Structural units of the nervous system, the neurons are excitable electrically. A range of anatomically distinct nerve cells have evolved to take part in various organismal functions. For instance, touch, light, sound and other sensory inputs are responded to by the sensory neurons. The spinal cord and brain send signals received by the motor neurons to commence the contraction of muscles and thereby impact the glands. A basic neuron structure comprises a cell body and neuronal processes – axon and dendrites.
There is a change in the voltage in the flow of neurons from the dendrites, soma to the axon. The environmental stimuli in sensory neurons activates ion channels generating action potentials flowing from the axon to the soma. Typically, the neurons pass signals in the form of action potentials through the axons which is the pathway of communication of nerve cells with other neurons. This is synaptic transmission.
Dendrites are receptive regions which have tapering extensions and are short. They assist to convey incoming signals to the cell body. Axons project from cone-shaped regions of the cell body known as axon hillocks. Extensions are the conducting areas of the neurons. Axons are the sites where the nerve impulses are generated and passed away from the cell body to the synapse. Neuron’s main biosynthetic center is the cell body. It comprises the neurotransmitters and other organelles required to generate chemicals and proteins.
Synapse in Neurons
In neurons, two types of structural synapses are electrical and chemical synapses. Observed in heart muscles, electrical synapses are as a result of membranes linked together through specific proteins enabling ion-flow from one to another cell. Chemical synapses on the other hand takes place as a result of the neural membranes coming in proximity however, leaves a space and remains distinct.
Neurotransmitters (chemicals) are used to communicate signals between cells in case of chemical synapse. The presynaptic terminal releases neurotransmitters into the synapse while postsynaptic terminal receives neurotransmitters. The space found in this region is the synaptic cleft.
The postsynaptic and presynaptic terminals, both have the molecular machinery required to perform signalling. The presynaptic terminal has many vesicles richly supplied with neurotransmitters. Arrival of an action potential at the presynaptic terminal causes the opening of the voltage gated Ca++ channels. This enables the influx of Ca++, in turn stimulating a string of molecules in the neuronal and vesicular membrane to activate. The molecules thus activated induce exocytosis of the vesicles, resulting in the release of neurotransmitters.
These structures then associate with the receptors found in the postsynaptic membrane inducing a physical modification. In turn, this alteration induces the receptor to serve as a pore in the membrane for ions to pass. The impact of the postsynaptic cell can be excitatory (depolarizing, eliciting an excitatory response) or inhibitory (hyperpolarizing, inhibits response), basis the ion type.
Neurons – PNS neurons and CNS neurons
There is only a slight difference in the composition of the neurons in the central nervous system (CNS) and peripheral nervous system (PNS).
PNS Neurons
A string of paired nerves extending from the spinal nerves emerging from the spinal cord and cranial nerves that take origin from the brain stem are the PNS neurons. These neurons are similar to the CNS neurons with a few differences only. The Schwann cells are the supportive cells surrounding the nerves in the PNS. Acetylcholine is the major neurotransmitter found here.
CNS Neurons
The CNS comprises neurons made of the white and gray matter. While the white matter comprises the nerve fibers, the long nerve extensions, the gray matter comprises the nerve cell bodies and a few short branches from such cell bodies. There are two main types of cells in the CNS – glial cells and functional cells depicting crucial roles.
The CNS has other supportive cells to the nerve tissues too, such as microglial cells, oligodendrocytes, astrocytes, choroid plexus epithelial cells and ependymal cells.
Neuron – Brief Structure
A basic neuron can be represented by a motor neuron wherein the cell body is found in the gray matter of the spinal cord and axon or nerve fibers extending to the muscle. Axons can stretch long enabling electrical impulses to be passed to distances throughout the body.
Components of the Nerve cell
Basically, the neurons consist of the soma (cell body), with an axon (single long nerve fiber) extending from one end of the cell body and at the other terminal is connected to another nerve cell body or to a structure needing nerve impulses such as skeletal muscles.
The nerve synapse is the interface between two neurons. Cell body in turn possesses many dendrites increasing the surface area causing other axons to link with the cell body. Typically, the cell body has links with other axons with various synapses on the cell body. Axons are ideally surrounded by a myelination or insulating protection, this is the myelin sheath.
Axon
These fine projections are much longer in diameter than that of the cell body. The axon passes signals away from the cell body. Their role and structure is almost the same in both CNS and PNS neurons.
Cell Body
It is the central part of the nerve cell containing the smooth and rough endoplasmic reticulum, nucleus of the cell, golgi apparatus, and ribosome. The soma or the cell body contains organelles wherein similar processes take place. However, it is the cell body majorly where the synthesis of proteins takes place.
Dendrites
They are the branched structures emerging from the cell body. These branching structures of the dendrites are referred to as the dendritic tree which is the region wherein the signals to the neurons take place through synapses with other axons.
The nerve impulses mediate from other neurons to the cell body and then are conducted along the axon of its own to other cell bodies. Nerve impulses are unidirectional.
Glial cells
These cells lay a protective layer, networked through the nerve cells in the CNS. It assists in the maintenance of the fluid content of the tissue engirdling the nerves. This protective layer, the myelin sheath and axons surrounded in this sheath are capable of conducting nerve impulses at higher speeds compared to those that are not surrounded.
The glial cells during the foetal development surround the axons many times. With maturity, glial cells lose most of its cytoplasm. The Nodes of Ranvier are gaps in the myelin sheath pivotal for the conduction of the impulses through the axon. These nodes are present periodically (at a distance of 1-2mm) along the surface of the glial cells
50% volume of the nervous system is accounted for by the glial cells. Their main role is to render support to the neuronal cells. The types of glial cells in PNS are Schwann cells and those in CNS are Oligodendrocytes.
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