Neurocytology

 

Professor Alfred Cuschieri

Department of Anatomy, University of Malta.

 

Objectives

On completion of this unit the student should be able to:

o       Classify neurons according to morphology and size

o       Distinguish between axons and dendrites according to their morphological and functional characteristics

o       Correlate the morphological characteristics of neuronal cell bodies with their respective functions

o       Distinguish between different types on neuronal transport

o       Name the main molecular channels present in the neuronal plasma membrane

o       Distinguish between various types of neuroglia according to their location, morphology and functional characteristics

o       Distinguish the different types of non-neuronal cells in the peripheral nervous system

 

Neurons

Neurons are the functional cells of the nervous system:

o       specialised for reception, transmission and integration of information

o       specialised for the generation and transmission of nerve impulses

o       have a cell body (soma)

o       have one or more processes, subdivided into:

·          dendrites

-          branching processes, usually multiple and often forming complex dendritic trees 

¨        one axon

-         arising from the soma at the axon hillock

-         often gives off collateral  branches at intervals

-         end in terminal arborisations

 

 

Morphological classifications

 

I Classification according to number of processes

1.    Multipolar

·          with a dendritic tree and an axon

·          the commonest type of neuron

2.    Bipolar with two processes

·          characteristic of the olfactory, visual, auditory and vestibular system

3.    Pseudounipolar having only a short axon that divides into two processes (proximal and distal) 

·          Found only in the peripheral nervous system

·          Characteristic of all sensory ganglia – dorsal root ganglia and the sensory ganglia of the cranial nerves (trigeminal (V), nervus intermedius (VII), glossopharyngeal (IX), vagus(X) and cranial accessory (XI)

4.    Unipolar neurons with one axon

·          Very rare, said to be found only in the mesencephalic nucleus of the trigeminal (unimportant)

 

 

II Classification according to length of axon

 

Neurons vary greatly in size, depending mainly on the    length of their axons.

1       Large neurons have a large cell body and a long axon that may extend over very long distances ranging from several centimeters to over 1 metre. The main examples are

·          Anterior horn cells – may extend for the whole length from the spinal cord to the distal muscles of limbs

·          Pyramidal cells from the cerebral motor cortex to the anterior horn cells

2       Small (internuncial) neurons have their axons extending over short distances within the central nervous system.  These are further subdivided according to axon length relative to the dendritic tree:

·          Golgi type I – where the axon length is greater than the distance covered by the dendritic tree

·          Golgi type II – very small neurons whose axon terminates in the vicinity of the cell body

 

 

The morphology of the neuronal cell body depends on the size of the axon. Classically the morphology of a large neuron is considered as “typical”.  However, these form a small minority of neurons.  The vast majority of neurons are of the small internuncial variety and do not conform to the classical description.

 

Morphology of large neurons

The cell body or Soma

The cell body is mainly responsible for the maintenance of the neuron and all its processes.  Neurons are never replaced, and therefore the main function of their cell bodies is the replacement of the organelles and particularly of the plasma membrane, which may be very extensive.  The main features of the soma are:

1.    A large, spherical, euchromatic, pale-staining nucleus with a large, prominent central nucleolus.

2.    Abundant rough endoplasmic reticulum and prominent Golgi complex – form the synthetic machinery of the cell

a.    for replacement of organelles and plasma membrane

b.    for synthesis of neurotransmitters

The RER appears as basophilic Nissl granules in light microscopy.

3.    Numerous lysosomes – for the destruction of old and worn out organelles and membranes by autophagocytosis

4.    Mitochondria – for APT generation.

5.    Neurotubules (microtubules) responsible for movement of vesicles (mainly containing worn-out or replacement plasma membrane) between the soma and its processes.

6.    Neurofilaments (10 nm or intermediate type   microfilaments) forming the cell cytoskeleton and responsible for maintenance of cell shape.

7.    Lipofuschin granules - pigmented inclusions related to cell ageing

 

The Axon Hillock

·          Is a specialised region where the soma tapers into the axon. 

·          Has a structure similar to that of axons, and contains neurotubules, neurofilaments, few mitochondria and vesicles

·          Is devoid of RER, Golgi or lysosomes

·          Continues into the initial segment of the axon, the site where an action potential (nerve impulse) is generated

 

The Dendritic Tree

·          Is considered to be the receptive area of the neuron

·          Consists numerous branching processes arising from the soma

·          May have dendritic spines – small protrusions, specialized synaptic sites

·          Has a structure similar to that of the cell body, including Nissl granules in its thicker branches

Axons

·          Contain neurotubules, neurofilaments, few mitochondria and vesicles, but are devoid of RER, Golgi and lysosomes

·          Transmit nerve impulses from the axon hillock to their terminations

·          Have an axolemma (plasma membrane) containing:

·          Sodium pumps

- pump out Na+ against a concentration gradient

- create a membrane potential of -80 mv

·          Electrically gated sodium channels

- Open on arrival of an electrical impulse

- Allow Na+ to flow out along concentration gradient, forming an action potential of about +40 mv

·          Have a number of collaterals arising at intervals

·          Have terminal arborisations

·          Usually end in terminal boutons (boutons termineaux) – small slightly dilated terminals

·          Exhibit:

·          Anterograde transport (away from the cell body)

·          Retrograde transport (towards the cell body)

·          Fast axon transport, 100-300mm/day, mediated by microtubules

·          Slow axon transport, 1-3 mm/day resulting from axoplasmic flow

·          Are often surrounded by a myelin sheath

 

 

Neuroglia

 

Neuroglia are the cells associated with the neurons in the central nervous system.  They consist of:

 

1.    Astrocytes

Astrocytes are small cells that isolate neurons from one another so that no two neurons are in contact except at synapses. There are two types:

a. Protoplasmic astrocytes

·          Are found in the grey matter

·          Surround and isolate dendrites, cell bodies

·          Are small cells with numerous short, thick, branched processes

·          Contain glial fibrillary acidic (10 nm intermediate filaments) forming their cyotskeleton

·          Often have perivascular feet ending on blood capillaries, and forming part of a continuous perivascular sheath

 

a.    Fibrous astrocytes

·          Are found in the white matter (nerve tracts)

·          Surround and isolate axons from one another

·          Are small cells with few, long, slightly branched processes

·          Contain abundant bundles of glial filaments

 

2.    Oligodendrocytes

·          form the myelin sheaths of axons in the CNS

·          Small cells with few delicate processes

·          Have multiple branches that form myelin sheath of several different axons

·          Do not have a basement membrane separating them from axons

 

3.    Microglia

·          Very small cells

·          Are the macrophages of the CNS

·          Belong to the Monocyte-Phagocyte (MP) system

·          Found in both grey and white matter

·          Proliferate and become actively phagocytic following CNS damage

 

4.    Ependyma

·          Form a cuboidal or columnar epithelial lining of the central canal and ventricles of the CNS

·          Have microvilli and cilia on their apical surface

·          Rest on the internal glial limiting membrane formed of end processes of protoplasmic astrocytes

 

·          They may be subdivided into 3 types:

-         Ependymocytes line the ventricles – have gap junctions between adjacent cells; allow CSF to communicate with intercellular spaces

-         Tancytes line the floor of the 3rd ventricle – have long basal processes extending to capillaries

-         Choroidal epithelial cells – line the chodoid plexuses; they have tight junctions between adjacent cells 

 

 

Non-Neuronal Cells In The Peripheral Nervous System

 

1.    Schwann cells

·          Form the myelin sheath around axons in the peripheral nervous system

·          Each Shwann cell forms myelin sheath of only one axon

2.    Satellite cells

·          Cells surrounding the neuronal cell bodies and dendrites in peripheral nerve ganglia  

3.    Endoneurium, perineurium and epineurium

These are connective tissue sheaths associated with peripheral nerves.  The endonerium is a very delicate sheath that surrounds individual axons and small groups of axons; the perineurium surrounds bundles of axons; and the epineurium is a thick sheath surrounding the whole nerve.

 

 

Grey Matter And White Matter

Grey matter consists of:

1.    Neuronal cell bodies 

2.    Neuropil consisting of dendritic trees, axon terminals and synapses

3.    Neuroglia, mainly protoplasmic astrocytes  and microglia

 

White matter consists of:

1.    Tracts of nerve axons with their associated myelin sheaths

2.    Neuroglia, mainly oligodendrocytes, fibrous astrocytes and microglia

 

The Peripheral Nervous System

The peripheral nervous system consists of nerves and galnglia. There are two types of ganglia in the peripheral nervous system:

1.    Sensory ganglia.  These are

a.    Dorsal root ganglia on the dorsal roots of all spinal nerves

b.    Sensory ganglia of the cranial nerves

¨                                Trigeminal ganglion (V)

¨                                Genicular ganglion (VII, nervus intermedius)

¨                                Spiral ganglion of cochlea (VIII, auditory)

¨                                Vestibular ganglion (VIII, vestibular)

¨                                Superior and inferior ganglia of glossopharngeal (IX) nerve

¨                                Superior and inferior ganglia of the vagus (X) nerve

¨        Autonomic ganglia. These are:

¨        Sympathetic ganglia of the sympathetic chain

¨        Parasympathetic ganglia arising from their respective cranial nerves:

¨        Ciliary - Oculomotor

¨        Perygopalatine - Facial

¨        Submandibular - Facial

¨        Otic - Glossopharyngeal

 

Sensory Ganglia consist of pseudounipolar neurons, with spherical cell bodies and single axons. They are large neurons showing the typical large, euchromatic nucleus with a prominent nucleolus and prominent Nissl granules.  The neuronal cell bodies are surrounded by a layer of satellite cells.  They occur in clusters of neurons, separted by bundles of axons.  They are surrounded by endo-, peri- and endo-neurium.  There are no synapses in sensory ganglia.

 

Autonomic ganglia consist of multpolar nerve cells.   These are the cell bodies of postganglionic neurons.  They have a dendritic tree on which synapse the nerve terminals of the preganglionic axons.  The postganglionic axons are characteristically unmyelinated, while the pre-ganglionic axons are myelinated.  The cell bodies are surrounded by satellite cells, and are interspersed among a network of axons, dendrites and synapse.