Outlines of Histology (Ukázka, strana 99)

3. Neurons which possess a single axon and single dendrite are bipolar neurons. They appear as intermediate neurons, e.g. in the retina of the eye, but also in the ganglia of the vestibulocochlear nerve (VIII.), as well as in the cerebrospinal ganglia of lower vertebrates (fishes).

4. From the bipolar neurons the pseudounipolar neurons developed by gradual fusing of the dendrite and axon. They are situated in spinal ganglia of higher vertebrates (including human). This sensory neuron possesses only single process, which then divides into two branches. They can be functionally considered as dendrite and axon (both are myelinated) and the stimuli do not pass through the cell body, which has a trophic function only.

5. Most neurons possess more than one dendrite – these are multipolar neurons. Typical star-shaped neurons belong mostly to motor neurons (e.g. situated in anterior horns of spinal cord), possess numerous branched dendrites capable of receiving stimuli from many other neu-

Fig. 91 Types of neurons – 98 –

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rons, and a single axon, conducting the stimulus to the effector (muscle, glands). They can be also pyramid-shaped (motor brain cortex), or pear shaped (Purkinje) cells of cerebellum).

Cytology of the neuron The cell body (also called perikaryon, or soma) is large, usually starshaped, containing a centrally located large, pale and vesicular nucleus with a conspicuous deeply staining nucleolus (similar to that of an oocyte).

There is no centrosome – that is why once a nerve cell has been differentiated, it loses the ability to divide. Golgi complex is located in the form of cisterns around the nucleus like a wreath. (This apparatus was discovered

by Camillo Golgi just in the nerve cell body in 1898). Numerous mitochondria witness the heavy energy demands of the neuron.

Fig. 92 Cell body of a neuron – 99 –

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Nissl bodies in the form of large basophilic granular areas consist of rough endoplasmic reticulum and polyribosomes and are sites of active protein synthesis. In exhausted neurons there is a temporary reduction of Nissl bodies (tigrolysis or chromatolysis). Nissl bodies also extend into dendrites but are absent from the axon hillock.

The neurofibrils are composed of neurofilaments (diameter 10 nm) and neurotubules (microtubules – 24 nm), which also extend into all processes.

The granules of a brown, age dependent pigment, called lipofuscin are found in the axon hillock. In some CNS ganglia black pigment granules of neuromelanin can also be observed.

Dendrites are afferent processes sometimes with rich branching which enlarges the area of reception of nerve impulses. Arborization and dendritic spines enable contacts with terminal arborizations of other neurons (there can be thousands of synapses). The larger dendrites contain most of the same organelles as the cell body.

The axon is usually the largest and the longest process of the neuron, reaching up to 100 cm in length (e.g., in motor neurons innervating the foot muscles) with a constant diameter along its course. It is covered by axolemma and it can branch in axon collaterals and at the end it ramifies into terminal arborizations.

Axoplasm contains no Nissl bodies or ribosomes (proteins are supplied from the cell body). It contains neurofilaments and neurotubules which are protein tubular fibers running parallel to longitudinal axis of the axon.

There are also microfilaments built of the protein actin – these can run crosswise to be fixed to the axolemma. In addition to the filaments the mitochondria, neurotransmitter vesicles and lysosomes, as well as axoplasmic reticulum cisterns (of unknown function) can also be observed.

Fig. 93 Axon sheaths – 100 –

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