmic transport. This process consumes energy, relies on neurofibrils of axon and branches
• Structural Classification of Neurons. Anaxonic neurons are small
• Structural Classification of Neurons. Anaxonic neurons are small. Only in CNS and special sense organs 
• Structural Classification of Neurons. Bipolar neurons have fine dendrites fusing to form single dendrite
• Structural Classification of Neurons. Bipolar neurons have fine dendrites fusing to form single dendrite. Between single dendrite and axon
• Structural Classification of Neurons. Bipolar neurons have fine dendrites fusing to form single dendrite. Relay sensory info with sight, smell, hearing
• Structural Classification of Neurons. Bipolar neurons have fine dendrites fusing to form single dendrite. Axons not myelinated
• Structural Classification of Neurons. Pseudounipolar neurons have continuous dendrites and axons, cell body off to one side
• Structural Classification of Neurons. Pseudounipolar neurons have continuous dendrites and axons, cell body off to one side. Axons myelinated
• Structural Classification of Neurons. Multipolar neurons with several dendrites and single axon
• Structural Classification of Neurons. Multipolar neurons with several dendrites and single axon. Most common in CNS
• Neurons classified by: sensory neurons, motor neurons, interneurons 
• Sensory Neurons forming afferent division of PNS, deliver info about external/internal environment to CNS
• Axons of sensory neurons, afferent fibers, extend between sensory receptor and spinal cord/brain 
• Somatic sensory neurons transmit info about environment and position
• Visceral sensory neurons transmit info of internal and status of systems 
• Interoceptors monitor digestive, respiratory, cardiovascular, urinary, reproductive systems and provide sensations of pressure, pain, taste
• Exteroceptors provide info about external environment in form of touch, temp, pressure, sight, smell, hearing
• Proprioceptors monitor position and movement of skeletal muscles/joints
• Motor Neurons forming efferent division of NS, stimulate/modify activity of peripheral tissue, organ, organ tissue
• Three categories of receptors:

  • proprioceptors
  • exteroceptors
  • interoceptors
• Motor Neurons. Axons traveling away from CNS are efferent fibers
• Motor Neurons. SNS consists of all somatic motor neurons innervating skeletal muscles