In all vertebrates and most invertebrate animals, the center of the nervous system is the brain. Location is near the sensory organs i.e., in the head for senses like vision. In the vertebrate’s body, it is the most complex organ. The cerebellum has 55-70 billion neuron and the cerebral cortex have approx. 14-16 billion neurons. One neuron is connected to thousand of other neurons by synapses. Axons are long fibers that are used as a communication tool between all neurons. The signals to recipient cells are carried by these axons. All the organs of an individual are under the control of the brain. By secretion of hormones and by generating signals brain act on the rest of the body. To the changes in the environment, our body reacts through these rapid and coordinated responses. Brain is treated as a biological computer in the modern world. There are similarities between the electronic computer and the brain in the way they collect information from the surrounding and respond according to that and also stores important information.  

Nervous System

Besides the central nervous system (CNS) nervous system has one more component i.e., the peripheral nervous system (PNS). Outside the brain and spinal cord, PNS consists of nerves and ganglia. PNS serves as a transmitter between spinal cord and body. CNS is connected to limbs by PNS. It is more prone to mechanical injuries and is exposed to toxins as it is not protected by the blood-brain barrier. There is no vertebral column and skull in PNS to protect it as present in CNS. Normally PNS has two parts, i.e., autonomic nervous system and somatic nervous system. Cranial nerves are a part of PNS in the somatic nervous system but exceptions are always there. The optic nerve and retina don’t have cranial nerves. Diencephalon’s tract is not a true peripheral nerve. CNS gives rise to cranial nerve ganglia. PNS have left with ten cranial nerves whose axons extend ahead of the brain. Involuntary control over smooth muscles and glands is plied by the autonomic nervous system. Sympathetic and parasympathetic is the two-system formed because of the connection between organs and the CNS. These two are the different functional states.

Peripheral Nervous System

 

Autonomic nervous system and somatic nervous system are the two divisions of the peripheral nervous system. The brain transmits signals to end organs like muscles and it is done voluntary by the somatic nervous system. Signals from senses such as touch, smell, and taste are sent to our brain and spinal cord through the sensory nervous system which is a part of the somatic nervous system. The functioning of the digestive system and heart and all those systems which are self-regulating and are not in voluntary control come under the autonomic nervous system.

Somatic Nervous System

Sensory nervous system and somatosensory nervous system are the parts of somatic nervous system which includes somatic nerves, sensory nerves and other nerves which have both somatic and sensory functions. Somatosensory data is carried by cranial nerves in the neck and head. Ten cranial nerves originated from the brainstem out of twelve and have the authority for the functions of anatomic structures of the head. Some exceptions can be seen in their functions. Organs of the thorax and abdomen send sensory information to the vagus nerve which is a unique cranial nerve. Trapezius and Sternocleidomastoid muscles get innervated by accessory nerves while both of these nerves don’t belong to the head. Somatosensory information is one of the responsibilities of spinal nerves for the rest of the body. Spinal cord give rise to spinal nerves. Single spinal cord arises from bunch of interconnected nerve roots. The functions of rest of the body is regulated by these nerves only. There is total of 31 pairs of spinal nerves are present in humans which 12 thoracic, 5 sacral, 8 cervical, 5 lumbar, and 1 coccygeal. In spinal Vertebrata there are many locations and the nerves in these locations are named after them like the nerves in the lumbar region are called as lumbar spinal nerves and so on. Spinal nerve roots come out beyond corresponding vertebrate in cervical region. Spinal nerve roots in region of thoracic to coccygeal comes out beneath corresponding Vertebrata. In the naming of these spinal nerve roots from C7 and T1 there are some complications. In dural sac the spinal nerve roots travel in sacral and lumber region. As cauda equina the spinal nerve travel below the level of L2.

Cervical Spinal Nerves

 

C1 to C4 are the four cervical spinal nerves. The nerves that serve the back of the head and neck arise by the splitting and recombination of these nerves. Motor interventions at the base of the skull are provided by the suboccipital nerve which is spinal nerve C1. Sensory and motor control is given by C2 and C3 to the back of the neck. Sensation to the back of the head is given by the greater occipital nerve and the lesser occipital nerve is responsible to give sensation behind the ears. Nerve roots C3, C4, and C5 give rise to the nerve which is very important for our survival and that is called as phrenic nerve. In the thoracic diaphragm, the nerve which is supplied is the phrenic nerve and of which breathing is enabled. Spontaneous breathing is not possible if we cut the spinal cord above C3.

Branchial plexus

Branchial plexus which is also called plexus brachialis is a bunch of tangled nerves which is formed by the last four cervical spinal nerves C5 to C8 and T1 which is the first thoracic spinal nerve used to serve the upper limb and upper back. Little variation is seen in people but the branchial plexus is very organized and predictable even if it seems to be tangled.

Lumbosacral plexus

The lumbosacral plexus is formed by the anterior division of the coccygeal nerve, lumbar nerve, and sacral nerves. From the twelfth thoracic branch, the first lumbar nerve is usually joined. It is divided into three parts that are-

• Sacral plexus
• Lumbar plexus
• Pudendal plexus

Autonomic Nervous System

Regulation of physiological functions is controlled by involuntary responses which are under the authority of the autonomic nervous system (ANS). Smooth muscles such as of heart, exocrine, bladder, endocrine-related organs are connected to the central nervous system of the brain and spinal cord by ganglionic neurons. Dilation, salivation of saliva, and pupil constriction are the most noticeable physiological effects of autonomic activity. In a sympathetic or in a parasympathetic state, the autonomic nervous system always remains active. Different kinds of neurotransmitters are released depending on the need of the body so to fulfill the need one state of the autonomic nervous system can overshadow the other state.

Sympathetic Nervous System  

In situations where physical danger and mental stress are encountered the sympathetic nervous system gets activated. During such conditions, the activities like digestion which is not so important for survival at that time are reduced and heart rate and blood flow are increased in some areas like muscles because of the release of norepinephrine and epinephrine which are neurotransmitters. Some parts of the body take a rest and some parts become active as the system is independent of each other.

Parasympathetic Nervous System

Parasympathetic system turns the body into a rest and digest stage when acetylcholine is used as a mediator which is a neurotransmitter. There is an increase in metabolic activities of digestion and salivation there is a decrease in sympathetic responses and heart rate when the body is dominated by the parasympathetic system. In humans, there are some voluntary controls in the parasympathetic system, unlike the sympathetic system. Defecation and urination are the best examples of this.

Enteric Nervous System

Enteric nervous system is a division of the autonomic nervous system. From the whole body, it can get signals but is usually located near the digestive tract. Without the input from parasympathetic and sympathetic branches, it can control the local systems. For the various functions related to the gastrointestinal system, the enteric system is responsible.

Functions:

• The connection between the spinal cord, brain, and the rest of the environment is the peripheral nervous system.
• Internal homeostasis is regulated by peripheral nervous system.
• Strength of muscle contractibility can be regulated by this system.
• Exocrine gland’s secretions are controlled by peripheral nervous system.
• Senses- Peripheral nervous system is responsible for letting us know what is happening around us. In the peripheral nervous system, the somatic nervous system is responsible for this function. As if the superpower of our body is our brain but without input from outside it can’t do anything. That’s why we need a peripheral nervous system. Our brain is similar to a computer, just like the computer needs peripheral devices like a keyboard, microphone, and camera to gather information our brain also needs information from outside to work. By entering and exiting the spinal cord the peripheral nervous system travels in our body. Cranial nerves are directly attached to the brain but peripheral nerves are not. Signals to ears, nose, and mouth are carried by cranial nerves. Sense of touch on your whole body is also given by cranial nerves. Through every part of the body, these peripheral nerves are entangled in it. It can also be founded at the toes and at the tips of fingers. To gain outside information, there are nerves in our feet and hands which are sensory nerves that give sensations to our brain through motor nerves.
• Movement- Signals from the muscles to the brain reaches by peripheral nerves that are branched towards outside the body. Scratching your nose, doing all tasks, juggling, and all activities are done with the help of these muscles.
• Unconscious Process- Without even our thinking autonomic nervous system starts working. We are alive because our brain is continuously working. The peripheral nervous system is the one that controls all these functions. For e.g., blood pressure, heart rate, digestion, etc.

Diseases

Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

Body nerves are the primary target of this disease. This is a neurological disorder. This is not necessary that the symptoms are shown from the start of this disease. Sometimes, the symptoms come back after vanishing for a long time. This disease needs to be cured as soon as it gets diagnosed in the body. In comparison with men and women, this disease is more seen in older adults. This disease is not easily diagnosable. In the U.S. there are approx. 40,000 people with the symptoms but it is not confirmed how many of them are suffering from this disease. 

Symptoms

Pain and numbness of different areas, tiredness, weakness in legs and arms, and reflexes get slow and it is not compulsory that everyone can have the same symptoms. Everyone is diagnosed with different symptoms. For to be sure that the disease is CIDP the symptoms should exist for 8 weeks.

Cause

The exact reason that causes this disease is not known. The reason behind the inflammation of nerve roots and nerves is not known. Due to the inflammation myelin can be destroyed which is the protective covering of nerves. The ability of the nerves to send the signals can be destroyed due to this swelling. Pain, numbness, and weakness are caused by this inflammation and destruction of myelin.

Ganglioneuroma

Autonomic nerve fibers have tumors that start from neural crest sympathogonia which is known as ganglioneuroma. Immature elements are not there in ganglioneuromas, they are fully differentiated neuronal tumors. The most prone area to this tumor is the abdomen but wherever sympathetic nervous tissue is found this tumor can be found there. Neck, posterior mediastinum, adrenal glands, and paraspinal retroperitoneum are the other prone areas to this tumor.

Symptoms

Main symptom is that the organ gets affected where it is developed. Difficulty in breathing, chest pain if the tumor is in the chest, bloating and abdominal pain is seen when the tumor is located in the abdomen, sensation in legs and arms are lost, pain in legs and arms, spinal deformity and spinal compression is seen when the tumor is in the spinal cord or near it. Increased sweating and body hair, high blood pressure, and diarrhea can be seen because due to the tumor some hormones are released which causes all this in the patient.

Cause

Exact cause is still not known and for ganglioneuroma, no risk factor is known. It can be caused by genetic problems like neurofibromatosis type1. Usually, people more than 10 are more prone to this disease. This is a slow growing disease.

Autonomic Neuropathy

Nerves which control the autonomic body functions seem to be damaged then this disease occurs i.e., autonomic neuropathy. Sexual functions, digestion, blood pressure, bladder function, and control of temperature can be disturbed due to autonomic neuropathy. Signals are sent from the brain to areas where autonomic nerves are present that communication channels get disturbed. Mostly, blood vessels, sweat glands, and the heart are the areas that get affected

Symptoms

The nerve which is damaged decides the symptoms for the patient. Sudden fall in blood pressure while standing causes dizziness and fainting. Urinary tract infections can be caused by this disease as the urinary bladder doesn’t get fully empty in this disease. The sensation of a full bladder is lost, bladder control is lost, problems with urination. Loss of appetite, constipation, nausea, vomiting, abdominal blotting, and heartburn can be seen in this disease. 

Cause

It can be caused as a side effect of other diseases including diabetes which is not controlled causes damage to the nerves in the whole body. The organs and the nerves get affected by irregular protein buildup. In cancer drugs are used for treatment, these drugs can cause autonomic neuropathy. Bacterias and viruses like HIV and other which causes Lyme disease and botulism can also cause autonomic neuropathy. The diseases which can be through heredity also cause this disease in an individual.

FAQs on Peripheral Nervous System

Question 1: Explain the two different divisions of nervous system.

Answer:

All the functions assigned to the nervous system are performed in a fraction of seconds. Central nervous system and peripheral nervous system are the two divisions of the nervous system. Spinal cord and brain are included in central nervous system and the nerves which originate from spinal cord and brain are included in peripheral nervous system.

Question 2: Define neurons and nerves.

Answer: 

Brain and spinal cord give rise to thread-like structures which are known as nerves. Nerves are the true protrusions of neurons. Electrochemical signals are conducted by neurons which are the basic and functional unit of our nervous system. Both are interconnected with each other and both are important for the nervous system to work properly.

Question 3: Describe cranial nerves.

Answer:

Brain stem and brain directly exhibit these nerves which reach throughout the body on all sides and are called cranial nerves. Information to mainly the neck and back of the head is transported by cranial nerves. It provides information in the form of signals to the whole body from the brain but its main assigned organs are the back of the head and neck. 

Question 4: Explain the main functions of peripheral nervous system.

Answer:

Main functions of peripheral nervous system are-

• To all the voluntary striated muscles it conveys motor response in our body.
• Providing external information on the environment to the brain and spinal cord.
• Responsible for the proper functioning of our body responses like walking, chewing food, and facial expressions.
• Heart rate, digestion are autonomous functions that are also regulated by the peripheral nervous system.  

Question 5: Give an example of the work done by the peripheral nervous system.

Answer:

There are many works that are done by the peripheral nervous system out of which one is an enlargement of the pupil when a person enters a dark or dim lighted room. Retina gets maximum light through the enlargement of the pupil. CNS is reached by the sensory receptors when a bright light suddenly falls on the eyes due to which the size of the pupil is reduced under bright light. In this process, the stimulus is mediated by PNS.