The neurons are the structural units of the nervous system, which are excitable electrically. Different types of nerve cells have evolved to perform various functions in organisms, such as responding to touch, light, sound and other sensory inputs. The spinal cord and brain send signals to the motor neurons, which then trigger the contraction of muscles and influence the glands. Generally, a neuron has a cell body and two processes - an axon and dendrites.

The flow of neurons from the dendrites to the axon experiences a change in voltage. Environmental stimuli in sensory neurons activate ion channels, triggering action potentials to flow from the axon to the soma. Action potentials are the form of communication that neurons use to pass signals to other neurons, which is known as 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, electrical synapses are observed in heart muscles as a result of membranes linked together through specific proteins enabling ion-flow from one to another cell. Chemical synapses, on the other hand, take place as a result of the neural membranes coming in proximity however, leaving a space and remaining distinct.

The presynaptic terminal releases neurotransmitters into the synapse, while the postsynaptic terminal receives the neurotransmitters. The space between the two terminals is known as the synaptic cleft, and is used for communication between cells in the case of a chemical synapse.

The post- and pre-synaptic terminals both possess the molecular machinery needed for signalling. The pre-synaptic terminal is endowed with numerous vesicles filled with neurotransmitters. When an action potential arrives at the pre-synaptic terminal, voltage-gated Ca++ channels open, allowing Ca++ influx and triggering a sequence of molecules in the neuronal and vesicular membranes to activate. The activated molecules induce exocytosis of the vesicles, thus releasing the neurotransmitters.

The physical modification of the receptors found in the postsynaptic membrane by the associated structures induces the receptor to act as a pore for ions to pass. This can then have an excitatory (depolarizing, eliciting an excitatory response) or inhibitory (hyperpolarizing, inhibits response) effect on the postsynaptic cell, depending on the type of ion.

Neurons - PNS neurons and CNS neurons

The composition of neurons in the Central Nervous System (CNS) and Peripheral Nervous System (PNS) is only slightly different.

PNS Neurons

The PNS neurons are 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. 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, and Acetylcholine is the major neurotransmitter found here.

CNS Neurons

The Central Nervous System (CNS) consists of neurons made up of white and gray matter. The white matter is composed of nerve fibers and long nerve extensions, while the gray matter is composed of nerve cell bodies and a few short branches from these cell bodies. The CNS is made up of two main types of cells: glial cells and functional cells, which play essential roles.

The Central Nervous System (CNS) also has other supportive cells to the nerve tissues, such as microglial cells, oligodendrocytes, astrocytes, choroid plexus epithelial cells, and ependymal cells.

Neuron - Brief Structure

A neuron is a specialized cell in the nervous system that transmits information to other cells in the form of electrical signals. It is composed of a cell body, dendrites, an axon, and axon terminals. The cell body contains the nucleus and other organelles, while the dendrites receive signals from other neurons. The axon carries signals away from the cell body, and the axon terminals transmit signals to other cells.

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 an axon or nerve fiber extending to a muscle. Axons can stretch long, enabling electrical impulses to be passed to distances throughout the body.

Components of the Nerve Cell

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 requiring nerve impulses such as skeletal muscles.

The nerve synapse is the interface between two neurons. The cell body possesses many dendrites, increasing the surface area and allowing for 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 surrounded by a myelin sheath, which is an insulating protection.

Axon

The diameter of these fine projections is much longer than that of the cell body. The axon passes signals away from the cell body and its role and structure is almost the same in both CNS and PNS neurons.

Cell Body

The soma or cell body is the central part of the nerve cell, containing the smooth and rough endoplasmic reticulum, nucleus of the cell, golgi apparatus, and ribosome. It is in the cell body where similar processes take place, and majorly where the synthesis of proteins occurs.

Dendrites are the branch-like structures of a neuron that receive electrical signals from other neurons.

The dendritic tree is a branching structure that emerges from the cell body of a neuron. It is the region in which signals are transmitted to the neuron through synapses with other axons.

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

The myelin sheath, a protective layer networked through the nerve cells in the Central Nervous System (CNS), assists in the maintenance of the fluid content of the tissue engirdling the nerves. Furthermore, the axons surrounded in this sheath are capable of conducting nerve impulses at higher speeds compared to those that are not surrounded.

Glial cells during foetal development surround the axons multiple times. As the organism matures, glial cells lose most of their cytoplasm. Nodes of Ranvier, gaps in the myelin sheath, are essential for the conduction of impulses along the axon. These nodes are spaced apart at intervals of 1-2mm along the glial cells’ surface.

Approximately 50% of the nervous system is composed of glial cells. Their primary role is to provide support to the neuronal cells. In the peripheral nervous system (PNS), the type of glial cells are Schwann cells, and in the central nervous system (CNS), the type of glial cells are Oligodendrocytes.