Growth of the axons toward their target cells: the growth cone
A neuron represents the basic structural and functional unit of the nervous system. A highly differentiated cell is involved that has lost its dividing ability (postmitotic cells). It is specialized excitability and excitation conduction.
Generalities
Nerve cells are able to transform chemical signal into electrical activity and vice versa. Today's dogma says that neurons are characterized by extreme longevity and can, with appropriate nourishment, attain an age of more than 100 years!
The human brain contains roughly 1011 nerve cells of around 100 various kinds. These are connected with each other via ca. 1011-14 synaptic contacts.
Morphologically, nerve cells differ from each other in the cytoplasmatic processes that go out from their cell bodies. The numerous, short and unmyelinized processes are termed dendrites ("little trees"). The dendrites of a single neuron can produce several thousand synapses. The more or less long, myelinized or unmyelinized processes that exhibit individual branchings (collaterals) are the axons.
Definition
A synapse represents the contact place between two neurons at which the information transfer takes place. Chemical synapses utilize neurotransmitters for the transmission of the neuronal impulses. In contrast, electrical synapses allow direct transmission of the electrical signals
In early developmental stages dendrites and axons cannot be distinguished from one another. The processes that arise possess a growth cone at their ends. Many of them will develop into dendrites (taking in information) and one of them will differentiate into an axon (passing on information).
- Cytoplasmic protrusions
- Cytoplasmic processes
- End branchings with growth cone
- Dendrite
- Axon
The originally apolar neuroblast then forms uniform cytoplasmic processes that develop further into functionally differing structures: into the numerous dendrites and into a single axon. Temporarily, all cellular processes exhibit a growth cone.
During their development axons of the neuroblasts extend outward over considerable distances. In this they often follow convoluted paths in order to establish the connections to their target organs outside the CNS (muscles, glands, receptors or synaptic connections with other neurons). The active path searching proceeds from the growth cone. This free end represents a temporary, mobile structure the task whose task it is to explore the extracellular surroundings. In addition, it reacts to various soluble and membrane-bound signal molecules. It appears the latter shows the growth cone the way via influencing the growth speed and expansion direction. The surface of the growth cone consists of flattened membrane sheets, the lamellipodia, from which fine, cell-adhesion-point exhibiting filopodia emerge. These processes contain actin filaments and myosin filaments that contract rhythmically. As soon as the growth cone reaches the target organ, it experiences alterations of the cytoskeleton, its growth comes to a stop, it flattens and forms a synapse. In this it is to be noted that the somatic sensory and motor neurons step into direct synaptic contact with their target organ while the vegetative nerve cells release their neurotransmitters from a certain distance from their targets.