Histology of The Human Nervous System by Fakeye, University of Ibadan
The human nervous system is one of the most complex systems in the body is made up of a large variety of cells which are generally grouped into two:
Neurons (nerve cells); and Glial cells. The human nervous system is divided into two anatomically: The Central nervous system(CNS); and the peripheral nervous system(PNS). CNS consists of the brain and spinal cord while the PNS consists of the peripheral nerves, cranial nerves, spinal nerves and the ganglia (nerve cells located just outside the CNS).
The neurons serve as the functional unit in both the PNS and CNS. Every cell has a certain ionic gradient maintained between the outer and inner layer of its membrane. Stimulus is an environmental change capable of causing a neuron or excitable cell to reverse the ionic gradient of its membrane. This change is known as membrane depolarization.
The two major characteristics of a human nervous system that allow the neuron to serve as the functional unit in both the CNS and PNS are its ability to interconnect with neighbouring neurons and cells, and it is an excitable cell (it responds to stimuli by membrane depolarization).
Perikaryon- this is the central part of the neuron and is the centre of growth in the neuron. It is spherical in shape and relatively large in size. Its nucleus is also spherical and euchromatic and has a prominent nucleolus. Most neurons are uninucleated however some binucleated neurons are found in the sympathetic and sensory ganglia. Subcellular structures found within the perikaryon include rough endoplasmic reticulum, ribosomes, microtubules etc. The perikaryon is the part to which all the processes of the cell are connected and it receives stimuli through the dendrites.
These are relatively short processes which are responsible for the reception of stimuli. They are usually more than one and the thickness of each dendrite is not even throughout. Dendrites have small projections with lengths ranging from 1-3 micrometres known as dendritic spines which occur in very large numbers (about 10^14 per neuron). The spines are responsible for receiving the stimuli through synapses located on their surfaces.
Axons are relatively long processes specialized in creating and conducting neural impulses to other cells. Axons are of uniform thickness almost throughout and are longer than dendrites. They originate from the axon hillock and the distal portion of the axon branches into terminal arborizations which end in bulb structures known as terminal boutons. Terminal boutons interact with other excitable cells at synapses.
Types of Neurons in human nervous system
Neurons can be grouped into types based either function or structure.
Based on structure there are 3 types namely: Unipolar neurons( which have only one process but are also called pseudounipolar because this process splits into two with the longer branch connecting to a peripheral ending and the shorter one to the CNS); Bipolar neuron which have only one dendrite and an axon; and Multipolar neurons which have at least two dendrites and one axon.
Histology of Human Nervous System continued…
Based on function there are three types: Motor neurons (which are in charge of the effector cells); Sensory neurons(which are in charge of receiving stimuli from their environment or through sensory epithelia); and Interneurons which are responsible for interconnecting neurons.
The integral proteins of the axolemma(membrane of the axon) pump Na+ ions out of the axoplasm(cytoplasm of the axon) so as to maintain a constant concentration of Na+ which is 0.1 of the concentration in the extracellular fluid. The cellular concentration of K+ is far greater than the extracellular concentration thus making a potential difference of -65mV across the axolemma( this is the resting membrane potential). When stimulated ion charges in the neuron open and extracellular Na+ ions flow in increasing the p.d. to +30mV(signifying the beginning of the nerve impulse which behaves like a wave). The +30mV potential closes the Na+ channels and opens the K+ channels allowing K+ ions to diffuse out and the p.d returns to -65mV thus ending the nerve impulse this entire process lasting about 5 milliseconds. This process, however, continues throughout the axolemma thus forming the nerve impulse which eventually travels to neighbouring cells.
Furthermore on the histology of human nervous system
Glial cells act as supporting cells in the nervous system. They are not all present in both the CNS and PNS. They are far more abundant than neurons although smaller in size. They include oligodendrocytes, astrocytes, ependymal cells, microglia, Schwann cells and satellite cells of ganglia.
These cells like neurons have a spherical perikaryon and also have a few numbers of processes. Unlike neurons, however, the perikaryon is smaller and the dendrites are wrapped around the axons of surrounding neurons. The cell is responsible for myelinating axons so as to speed up nerve impulses. Myelin is a fatty compound which insulates most of the axon thereby speeding up nerve impulses. Myelin sheaths occur with nodes between them. This speeds up nerve impulses by making them jump from node to node instead of travelling the whole length of the axon. Oligodendrocytes are found within the CNS alone and are the predominant glial cells in CNS white matter.
These are found in the CNS and have a large number of processes. They are the most numerous glial cells in the body.They are star-shaped glial cells and occupy interneuron spaces. Their main functions are providing mechanical support, they deal with the exchange of metabolites between blood vessels and neurons and they regulate the composition of the intercellular environment of the CNS.
These are located in the CNS. They are columnar or cuboidal cells lining the ventricles of the brain and central canal of the spinal cord. They possess cilia on their apical ends which facilitate movement of cerebrospinal fluid. Like epithelial cells, they are joined by similar junctional complexes. However, they lack basal lamina.
Unlike other glial cells which originate from the neural tube, these originate from the bone marrow. They are found in both the grey and white matter of the CNS and constitute the major immune defence mechanism in the CNS
These are found only in the PNS and are responsible for myelination in the PNS. However, unlike oligodendrocytes, once the cell is capable of myelinating only one segment of one axon.
These are located in covering large neuronal cell bodies in the ganglia of the PNS. However, their function is not fully understood yet.