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Neurons
By Han Shi & Melinda Song
Neuron Structure
The Soma (cell body)• Contains large, round nucleus.• Surrounding cytoplasm consists of
perikaryon, which contains organelles that provide energy and synthesize organic materials, esp. neurotransmitter.
• Contains mitochondrion, free and fixed ribosomes, golgi apparatus, and nissl bodies.
• Most neurons lack the centrosome.
The Dendrites• Extend out from the soma. • Highly branched branch contains
dendritic spines, where information is received from other neurons via synaptic connections.
• The majority of synapses occur on the dendrites.
The Axon• Long cytoplasmic process capable of
propagating an action potential.• The axoplasm contains neurofibrils,
neurotubules, small vesicles, lysosomes, mitochondria, and various enzymes.
• An axon may lengthen and produce branches called collaterals, which allows a single neuron to communicate with several other cells.
The Axon (cont…)• Telodendria – main axon trunk• Synaptic knobs form synaptic
connections with other cells. • These connections usually involve
release of neurotransmitters. • Triggered by action potential
Axoplasmic Transport• Transport continual supply of
neurotransmitter along with enzymes and lysosomes.
• Travel from the soma, through the axon, to the synaptic knob.
• Occurs along neurotubules and involves proteins called kinesins.
Axoplasmic Transport (cont…)
• Retrograde flow transports substances to the soma if debris or unusual chemicals appear in the synaptic knob.
• May alter activities in the cell by turning appropriate genes on or off.
• Rabies results from the transport in retrograde flow, which carries the rabies virus to the central nervous system from the synaptic knob.
The Synapse• A specialized site of intercellular
communication. • May occur on the dendrite, the soma, or along
the length of the axon of the receiving cell. • Neuroeffector junction
How Are Neurons Classified?• There are four types of neurons
organized by shapes• Anaxonic Neuron• Unipolar Neuron• Bipolar Neuron• Multipolar Neuron
Anaxonic Neurons• Have no myelination (membraneous
wrapping accelerating action potentials)
• Without axon• In central nervous system, special
organs• Scientists have yet to understood all
aspects of mechanism
Unipolar Neurons• Also called Pseudounipolar neurons• Base of dendrites sends directional
signal to the axons• Is myelinated• Continous connection between axon
and dendrites• Can be over 1 meter long• In sensory endings in peripheral
nervous system
Bipolar Neurons• Contain one dendrites ending and
one axon
• Are not myelinated• Very rarely found in sense organs
for aural, sight, and smell
• <30 mm in length
Multipolar Neurons• One end has many dendrites
branched off and one axon
• Are myelinated• Found in central nervous system
and are motor-based (for muscles)
• Is the most common
Functional Classification• Neurons can additionally be
organized into groups based on their usage
• Sensory neurons
• Motor neurons
• Interneurons
Sensory Neurons• Make up AFFERENT division of
peripheral nervous system• Relays sensed perceptions to central
nervous system• In sensory ganglia• Unipolar, fibers stretch from receptor to
the brain• More than 10 million neurons in each
human body
2 Types of Sensory Neurons• Somatic sensory neurons
– Gathers information about environment outside
• Visceral sensory neurons– Detects body’s internal rhythms and
statuses
Receptors• Exteroceptors
– Observe the external world by relaying sight, smell, touch, hearing, and taste
• Proprioceptors– Provides information about flexions and
articulations of muscles and joints• Interceptors
– Screens digestive, respiratory, urinary, reproductive systems, heart, and gives pain and extreme pressure senses
• Activates the activity of peripheral part• Long axon (called EFFERENT fiber
because it leaves central nervous system)
• Brings instruction from the central nervous system to the effectors
• Two types– Somatic motor neurons– Visceral motor neurons
Motor neurons
• Somatic motor neurons stimulate skeletal muscle (voluntary)
• Visceral motor neurons stimulate all other tissue– Axons of this type are inside the
peripheral autonomic ganglia– Preganglionic fibers connect central
nervous system to ganglions– Postganglionic fibers connect
ganglions to effectors
Interneurons• Positioned in the brain area and
spinal cord
• 20 billion interneurons, the most of all types
• Distributes, or processes information between motor and sensory neurons
Bibliography• http://www.3dscience.com/3D_Models/Biology/Cells/Neuron.php
• http://cwx.prenhall.com/bookbind/pubbooks/martinidemo/chapter12/medialib/CH12/html/ch12_3_2.html