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The human nervous system is a complex network of neurons that extends throughout the body and performs the role of communication and integration. The human nervous system has a sensory division as well as a motor division. The sensory division detects the internal and environmental stimuli and triggers the motor division which brings about the motor response by influencing the muscles or glands. In addition to this, the nervous system is also involved in learning and adapting to environmental stimuli and producing responses in a better way over time. It helps to memorize things and controls cognitive and behavioral functions.
Introduction to the functions of the human nervous system
The human nervous system has two main physiological divisions which include the somatic nervous system which is concerned with the voluntary actions of the body and the autonomic nervous system which is concerned with the involuntary actions of the body. These systems control all the functions of the human body either voluntary or involuntary. They make multiple connections with each other and influence the individual responses of each division. The anatomical divisions of the nervous system include the central nervous system and the peripheral nervous system. Each division of the nervous system interacts with the other division in an integrated and complex way to control the body’s response to stimuli.
Major physiological functions of the nervous system
The nervous system controls a wide range of the activities and body’s responses. Some major functions of the nervous system are as follows:
The nervous system performs the duty of sensory perception. We need sensory information to sense any danger, maintain an appropriate response to the environment, maintain the internal body environment (homeostasis), time out activities, obtain food resources, see and apprehend the surroundings, understand the meaning of words, and so on. Sensory perception involves receptors that detect the stimulus and stimulate the sensory neurons which send signals to the CNS and make an understanding of the stimulus and trigger an appropriate body response. Vision, hearing, touch, smell, balance, pain, heat, and cold, etc all come under sensory perception.
If there are no sensory receptors in the retina of the eyes, we will not be able to see; if there are no sensory receptors in the nose we will not be able to smell. If there are no sensory receptors in the taste buds we will not be able to detect taste. If there are no pain receptors in the body, we will not be able to sense pain and if we don’t sense pain we will not be able to take appropriate steps to save ourselves from the damaging agents causing pain. If there are no heat and cold receptors, we will not be able to make appropriate steps to save ourselves from extreme hot or cold. The importance of sensory perception is evident here; if there is a disturbance in the sensory perception, the normal physiological functioning of the body is greatly compromised.
Control of mobility
Humans are mobile organisms. They can move from one place to another and all this mobility is under the control of the somatic or voluntary division of the nervous system which controls the skeletal muscles of the body. The mobility of human beings isn’t random; it occurs in an organized and programmed way. Even in simple walking, there is a specific pattern of alternative muscles’ contractions and relaxations to allow the movement in a specific range and direction. All this programmed contractility of skeletal muscles is under the control of the nervous system. Just consider the example of bending the elbow which involves the contraction of the biceps and relaxation of the tricep muscle. This programmed contraction and relaxation are under nervous control. If both the bicep and tricep contract at the same time or both relax at the same time, normal movement will not occur. The nervous system stimulates the appropriate muscles at a joint to bring specific movements. The higher levels of movements such as walking, running, swimming, etc which involve multiple groups of muscles require more integrated control which is also the job of the nervous system.
Control of breathing
Breathing is a continuous process. It continues whether we are awake or asleep. It is an automatic process that is controlled subconsciously by the respiratory center in the brain stem. We can hold breathing for a short time but can’t stop it for longer durations. There are sensory receptors in the carotid bodies and aorta which detect the CO2 concentration in the body and trigger the respiratory center to breathe more fast and deep. Increased CO2 partial pressure in the blood is the strongest stimulus for breathing. Breathing involves two phases; inhalation and exhalation, both having a specific duration and controlled by the inspiratory signals arising in the CNS. The breathing phenomenon is produced by the diaphragm and the intercostal muscles. These muscles are controlled by dorsal and ventral neural groups in the medulla oblongata. Dorsal neural groups contain inspiratory neurons while ventral neuronal groups have both inspiratory and expiratory neurons. These neuronal groups also make multiple connections which form the basis of reciprocal inhibition. Inspiratory neurons produce inspiratory ramp signals which stimulate the contraction of the diaphragm and intercostal muscles to increase the space of the chest cavity and air moves inside. When the inspiratory signal is over, the diaphragm and intercostal muscles relax to cause expiration. The duration of each phase and overall breathing rate is tightly regulated to meet the ventilatory needs of the body.
Control of the myocardial contractility
Similar to breathing, myocardial contractility is crucial for life. The control center for the regulation of myocardial contractility is located in the brainstem. The heart contracts and pumps blood throughout life without getting fatigued. The heart stops when life stops. However, this myocardial contractility is also tightly regulated to ensure that the vital body organs receive adequate blood and nutrition. Myocardial contractility isn’t under our voluntary control. Its rate and strength of contraction are regulated by the autonomic nervous system. Sympathetic discharge increases heart rate and strength of contraction and raises the cardiac output. Parasympathetic discharge slows down the heart rate and decreases cardiac output. The balance of sympathetic and parasympathetic discharge maintains normal cardiac output and blood pressure and also makes adjustments in the heart rate and cardiac output at the time of crisis. For example, when blood pressure falls due to hypovolemia, peripheral vasodilation, or shock, sympathetic discharge increases to increase cardiac output and heart rate while when there is hypertension due to kidney stenosis or other conditions, parasympathetic discharge increases to bring blood pressure towards normal.
Control of hormonal secretions
The nervous system plays a key role in the hormonal regulation of the body. Two branches of the nervous system, the sympathetic nervous system, and the parasympathetic nervous system control the levels of stress hormones in the body. When the body is under a state of stress, the sympathetic system becomes active which increases the release of epinephrine and norepinephrine. Epinephrine and norepinephrine bring about the “fight or flight” response enabling the body to either counter the danger or fly away from the threat. However, when sympathetic system activity declines and the parasympathetic system takes hold, the levels of stress hormones decline. The sympathetic and parasympathetic systems also influence the release of insulin from the pancreatic beta cells. Sympathetic stimulation decreases insulin release while parasympathetic stimulation increases insulin release.
Hypothalamus is the part of the brain that forms a strong association with the endocrine system. It controls the activities of multiple organs and glands of the body through its neurotransmitters and hormones. The pituitary gland is greatly under the control of the hypothalamus which in turn controls the secretions of the thyroid gland, adrenal gland, and gonadal hormones. Hypothalamus receives input signals from the central as well as peripheral nervous systems and also senses the chemical signals from the blood. It releases two main kinds of hormones, releasing hormones and inhibitory hormones controlling the secretions of other hormones.
The hypothalamus controls the functioning of thyroid hormone through thyrotropin-releasing hormone. The thyrotropin-releasing hormone acts on the anterior pituitary gland and stimulates the secretion of thyroid-stimulating hormone (TSH) which stimulates the thyroid gland to increase the production of principal thyroid hormones, thyroxine, and triiodothyronine. Similarly, the hypothalamus releases a corticotropin-releasing hormone which acts on the anterior pituitary gland to increase the secretion of adrenocorticotrophin hormone (ACTH). ACTH stimulates the adrenal gland to increase the secretion of the stress hormone, cortisol. Hypothalamus also releases the gonadotropin-releasing hormone in a pulsatile pattern which increases the production of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones in turn control gonadal steroidogenesis and normal reproductive function.
Inhibitory hormones of the hypothalamus include dopamine and somatostatin which suppress the release of prolactin and growth hormone. Moreover, the hypothalamus also synthesizes oxytocin and anti-diuretic hormone which are stored in the posterior pituitary and released according to the requirements.
Body temperature regulation
Temperature regulation is the integrated response produced by the hypothalamus, autonomic nervous system, and endocrine system. It is controlled primarily by the hypothalamus. The anterior hypothalamic preoptic area is rich in heat-sensitive and cold-sensitive neurons. These neurons act as temperature sensors to regulate the body temperature. The heat-sensitive neurons increase their firing rate by 2 to 10 fold for every 10-degree Celsius increase in temperature while cold-sensitive neurons fire when the body temperature declines. When the preoptic area is heated, the entire body breaks out in profuse sweating and skin blood vessels dilate to increase the heat loss. In addition, heat production in the body is also dramatically reduced. Heat and cold receptors are also present in the peripheral tissues which detect temperature changes and send the signals to the hypothalamus. The posterior hypothalamic area integrates central and peripheral information to regulate the temperature. If body temperature declines, it promotes skin vasoconstriction to decrease heat loss, inhibits sweating, and increases shivering & nonshivering thermogenesis to increase the body temperature.
Thirst and fluid balance
Thirst and fluid balance are maintained by a complex feedback loop where changes in the osmotic state in the body trigger changes in the thirst and fluid intake which affect the fluid load in the body. The osmotic state of the body is sensed by various sensors such as stretch receptors of blood vessels, carotid and aortic bodies, baroreceptors, and osmoreceptors of the hypothalamus. These receptors monitor changes in the plasma osmolality, blood volume, and blood pressure and stimulate the thirst center in the hypothalamus. The thirst center in turn increases the fluid intake to bring back the fluid levels to normal. Moreover, the hypothalamic hormone ADH also influences the fluid balance in the body. If fluid levels in the body are low, ADH production is increased which increases fluid reabsorption from the kidney tubules and reduces the urinary output.
Memory and cognition
Memory is the ability of the brain to store information while cognition refers to the ability of the body to integrate information, think, and find solutions to problems. Memory is stored in different brain regions primarily the hippocampus. It is stored in the form of axonal modifications and is divided into short-term and long-term memory. Short-term memory is a temporary memory that allows us to handle and manipulate information for a short period of time and is lost after a short duration if not rehearsed. Long-term memory is the information stored in the brain for prolonged periods and isn’t wiped away in a short duration. Both short-term and long-term memories are critical brain functions that impart a great impact on human lives. Similarly, cognition is also a uniquely human ability to interlink information, think, and manipulate information. It’s also a neuronal phenomenon that is poorly understood.
Hunger and appetite regulation
Hunger and appetite regulation is a complex interplay of hormones and the nervous system. Hypothalamus is the key regulator of hunger and energy balance in the body. Hunger is signaled by various hormones such as ghrelin, leptin, and insulin. Hypothalamus receives these signals and adjusts the food intake accordingly to maintain the energy balance. The peripheral nervous system is also involved in appetite and hunger regulation. The vagus nerve sends signals to the brain about the status of nutrient absorption and metabolism from GIT. Then hypothalamus adjusts the food intake accordingly.
The nervous system controls the timing and quality of sleep. It controls mood and behavior and helps to learn and adapt to new experiences. It gives a sense of self-awareness and consciousness and helps to understand one’s feelings, emotions, and sensations. The nervous system forms the basis of communication and helps to speak and interact with others. It helps to sense time and estimate the duration of events. In short, the nervous system plays a life-sustaining role in human biology.
The nervous system is a neuronal network scattered throughout the body. It has different anatomical and physiological parts. It’s divided into the somatic nervous system concerned with the voluntary processes of the body and the autonomic nervous system concerned with involuntary processes of the body.
Both these systems are linked to each other to bring out functional integration in the body. It can also be divided into sensory and motor divisions; the sensory part of the nervous system detects the various internal and external stimuli and sends impulses to the central nervous system which in turn triggers the motor part to produce the response.
The nervous system performs a wide range of physiological activities without which human life is impossible. It helps us to see, smell, taste, hear, and touch.
Our hot and cold sensation is due to the virtue of the nervous system. It helps us to sense the pain of tissue damage and take appropriate steps to avoid tissue damage. It controls our mobility. During locomotion, specific groups of body muscles contract and relax in a specific fashion to produce the movement. These muscle movements are under the control of the nervous system.
The nervous system controls the rate and depth of breathing according to the body’s needs. It maintains the cardiac output and heart rate. The nervous system also controls the functioning of many endocrine glands of the body.
The hypothalamus controls the release of anterior pituitary hormones which in turn control the secretions of thyroid and adrenal hormones. Dopamine and somatostatin produced by the hypothalamus inhibit the pituitary release of prolactin and growth hormone. Hypothalamus releases ADH which plays its role in fluid regulation in the body.
Hypothalamus also plays its role in appetite, hunger, and thirst regulation. It is the key component of CNS involved in body temperature regulation.
The nervous system regulates emotions, memory, and cognitive functions. In short, the nervous system is crucial to maintain human biology.
Ludwig PE, Reddy V, Varacallo M. Neuroanatomy, Central Nervous System (CNS) [Updated 2022 Oct 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK442010/