Nerve, neuroglia, synapse, nerve transmission

The nervous system is the body's primary system for communicating and controlling. However, none of the bodily systems can work by themselves. All are interconnected and function as a single system to maintain the body's normal conditions. The nervous system and the endocrine system work together to communicate with the billions of cells that make up the human body. Both systems transfer data from one body part to another. The endocrine system secretes chemicals into the blood stream, while the nervous system transmits information more slowly by sending nerve impulses.A person's internal harmony (homeostasis) and the balance between the individual and the environment are maintained by the nerve impulse given to the various organs by the nervous system. The nervous system is in charge of our actions, perceptions, memories, and all voluntary motions in addition to aiding in homeostasis.

Structure of the Nervous System

Structurally, the nervous system can be divided into two major parts:

• The central nervous system (CNS), which consists of:
  • The brain,
  • The spinal cord.
• The peripheral nervous system (PNS), which consists of all nervous tissue outside the CNS:
  • The spinal nerves,
  • The cranial nerves,
  • Autonomic nervous system (ANS), which consists of sympathetic and para sympathetic system.

Functions of the Nervous System

• It communicates with the body's cells via electrical impulses to signal the body.
• It controls and keeps up bodily processes.
• Through cranial and spinal nerves, sensory neurons of the nervous system transmit sensory data to the brain and spinal cord.
• The nervous system interprets sensory input and decides what action to take at any given time.
• The nervous system responds to sensory stimuli with a motor action.
• In times of stress or emergency, it defends the body.

Nervous Cells and Tissue

The nervous tissue consists of two types of cells: neurons and neuroglias. Neurons conduct impulses, whereas neuroglia supports neurons. Together the two types of cells form the structures of both the CNS and PNS.

Neurons

The structural and operational unit of the nervous system is the neuron or nerve cell. They are highly specialized cells that transport messages from one area of the body to another via nerve impulses. Due to their high metabolic rate, neurons need constant and plentiful amounts of oxygen and glucose. Without oxygen, neurons cannot last more than five minutes. The size and structure of neurons varies. They transmit impulse or action potential signals from neurons to other neurons as well as to other body cells. There are three components to each neuron:

• A cell body,
• Dendrites,
• Axon.

The main structure of neurons is called a cell body, or perikaryon, which houses the majority of the cytoplasm as well as many organelles like the mitochondria, golgi apparatus, and nucleus that are typically found in cells. Nissl granules, which are also found in the cell body, aid in the neuron's protein production. In the cell body, close to the axon hillock, where the axon joins the cell body, neurofibrils can be seen.

The diffusely branching, short, tapering projections of the cell body are called dendrites. Most neurons have shorter processes than axons. They are receptive or input areas with a large surface area for receiving signals from other neurons and sending them to the cell bodies of the neurons. Nissl bodies, mitochondria, and other organelles can be found in their cytoplasm. Dendrites are not myelinated like axons are.

In order to transmit impulses away from the cell bodies of the neurons, the axons are one long projection from the cell body. Myelin sheath, an insulating substance, surrounds it. Schwann cells produce the white, fatty myelin sheath, a substance with a protein and lipid composition that wraps around some axons outside the central nervous system.

Myelinated axons are those that have a myelin coating, while unmyelinated axons do not. The myelin coating electrically insulates the neuron's axon, shields it from damage, and speeds up the transmission of nerve impulses. Nodes of Ranvier are depressions between adjacent Schwann cells that facilitate faster nerve impulse transmission.

Neurilemma, the name for the Schwann cell's outer cell membrane, is only present around axons in the PNS. It is crucial for the regeneration of damaged and cut axons. As a result, compared to the peripheral nervous system, cell regeneration occurs much less frequently in the brain and spinal cord.

Neuroglia

The Greek term glia, which means glue, is the source of the English phrase neuroglia. When these cells were given their names, it was believed that their main function was to hold or "glue" together CNS cells. In general, neuroglia are 5 to 50 times more abundant and smaller than neurons. These unique cells of the connective tissue do not transmit impulses. Neuroglia come in six different subtypes:

• Astrocytes,
• Oligodendrocytes neurons,
• Microglia,
• Ependymal cells,
• Schwann cells,
• Satellite cells.

Of the six types of neuroglia, four- astrocytes, oligodendrocytes, microglia, and ependymal cells- are found only in the CNS. The other two types-Schwann cells and satellite cells are found in the PNS. One major function of neuroglia cells is to hold the functioning neurons together and protect them.

Neuroglia of the CNS

• Astrocytes: They are named for their "star" shape and are the largest and most abundant glial cells. They have many processes which create a supporting network for neurons also help to transport nutrients from blood to neurons.
• Oligo Dendrocytes: They are smaller and contain fewer dendrites and a short axon. They produce myelin sheath.
• Microglia: They are small cells which thought to protect nerve cells against infection.
• Ependymal Cells: They form a sheet that lines the ventricles of the brain and the central canal of the spinal cord. These cells produce and help in the circulation of cerebrospinal fluid.

Neuroglia of the PNS

• Satellite Cells: These cells surround neuron cell bodies within ganglia and regulate the exchange of materials between neuronal cell bodies and interstitial fluid.
• Schwann Cells: They surround and form myelin sheaths around the larger nerve fibers in the PNS. These cells play vital role in axon regeneration in the PNS.

Grey and White Matter

There are areas of the spinal cord or any section of the brain that seem white, and there are other areas that are a deeper, grey tint. The white and gray matter are made up of these, respectively. Microscopic analysis reveals that only the grey matter, which also comprises dendrites and axons beginning from or ending on the cell bodies, contains the neuronal cell bodies. The majority of the grey matter's fibers are unmyelinated. On the other hand, myelinated fibers make up the majority of the white matter. This area appears yellowish due to the myelin's ability to reflect light. Both the grey and white matter contain neuroglia and blood vessels.

The arrangement of the grey and white matter differs at different regions in the brain and spinal cord. In the spinal cord and brainstem, the white matter is on the outside, whereas the grey matter forms one or more masses embedded within the white matter. In the cerebrum and cerebellum, there is an extensive, but thin, layer of grey matter on the surface. This layer is called the cortex. Deep to the cortex, there is white matter, but within the latter, several isolated masses of grey matter are present.

Isolated spherical masses of grey matter, present anywhere in the CNS, are referred to as nuclei whereas aggregations of the cell bodies of neurons, found outside the CNS, are referred to as ganglia. Fibers connecting two grey matters nuclei form aggregation which is called as tract.