The Nervous System is a complex network of nerves and cells that is spread out all over the body transmitting signals to and from the brain and spinal cord. The signals help in building up communication within the body. Thus all the actions and reactions happen in the body and extend towards the surrounding. It controls both our voluntary and involuntary actions.
The Nervous System of our body is divided into two parts:
Central Nervous System (CNS): It is the main control room of the Nervous System which controls all the movements, actions and reactions that happen in the body. It consists of the brain and the spinal cord.
Peripheral Nervous System (PNS): The rest of the parts that constitute the Nervous System other than the CNS make up the PNS. Its main function is transmission of signals. This PNS is further divided into three parts:
Somatic Nervous System: It controls the voluntary reactions of our body.
Autonomic Nervous System (ANS): It controls our body’s involuntary reactions. This ANS is again subdivided into three parts, namely:
Sympathetic Nervous System (SNS): It takes control of the nerves of our inner organs which is not under our conscious control such as increase in heartbeat, breathing, and decrease in digestion etc. due to certain stress, danger, excitement, embarrassment, etc.
Parasympathetic Nervous System (PNS): It relaxes the smooth involuntary muscles of the internal organs and glands after going through some kind of stress. It neutralises the situation caused by SNS by decreasing the heart rate and breathing while increasing the digestion process.
Enteric Nervous System (ENS): It is a complex network of nerve fibres which receives the signals and transmits it to the SNS and PNS.
The CNS is majorly made up of brain and the spinal cord. The brain is divided into four parts known as cerebrum, cerebellum, diencephalon and medulla oblongata. It weighs around 1.3 kilograms and is made up of a grey and white matter. The grey matter is composed of gray nerve cell bodies; neurons also known as nerve cells, and its helper cells, neuroglia also known as glial cells. The white matter is a part of the neuron known as axons which are covered with a covering known as the myelin sheath.
The spinal cord is extended from the brain through the medulla oblongata and it has a series of 31 segments.
The brain and spinal cord are covered by a three-tissue-layered membrane known as meninges. The outermost layer is known as the dura mater, the middle layer is known as the arachnoid and the innermost is the pia mater. It protects both the brain and spinal cord against bacteria and microorganisms.
There is also a fluid known as the Cerebrospinal Fluid (CSF) which circulates and fills the cavities in the brain and spinal cord. It acts as a shock absorber between the brain and skull, and between spinal cord and vertebrae. It reduces the weight of brain and spinal cord by creating buoyancy. It also helps in removing the waste products from the CNS.
Neurons: These are the small nerve cells that spread all around the body to receive and transmit signals. It is made up of three parts known as the cell of the body, dendrites which are small and thick extensions from the cell body and the axons which are the long thin processes from the cell body.
Neurons can be divided in two ways; according to the number of axons and dendrites, and according to its function. According to the number of axons and dendrites, neuron is divided into three types:
Multipolar neurons: It has only one axon but a number of dendrites. Such neurons are available mostly in the brain and spinal cord.
Bipolar neurons: This type of neuron has only one axon and one dendrite. It is found in the retina of the eye, the inner ear and the olfactory area (smelling area of nose).
Unipolar neurons: There is only one extension from the cell body and it further divides into two parts, one axon and one dendrite. This neuron is seen in the spinal cord.
According to its function also, it is divided into three types:
Afferent neurons: It receives sensory signals from the receptors of the body, which are a group of cells that can detect the changes, to the CNS. They are also known as sensory neurons.
Efferent neurons: It transmits signals from the CNS to the effectors of the body such as muscles and glands in order to react to the changes. They are also known as motor neurons.
Interneurons: It does the integration of information from afferent neurons and its response through the efferent neurons.
The Peripheral Nervous System consists of nerves and neurons that are extended or found outside the CNS. Nerves carry signals between the CNS and rest of the body. It is made of bundled-up axons. Individual axons bundle-up together to form fascicles and is wrapped in by a layer of connective tissue known as perineurium. Then, a number of fascicles are wrapped together into a bundle covered with another connective tissue layer known as epineurium. This forms a whole nerve. The layers of connective tissue protect the nerves and also help in increasing the speed of its signal transmission. There are afferent nerves that carry signals in only one direction i.e. from the sensory receptors to the CNS, and then there are the other nerves known as efferent nerves that also carry signals in only one direction but from CNS to the effectors. There are other nerves that carry signals in both the ways, and are known as mixed nerves. There are two major types of nerves in the PNS. They are:
Cranial nerves: The cranial nerves are extended from the brain and there are 12 pairs of cranial nerves. They connect and communicate the sense organs, muscles of the head, neck and shoulders, the heart and the gastrointestinal tract with the brain.
Spinal Nerves: These are mixed nerves and there are 31 pairs of spinal nerves which are extended from the spinal cord. They are divided into 5 groups; 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and a pair of coccygeal nerves.
The functions of Nervous System are sensory, integration and motor. The sensory function has the nerves collecting information from the sensory receptors of the body and transmitting it to the CNS. In the process of integration, the signal received in the CNS is processed, evaluated and compared for decision making. Then in the motor function, the evaluation takes the form of an action which is then transmitted to the effector cells which either release hormones or move a part of the body in response to the change occurred in the body condition.
Any dysfunction in any part of the Nervous System will lead to one or the other disorder of the Nervous System which is known as a neurological disorder and give rise to diseases. Some of the major ones are:
Alzheimer’s disease: It is a progressive disease related to the dysfunction of the brain cells. Here the victim suffers from impaired memory, thinking and behaviour, confusion, restlessness, inability to follow directions and language deterioration.
Parkinson’s disease: It is caused due to the death of dopamine (chemicals that send out signals from neuron to the body cells) producing brain cells. The patient shows the symptoms such as, trembling of limbs and face, stiffness of the limbs and trunk, slowness of movement, impaired balance and coordination.
Stroke (Brain Attack): It is caused due to the disruption of blood flow in the brain which is due to the death of brain cells. A brain attack has several symptoms such as, weakness of one side of the limbs or the face, confusion or difficulty while speaking, vision problems, dizziness, impaired balance, severe headaches etc.