Human Nervous System

The nervous system is defined as a systematic collection of cells designed for the transmission of electrochemical inputs from sensory neurons to the region where a reaction occurs through a network. The nervous system consists of a network of neurons and synapses that transmit information from the brain and spinal cord to other portions of the body. Organisms adjust their movements and orientations in reaction to changes in the external environment. The message that an organism gains from the external environment are defined as a stimulus. The response of the nervous system is defined as the conduction of the stimulus in the form of an electrochemical message. Some of the most common stimuli include the following sound, light, air, heat, smell, taste, and gravity. 

The article defines the human nervous system along with the nervous system diagram, the article specifically discusses the nervous system function and the nerves in the human body. The article also describes the classification of the human nervous system along with a discussion of the central nervous system diagram. The nervous system facts function and diseases are briefly discussed in the article.

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Nervous System Function and Description

We have developed a basic understanding of the nervous system and have seen a simple nervous system diagram. The nervous system function can be better understood by understanding the structure and functions of nerves in the human body. Let us see the structural details of the nerves.

• The nerve is made up of nerve fibre, which contains neurons.

• The sensory nerve contains the afferent fibres which have the characteristic feature of the long dendrites.

• The motor nerves constitute the efferent fibres that have characteristics of long axons. The motor neurons are an example of such nerves. 

• A mixed nerve contains both the sensory and motor neurons.

• The epineurium, also known as the connective tissue sheath, is the protective covering of the nerves, it is important to not be confused with the myelin sheath, which is the outer covering of neurons.  

• A bundle of nerve fibres is known as fasciculus. It is encircled by a layer of connective tissue called the perineurium.

• Perineurium contains blood vessels.

• The solitary nerve fibre (axon), along with myelin and neurilemma, is surrounded by connective tissue called the endoneurium within the fasciculus.

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Neuron: Structure and Function

Neurons are defined as the basic structural and functional units of the nervous system. They are the primary elements that are responsible for the transmission of the signal. The nervous system function is dependent on the functioning of neurons. The neurons have the following important parts.

• Dendrites- They are responsible for the receiving of the signal.

• Cell Body- It is the part where the signal from dendrites integrate, it contains the nucleus and cytoplasm. It is the metabolic centre of the cell.

• Axon- It is the long-tailed structure of neurons, it is myelinated by oligodendrocytes in CNS and Schwann cells in PNS. It carries the electrical impulse out from the cell body.

• Axon Terminals- They are the knob like structure present at the end of the axon, they contain various neurotransmitters and VGIC (voltage-gated ion channels. They are responsible for transmitting the electrical signal into a chemical signal in the synapse.

• Synapse - It is the space between two neurons. It is responsible for the unidirectional movement of the signal.

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General Mechanism of Nervous System Function

The nerve impulse is generated by the stimulus which is then sensed by the dendrites of the sensory neurons. The impulse is then carried out as an electrochemical signal from neurons to neurons to the brain. An appropriate response is generated by the brain which is carried as an electrochemical signal to the motor neurons which relay the information to the affected organ.

The Classification of the Nervous System

Since we have developed a basic understanding of the nervous system and the nerves in the human body, let us look into the classification of the nervous system. The classification of the nervous system is very important as it helps in understanding the mechanism of the transmission of the stimulus throughout the human nervous system. It is also important to keep in mind that the nervous system defined here is for the organisms that belong to highly evolved categories of evolution like vertebrates.

The nervous system is broadly classified into the Central Nervous System and the Peripheral Nervous System. The Central Nervous System is further classified into the Autonomic Nervous System and the Somatic Nervous System. The Autonomic Nervous System is further classified into two groups Sympathetic and Parasympathetic Nervous System. The classification of the nervous system diagram mentioned below will help in a better understanding of the classification.

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Central Nervous System

The Central Nervous System (CNS) is commonly referred to as the body's central processing unit. It is made up of two parts: the brain and the spinal cord. The Central Nervous System (CNS) is responsible for integrating and responding to the information, and commands. CNS is also responsible for coordinating as well as influencing all other activities within the body. The brain is divided into three main parts namely, the forebrain, midbrain and hindbrain. These are all important for the proper functioning of the Central Nervous System. The spinal cord is best defined as the nerve bundle that is responsible for spinal reflex actions and the conduction of impulses to the brain and from the brain. The central nervous system diagram mentioned below will help in better understanding.

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The cells of the brain are an important part of the nervous system although neurons are the cells that are directly involved in the stimulus transmission, these cells help in the proper functioning of the nervous system. These are also known as supportive glial cells.

Astrocytes- They are involved in providing nourishment to the nerves in the human body. They are the most abundant cells of the CNS, they are also involved in the protection of the cell body of the neuron. Astrocytes are responsible for the formation of the blood-brain barrier.

Microglial cells- They are specialised macrophages, they are the least abundant cells of the CNS. One of the important points to note here is that they are of non-nervous origin, they are of mesodermal origin.

Ependymal cells- These cells are responsible for the production of the cerebrospinal fluid, they are also responsible for lining the brain cavity.

Oligodendrocytes- They are the most important cell of the CNS, they myelinate the neuron. Myelination is the process of enveloping the neuron with a layer of the plasma membrane, this is done to lower the capacitance of the neuron and increase the resistance. This results in the saltatory movement of the impulse. An important modification to note is the increased concentration of the sphingolipid in the plasma membrane in the nerves in the human body.

Peripheral Nervous System

The nerve that extends out through the central nervous system makes up the peripheral nervous system. The transmission and response network between the CNS as well as the internal organs is formed by such nerves. There are two subsystems of the peripheral nervous system namely the Autonomic and somatic nervous systems. The somatic nervous system is made up of nerves that extend from the brain to the skin and muscles. Nerves that connect the CNS to visceral organs like the heart, stomach, and intestines make up the autonomic nervous system.

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Autonomic and Somatic Nervous System

The autonomic nervous system deals with the involutory processes of the body, that it functions in the involuntary responses. The ANS relays information to internal organs. The ANS is further classified as the sympathetic and parasympathetic nervous systems. The Sympathetic nervous system works in the condition of the emergency response (fight or flight) while the parasympathetic works when the body is at rest or with normal functions. The autonomic nervous system diagram with the classification of parasympathetic and sympathetic nervous system diagrams will help in understanding.

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The somatic nervous system refers to the subset of the Peripheral Nervous System (PNS), this subset is responsible for controlling and coordinating the voluntary action of the body. It achieves the nervous system function with help of nerves attached to teh skeletal muscle.

In conclusion of the article, we have learnt about the human nervous system along with its simple nervous system diagram. We also have learnt about cells of the nervous system, we have also learnt about the subsets of the human nervous system.