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